Which human metabolites have we MIST? Retrospective analysis, practical aspects, and perspectives for metabolite identification and quantification in pharmaceutical development

Fluorine-proton correlation from isolated trifluoromethyl groups using unresolved J-couplings

Fluorine-containing compounds are rare in biological systems, so fluorine NMR spectroscopy can selectively detect and quantify fluorinated xenobiotics in crude biological extracts. The high sensitivity of fluorine NMR allows the detection of compounds containing isolated trifluoromethyl groups at nanogramme levels. However, it only provides limited structural information about trifluoromethyl-containing compounds owing to the difficulty of interpreting fluorine chemical shifts and the low sensitivity of HOESY experiments used to correlate fluorine nuclei with protons in the same compound. This paper demonstrates that long-range fluorine-proton J-couplings can be used to correlate isolated trifluoromethyl groups with nearby protons with significantly higher sensitivity than HOESY. Fluorine-observe fluorine-proton HMQC can even give correlations when the fluorine-proton J-couplings are less than the observed fluorine resonance linewidth, so it provides a useful alternative source of structural information about fluorinated xenobiotics.

The early estimation of circulating drug metabolites in humans

INTRODUCTION

An evolution in bioanalytical methodologies to identify and quantify drug metabolites has led to a wealth of biotransformation information during preclinical and early clinical testing phases. However, this abundance of metabolism data has not clarified how to select the most important circulating human metabolites for safety assessment. Consequently, more stringent regulatory expectations for a comprehensive approach to human metabolism have led pharmaceutical sponsors to employ a variety of novel methods to estimate circulating drug metabolites in humans and animals.

AREAS COVERED

This review provides context for 'why' human circulating metabolites must be qualified for safety in animals. A detailed overview is also presented concerning 'where,' 'how' and 'when' to conduct these assessments during drug development.

EXPERT OPINION

A human metabolite qualification strategy is now a required element of the drug safety package submitted with a new drug application (NDA). The important question is whether or not this additional information, about metabolite safety, is making human drugs any safer. Currently, this is a debatable issue, especially because stand-alone metabolite testing is fraught with its own challenges. As drug development moves into the twenty-first century, there is a pressing need for more sophisticated methodologies to address human drug and metabolite safety.

Coupling of UHPLC with fast fraction collection–microplate scintillation counting and MS for radiolabeled metabolite profiling

Background: To further expand the use of fraction collection (FC)–microplate scintillation counting (MSC) in detecting trace amount of radioactivity in absorption, distribution, metabolism and excretion (ADME) studies and improve the resolution of UHPLC–FC–MSC, we report the coupling of UHPLC with MS and faster FC (1.2 s/fraction) followed by MSC using 384-deep-well LumaPlate™ (PerkinElmer, MA, USA) for profiling of radiolabeled metabolites in plasma, urine, bile and feces. Results: Collection of 1.2 s/well clearly improved the resolution of the reconstructed radiochromatograms and, at the same time, provided sufficient detection sensitivity that allowed for more accurate integration of peaks, which is required for radiolabeled ADME studies. The introduction of a reversed gradient as a make-up solvent mixture ensured more uniform drops collected in each well, with resolution maintained throughout the UHPLC run. Less sample injection and more frequent FC resulted in less quenching by matrix and accurate integration of peak. Conclusion: UHPLC–FC–MSC–MS is suitable for metabolite profiling in ADME studies and offers higher resolution, higher sensitivity, shorter LC running time, reduced matrix effect and more environmentally friendly experiments compared with conventional online flow scintillation analysis.

Assessment of exposure of metabolites in preclinical species and humans at steady state from the single-dose radiolabeled absorption, distribution, metabolism, and excretion studies: a case study

The exposure of a drug candidate and its metabolites in humans and preclinical species during drug development needs to be determined to ensure that the safety of drug-related components in humans is adequately assessed in the standard toxicology studies. The in vivo radiolabeled studies in preclinical species and human volunteers provide the total fate of the drug-derived radioactivity including the relative abundance of metabolites. Here, we describe how the single-dose radiolabeled human studies could provide the exposure of circulating metabolites at steady state using a case study of an extensively metabolized drug, lixivaptan. After an oral dose of [(14)C]lixivaptan to humans, a total of nine metabolites were detected in the systemic circulation; eight of them exceeded 10% of the parent exposure (2-41% of total radioactivity). The plasma samples were profiled for all subjects at each time point by high-performance liquid chromatography, and metabolites were quantified using a radioactive detector. On the basis of single-dose area under the concentration-time curve (AUC) values, exposure of six human metabolites was greater at least in one preclinical species used in toxicology evaluation. On the basis of the t(1/2) of lixivaptan and two major metabolites from a single dose in humans, their AUC and C(max) values were simulated at the steady state. The simulated exposure (C(max) and AUC) values of parent drug and the two most abundant metabolites were similar to those from a 7-day clinical study obtained using a validated liquid chromatography-mass spectrometry assay, suggesting that a well designed single-dose radiolabeled human study can help in addressing the metabolites in safety testing-related issues.

