On-line liquid chromatography-accurate radioisotope counting coupled with a radioactivity detector and mass spectrometer for metabolite identification in drug discovery and development

Uptake and metabolism of 14C-triclosan in celery under hydroponic system

As triclosan is used extensively as an antimicrobial agent, it inevitably enters agroecosystems, when sewage and treated wastewater are applied to agricultural fields. As a result, triclosan can be accumulated into crops and vegetables. Currently, limited information is available on the metabolism of triclosan in vegetables. In this study, the fate of ¹⁴C-triclosan in celery under a hydroponic system was investigated in a 30-day laboratory test. Most (97.7 %) of the ¹⁴C-triclosan accumulated in celery. The bioconcentration factors of triclosan were up to 3140 L kg^-1 at day 30. The concentration of ¹⁴C-triclosan in roots (17.8 mg kg^-1) was 57- and 127-fold higher than that in stems (0.31 mg kg^-1) and leaves (0.14 mg kg^-1), respectively, at day 30, suggesting a higher accumulation of triclosan in celery roots and negligible transport to stems and leaves. Moreover, triclosan, as well as its eight metabolites, was detected and identified in celery tissues and the growth medium using ¹⁴C-labelling and LC-Q-TOF-MS analysis methods. Phase I metabolites in the growth medium were from hydroxylation, dechlorination, nitration, and nitrosylation. Phase II metabolism was the major pathway in celery tissues. Monosaccharide, disaccharide, and sulfate conjugates of triclosan were putatively identified. The results represent an important step toward a better evaluation of the behavior of triclosan in vegetables, with notable implications for environmental and human risk assessments of triclosan.

Absorption, distribution, metabolism, and excretion of [14C]TPN729 after oral administration to rats

TPN729, a novel phosphodiesterase type 5 (PDE5) inhibitor for the treatment of erectile dysfunction (ED), is in phase II clinical trials in China. Previous studies suggested that TPN729 possesses promising therapeutic value. In previous non-radiolabeled rat excretion studies, the recovery of TPN729 and its major metabolites accounted for approximately 8.58% of the administration dose in urine and feces by 48 h post-dose.To solve this problem and further study the metabolism of TPN729 in rats, we used the radio-isotopic tracing technique for the first time. In this study, the mass balance, tissue distribution, and metabolism of TPN729 were evaluated in rats after a single oral dose of 25 mg/kg [¹⁴C]TPN729 (150 μCi/kg).At 168 h post-dose, the mean total radioactivity recovery of the dose was 92.13%. Feces was the major excretion route, accounting for 74.63% of the dose, and urine excretion accounted for 17.50%. After oral administration of [¹⁴C]TPN729, radioactivity was widely distributed in all examined tissues, and a higher radioactivity concentration was observed in the stomach, large intestine, lung, liver, small intestine, and eyes. The concentration of drug-related materials were similar in plasma and blood cells. A total of 51 metabolites were identified in rat plasma, urine, feces, and bile, and the predominant metabolically susceptible position of TPN729 was the pyrrolidine moiety. The main metabolic pathways were N-dealkylation, oxidation, dehydrogenation, and glucuronidation.In summary, we solved the previous problem of low drug recovery, elucidated the major excretion pathway, determined the tissue distribution patterns, and investigated the metabolism of TPN729 in rats by using a radioisotopic tracing technique.

Combination of high specific activity carbon-14 labeling and high resolution mass spectrometry to study pesticide metabolism in crops: Metabolism of cycloxaprid in rice

