Metabolite Bioanalysis in Drug Development: Recommendations from the IQ Consortium Metabolite Bioanalysis Working Group

The intent of this perspective is to share the recommendations of the International Consortium for Innovation and Quality in Pharmaceutical Development Metabolite Bioanalysis Working Group on the fit-for-purpose metabolite bioanalysis in support of drug development and registration. This report summarizes the considerations for the trigger, timing, and rigor of bioanalysis in the various assessments to address unique challenges due to metabolites, with respect to efficacy and safety, which may arise during drug development from investigational new drug (IND) enabling studies, and phase I, phase II, and phase III clinical trials to regulatory submission. The recommended approaches ensure that important drug metabolites are identified in a timely manner and properly characterized for efficient drug development.

Considerations for Human ADME Strategy and Design Paradigm Shift(s) - An Industry White Paper

The human absorption, distribution, metabolism, and excretion (hADME) study is the cornerstone of the clinical pharmacology package for small molecule drugs, providing comprehensive information on the rates and routes of disposition and elimination of drug-related material in humans through the use of 14 C-labeled drug. Significant changes have already been made in the design of the hADME study for many companies, but opportunity exists to continue to re-think both the design and timing of the hADME study in light of the potential offered by newer technologies, that enable flexibility in particular to reducing the magnitude of the radioactive dose used. This paper provides considerations on the variety of current strategies that exist across a number of pharmaceutical companies and on some of the ongoing debates around a potential move to the so called "human first/human only" approach, already adopted by at least one company. The paper also provides a framework for continuing the discussion in the application of further shifts in the paradigm.

High-Throughput Metabolic Soft-Spot Identification in Liver Microsomes by LC/UV/MS: Application of a Single Variable Incubation Time Approach

CYP-mediated fast metabolism may lead to poor bioavailability, fast drug clearance and significant drug interaction. Thus, metabolic stability screening in human liver microsomes (HLM) followed by metabolic soft-spot identification (MSSID) is routinely conducted in drug discovery. Liver microsomal incubations of testing compounds with fixed single or multiple incubation time(s) and quantitative and qualitative analysis of metabolites using high-resolution mass spectrometry are routinely employed in MSSID assays. The major objective of this study was to develop and validate a simple, effective, and high-throughput assay for determining metabolic soft-spots of testing compounds in liver microsomes using a single variable incubation time and LC/UV/MS. Model compounds (verapamil, dextromethorphan, buspirone, mirtazapine, saquinavir, midazolam, amodiaquine) were incubated at 3 or 5 µM with HLM for a single variable incubation time between 1 and 60 min based on predetermined metabolic stability data. As a result, disappearances of the parents were around 20-40%, and only one or a few primary metabolites were generated as major metabolite(s) without notable formation of secondary metabolites. The unique metabolite profiles generated from the optimal incubation conditions enabled LC/UV to perform direct quantitative estimation for identifying major metabolites. Consequently, structural characterization by LC/MS focused on one or a few major primary metabolite(s) rather than many metabolites including secondary metabolites. Furthermore, generic data-dependent acquisition methods were utilized to enable Q-TOF and Qtrap to continuously record full MS and MS/MS spectral data of major metabolites for post-acquisition data-mining and interpretation. Results from analyzing metabolic soft-spots of the seven model compounds demonstrated that the novel MSSID assay can substantially simplify metabolic soft-spot identification and is well suited for high-throughput analysis in lead optimization.

