 Instead of the 5-thiazolyl end group in ritonavir, lopinavir has a phenoxyacetyl group and the 2-isopropylthiazolyl group in ritonavir was replaced by a modified valine in which the amino terminal had a six-membered cyclic urea attached.Fosamprenavir was marketed in 2003 and is a phosphoester prodrug that is rapidly and extensively metabolized to amprenavir.The solubility and bioavailability is better than of amprenavir which results in reduced daily pill burden.