The study of pesticide metabolism in crops is critical for assessing the mode of action and environmental risks of pesticides. However, the study of pesticide metabolism in crops is usually complicated and it is often a daunting challenge to accurately screen the metabolites of novel pesticides in complex matrices. This study demonstrated a combined use of high-specific activity carbon-14 labeling and high-resolution mass spectrometry (HSA-¹⁴C-HRMS) for metabolism profiling of a novel neonicotinoid cycloxaprid in rice. By generating the characteristic radioactive peaks on the liquid chromatogram, the use of ¹⁴C can eliminate the severe interference of complex matrices and quickly probe target compounds; by producing ion pairs with unique abundance ratios on HRMS, high-specific activity labeling can effectively exclude false matrix positives and promote the elucidation of metabolite structure. The structures of 15 metabolites were identified, three of which were further confirmed by authentic standards. Based on these metabolites, a metabolic profile of cycloxaprid was established, which includes denitrification, demethylation, imidazolidine hydroxylation and ring cleavage olefin formation, oxidation and carboxylation reactions. The strategy of combining high-specific activity ¹⁴C labeling and HRMS offers unique advantages and provides a powerful solution for profiling unknown metabolites of novel pesticides in complex matrices, especially when traditional non-labeling methods are not feasible.

Absorption, distribution, metabolism, and excretion of [14C]NBP (3-n-butylphthalide) in rats

3-n-Butylphthalide (NBP) has a considerable neuroprotective effect and is currently used for the treatment of ischemic stroke. NBP was launched on the market in 2004. However, information on its metabolism in humans and preclinical animal models is insufficient. Although the metabolism of unradiolabeled NBP in humans has been reported, the quantitative metabolite profile, blood-to-plasma radioactivity concentration ratio (B/P), and tissue distribution of this drug remain unclear. We evaluated the pharmacokinetics, tissue distribution, mass balance, and metabolism of NBP in rats after a single oral dose of 60 mg/kg (100 μCi/kg) [¹⁴C]NBP to understand the biotransformation of NBP comprehensively and to provide preclinical drug metabolism data prior to human mass balance studies with [¹⁴C]NBP in the near future. NBP absorption was rapid (T_max = 0.75 h) and declined with a terminal half-life of 9.73 h. In rats, the B/P was 0.63 during the 48 h postdose period, indicating that drug-related substances did not tend to be distributed into blood cells. Tissue distribution was determined by using the oxidative combustion method. NBP-related components were widely distributed throughout the body, and high concentrations were detected in the stomach, small intestine, fat, bladder, kidney, liver and ovary. At 168 h after oral administration, the mean cumulative recovered radioactivity was 99.85% of the original dose, and was 85.12% in urine and 14.73% in feces. Metabolite profiles were detected via radiochromatography. A total of 49 metabolites were identified in rat plasma, urine, and feces. The main metabolic pathways were oxidation, glucuronidation, and sulfation. Overall, NBP was absorbed rapidly, distributed throughout the body, and excreted in the form of metabolites. Urine was the main excretion route, and the absorption, distribution, metabolism and excretion of NBP showed no significant gender difference between male and female rats.

Liquid Chromatography Analysis of Common Nutritional Components, in Feed and Food

Food and feed laboratories share several similarities when facing the implementation of liquid-chromatographic analysis. Using the experience acquired over the years, through application chemistry in food and feed research, selected analytes of relevance for both areas were discussed. This review focused on the common obstacles and peculiarities that each analyte offers (during the sample treatment or the chromatographic separation) throughout the implementation of said methods. A brief description of the techniques which we considered to be more pertinent, commonly used to assay such analytes is provided, including approaches using commonly available detectors (especially in starter labs) as well as mass detection. This manuscript consists of three sections: feed analysis (as the start of the food chain); food destined for human consumption determinations (the end of the food chain); and finally, assays shared by either matrices or laboratories. Analytes discussed consist of both those considered undesirable substances, contaminants, additives, and those related to nutritional quality. Our review is comprised of the examination of polyphenols, capsaicinoids, theobromine and caffeine, cholesterol, mycotoxins, antibiotics, amino acids, triphenylmethane dyes, nitrates/nitrites, ethanol soluble carbohydrates/sugars, organic acids, carotenoids, hydro and liposoluble vitamins. All analytes are currently assayed in our laboratories.

