Addressing the selectivity and toxicity of antiviral nucleosides

Feng, Joy Y

Despite our collective knowledge of nucleoside analogassociated mitochondrial toxicity, safety concerns continued in the discovery of HCV antivirals. A number of HCV nucleoside/tide analogs failed in Phase II due to toxicity or association with a toxic analog. These include valopicitabine (NM283, gastrointestinal toxicity, Figure 1 ), 17 balapiravir (R1626, hematologic toxicity, Figure 1 ), 17 BMS-986094 (cardiac and kidney toxicity, Figure 1 ), 18 IDX184 (clinical hold due to association with BMS-986094, Figure 1 ), 19 and PSI-938 (liver toxicity, Figure 1 ). 17 With the exception of PSI-938, the TP forms of these compounds are substrates of mitochondrial RNA polymerase (POLRMT). Balapiravir is a specific mitochondria toxin, inhibits POLRMT, reduces mitochondrial protein synthesis, and eventually decreases mitochondrial respiration. 20, 21 Our data showed that 2 0 CMeG-TP is incorporated into RNA by POLRMT, decreases mitochondria protein synthesis, and decreases mitochondrial cellular respiration. 20, 22 Furthermore, Jin et al. demonstrated that the toxicity of BMS-986094 is directly correlated to its 2 0 CMeGMP nucleotide moiety rather than the naphthalene-phosphorylamino propanoate prodrug moiety. 21 Interestingly, when compared to selective mitochondria inhibitors such as ddC, 4-azidoC, and chloramphenicol ( Figure 1 ), the inhibition profile of BMS-986094 is distinctly different for mitochondrial protein synthesis and respiration, indicating it may hit other off-targets such as RNA polymerase I. 20 As of today, the only approved nucleotide prodrug for the treatment of HCV is sofosbuvir ( Figure 1 ), a 2 0 F, 2 0 CMe uridine prodrug that is well tolerated in patients. 23 The TP form of sofosbuvir showed no inhibition of any of the human polymerases tested and is a poor substrate of POLRMT. 20 The relevance of POLRMT inhibition as a marker for nucleotide toxicity has been further supported by works from multiple groups. 20, 21, 22, 24 A clear outlier is PSI-938, a 2 0 F, 2 0 CMe guanosine prodrug, which showed liver toxicity in clinical trials but is a poor substrate of POLMRT and showed no toxicity in any of the in vitro studies conducted so far. 20 Host polymerases have been generally regarded as the primary off-targets for this class of compounds. However, the observed toxicities tend to be highly unpredictable. 7, 25 These may be attributed to complex uptake, distribution, and accumulation of different compound in different organs. For example, three closely related analogs, FIAU, FMAU, and FIAC ( Figure 1 ), showed distinctly different toxicities in animals and human. FIAU showed no toxicity in nonclinical animal species including mice, rats, dogs, and monkeys, but conflicting toxicity findings from two separate studies in woodchucks. 15, 26 In contrast, FIAU-induced hepatotoxicity was observed in clinical trials in week 13 and beyond under potentially much lower exposures than the animal models. 15, 26 In addition, FMAU, a metabolite of FIAU, was tested in a Phase I clinical trial for the treatment of murine leukemia. It caused severe neurologic toxicity and two deaths among eight patients after 8-10 days of treatment. Toxicity, primarily hematopoietic, was observed with FIAC, the cytidine analog and known metabolic precursor of FIAU, when used to treat HSV and vesicular stomatitis virus infection. 26 Interestingly, while the more recent HCV nucleotide antivirals, e.g. BMS-986094, PSI-938, and VX-135 (also known as ALS-2200, Fi