HIV-1 viral load, phenotype, and resistance in a subset of drug-naive participants from the Delta trial. The National Virology Groups. Delta Virology Working Group and Coordinating Committee.

Background: The Delta trial showed that combination therapy (zidovudine plus didanosine and zidovudine plus zalcitabine) substantially lengthened life and reduced disease progression compared with zidovudine monotherapy. We did a nested virological study in three countries (France, the Netherlands, and the UK) to investigate changes in markers for viral load and antiretroviral-drug resistance during therapy.

Methods: 240 zidovudine-naive HIV-1-infected patients were randomly assigned zidovudine only (n = 87), zidovudine plus didanosine (n = 80), or zidovudine plus zalcitabine (n = 73). Viral load in peripheral-blood mononuclear cells and plasma was measured by quantitative culture. Plasma HIV-1 RNA was measured by reverse-transcriptase PCR amplification, and serum p24 antigen by ELISA. Resistance to antiretroviral drugs was measured phenotypically by culture and genotypically by detection and quantification of drug-related point mutations in the pol gene. Analyses were done by intention to treat.

Findings: The reduction in viral load was greatest 4-12 weeks after the start of therapy and was most pronounced in the combination-therapy study groups (median reductions of RNA at 4 weeks 1.58, 1.28, and 0.49 log10 copies/mL for zidovudine plus didanosine, zidovudine plus zalcitabine, and zidovudine only, respectively). RNA levels at 8 weeks were predictive of disease progression and death after allowance for baseline values. At 48 weeks, the proportion of participants with phenotypic zidovudine resistance was similar in all three groups: didanosine and zalcitabine resistance were rare; zidovudine genomic resistance correlated with phenotypic resistance (r = 0.54, p < 0.0001) and developed earlier in the combined-therapy groups. However, participants in the zidovudine monotherapy group had higher circulating loads of resistant virus than those in the combined-therapy groups.

Interpretation: Combined antiretroviral therapy was more efficient at lowering virus load than monotherapy. Although zidovudine resistance was common in monotherapy and combined-therapy groups, circulating concentrations of resistant virus were substantially lower in the combination groups, which is likely to be a result of the continued antiviral activity of didanosine or zalcitabine.