Uptake and distribution of 2',3'-dideoxyinosine and its derivatives in a human monocytoid cell line

The dideoxynucleoside analogue 2',3'-dideoxyinosine (ddI) has been used in the clinic as an alternative drug to zidovudine (AZT) in the treatment of patients with the acquired immunodeficiency syndrome (AIDS). However, it shows significant and variable toxicity in patients. It is known that various dideoxynucleoside analogues can cause the termination of the DNA chain following incorporation of the corresponding triphosphate metabolite by the polymerases. In the case of ddI, the presumed active metabolite is 2',3'-dideoxyadenosine 5'-triphosphate (ddATP). In order to understand the molecular basis for the toxicity of ddI, we evaluated the relationship between the intracellular formation of ddATP, its incorporation into cellular DNA and the effects on the growth of U937 cells, a human monocytoid cell line. Dideoxyinosine was not significantly toxic to U937 cells at concentrations as high as 500 microM in a 72 hrs. growth inhibition assay. The results of the uptake of 3HddI in this cell line showed a proportional increase in total metabolites with increasing concentrations of the drug (1-20 microM) after a 24 hrs. exposure. Incubation with 10 microM 3HddI resulted in the formation of low levels of ddATP within a period of 2 hrs. A significant amount of ddI-derived radioactivity was found in both DNA and RNA after exposure to 10 microM 3HddI for 24 to 72 hrs. However, no evidence of incorporation of ddATP into the cellular DNA fraction was obtained in these experimental conditions. Therefore, the lack of significant toxicity of ddI to U937 cells can be explained, at least in part, by its inability to incorporate ddATP into its cellular DNA at the doses studied.