Intracellular metabolism of beta-L-2 ',3 '-dideoxyadenosine: Relevance to its limited antiviral activity

The intracellular metabolism of the beta-L- enantiomer of 2',3'-dideoxyaden osine (beta-L-ddA) was investigated in HepG2 cells, human peripheral blood mononuclear cells (PBMC), and primary cultured human hepatocytes in an effo rt to understand the metabolic basis of its limited activity on the replica tion of human immunodeficiency virus and hepatitis B virus. Incubation of c ells with 10 mu M [2',3',8-H-3]-beta-L-ddA resulted in an increased intrace llular concentration of beta-L-ddA with time, demonstrating that these cell s were able to transport beta-L-ddA However, it did not result in the phosp horylation of beta-L-ddA to its pharmacologically active 5'-triphosphate (b eta-L-ddATP). Five other intracellular metabolites were detected and identi fied as beta-L-2',3'-dideoxyribonolactone, hypoxanthine, inosine, ADP, and ATP, with the last being the predominant metabolite, reaching levels as hig h as 5.14 +/- 0.95, 8.15 +/- 2.64, and 15.60 +/- 1.74 pmol/10(6) cells at 8 , 4, and 2 h in HepG2 cells, PBMC, and hepatocytes, respectively. In additi on, a beta-glucuronic derivative of beta-L-ddA was detected in cultured hep atocytes, accounting for 12.5% of the total metabolite pool. Coincubation o f hepatocytes in primary culture with beta-L-ddA in the presence of increas ing concentrations of 5'-methylthioadenosine resulted in decreased phosphor olysis of beta-L-ddA and formation of associated metabolites. These results indicate that the limited antiviral activity of beta-L-ddA is the result o f its inadequate phosphorylation to the nucleotide level due to phosphoroly sis and catabolism of beta-L-ddA by methylthioadenosine phosphorylase (EC 2 .4.2.28).