Effects of thiabendazole (TBZ) on mitochondrial function in renal cortex of ICR mice

In vitro and in silico identification and characterization of thiabendazole as a mechanism-based inhibitor of CYP1A2 and simulation of possible pharmacokinetic drug-drug interactions

Thiabendazole (TBZ) and its major metabolite 5-hydroxythiabendazole (5OH-TBZ) were screened for potential time-dependent inhibition (TDI) against CYP1A2. Screen assays were carried out in the absence and presence of NADPH. TDI was observed with both compounds, with k(inact) and K(I) values of 0.08 and 0.02 min(-1) and 1.4 and 63.3 microM for TBZ and 5OH-TBZ, respectively. Enzyme inactivation was time-, concentration-, and NADPH-dependent. Inactivation by TBZ was irreversible by dialysis and oxidation by potassium ferricyanide, and there was no protection by glutathione. 5OH-TBZ was a weak TDI of CYP1A2, and enzyme activity was recovered by dialysis. IC(50) determination of TBZ and 5OH-TBZ showed both compounds to be potent inhibitors, with IC(50) values of 0.83 and 13.05 microM, respectively. IC(50) shift studies also demonstrated that TBZ was a TDI of CYP1A2. In silico methods identified the thiazole group as a TDI fragment and predicted it as the site of metabolism. The observation pointed to epoxidation of the thiazole and the benzyl rings of TBZ as possible routes of metabolism and mechanisms of TDI. Drug-drug interaction (DDI) simulation studies using SimCyp showed good predictions for competitive inhibition. However, predictions for mechanism-based inhibition (MBI)-based DDI were not in agreement with clinical observations. There was no TBZ accumulation upon chronic administration of the drug. The in vitro MBI findings might therefore not be capturing the in vivo situation in which the proposed bioactivation route is minor. This might be the case for TBZ in which, in vivo, UDP glucuronosyltransferases and sulfanotransferase metabolize and eliminate the 5OH-TBZ.

A rapid assay for mitochondrial DNA damage and respiratory chain inhibition in the yeast Saccharomyces cerevisiae

There is a need for a rapid assay to identify agents that damage mitochondria because the mitochondrion may be an important target for numerous environmental mitotoxins. Certainly at least one chemotherapeutic regimen (CHOP therapy) that includes doxorubicin can induce cardiomyopathy through mitochondrial genotoxicity in cardiac muscle cells. Yeast cells (1.5 x 10(6)-10(7)) in water are spread on a YEPD plate, and, when the suspension of cells has dried, a small well (12 mm diameter) is cut into the agar; 200-400 microl of a solution of the presumptive mitochondrial genotoxin is placed in the well, and the plates are incubated for 2 days. The genotoxin forms a concentration gradient through the agar and affects the growing cells. An overlay containing tetrazolium chloride is added, and the plates are incubated for 6-24 hr. Respiring cells turn red, and nonrespiring cells, with damaged DNA or inhibited respiratory chains, that are adjacent to the well, are white. A white ring, or a more lightly colored red ring, around the well indicates the presence of cells with lowered respiratory activity which may be fully reversible when the mitochondrial genotoxin is removed. In preliminary experiments, doxorubicin (= adriamycin) shows strong activity with this assay; cyclophosphamide is negative, and 4-hydroxycyclophosphamide, a metabolite of cyclophosphamide, is weakly positive. Ethidium bromide, methotrexate, 5-fluorouracil, and 5-fluorocytosine also are mitochondrial genotoxins. Antifungal agents similar to 5-fluorocytosine and anthelmintic compounds such as pyrvinium iodide can be powerful mitochondrial genotoxins.