An Investigation into 3'-Azido-3'-Deoxythymidine Toxicity: Inhibtion of Thymidine Phosphorylation

3'-azido-3'-deoxythymidine (AZT) has been a staple of AIDS therapy for over two decades. Long-term use of high dosage AZT, as seen in the late 1980's and early 1990's, was associated with tissue toxicities, including hepatotoxicity and cardiomyopathy. Low dosage AZT therapy used in modern regimens is associated with lipodystrophy. Mitochondria in the affected tissues are dysfunctional and have depleted mitochondrial DNA. The toxicity of AZT is hypothesized to be due to AZT-5'-triphosphate (AZT-TP) inhibition of the mitochondrial DNA polymerase γ, leading to mitochondrial DNA depletion. In this work with isolated rat liver mitochondria and previous work with isolated rat heart mitochondria, AZT was phosphorylated to AZT-5'-monophosphate (AZT-MP), but no AZT-TP was detectable over two hours of incubation. However, in these mitochondria, AZT was shown to be a potent competitive inhibitor of thymidine phosphorylation. From this work, an alternative mechanism for AZT toxicity was proposed, wherein AZT's inhibition of thymidine phosphorylation leads to a depletion of the TTP pool, which causes mitochondrial DNA depletion. The 3T3-F442a cell culture model was used to further investigate this mechanism of toxicity. These cells are mouse pre-adipocytes that can be differentiated into adipocytes, making these cells a good model for lipodystrophy. The 3T3-F442a cells were grown and differentiated in the presence of 1 and 10 Ì_å_M AZT for 12 days. Samples were analyzed to determine mitochondrial DNA content and deoxynuceloside-triphosphate (dNTP) levels. Both 1 and 10 Ì_å_M AZT were toxic to mitochondria but caused a significant increase in mitochondrial DNA content relative to untreated cells. This is the opposite of the expected effect and may reflect a compensatory mechanism to overcome toxicity. Also, AZT did not cause any trend of variation from the untreated cells in the dNTP pools during the 12 days of treatment. These results do not provide any further evidence to advance the proposed mechanism of AZT toxicity due to AZT inhibition of thymidine phosphorylation.