Integrated quantitative and qualitative workflow for in vivo bioanalytical support in drug discovery using hybrid Q-TOF-MS

Background: UHPLC coupled with orthogonal acceleration hybrid quadrupole-TOF (Q-TOF)-MS is an emerging technique offering new strategies for the efficient screening of new chemical entities and related molecules at the early discovery stage within the pharmaceutical industry. In the first part of this article, we examine the main instrumental parameters that are critical for the integration of UHPLC–Q-TOF technology to existing bioanalytical workflows, in order to provide simultaneous quantitative and qualitative bioanalysis of samples generated following in vivo studies. Material & Methods: Three modern Q-TOF mass spectrometers, including Bruker maXis™, Agilent 6540 and Sciex TripleTOF™ 5600, all interfaced with UHPLC systems, are evaluated in the second part of the article. The scope of this work is to demonstrate the potential of Q-TOF for the analysis of typical small molecules, therapeutic peptides (molecular weight <6000 Da), and enzymatically (i.e., trypsin, chymotrypsin and pepsin) cleaved peptides from larger proteins. Results & Discussion: This work focuses mainly on full-scan TOF data obtained under ESI conditions, the major mode of TOF operation in discovery bioanalytical research, where the compounds are selected based on their pharmacokinetic/pharmacodynamic behaviors using animal models prior to selecting a few desirable candidates for further development. Finally, important emerging TOF technologies that could potentially benefit bioanalytical research in the semi-quantification of metabolites without synthesized standards are discussed. Particularly, the utility of captive spray ionization coupled with TripleTOF 5600 was evaluated for improving sensitivity and providing normalized MS response for drugs and their metabolites. The workflow proposed compromises neither the efficiency, nor the quality of pharmacokinetic data in support of early drug discovery programs.

Meeting the MIST regulations: human metabolism in Phase I using AMS and a tiered bioanalytical approach

The metabolites in safety testing and ICH-M3 guidance documents emphasize the importance of metabolites when considering safety aspects for new drugs. Both guidances state that relevant metabolites should have safety coverage in humans (although the guidelines have different definitions of relevant metabolites). Not having safety coverage for important metabolites in humans may cause significant delay in the overall pharmaceutical development program. This article discusses the regulatory background regarding safety and metabolites, as well as outlines an integrated strategy taken by one pharmaceutical company, Lundbeck A/S. Lundbeck uses metabolite exposure data from first-in-man studies, obtained using an accelerator MS approach followed by a two-tiered bioanalytical investigation. This enables early availability of key data on this aspect and, overall, represents a powerful risk mitigation strategy.

Use of relative 12C/14C isotope ratios to estimate metabolite concentrations in the absence of authentic standards

Background: There is considerable interest in the determination of relative abundances of human metabolites in plasma (and potentially excreta) with reasonable accuracy early on in the drug development process in order to make scientifically sound decisions with regard to the presence of potentially active or toxic disproportionate metabolites. At this point, authentic metabolite standards are generally not available. Results: A new methodology is proposed for the estimation of metabolite concentrations in the absence of authentic standards. A reference sample containing radiolabeled metabolites of interest is produced by incubating the 14C-labeled drug in vitro, and mixed with a sample to be quantitated containing the unlabeled metabolites. The 12C/14C isotope ratio is measured with high-resolution ESI–MS for each metabolite, and used as a basis for quantitation of the cold metabolite based on the concentration of radioactive metabolite, determined from independent analysis of the radioactive sample with LC-radiochemical detection. The 14C-labeled metabolite serves as an isotopically labeled internal standard, which corrects for any variations in injection volume, sample preparation, MS intensity drift, matrix effects and/or saturation of electrospray ionization. The approach was validated by the analysis of solutions containing variable amounts of the analyte with a fixed amount of radioactive standard on a QToF Synapt® G2 MS system. The same methodology was also successfully applied to first-in-human plasma samples analyzed on a LTQ-Orbitrap®. Conclusion: The metabolite abundances obtained by 12C/14C isotope ratio measurements showed suitable accuracy and precision and were very close to those obtained with matrix mixing. The parent drug concentrations also corresponded well with the bioanalytical results obtained with a validated LC–MS/MS method.

