In vitro investigation of the glutathione transferase M1 and T1 null genotypes as risk factors for troglitazone-induced liver injury.

The double null mutation of glutathione transferase, GSTM1 and GSTT1, is reported to influence troglitazone-associated abnormal increases of alanine aminotransferase and aspartate aminotransferase. However, no nonclinical data with a bearing on the clinical outcomes and underlying mechanisms have hitherto been reported. To investigate whether deficiency in GSTM1 and/or GSTT1 is related to troglitazone hepatotoxicity in vitro, the covalent binding level (CBL) (an index of reactive metabolite formation) and cytotoxicity of troglitazone and rosiglitazone, another thiazolidinedione but with low hepatotoxicity, were examined using human liver samples phenotyped for cytochrome P450s and genotyped for GSTM1 and GSTT1. Despite addition of GSH, CBLs of troglitazone and rosiglitazone in human liver microsomes were correlated with CYP3A (or CYP2C8) and CYP2C8 activities, respectively. With addition of recombinant GSTM1, the microsomal CBLs of troglitazone and rosiglitazone decreased. However, the CBLs of troglitazone in GSTM1/GSTT1 wild-type hepatocytes were unexpectedly higher than those in null hepatocytes. Although this discrepancy has not been fully explained, the GSTM1 and GSTT1 null mutations increased the cytotoxicity of troglitazone, independent of CYP3A or CYP2C8 activities. Furthermore, a GSH adduct of troglitazone, M2, limited to GSTM1 wild-type hepatocytes was detected. Of clear interest, GSTM1 and/or GSTT1 null mutation-dependent cytotoxicity and higher exposure to the reactive metabolite trapped as M2 as for troglitazone were not observed for rosiglitazone. This result might at least partly explain the findings related to clinical hepatotoxicity, suggesting that measurement of GSH adducts or cytotoxicity using GSTM1- and GSTT1-genotyped hepatocytes might offer an important in vitro system to assist in better prediction of idiosyncratic hepatotoxicity.