INDIGO’s new MitoVir platform provides novel approach to evaluating mitochondrial toxicity

The new method may provide a means of predicting adverse side effects to antiviral drugs in the safety of a laboratory setting, rather than in clinical trials where patients may be placed at risk

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STATECOLLEGE, Pa.—Various clinical studies involving antiviral compounds haverecently been paused or ended because of mitochondrial toxicity, such as thatwhich led to the observation of hepatotoxicity during clinical trials of theantiviral drug candidate fialuridine aimed at hepatitis B. According to INDIGOBiosciences, some of these efforts might have fared better if the companies hadbeen able to use its MitoVir Platform, a series of assays to evaluatemitochondrial toxicity by examining inhibition of the ribonucleosidetriphosphates that are substrates for the human mitochondrial RNA polymerase(POLRMT).
Theplatform's origins come out of research conducted by Dr. Jamie Arnold, Dr.Craig Cameron and their colleagues at Pennsylvania State University, demonstratingthat antiviral drugs may not only enter into viral RNA, but also into healthymitochondrial RNA. 
After nucleosidereverse transcriptase inhibitors (NRTIs) enter a cell, for example, the NRTIsand their phosphorylated metabolites partition between the cytoplasm andmitochondria facilitated by nucleoside and nucleotide transporters, INDIGOnotes. In mitochondria, many triphosphorylated NRTIs serve as substrates forthe mitochondrial DNA polymerase gamma.
"Thecellular mitochondria that are responsible for making ATP are affected by thesecompounds, which is why we see adverse side effects," Cameron said, notingthat some individuals are more sensitive to the effects of antiviral drugs. "Wehope this method will prevent adverse events in the clinic, at least untilprescreening of patients to identify those with mitochondrial diseases becomesa part of the process."
Theuse of MitoVir was discussed in a journal article for which Arnold served aslead, which was titled "Sensitivity of Mitochondrial Transcription andResistance of RNA Polymerase II Dependent Nuclear Transcription to AntiviralRibonucleosides" and appeared in the Nov. 15, 2012, issue of  in the journal PLOS Pathogens.
Asnoted in the article, maintenance, replication and expression of themitochondrial genome (mtDNA) is absolutely essential for cell function becausemtDNA encodes 13 proteins required for oxidative phosphorylation, as well asthe tRNAs and rRNAs essential to their production.
"However,cells have far more mtDNA than required to support oxidative phosphorylation;therefore, a substantial reduction in mtDNA copy number is required formanifestation of a clinically apparent phenotype," the authors noted. "Therefore,adverse effects of drugs that interfere with replication and/or expression ofmtDNA may not be readily apparent in mammalian cells or animal models on thetimescale of preclinical testing. In addition, because mammalian cell lines areoften grown in high concentrations of glucose, oxidative phosphorylation is notnecessarily required for ATP production in cells, the so called Crabtree effect."
Underthese conditions, according to the authors, even severe mitochondrialdysfunction would have little to no impact on intracellular ATP levels.
TheMitoVir Platform includes three assays: ATP production, mitochondrial RNAproduction and POLRMT activity, and INDIGO President and CEO Jack Vanden Heuvelsays of them, "We now have a method of predicting adverse side effects toantiviral drugs in the safety of a laboratory setting, rather than in clinicaltrials where patients may be placed at risk. We're certain this will be asignificant advance in the development of this important class of antiviralcompounds."

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