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Molecular properties of HIV-1 resistant to 3TC and FTC.
Schinazi RF; Lloyd RM; McMillan A; Mathez DP; Leibowitch J; Mellors J;
December 30, 1995
Natl Conf Hum Retroviruses Relat Infect (1st). 1993 Dec 12-16;:136.

The in vitro and in vivo development of resistant viruses to L-oxathiolane cytosine nucleosides in cell culture was first described by our group (AAC 37:875,1993). These variants were highly (greater than 1,000-fold) resistant to 3TC, and FTC, but remained susceptible to other nucleosides and NNRT inhibitors. Unlike AZT-TP, HIV-1 RT derived from these viruses were less susceptible to FTC-TP and 3TC-TP relative to parental virus enzyme. DNA sequence analysis of the RT gene amplified from resistant viruses consistently identified mutations at codon 184 from Met (ATG) to Val (GTG or GTA) or Ile (ATA); the resistance site in this highly conserved polymerase motif (YMDD) adjacent to the putative catalytic site of the HIV-1 RT was confirmed by site- directed mutagenesis. In vitro studies indicated that (-)- FTC-resistant HIV-1 (EC50 greater than 100 micromolar) reverted to more susceptible virus within 7 weeks of passage in the absence of (-)-FTC. The EC50 plateaus at ca. 1 micromolar, suggesting a preferred intermediate RT conformation for this virus. Sequencing analysis of amplified RT from several patients who had received 3TC therapy demonstrated only the Met-184-to-Val (GTG) mutation. Virus obtained from 3/3 of these patients were highly resistant to (-)-FTC and 3TC. One patient also had the Thr- 215 (ACC) to Tyr (TAC) mutation associated with AZT- resistance, suggesting that double-mutated virus can be isolated in humans. A co-culture assay which measures the frequencies of infectious primary cells from HIV positive patients by the limiting dilution technique was used to study drug interactions. An advantage of this assay is that it utilizes a variety of wild-type viruses, not selected by in vitro propagation. Combinations of AZT with (-)-FTC 3TC, ddC, and ddI produced synergistic interactions. Synergistic interactions were also found with AZT and (-)-FTC or ddC in cells bearing AZT-resistant HIV.

Chromatography, High Pressure Liquid Didanosine/*PHARMACOKINETICS/THERAPEUTIC USE Drug Interactions Drug Therapy, Combination Human HIV Infections/DRUG THERAPY Rifabutin/*PHARMACOLOGY/THERAPEUTIC USE ABSTRACT