Pharmacokinetics and Disposition of Contezolid in Humans-Resolution of a Disproportionate Human Metabolite for Clinical Development

Contezolid (MRX-I), a novel oxazolidinone antibiotic, was recently approved for the treatment of serious Gram-positive infections. The pharmacokinetics and disposition of [¹⁴C]contezolid were investigated in a single-dose human mass balance study. Cross-species comparison of plasma exposure for contezolid and metabolites was performed, and the safety of the disproportionate metabolite in human was evaluated with additional nonclinical studies. After an oral administration of 99.1 μCi/602 mg dose of [¹⁴C]contezolid, approximately 91.5% of the radioactivity was recovered in 0-168 h postdose, mainly in urine and followed by feces. The principal metabolic pathway of contezolid in human comprised an oxidative ring opening of 2,3-dihydropyridin-4-one fragment into polar metabolites MRX445-1 and MRX459, with recovery of approximately 48% and 15% of the dose, respectively, in urine and feces. Contezolid, MRX445-1, and MRX459 accounted for 68.0%, 19.5%, and 4.84% of the plasma exposure of the total radioactivity, respectively. Metabolites MRX445-1 and MRX459 were observed in disproportionately higher amounts in human plasma as compared to that rat or dog, the rodent and nonrodent species used for the general nonclinical safety assessment of this molecule. This discrepancy was resolved with additional nonclinical studies, wherein the primary metabolite, MRX445-1, was further characterized. The no observed adverse effect level (NOAEL) of MRX445-1 was determined as 360 mg/kg/day in 14-day repeat-dose test in pregnant and non-pregnant SD rats. Furthermore, MRX445-1 exhibited no antibacterial activity in vitro. Thus, MRX445-1 is not expected to exert clinically relevant pharmacology and toxicity.

Case Study 4: Application of Basic Enzyme Kinetics to Metabolism Studies-Real-Life Examples

An appreciation of enzyme kinetic principles can be applied in a number of drug metabolism applications. The concept for this chapter arose from a simple discussion on selecting appropriate time points to most efficiently assess metabolite profiles in a human Phase 1a clinical study (Subheading 4). By considering enzyme kinetics, a logical approach to the issue was derived. The dialog was an important learning opportunity for the participants in the discussion, and we have endeavored to capture this experience with other questions related to determination of K_m and V_max parameters, a consideration of the value of hepatocytes vs. liver microsomes, and enzyme inhibition parameters.

An integrated assessment of the ADME properties of the CDK4/6 Inhibitor ribociclib utilizing preclinical in vitro, in vivo, and human ADME data

Ribociclib (LEE011, Kisqali ®) is a highly selective small molecule inhibitor of cyclin-dependent kinases 4 and 6 (CDK4/6), which has been approved for the treatment of advanced or metastatic breast cancer. A human ADME study was conducted in healthy male volunteers following a single oral dose of 600 mg [¹⁴ C]-ribociclib. Mass balance, blood and plasma radioactivity, and plasma ribociclib concentrations were measured. Metabolite profiling and identification was conducted in plasma, urine, and feces. An assessment integrating the human ADME results with relevant in vitro and in vivo non-clinical data was conducted to provide an estimate of the relative contributions of various clearance pathways of the compound. Ribociclib is moderately to highly absorbed across species (approx. 59% in human), and is extensively metabolized in vivo, predominantly by oxidative pathways mediated by CYP3A4 (ultimately forming N-demethylated metabolite M4) and, to a lesser extent, by FMO3 (N-hydroxylated metabolite M13). It is extensively distributed in rats, based on QWBA data, and is eliminated rapidly from most tissues with the exception of melanin-containing structures. Ribociclib passed the placental barrier in rats and rabbits and into milk of lactating rats. In human, 69.1% and 22.6% of the radiolabeled dose were excreted in feces and urine, respectively, with 17.3% and 6.75% of the ¹⁴ C dose attributable to ribociclib, respectively. The remainder was attributed to numerous metabolites. Taking into account all available data, ribociclib is estimated to be eliminated by hepatic metabolism (approx. 84% of total), renal excretion (7%), intestinal excretion (8%), and biliary elimination (1%).