Are multimetabolite methods fit-for-purpose for supporting toxicology studies?

The measurement of parent drug in biological samples is an integral part of the drug-development process from discovery support through to late phase clinical development. At some point during this process it may also be necessary to measure metabolites of the drug. The regulatory guidelines on metabolite safety testing outline metabolite exposure data that is required to support drug registration. There are also a number of publications that describe strategies for validating bioanalytical methods used to quantify metabolites based on the development status of the drug. Despite current regulatory and scientific thinking on this subject, there still seems to be a consensus in parts of the industry that drug metabolites should be measured whenever possible, provided that it is technically feasible to do so, rather than basing this decision on the development requirements of the drug. One consequence of this strategy is that often several metabolites are quantified when supporting early development studies (e.g., regulatory toxicology studies) using bioanalytical methods that have been fully validated to measure all the metabolites. This approach may be regarded as a questionable use of resources at a time when a more targeted approach to drug-development is probably a better option.

Prediction of relative in vivo metabolite exposure from in vitro data using two model drugs: dextromethorphan and omeprazole

Metabolites can have pharmacological or toxicological effects, inhibit metabolic enzymes, and be used as probes of drug-drug interactions or specific cytochrome P450 (P450) phenotypes. Thus, better understanding and prediction methods are needed to characterize metabolite exposures in vivo. This study aimed to test whether in vitro data could be used to predict and rationalize in vivo metabolite exposures using two model drugs and P450 probes: dextromethorphan and omeprazole with their primary metabolites dextrorphan, 5-hydroxyomeprazole (5OH-omeprazole), and omeprazole sulfone. Relative metabolite exposures were predicted using metabolite formation and elimination clearances. For dextrorphan, the formation clearances of dextrorphan glucuronide and 3-hydroxymorphinan from dextrorphan in human liver microsomes were used to predict metabolite (dextrorphan) clearance. For 5OH-omeprazole and omeprazole sulfone, the depletion rates of the metabolites in human hepatocytes were used to predict metabolite clearance. Dextrorphan/dextromethorphan in vivo metabolite/parent area under the plasma concentration versus time curve ratio (AUC(m)/AUC(p)) was overpredicted by 2.1-fold, whereas 5OH-omeprazole/omeprazole and omeprazole sulfone/omeprazole were predicted within 0.75- and 1.1-fold, respectively. The effect of inhibition or induction of the metabolite's formation and elimination on the AUC(m)/AUC(p) ratio was simulated. The simulations showed that unless metabolite clearance pathways are characterized, interpretation of the metabolic ratios is exceedingly difficult. This study shows that relative in vivo metabolite exposure can be predicted from in vitro data and characterization of secondary metabolism of probe metabolites is critical for interpretation of phenotypic data.

Humanized mice with ectopic artificial liver tissues

"Humanized" mice offer a window into aspects of human physiology that are otherwise inaccessible. The best available methods for liver humanization rely on cell transplantation into immunodeficient mice with liver injury but these methods have not gained widespread use due to the duration and variability of hepatocyte repopulation. In light of the significant progress that has been achieved in clinical cell transplantation through tissue engineering, we sought to develop a humanized mouse model based on the facile and ectopic implantation of a tissue-engineered human liver. These human ectopic artificial livers (HEALs) stabilize the function of cryopreserved primary human hepatocytes through juxtacrine and paracrine signals in polymeric scaffolds. In contrast to current methods, HEALs can be efficiently established in immunocompetent mice with normal liver function. Mice transplanted with HEALs exhibit humanized liver functions persistent for weeks, including synthesis of human proteins, human drug metabolism, drug-drug interaction, and drug-induced liver injury. Here, mice with HEALs are used to predict the disproportionate metabolism and toxicity of "major" human metabolites using multiple routes of administration and monitoring. These advances may enable manufacturing of reproducible in vivo models for diverse drug development and research applications.

