Metabolism, Pharmacokinetics, Tissue Distribution, and Stability Studies of the Prodrug Analog of an Anti-Hepatitis B Virus Dinucleoside Phosphorothioate

Abstract

The alkoxycarbonyloxy dinucleotide prodrug R_p, S_p-2 is an orally bioavailable anti-hepatitis B virus agent. The compound is efficiently metabolized to the active dinucleoside phosphorothioate R_p, S_p-1 by human liver microsomes and S9 fraction without cytochrome P450-mediated oxidation or conjugation. The conversion of R_p, S_p-2 to R_p, S_p-1 appears to be mediated by liver esterases, occurs in a stereospecific manner, and is consistent with our earlier reported studies of serum-mediated hydrolytic conversion of R_p, S_p-2 to R_p, S_p-1. However, further metabolism of R_p, S_p-1 does not occur. The presence of a minor metabolite, the desulfurized product 10 was noted. The prodrug R_p, S_p-2 was quite stable in simulated gastric fluid, whereas the active R_p, S_p-1 had a half-life of <15 min. In simulated intestinal fluid, the prodrug 2 was fully converted to 1 in approximately 3 h, whereas 1 remained stable. To ascertain the tissue distribution of the prodrug 2 in rats, the synthesis of ³⁵S-labeled R_p, S_p-2 was undertaken. Tissue distribution studies of orally and intravenously administered radiolabeled [³⁵S]2 demonstrated that the radioactivity concentrates in the liver, with the highest liver/plasma ratio in the intravenous group at 1 h being 3.89 (females) and in the oral group at 1 h being 2.86 (males). The preferential distribution of the dinucleotide 1 and its prodrug 2 into liver may be attributed to the presence of nucleoside phosphorothioate backbone because phosphorothioate oligonucleotides also reveal a similar tissue distribution profile upon intravenous administration.