The Application of Mass Spectrometry in Drug Metabolism and Pharmacokinetics

Drug metabolism and pharmacokinetics (DMPK) are fundamental in drug discovery. New chemical entities (NCEs) are typically evaluated with various in vitro and in vivo assays, which are time-consuming and labor intensive. These experiments are essential in identifying potential new drugs. Recently, mass spectrometry (MS) has played a key role in examining the drug-like properties of NCEs. Quantitative and qualitative mass spectrometry approaches are routinely utilized to obtain high-quality data in an efficient, timely, and cost-effective manner. Especially, liquid chromatography (LC) coupled with MS technology has been refined for metabolite identification (Met ID), which is critical for lead optimization. These qualitative and quantitative MS approaches and their specific utility in DMPK characterization will be described in this chapter.

Metabolite Profiling and Characterization of LW6, a Novel HIF-1α Inhibitor, as an Antitumor Drug Candidate in Mice

A novel HIF (hypoxia-inducible factor)-1α inhibitor, the (aryloxyacetylamino)benzoic acid derivative LW6, is an anticancer agent that inhibits the accumulation of HIF-1α. The aim of this study was to characterize and determine the structures of the metabolites of LW6 in ICR mice. Metabolite identification was performed using a predictive multiple reaction monitoring-information dependent acquisition-enhanced product ion (pMRM-IDA-EPI) method in negative ion mode on a hybrid triple quadrupole-linear ion trap mass spectrometer (QTRAP). A total of 12 metabolites were characterized based on their MS/MS spectra, and the retention times were compared with those of the parent compound. The metabolites were divided into five structural classes based on biotransformation reactions: amide hydrolysis, ester hydrolysis, mono-oxidation, glucuronidation, and a combination of these reactions. From this study, 2-(4-((3r,5r,7r)-adamantan-1-yl)phenoxy)acetic acid (APA, M7), the metabolite produced via amide hydrolysis, was found to be a major circulating metabolite of LW6 in mice. The results of this study can be used to improve the pharmacokinetic profile by lowering the clearance and increasing the exposure relative to LW6.

Molecular imaging identifies age-related attenuation of acetylcholine in retrosplenial cortex in response to acetylcholinesterase inhibition

The neurotransmitter of the cholinergic system, acetylcholine plays a major role in the brain's cognitive function and is involved in neurodegenerative disorders. Here, we present age-related alterations of acetylcholine levels after administration of the acetylcholinesterase inhibitor drug tacrine in normal mice. Using a quantitative, robust and molecular-specific mass spectrometry imaging method we found that tacrine administration significantly raised acetylcholine levels in most areas of sectioned mice brains, inter alia the striatum, hippocampus and cortical areas. However, acetylcholine levels in retrosplenial cortex were significantly lower in 14-month-old than in 12-week-old animals following its administration, indicating that normal aging affects the cholinergic system's responsivity. This small brain region is interconnected with an array of brain networks and is involved in numerous cognitive tasks. Simultaneous visualization of distributions of tacrine and its hydroxylated metabolites in the brain revealed a significant decrease in levels of the metabolites in the 14-month-old mice. The results highlight strengths of the imaging technique to simultaneously investigate multiple molecular species and the drug-target effects in specific regions of the brain. The proposed approach has high potential in studies of neuropathological conditions and responses to neuroactive treatments.

Development of a new method for the quantitation of metabolites in the absence of chemically synthetized authentic standards

A new method for the quantification of metabolites in the absence of a chemically synthetized authentic standard is described herein. Metabolites to be used as reference standards were obtained biologically from microsomes incubation. The method is a stepwise process in which, only the radiolabeled (¹⁴C) and non-radiolabeled parent compound are required. Briefly, the separation and principles of equimolar detection of LC-radioactivity were applied and, a calibration curve of the ¹⁴C-parent compound was used to quantify the formation of its ¹⁴C-metabolite. In turn, serial dilutions of this ¹⁴C-metabolite were the base for the calibration curve that allowed the quantification of the non-radiolabeled metabolite. This method was applied in plasma samples obtained from a dog pharmacokinetic study in which, a PharmaMar compound (lurbinectedin) and its N-desmethylated metabolite were quantified and, the results compared to those obtained by the classical approach (with the chemically synthetized N-desmethylated metabolite). Plasma concentrations obtained with the two methods were very similar, with standard relative errors between -11% to -4%. Similar, main pharmacokinetic parameters were calculated with the concentrations obtained either thru this method or by using a chemically synthetized authentic standard.