The effect of CYP3A4 inhibition or induction on the pharmacokinetics and pharmacodynamics of orally administered ruxolitinib (INCB018424 phosphate) in healthy volunteers

Ruxolitinib, a selective Janus kinase (JAK) 1&2 inhibitor in development for the treatment of myeloproliferative neoplasms, is primarily metabolized by CYP3A4. The effects of inhibition or induction of CYP3A4 on single oral dose ruxolitinib pharmacokinetics (PK) and pharmacodynamics (PD) were evaluated in healthy volunteers. Coadministration of ketoconazole (a potent CYP3A4 inhibitor) and erythromycin (a moderate CYP3A4 inhibitor) increased total ruxolitinib plasma exposure (AUC(0-∞)) by 91% and 27%, respectively, and ruxolitinib PD, as measured by the inhibition of interleukin (IL)-6-stimulated STAT3 phosphorylation in whole blood, was generally consistent with the PK observed. Pretreatment with rifampin, a potent CYP3A4 inducer, decreased ruxolitinib AUC(0-∞) by 71% while resulting in only a 10% decrease in the overall PD activity. This apparent PK/PD discrepancy may be explained, in part, by an increase in the relative abundance of ruxolitinib active metabolites with the rifampin coadministration. The collective PK/PD data suggest that starting doses of ruxolitinib should be reduced by 50% if coadministered with a potent CYP3A4 inhibitor, whereas adjustments in ruxolitinib starting doses may not be needed when coadministered with inducers or mild/moderate inhibitors of CYP3A4. All study doses of ruxolitinib were generally safe and well tolerated when given alone and in combination with ketoconazole, erythromycin, or rifampin.

Analysis of polar metabolites by hydrophilic interaction chromatography–MS/MS

Increasing emphasis has been placed on quantitative characterization of drug metabolites during drug discovery and development. Due to the more polar nature of drug metabolites, quantitative analysis using traditional reversed-phase liquid chromatography tandem mass spectrometry (RPLC–MS/MS) can be quite challenging. As an alternative chromatographic mode, hydrophilic interaction chromatography (HILIC) offers unique advantages for analysis of polar metabolites, providing better retention/separation, higher sensitivity, higher efficiency and potential for ultra-fast analysis to improve throughput. In this article, selected case studies from the authors’ own laboratory, and examples from current literature, will be discussed to demonstrate some practical considerations for method development of HILIC–MS/MS assays. The effectiveness of using HILIC–MS/MS for mitigating analytical challenges associated with quantitation of polar metabolites, including phase I and II metabolites of drugs, as well as endogenous metabolites, will be exhibited.

Characterization of diverse natural variants of CYP102A1 found within a species of Bacillus megaterium

An extreme diversity of substrates and catalytic reactions of cytochrome P450 (P450) enzymes is considered to be the consequence of evolutionary adaptation driven by different metabolic or environmental demands. Here we report the presence of numerous natural variants of P450 BM3 (CYP102A1) within a species of Bacillus megaterium. Extensive amino acid substitutions (up to 5% of the total 1049 amino acid residues) were identified from the variants. Phylogenetic analyses suggest that this P450 gene evolve more rapidly than the rRNA gene locus. It was found that key catalytic residues in the substrate channel and active site are retained. Although there were no apparent variations in hydroxylation activity towards myristic acid (C₁₄) and palmitic acid (C₁₆), the hydroxylation rates of lauric acid (C₁₂) by the variants varied in the range of >25-fold. Interestingly, catalytic activities of the variants are promiscuous towards non-natural substrates including human P450 substrates. It can be suggested that CYP102A1 variants can acquire new catalytic activities through site-specific mutations distal to the active site.

Best practices in a tiered approach to metabolite quantification: views and recommendations of the European Bioanalysis Forum

The relationship between the exposure to drug metabolites and overall drug safety has become an integral part of the drug-development process. In-depth discussions in the scientific community, as well as recent guidelines on Drug Safety Testing of Metabolites from the US FDA (often referred to as the MIST guidance and ICH M3(R2) from the International Conference on Harmonization (ICH), has brought clarity to the regulatory requirements of the sponsor company in providing documentation on circulating levels of qualifying metabolites. However, less attention has been given to the challenges now faced by the bioanalytical community in supporting these new guidance policies. In this paper, the European Bioanalysis Forum (EBF) is providing a recommendation on which quality standards to apply when assessing the (relative) abundance or absolute concentrations of metabolites. This paper is the result of both an intensive consultation within the EBF (through internal surveys amongst EBF member companies and discussions) and consultation of the broader bioanalytical community (through discussions at international conferences). These recommendations will provide an increased understanding of how to apply a tiered approach to metabolite quantification as part of the bioanalytical strategy. As such, it aims to provide support to the bioanalytical community on the appropriate level of validation required at each stage of the drug-development process.

