Researchers from the University of Colorado have a proposed a new explanation for why people with advanced HIV disease tend to suffer more serious toxicities when treated with drug combinations that include nucleoside analogues. Writing in the March 1st edition of Clinical Infectious Diseases, they reason that excessive intracellular concentrations of phosphorylated NRTIs might be responsible for the disruption of mitochondrial processes and the consequent toxicities observed in affected tissues. Furthermore, they propose that increased cellular activation as a result of advanced HIV disease may be responsible for the over-production of NRTI phosphates, which are then shuttled into the mitochondria in preference to normal nucleotides.
All nucleoside analogues must be converted from their inactive monophosphate form into an active triphosphate form within cells, a process called phosphorylation. Nucleoside analogue triphosphates are produced at higher levels when lymphocytes are activated, due to higher production within the cell of the kinases that carry out phosphorylation. Nucleoside analogue triphosphates compete with nucleotides for incorporation into viral DNA, but also get incorporated into mitochondrial DNA within human cells. This causes loss of efficiency in mitochondria, leading to reduced cellular energy supply and malfunction.
Although it is widely accepted that toxicities tend to be more severe in people with advanced HIV disease, nobody has been able to explain why. In a review of the literature on nucleoside analogue toxicity, pharmacologist Dr Peter Anderson of the University of Colorado and colleagues offer a potential explanation for the greater severity of nucleoside analogue toxicities in advanced HIV disease and during treatment of hepatitis C infection.
The authors note that people on therapy with CD4 cell counts below 100 tend to have more problems with NRTI-associated toxicities such as peripheral neuropathy, pancreatitis and fat wasting. They cite evidence for the association of higher intracellular NRTI phosphate levels with severity of HIV disease. A few studies also suggest that women have higher concentrations of phosphorylated NRTIs than men, which may explain epidemiological findings that women also have a greater susceptibility to developing NRTI toxicity. Finally, they make a link between cellular activation and NRTI side-effects, pointing to increased levels of chemical markers of activation such as interferon (IFN) and tumor necrosis factor (TNF) found in people with advanced HIV disease. What is missing from the literature, though, are data showing a direct correlation between increased levels of cellular activation markers and increased intracellular levels of phosphorylated NRTIs or increased NRTI toxicity.
If an elevated state of cellular activation is indeed the mechanism for increased NRTI phosphorylation and consequent toxicity, there may be important clinical implications. The authors question if the strategy of delaying initiation of HIV therapy to minimize lifetime drug exposure and thereby limit toxicity may be flawed. The increased activation state seen in persons with advanced disease may actually make those who wait more susceptible to NRTI toxicity problems. The authors note that in the HOPS cohort, individuals who began treatment with CD4 cell counts below 100 cells/mm3 were significantly more likely to develop peripheral neuropathy, whilst a nadir CD4 count below 100 cells/mm3 was also the strongest predictor of lipoatrophy
Alpha interferon in hepatitis C: does it increase risk of NRTI toxicity?
The impact of drug-drug interactions on NRTI toxicity also needs more study. One of the activation markers the authors cite is the proinflammatory cytokine, alpha interferon(IFN). Therapy for hepatitis C virus (HCV) infection typically employs a form of alpha IFN given in combination with ribaviran. There have been reports of increased NRTI-associated toxicity in some co-infected individuals receiving HIV and HCV therapy simultaneously, but they have mostly been attributed to pharmacologic interactions with ribaviran. The authors suggest that the immune activator alpha IFN, by revving up NRTI phosphorylation in cells, may be the real culprit behind increased NRTI toxicity seen in dually treated people.
The associations are intriguing, but much is still not clear and more research is needed. Some studies have found increased markers of activation and evidence of mitochondrial DNA depletion in people with advanced HIV disease who have never been on therapy. So the extent to which mitochondrial toxicity is due to NRTIs or to HIV itself remains uncertain and in need of more study. Interestingly, as yet there is no evidence that genetic polymorphisms in mitochondrial polymerase gamma contribute to susceptibility for NRTI toxicity.
The available data are mostly from small studies, many from the pre-HAART era. And while the technology for determining intracellular levels of phosphorylated NRTIs is improving, the limitation of only being able to look easily at cells found in the bloodstream persists. What may be happening in the tissues where the clinical problems are seen is still mostly speculation. If the overall number of people in whom these questions have been studied is small, the number of women studied is smaller still. The evidence cited for women having higher intracellular phosphate concentrations came from just four women in one study, five in another and only one woman in a third. At a time when the number of people taking NRTIs is about to jump from the low hundred-thousands to the millions (with the number of women treated going up even more dramatically), the need to better understand the mechanisms and risks of NRTI toxicity is urgent.
Reference
Anderson PL, Kakuda TN, Lichtenstein KA. The cellular pharmacology of nucleoside- and nucleotide analogue reverse-transcriptase inhibitors and its relationship to clinical toxicities. Clinical Infectious Diseases, 2004;38:743-53