Interspecies Differences in the Metabolism of a Multiester Prodrug by Carboxylesterases

Abstract

The penta-ethyl ester prodrug of the chelating agent diethylene triamine pentaacetic acid (DTPA) referred to as C2E5, is being developed as an orally bioavailable radionuclide decorporation agent. The predicted human efficacy obtained in these experimental animals is confounded by interspecies variations of metabolism. Therefore, in the present study, carboxylesterase-mediated metabolism of [14C]-C2E5 was compared in the S9 intestinal and hepatic fractions of human, dog and rat and their respective plasma. Intestinal hydrolysis of C2E5, resulting in the formation of the tetraethyl ester of DTPA (C2E4), was only detected in human and rat. The primary metabolite in human and dog hepatic fractions was C2E4 whereas the predominant species identified in rat hepatic fractions was the triethyl ester (C2E3). Hepatic hydrolysis of C2E5 causes the formation of C2E4 in human, dog and rat and C2E3 in rat only. Minimal C2E5 hydrolysis was observed in human and dog plasma whereas in rat plasma C2E5 converted to C2E3 rapidly, followed by slower further metabolism. Both recombinant CES1 and CES2 play roles in C2E5 metabolism. Together, these data suggest that dogs may be the most appropriate species for predicting human C2E5 metabolism whereas rats might be useful for clarifying the potential toxicity of C2E5 metabolites.

Biotransformation Capacity of Carboxylesterase in Skin and Keratinocytes for the Penta-Ethyl Ester Prodrug of DTPA

“…However, CES1 hydrolyzed C2E5 to a greater extent compared with CES2 (Fu et al, 2015). The results in the current study agree with our previous findings (Figs.…”

“…The reactions were terminated by adding an equal volume of ice-cold acetonitrile, followed by centrifugation at 14,000g for 15 minutes at 4°C. The supernatant was transferred to high-performance liquid chromatography vials for analysis as previously described (Fu et al, 2015). Representative radio-chromatograms illustrate a C2E5 peak generated from the boiled S9 fractions and C2E5 and the presence of a metabolite, C2E4, in human skin S9 fractions (Supplemental Figs.…”

“…Xenobiotic-metabolizing enzymes are located in the epidermis and dermis, where hair follicles and sebaceous and sweat glands are located (Scheuplein and Blank, 1971;Sugibayashi et al, 1999). The study of dermal metabolism is complicated by significant interspecies differences in xenobiotic metabolism (Inoue et al, 1980;Prusakiewicz et al, 2006;Fu et al, 2015). Therefore, ideally, metabolism should be investigated in human skin tissues.…”

“…1), to enhance clearance (decorporation) of transuranic radionuclides (Zhang et al, 2013b). C2E5 is metabolized by CESs (Fu et al, 2015) and the physiochemical properties of C2E5 (CLogP of 4.7, with a molecular weight of 533 Daltons) suggest that it would be a good candidate for transdermal delivery. Evidence supporting transdermal application of C2E5 was reported in rat in vivo transdermal pharmacokinetics and efficacy studies (Zhang et al, 2013b).…”

Prochelator strategies for site-selective activation of metal chelators

“…The enzymes responsible for the metabolism of 33 to DTPA are the carboxylesterases (CESs), which could sequentially cleave the five ester moieties to form the metabolites and DTPA. 35,36 3 Oxidative stress-activated prochelators…”