NMR spectroscopy as a tool to close the gap on metabolite characterization under MIST

Withdrawals from the market due to unforeseen adverse events have triggered changes in the way therapeutics are discovered and developed. This has resulted in an emphasis on truly understanding the efficacy and toxicity profile of new chemical entities (NCE) and the contributions of their metabolites to on-target pharmacology and off-target receptor-mediated toxicology. Members of the pharmaceutical industry, scientific community and regulatory agencies have held dialogues with respect to metabolites in safety testing (MIST); and both the US FDA and International Conference on Harmonisation have issued guidances with respect to when and how to characterize metabolites for human safety testing. This review provides a brief overview of NMR spectroscopy as applied to the structure elucidation and quantification of drug metabolites within the drug discovery and development process. It covers advances in this technique, including cryogenic cooling of detection circuitry for enhanced sensitivity, hyphenated LC-NMR techniques, improved dynamic range through new solvent-suppression pulse sequences and quantitation. These applications add to the already diverse NMR toolkit and further anchor NMR as a technique that is directly applicable to meeting the requirements of MIST guidelines.

Acyl glucuronides: the good, the bad and the ugly

Acyl glucuronidation is the major metabolic conjugation reaction of most carboxylic acid drugs in mammals. The physiological consequences of this biotransformation have been investigated incompletely but include effects on drug metabolism, protein binding, distribution and clearance that impact upon pharmacological and toxicological outcomes. In marked contrast, the exceptional but widely disparate chemical reactivity of acyl glucuronides has attracted far greater attention. Specifically, the complex transacylation and glycation reactions with proteins have provoked much inconclusive debate over the safety of drugs metabolised to acyl glucuronides. It has been hypothesised that these covalent modifications could initiate idiosyncratic adverse drug reactions. However, despite a large body of in vitro data on the reactions of acyl glucuronides with protein, evidence for adduct formation from acyl glucuronides in vivo is limited and potentially ambiguous. The causal connection of protein adduction to adverse drug reactions remains uncertain. This review has assessed the intrinsic reactivity, metabolic stability and pharmacokinetic properties of acyl glucuronides in the context of physiological, pharmacological and toxicological perspectives. Although numerous experiments have characterised the reactions of acyl glucuronides with proteins, these might be attenuated substantially in vivo by rapid clearance of the conjugates. Consequently, to delineate a relationship between acyl glucuronide formation and toxicological phenomena, detailed pharmacokinetic analysis of systemic exposure to the acyl glucuronide should be undertaken adjacent to determining protein adduct concentrations in vivo. Further investigation is required to ascertain whether acyl glucuronide clearance is sufficient to prevent covalent modification of endogenous proteins and consequentially a potential immunological response.

Rationalization and prediction of in vivo metabolite exposures: the role of metabolite kinetics, clearance predictions and in vitro parameters

IMPORTANCE OF THE FIELD

Due to growing concerns over toxic or active metabolites, significant efforts have been focused on qualitative identification of potential in vivo metabolites from in vitro data. However, limited tools are available to quantitatively predict their human exposures.

AREAS COVERED IN THIS REVIEW

Theory of clearance predictions and metabolite kinetics is reviewed together with supporting experimental data. In vitro and in vivo data of known circulating metabolites and their parent drugs were collected and the predictions of in vivo exposures of the metabolites were evaluated.

WHAT THE READER WILL GAIN

The theory and data reviewed will be useful in early identification of human metabolites that will circulate at significant levels in vivo and help in designing in vivo studies that focus on characterization of metabolites. It will also assist in rationalization of metabolite-to-parent ratios used as markers of specific enzyme activity.

TAKE HOME MESSAGE

The relative importance of a metabolite in comparison to the parent compound as well as other metabolites in vivo can only be predicted using the metabolite's in vitro formation and elimination clearances, and the in vivo disposition of a metabolite can only be rationalized when the elimination pathways of that metabolite are known.

Approaches for the rapid identification of drug metabolites in early clinical studies

Understanding the metabolism of a novel drug candidate in drug discovery and drug development is as important today as it was 30 years ago. What has changed in this period is the technology available for proficient metabolite characterization from complex biological sources. High-efficiency chromatography, sensitive MS and information-rich NMR spectroscopy are approaches that are now commonplace in the modern laboratory. These advancements in analytical technology have led to unequivocal metabolite identification often being performed at the earliest opportunity, following the first dose to man. For this reason an alternative approach is to shift from predicting and extrapolating possible human metabolism from in silico and nonclinical sources, to actual characterization at steady state within early clinical trials. This review provides an overview of modern approaches for characterizing drug metabolites in these early clinical studies. Since much of this progress has come from technology development over the years, the review is concluded with a forward-looking perspective on how this progression may continue into the next decade.

