A mechanistic biomarker investigation of fialuridine hepatotoxicity using the chimeric TK-NOG Hu-liver mouse model and in vitro micropatterned hepatocyte cocultures.

Fialuridine (FIAU) is a nucleoside-based drug that caused liver failure and deaths in a human clinical trial that were not predicted by nonclinical safety studies. A recent report concluded that a TK-NOG humanized liver (hu-liver) mouse model detected human-specific FIAU liver toxicity, and broader use of that model could improve drug safety testing. We further evaluated this model at similar dose levels to assess FIAU sensitivity and potential mechanistic biomarkers. Although we were unable to reproduce the marked acute liver toxicity with two separate studies (including one with a "sensitized" donor), we identified molecular biomarkers reflecting the early stages of FIAU mitochondrial toxicity, which were not seen with its stereoisomer (FIRU). Dose dependent FIAU-induced changes in hu-liver mice included more pronounced reductions in mitochondrial to nuclear DNA (mtDNA/nucDNA) ratios in human hepatocytes compared to mouse hepatocytes and kidneys of the same animals. FIAU treatment also triggered a p53 transcriptional response and opposing changes in transcripts of nuclear- and mitochondrial-encoded mitochondrial proteins. The time dependent accumulation of FIAU into mtDNA is consistent with the ≥9-week latency of liver toxicity observed for FIAU in the clinic. Similar changes were observed in an in vitro micro-patterned hepatocyte coculture system. In addition, FIAU-dependent mtDNA/nucDNA ratio and transcriptional alterations, especially reductions in mitochondrially encoded transcripts, were seen in livers of non-engrafted TK-NOG and CD-1 mice dosed for a shorter period. Conclusion: These mechanistic biomarker findings can be leveraged in an in vitro model and in a more routine preclinical model (CD-1 mice) to identify nucleosides with such a FIAU-like mitochondrial toxicity mechanistic liability potential. Further optimization of the TK-NOG hu-liver mouse model is necessary before broader adoption for drug safety testing.

Use of a Bile Salt Export Pump Knockdown Rat Susceptibility Model to Interrogate Mechanism of Drug-Induced Liver Toxicity.

Inhibition of the bile salt export pump (BSEP) may be associated with clinical drug-induced liver injury, but is poorly predicted by preclinical animal models. Here we present the development of a novel rat model using siRNA knockdown (KD) of Bsep that displayed differentially enhanced hepatotoxicity to 8 Bsep inhibitors and not to 3 Bsep noninhibitors when administered at maximally tolerated doses for 7 days. Bsep KD alone resulted in 3- and 4.5-fold increases in liver and plasma levels, respectively, of the sum of the 3 most prevalent taurine conjugated bile acids (T3-BA), approximately 90% decrease in plasma and liver glycocholic acid, and a distinct bile acid regulating gene expression pattern, without resulting in hepatotoxicity. Among the Bsep inhibitors, only asunaprevir and TAK-875 resulted in serum transaminase and total bilirubin increases associated with increases in plasma T3-BA that were enhanced by Bsep KD. Benzbromarone, lopinavir, and simeprevir caused smaller increases in plasma T3-BA, but did not result in hepatotoxicity in Bsep KD rats. Bosentan, cyclosporine A, and ritonavir, however, showed no enhancement of T3-BA in plasma in Bsep KD rats, as well as Bsep noninhibitors acetaminophen, MK-0974, or clarithromycin. T3-BA findings were further strengthened through monitoring TCA-d4 converted from cholic acid-d4 overcoming interanimal variability in endogenous bile acids. Bsep KD also altered liver and/or plasma levels of asunaprevir, TAK-875, TAK-875 acyl-glucuronide, benzbromarone, and bosentan. The Bsep KD rat model has revealed differences in the effects on bile acid homeostasis among Bsep inhibitors that can best be monitored using measures of T3-BA and TCA-d4 in plasma. However, the phenotype caused by Bsep inhibition is complex due to the involvement of several compensatory mechanisms.

Antibiotic-Induced Elevations of Plasma Bile Acids in Rats Independent of Bsep Inhibition.

Drug-induced liver injury (DILI) is a common toxicity observed in drug development and can lead to withdrawal of approved drugs from the market. To better understand the numerous mechanisms of DILI, recent efforts have focused on transporter inhibition, specifically liver canalicular bile salt export pump (Bsep) as one mechanism of DILI, and on the potential use of plasma bile acids as monitorable mechanism-based biomarkers of Bsep inhibition. To explore alternative mechanisms of bile acid increases in plasma, 6 antibiotic and 2 nonantibiotic drugs unlikely to be Bsep inhibitors were evaluated in rat studies. Surprisingly, all 6 antibiotics demonstrated 2- to 14-fold increases of plasma taurocholic acid (TCA). Also, unconjugated primary bile acids and secondary bile acids (both taurine-conjugated and unconjugated) were decreased in rat plasma after antibiotic treatments, but not with the nonantibiotic drugs. These results suggest alternative mechanisms of bile acids regulation such as attenuation of bacterial deconjugation of bile acids following reduction of gut microflora by antibiotics. Measurements of TCA transport in rat hepatocytes and Bsep-containing membrane vesicles suggest that inhibition of uptake into hepatocytes could also contribute to increases in plasma bile acid concentrations, while excluding inhibition of Bsep as a mechanism. These studies further demonstrate that there are several mechanisms that can lead to conjugated bile acid increases in plasma. By carefully considering the time course and magnitude of changes of individual bile acids relative to any changes seen in transaminases and bilirubin, interpretations and conclusions of the involvement of Bsep inhibition are enabled.

Role of chronic toxicology studies in revealing new toxicities.

Chronic (>3 months) preclinical toxicology studies are conducted to support the safe conduct of clinical trials exceeding 3 months in duration. We have conducted a review of 32 chronic toxicology studies in non-rodents (22 studies in dogs and 10 in non-human primates) and 27 chronic toxicology studies in rats dosed with Merck compounds to determine the frequency at which additional target organ toxicities are observed in chronic toxicology studies as compared to subchronic studies of 3 months in duration. Our review shows that majority of the findings are observed in the subchronic studies since additional target organs were not observed in 24 chronic non rodent studies and in 21 chronic rodent studies. However, 6 studies in non rodents and 6 studies in rodents yielded new findings that were not seen in studies of 3-month or shorter duration. For 3 compounds the new safety findings did contribute to termination of clinical development plans. Although the incidence of compound termination associated with chronic toxicology study observations is low (∼10%), the observations made in these studies can be important for evaluating human safety risk.