Pharmacology_Katzung

Pharmacology_Katzung. intrinsic factor–vitamin B12 complex is subsequently absorbed in the distal ileum by a highly selective receptor-mediated transport system. Vitamin B12 deficiency in humans most often results from malabsorption of vitamin B12 due either to lack of intrinsic factor or to loss or malfunction of the absorptive mechanism in the distal ileum. Nutritional deficiency is rare but may be seen in strict vegetarians after many years without meat, eggs, or dairy products.


Pharmacology_Katzung

Pharmacology_Katzung. Once absorbed, vitamin B12 is transported to the various cells of the body bound to a family of specialized glycoproteins, transcobalamin I, II, and III. Excess vitamin B12 is stored in the liver.


Pharmacology_Katzung

Pharmacology_Katzung. Two essential enzymatic reactions in humans require vitamin B12 (Figure 33–2). In one, methylcobalamin serves as an intermediate in the transfer of a methyl group from N5-methyltetrahydrofolate to homocysteine, forming methionine (Figure 33–2A; Figure 33–3, section 1). Without vitamin B12, conversion of the major dietary and storage folate—N 5-methyltetrahydrofolate—to tetrahydrofolate, the precursor of folate cofactors, cannot occur. As a result, vitamin B12 deficiency leads to deficiency of folate cofactors necessary for several biochemical reactions involving the transfer of one-carbon groups. In particular, the depletion of tetrahydrofolate prevents synthesis of adequate supplies of the deoxythymidylate (dTMP) and purines required for DNA synthesis in rapidly dividing cells, as shown in Figure 33–3, section 2. The accumulation of folate as N5-methyltetrahydrofolate and the associated depletion of tetrahydrofolate cofactors in vitamin B12 deficiency have been referred to as the


Pharmacology_Katzung

Pharmacology_Katzung. in Figure 33–3, section 2. The accumulation of folate as N5-methyltetrahydrofolate and the associated depletion of tetrahydrofolate cofactors in vitamin B12 deficiency have been referred to as the “methylfolate trap.” This is the biochemical step whereby vitamin B12 and folic acid metabolism are linked, and it explains why the megaloblastic anemia of vitamin B12 deficiency can be partially corrected by ingestion of large amounts of folic acid. Folic acid can be reduced to dihydrofolate by the enzyme dihydrofolate


Pharmacology_Katzung

Pharmacology_Katzung. A. Methyl transfer N5-Methyltetrahydrofolate B. Isomerization of L-Methylmalonyl-CoA FIGURE 33–2 Enzymatic reactions that use vitamin B12. N5 , N10-Methylenetetrahydrofolate Thymidylate synthase Serine transhydroxymethylase Folate reductase Dihydrofolate Glycine Serine Folic acid Tetrahydrofolate Methylcobalamin Cobalamin Methionine Homocysteine N5-Methyltetrahydrofolate 231 FIGURE 33–3 Enzymatic reactions that use folates. Section 1 shows the vitamin B12–dependent reaction that allows most dietary folates to enter the tetrahydrofolate cofactor pool and becomes the “folate trap” in vitamin B12 deficiency. Section 2 shows the deoxythymidine monophosphate (dTMP) cycle. Section 3 shows the pathway by which folic acid enters the tetrahydrofolate cofactor pool. Double arrows indicate pathways with more than one intermediate step. dUMP, deoxyuridine monophosphate.


Pharmacology_Katzung

Pharmacology_Katzung. reductase (Figure 33–3, section 3) and thereby serve as a source of the tetrahydrofolate required for synthesis of the purines and dTMP required for DNA synthesis. Vitamin B12 deficiency causes the accumulation of homocysteine due to reduced formation of methylcobalamin, which is required for the conversion of homocysteine to methionine (Figure 33–3, section 1). The increase in serum homocysteine can be used to help establish a diagnosis of vitamin B12 deficiency (Table 33–2). There is evidence from observational studies that elevated serum homocysteine increases the risk of atherosclerotic cardiovascular disease. However, randomized clinical trials have not shown a definitive reduction in cardiovascular events (myocardial infarction, stroke) in patients receiving vitamin supplementation that lowers serum homocysteine.


Pharmacology_Katzung

Pharmacology_Katzung. The other reaction that requires vitamin B12 is isomerization of methylmalonyl-CoA to succinyl-CoA by the enzyme methylmalonyl-CoA mutase (Figure 33–2B). In vitamin B12 deficiency, this conversion cannot take place and the substrate, methylmalonyl-CoA, as well as methylmalonic acid accumulate. The increase in serum and urine concentrations of methylmalonic acid can be used to support a diagnosis of vitamin B12 deficiency (Table 33–2). In the past, it was thought that abnormal accumulation of methylmalonyl-CoA causes the neurologic manifestations of vitamin B12 deficiency. However, newer evidence implicates the disruption of the methionine synthesis pathway as the cause of neurologic problems. Whatever the biochemical explanation for neurologic damage, the important point is that administration of folic acid in the setting of vitamin B12 deficiency will not prevent neurologic manifestations even though it will largely correct the anemia caused by the vitamin B12 deficiency.


Pharmacology_Katzung

Pharmacology_Katzung. Vitamin B12 is used to treat or prevent deficiency. The most characteristic clinical manifestation of vitamin B12 deficiency is megaloblastic, macrocytic anemia (Table 33–2), often with associated mild or moderate leukopenia or thrombocytopenia (or both), and a characteristic hypercellular bone marrow with an accumulation of megaloblastic erythroid and other precursor cells. The neurologic syndrome associated with vitamin B12 deficiency usually begins with paresthesias in peripheral nerves and weakness and progresses to spasticity, ataxia, and other central nervous system dysfunctions. Correction of vitamin B12 deficiency arrests the progression of neurologic disease, but it may not fully reverse neurologic symptoms that have been present for several months. Although most patients with neurologic abnormalities caused by vitamin B12 deficiency have megaloblastic anemia when first seen, occasional patients have few if any hematologic abnormalities.


Pharmacology_Katzung

Pharmacology_Katzung. Once a diagnosis of megaloblastic anemia is made, it must be determined whether vitamin B12 or folic acid deficiency is the cause. (Other causes of megaloblastic anemia are very rare.) This can usually be accomplished by measuring serum levels of the vitamins. The Schilling test, which measures absorption and urinary excretion of radioactively labeled vitamin B12, can be used to further define the mechanism of vitamin B12 malabsorption when this is found to be the cause of the megaloblastic anemia. The most common causes of vitamin B12 deficiency are pernicious anemia, partial or total gastrectomy, and conditions that affect the distal ileum, such as malabsorption syndromes, inflammatory bowel disease, or small bowel resection. Strict vegans eating a diet free of meat and dairy products may become B12 deficient.


Pharmacology_Katzung

Pharmacology_Katzung. Pernicious anemia results from defective secretion of intrinsic factor by the gastric mucosal cells. Patients with pernicious anemia have gastric atrophy and fail to secrete intrinsic factor (as well as hydrochloric acid). These patients frequently have autoantibodies to intrinsic factor. Historically, the Schilling test demonstrated diminished absorption of radioactively labeled vitamin B12, which is corrected when intrinsic factor is administered with radioactive B12, since the vitamin can then be normally absorbed. This test is now rarely performed due to use of radioactivity in the assay.


Pharmacology_Katzung

Pharmacology_Katzung. Vitamin B12 deficiency also occurs when the region of the distal ileum that absorbs the vitamin B12–intrinsic factor complex is damaged, as when the ileum is involved with inflammatory bowel disease or when the ileum is surgically resected. In these situations, radioactively labeled vitamin B12 is not absorbed in the Schilling test, even when intrinsic factor is added. Rare cases of vitamin B12 deficiency in children have been found to be secondary to congenital deficiency of intrinsic factor or to defects of the receptor sites for vitamin B12–intrinsic factor complex located in the distal ileum. Alternatives to the Schilling test include testing for intrinsic factor antibodies and testing for elevated homocysteine and methylmalonic acid levels (Figure 33–2) to make a diagnosis of pernicious anemia with high sensitivity and specificity.


Pharmacology_Katzung

Pharmacology_Katzung. Almost all cases of vitamin B12 deficiency are caused by malabsorption of the vitamin; therefore, parenteral injections of vitamin B12 are required for therapy. For patients with potentially reversible diseases, the underlying disease should be treated after initial treatment with parenteral vitamin B12. Most patients, however, do not have curable deficiency syndromes and require lifelong treatment with vitamin B12.


Pharmacology_Katzung

Pharmacology_Katzung. Vitamin B12 for parenteral injection is available as cyanocobalamin or hydroxocobalamin. Hydroxocobalamin is preferred because it is more highly protein-bound and therefore remains longer in the circulation. Initial therapy should consist of 100–1000 mcg of vitamin B12 intramuscularly daily or every other day for 1–2 weeks to replenish body stores. Maintenance therapy consists of 100–1000 mcg intramuscularly once a month for life. If neurologic abnormalities are present, maintenance therapy injections should be given every 1–2 weeks for 6 months before switching to monthly injections. Oral vitamin B12–intrinsic factor mixtures and liver extracts should not be used to treat vitamin B12 deficiency; however, oral doses of 1000 mcg of vitamin B12 daily are usually sufficient to treat patients with pernicious anemia who refuse or cannot tolerate the injections. After pernicious anemia is in remission following parenteral vitamin B12 therapy, the vitamin can be administered intranasally as


Pharmacology_Katzung

Pharmacology_Katzung. with pernicious anemia who refuse or cannot tolerate the injections. After pernicious anemia is in remission following parenteral vitamin B12 therapy, the vitamin can be administered intranasally as a spray or gel.


Pharmacology_Katzung

Pharmacology_Katzung. Reduced forms of folic acid are required for essential biochemical reactions that provide precursors for the synthesis of amino acids, purines, and DNA. Folate deficiency is relatively common, even though the deficiency is easily corrected by administration of folic acid. The consequences of folate deficiency go beyond the problem of anemia because folate deficiency is implicated as a cause of congenital malformations in newborns and may play a role in vascular disease (see Box: Folic Acid Supplementation: A Public Health Dilemma).