Assessment of chimeric mice with humanized liver as a tool for predicting circulating human metabolites

The ability to predict circulating human metabolites of a candidate drug before first-in-man studies are carried out would provide a clear advantage in drug development. A recent report demonstrated that while in vitro studies using human liver preparations reliably predict primary human metabolites in plasma, the predictability of secondary metabolites, formed by multiple reactions, was low, with total success rates of < or =65%. Here, we assess the use of chimeric mice with humanized liver as an animal model for the prediction of human metabolism in vivo. Metabolism studies with debrisoquine and (S)-warfarin demonstrated significantly higher concentrations of their primary human abundant metabolites in serum or plasma in chimeric mice than in control mice. Humanized chimeric mice were also capable of producing human-specific metabolites of several in-house compounds which were generated through more than one metabolism reaction. This model is closer to in vivo human physiology and therefore appears to have an advantage over in vitro systems in predicting complex metabolites in human plasma. However, prediction of human metabolites failed for other compounds which were highly metabolized in mice. Although requiring careful consideration of compound suitability, this model represents a potential tool for predicting human metabolites in combination with conventional in vitro systems.

Looking back through the MIST: a perspective of evolving strategies and key focus areas for metabolite safety analysis

The publication of the US FDA MIST guidance document in 2008 reignited the debate around the most appropriate strategies to underwrite metabolite safety for novel compounds. Whilst some organizations have suggested that the guidelines necessitate a paradigm shift to more thorough metabolite analysis during early development, an evaluation of historical practices shows that the principles of the guidelines have always largely underpinned metabolism studies within the pharmaceutical industry. Therefore, it is argued that existing practices, when coupled to appropriate emerging analytical tools and a case-by-case consideration of the relevance of the generated metabolism data in terms of structure, physicochemisty, abundance and activity, represent a fit-for-purpose approach to metabolite-safety assessments.

Addressing metabolite safety during first-in-man studies using 14C-labeled drug and accelerator mass spectrometry

Active drug metabolites formed in humans but present in relatively low abundance in preclinical species can lead to unpredicted adverse effects during clinical use. The regulatory guidelines in recent years have therefore required that the metabolism of a drug be quantitatively compared between preclinical species and human at the earliest practicable stage of drug development. Amongst the variety of methods available, inclusion of low radioactive doses of 14C drug in first-in-man studies coupled to the sensitive analytical technology of accelerator MS (AMS) has found utility. Measurement of 14C by AMS allows for quantification of metabolites, even if their structures are unknown, and, when used in conjunction with LC–MS, can provide both quantitative and structural data. This review examines a typical approach to using AMS and associated analytical methods in addressing the regulatory guidelines and discusses a number of possible scenarios including the question of steady state.

Overview of metabolite safety testing from an industry perspective

Regulatory guidelines on MIST were initially established in 2005 and finalized in 2008 by the US FDA and this has led to much discussion and debate on how to apply these recommendations in today’s resource-constrained pharmaceutical environment. There are four aspects of MIST that impact on the field of bioanalysis: definition of a disproportionate human metabolite, establishment of nonclinical (animal) safety coverage for important human metabolites, degree of rigor in validation of bioanalytical methods to quantify metabolites when synthetic standards are available, and semiquantitation of metabolites when synthetic standards are not available. In this manuscript, each of these points has been addressed from a pharmaceutical industry standpoint, including a perspective on the necessary convergence of the fields of metabolite safety testing and bioanalysis.

A radiocalibration method with pseudo internal standard to estimate circulating metabolite concentrations

Background: It has become important for metabolism scientists to identify and quantify prominent circulating human metabolites in order to develop a metabolite safety-qualification package that meets regulatory standards. Often these metabolites cannot be analyzed using traditional bioanalytical methods because a standard is not available. Results: A radiocalibration method is described that can estimate circulating metabolite concentrations in nonradioactive human and animal plasma. The key to this method is application of a pseudo internal standard (PIS) that is present in both radioactive reference and nonradioactive (i.e., unknown) samples. Metabolite exposure in the unknown samples is estimated from measured PIS exposure using a relative molar ratio established between the metabolite and PIS (usually parent drug). Conclusion: Two case studies demonstrate that the method can be used to establish human metabolite safety coverage in animal plasma and method validation is demonstrated by comparing estimated metabolite concentrations in human plasma with concentrations obtained directly using a metabolite calibration curve.

