Intracellular activation of 2 ',3 '-dideoxyinosine and drug interactions in vitro

Didanosine (2',3'-dideoxyinosine; ddI) requires intracellular metabolism to its active triphosphate, 2',3'-dideoxyadenosine 5'-triphosphate (ddATP), t o inhibit the replication of human immunodeficiency virus (HIV). We have in vestigated the metabolism of ddI to ddATP in the presence and absence of a range of compounds. In addition, we determined the levels of the endogenous competitor of ddATP, 2'-deoxyadenosine 5'-triphosphate (dATP), and calcula ted ddATP/dATP ratios. None of the nucleoside analogs studied had any effec t on ddI phosphorylation at 1 and 10 mu M concentrations. At 100 mu M conce ntrations, ddC reduced total ddA phosphates (82% of control total ddA phosp hates; p < 0.001). ZDV significantly decreased the levels of dATP, whereas ddC significantly increased dATP pools (e.g., at 100 mu M ZDV, 82% of contr ol dATP levels; p < 0.001). Hence, the ddATP/dATP ratio was increased in th e presence of ZDV, but was decreased in the presence of ddC. Neither d4T no r 3TC affected the ddATP/dATP ratio. Deoxyinosine (dI) significantly reduce d ddA phosphate production at 100 mu M concentrations, with ddATP reduced t o undetectable levels (p < 0.001). Hydroxyurea (HU) did not affect the acti vation of ddI, but significantly reduced dATP pools at 100 mu M concentrati ons (67% of control dATP levels; p < 0.001), enhancing the ddATP/dATP ratio , ddA phosphate production was significantly reduced by pentoxyfylline (PXF ) at 10 and 100 mu M concentrations. dATP levels were unaffected, but the d dATP/dATP ratio was reduced. Finally, 8-aminoguanosine (8-AMG) had no effec t on either ddI activation or dATP pools. These studies demonstrate the imp ortance of determining both the active TP and the competing endogenous TP, as changes to the resulting ratio could alter the efficacy of the nucleosid e analog in question.