Pharmacology_Katzung

Pharmacology_Katzung. Starting in January 1998, all products made from enriched grains in the United States and Canada were required to be supplemented with folic acid. These rulings were issued to reduce the incidence of congenital neural tube defects (NTDs). Epidemiologic studies show a strong correlation between maternal folic acid deficiency and the incidence of NTDs such as spina bifida and anencephaly. The requirement for folic acid supplementation is a public health mea-sure aimed at the significant number of women who do not receive prenatal care and are not aware of the importance of adequate folic acid ingestion for preventing birth defects in their infants. Observational studies from countries that supplement grains with folic acid have found that supplementation is associated with a significant (20–25%) reduction in NTD rates. Observational stud-ies also suggest that rates of other types of congenital anomalies (heart and orofacial) have fallen since supplementation began. There may be an added


Pharmacology_Katzung

Pharmacology_Katzung. reduction in NTD rates. Observational stud-ies also suggest that rates of other types of congenital anomalies (heart and orofacial) have fallen since supplementation began. There may be an added benefit for adults. N5-Methyl-tetrahydrofolate is required for the conversion of homocysteine to methionine (Figure 33–2; Figure 33–3, reaction 1). Impaired synthesis of N5-methyltetrahydrofolate results in elevated serum concentrations of homocysteine. Data from several sources sug-gest a positive correlation between elevated serum homocysteine and occlusive vascular diseases such as ischemic heart disease and stroke. Clinical data suggest that the folate supplementation pro-gram has improved the folate status and reduced the prevalence of hyperhomocysteinemia in a population of middle-aged and older adults who did not use vitamin supplements. There is also evidence that adequate folic acid protects against several cancers, including colorectal, breast, and cervical cancer. Although the


Pharmacology_Katzung

Pharmacology_Katzung. and older adults who did not use vitamin supplements. There is also evidence that adequate folic acid protects against several cancers, including colorectal, breast, and cervical cancer. Although the potential benefits of supplemental folic acid during pregnancy are compelling, the decision to require folic acid in grains was controversial. As described in the text, ingestion of folic acid can partially or totally correct the anemia caused by vitamin B12 deficiency. However, folic acid supple-mentation does not prevent the potentially irreversible neu-rologic damage caused by vitamin B12 deficiency. People with pernicious anemia and other forms of vitamin B12 deficiency are usually identified because of signs and symptoms of ane-mia, which typically occur before neurologic symptoms. Some opponents of folic acid supplementation were concerned that increased folic acid intake in the general population would mask vitamin B12 deficiency and increase the prevalence of neurologic disease in


Pharmacology_Katzung

Pharmacology_Katzung. of folic acid supplementation were concerned that increased folic acid intake in the general population would mask vitamin B12 deficiency and increase the prevalence of neurologic disease in the elderly population. To put this in perspective, approximately 4000 pregnancies, including 2500 live births, in the United States each year are affected by NTDs. In contrast, it is estimated that more than 10% of the elderly population in the United States, or several million people, are at risk for the neuropsychiatric complications of vitamin B12 defi-ciency. In acknowledgment of this controversy, the FDA kept its requirements for folic acid supplementation at a somewhat low level. There is also concern based on observational and prospective clinical trials that high folic acid levels can increase the risk of some diseases, such as colorectal cancer, for which folic acid may exhibit a bell-shaped curve. Further research is needed to more accurately define the optimal level of folic acid


Pharmacology_Katzung

Pharmacology_Katzung. the risk of some diseases, such as colorectal cancer, for which folic acid may exhibit a bell-shaped curve. Further research is needed to more accurately define the optimal level of folic acid fortification in food and recommendations for folic acid supple-mentation in different populations and age groups.


Pharmacology_Katzung

Pharmacology_Katzung. Folic acid (pteroylglutamic acid) is composed of a heterocycle (pteridine), p-aminobenzoic acid, and glutamic acid (Figure 33–4). Various numbers of glutamic acid moieties are attached to the pteroyl portion of the molecule, resulting in monoglutamates, triglutamates, or polyglutamates. Folic acid undergoes reduction, catalyzed by the enzyme dihydrofolate reductase (“folate reductase”), to give dihydrofolic acid (Figure 33–3, section 3). Tetrahydrofolate is subsequently transformed to folate cofactors possessing one-carbon units attached to the 5-nitrogen, to the 10-nitrogen, or to both positions (Figure 33–3). Folate cofactors are interconvertible by various enzymatic reactions and serve the important biochemical function of donating one-carbon units at various levels of oxidation. In most of these, tetrahydrofolate is regenerated and becomes available for reutilization.


Pharmacology_Katzung

Pharmacology_Katzung. The average American diet contains 500–700 mcg of folates daily, 50–200 mcg of which is usually absorbed, depending on metabolic requirements. Pregnant women may absorb as much as 300–400 mcg of folic acid daily. Various forms of folic acid are


Pharmacology_Katzung

Pharmacology_Katzung. FIGURE 33–4 The structure of folic acid. (Reproduced, with permission, from Murray RK et al: Harper’s Biochemistry, 24th ed. McGraw-Hill, 1996. Copyright © The McGraw-Hill Companies, Inc.) present in a wide variety of plant and animal tissues; the richest sources are yeast, liver, kidney, and green vegetables. Normally, 5–20 mg of folates is stored in the liver and other tissues. Folates are excreted in the urine and stool and are also destroyed by catabolism, so serum levels fall within a few days when intake is diminished. Because body stores of folates are relatively low and daily requirements high, folic acid deficiency and megaloblastic anemia can develop within 1–6 months after the intake of folic acid stops, depending on the patient’s nutritional status and the rate of folate utilization.


Pharmacology_Katzung

Pharmacology_Katzung. Unaltered folic acid is readily and completely absorbed in the proximal jejunum. Dietary folates, however, consist primarily of polyglutamate forms of N 5-methyltetrahydrofolate. Before absorption, all but one of the glutamyl residues of the polyglutamates must be hydrolyzed by the enzyme α-1-glutamyl transferase (“conjugase”) within the brush border of the intestinal mucosa. The monoglutamate N 5-methyltetrahydrofolate is subsequently transported into the bloodstream by both active and passive transport and is then widely distributed throughout the body. Inside cells, N 5-methyltetrahydro-folate is converted to tetrahydrofolate by the demethylation reaction that requires vitamin B12 (Figure 33–3, section 1).


Pharmacology_Katzung

Pharmacology_Katzung. Tetrahydrofolate cofactors participate in one-carbon transfer reactions. As described earlier in the discussion of vitamin B12, one of these essential reactions produces the dTMP needed for DNA synthesis. In this reaction, the enzyme thymidylate synthase catalyzes the transfer of the one-carbon unit of N 5, N10-methylenetetrahydrofolate to deoxyuridine monophosphate (dUMP) to form dTMP (Figure 33–3, section 2). Unlike all the other enzymatic reactions that use folate cofactors, in this reaction the cofactor is oxidized to dihydrofolate, and for each mole of dTMP produced, 1 mole of tetrahydrofolate is consumed. In rapidly proliferating tissues, considerable amounts of tetrahydrofolate are consumed in this reaction, and continued DNA synthesis requires continued regeneration of tetrahydrofolate by reduction of dihydrofolate, catalyzed by the enzyme dihydrofolate reductase. The tetrahydrofolate thus produced can then reform the cofactor N 5, N10-methylenetetrahydrofolate by the action


Pharmacology_Katzung

Pharmacology_Katzung. by reduction of dihydrofolate, catalyzed by the enzyme dihydrofolate reductase. The tetrahydrofolate thus produced can then reform the cofactor N 5, N10-methylenetetrahydrofolate by the action of serine transhydroxymethylase and thus allow for the continued synthesis of dTMP. The combined catalytic activities of dTMP synthase, dihydrofolate reductase, and serine transhydroxymethylase are referred to as the dTMP synthesis cycle. Enzymes in the dTMP cycle are the targets of two anti-cancer drugs: methotrexate inhibits dihydrofolate reductase, and a metabolite of 5-fluorouracil inhibits thymidylate synthase (see Chapter 54).


Pharmacology_Katzung

Pharmacology_Katzung. Cofactors of tetrahydrofolate participate in several other essential reactions. N 5-Methylenetetrahydrofolate is required for the vitamin B12-dependent reaction that generates methionine from homocysteine (Figure 33–2A; Figure 33–3, section 1). In addition, tetrahydrofolate cofactors donate one-carbon units during the de novo synthesis of essential purines. In these reactions, tetrahydrofolate is regenerated and can reenter the tetrahydrofolate cofactor pool.


Pharmacology_Katzung

Pharmacology_Katzung. Folate deficiency results in a megaloblastic anemia that is microscopically indistinguishable from the anemia caused by vitamin B12 deficiency (see above). However, folate deficiency does not cause the characteristic neurologic syndrome seen in vitamin B12 deficiency. In patients with megaloblastic anemia, folate status is assessed with assays for serum folate or for red blood cell folate. Red blood cell folate levels are often of greater diagnostic value than serum levels, because serum folate levels tend to be labile and do not necessarily reflect tissue levels.


Pharmacology_Katzung

Pharmacology_Katzung. Folic acid deficiency is often caused by inadequate dietary intake of folates. Patients with alcohol dependence and patients with liver disease can develop folic acid deficiency because of poor diet and diminished hepatic storage of folates. Pregnant women and patients with hemolytic anemia have increased folate requirements and may become folic acid-deficient, especially if their diets are marginal. Evidence implicates maternal folic acid deficiency in the occurrence of fetal neural tube defects. (See Box: Folic Acid Supplementation: A Public Health Dilemma.) Patients with malabsorption syndromes also frequently develop folic acid deficiency. Patients who require renal dialysis are at risk of folic acid deficiency because folates are removed from the plasma during the dialysis procedure.


Pharmacology_Katzung

Pharmacology_Katzung. Folic acid deficiency can be caused by drugs. Methotrexate and, to a lesser extent, trimethoprim and pyrimethamine, inhibit dihydrofolate reductase and may result in a deficiency of folate cofactors and ultimately in megaloblastic anemia. Long-term therapy with phenytoin also can cause folate deficiency, but it only rarely causes megaloblastic anemia.


Pharmacology_Katzung

Pharmacology_Katzung. Parenteral administration of folic acid is rarely necessary, since oral folic acid is well absorbed even in patients with malabsorption syndromes. A dose of 1 mg folic acid orally daily is sufficient to reverse megaloblastic anemia, restore normal serum folate levels, and replenish body stores of folates in almost all patients. Therapy should be continued until the underlying cause of the deficiency is removed or corrected. Therapy may be required indefinitely for patients with malabsorption or dietary inadequacy. Folic acid supplementation to prevent folic acid deficiency should be considered in high-risk patients, including pregnant women, patients with alcohol dependence, hemolytic anemia, liver disease, or certain skin diseases, and patients on renal dialysis.


Pharmacology_Katzung

Pharmacology_Katzung. The hematopoietic growth factors are glycoprotein hormones that regulate the proliferation and differentiation of hematopoietic progenitor cells in the bone marrow. The first growth factors to be identified were called colony-stimulating factors because they could stimulate the growth of colonies of various bone marrow progenitor cells in vitro. Many of these growth factors have been purified and cloned, and their effects on hematopoiesis have been extensively studied. Quantities of these growth factors sufficient for clinical use are produced by recombinant DNA technology. Of the known hematopoietic growth factors, erythropoietin (epoetin alfa and epoetin beta), granulocyte colony-stimulating factor (G-CSF), granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin 11 (IL-11), and thrombopoietin receptor agonists (romiplostim and eltrombopag) are currently in clinical use.


Pharmacology_Katzung

Pharmacology_Katzung. The hematopoietic growth factors and drugs that mimic their action have complex effects on the function of a wide variety of cell types, including nonhematologic cells. Their usefulness in other areas of medicine, particularly as potential anti-cancer and anti-inflammatory drugs, is being investigated.


Pharmacology_Katzung

Pharmacology_Katzung. Erythropoietin, a 34to 39-kDa glycoprotein, was the first human hematopoietic growth factor to be isolated. It was originally purified from the urine of patients with severe anemia. Recombinant human erythropoietin (rHuEPO, epoetin alfa) is produced in a mammalian cell expression system. After intravenous administration, erythropoietin has a serum half-life of 4–13 hours in patients with chronic renal failure. It is not cleared by dialysis. It is measured in international units (IU). Darbepoetin alfa is a modified form of erythropoietin that is more heavily glycosylated as a result of changes in amino acids. Darbepoetin alfa has a twofold to threefold longer half-life than epoetin alfa. Methoxy polyethylene glycol–epoetin beta is an isoform of erythropoietin covalently attached to a long polyethylene glycol polymer. This long-lived recombinant product is administered as a single intravenous or subcutaneous dose at 2-week or monthly intervals, whereas epoetin alfa is generally


Pharmacology_Katzung

Pharmacology_Katzung. a long polyethylene glycol polymer. This long-lived recombinant product is administered as a single intravenous or subcutaneous dose at 2-week or monthly intervals, whereas epoetin alfa is generally administered three times a week and darbepoetin is administered weekly.


