Deuterium Isotope Effects on Drug Pharmacokinetics. I. System- Dependent Effects of Specific Deuteration with Aldehyde Oxidase Cleared Drugs Raman Sharma, Timothy J. Strelevitz, Hongying Gao, Alan J. Clark, Klaas Schildknegt, R. Scott Obach, Sharon L. Ripp, Douglas K. Spracklin, Larry M. Tremaine, and Alfin D. N. Vaz Departments of Pharmacokinetics Dynamics and Metabolism (R.S., T.J.S., H.G., A.J.C., R.S.O., S.L.R., D.K.S., L.M.T., A.D.N.V.) and Pharmaceutical Sciences (K.S.), Pfizer Global Research and Development, Groton, Connecticut Received September 14, 2011; accepted December 15, 2011 ABSTRACT: The pharmacokinetic properties of drugs may be altered by kinetic deuterium isotope effects. With specifically deuterated model sub- strates and drugs metabolized by aldehyde oxidase, we demon- strate how knowledge of the enzyme’s reaction mechanism, spe- cies differences in the role played by other enzymes in a drug’s metabolic clearance, and differences in systemic clearance mech- anisms are critically important for the pharmacokinetic application of deuterium isotope effects. Ex vivo methods to project the in vivo outcome using deuterated carbazeran and zoniporide with hepatic systems demonstrate the importance of establishing the extent to which other metabolic enzymes contribute to the metabolic clear- ance mechanism. Differences in pharmacokinetic outcomes in guinea pig and rat, with the same metabolic clearance mechanism, show how species differences in the systemic clearance mecha- nism can affect the in vivo outcome. Overall, to gain from the application of deuteration as a strategy to alter drug pharmacoki- netics, these studies demonstrate the importance of understand- ing the systemic clearance mechanism and knowing the identity of the metabolic enzymes involved, the extent to which they contrib- ute to metabolic clearance, and the extent to which metabolism contributes to the systemic clearance. Introduction Deuteration of drugs to enhance their pharmacokinetic, pharmaco- dynamic, or toxicological properties has gained momentum as judged by a search of the SciFinder database with the search term “deuterated drugs.” Of 179 registries retrieved, 151 are since 2005 with an exponential growth since 2006. These include deuterated versions of patented and off-patent drugs with claims of increased efficacy, de- creased toxicity, reduced interpatient variability, and decreased drug dose or dosing frequency. Belleau et al. (1961) were among the first to demonstrate the pharmacodynamic effect of deuteration with - dideuterated p-tyramine. The effect was attributed to decreased me- tabolism of p-tyramine by monoamine oxidases. Several reports that have examined the effect of deuteration on the pharmacokinetic and pharmacodynamic properties of drugs reveal results that include little to no effect (Tanabe et al., 1970; Farmer et al., 1979; Taylor et al., 1983; Burm et al., 1988; Dunsaed et al., 1995); increased systemic exposure, a pharmacodynamic effect, and receptor selectivity (Dyck et al., 1988; Schneider et al., 2006, 2007); and decreased toxicity (Najjar et al., 1978). However, in these studies the mechanisms underlying the observed effects or lack thereof were not examined. With the use of formyl-deuterated N-methylformamide, the hepato- toxicity was shown to be due to oxidative metabolism at the formyl carbon (Threadgill et al., 1989). Pohl and Gillette (1984 –1985) out- lined the kinetic basis for use of deuterated compounds to determine toxic metabolic pathways, and, in addition, Nelson and Trager (2003) have reviewed distinctions between “intrinsic KDIEs” and “observed KDIEs” in enzyme reaction mechanisms with particular emphasis on cytochrome P450 reactions. Foster (1984) and Kushner et al. (1999) have also discussed the application of deuterated drugs to drug phar- macokinetics, pharmacodynamics, and toxicity. A KDIE on the intrinsic metabolic clearance (CL int or V max /K m ) is fundamental to the application of a deuteration strategy to alter drug pharmacokinetics. Multiple factors mute the magnitude of this isotope effect. These include substantial contribution to the metabolic clear- ance by conjugating enzymes (UDP-glucuronosyltransferases, sulfo- transferases, and glutathione transferases) and heteroatom oxidizing enzymes (flavin monooxygenases), in which carbon-hydrogen bonds are not broken; aspects of enzyme reaction mechanisms such as “metabolic switching” due to deuterium substitution, particularly im- portant with cytochrome P450 cleared molecules (Miwa and Lu, 1987; Nelson and Trager, 2003); rate-limiting product release from enzymes, which mask intrinsic KDIEs (Ling and Hanzlik, 1989; Hall and Hanzlik, 1990; Bell-Parikh and Guengerich, 1999); and other biological processes such as organ blood flow-limited clearance, renal and/or biliary clearance by passive or active transport involving uptake or efflux pumps and enterohepatic recycling. Consequently, a Article, publication date, and citation information can be found at http://dmd.aspetjournals.org. http://dx.doi.org/10.1124/dmd.111.042770. ABBREVIATIONS: KDIE, kinetic deuterium isotope effect; LC, liquid chromatography; MS, mass spectrometry; QC, quality control; MRM, multiple reaction monitoring; AO, aldehyde oxidase; AUC, area under the curve. 1521-009X/12/4003-625–634$25.00 DRUG METABOLISM AND DISPOSITION Vol. 40, No. 3 Copyright © 2012 by The American Society for Pharmacology and Experimental Therapeutics 42770/3752997 DMD 40:625–634, 2012 625 at ASPET Journals on May 7, 2015 dmd.aspetjournals.org Downloaded from