Unique intracellular activation of the potent anti-human immunodeficiency virus agent 1592U89.

The anabolism of 1592U89, (-)-(1S,4R)-4-[2-amino-6-(cyclopropylamino)-9H-purin-9-yl]-2-cyclo pentene-1-methanol, a selective inhibitor of human immunodeficiency virus (HIV), was characterized in human T-lymphoblastoid CD4+ CEM cells. 1592U89 was ultimately anabolized to the triphosphate (TP) of the guanine analog (-)-carbovir (CBV), a potent inhibitor of HIV reverse transcriptase. However, less than 2% of intracellular 1592U89 was converted to CBV, an amount insufficient to account for the CBV-TP levels observed. 1592U89 was anabolized to its 5'-monophosphate (MP) by the recently characterized enzyme adenosine phosphotransferase, but neither its diphosphate (DP) nor its TP was detected. The MP, DP, and TP of CBV were found in cells incubated with either 1592U89 or CBV, with CBV-TP being the major phosphorylated species. We confirmed that CBV is phosphorylated by 5'-nucleotidase and that mycophenolic acid increased the formation of CBV-TP from CBV 75-fold. However, mycophenolic acid did not stimulate 1592U89 anabolism to CBV-TP. The adenosine deaminase inhibitor erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA) did not inhibit CBV-TP formation from CBV or 1592U89, whereas the adenylate deaminase inhibitor 2'-deoxycoformycin selectively inhibited 1592U89 anabolism to CBV-TP and reversed the antiviral activity of 1592U89. 1592U89-MP was not a substrate for adenylate deaminase but was a substrate for a distinct cytosolic deaminase that was inhibited by 2'-deoxycoformycin-5'-MP. Thus, 1592U89 is phosphorylated by adenosine phosphotransferase to 1592U89-MP, which is converted by a novel cytosolic enzyme to CBV-MP. CBV-MP is then further phosphorylated to CBV-TP by cellular kinases. This unique activation pathway enables 1592U89 to overcome the pharmacokinetic and toxicological deficiencies of CBV while maintaining potent and selective anti-HIV activity.