Pharmacology_Katzung

Pharmacology_Katzung. Erythropoietin stimulates erythroid proliferation and differentiation by interacting with erythropoietin receptors on red cell progenitors. The erythropoietin receptor is a member of the JAK/ STAT superfamily of cytokine receptors that use protein phosphorylation and transcription factor activation to regulate cellular function (see Chapter 2). Erythropoietin also induces release of reticulocytes from the bone marrow. Endogenous erythropoietin is produced primarily in the kidney. In response to tissue hypoxia, more erythropoietin is produced through an increased rate of transcription of the erythropoietin gene. This results in correction of the anemia, provided that the bone marrow response is not impaired by red cell nutritional deficiency (especially iron deficiency), primary bone marrow disorders (see below), or bone marrow suppression from drugs or chronic diseases.


Pharmacology_Katzung

Pharmacology_Katzung. Normally, an inverse relationship exists between the hematocrit or hemoglobin level and the serum erythropoietin level. Nonanemic individuals have serum erythropoietin levels of less than 20 IU/L. As the hematocrit and hemoglobin levels fall and anemia becomes more severe, the serum erythropoietin level rises exponentially. Patients with moderately severe anemia usually have erythropoietin levels in the 100–500 IU/L range, and patients with severe anemia may have levels of thousands of IU/L. The most important exception to this inverse relationship is in the anemia of chronic renal failure. In patients with renal disease, erythropoietin levels are usually low because the kidneys cannot produce the growth factor. These are the patients most likely to respond to treatment with exogenous erythropoietin. In most primary bone marrow disorders (aplastic anemia, leukemias, myeloproliferative and myelodysplastic disorders, etc) and most nutritional and secondary anemias, endogenous


Pharmacology_Katzung

Pharmacology_Katzung. erythropoietin. In most primary bone marrow disorders (aplastic anemia, leukemias, myeloproliferative and myelodysplastic disorders, etc) and most nutritional and secondary anemias, endogenous erythropoietin levels are high, so there is less likelihood of a response to exogenous erythropoietin (but see below).


Pharmacology_Katzung

Pharmacology_Katzung. The availability of erythropoiesis-stimulating agents (ESAs) has had a significant positive impact for patients with several types of anemia (Table 33–4). The ESAs consistently improve the hematocrit and hemoglobin level, often eliminate the need for transfusions, and reliably improve quality of life indices. The ESAs are used routinely in patients with anemia secondary to chronic kidney disease. In patients treated with an ESA, an increase in reticulocyte count is usually observed in about 10 days and an increase in hematocrit and hemoglobin levels in 2–6 weeks. Dosages of ESAs are adjusted to maintain a target hemoglobin up to, but not exceeding, 10–12 g/dL. To support the increased erythropoiesis, nearly all patients with chronic kidney disease require oral or parenteral iron supplementation. Folate supplementation may also be necessary in some patients.


Pharmacology_Katzung

Pharmacology_Katzung. In selected patients, erythropoietin is also used to reduce the need for red blood cell transfusion in patients undergoing myelosuppressive cancer chemotherapy who have a hemoglobin level of less than 10 g/dL, and for selected patients with low-risk myelodysplastic syndromes and anemia requiring red blood cell transfusion. Patients who have disproportionately low serum erythropoietin levels for their degree of anemia are most likely to respond to treatment. Patients with endogenous erythropoietin levels of less than 100 IU/L have the best chance of response, although patients with erythropoietin levels between 100 and 500 IU/L respond occasionally. Methoxy polyethylene glycol– epoetin beta should not be used for treatment of anemia caused by cancer chemotherapy because a clinical trial found significantly more deaths among patients receiving this form of erythropoietin.


Pharmacology_Katzung

Pharmacology_Katzung. Erythropoietin is one of the drugs commonly used illegally by endurance athletes to enhance performance. Other methods such as autologous transfusion of red cells or use of androgens also have been used to increase hemoglobin. “Blood doping” constitutes a serious health risk to athletes and as a form of cheating is universally banned and routinely tested for in athletic events. The most common adverse effects of erythropoietin are hypertension and thrombotic complications. ESAs increase the risk of serious cardiovascular events, thromboembolic events, stroke, and mortality in clinical studies when given to support hemoglobin TABLE 33–4 Clinical uses of hematopoietic growth factors and agents that mimic their actions.


Pharmacology_Katzung

Pharmacology_Katzung. TABLE 33–4 Clinical uses of hematopoietic growth factors and agents that mimic their actions. levels greater than 11 g/dL. In addition, a meta-analysis of 51 placebo-controlled trials of ESAs in cancer patients reported an increased rate of all-cause mortality and venous thrombosis in those receiving an ESA. Based on the accumulated evidence, it is recommended that the hemoglobin level not exceed 11 g/dL in patients with chronic kidney disease receiving an ESA, and that ESAs be used conservatively in cancer patients (eg, when hemoglobin levels are <10 g/dL) and with the lowest dose needed to avoid transfusion. It is further recommended that ESAs not be used when a cancer therapy is being given with curative intent.


Pharmacology_Katzung

Pharmacology_Katzung. Allergic reactions to ESAs have been infrequent. There have been a small number of cases of pure red cell aplasia (PRCA) accompanied by neutralizing antibodies to erythropoietin. PRCA was most commonly seen in dialysis patients treated subcutaneously for a long period with a particular form of epoetin alfa (Eprex with a polysorbate 80 stabilizer rather than human serum albumin) that is not available in the United States. After regulatory agencies required that Eprex be administered intravenously rather than subcutaneously, the rate of ESA-associated PRCA diminished. However, rare cases have still been seen with all ESAs administered subcutaneously for long periods to patients with chronic kidney disease.


Pharmacology_Katzung

Pharmacology_Katzung. G-CSF and GM-CSF, the two myeloid growth factors currently available for clinical use, were originally purified from cultured human cell lines (Table 33–4). Recombinant human G-CSF (rHuG-CSF; filgrastim) is produced in a bacterial expression system. It is a nonglycosylated peptide of 175 amino acids, with a molecular weight of 18 kDa. Tbo-filgastrim is similar to filgrastim, with minor structural differences and equivalent activity. Recombinant human GM-CSF (rHuGM-CSF; sargramostim) is produced in a yeast expression system. It is a partially glycosylated peptide of 127 amino acids, comprising three molecular species with molecular weights of 15,500, 15,800, and 19,500. These preparations have serum half-lives of 2–7 hours after intravenous or subcutaneous administration. Pegfilgrastim, a covalent conjugation product of filgrastim and a form of polyethylene glycol, has a much longer serum half-life than recombinant G-CSF, and it can be injected once per myelosuppressive chemotherapy


Pharmacology_Katzung

Pharmacology_Katzung. a covalent conjugation product of filgrastim and a form of polyethylene glycol, has a much longer serum half-life than recombinant G-CSF, and it can be injected once per myelosuppressive chemotherapy cycle instead of daily for several days. Lenograstim, used widely in Europe, is a glycosylated form of recombinant G-CSF.


Pharmacology_Katzung

Pharmacology_Katzung. The myeloid growth factors stimulate proliferation and differentiation by interacting with specific receptors found on myeloid progenitor cells. Like the erythropoietin receptor, these receptors are members of the JAK/STAT superfamily (see Chapter 2). G-CSF stimulates proliferation and differentiation of progenitors already committed to the neutrophil lineage. It also activates the phagocytic activity of mature neutrophils and prolongs their survival in the circulation. G-CSF also has a remarkable ability to mobilize hematopoietic stem cells, ie, to increase their concentration in peripheral blood. This biologic effect underlies a major advance in transplantation—the use of peripheral blood stem cells (PBSCs) rather than bone marrow stem cells for autologous and allogeneic hematopoietic stem cell transplantation (see below).


Pharmacology_Katzung

Pharmacology_Katzung. GM-CSF has broader biologic actions than G-CSF. It is a multipotential hematopoietic growth factor that stimulates proliferation and differentiation of early and late granulocytic progenitor cells as well as erythroid and megakaryocyte progenitors. Like G-CSF, GM-CSF also stimulates the function of mature neutrophils. GM-CSF acts together with interleukin-2 to stimulate T-cell proliferation and appears to be a locally active factor at the site of inflammation. GM-CSF mobilizes peripheral blood stem cells, but it is significantly less efficacious and more toxic than G-CSF in this regard. A. Cancer Chemotherapy-Induced Neutropenia


Pharmacology_Katzung

Pharmacology_Katzung. A. Cancer Chemotherapy-Induced Neutropenia Neutropenia is a common adverse effect of the cytotoxic drugs used to treat cancer and increases the risk of serious infection in patients receiving chemotherapy. Unlike the treatment of anemia and thrombocytopenia, transfusion of neutropenic patients with granulocytes collected from donors is performed rarely and with limited success. The introduction of G-CSF in 1991 represented a milestone in the treatment of chemotherapy-induced neutropenia. This growth factor dramatically accelerates the rate of neutrophil recovery after dose-intensive myelosuppressive chemotherapy (Figure 33–5). It reduces the duration of neutropenia and usually raises the nadir count, the lowest neutrophil count seen following a cycle of chemotherapy.


Pharmacology_Katzung

Pharmacology_Katzung. The ability of G-CSF to increase neutrophil counts after myelosuppressive chemotherapy is nearly universal, but its impact on clinical outcomes is more variable. Many, but not all, clinical trials and meta-analyses have shown that G-CSF reduces episodes of febrile neutropenia, requirements for broad-spectrum antibiotics, infections, and days of hospitalization. Clinical trials have not shown improved survival in cancer patients treated with G-CSF. Clinical guidelines for the use of G-CSF after cytotoxic chemotherapy recommend reserving G-CSF for patients at high risk for febrile neutropenia based on age, medical history, and disease characteristics; patients receiving dose-intensive chemotherapy regimens that carry a greater than 20% risk of causing febrile 30.0 25.0 20.0 15.0 10.0 5.0


Pharmacology_Katzung

Pharmacology_Katzung. FIGURE 33–5 Effects of granulocyte colony-stimulating factor (G-CSF; red line) or placebo (green line) on absolute neutrophil count (ANC) after cytotoxic chemotherapy for lung cancer. Doses of chemotherapeutic drugs were administered on days 1 and 3. G-CSF or placebo injections were started on day 4 and continued daily through day 12 or 16. The first peak in ANC reflects the recruitment of mature cells by G-CSF. The second peak reflects a marked increase in new neutrophil production by the bone marrow under stimulation by G-CSF. (Normal ANC is 2.2–8.6 × 109/L.) (Reproduced, with permission, from Crawford J et al: Reduction by granulocyte colony-stimulating factor of fever and neutropenia induced by chemotherapy in patients with small-cell lung cancer. N Engl J Med 1991;325:164. Copyright © 1991 Massachusetts Medical Society. Reprinted with permission from Massachusetts Medical Society.) neutropenia; patients with a prior episode of febrile neutropenia after cytotoxic chemotherapy;


Pharmacology_Katzung

Pharmacology_Katzung. © 1991 Massachusetts Medical Society. Reprinted with permission from Massachusetts Medical Society.) neutropenia; patients with a prior episode of febrile neutropenia after cytotoxic chemotherapy; patients at high risk for febrile neutropenia; and patients who are unlikely to survive an episode of febrile neutropenia. Pegfilgrastim is an alternative to G-CSF for prevention of chemotherapy-induced febrile neutropenia. Pegfilgrastim can be administered once per chemotherapy cycle, and it may shorten the period of severe neutropenia slightly more than G-CSF.


