Human immunodeficiency virus type 1 drug susceptibility during zidovudine (AZT) monotherapy compared with AZT plus 2',3'-dideoxyinosine or AZT plus 2',3'-dideoxycytidine combination therapy. The protocol 34,225-02 Collaborative Group.

Human immunodeficiency virus type 1 (HIV-1) isolates obtained prior to and during a combination therapy trial comparing zidovudine (AZT; 3'-azidothymidine) monotherapy with AZT plus 2',3'-dideoxyinosine (ddI) or AZT plus 2',3'-dideoxycytidine (ddC) were assessed for the development of drug resistance. Drug susceptibility was measured by using two different phenotypic assays, one that requires infection of peripheral blood mononuclear cells with HIV-1 isolated from cocultures and a second based on infection of HeLa CD4+ cells with recombinant virus containing the reverse transcriptase (RT) of the clinical isolate. In addition, genotypic assessment of resistance was obtained by DNA sequencing of the RT coding region. No difference in the development of AZT resistance was noted in isolates from individuals receiving AZT monotherapy or combination therapy. However, a low frequency of ddI or ddC resistance was seen in isolates from the combination arms, which may at least partially explain the enhanced efficacy observed with these drug combinations compared with monotherapy. It was noted from DNA sequencing that a relatively high frequency of the nonnucleoside RT inhibitor resistance mutation, codon 181 changed from encoding Tyr to encoding Cys, was present in some isolates both before and during nucleoside analog combination therapy. Since these patients were unlikely to have access to nonnucleoside RT inhibitors, it is probable that this mutation preexisted at a reasonable level in the wild-type virus population. Comparisons of the AZT susceptibility assays indicated a good correlation between the phenotypic and genotypic determinations. However, direct numerical comparisons between the phenotypic assays were not reliable, suggesting that valid comparisons of different resistance data sets will require the use of the same assay procedure.

Effects of discontinuation of zidovudine treatment on zidovudine sensitivity of human immunodeficiency virus type 1 isolates.

Zidovudine treatment of individuals infected with human immunodeficiency virus type 1 (HIV-1) results in HIV-1 isolates with a reduced zidovudine sensitivity in vitro. This reduction is due to mutations causing amino acid substitutions at five codons (41, 67, 70, 215, and 219) on the reverse transcriptase enzyme of HIV. HIV-1 isolates were obtained 8 to 69 weeks after therapy discontinuation from 10 patients at different stages of disease. Zidovudine sensitivity was determined by the HeLa CD4+ plaque assay. The presence of the resistance-conferring mutations was determined by using a selective polymerase chain reaction. Sensitivity could be determined for six isolate pairs: one showed a decline in the 50% inhibitory zidovudine concentration after therapy discontinuation; four pairs did not show a change. The majority of changes in the five codons in isolates from all 10 patients were the result of a relative increase in the wild-type sequence. Complete changes from mutant to the wild type were seen for only two codons in isolates from two patients. This study of isolates from a small group of individuals at different stages of disease, who had been taking zidovudine for 1 to 2 years, shows that a period of 1 year without zidovudine may be required to achieve a change from a mutant or mixed virus population to a wild-type virus population.

HIV with reduced sensitivity to zidovudine (AZT) isolated during prolonged therapy.

The drug sensitivities of human immunodeficiency virus (HIV) isolates from a group of patients with acquired immunodeficiency syndrome (AIDS) or AIDS-related complex (ARC) who were receiving zidovudine (3'-azido-3'-deoythymidine, AZT) therapy were tested by means of a newly developed plaque assay in CD4+ HeLa cells. Fifty percent inhibitory dose (ID50) values of 18 isolates from untreated individuals ranged between 0.01 microM and 0.05 microM. In contrast, most isolates from patients who had received zidovudine for 6 months or more exhibited decreased sensitivity characterized by changes in ID50 or ID95 values (or both), with isolates from several patients (5/15) showing 100-fold increases in ID50. The latter isolates were also insensitive to 3'-azido-2',3'-dideoxyuridine; however, the isolates were still sensitive to 2',3'-dideoxycytidine, 2',3'-dideoxy-2',3'-didehydrothymidine, or phosphonoformate. It cannot be determined from this small sample of patients whether development of a less sensitive virus phenotype results in clinical resistance. Appearance of such variants was not associated with a consistent increase in viral p24 concentrations in patient plasma and did not herald any sudden deterioration in clinical status. More extensive studies are required to determine the clinical significance. Thus, it would be premature to alter any treatment protocols for HIV-infected individuals at present.