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Fasiglifam (TAK-875), a Free Fatty Acid Receptor 1 (FFAR1) agonist in development for the treatment of type 2 diabetes, was voluntarily terminated in phase 3 due to adverse liver effects. A mechanistic investigation described in this manuscript focused on the inhibition of bile acid (BA) transporters as a driver of the liver findings. TAK-875 was an in vitro inhibitor of multiple influx (NTCP and OATPs) and efflux (BSEP and MRPs) hepatobiliary BA transporters at micromolar concentrations. Repeat dose studies determined that TAK-875 caused a dose-dependent increase in serum total BA in rats and dogs. Additionally, there were dose-dependent increases in both unconjugated and conjugated individual BAs in both species. Rats had an increase in serum markers of liver injury without correlative microscopic signs of tissue damage. Two of 6 dogs that received the highest dose of TAK-875 developed liver injury with clinical pathology changes, and by microscopic analysis had portal granulomatous inflammation with neutrophils around a crystalline deposition. The BA composition of dog bile also significantly changed in a dose-dependent manner following TAK-875 administration. At the highest dose, levels of taurocholic acid were 50% greater than in controls with a corresponding 50% decrease in taurochenodeoxycholic acid. Transporter inhibition by TAK-875 may cause liver injury in dogs through altered bile BA composition characteristics, as evidenced by crystalline deposition, likely composed of test article, in the bile duct. In conclusion, a combination of in vitro and in vivo evidence suggests that BA transporter inhibition could contribute to TAK-875-mediated liver injury in dogs., (© The Author 2017. Published by Oxford University Press on behalf of the Society of Toxicology.)