Pharmacology_Katzung

Pharmacology_Katzung. Like G-CSF and pegfilgrastim, GM-CSF also reduces the duration of neutropenia after cytotoxic chemotherapy. It has been more difficult to show that GM-CSF reduces the incidence of febrile neutropenia, probably because GM-CSF itself can induce fever. In the treatment of chemotherapy-induced neutropenia, G-CSF 5 mcg/kg daily or GM-CSF 250 mcg/m2 daily is usually started within 24–72 hours after completing chemotherapy and is continued until the absolute neutrophil count is greater than 10,000 cells/μL. Pegfilgrastim is given as a single dose of 6 mg.


Pharmacology_Katzung

Pharmacology_Katzung. The utility and safety of the myeloid growth factors in the postchemotherapy supportive care of patients with acute myeloid leukemia (AML) have been the subject of a number of clinical trials. Because leukemic cells arise from progenitors whose proliferation and differentiation are normally regulated by hematopoietic growth factors, including GM-CSF and G-CSF, there was concern that myeloid growth factors could stimulate leukemic cell growth and increase the rate of relapse. The results of randomized clinical trials suggest that both G-CSF and GM-CSF are safe following induction and consolidation treatment of myeloid and lymphoblastic leukemia. There has been no evidence that these growth factors reduce the rate of remission or increase relapse rate. On the contrary, the growth factors accelerate neutrophil recovery and reduce infection rates and days of hospitalization. Both G-CSF and GM-CSF have FDA approval for treatment of patients with AML. B. Other Applications


Pharmacology_Katzung

Pharmacology_Katzung. B. Other Applications G-CSF and GM-CSF have also proved to be effective in treating the neutropenia associated with congenital neutropenia, cyclic neutropenia, myelodysplasia, and aplastic anemia. Many patients with these disorders respond with a prompt and sometimes dramatic increase in neutrophil count. In some cases, this results in a decrease in the frequency of infections. Because neither G-CSF nor GM-CSF stimulates the formation of erythrocytes and platelets, they are sometimes combined with other growth factors for treatment of pancytopenia.


Pharmacology_Katzung

Pharmacology_Katzung. The myeloid growth factors play an important role in autologous stem cell transplantation for patients undergoing high-dose chemotherapy. High-dose chemotherapy with autologous stem cell support is increasingly used to treat patients with tumors that are resistant to standard doses of chemotherapeutic drugs. The high-dose regimens produce extreme myelosuppression; the myelosuppression is then counteracted by reinfusion of the patient’s own hematopoietic stem cells (which are collected prior to chemotherapy). The administration of G-CSF or GM-CSF early after autologous stem cell transplantation reduces the time to engraftment and to recovery from neutropenia in patients receiving stem cells obtained either from bone marrow or from peripheral blood. These effects are seen in patients being treated for lymphoma or for solid tumors. G-CSF and GM-CSF are also used to support patients who have received allogeneic bone marrow transplantation for treatment of hematologic malignancies or bone


Pharmacology_Katzung

Pharmacology_Katzung. treated for lymphoma or for solid tumors. G-CSF and GM-CSF are also used to support patients who have received allogeneic bone marrow transplantation for treatment of hematologic malignancies or bone marrow failure states. In this setting, the growth factors speed the recovery from neutropenia without increasing the incidence of acute graft-versus-host disease.


Pharmacology_Katzung

Pharmacology_Katzung. Perhaps the most important role of the myeloid growth factors in transplantation is for mobilization of PBSCs. Stem cells collected from peripheral blood have nearly replaced bone marrow as the hematopoietic preparation used for autologous and allogeneic transplantation. The cells can be collected in an outpatient setting with a procedure that avoids much of the risk and discomfort of bone marrow collection, including the need for general anesthesia. In addition, there is evidence that PBSC transplantation results in more rapid engraftment of all hematopoietic cell lineages and in reduced rates of graft failure or delayed platelet recovery.


Pharmacology_Katzung

Pharmacology_Katzung. G-CSF is the cytokine most commonly used for PBSC mobilization because of its increased efficacy and reduced toxicity compared with GM-CSF. To mobilize stem cells for autologous transplantation, donors are given 5–10 mcg/kg daily subcutaneously for 4 days. On the fifth day, they undergo leukapheresis. The success of PBSC transplantation depends on transfusion of adequate numbers of stem cells. CD34, an antigen present on early progenitor cells and absent from later, committed, cells, is used as a marker for the requisite stem cells. The goal is to infuse at least 5 × 106 CD34 cells/kg; this number of CD34 cells usually results in prompt and durable engraftment of all cell lineages. It may take several separate leukaphereses to collect enough CD34 cells, especially from older patients and patients who have been exposed to radiation therapy or chemotherapy.


Pharmacology_Katzung

Pharmacology_Katzung. For patients with multiple myeloma or non-Hodgkin’s lymphoma who respond suboptimally to G-CSF alone, the novel hematopoietic stem cell mobilizer plerixafor can be added to G-CSF. Plerixafor is a bicyclam molecule originally developed as an anti-HIV drug because of its ability to inhibit the CXC chemokine receptor 4 (CXCR4), a co-receptor for HIV entry into CD4+ T lymphocytes (see Chapter 49). Early clinical trials of plerixafor revealed a remarkable ability to increase CD34 cells in peripheral blood. Plerixafor mobilizes CD34 cells by preventing chemokine stromal cell-derived factor-1α (CXCL12) from binding to CXCR4 and directing the CD34 cells to “home” to the bone marrow. Plerixafor is administered by subcutaneous injection after 4 days of G-CSF treatment and 11 hours prior to leukapheresis; it can be used with G-CSF for up to 4 continuous days. Plerixafor is eliminated primarily by the renal route and must be dose-adjusted for patients with renal impairment. The drug is well


Pharmacology_Katzung

Pharmacology_Katzung. it can be used with G-CSF for up to 4 continuous days. Plerixafor is eliminated primarily by the renal route and must be dose-adjusted for patients with renal impairment. The drug is well tolerated; the most common adverse effects associated with its use are injection site reactions, gastrointestinal disturbances, dizziness, fatigue, and headache.


Pharmacology_Katzung

Pharmacology_Katzung. Although the three growth factors have similar effects on neutrophil counts, G-CSF and pegfilgrastim are used more frequently than GM-CSF because they are better tolerated. G-CSF and pegfilgrastim can cause bone pain, which clears when the drugs are discontinued. GM-CSF can cause more severe side effects, particularly at higher doses. These include fever, malaise, arthralgias, myalgias, and a capillary leak syndrome characterized by peripheral edema and pleural or pericardial effusions. Allergic reactions may occur but are infrequent. Splenic rupture is a rare but serious complication of the use of G-CSF for PBSC mobilization.


Pharmacology_Katzung

Pharmacology_Katzung. Patients with thrombocytopenia have a high risk of hemorrhage. Although platelet transfusion is commonly used to treat thrombocytopenia, this procedure can cause adverse reactions in the recipient; furthermore, a significant number of patients fail to exhibit the expected increase in platelet count. Thrombopoietin (TPO) and IL-11 both appear to be key endogenous regulators of platelet production. A recombinant form of IL-11 was the first agent to gain FDA approval for treatment of thrombocytopenia. Recombinant human thrombopoietin and a pegylated form of a shortened human thrombopoietin protein underwent extensive clinical investigation in the 1990s. However, further development was abandoned after autoantibodies to the native thrombopoietin formed in healthy human subjects and caused thrombocytopenia. Efforts shifted to investigation of novel, nonimmunogenic agonists of the thrombopoietin receptor, which is known as Mpl. Two thrombopoietin agonists (romiplostim and eltrombopag) are


Pharmacology_Katzung

Pharmacology_Katzung. Efforts shifted to investigation of novel, nonimmunogenic agonists of the thrombopoietin receptor, which is known as Mpl. Two thrombopoietin agonists (romiplostim and eltrombopag) are approved for treatment of thrombocytopenia.


Pharmacology_Katzung

Pharmacology_Katzung. Interleukin-11 is a 65to 85-kDa protein produced by fibroblasts and stromal cells in the bone marrow. Oprelvekin, the recombinant form of IL-11 approved for clinical use (Table 33–4), is produced by expression in Escherichia coli. The half-life of IL-11 is 7–8 hours when the drug is injected subcutaneously.


Pharmacology_Katzung

Pharmacology_Katzung. Romiplostim is a thrombopoietin agonist peptide covalently linked to antibody fragments that serve to extend the peptide’s half-life. The Mpl-binding peptide has no sequence homology with human thrombopoietin, and there is no evidence in animal or human studies that the Mpl-binding peptide or romiplostim induces antibodies to thrombopoietin. After subcutaneous administration, romiplostim is eliminated by the reticuloendothelial system with an average half-life of 3–4 days. Its half-life is inversely related to the serum platelet count; it has a longer half-life in patients with thrombocytopenia and a shorter half-life in patients whose platelet counts have recovered to normal levels. Romiplostim is approved for therapy of patients with chronic immune thrombocytopenia who have had an inadequate response to other therapies.


Pharmacology_Katzung

Pharmacology_Katzung. Eltrombopag is an orally active small nonpeptide thrombopoietin agonist molecule approved for therapy of patients with chronic immune thrombocytopenia who have had an inadequate response to other therapies, and for treatment of thrombocytopenia in patients with hepatitis C to allow initiation of interferon therapy. Following oral administration, peak eltrombopag levels are observed in 2–6 hours and the half-life is 26–35 hours. Eltrombopag is excreted primarily in the feces. Interleukin-11 acts through a specific cell surface cytokine receptor to stimulate the growth of multiple lymphoid and myeloid cells. It acts synergistically with other growth factors to stimulate the growth of primitive megakaryocytic progenitors and, most importantly, increases the number of peripheral platelets and neutrophils.


Pharmacology_Katzung

Pharmacology_Katzung. Romiplostim has high affinity for the human Mpl receptor. Eltrombopag interacts with the transmembrane domain of the Mpl receptor. Both drugs induce signaling through the Mpl receptor pathway and cause a dose-dependent increase in platelet count. Romiplostim is administered once weekly by subcutaneous injection. Eltrombopag is an oral drug. For both drugs, peak platelet count responses are observed in approximately 2 weeks.


Pharmacology_Katzung

Pharmacology_Katzung. Interleukin-11 is approved for the secondary prevention of thrombocytopenia in patients receiving cytotoxic chemotherapy for treatment of nonmyeloid cancers. Clinical trials show that it reduces the number of platelet transfusions required by patients who experience severe thrombocytopenia after a previous cycle of chemotherapy. Although IL-11 has broad stimulatory effects on hematopoietic cell lineages in vitro, it does not appear to have significant effects on the leukopenia caused by myelosuppressive chemotherapy. Interleukin-11 is given by subcutaneous injection at a dose of 50 mcg/kg daily. It is started 6–24 hours after completion of chemotherapy and continued for 14–21 days or until the platelet count passes the nadir and rises to more than 50,000 cells/μL.


