Combinations of antiretroviral drugs that prevent or delay the appearance of drug-resistant human immunodeficiency virus-type 1 (HIV-1) mutants are urgently required. Mutants resistant to 3'-azidothymidine (AZT, zidovudine) became phenotypically sensitive in vitro by mutation of residue 184 of viral reverse transcriptase to valine, which also induced resistance to (-)2'-deoxy-3'-thiacytidine (3TC). Furthermore, AZT-3TC coresistance was not observed during extensive in vitro selection with both drugs. In vivo AZT-3TC combination therapy resulted in a markedly greater decreased in serum HIV-1 RNA concentrations than treatment with AZT alone, even though valine-184 mutants rapidly emerged. Most samples assessed from the combination group remained AZT sensitive at 24 weeks of therapy, consistent with in vitro mutation studies.

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The safety, pharmacokinetics, and antiretroviral activity of lamivudine alone and in combination with zidovudine was studied in pregnant women infected with human immunodeficiency virus type 1 (HIV-1) and their neonates. Women received the drugs orally from week 38 of pregnancy to 1 week after delivery. Neonate therapy began 12 h after delivery and continued for 1 week. Both treatment regimens were well-tolerated in women and newborns. Lamivudine and zidovudine pharmacokinetics in pregnant women were similar to those in nonpregnant adults. Lamivudine and zidovudine freely crossed the placenta and were secreted in breast milk. Neonatal lamivudine clearance was about half that in pediatric patients; zidovudine clearance was consistent with previous reports. HIV-1 RNA could be quantified in 17 of the 20 women. At the onset of labor/delivery, mean virus load had decreased by approximately 1.5 log10 copies/mL in both treatment cohorts. Although not definitive for HIV-1 infection status, all neonates had HIV-1 RNA levels below the limit of quantification at birth and at ages 1 and 2 weeks.

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Cross-resistance was not commonly observed in this lamivudine-treated cohort. M184V per se is not expected to compromise subsequent treatment with NRTI such as didanosine-stavudine or combinations containing abacavir.