Degradation and metabolic profiling for benzene kresoxim-methyl using carbon-14 tracing

Benzene kresoxim-methyl (BKM) is an effective strobilurin fungicide for controlling fungal pathogens but limited information is available on its degradation and metabolism. This study explored the degradation and metabolic profiling for BKM in soils by carbon-14 tracing and HPLC-TOF-MS² analyzing. Results indicated that 88%-98% of ¹⁴C-BKM remained as parent or incomplete intermediates after 100 days. Three main radioactive metabolites (M1 to M3, ≥90%) and three subordinate radioactive metabolites (Ma to Mc, ≤2%) were observed, along with a non-radioactive metabolite M4. The main intermediates were further confirmed by self-synthesizing their authentic standards, and BKM was proposed to degrade via pathways including: 1) the oxidative cleavage of the acrylate double bond to give BKM-enol (M1); 2) the hydrolysis of the methyl ester to give BKM acid (M2); 3) the cleavage of M1 and M2 to yield Mc, which could be decarboxylated to give M3; and 4) the ether cleavage between aromatic rings to form M4. This study builds a solid metabolic profiling method for strobilurins and gives a deeper insight into the eventual fate of BKM by demonstrating its transformation pathways for the first time, which may also be beneficial for understanding the risks of other analogous strobilurins.

Studies on the anoxic dissipation and metabolism of pyribambenz propyl (ZJ0273) in soils using position-specific radiolabeling

Pyribambenz propyl (ZJ0273) is a polycyclic herbicide with increasing use, although studies show that it tends to be persistent in soil and pose phytotoxicity to rotational crops. This study employed an improved ring-specific (14)C labeling method to characterize its anoxic metabolism, with (14)C positioned on the benzoate, pyrimidyl or benzyl rings. Separation and identification of the metabolites were achieved by liquid chromatography (LC), ultralow-level liquid scintillation spectrometry, and LC-mass spectrometry (MS). Results show that the anoxic degradation follows first-order kinetics and the half-lives are approximately 38.7, 50.2 and 70.7d for loamy, saline and clayey soils, respectively. A total of five radioactive intermediates (M1-M5) were detected, and due to the loss of radiolabels, different radiochromatograms were obtained from different labels, i.e., radioactive M5 was only detected for pyrimidinyl-(14)C; M3 and M4 were only detected for pyrimidinyl-(14)C and benzyl-(14)C, while M1 and M2 were detected for all labels. Based on their appearance pattern and fragmentations from LC-MS, the structures of M1-M5 were identified, and they were proposed to form by reactions such as de-estering, hydrolysis, acylation, CN cleavage, and demethylation. All metabolites have been previously detected in aerobic soils except M4, which is a demethylation product from M3, and identified as 2-(4-hydroxy-6-methoxypyrimidin-2-yloxy)benzoic acid. The results show that ZJ0273 is more persistent in anoxic soils, and its degradation pathways and intermediates are different from aerobic metabolism and differ with the soil types, suggesting that soil-specific and farming practices may be important considerations in the use of this herbicide. The ring-specific labeling provides full molecular information about the referred compound and guarantees the reliability of the results, and can be used as an effective tool for metabolite profiling of polycyclic compounds.

Bioanalytical aspects of clinical mass balance studies in oncology

Clinical mass balance studies aim to investigate the absorption, distribution, metabolism and excretion (ADME) of a(n) (often radiolabeled) drug, following a single administration to humans. They are perfectly suited to determine the disposition and major metabolic pathways of a drug, the exposure to the parent drug and its metabolites, and the rate and route of elimination. A mass balance study, however, poses interesting challenges to the analysis of parent drug and metabolites in different biological matrices. Using recent clinical mass balance studies in oncology as an example, this review focuses on the aspects of mass balance studies, from bioanalytical assay development, analysis of clinical samples to reporting of study results. Along the way, it discusses bioanalytical problems and practical solutions.

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.

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.

