5-Chloro-2',3'-dideoxy-3'-fluorouridine (935U83) is a selective anti-human immunodeficiency virus (HIV) agent. When tested in phytohemagglutinin-stimulated normal human peripheral blood lymphocytes against fresh clinical isolates of HIV type 1 (HIV-1) obtained from patients naive to AZT (3'-azido-3'-deoxythymidine [zidovudine]), 935U83 inhibited virus growth with an average 50% inhibitory concentration (IC50) of 1.8 microM; corresponding IC50s were 0.10 microM for FLT (3'-deoxy-3'-fluorothymidine) and 0.23, 0.49, and 0.03 microM for the approved agents AZT, ddI (2',3'-dideoxyinosine), and ddC (2',3'-dideoxycytosine), respectively. Importantly, 935U83 retained activity against HIV strains that were resistant to AZT, ddI, or ddC. Of additional interest, we were unable to generate virus which was resistant to 935U83 by passaging either HXB2 (AZT-sensitive) or RTMC (AZT-resistant) strains in the presence of high concentrations of 935U83. The anabolic profile of 935U83 was similar to that of AZT, and 935U83 triphosphate was a potent inhibitor of HIV-1 reverse transcriptase. Pharmacokinetic evaluation showed good oral bioavailability (86% in mice and 60% in monkeys) and less extensive metabolism to the glucuronide relative to AZT. 935U83 showed low toxicity. In an in vitro assay for toxicity to a human erythrocyte progenitor, erythroid burst-forming unit (BFU-E), the IC50 for 935U83 (> 400 microM) was more than 1,000-fold those of FLT (0.07 microM) and AZT (0.30 microM). Mild reversible reductions in erythrocytes and associated parameters were seen in mice dosed orally with 2,000 mg of 935U83 per kg per day for 1 and 6 months. In monkeys dosed orally with up to 700 mg/kg/day for 1 and 6 months, the only possible treatment-related finding was cataracts in 1 of 12 animals given the intermediate dose of 225 mg/kg/day. At the highest doses in mice and monkeys, maximal concentrations in plasma were more than 100-fold the anti-HIV IC50s against clinical isolates. This safety profile in animals compares very favorably with that of any of the anti-HIV drugs approved to date and has led us to begin evaluation of 935U83 in patients with HIV infection.