Pharmacology_Katzung

Pharmacology_Katzung. In patients with chronic immune thrombocytopenia who failed to respond adequately to previous treatment with steroids, immunoglobulins, or splenectomy, romiplostim and eltrombopag significantly increase platelet count in most patients. Both drugs are used at the minimal dose required to maintain platelet counts of greater than 50,000 cells/μL. The most common adverse effects of IL-11 are fatigue, headache, dizziness, and cardiovascular effects. The cardiovascular effects include anemia (due to hemodilution), dyspnea (due to fluid accumulation in the lungs), and transient atrial arrhythmias. Hypokalemia also has been seen in some patients. All of these adverse effects appear to be reversible.


Pharmacology_Katzung

Pharmacology_Katzung. Eltrombopag is potentially hepatotoxic and liver function must be monitored, particularly when used in patients with hepatitis C. Portal vein thrombosis also has been reported with eltrombopag and romiplostim in the setting of chronic liver disease. In patients with myelodysplastic syndromes, romiplostim increases the blast count and risk of progression to acute myeloid leukemia. Marrow fibrosis has been observed with thrombopoietin agonists but is generally reversible when the drug is discontinued. Rebound thrombocytopenia has been observed following discontinuation of TPO agonists. Ferrous gluconate and ferrous fumarate: Oral iron preparations Iron dextran, iron sucrose complex, sodium ferric gluconate complex, ferric carboxymaltose, and ferumoxytol: Parenteral preparations can cause pain, hypersensitivity reactions Required for biosynthesis of heme and heme-containing proteins, including hemoglobin and myoglobin Adequate supplies required for normal heme synthesis


Pharmacology_Katzung

Pharmacology_Katzung. Required for biosynthesis of heme and heme-containing proteins, including hemoglobin and myoglobin Adequate supplies required for normal heme synthesis Iron deficiency, which manifests as preparation Complicated endogenous system for absorbing, storing, and transporting iron • Toxicity: Acute overdose results in necrotizing gastroenteritis, abdominal pain, bloody diarrhea, shock, lethargy, and hemochromatosis, with damage to the heart, liver, pancreas, and other organs Preferred route of administration is IM or SC • Toxicity: Rapid IV administration may cause hypotension • neurotoxicity and increased susceptibility to certain infections have occurred with long-term use • Deferasirox: Orally administered iron chelator for treatment of hemochromatosis


Pharmacology_Katzung

Pharmacology_Katzung. Anemia, especially anemia associated with chronic renal failure, HIV infection, cancer, and prematurity • prevention of the need for transfusion in patients undergoing certain types of elective surgery IV or SC administration 1–3 times per week •EpoetinalfaAgonist of erythropoietin receptors expressed by red cell progenitors Stimulates erythroid proliferation and differentiation, and induces the release of reticulocytes from the bone marrow Darbepoetin alfa: Long-acting glycosylated form administered weekly Methoxy polyethylene glycol-epoetin beta: Long-acting form administered 1–2 times per month • Toxicity: Hypertension, thrombotic complications, and, very rarely, pure red cell aplasia • to reduce the risk of serious cerebrovascular events, hemoglobin levels should be maintained <12 g/dL Pegfilgrastim: Long-acting form of filgrastim that is covalently linked to a type of polyethylene glycol Tbo-filgrastim: Similar to filgrastim


Pharmacology_Katzung

Pharmacology_Katzung. Pegfilgrastim: Long-acting form of filgrastim that is covalently linked to a type of polyethylene glycol Tbo-filgrastim: Similar to filgrastim GM-CSF (sargramostim): Myeloid growth factor that acts through a distinct GM-CSF receptor to stimulate proliferation and differentiation of early and late granulocytic progenitor cells, and erythroid and megakaryocyte progenitors; clinical uses are similar to those of G-CSF, but it is more likely than G-CSF to cause fever, arthralgia, myalgia, and capillary leak syndrome Plerixafor: Antagonist of CXCR4 used in combination with G-CSF for mobilization of peripheral blood cells prior to autologous transplantation in patients with multiple myeloma or non-Hodgkin’s lymphoma who responded suboptimally to G-CSF alone Romiplostim: Subcutaneously administered thrombopoietin agonist approved for treatment of chronic immune thrombocytopenia with insufficient response to corticosteroids, intravenous immunoglobulin, or splenectomy.


Pharmacology_Katzung

Pharmacology_Katzung. Eltrombopag: Orally active thrombopoietin agonist approved for treatment of chronic immune thrombocytopenia with insufficient response to corticosteroids, intravenous immunoglobulin, or splenectomy; and for treatment of thrombocytopenia in hepatitis C to allow the use of interferon-based therapies. Deferoxamine Generic, Desferal Epoetin alfa Erythropoietin (EPO), Epogen, Procrit Filgrastim (G-CSF) Neupogen, Granix Folic acid (folacin, pteroylglutamic acid) Generic Oral: See Table 33–3. Iron dextran (parenteral) INFeD, Dexferrum Oral, parenteral Generic cyanocobalamin or hydroxocobalamin Nasal Nascobal, CaloMist Aapro MS et al, European Organisation for Research and Treatment of Cancer: 2010 update of EORTC guidelines for the use of granulocyte-colony stimulating factor to reduce the incidence of chemotherapy-induced febrile neutropenia in adult patients with lymphoproliferative disorders and solid tumours. Eur J Cancer 2011;47:8.


Pharmacology_Katzung

Pharmacology_Katzung. Albaramki J et al: Parenteral versus oral iron therapy for adults and children with chronic kidney disease. Cochrane Database Syst Rev 2012;(1):CD007857. Auerbach M, Al Talib K: Low-molecular weight iron dextran and iron sucrose have similar comparative safety profiles in chronic kidney disease. Kidney Int 2008;73:528. Barzi A, Sekeres MA: Myelodysplastic syndromes: A practical approach to diagnosis and treatment. Cleve Clin J Med 2010;77:37. Brittenham GM: Iron-chelating therapy for transfusional iron overload. N Engl J Med 2011;364:146. Clark SF: Iron deficiency anemia: diagnosis and management. Curr Opin Gastroenterol 2009;25:122. Darshan D, Fraer DM, Anderson GJ: Molecular basis of iron-loading disorders. Expert Rev Mol Med 2010;12:e36. Gertz MA: Current status of stem cell mobilization. Br J Haematol 2010;150:647. Kessans MR, Gatesman ML, Kockler DR: Plerixafor: A peripheral blood stem cell mobilizer. Pharmacotherapy 2010;30:485.


Pharmacology_Katzung

Pharmacology_Katzung. Gertz MA: Current status of stem cell mobilization. Br J Haematol 2010;150:647. Kessans MR, Gatesman ML, Kockler DR: Plerixafor: A peripheral blood stem cell mobilizer. Pharmacotherapy 2010;30:485. McKoy JM et al: Epoetin-associated pure red cell aplasia: Past, present, and future considerations. Transfusion 2008;48:1754. Rees DC, Williams TN, Gladwin MT: Sickle-cell disease. Lancet 2010;376:2018. Rizzo JD et al: American Society of Clinical Oncology/American Society of Hematology clinical practice guideline update on the use of epoetin and darbepoetin in adult patients with cancer. J Clin Oncol 2010;28:4996. Sauer J, Mason JB, Choi SW: Too much folate: A risk factor for cancer and cardiovascular disease? Curr Opin Clin Nutr Metab Care 2009;12:30. Solomon LR: Disorders of cobalamin (vitamin B12) metabolism: Emerging concepts in pathophysiology, diagnosis and treatment. Blood Rev 2007;21:113. Stasi R et al: Thrombopoietic agents. Blood Rev 2010;24:179.


Pharmacology_Katzung

Pharmacology_Katzung. Stasi R et al: Thrombopoietic agents. Blood Rev 2010;24:179. Wolff T et al: Folic acid supplementation for the prevention of neural tube defects: An update of the evidence for the U.S. Preventive Services Task Force. Ann Intern Med 2009;150:632.


Pharmacology_Katzung

Pharmacology_Katzung. Wolff T et al: Folic acid supplementation for the prevention of neural tube defects: An update of the evidence for the U.S. Preventive Services Task Force. Ann Intern Med 2009;150:632. This patient’s megaloblastic anemia appears to be due to vitamin B12 (cobalamin) deficiency secondary to inadequate dietary B12. It is important to measure serum concentrations of both folic acid and cobalamin because megaloblastic anemia can result from deficiency of either nutrient. It is especially important to diagnose vitamin B12 deficiency because this deficiency, if untreated, can lead to irreversible neurologic damage. Folate supplementation, which can com-pensate for vitamin B12–derived anemia, does not prevent B12-deficiency neurologic damage. To correct this patient’s vitamin B12 deficiency, she would probably be treated paren-terally with cobalamin because of her neurologic symptoms, followed by oral supplementation to maintain her body stores of vitamin B12.


Pharmacology_Katzung

Drugs Used in Disorders of Coagulation

Pharmacology_Katzung. Drugs Used in Disorders of Coagulation


Pharmacology_Katzung

Pharmacology_Katzung. James L. Zehnder, MD and edema and is tender to touch. Oxygen saturation by fingertip pulse oximeter while breathing room air is 87% (normal > 90%). Ultrasound reveals a deep vein thrombosis in the left lower extremity; chest computed tomography scan confirms the presence of pulmonary emboli. Laboratory blood tests indicate elevated d-dimer levels. What therapy is indicated acutely? What are the long-term therapy options? How long should she be treated? Should this indi-vidual use oral contraceptives? A 25-year-old woman presents to the emergency depart-ment complaining of acute onset of shortness of breath and pleuritic pain. She had been in her usual state of health until 2 days prior when she noted that her left leg was swollen and red. Her only medication was oral contraceptives. Family history was significant for a history of “blood clots” in mul-tiple members of the maternal side of her family. Physical examination demonstrates an anxious woman with stable vital signs. The left


Pharmacology_Katzung

Pharmacology_Katzung. history was significant for a history of “blood clots” in mul-tiple members of the maternal side of her family. Physical examination demonstrates an anxious woman with stable vital signs. The left lower extremity demonstrates erythema


Pharmacology_Katzung

Pharmacology_Katzung. Hemostasis refers to the finely regulated dynamic process of maintaining fluidity of the blood, repairing vascular injury, and limiting blood loss while avoiding vessel occlusion (thrombosis) and inadequate perfusion of vital organs. Either extreme—excessive bleeding or thrombosis—represents a breakdown of the hemostatic mechanism. Common causes of dysregulated hemostasis include hereditary or acquired defects in the clotting mechanism and secondary effects of infection or cancer. Atrial fibrillation is associated with stasis of blood in the atria, formation of clots, and increased risk of occlusive stroke. Because of the high prevalence of chronic atrial fibrillation, especially in the older population, use of anticoagulants is common. Guidelines for the use of oral anticoagulants (CHA2DS2-VASC score, see January C et al reference) are based on various risk factors (congestive heart failure, hypertension, age, diabetes, history of stroke, vascular disease, and sex). The drugs used to


Pharmacology_Katzung

Pharmacology_Katzung. score, see January C et al reference) are based on various risk factors (congestive heart failure, hypertension, age, diabetes, history of stroke, vascular disease, and sex). The drugs used to inhibit thrombosis and to limit abnormal bleeding are the subjects of this chapter.


