EFFECT OF NUCLEOSIDE ANALOGS ON NEURITE REGENERATION AND MITOCHONDRIAL-DNA SYNTHESIS IN PC-12 CELLS

The effects of several anti-human immunodeficiency virus nucleoside an alogs were examined on neurite regeneration and mitochondrial DNA (mtD NA) synthesis in nerve growth factor-primed PC-12 cells. Under pharmac ologically relevant concentrations, the exposure of cells to 2',3'-did eoxyinosine (ddl), 2',3'-dideoxycytidine (ddC) and 2',3'-didehydro-3'- deoxythymidine (d4T) led to a marked dose-dependent inhibition of neur ite regeneration with a 50% inhibitory concentration approximating 1,5 and 15 mu M, respectively. In contrast, 3'-azido-3'-deoxythymidine (A ZT) and beta-L-2',3'-dideoxy-3'-thiacytidine (3TC) had no effect on ne urite regeneration. Inhibition of mtDNA synthesis by ddl was dose depe ndent, and ddC at a concentration of 10 mu M strongly reduced mtDNA co ntent by >75%. However, no inhibition of mtDNA synthesis was detected in cells exposed to 10 mu M 3TC or d4T and to 25 mu M AZT, suggesting a lack of definite correlation between mtDNA depletion and blockage of neurite regeneration. High performance liquid chromatographic analysi s demonstrated that AZT, ddC, 3TC and d4T were anabolized to their res pective monophosphate, diphosphate and triphosphate derivatives in the PC-12 cells. In addition, d4T was phosphorylated to form its monophos phate, diphosphate and triphosphate derivatives in isolated mitochondr ia, whereas ddC was metabolized only to its monophosphate form and no phosphorylated metabolites of 3TC were detected under the same conditi ons. In summary, the peripheral neuropathy induced by ddC and ddl in p atients with acquired immune deficiency syndrome may be accounted for by the depletion of mtDNA content in the neurons. As for d4T, some oth er mechanism(s) may be involved in its clinical neurotoxicity. Both AZ T and 3TC lacked any substantial toxicity in our in vitro model, which is in agreement with the clinical action of these drugs.