Biological comparison of wild-type and zidovudine-resistant isolates of human immunodeficiency virus type 1 from the same subjects: susceptibility and resistance to other drugs.

We used a viral endpoint dilution assay to show changes in the proportion of zidovudine (azidothymidine; AZT)-resistant viruses within a heterogeneous mixture of human immunodeficiency virus type 1 (HIV-1) quasispecies isolated from patients on long-term AZT therapy. Several HIV-1 isolates, which could replicate in 10 microM AZT, were susceptible to both 2',3'-dideoxycytidine and a novel cytosine analog BCH-189, in which a sulfur atom replaces the 3' carbon of the pentose ring. In certain instances, cross-resistance was seen with 3'-didehydro-2',3'-dideoxythymidine. Although most strains of AZT-resistant HIV-1 displayed reduced susceptibility to 3'-azido-2',3'-dideoxyuridine, two strains were identified for which this was not the case.