Pharmacology_Katzung

Pharmacology_Katzung. The vascular endothelial cell layer lining blood vessels has an anticoagulant phenotype, and circulating blood platelets and clotting factors do not normally adhere to it to an appreciable extent. In the setting of vascular injury, the endothelial cell layer rapidly undergoes a series of changes resulting in a more procoagulant phenotype. Injury exposes reactive subendothelial matrix proteins such as collagen and von Willebrand factor, which results in platelet adherence and activation, and secretion and synthesis of vasoconstrictors and platelet-recruiting and activating molecules. Thus, thromboxane A2 (TXA2) is synthesized from arachidonic acid within platelets and is a platelet activator and potent vasoconstrictor. Products secreted from platelet granules include adenosine diphosphate (ADP), a powerful inducer of platelet aggregation, and serotonin (5-HT), which stimulates


Pharmacology_Katzung

Pharmacology_Katzung. FIGURE 34–1 Thrombus formation at the site of the damaged vascular wall (EC, endothelial cell) and the role of platelets and clotting factors. Platelet membrane receptors include the glycoprotein (GP) Ia receptor, binding to collagen (C); GP Ib receptor, binding von Willebrand factor (vWF); and GP IIb/IIIa, which binds fibrinogen and other macromolecules. Antiplatelet prostacyclin (PGI2) is released from the endothelium. Aggregating substances released from the degranulating platelet include adenosine diphosphate (ADP), thromboxane A2 (TXA2), and serotonin (5-HT). Production of factor Xa by intrinsic and extrinsic pathways is detailed in Figure 34–2. (Redrawn and reproduced, with permission, from Simoons ML, Decker JW: New directions in anticoagulant and antiplatelet treatment. [Editorial.] Br Heart J 1995;74:337.) aggregation and vasoconstriction. Activation of platelets results in a conformational change in the αIIbβIII integrin (IIb/IIIa) receptor, enabling it to bind fibrinogen,


Pharmacology_Katzung

Pharmacology_Katzung. Br Heart J 1995;74:337.) aggregation and vasoconstriction. Activation of platelets results in a conformational change in the αIIbβIII integrin (IIb/IIIa) receptor, enabling it to bind fibrinogen, which cross-links adjacent platelets, resulting in aggregation and formation of a platelet plug (Figure 34–1). Simultaneously, the coagulation system cascade is activated, resulting in thrombin generation and a fibrin clot, which stabilizes the platelet plug (see below). Knowledge of the hemostatic mechanism is important for diagnosis of bleeding disorders. Patients with defects in the formation of the primary platelet plug (defects in primary hemostasis, eg, platelet function defects, von Willebrand disease) typically bleed from surface sites (gingiva, skin, heavy menses) with injury. In contrast, patients with defects in the clotting mechanism (secondary hemostasis, eg, hemophilia A) tend to bleed into deep tissues (joints, muscle, retroperitoneum), often with no apparent inciting event,


Pharmacology_Katzung

Pharmacology_Katzung. patients with defects in the clotting mechanism (secondary hemostasis, eg, hemophilia A) tend to bleed into deep tissues (joints, muscle, retroperitoneum), often with no apparent inciting event, and bleeding may recur unpredictably.


Pharmacology_Katzung

Pharmacology_Katzung. The platelet is central to normal hemostasis and thromboembolic disease, and is the target of many therapies discussed in this chapter. Platelet-rich thrombi (white thrombi) form in the high flow rate and high shear force environment of arteries. Occlusive arterial thrombi cause serious disease by producing downstream ischemia of extremities or vital organs, and they can result in limb amputation or organ failure. Venous clots tend to be more fibrin-rich, contain large numbers of trapped red blood cells, and are recognized pathologically as red thrombi. Deep venous thrombi (DVT) can cause severe swelling and pain of the affected extremity, but the most feared consequence is pulmonary embolism (PE). This occurs when part or all of the clot breaks off from its location in the deep venous system and travels as an embolus through the right side of the heart and into the pulmonary arterial circulation. Occlusion of a large pulmonary artery by an embolic clot can precipitate acute right


Pharmacology_Katzung

Pharmacology_Katzung. system and travels as an embolus through the right side of the heart and into the pulmonary arterial circulation. Occlusion of a large pulmonary artery by an embolic clot can precipitate acute right heart failure and sudden death. In addition lung ischemia or infarction will occur distal to the occluded pulmonary arterial segment. Such emboli usually arise from the deep venous system of the proximal lower extremities or pelvis. Although all thrombi are mixed, the platelet nidus dominates the arterial thrombus and the fibrin tail dominates the venous thrombus.


Pharmacology_Katzung

Pharmacology_Katzung. Blood coagulates due to the transformation of soluble fibrinogen into insoluble fibrin by the enzyme thrombin. Several circulating proteins interact in a cascading series of limited proteolytic reactions (Figure 34–2). At each step, a clotting factor zymogen undergoes limited proteolysis and becomes an active protease (eg, factor VII is converted to factor VIIa). Each protease factor activates the next clotting factor in the sequence, culminating in the formation of thrombin (factor IIa). Several of these factors are targets for drug therapy (Table 34–1). Thrombin has a central role in hemostasis and has many functions. In clotting, thrombin proteolytically cleaves small peptides Clotting in the Lab TABLE 34–1 Blood clotting factors and drugs that affect them.1 Tissue factor, VIIa


Pharmacology_Katzung

Pharmacology_Katzung. Clotting in the Lab TABLE 34–1 Blood clotting factors and drugs that affect them.1 Tissue factor, VIIa FIGURE 34–2 A model of blood coagulation. With tissue factor (TF), factor VII forms an activated complex (VIIa-TF) that catalyzes the activation of factor IX to factor IXa. Activated factor XIa also catalyzes this reaction. Tissue factor pathway inhibitor inhibits the catalytic action of the VIIa-TF complex. The cascade proceeds as shown, resulting ultimately in the conversion of fibrinogen to fibrin, an essential component of a functional clot. The two major anticoagulant drugs, heparin and warfarin, have very different actions. Heparin, acting in the blood, directly activates anticlotting factors, specifically antithrombin, which inactivates the factors enclosed in rectangles. Warfarin, acting in the liver, inhibits the synthesis of the factors enclosed in circles. Proteins C and S exert anticlotting effects by inactivating activated factors Va and VIIIa.


Pharmacology_Katzung

Pharmacology_Katzung. from fibrinogen, allowing fibrinogen to polymerize and form a fibrin clot. Thrombin also activates many upstream clotting factors, leading to more thrombin generation, and activates factor XIII, a transaminase that cross-links the fibrin polymer and stabilizes the clot. Thrombin is a potent platelet activator and mitogen. Thrombin also exerts anticoagulant effects by activating the protein C pathway, which attenuates the clotting response (Figure 34–2). It should therefore be apparent that the response to vascular injury is a complex and precisely modulated process that Plasminogen Thrombolytic enzymes, aminocaproic acid 1See Figure 34–2 and text for additional details. ensures that under normal circumstances, repair of vascular injury occurs without thrombosis and downstream ischemia—that is, the response is proportionate and reversible. Eventually vascular remodeling and repair occur with reversion to the quiescent resting anticoagulant endothelial cell phenotype.


Pharmacology_Katzung

Pharmacology_Katzung. Initiation of Clotting: The Tissue Factor-VIIa Complex


Pharmacology_Katzung

Pharmacology_Katzung. The main initiator of blood coagulation in vivo is the tissue factor (TF)–factor VIIa pathway (Figure 34–2). Tissue factor is a trans-membrane protein ubiquitously expressed outside the vasculature but not normally expressed in an active form within vessels. The exposure of TF on damaged endothelium or to blood that has extravasated into tissue binds TF to factor VIIa. This complex, in turn, activates factors X and IX. Factor Xa along with factor Va forms the prothrombinase complex on activated cell surfaces, which catalyzes the conversion of prothrombin (factor II) to thrombin (factor IIa). Thrombin, in turn, activates upstream clotting factors, primarily factors V, VIII, and XI, resulting in amplification of thrombin generation. The TF-factor VIIa-catalyzed activation of factor Xa is regulated by tissue factor pathway inhibitor (TFPI). Thus after initial activation of factor X to Xa by TF-VIIa, further propagation of the clot occurs by feedback amplification of thrombin through the


Pharmacology_Katzung

Pharmacology_Katzung. by tissue factor pathway inhibitor (TFPI). Thus after initial activation of factor X to Xa by TF-VIIa, further propagation of the clot occurs by feedback amplification of thrombin through the intrinsic pathway factors VIII and IX. (This provides an explanation for why patients with deficiency of factor VIII or IX—hemophilia A and hemophilia B, respectively—have a severe bleeding disorder.)


Pharmacology_Katzung

Pharmacology_Katzung. It is also important to note that the coagulation mechanism in vivo does not occur in solution, but is localized to activated cell surfaces expressing anionic phospholipids such as phosphatidylserine, and is mediated by Ca2+ bridging between the anionic phospholipids and γ-carboxyglutamic acid residues of the clotting factors. This is the basis for using calcium chelators such as ethylenediamine tetraacetic acid (EDTA) or citrate to prevent blood from clotting in a test tube.


Pharmacology_Katzung

Pharmacology_Katzung. Antithrombin (AT) is an endogenous anticoagulant and a member of the serine protease inhibitor (serpin) family; it inactivates the serine proteases IIa, IXa, Xa, XIa, and XIIa. The endogenous anticoagulants protein C and protein S attenuate the blood clotting cascade by proteolysis of the two cofactors Va and VIIIa. From an evolutionary perspective, it is of interest that factors V and VIII have an identical overall domain structure and considerable homology, consistent with a common ancestor gene; likewise the serine proteases are descendants of a trypsin-like common ancestor. Thus, the TF-VIIa initiating complex, serine proteases, and cofactors each have their own lineage-specific attenuation mechanism (Figure 34–2). Defects in natural anticoagulants result in an increased risk of venous thrombosis. The most common defect in the natural anticoagulant system is a mutation in factor V (factor V Leiden), which results in resistance to inactivation by the protein C/protein S mechanism.


Pharmacology_Katzung

Pharmacology_Katzung. Fibrinolysis refers to the process of fibrin digestion by the fibrin-specific protease, plasmin. The fibrinolytic system is similar to the coagulation system in that the precursor form of the serine protease plasmin circulates in an inactive form as plasminogen. In response to injury, endothelial cells synthesize and release tissue plasminogen activator (t-PA), which converts plasminogen to plasmin (Figure 34–3). Plasmin remodels the thrombus and limits its extension by proteolytic digestion of fibrin.


Pharmacology_Katzung

Pharmacology_Katzung. Both plasminogen and plasmin have specialized protein domains (kringles) that bind to exposed lysines on the fibrin clot and impart clot specificity to the fibrinolytic process. It should be noted that this clot specificity is only observed at physiologic levels of t-PA. At the pharmacologic levels of t-PA used in thrombolytic therapy, clot specificity is lost and a systemic lytic state is created, with attendant increase in bleeding risk. As in the coagulation cascade, there are negative regulators of fibrinolysis: endothelial cells synthesize and release plasminogen activator inhibitor (PAI), which inhibits t-PA; in addition α2 antiplasmin circulates in the blood at high concentrations and under physiologic conditions will rapidly inactivate any plasmin that is not clot-bound. However, this regulatory system is overwhelmed by therapeutic doses of plasminogen activators.


