Metabolism, mitochondrial uptake and toxicity of 2 ',3 '-dideoxycytidine

2',3'-Dideoxycytidine (ddCyd) is a prescription anti-retroviral drug that c auses mitochondrial toxicity and peripheral neuropathy. ddCyd is actively p hosphorylated by cytosolic deoxycytidine kinase and nucleoside (di)phosphat e kinase to the 5'-triphosphate derivative. However, 2',3'-dideoxycytidine 5'-diphosphocholine (ddCDP-choline) was also found in human cells incubated with ddCyd. In this paper we show that ddCDP-choline is produced from dide oxyCTP (ddCTP) and phosphocholine by phosphocholine cytidylyltransferase. d CTP and CTP appear to activate this synthesis in a concentration-dependent manner. Although ddCTP and ddCDP-choline can both enter the mitochondria, d dCDP-choline uptake is more efficient than ddCTP uptake. These data suggest that ddCDP-choline is the ddCyd metabolite that is probably responsible fo r mitochondrial toxicity. The uptake of ddCTP and ddCDP-choline by mitochon dria is inhibited by 3.0 mM L-carnitine in the cell-free system investigate d; when added to U937 cells grown in the presence of 0.25 mu M ddCyd, 3.0 m M L-carnitine partially abrogated the mitochondrial toxicity of ddCyd.