Metabolites characterization of a novel DPP-4 inhibitor, imigliptin in humans and rats using ultra-high performance liquid chromatography coupled with synapt high-resolution mass spectrometry

Imigliptin has been reported as a novel dipeptidyl-peptidase-IV (DPP-4) inhibitor to treat type 2 Diabetes Mellitus (T2DM), and is currently being tested in clinical trials. In the first human clinical study, imigliptin was well tolerated and proved to be a potent DPP-4 inhibitor. Considering its potential therapeutic benefits and promising future, it is of great importance to study the metabolite profiles in the early stage of drug development. In the present study, a robust and reliable analytical method based on the ultra-high performance liquid chromatography/quadrupole time-of-flight mass spectrometry (UHPLC/Q-TOF MS) method combined with MassLynx software was established to investigate the characterization of metabolites of imigliptin in human and rat plasma, urine and feces after oral administration. As a result, a total of 9 metabolites were identified in humans, including 6, 9 and 8 metabolites in human plasma, urine, and feces, respectively. A total of 11 metabolites were identified in rats, including 7, 10 and 8 metabolites in rat plasma, urine, and feces, respectively. In addition, 6 of the metabolites detected in humans and rats were phase I metabolites, including demethylation, carboxylation, hydroxylation and dehydrogenation metabolites, and 5 of the metabolites were phase II metabolites, including acetylation and glucuronidation. There was no human metabolite detected compared to those in rats. The major metabolites detected in human plasma (M1 and M2) were products resulting from acetylation, and hydroxylation followed by dehydrogenation. M1 was the major metabolite in rat plasma. M2 and the parent drug were the major drug-related substances in human urine. The parent drug was the major drug-related substances in rat urine. M2, M5 (hydroxylation product) and M6 (2 × hydroxylation and acetylation product) were the predominant metabolites in human feces. M2 and M5 were the major metabolites in rat feces. In addition, renal clearance was the major route of excretion for imigliptin.

Pharmacokinetics and Disposition of Momelotinib Revealed a Disproportionate Human Metabolite-Resolution for Clinical Development

Momelotinib (MMB), a small-molecule inhibitor of Janus kinase (JAK)1/2 and of activin A receptor type 1 (ACVR1), is in clinical development for the treatment of myeloproliferative neoplasms. The pharmacokinetics and disposition of [¹⁴C]MMB were characterized in a single-dose, human mass-balance study. Metabolism and the pharmacologic activity of key metabolites were elucidated in multiple in vitro and in vivo experiments. MMB was rapidly absorbed following oral dosing with approximately 97% of the radioactivity recovered, primarily in feces with urine as a secondary route. Mean blood-to-plasma [¹⁴C] area under the plasma concentration-time curve ratio was 0.72, suggesting low association of MMB and metabolites with blood cells. [¹⁴C]MMB-derived radioactivity was detectable in blood for ≤48 hours, suggesting no irreversible binding of MMB or its metabolites. The major circulating human metabolite, M21 (a morpholino lactam), is a potent inhibitor of JAK1/2 and ACVR1 in vitro. Estimation of pharmacological activity index suggests M21 contributes significantly to the pharmacological activity of MMB for the inhibition of both JAK1/2 and ACVR1. M21 was observed in disproportionately higher amounts in human plasma than in rat or dog, the rodent and nonrodent species used for the general nonclinical safety assessment of this molecule. This discrepancy was resolved with additional nonclinical studies wherein the circulating metabolites and drug-drug interactions were further characterized. The human metabolism of MMB was mediated primarily by multiple cytochrome P450 enzymes, whereas M21 formation involved initial P450 oxidation of the morpholine ring followed by metabolism via aldehyde oxidase.