Utility of multivariate analysis in support of in vitro metabolite identification studies: retrospective analysis using the antidepressant drug nefazodone

The utility of multivariate analysis in in vitro metabolite identification studies was examined with nefazodone, an antidepressant drug with a well-established metabolic profile. The chromatographic conditions were purposefully chosen to reflect those utilized in high-throughput screening for microsomal stability of new chemical entities. Molecular ion, retention time information on groups of human liver microsomal samples with/without nefazodone was evaluated by principal component analysis (PCA). Resultant scores and loadings plots from the PCA revealed the segregation and the ions of interest that designated the drug and its corresponding metabolites. Subsequent acquisition of tandem mass spectrometry (MS/MS) spectra for targeted ions permitted the interrogation and interpretation of spectra to identify nefazodone and its metabolites. A comparison of nefazodone metabolites identified by PCA versus those found by traditional metabolite identification approaches resulted in very good correlation when utilizing similar analytical methods. Fifteen metabolites of nefazodone were identified in beta-nicotinamide adenine dinucleotide phosphate (NADPH)-supplemented human liver microsomal incubations, representing nearly all primary metabolites previously reported. Of the 15 metabolites, eight were derived from the N-dealkylation and N-dephenylation of the N-substituted 3-chlorophenylpiperazine motif in nefazodone, six were derived from mono- and bis-hydroxylation, and one was derived from the Baeyer Villiger oxidation of the ethyltriazolone moiety in nefazodone.

Comparison of triple quadrupole, hybrid linear ion trap triple quadrupole, time-of-flight and LTQ-Orbitrap mass spectrometers in drug discovery phase metabolite screening and identification in vitro--amitriptyline and verapamil as model compounds

Liquid chromatography in combination with mass spectrometry (LC/MS) is a superior analytical technique for metabolite profiling and identification studies performed in drug discovery and development laboratories. In the early phase of drug discovery the analytical approach should be both time- and cost-effective, thus providing as much data as possible with only one visit to the laboratory, without the need for further experiments. Recent developments in mass spectrometers have created a situation where many different mass spectrometers are available for the task, each with their specific strengths and drawbacks. We compared the metabolite screening properties of four main types of mass spectrometers used in analytical laboratories, considering both the ability to detect the metabolites and provide structural information, as well as the issues related to time consumption in laboratory and thereafter in data processing. Human liver microsomal incubations with amitriptyline and verapamil were used as test samples, and early-phase 'one lab visit only' approaches were used with all instruments. In total, 28 amitriptyline and 69 verapamil metabolites were found and tentatively identified. Time-of-flight mass spectrometry (TOFMS) was the only approach detecting all of them, shown to be the most suitable instrument for elucidating as comprehensive metabolite profile as possible leading also to lowest overall time consumption together with the LTQ-Orbitrap approach. The latter however suffered from lower detection sensitivity and false negatives, and due to slow data acquisition rate required slower chromatography. Approaches with triple quadrupole mass spectrometry (QqQ) and hybrid linear ion trap triple quadrupole mass spectrometry (Q-Trap) provided the highest amount of fragment ion data for structural elucidation, but, in addition to being unable to produce very high-important accurate mass data, they suffered from many false negatives, and especially with the QqQ, from very high overall time consumption.

Early human ADME using microdoses and microtracers: bioanalytical considerations

Quantitative assessment of metabolites of drug candidates in early-phase clinical development presents an analytical challenge when methods, standards and assays are not yet available. Radioisotopic labeling, principally with radiocarbon (14C), is the preferred method for discovering and quantifying the absolute yields of metabolites in the absence of reference material or a priori knowledge of the human metabolism. However, the detection of 14C is inefficient by decay counting methods and, as a result, high radiological human 14C-doses had been needed to assure sensitive detection of metabolites over time. High radiological doses and the associated costs have been a major obstacle to the routine (and early) use of 14C despite the recognized advantages of a 14C-tracer for quantifying drug metabolism and disposition. Accelerator mass spectrometry eliminates this long-standing problem by reducing radioactivity levels while delivering matrix-independent quantitation to attomole levels of sensitivity in small samples or fractionated isolates. Accelerator mass spectrometry and trace 14C-labeled drugs are now used to obtain early insights into the human metabolism of a drug candidate in ways that were not previously practical. With this article we describe some of our empirically based approaches for regualted bioanalysis and offer perspectives on current applications and opportunities for the future.