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

With the recent publication of the FDA guidance on metabolites in safety testing (MIST), a reflection is provided that describes the impact of this guidance on the processes of drug metabolite identification and quantification at various stages of drug development. First, a retrospective analysis is described that was conducted on 12 human absorption, metabolism, and excretion (AME) trials with the application of these MIST criteria. This analysis showed that the number of metabolites requiring identification, (semi)-quantification, and coverage in the toxicology species would substantially increase. However, a significant proportion of these metabolites were direct or indirect conjugates, a class of metabolites that was specifically addressed in the guidance as being largely innocuous. The nonconjugated metabolites were all covered in at least one toxicology animal species, with no need for additional safety evaluation. Second, analytical considerations pertaining to the efficient identification of metabolites are discussed. Topics include software-assisted detection and structural identification of metabolites, the emerging hyphenation of ultraperformance liquid chromatography (UPLC) with radioactivity detection, and the various ways to estimate metabolite abundance in the absence of an authentic standard. Technical aspects around the analysis of metabolite profiles are also presented, focusing on precautions to be taken in order not to introduce artifacts. Finally, a tiered approach for metabolite quantification is proposed, starting with quantification of metabolites prior to the multiple ascending dose study (MAD) in humans in only specific cases (Tier A). The following step is the identification and quantification of metabolites expected to be of pharmacological or toxicological relevance (based on MIST and other complementary criteria) in selected samples from the MAD study and preclinical studies in order to assess metabolite exposure coverage (Tier B). Finally, a metabolite quantification strategy for the studies after the MAD phase (Tier C) is proposed.

High-performance liquid chromatography-tandem mass spectrometry in the identification and determination of phase I and phase II drug metabolites

Applications of tandem mass spectrometry (MS/MS) techniques coupled with high-performance liquid chromatography (HPLC) in the identification and determination of phase I and phase II drug metabolites are reviewed with an emphasis on recent papers published predominantly within the last 6 years (2002–2007) reporting the employment of atmospheric pressure ionization techniques as the most promising approach for a sensitive detection, positive identification and quantitation of metabolites in complex biological matrices. This review is devoted to in vitro and in vivo drug biotransformation in humans and animals. The first step preceding an HPLC-MS bioanalysis consists in the choice of suitable sample preparation procedures (biomatrix sampling, homogenization, internal standard addition, deproteination, centrifugation, extraction). The subsequent step is the right optimization of chromatographic conditions providing the required separation selectivity, analysis time and also good compatibility with the MS detection. This is usually not accessible without the employment of the parent drug and synthesized or isolated chemical standards of expected phase I and sometimes also phase II metabolites. The incorporation of additional detectors (photodiode-array UV, fluorescence, polarimetric and others) between the HPLC and MS instruments can result in valuable analytical information supplementing MS results. The relation among the structural changes caused by metabolic reactions and corresponding shifts in the retention behavior in reversed-phase systems is discussed as supporting information for identification of the metabolite. The first and basic step in the interpretation of mass spectra is always the molecular weight (MW) determination based on the presence of protonated molecules [M+H]⁺ and sometimes adducts with ammonium or alkali-metal ions, observed in the positive-ion full-scan mass spectra. The MW determination can be confirmed by the [M-H]^- ion for metabolites providing a signal in negative-ion mass spectra. MS/MS is a worthy tool for further structural characterization because of the occurrence of characteristic fragment ions, either MSⁿ analysis for studying the fragmentation patterns using trap-based analyzers or high mass accuracy measurements for elemental composition determination using time of flight based or Fourier transform mass analyzers. The correlation between typical functional groups found in phase I and phase II drug metabolites and corresponding neutral losses is generalized and illustrated for selected examples. The choice of a suitable ionization technique and polarity mode in relation to the metabolite structure is discussed as well.

Detection limit of measurement of pharmaceuticals labeled with short-lived isotopes in HPLC with flow-through gamma-counter

This paper proposes a method for estimating the detection limit, which is defined as 3.3 times the standard deviation (S.D.) of blank measurements under the situations where the repetition of measurement is difficult or impossible because of a short half-life of radioactivity. The FUMI theory, which can estimate an S.D. value without repetition in various instrumental analyses, is adopted and proved here to be available in a radio-HPLC system as well. (99m)Tc-ECD (T(1/2)=360.6 min) that is a lipophilic compound for the diagnosis of regional brain perfusion is taken as an example.

Cytostar-T plates—A valid alternative for microplate scintillation counting of low radioactivity in combination with high-performance liquid chromatography in drug metabolism studies?

