Background:  A prominent and consistent finding in patients who suffer serious nucleoside analog (NRTI) toxicities is a state of pro-inflammation including, high serum and/or tissue cytokines. Also, patients who initiate NRTI during the pro-inflammatory state of advanced HIV disease experience elevated rates of NRTI-toxicity. NRTI-toxicity presumably results from concentration-dependent intracellular NRTI-triphosphate inhibition of mitochondrial DNA synthesis. The objective of this work was to address the hypothesis that cytokines cause increased intracellular NRTI-triphosphate concentrations.

Methods: Peripheral blood mononuclear cells (PBMC) were harvested from 500 mL of blood from non-HIV-infected volunteers. PBMC were immediately aliquoted equally into complete media. Triplicate controls contained 10 mg/mL zidovudine (ZDV) or 1 mg/mL lamivudine (3TC). Triplicate experiments contained physiologically relevant and increasing concentrations of interferon-α (IFN) and tumor necrosis factor-α (TNF) added to the same ZDV or 3TC concentrations. After 12 hours of incubation, cells were recounted and tested for viability. Triphosphates were quantified with LC-MS-MS. Data were described as mean (SD) and analyzed with Student’s t-tests.

Results:  ZDV-triphosphate was 65 (0.9) fmol/million cells in controls. Each TNF concentration of 0.02, 0.2, and 2 ng/mL increased ZDV-triphosphate to a similar extent; overall mean 79 (6.4) fmol/million cells (p = 0.005). Each IFN concentration of 0.1, 1, and 10 units/mL increased ZDV-triphosphate to a similar extent; overall mean 76.1 (4) fmol/million cells (p = 0.001). 3TC-triphosphate was 1.36 (0.14) pmol/million cells in controls. Each of the same IFN concentrations increased 3TC-triphosphate to a similar extent; overall mean 1.83 (0.14) pmol/million cells (p < 0.001). Only the highest TNF concentration increased 3TC-triphosphate to 1.68 (0.08) pmol/million cells (p = 0.03). The cytokines did not affect cell counts and viability.

Conclusions:  TNF and IFN increased ZDV-triphosphate by about 1.2-fold. TNF and IFN increased 3TC-triphosphate by 1.24- and 1.35-fold, respectively. These cytokines exert actions on tissues other than PBMC, such as on fat for TNF. Given the narrow therapeutic index of some NRTI, these data provide a rational pharmacologic explanation for increased NRTI toxicity in subjects with a pro-inflammatory state. Lastly, the IFN findings raise the possibility of intracellular interactions between IFN-α, for hepatitis C therapy, and NRTI.

Keywords: Nucleoside analog; pharmacology; toxicity