Molinate Metabolism, Kinetics, and PBPK Model

Molinate has been widely used as a pre-emergent herbicide in the rice fields of California's Central Valley. In rat studies, the metabolite molinate sulfoxide was generated after molinate exposure and it is the sulfoxide, sulphone or both that are suspected of causing testicular toxicity. The sulfoxide is generated in the liver and can be transported through the blood stream to the testis. In similar fashion to the rat, humans qualitatively produce the same molinate metabolites. Therefore to extrapolate the reproductive risk to humans post molinate exposure this study outlined the development of a preliminary physiologically-based pharmacokinetic (PBPK) model for validation in rats and extrapolation to humans.;Previous lab studies have implicated the sulfoxidation pathway of molinate metabolism to induce testicular toxicity. Once molinate is metabolized to molinate sulfoxide it undergoes phase II metabolism either spontaneously, enzyme catalyzed or both to form glutathioneconjugated molinate. This research has compared the metabolic capability of rat and human liver cytosol to form a glutathione conjugated metabolite of molinate. The glutathione conjugation of molinate sulfoxide in rat cytosol is described by the constants Km which is the equivalent to the substrate concentration at which the rate of conversion is one-half of Vmax, and the Vmax which is the maximum rate an enzyme can catalyze a reaction (Km of 305 micromolar (muM ) and Vmax of 4.21 nanomole/minute/milligram (nmol/min/mg) cytosol); whereas, the human values are (91 muM and 0.32 nmol/min/mg protein for Km and Vmax), respectively. Using the same 1 mM glutathione concentration the in vitro bimolecular nonenzymatic rate constant of 3.02 x 10-6 /muM * min. was calculated for glutathione conjugation of molinate sulfoxide. Specific activity for rat and human glutathione transferase was calculated to equal 1.202 +/- 0.25 and 0.809 +/- 0.45 imol/min/mg protein, respectively, by 1-chloro-2 and 4-dinitrobenzene assay. Compared to a conventional glutathione depletion model (buthionine sulfoximine + diethylmaleate combination), molinate alone is nearly as effective in reducing glutathione levels by approximately 90 % in the liver and 25 % in the testes. The impact of molinate sulfoxide's ability to adduct glutathione transferase and inhibit the production of the glutathione conjugated metabolite was examined and found to be negligible.;The preliminary seven-compartment physiologically-based pharmacokinetic model for molinate, constructed for an adult male Sprague-Dawley rat, employed flow-limited rate equations, including blood, kidney, liver, rapid perfused, slowly perfused tissues, and diffusion-limited fat. The systemic circulation connected the various compartments. The simulations favorably predicted the molinate blood concentrations of rat blood and testes compartment with profiles obtained from 10 and 100 mg/kg oral or 1.5 and 15 mg/kg intravenous (I.V.) doses. Human physiological parameters were substituted into the oral dosed model and the simulations closely predicted the molinate blood concentration obtained from a 5.06 mg oral dose. A sensitivity analysis determined for an oral dose that peak blood molinate concentrations were most responsive to the blood flows to kidney and fat compartments. The testicular molinate sulfoxide concentrations depended on molinate sulfoxide partition coefficients for the testes compartment and the Km for glutathione conjugation of molinate sulfoxide in the liver compartment.