Pharmacology_Katzung

Pharmacology_Katzung. If the coagulation and fibrinolytic systems are pathologically activated, the hemostatic system may careen out of control, leading to generalized intravascular clotting and bleeding. This process is called disseminated intravascular coagulation (DIC) and may CHAPTER 34 Drugs Used in Disorders of Coagulation 611 t-PA, urokinase Fibrinogen Fibrin clot products, products FIGURE 34–3 Schematic representation of the fibrinolytic system. Plasmin is the active fibrinolytic enzyme. Several clinically useful activators are shown on the left in bold. Anistreplase is a combination of streptokinase and the proactivator plasminogen. Aminocaproic acid (right) inhibits the activation of plasminogen to plasmin and is useful in some bleeding disorders. t-PA, tissue plasminogen activator.


Pharmacology_Katzung

Pharmacology_Katzung. follow massive tissue injury, advanced cancers, obstetric emergencies such as abruptio placentae or retained products of conception, or bacterial sepsis. The treatment of DIC is to control the underlying disease process; if this is not possible, DIC is often fatal. Regulation of the fibrinolytic system is useful in therapeutics. Increased fibrinolysis is effective therapy for thrombotic disease. Tissue plasminogen activator, urokinase, and streptokinase all activate the fibrinolytic system (Figure 34–3). Conversely, decreased fibrinolysis protects clots from lysis and reduces the bleeding of hemostatic failure. Aminocaproic acid is a clinically useful inhibitor of fibrinolysis. Heparin and the oral anticoagulant drugs do not affect the fibrinolytic mechanism.


Pharmacology_Katzung

Pharmacology_Katzung. The ideal anticoagulant drug would prevent pathologic thrombosis and limit reperfusion injury yet allow a normal response to vascular injury and limit bleeding. Theoretically this could be accomplished by preservation of the TF-VIIa initiation phase of the clotting mechanism with attenuation of the secondary intrinsic pathway propagation phase of clot development. At this time such a drug does not exist; all anticoagulants and fibrinolytic drugs have an increased bleeding risk as their principle toxicity.


Pharmacology_Katzung

Pharmacology_Katzung. The indirect thrombin inhibitors are so-named because their antithrombotic effect is exerted by their interaction with a separate protein, antithrombin. Unfractionated heparin (UFH), also known as high-molecular-weight (HMW) heparin, lowmolecular-weight (LMW) heparin, and the synthetic pentasaccharide fondaparinux bind to antithrombin and enhance its inactivation of factor Xa (Figure 34–4). Unfractionated heparin and to a lesser extent LMW heparin also enhance antithrombin’s inactivation of thrombin. Chemistry & Mechanism of Action


Pharmacology_Katzung

Pharmacology_Katzung. Heparin is a heterogeneous mixture of sulfated mucopolysaccharides. It binds to endothelial cell surfaces and a variety of plasma proteins. Its biologic activity is dependent upon the endogenous anticoagulant antithrombin. Antithrombin inhibits clotting factor proteases, especially thrombin (IIa), IXa, and Xa, by forming equimolar stable complexes with them. In the absence of heparin, these reactions are slow; in the presence of heparin, they are accelerated 1000-fold. Only about a third of the molecules in commercial heparin preparations have an accelerating effect because the remainder lack the unique pentasaccharide sequence needed for high-affinity binding to antithrombin. The active heparin molecules bind tightly to antithrombin and cause a conformational change in this inhibitor. The conformational change of antithrombin exposes its active site for more rapid interaction with the proteases (the activated clotting factors). Heparin functions as a cofactor for the


Pharmacology_Katzung

Pharmacology_Katzung. inhibitor. The conformational change of antithrombin exposes its active site for more rapid interaction with the proteases (the activated clotting factors). Heparin functions as a cofactor for the antithrombin-protease reaction without being consumed. Once the antithrombin-protease complex is formed, heparin is released intact for renewed binding to more antithrombin.


Pharmacology_Katzung

Pharmacology_Katzung. The antithrombin binding region of commercial unfractionated heparin consists of repeating sulfated disaccharide units composed of d-glucosamine-l-iduronic acid and d-glucosamined-glucuronic acid. High-molecular-weight fractions of heparin with high affinity for antithrombin markedly inhibit blood coagulation by inhibiting all three factors, especially thrombin and factor Xa. Unfractionated heparin has a molecular weight range of 5000–30,000 Da. In contrast, the shorter-chain, low-molecularweight fractions of heparin inhibit activated factor X but have less effect on thrombin than the HMW species. Nevertheless, numerous studies have demonstrated that LMW heparins such as enoxaparin, dalteparin, and tinzaparin are effective in several thromboembolic conditions. In fact, these LMW heparins—in comparison with UFH—have equal efficacy, increased bioavailability from the subcutaneous site of injection, and less frequent dosing requirements (once or twice daily is sufficient).


Pharmacology_Katzung

Pharmacology_Katzung. USP heparin is harmonized to the World Health Organization International Standard (IS) unit dose. Enoxaparin is obtained from the same sources as regular UFH, but doses are specified in milligrams. Fondaparinux also is specified in milligrams. Dalteparin, tinzaparin, and danaparoid (an LMW heparinoid containing heparan sulfate, dermatan sulfate, and chondroitin sulfate), on the other hand, are specified in anti-factor Xa units. Monitoring of Heparin Effect


Pharmacology_Katzung

Pharmacology_Katzung. Monitoring of Heparin Effect Close monitoring of the activated partial thromboplastin time (aPTT or PTT) is necessary in patients receiving UFH. Levels of UFH may also be determined by protamine titration (therapeutic levels 0.2–0.4 unit/mL) or anti-Xa units (therapeutic levels 0.3–0.7 unit/mL). Weight-based dosing of the LMW heparins results in predictable pharmacokinetics and plasma levels in patients with normal renal function. Therefore, LMW heparin levels are not generally measured except in the setting of renal insufficiency, obesity, and pregnancy. LMW heparin levels can be determined by anti-Xa units. For enoxaparin, peak therapeutic levels should be 0.5–1 unit/mL for twice-daily dosing, determined 4 hours after administration, and approximately 1.5 units/mL for once-daily dosing.


Pharmacology_Katzung

Pharmacology_Katzung. FIGURE 34–4 Differences between low-molecular-weight (LMW) heparins and high-molecular-weight heparin (unfractionated heparin). Fondaparinux is a small pentasaccharide fragment of heparin. Activated antithrombin III (AT III) degrades thrombin, factor X, and several other factors. Binding of these drugs to AT III can increase the catalytic action of AT III 1000-fold. The combination of AT III with unfractionated heparin increases degradation of both factor Xa and thrombin. Combination with fondaparinux or LMW heparin more selectively increases degradation of Xa. A. Bleeding and Miscellaneous Effects


Pharmacology_Katzung

Pharmacology_Katzung. A. Bleeding and Miscellaneous Effects The major adverse effect of heparin is bleeding. This risk can be decreased by scrupulous patient selection, careful control of dosage, and close monitoring. Elderly women and patients with renal failure are more prone to hemorrhage. Heparin is of animal origin and should be used cautiously in patients with allergy. Increased loss of hair and reversible alopecia have been reported. Long-term heparin therapy is associated with osteoporosis and spontaneous fractures. Heparin accelerates the clearing of postprandial lipemia by causing the release of lipoprotein lipase from tissues, and long-term use is associated with mineralocorticoid deficiency. B. Heparin-Induced Thrombocytopenia


Pharmacology_Katzung

Pharmacology_Katzung. B. Heparin-Induced Thrombocytopenia Heparin-induced thrombocytopenia (HIT) is a systemic hyper-coagulable state that occurs in 1–4% of individuals treated with UFH. Surgical patients are at greatest risk. The reported incidence of HIT is lower in pediatric populations outside the critical care setting; it is relatively rare in pregnant women. The risk of HIT may be higher in individuals treated with UFH of bovine origin compared with porcine heparin and is lower in those treated exclusively with LMW heparin.


Pharmacology_Katzung

Pharmacology_Katzung. Morbidity and mortality in HIT are related to thrombotic events. Venous thrombosis occurs most commonly, but occlusion of peripheral or central arteries is not infrequent. If an indwelling catheter is present, the risk of thrombosis is increased in that extremity. Skin necrosis has been described, particularly in individuals treated with warfarin in the absence of a direct thrombin inhibitor, presumably due to acute depletion of the vitamin K– dependent anticoagulant protein C occurring in the presence of high levels of procoagulant proteins and an active hypercoagulable state.


Pharmacology_Katzung

Pharmacology_Katzung. The following points should be considered in all patients receiving heparin: Platelet counts should be performed frequently; thrombocytopenia appearing in a time frame consistent with an immune response to heparin should be considered suspicious for HIT; and any new thrombus occurring in a patient receiving heparin therapy should raise suspicion of HIT. Patients who develop HIT are treated by discontinuance of heparin and administration of the direct thrombin inhibitor argatroban.


Pharmacology_Katzung

Pharmacology_Katzung. Heparin is contraindicated in patients with HIT, hypersensitivity to the drug, active bleeding, hemophilia, significant thrombocytopenia, purpura, severe hypertension, intracranial hemorrhage, infective endocarditis, active tuberculosis, ulcerative lesions of the gastrointestinal tract, threatened abortion, visceral carcinoma, or advanced hepatic or renal disease. Heparin should be avoided in patients who have recently had surgery of the brain, spinal cord, or eye; and in patients who are undergoing lumbar puncture or regional anesthetic block. Despite the apparent lack of placental transfer, heparin should be used in pregnant women only when clearly indicated.


Pharmacology_Katzung

Pharmacology_Katzung. The indications for the use of heparin are described in the section on clinical pharmacology. A plasma concentration of heparin of 0.2–0.4 unit/mL (by protamine titration) or 0.3–0.7 unit/mL (anti-Xa units) is considered to be the therapeutic range for treatment of venous thromboembolic disease. This concentration generally corresponds to a PTT of 1.5–2.5 times baseline. However, the use of the PTT for heparin monitoring is problematic. There is no standardization scheme for the PTT as there is for the prothrombin time (PT) and its international normalized ratio (INR) in warfarin monitoring. The PTT in seconds for a given heparin concentration varies between different reagent/ instrument systems. Thus, if the PTT is used for monitoring, the laboratory should determine the clotting time that corresponds to the therapeutic range by protamine titration or anti-Xa activity, as listed above.


Pharmacology_Katzung

Pharmacology_Katzung. In addition, some patients have a prolonged baseline PTT due to factor deficiency or inhibitors (which could increase bleeding risk) or lupus anticoagulant (which is not associated with bleeding risk but may be associated with thrombosis risk). Using the PTT to assess heparin effect in such patients is problematic. An alternative is to use anti-Xa activity to assess heparin concentration, a test now widely available on automated coagulation instruments. This approach measures heparin concentration; however, it does not provide the global assessment of intrinsic pathway integrity of the PTT.


