Pre- and postexposure chemoprophylaxis: evidence that 3'-azido-3'-dideoxythymidine inhibits feline leukemia virus disease by a drug-induced vaccine response.

The benefits of postexposure 3'-azido-3'-dideoxythymidine (AZT) prophylaxis following human immunodeficiency virus exposure are unknown. We describe a comprehensive assessment of pre- and postexposure AZT therapy in the feline leukemia virus (FeLV)-cat model for AIDS which included in vitro testing, an in vivo dose-response titration, a postexposure treatment study, plasma drug concentration determinations, and evaluation of the immune response to FeLV. In in vitro studies, AZT prevented FeLV infection of a feline T-lymphoid cell line, giving 50 and 90% inhibition concentrations of 4.6 and 11.1 mM, respectively. In all of the in vivo efficacy studies, AZT was administered by continuous subcutaneous infusion for 28 days. AZT toxicity was excessive at a dosage of 120 mg/kg of body weight per day, causing acute anemia, but AZT was tolerable at 60 mg/kg/day. In preexposure studies, AZT was efficacious in preventing chronic antigenemia at a dosage of > or = 15 mg/kg/day, at which plasma AZT concentrations averaged between 0.51 and 0.81 micrograms/ml (2.13 and 3.03 microM). As a postexposure treatment, at 60 mg/kg/day, AZT prevented chronic FeLV antigenemia when treatment was started up to 96 h post-virus inoculation (p.i.), but not when treatment was started at 192 h p.i. The 4-day period between 96 and 192 h p.i. appears to be critical for establishing chronic viremia. It is presumed that the increase in virus load between 4 and 8 days p.i. was able to overwhelm the immunologic functions responsible for containment of FeLV infection, even though AZT therapy effectively controlled viremia during the treatment period. The antibody response to FeLV varied depending on the time of AZT treatment initiation relative to virus challenge. When AZT treatment was started 48 h before or 8 h after FeLV challenge, antibodies to FeLV were not detected until after AZT treatment was discontinued at 28 days p.i. Following AZT treatment, however, antibody titers rapidly increased at a rate suggestive of a secondary immune response. When AZT treatment was initiate at later time points relative to virus challenge (24, 48, and 96 h p.i.), antibodies to FeLV became detectable during the treatment period. These results indicate that AZT treatment does not completely prevent FeLV infection, even when treatment begins before virus challenge, and that immune sensitization to FeLV proceeds during the prophylactic drug treatment period.