Absolute quantification of imipramine and its metabolites in vivo utilizing calibrators from radiolabeled in vitro incubations

BACKGROUND

We have demonstrated the use of a single-point calibration approach, derived from in vitro metabolite identification studies utilizing radiolabeled imipramine, that allows for the quantitation of metabolites from in vivo studies in the absence of metabolite synthetic standards.

RESULTS

From the in vivo study of imipramine in rats, the concentration of parent and metabolites were determined using the single-point calibration approach. Sixty seven percent of the dosed imipramine was recovered within 24 h, with 95 and 5% of drug-related material detected in feces and urine, respectively.

CONCLUSION

Using a novel single-point calibration approach from radiolabeled in vitro studies, we quantified metabolites in vivo and determined various disposition pathways.

Determination of species-difference in microsomal metabolism of amitriptyline using a predictive MRM-IDA-EPI method

We investigated to compare species differences in amitriptyline (AMI) metabolism among mouse, rat, dog, and human liver microsomes. We developed a method for simultaneous determination of metabolic stability and metabolite profiling using predictive multiple reaction monitoring information-dependent acquisition-enhanced product ion (MRM-IDA-EPI) scanning. In the cofactor-dependent microsomal metabolism study, AMI was metabolized more rapidly in rat and human liver microsomes incubated with NADPH than UDPGA. AMI incubated with NADPH+UDPGA in rat, dog, or mouse liver microsomes disappeared rapidly with a half-life of 3.5, 8.4, or 9.2 min, respectively, but slowly in human liver microsomes with a half-life of 96 min. In total, 9, 10, 11, and 6 putative metabolites of AMI were detected in mouse, rat, dog, and human liver microsomes, respectively, based on mass spectrometric analyses. Kinetic analysis of metabolites in liver microsomes from each species over 120 min showed common metabolic routes of AMI, such as N-demethylation, hydroxylation, and glucuronidation, and subtle interspecies differences in AMI metabolism. The main metabolic routes in mouse, rat, dog, and human liver microsomes were hydroxylation followed by glucuronide conjugation, methyl hydroxylation, and N-demethylation, respectively. The MRM-IDA-EPI method can provide quantitative and qualitative information about metabolic stability and metabolite profiling simultaneously. Moreover, time course analysis of metabolites can not only eliminate false identification of metabolites, but also provide a rationale for proposed metabolic pathways. The MRM-IDA-EPI method combined with time course analysis of metabolites is useful for investigating drug metabolism at the early drug discovery stage.

Comparison of minipig, dog, monkey and human drug metabolism and disposition

INTRODUCTION

This article gives an overview of the drug metabolism and disposition (ADME) characteristics of the most common non-rodent species used in toxicity testing of drugs (minipigs, dogs, and monkeys) and compares these to human characteristics with regard to enzymes mediating the metabolism of drugs and the transport proteins which contribute to the absorption, distribution and excretion of drugs.

METHODS

Literature on ADME and regulatory guidelines of relevance in drug development of small molecules has been gathered.

RESULTS

Non-human primates (monkeys) are the species that is closest to humans in terms of genetic homology. Dogs have an advantage due to the ready availability of comprehensive background data for toxicological safety assessment and dogs are easy to handle. Pigs have been used less than dogs and monkeys as a model in safety assessment of drug candidates. However, when a drug candidate is metabolised by aldehyde oxidase (AOX1), N-acetyltransferases (NAT1 and NAT2) or cytochrome (CYP2C9-like) enzymes which are not expressed in dogs, but are present in pigs, this species may be a better choice than dogs, provided that adequate exposure can be obtained in pigs. Conversely, pigs might not be the right choice if sulfation, involving 3-phospho-adenosyl-5-phosphosulphate sulphotransferase (PAPS) is an important pathway in the human metabolism of a drug candidate.

