UC-38, the lead compound for a group of structurally related non- nucleoside reverse transcriptase inhibitors shown to be potent inhibitors of HIV-induced cytopathology in the NCI primary in vitro anti-HIV drug screen, was found to be rapidly and extensively metabolized in vivo. Previously, this project has employed pharmaco-kinetic and metabolic considerations to design structural congeners of UC-38 which showed low metabolism and improved oral bioavailability while retaining high in vitro anti-HIV activity. A number of congeners which met these criteria were designed, synthesized and evaluated in murine and canine pharmacokinetic models. New anti-HIV assays have been developed, which allow compounds to be tested for activity against specific HIV strains producing mutant types of reverse transcrip-tase, since the initial selection of congeners for in vivo pharmacokinetic evaluation. The objective of this project was to evaluate the pharmacokinetic and metabolic properties of a group of UC-38 congeners which had shown activity against mutant strains of HIV in order to select a single candidate for more detailed preclinical study. Compounds were initially screened using a mouse liver homogenate assay for the assessment of in vitro metabolism. Results of the mouse liver homogenate assays were consistent with earlier studies conducted with hamster liver homogenate. Modification of the isopropyl ester moiety of UC-38 appeared to be the primary determinant of the extent of metabolism by the homogenate. In general, primary and secondary esters were metabolized most rapidly followed by ethers. O-alkyloximes and esters of 2,4-dimethyl-3-pentanol were relatively resistant to metabolism. Intravenous and oral murine pharmacokinetic studies have been completed on four of the compounds which were resistant to metabolism in vitro. Following IV injection of each of the compounds in mice, plasma concentration-time profiles exhibited triexponential behavior. High total body volumes of distribution suggest that the compounds are all highly distributed into deep tissue compartments. The plasma total body clearance values were 25 and 35 ml/min/kg for two t- butoximes and 27 and 51 ml/min/kg for the two esters. Following oral administration, plasma concentrations of the two t-butoximes were above 0.1 micro m through 24 hours. The two esters, however, were below the limit of detection (0.1 micro m) after 8 to 12 hours. The oral bioavailabilities for the t-butoximes were 32% and 18%, whereas the bioavailabilities of the esters were 13% and 8%. Additional congeners selected by the liver homogenate assay are under study.
