Emergence of mutant HIV reverse transcriptase conferring resistance to AZT.

Abstract

(Krc,eived 2 Ortobrr IYYI) The likelihood of emergence of drug resistance is a function of mutation rate and the number of replicative events. The mutation rate of HIV is presumed to be at least as high as any virus with a single stranded RNA genome. The number of replicative events in a host infected with HIV are incalculably high as a result of years of persistent replication that is poorly restricted by immune surveillance. Viral chemo- therapy in humans has selected for drug resistant variants of influenza A virus, herpes simplex virus. varicella zoster virus, cytomegalovirus and rhinovirus. ' The prospect of drug resistant AZT therefore did not appear as a surprise to many. In the initial study of HIV susceptibility to AZT (zidovudine),' isolation of HIV was attempted with 186 peripheral blood mononuclear cell specimens from subjects at different stages of infection who h,ad received therapy with AZT for various periods. A total of 46 isolates were obtained from 33 individuals. The use of MT2 lympho- blastoid cells for the isolation procedure permitted the preparation of large stocks of virus that could be frozen in riliquots and titrated. Susceptibility to AZT with these low-passage clinical isolates of HIV could not be tested reliably with the described assays of inhibition of cytopathology or production of p24 antigen.j This dilemma was resolved by the USIZ of a CD4-expressing HeLa cell line (obtained from B. Chesebro, National Institute of Allergy and Infectious Diseases, Rocky Mountain Laboratories, Hamilton, Mont.) that readily permitted plaque reduction assays with all isolates te~ted.~ In addition, this system permitted assay of a wide range of compounds, including nucleosides, interferons, and recombinant soluble CD4. Study of these original isolates permitted several conclusions: (I) Isolates from subjects not treated with AZT display a narrow range of susceptibility to AZT, with