DISCUSSION

In general, the species selection should be based on comparison between in vitro studies with human cell-based systems and animal-cell-based systems. Results from pharmacokinetic studies are also important for decision-making by establishing the obtainable exposure level in the species. Access to genetically humanized mouse models and highly sensitive analytical methods (accelerator mass spectrometry) makes it possible to improve the chance of finding all metabolites relevant for humans before clinical trials have been initiated and, if necessary, to include another animal species before long term toxicity studies are initiated. In conclusion, safety testing can be optimized by applying knowledge about species ADME differences and utilising advanced analytical techniques.

The debate on animal ADME studies in drug development: an update

The preparation and release of the International Conference on Harmonisation guideline on safety evaluation of human metabolites and the technical progresses in bioanalysis have triggered an intense debate on the value of absorption, distribution, metabolism and excretion radiolabelled studies in animals. Some authors have radically challenged the traditional approach whereas others, while accepting the need of significant changes, argue that these studies remain an irreplaceable component of the preclinical registration dossier. This paper reviews some of the representative positions and describes the potential evolution.

Case study 3. Application of basic enzyme kinetics to metabolism studies: real-life examples

An appreciation of the principles of enzyme kinetics can be applied in a number of drug metabolism applications. The concept for this chapter arose from a simple discussion on selecting appropriate time points to most efficiently assess metabolite profiles in a human Phase 1a clinical study (Subheading 4). By considering enzyme kinetics, a logical approach to the issue was derived. The dialog was an important learning opportunity for the participants in the discussion, and we have endeavored to capture this experience with other questions related to determination of K m and V max parameters, a consideration of the value of hepatocytes versus liver microsomes and enzyme inhibition parameters.

Data-driven approach for cross-species comparative metabolite exposure assessment: how to establish fundamental bioanalytical parameters for the peak area ratio method

We previously developed an high-performance LC–MS peak area ratio approach to demonstrate whether an animal species used in a toxicology study has greater exposures to drug metabolites relative to humans, meeting regulatory guidances regarding safety assessment of drug metabolites. Herein we explain the underlying bioanalytical principals, how to establish all fundamental bioanalytical parameters, and how to evaluate data quality in sample analysis, in the absence of authentic standards of analyte(s). A data-driven tiered approach was used in which data from the peak area ratio method can stand based on statistical analysis, as well as assuring that fundamental elements of bioanalytical method and bioanalysis are met. This strategy offers considerable time- and resource-saving advantage while providing high confidence in the safety assessment of human metabolites in drug development.

Matrix effects in metabolite quantification for MIST assessment: the impact of phospholipid removal and HPLC column particle size

Background: This Research article investigates the impact of phospholipid removal and high-performance liquid chromatography column particle size on the accuracy of determining the relative abundance of human metabolites using mass spectrometry peak areas in the context of assessing metabolite abundance for Metabolites in Safety Testing assessment. Results/Methodology: Plasma samples spiked with 20 compounds, representing ten pairs of drugs and metabolites, were prepared using phospholipid removal plates (Ostro™) or standard protein precipitation techniques and analyzed by liquid chromatography–tandem mass spectrometry using high-performance liquid chromatography columns containing either 2.5 or 3.5 µm particles. Removal of phospholipids significantly reduced matrix effects for samples analyzed on the larger particle size columns while preventing phospholipid build up on the analytical columns. In addition, quantitative accuracy and linearity were not affected by phospholipid removal. Conclusion: Both sample preparation strategies and column particle sizes should be considered in order to reduce the inaccuracy as a result of matrix effects in assessing metabolite abundance using mass spectrometry peak areas.

Reflecting on a decade of metabolite screening and monitoring

The last 10 years have witnessed robust debate within the bioanalytical community and regulatory authorities on the topic of metabolite monitoring and safety assessment. Of particular interest to regulated bioanalytical laboratories was the acceptance by the US FDA and other major regulatory bodies of a tiered approach to bioanalytical assay validation. The tiered approach defines a sliding scale of regulatory rigor for the evaluation of significant human metabolites that encompasses a range of assessments from semi-quantitative assays to fully validated assays, all of which can be used in support of regulatory submissions. This article describes the utilization of a tiered approach at Bristol-Myers Squibb and the decision trees guiding the selection of the appropriate level of assay qualification. Case studies illustrate how decisions are made, how different scientific situations influence the assay choice, and what criteria may be set to continue or discontinue metabolite monitoring in later drug development.