A holistic strategy for characterizing the safety of metabolites through drug discovery and development

The subject of metabolites in safety testing has had much debate in the recent past and has shown itself to be a complex issue with no simple solutions to providing absolute assurance of drug safety. Much of the attention has focused on the ability to identify metabolites and then demonstrate that their risk has been adequately characterized, either through their exposure in toxicology species or, failing this, by direct safety testing. In this review, we summarize our forward operational strategy that combines the principles summarized in the FDA Guidance, together with discussions at scientific meetings and literature opinions. It is a balance between the primary goal of assuring patient safety with one of reasonable investment. A key principle in striking this balance is to build stepwise information on metabolites through the drug discovery and development continuum. This allows assessments to be made from early nonclinical studies onward as to whether or not metabolite safety is underwritten by exposure in toxicology species. This strategy does not require absolute quantitation of the metabolites in early clinical trials but relies upon comparison of relative exposures between animals and humans using the capabilities of modern analytical techniques. Through this strategy, human disproportionate metabolites can be identified to allow a decision regarding the need for absolute quantitation and direct safety testing of the metabolite. Definitive radiolabeled studies would be initiated following proof of pharmacology or efficacy in humans, and nonclinical safety coverage would be adequately assessed prior to large-scale clinical trials. In cases where metabolite safety is not supported through the parent compound toxicology program, approaches for the direct safety testing of metabolites with regard to general and reproductive toxicology, safety pharmacology, and genetic safety have been defined.

A microfluidic hepatic coculture platform for cell-based drug metabolism studies

Within the global pharmaceutical and biotech industries, there is significant interest in identifying in vitro screening systems that are more human-relevant-i.e., that offer greater utility in predicting subcellular and cellular physiological responses in humans in vivo-and that thereby allow investigators to reduce the incidence of costly late-stage failures during pharmaceutical clinical trials, as well as to reduce the use of animals in drug testing. Currently incumbent in vitro screening methods, such as culturing human hepatocytes in suspension, while useful, are limited by a lack of long term cellular function. In order to address this limitation, we have established an integrated, microfluidic, in vitro platform that combines the patented HmuREL((R)) microdevice with a hepatic coculture system. In the present report, we use this platform to study clearance and metabolite generation of a battery of molecular entities. The results show that the flow-based coculture system is capable of clearing, with improved resolution and predictive value, compounds with high, medium, and low clearance values. In addition, when coculture is coupled with flow, higher metabolite production rates are obtained than in static systems.

Human radiolabeled mass balance studies: objectives, utilities and limitations

The determination of metabolic pathways of a drug candidate through the identification of circulating and excreted metabolites is vitally important to understanding its physical and biological effects. Knowledge of metabolite profiles of a drug candidate in animals and humans is essential to ensure that animal species used in toxicological evaluations of new drug candidates are appropriate models of humans. The recent FDA final guidance recommends that human oxidative metabolites whose exposure exceeds 10% of the parent AUC at steady-state should be assessed in at least one of the preclinical animal species used in toxicological assessment. Additional toxicological testing on metabolites that have higher exposure in humans than in preclinical species may be required. The metabolite profiles in laboratory animals and humans are generally accomplished by mass balance and excretion studies in which radiolabeled drugs are administered to these species. The biological fluids are collected, analysed for total radioactivity and evaluated for a quantitative profile of metabolites. Thus, these studies not only determine the rates and routes of excretion but also provide very critical information on the metabolic pathways of drugs in preclinical species and humans. In addition, these studies are required by regulatory agencies for the new drug approval process. Despite the usefulness of these radiolabeled mass balance studies, there is little concrete guidance on how to perform or assess these complex studies. This article examines the objectives, utilities and limitations of these studies and how these studies could be used for the determination of the metabolite exposure in animals and humans.

Mass defect filter technique and its applications to drug metabolite identification by high-resolution mass spectrometry

Identification of drug metabolites by liquid chromatography/mass spectrometry (LC/MS) involves metabolite detection in biological matrixes and structural characterization based on product ion spectra. Traditionally, metabolite detection is accomplished primarily on the basis of predicted molecular masses or fragmentation patterns of metabolites using triple-quadrupole and ion trap mass spectrometers. Recently, a novel mass defect filter (MDF) technique has been developed, which enables high-resolution mass spectrometers to be utilized for detecting both predicted and unexpected drug metabolites based on narrow, well-defined mass defect ranges for these metabolites. This is a new approach that is completely different from, but complementary to, traditional molecular mass- or MS/MS fragmentation-based LC/MS approaches. This article reviews the mass defect patterns of various classes of drug metabolites and the basic principles of the MDF approach. Examples are given on the applications of the MDF technique to the detection of stable and chemically reactive metabolites in vitro and in vivo. Advantages, limitations, and future applications are also discussed on MDF and its combinations with other data mining techniques for the detection and identification of drug metabolites.