Characterization of HIV isolates arising after prolonged zidovudine therapy

Human immunodeficiency virus type 1 (HIV-1) was isolated from five patients with late-stage disease treated with zidovudine (ZDV) for more than 1 year. Peripheral blood mononuclear cells (PBMCs) were used for all virus isolations and to assay for drug resistance. The isolates exhibited a 10- to 100-fold decrease in ZDV susceptibility compared to pretreatment isolates. Multiple clones of a 618 bp segment of the HIV reverse transcriptase gene encompassing codons 60-250 were sequenced for each isolate. The association of alterations at codons Asp⁶⁷ → Asn, Lys⁷⁰ → Arg, Thr²¹⁵ → Phe or Tyr, and Lys²¹⁹ → Gln with ZDV resistance has been previously noted (ref. 5). In this study, the most frequent alteration was Thr²¹⁵ → Tyr although genotypic mixtures of Thr/Tyr and Phe/Tyr were also observed. One isolate with a Tyr²¹⁵ alteration and unaltered codons at 67, 70, and 219 had high-level ZDV resistance. Alterations at codons 67, 70, and 219 did not appear to increase resistance when seen in combination with Tyr²¹⁵. Virus isolates obtained from each patient by cultivation with either 0 or 4 μM ZDV were compared and found to have similar alterations at codons 67, 70, 215, and 219, although one instance of apparent in vitro selection for Tyr²¹⁵ over Phe²¹⁵ was observed. Assays using PBMCs for virus propagation will permit susceptibility testing of HIV isolates from most patients on antiretroviral drugs to investigate the clinical significance of drug resistance.