Single laboratory method validation comparing MS3 with FI/MS fingerprinting, and quantitation strategies for the accurate determination of ochratoxins in beer

The current mycotoxin safety concerns demand the availability of reliable and selective control systems to measure the content of mycotoxins present in daily food. To address this, we report a prospective analytical tool involving the application of a flow injection mass spectrometry (FI/MS) tandem artificial neural network (ANN) strategy to predict the amount of ochratoxin A and B (OTA and OTB) in beer. Triple stage mass spectrometry (MS3) aided by chromatographic separation was applied as a reference method for comparison. 0.1% formic acid and methanol were used to convert ochratoxins into their respective ions under negative MS polarity. For experiments involving MS3, ochratoxins were separated by reversed-phase liquid chromatography in a 6 min run, ionised using electrospray ionisation, and detected by tandem mass spectrometry. Analyte-specific mass-to-charge ratios were used to perform quantitation in MS3 mode. For experiments involving FI/MS, no chromatographic separation was performed. Approximately 2% of the mass spectra was used for model construction. ANN models representing each ochratoxin were individually trained and validated using three sets of matrix-matched and matrix-free calibration curves at nine concentration levels of 2.5, 10, 25, 50, 100, 200, 300, 400 and 500 μg/l. Quintuplicate analyses were made in FI/ MS mode providing a total of 270 spectra for both OTA and OTB. Single measurement was made for each sample in MS3 mode. A root-mean-square error value of <1% was reported for both ochratoxin models in beer. Limits of quantitation were determined to be 0.2 μg/kg for both MS3 and FI/MS mode. Recovery assessment was performed over two days using beer blanks (n=6) spiked at three concentration levels of 5, 100 and 200 μg/kg. Extraction using acetonitrile provided excellent recovery ranges of 88 to 102% for both MS techniques. Relative standard deviations of 10% or better were achieved for interday spike recovery experiments. The successful utilisation of FI/MS without performing chromatographic separation implies the availability of a new analytical tool relevant to the field of mycotoxin analysis, possibly offering analyte specificity exceeding the capability of MS3 through chemometry.

Meeting report: metabolites in safety testing (MIST) symposium-safety assessment of human metabolites: what's REALLY necessary to ascertain exposure coverage in safety tests?

In the 2012 AAPS metabolites in safety testing (MIST) symposium held in Chicago, IL, USA, on October 15, 2012, regulatory experts and industrial scientists joined together to discuss their perspectives and strategies in addressing contemporary MIST recommendations (FDA 2008, International Conference on Harmonization (ICH) M3(R2), ICH M(R2) Q&A). Overall, these regulatory guidances indicate that metabolites identified in human plasma should circulate at similar or greater concentrations in at least one of the animal species used in nonclinical safety assessment of the parent drug. However, synthetic standards for the metabolites often do not exist or they are intractable to synthesize, thus introducing multiple challenges in drug development for the quantitative comparison of metabolites between human and animals. A tiered bioanalytical strategy for metabolite analysis is a prevalent approach to demonstrate coverage in animals. Recent developments in bioanalytical methodology have yielded several time- and resource-sparing strategies to provide fit-for-purpose approaches that can enable critical decisions related to metabolite quantification and monitoring in plasma. This report summarizes the presentations and panel discussions at the symposium.

Metabolite structure analysis by high-resolution MS: supporting drug-development studies

Effective characterization of drug metabolites in complex biological matrices is facilitated by mass spectrometers with high resolving power, mass accuracy and sensitivity. This review begins with an overview of high-resolution MS terminology and the different types of instrumentation that are currently available. Metabolite structure analysis offers unique challenges and, therefore, the different types of approaches used to solve problems are highlighted through specific examples. Overall, this review describes the value that high-resolution MS brings to drug-metabolism studies.

