Effect of nucleoside analogs on neurite regeneration and mitochondrial DNA synthesis in PC-12 cells

The effects of several anti-human immunodeficiency virus nucleoside analogs were examined on neurite regeneration and mitochondrial DNA (mtDNA) synthesis in nerve growth factor-primed PC-12 cells. Under pharmacologically relevant concentrations, the exposure of cells to 2',3'-dideoxyinosine (ddI), 2',3'-dideoxycytidine (ddC) and 2',3'-didehydro-3'-deoxythymidine (d4T) led to a marked dose-dependent inhibition of neurite regeneration with a 50% inhibitory concentration approximating 1, 5 and 15 microM, respectively. In contrast, 3'-azido-3'-deoxythymidine (AZT) and beta-L-2',3'-dideoxy-3'-thiacytidine (3TC) had no effect on neurite regeneration. Inhibition of mtDNA synthesis by ddI was dose dependent, and ddC at a concentration of 10 microM strongly reduced mtDNA content by >75%. However, no inhibition of mtDNA synthesis was detected in cells exposed to 10 microM 3TC or d4T and to 25 microM AZT, suggesting a lack of definite correlation between mtDNA depletion and blockage of neurite regeneration. High performance liquid chromatographic analysis demonstrated that AZT, ddC, 3TC and d4T were anabolized to their respective monophosphate, diphosphate and triphosphate derivatives in the PC-12 cells. In addition, d4T was phosphorylated to form its monophosphate, diphosphate and triphosphate derivatives in isolated mitochondria, whereas ddC was metabolized only to its monophosphate form and no phosphorylated metabolites of 3TC were detected under the same conditions. In summary, the peripheral neuropathy induced by ddC and ddI in patients with acquired immune deficiency syndrome may be accounted for by the depletion of mtDNA content in the neurons. As for d4T, some other mechanism(s) may be involved in its clinical neurotoxicity. Both AZT and 3TC lacked any substantial toxicity in our in vitro model, which is in agreement with the clinical action of these drugs.