ABSTRACT:An improved mass defect filter (MDF) method employing both drug and core structure filter templates was applied to the processing of high resolution liquid chromatography/mass spectrometry (LC/ MS) data for the detection and structural characterization of oxidative metabolites with mass defects similar to or significantly different from those of the parent drugs. The effectiveness of this approach was investigated using nefazodone as a model compound, which is known to undergo multiple common and uncommon oxidative reactions. Through the selective removal of all ions that fall outside of the preset filter windows, the MDF process facilitated the detection of all 14 nefazodone metabolites presented in human liver microsomes in the MDF-filtered chromatograms. The capability of the MDF approach to remove endogenous interferences from more complex biological matrices was examined by analyzing omeprazole metabolites in human plasma. The unprocessed mass chromatogram showed no distinct indication of metabolite peaks; however, after MDF processing, the metabolite peaks were easily identified in the chromatogram. Compared with precursor ion scan and neutral loss scan techniques, the MDF approach was shown to be more effective for the detection of metabolites in a complex matrix. The comprehensive metabolite detection capability of the MDF approach, together with accurate mass determination, makes high resolution LC/MS a useful tool for the screening and identification of both common and uncommon drug metabolites.The identification of drug metabolites, particularly metabolites formed through oxidation, reduction, or hydrolysis reactions, has become an integral part of the drug discovery and development process. These metabolites may have intrinsic pharmacological activity or display specific toxicity (Parkinson, 1996;Guengerich, 2000). In addition, most clinical drug-drug interactions are associated with oxidative biotransformation mediated by cytochrome P450 (Bjornsson et al., 2003). Although analytical sensitivity and the processing of data for liquid chromatography/mass spectrometry (LC/MS) have been tremendously improved in the last decade (Clarke et al., 2001;Kostiainen et al., 2003;Liu and Hop, 2005), the detection and identification of drug metabolites in complex biological matrices continue to be a challenge.Traditionally, detection of common or expected metabolites has been conducted on LC/MS data by generating extracted or reconstructed ion chromatograms corresponding to the expected protonated molecules of drug metabolites (Plumb et al., 2003). Over the last decade, product ion scanning techniques that use rule-based algorithms to generate a list of potential metabolite masses have been developed and continuously improved for rapid screening for common metabolites (Yu et al., 1999;Gangl et al., 2002;Lafaye et al., 2003). The technique employs a survey mode to search for the metabolites that are listed in the acquisition method. Both the detection of expected metabolites and the acquisition of their pr...

Organic anion-transporting polypeptides (OATP) 1B1, 1B3, and 2B1 can serve as the loci of drug-drug interactions (DDIs). In the present work, the cynomolgus monkey was evaluated as a potential model for studying OATP-mediated DDIs. Three cynomolgus monkey OATPs (cOATPs), with a high degree of amino acid sequence identity (91.9, 93.5, and 96.6% for OATP1B1, 1B3, and 2B1, respectively) to their human counterparts, were cloned, expressed, and characterized. The cOATPs were stably transfected in human embryonic kidney cells and were functionally similar to the corresponding human OATPs (hOATPs), as evident from the similar uptake rate of typical substrates (estradiol-17b-D-glucuronide, cholecystokinin octapeptide, and estrone-3-sulfate). Moreover, six known hOATP inhibitors exhibited similar IC 50 values against cOATPs. To further evaluate the appropriateness of the cynomolgus monkey as a model, a known hOATP substrate [rosuvastatin (RSV)]-inhibitor [rifampicin (RIF)] pair was examined in vitro; the monkey-derived parameters (RSV K m and RIF IC 50 ) were similar (within 3.5-fold) to those obtained with hOATPs and human primary hepatocytes. In vivo, the area under the plasma concentration-time curve of RSV (3 mg/kg, oral) given 1 hour after a single RIF dose (15 mg/kg, oral) was increased 2.9-fold in cynomolgus monkeys, consistent with the value (3.0-fold) reported in humans. A number of in vitro-in vivo extrapolation approaches, considering the fraction of the pathways affected and free versus total inhibitor concentrations, were also explored. It is concluded that the cynomolgus monkey has the potential to serve as a useful model for the assessment of OATP-mediated DDIs in a nonclinical setting. IntroductionDrug-drug interactions (DDIs) have often been attributed to cytochrome P450 (P450) enzymes because of their prominent role in the metabolic clearance of drugs (Vuppugalla et al., 2010). More recently, however, attention has turned to active transport processes in different organs and the close interplay between drug transport and metabolism at the cellular level. In particular, organic anion-transporting polypeptides (OATPs) are known to mediate the active uptake of numerous drugs into hepatocytes and hence govern their overall clearance, pharmacokinetic profile, and liver-toplasma ratio (Giacomini et al., 2010;Fenner et al., 2012;Yoshida et al., 2012).OATPs can also serve as the loci of important DDIs leading to changes in systemic and local drug concentrations, possibly resulting in altered efficacy and enhanced toxicity (Giacomini et al., 2010;Yoshida et al., 2012). For example, cyclosporine A (CsA) increases the area under the concentration-time curve (AUC) (∼15-fold) and C max (∼14-fold) of atorvastatin in s This article has supplemental material available at jpet.aspetjournals.org.ABBREVIATIONS: AUC, area under the concentration-time curve; CCK-8, cholecystokinin octapeptide; CI, confidence interval; cOATP, cynomolgus organic anion-transporting polypeptide; CsA, cyclosporine A; DDI, drug-drug inter...