 Lamivudine was developed as the sulfur analogue of zalcitabine (see table 2).It was initially synthesized as a racemic mixture (BCH-189) and analysis showed that both positive and negative enantiomers of BCH-189 (2',3'-dideoxy-3'-thiacytidine) had in vitro activity against HIV.Lamivudine is the negative enantiomer and is a pyrimidine nucleoside analogue.