Dietary organic isothiocyanates as modulators of ATP-binding cassette transporters
Organic isothiocyanates (ITCs), present in cruciferous vegetables, possess potent anticarcinogenic activity. In this dissertation, the bioavailability and pharmacokinetics of ITCs and the interactions of ITCs with the three ATP-binding cassette (ABC) transporters, P-glycoprotein (P-gp), multidrug resistance-associated protein 2 (MRP2) and breast cancer resistance protein (BCRP), were investigated in order to understand the pharmacokinetic characteristics of ITCs and potential food-drug interactions. Phenethyl isothiocyanate (PEITC) is being evaluated in clinical trials as a lung cancer preventive agent. A novel LC/MS/MS assay utilizing ammonia derivatization was developed in order to measure the unchanged PEITC concentrations in biological samples. This assay represents the first reported analytical method with the sensitivity and specificity to determine PEITC in clinical samples. Using this assay, the dose-dependent oral bioavailability and pharmacokinetics of PEITC were characterized in rats. PEITC exhibited high oral bioavailability, low clearance, extensive distribution, and non-linear elimination and distribution at high doses. 14 C-PEITC accumulation studies indicated that PEITC and/or its cellular metabolites were substrates of MRP2, but not P-gp. Intracellular glutathione (GSH) played an important role in both intracellular accumulation and transcellular transport of PEITC and/or cellular metabolites mediated by MRP2. Several ITCs were demonstrated to be BCRP inhibitors as they increased both the intracellular accumulation and the cytoxocity of mitoxantrone, a BCRP substrate, in BCRP-overexpressing cells, with little or no effect in BCRP-negative cells. The underlying mechanisms may include the inhibition of BCRP ATPase by ITCs and transport of ITCs by BCRP, since PEITC was transported by BCRP in a saturable and ATP-dependent manner. In vivo studies in rats, with the BCRP substrate topotecan, showed that benzyl ITC (BITC) increased the mean oral bioavailability of topotecan and increased its elimination in a dose-dependent manner; PEITC, however, failed to alter the oral bioavailability or pharmacokinetics of topotecan. In conclusion, this dissertation characterized the pharmacokinetics of PEITC and demonstrated that ITCs are inhibitors and substrates of MRP2 and BCRP. This information may help develop ITCs as chemopreventive agents and understand the membrane transport of ITCs and potential food-drug interactions.