Pharmacology_Katzung

Pharmacology_Katzung. The following strategy is recommended: prior to initiating anticoagulant therapy of any type, the integrity of the patient’s hemostatic system should be assessed by a careful history of prior bleeding events, as well as baseline PT and PTT. If there is a prolonged clotting time, the cause of this (deficiency or inhibitor) should be determined prior to initiating therapy, and treatment goals stratified to a risk-benefit assessment. In high-risk patients measuring both the PTT and anti-Xa activity may be useful. When intermittent heparin administration is used, the aPTT or anti-Xa activity should be measured 6 hours after the administered dose to maintain prolongation of the aPTT to 2–2.5 times that of the control value. However, LMW heparin therapy is the preferred option in this case, as no monitoring is required in most patients.


Pharmacology_Katzung

Pharmacology_Katzung. Continuous intravenous administration of heparin is accomplished via an infusion pump. After an initial bolus injection of 80–100 units/kg, a continuous infusion of about 15–22 units/kg per hour is required to maintain the anti-Xa activity in the range of 0.3–0.7 units/mL. Low-dose prophylaxis is achieved with subcutaneous administration of heparin, 5000 units every 8–12 hours. Because of the danger of hematoma formation at the injection site, heparin must never be administered intramuscularly. Prophylactic enoxaparin is given subcutaneously in a dosage of 30 mg twice daily or 40 mg once daily. Full-dose enoxaparin therapy is 1 mg/kg subcutaneously every 12 hours. This corresponds to a therapeutic anti-factor Xa level of 0.5–1 unit/mL. Selected patients may be treated with enoxaparin 1.5 mg/kg once a day, with a target anti-Xa level of 1.5 units/mL.


Pharmacology_Katzung

Pharmacology_Katzung. The prophylactic dosage of dalteparin is 5000 units subcutaneously once a day; therapeutic dosing is 200 units/kg once a day for venous disease or 120 units/kg every 12 hours for acute coronary syndrome. LMW heparin should be used with caution in patients with renal insufficiency or body weight greater than 150 kg. Measurement of the anti-Xa level is useful to guide dosing in these individuals. The synthetic pentasaccharide molecule fondaparinux avidly binds antithrombin with high specific activity, resulting in efficient inactivation of factor Xa. Fondaparinux has a long half-life of 15 hours, allowing for once-daily dosing by subcutaneous administration. Fondaparinux is effective in the prevention and treatment of venous thromboembolism and does not appear to cross-react with pathologic HIT antibodies in most individuals. Reversal of Heparin Action


Pharmacology_Katzung

Pharmacology_Katzung. Reversal of Heparin Action Excessive anticoagulant action of heparin is treated by discontinuance of the drug. If bleeding occurs, administration of a specific antagonist such as protamine sulfate is indicated. Protamine is a highly basic, positively charged peptide that combines with negatively charged heparin as an ion pair to form a stable complex devoid of anticoagulant activity. For every 100 units of heparin remaining in the patient, 1 mg of protamine sulfate is given intravenously; the rate of infusion should not exceed 50 mg in any 10-minute period. Excess protamine must be avoided; it also has an anticoagulant effect. Neutralization of LMW heparin by protamine is incomplete. Limited experience suggests that 1 mg of protamine sulfate may be used to partially neutralize 1 mg of enoxaparin. Protamine will not reverse the activity of fondaparinux. Excess danaparoid can be removed by plasmapheresis.


Pharmacology_Katzung

Pharmacology_Katzung. The clinical use of the coumarin anticoagulants began with the discovery of an anticoagulant substance formed in spoiled sweet clover silage, which caused hemorrhagic disease in cattle. At the behest of local farmers, a chemist at the University of Wisconsin identified the toxic agent as bishydroxycoumarin. Dicumarol, a synthesized derivative, and its congeners, most notably warfarin (Wisconsin Alumni Research Foundation, with “-arin” from coumarin added; Figure 34–5), were initially used as rodenticides. In the 1950s, warfarin (under the brand name Coumadin) was introduced as an antithrombotic agent in humans. Warfarin is one of the most commonly prescribed drugs.


Pharmacology_Katzung

Pharmacology_Katzung. Warfarin is generally administered as the sodium salt and has 100% oral bioavailability. Over 99% of racemic warfarin is bound to plasma albumin, which may contribute to its small volume of distribution (the albumin space), its long half-life in plasma (36 hours), and the lack of urinary excretion of unchanged drug. Warfarin used clinically is a racemic mixture composed of equal amounts of two enantiomorphs. The levorotatory S-warfarin is four times more potent than the dextrorotatory R-warfarin. This observation is useful in understanding the stereoselective nature of several drug interactions involving warfarin. Mechanism of Action Coumarin anticoagulants block the γ-carboxylation of several glutamate residues in prothrombin and factors VII, IX, and X as well as the endogenous anticoagulant proteins C and S (Figure 34–2, Table 34–1). The blockade results in incomplete coagulation


Pharmacology_Katzung

Pharmacology_Katzung. FIGURE 34–5 Structural formulas of several oral anticoagulant drugs and of vitamin K. The carbon atom of warfarin shown at the asterisk is an asymmetric center. factor molecules that are biologically inactive. The protein carboxylation reaction is coupled to the oxidation of vitamin K. The vitamin must then be reduced to reactivate it. Warfarin prevents reductive metabolism of the inactive vitamin K epoxide back to its active hydroquinone form (Figure 34–6). Mutational change of the gene for the responsible enzyme, vitamin K epoxide reductase (VKORC1), can give rise to genetic resistance to warfarin in humans and rodents.


Pharmacology_Katzung

Pharmacology_Katzung. There is an 8to 12-hour delay in the action of warfarin. Its anticoagulant effect results from a balance between partially inhibited synthesis and unaltered degradation of the four vitamin K–dependent clotting factors. The resulting inhibition of coagulation is dependent on their degradation half-lives in the circulation. These half-lives are 6, 24, 40, and 60 hours for factors VII, IX, X, and II, respectively. Importantly, protein C has a short half-life similar to factor VIIa. Thus the immediate effect of warfarin is to deplete the procoagulant factor VII and anticoagulant protein C, which can paradoxically create a transient hypercoagulable state due to residual activity of the longer half-life procoagulants in the face of protein C depletion (see below). For this reason in patients with active hypercoagulable states, such as acute DVT or PE, UFH or LMW heparin is always used to achieve immediate anticoagulation until adequate warfarin-induced depletion of the procoagulant clotting


Pharmacology_Katzung

Pharmacology_Katzung. active hypercoagulable states, such as acute DVT or PE, UFH or LMW heparin is always used to achieve immediate anticoagulation until adequate warfarin-induced depletion of the procoagulant clotting factors is achieved. The duration of this overlapping therapy is generally 5–7 days.


Pharmacology_Katzung

Pharmacology_Katzung. FIGURE 34–6 Vitamin K cycle–metabolic interconversions of vitamin K associated with the synthesis of vitamin K–dependent clotting factors. Vitamin K1 or K2 is activated by reduction to the hydroquinone form (KH2). Stepwise oxidation to vitamin K epoxide (KO) is coupled to prothrombin carboxylation by the enzyme carboxylase. The reactivation of vitamin K epoxide is the warfarin-sensitive step (warfarin). The R on the vitamin K molecule represents a 20-carbon phytyl side chain in vitamin K1 and a 30to 65-carbon polyprenyl side chain in vitamin K2.


Pharmacology_Katzung

Pharmacology_Katzung. Warfarin crosses the placenta readily and can cause a hemorrhagic disorder in the fetus. Furthermore, fetal proteins with γ-carboxyglutamate residues found in bone and blood may be affected by warfarin; the drug can cause a serious birth defect characterized by abnormal bone formation. Thus, warfarin should never be administered during pregnancy. Cutaneous necrosis with reduced activity of protein C sometimes occurs during the first weeks of therapy in patients who have inherited deficiency of protein C. Rarely, the same process causes frank infarction of the breast, fatty tissues, intestine, and extremities. The pathologic lesion associated with the hemorrhagic infarction is venous thrombosis, consistent with a hypercoagulable state due to warfarininduced depletion of protein C.


Pharmacology_Katzung

Pharmacology_Katzung. Treatment with warfarin should be initiated with standard doses of 5–10 mg. The initial adjustment of the prothrombin time takes about 1 week, which usually results in a maintenance dosage of 5–7 mg/d. The prothrombin time (PT) should be increased to a level representing a reduction of prothrombin activity to 25% of normal and maintained there for long-term therapy. When the activity is less than 20%, the warfarin dosage should be reduced or omitted until the activity rises above 20%. Inherited polymorphisms in 2CYP2C9 and VKORC1 have significant effects on warfarin dosing; however, algorithms incorporating genomic information to predict initial warfarin dosing were no better than standard clinical algorithms in two of three large randomized trials examining this issue (see Chapter 5).


Pharmacology_Katzung

Pharmacology_Katzung. The therapeutic range for oral anticoagulant therapy is defined in terms of an international normalized ratio (INR). The INR is the prothrombin time ratio (patient prothrombin time/mean of normal prothrombin time for lab)ISI, where the ISI exponent refers to the International Sensitivity Index and is dependent on the specific reagents and instruments used for the determination. The ISI serves to relate measured prothrombin times to a World Health Organization reference standard thromboplastin; thus the prothrombin times performed on different properly calibrated instruments with a variety of thromboplastin reagents should give the same INR results for a given sample. For most reagent and instrument combinations in current use, the ISI is close to 1, making the INR roughly the ratio of the patient prothrombin time to the mean normal prothrombin time. The recommended INR for prophylaxis and treatment of thrombotic disease is 2–3. Patients with some types of artificial heart valves (eg,


Pharmacology_Katzung

Pharmacology_Katzung. patient prothrombin time to the mean normal prothrombin time. The recommended INR for prophylaxis and treatment of thrombotic disease is 2–3. Patients with some types of artificial heart valves (eg, tilting disk) or other medical conditions increasing thrombotic risk have a recommended range of 2.5–3.5. While a prolonged INR is widely used as an indication of integrity of the coagulation system in liver disease and other disorders, it has been validated only in patients in steady state on chronic warfarin therapy.


Pharmacology_Katzung

Pharmacology_Katzung. Occasionally patients exhibit warfarin resistance, defined as progression or recurrence of a thrombotic event while in the therapeutic range. These individuals may have their INR target raised (which is accompanied by an increase in bleeding risk) or be changed to an alternative form of anticoagulation (eg, daily injections of LMW heparin or one of the newer oral anticoagulants). Warfarin resistance is most commonly seen in patients with advanced cancers, typically of gastrointestinal origin (Trousseau’s syndrome). LMW heparin is superior to warfarin in preventing recurrent venous thromboembolism in patients with cancer.


Pharmacology_Katzung

Pharmacology_Katzung. The coumarin anticoagulants often interact with other drugs and with disease states. These interactions can be broadly divided into pharmacokinetic and pharmacodynamic effects (Table 34–2). Pharmacokinetic mechanisms for drug interaction with warfarin mainly involve cytochrome P450 CYP2C9 enzyme induction, enzyme inhibition, and reduced plasma protein binding. Pharmacodynamic mechanisms for interactions with warfarin are synergism (impaired hemostasis, reduced clotting factor synthesis, as in hepatic disease), competitive antagonism (vitamin K), and an altered physiologic control loop for vitamin K (hereditary resistance to oral anticoagulants).