The determination of radioactivity from metabolite patterns in ADME studies in a low radioactivity/residue situation is a very challenging process requiring special technologies. The recently introduced TopCount technology uses LumaPlates for the collection of the column effluent after HPLC separation to subsequently determine radioactivity for the generation of the metabolite profile. Samples from drug metabolism studies were used to compare the performance of the widely used LumaPlates with Cytostar-T plates regarding sensitivity and recovery of metabolites for structure elucidation by MS. Optimized counting parameters were investigated for the Cytostar-T plates. This had led to higher sensitivity and therefore to a preferential signal to noise ratio. Metabolites which were collected into Cytostar-T instead of LumaPlates could be easily recovered and directly used for structure elucidation by MS. The full scan mass spectra of recovered metabolites showed higher quality allowing the characterization of metabolites without any further sample pre-treatment. This is a major advantage which could further speed-up the structure elucidation process of metabolites in complex biological matrices.

New analytical strategies in studying drug metabolism

Identification and elucidation of the structures of metabolites play major roles in drug discovery and in the development of pharmaceutical compounds. These studies are also important in toxicology or doping control with either pharmaceuticals or illicit drugs. This review focuses on: new analytical strategies used to identify potential metabolites in biological matrices with and without radiolabeled drugs; use of software for metabolite profiling; interpretation of product spectra; profiling of reactive metabolites; development of new approaches for generation of metabolites; and detection of metabolites with increased sensitivity and simplicity. Most of the new strategies involve mass spectrometry (MS) combined with liquid chromatography (LC).

Analytical strategies for identifying drug metabolites

With the dramatic increase in the number of new chemical entities (NCEs) arising from combinatorial chemistry and modern high-throughput bioassays, novel bioanalytical techniques are required for the rapid determination of the metabolic stability and metabolites of these NCEs. Knowledge of the metabolic site(s) of the NCEs in early drug discovery is essential for selecting compounds with favorable pharmacokinetic credentials and aiding medicinal chemists in modifying metabolic "soft spots". In development, elucidation of biotransformation pathways of a drug candidate by identifying its circulatory and excretory metabolites is vitally important to understand its physiological effects. Mass spectrometry (MS) and nuclear magnetic resonance (NMR) have played an invaluable role in the structural characterization and quantification of drug metabolites. Indeed, liquid chromatography (LC) coupled with atmospheric pressure ionization (API) MS has now become the most powerful tool for the rapid detection, structure elucidation, and quantification of drug-derived material within various biological fluids. Often, however, MS alone is insufficient to identify the exact position of oxidation, to differentiate isomers, or to provide the precise structure of unusual and/or unstable metabolites. In addition, an excess of endogenous material in biological samples often suppress the ionization of drug-related material complicating metabolite identification by MS. In these cases, multiple analytical and wet chemistry techniques, such as LC-NMR, enzymatic hydrolysis, chemical derivatization, and hydrogen/deuterium-exchange (H/D-exchange) combined with MS are used to characterize the novel and isomeric metabolites of drug candidates. This review describes sample preparation and introduction strategies to minimize ion suppression by biological matrices for metabolite identification studies, the application of various LC-tandem MS (LC-MS/MS) techniques for the rapid quantification and identification of drug metabolites, and future trends in this field.

Metabolism, Pharmacokinetics, and Excretion of the Substance P Receptor Antagonist CP-122,721 in Humans: Structural Characterization of the Novel Major Circulating Metabolite 5-Trifluoromethoxy Salicylic Acid by High-Performance Liquid Chromatography-Tandem Mass Spectrometry and NMR Spectroscopy

