Acute decrease in HIV-1 viral load after initiation of zidovudine therapy: Implications for interrupting vertical transmission

Use of zidovudine (AZT) in pregnant women has recently been reported to reduce the rate of vertical transmission of human immunodeficiency virus type 1 (HIV-1) possibly through a reduction in maternal viral load. To determine how quickly AZT is able to reduce viral load, infectious virus and plasma HIV-1 RNA were sequentially measured in an HIV-1-infected patient at short intervals (hours) after initiation of oral AZT. Peripheral blood samples were collected at baseline, 1, 2, 4, 8, 12, 24, 48 hours, and 1 week after initiation of AZT therapy (500 mg/day) for quantitative plasma HIV-1 RNA levels, p24 antigen levels, and AZT levels. Quantitative HIV-1 peripheral blood mononuclear cell and plasma cultures, CD4 cell counts, and MT-2 cell assays for syncytium-inducing phenotype were performed at baseline, 1, 2 days, and 1 week after initiation of AZT therapy. A significant drop in viral load did not occur until after 24-48 hours. AZT should probably be administered at least 2 days prior to anticipated delivery, if a reduced maternal viral load is responsible for AZT's efficacy in preventing vertical transmission.