Evidence that high-dosage zidovudine at time of retrovirus exposure reduces antiviral efficacy.

The antiviral efficacy of prophylactic 3'-azido-3'-deoxythymidine (AZT) therapy administered by continuous infusion or intermittent injection was compared in pediatric cats infected with feline leukemia virus. A 4-week treatment regimen of AZT was initiated at -48, 8, or 96 h postinfection (p.i.). For AZT therapy begun at -48 h p.i., significant efficacy was attained when therapy was given by continuous infusion but not by intermittent injection. However, when AZT therapy was delayed until 96 h p.i., both continuous infusion and intermittent injection gave complete protection. The results suggest that intermittent AZT administration is less efficacious than continuous infusion. Higher peak AZT concentrations in plasma associated with intermittent injection compared with those associated with continuous infusion may be immunotoxic, thus reducing the drug-induced vaccine effect. Furthermore, AZT toxicity seemed to be restricted to a window of sensitivity close to the time of virus challenge because delaying the start of AZT therapy until 96 h p.i. was highly efficacious, regardless of the method of administration.