The metabolism, pharmacokinetics, and excretion of a potent and selective substance P receptor antagonist, CP-122,721 [(+)-(2S,3S)-3-(2-methoxy-5-trifluoromethoxybenzylamino)-2-phenylpiperidine], have been studied in six healthy male human subjects [four extensive metabolizers (EMs) and two poor metabolizers (PMs) of CYP2D6) following oral administration of a single 30-mg dose of [14C]CP-122,721. Approximately 84% of the administered radioactivity was recovered from the urine and feces of the subjects over a period of 312 h. Approximately 80% of the dose for EM subjects was recovered within 48 h. PM subjects, however, excreted only about 45% of the dose in 48 h and required the full 312 h to achieve nearly 80% recovery. Absorption of CP-122,721 was rapid in both extensive and poor metabolizers, as indicated by the rapid appearance of radioactivity in serum. The serum concentrations of total radioactivity were always much greater than those of unchanged drug indicating early formation of metabolites. The average CP-122,721 t1/2 was 6.7 h and 45.0 h for EM and PM subjects, respectively. The serum concentrations of CP-122,721 reached a peak of 7.4 and 69.8 ng/ml for extensive and poor metabolizers, respectively. The major metabolic pathways of CP-122,721 were due to O-demethylation, aromatic hydroxylation, and indirect glucuronidation. The minor metabolic pathways included aliphatic oxidation at the piperidine moiety, O-dealkylation of the trifluoromethoxy group, N-dealkylation, and oxidative deamination. In addition to the major human circulating metabolite 5-trifluoromethoxy salicylic acid (TFMSA), all other circulating metabolites of CP-122,721 were glucuronide conjugates of oxidized metabolites. TFMSA was identified using high pressure liquid chromatography/tandem mass spectrometry and NMR and mechanisms were proposed for its formation. There are no known circulating active metabolites of CP-122,721.

A microplate solid scintillation counter as a radioactivity detector for high performance liquid chromatography in drug metabolism: Validation and applications

Sensitive radioactivity detection following high performance liquid chromatography (HPLC) separation remains a challenge in many drug metabolism studies with radiolabeled compounds. In this work, solid scintillation counting (SSC) after fraction collection into 96-well plates was evaluated as an off-line radioactivity detection method, in comparison with conventional liquid scintillation counting (LSC). The impact of counting time and biological matrix on the quantification of radiolabeled metabolites and parent drug in samples from animal and human absorption, distribution, metabolism and excretion (ADME) studies was investigated. Three different approaches were used to test whether reliable quantification by off-line SSC detection, which requires an approximately constant counting yield during the entire chromatographic run, can be realized: (i) the measurement of radioactivity-spiked biological blank samples without HPLC separation as an extreme case of biological background, (ii) the measurement of radioactivity-spiked HPLC fractions of biological blank samples and (iii) the comparison of radiochromatograms obtained by off-line SSC and LSC of real samples from ADME studies with radiolabeled compounds. Situations in which variations in SSC yield during an HPLC run are likely to lead to significant errors in quantitation were identified and are discussed. However, examples from a number of animal or human ADME studies showed that in the majority of cases off-line SSC provides very similar quantitative data, compared with the reference method of off-line LSC radioactivity detection. Approaches for validation of the off-line SSC approach in critical cases are discussed. The main advantages of off-line SSC, compared with off-line LSC, are lower detection limits and a substantially higher throughput. Several applications of off-line SSC detection in ADME studies are shown.

Investigation of low-abundant in vitro metabolites of stable isotope-labelled BAL4815 by accurate mass capillary-LC-ESI-qTof-MS and MS/MS

The metabolic profile of BAL4815, an antifungal azole drug, was determined using in vitro rat hepatocyte incubations and subsequent analysis by capillary LC-qTof-MS and MS/MS including accurate mass determination. For the detection of the metabolites, a mixture of the drug and its deuterium-labelled analogue was used for incubations. Metabolic stability of BAL4815 was high in cultured rat hepatocytes. However, several low-abundant metabolites were detected by the use of capillary LC-qTof-MS and manual investigation of the data. The peak intensity of the most abundant metabolite was close to the limit of detection. Except for an apparent oxidation product, the masses of the other detected metabolites could not be assigned to a single and frequently occurring biotransformation. Accurate mass determination and possible elemental compositions suggested that metabolism occurred through a combination of glutathionylation and defluorination. This was verified using accurate mass MS/MS. The use of accurate mass measurements and the derived suggestions for the elemental compositions were essential to elucidate this atypical metabolic pathway. A mass accuracy better than 8 ppm could be achieved for most assigned MS and MS/MS signals with intensities less than 6 cps in the spectra.