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.

Atypical antipsychotics: trends in analysis and sample preparation of various biological samples

Atypical antipsychotics are increasingly popular and increasingly prescribed. In some countries, they can even be obtained over-the-counter, without a prescription, making their abuse quite easy. Although atypical antipsychotics are thought to be safer than typical antipsychotics, they still have severe side effects. Intoxications are not rare and some of them have a fatal outcome. Drug interactions involving atypical antipsychotics complicate patient management in clinical settings and the determination of the cause of death in fatalities. In view of the above, analytical strategies that can efficiently isolate atypical antipsychotics from a variety of biological samples and quantify them accurately, sensitively and reliably, are of utmost importance both for the clinical, as well as for the forensic toxicologist. In this review, we will present and discuss novel analytical strategies that have been developed from 2004 to the present day for the determination of atypical antipsychotics in various biological samples.

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.

Preclinical in vivo ADME studies in drug development: a critical review

INTRODUCTION

The last two decades have brought many fundamental changes to the drug development process. One such change is the importance of preclinical pharmacokinetics, which has become an essential part of early drug discovery. Furthermore, bioanalytical methods have become more sensitive and the identification and quantitation of metabolites can now be carried out on limited amount of biological material. There has also been a change in regulatory expectations, which are now particularly focused on the safety of human metabolites.

AREAS COVERED

The focus of this paper is on some 'traditional' in vivo ADME studies: excretion balance, metabolic profile and WBA in the toxicological species. These studies, performed with radiolabeled material, have a long history: and are a regular presence in submission dossiers. This paper reviews their value in the perspective of the contemporary drug development process.

EXPERT OPINION

These experiments may sometimes still be relevant to explain toxicological findings or for other special purposes but should not be considered required pieces of the registration dossiers. An appropriate investigation of samples coming from safety evaluation and human Phase I studies and the knowledge generated during the lead optimization phase provide, in most instances, all the DMPK information needed to take decisions in the drug development process.

Evaluation of relative LC/MS response of metabolites to parent drug in LC/nanospray ionization mass spectrometry: potential implications in MIST assessment

There is an increasing demand for quantitative data on metabolite exposure triggered by regulatory guidances. This contribution describes the accuracy of nanoelectrospray ionization mass spectrometry response of drug compounds and their metabolites from biological matrices compared with radiometric quantification. This is a comprehensive investigation of a set of real-life pharmaceutical compounds in relevant matrices such as urine, bile, feces and plasma. The data suggest that nanoelectrospray mass spectrometry can be used for semi-quantitation of metabolites in the absence of reference standards. Therefore, this approach is suitable to screen out non-relevant metabolites early in development as long as an adequate analytical error margin is applied thus balancing risks and resources.

Small molecule quantification by liquid chromatography-mass spectrometry for metabolites of drugs and drug candidates

Identification and quantification of the metabolites of drugs and drug candidates are routinely performed using liquid chromatography-mass spectrometry (LC-MS). The best practice is to generate a standard curve with the metabolite versus the internal standard. However, to avoid the difficulties in metabolite synthesis, standard curves are sometimes prepared using the substrate, assuming that the signal for substrate and the metabolite will be equivalent. We have tested the errors associated with this assumption using a series of very similar compounds that undergo common metabolic reactions using both conventional flow electrospray ionization LC-MS and low-flow captive spray ionization (CSI) LC-MS. The differences in standard curves for four different types of transformations (O-demethylation, N-demethylation, aromatic hydroxylation, and benzylic hydroxylation) are presented. The results demonstrate that the signals of the substrates compared with those of the metabolites are statistically different in 18 of the 20 substrate-metabolite combinations for both methods. The ratio of the slopes of the standard curves varied up to 4-fold but was slightly less for the CSI method.