Incorporation of a nanosplitter interface into an LC-MS-RD system to facilitate drug metabolism studies

In the work reported here, a novel interface, the nanosplitter, is incorporated into the drug metabolism laboratory in order to enhance the analytical capabilities of detecting and identifying drug-related metabolites to support drug metabolism studies during the drug development process. When an existing LC-MS-radiometric detector (RD) system is coupled with this nanosplitter, the system becomes capable of performing dynamic microspray under a typical analytical LC method. With the superior MS sensitivity offered by this system, most of the analytical LC methods developed for metabolite profiling can then be easily adopted for metabolite identification work. The improvement of these analytical capabilities can streamline the entire process of the drug metabolism study. In the experiments presented here, the nanosplitter interface coupled with analytical HPLC systems (e.g. 4.6 x 250 mm column @ 1 ml/min) demonstrated significant increases in MS signal (2x to 40x peak area) when compared to the standard LC-MS interface for both in vitro and in vivo metabolism studies. Furthermore, this signal gain facilitated the MS detection of additional metabolites (observed in the radiometric trace) that were below the MS level of detection when using the standard interface.

Analysis of low level radioactive metabolites in biological fluids using high-performance liquid chromatography with microplate scintillation counting: method validation and application

TopCount, a microplate scintillation counter (MSC), has been recently employed as an off-line liquid radiochromatographic detector for radioactive metabolite profile analysis. The present study was undertaken to validate TopCount for metabolite profiling with respect to sensitivity, accuracy, precision and radioactivity recovery. Matrix effects of various human samples on TopCount performance and capability of MSC for volatile metabolite analysis were also investigated. TopCount had a limit of detection (LOD) of 5 DPM and a limit of quantification (LOQ) of 15 DPM for [(14)C]-labeled compounds at a 10min counting time. It was two-fold more sensitive than a liquid scintillation counter (LSC), and 50-100-fold more sensitive than a radioactivity flow detector (RFD). TopCount had comparable accuracy and precision to RFD, and comparable precision to LSC for determining relative abundance of metabolites. Human liver microsome incubation (up to 1 mL), plasma (up to 1 mL), urine (up to 2 mL) and feces (up to 50mg) had no significant quenching effects on TopCount performance. Benzoic acid, a volatile metabolite, was detected by TopCount, but not by Microbeta counter after microplates were dried under vacuum. Radioactivity recovery in HPLC-MSC analysis was reliably determined using an LSC-based method. Examples of using HPLC-MSC for analysis of low levels of radioactive metabolites are presented, including determination of plasma metabolite profile, in vitro reactive metabolites trapped by [(3)H]glutathione, and metabolite concentrations in an enzyme kinetic experiment. The data from this study strongly suggest that HPLC in combination with TopCount is a viable alternative analytical tool for detection and quantification of low levels of radioactive metabolites in biological fluids.

A SIMPLE SEQUENTIAL INCUBATION METHOD FOR DECONVOLUTING THE COMPLICATED SEQUENTIAL METABOLISM OF CAPRAVIRINE IN HUMANS

Capravirine, a non-nucleoside reverse transcriptase inhibitor for the treatment of human immunodeficiency virus type 1, undergoes extensive oxygenations to numerous sequential metabolites in humans. Because several possible oxygenation pathways may be involved in the formation and/or sequential metabolism of a single metabolite, it is very difficult or even impossible to determine the definitive pathways and their relative contributions to the overall metabolism of capravirine using conventional approaches. For this reason, a human liver microsome-based "sequential incubation" method has been developed to deconvolute the complicated sequential metabolism of capravirine. In brief, the method includes three fundamental steps: 1) 30-min primary incubation of [(14)C]capravirine, 2) isolation of (14)C metabolites from the primary incubate, and 3) 30-min sequential incubation of each isolated (14)C metabolite supplemented with an ongoing (30 min) microsomal incubation with nonlabeled capravirine. Based on the extent of both the disappearance of the isolated precursor (14)C metabolites and the formation of sequential (14)C metabolites, definitive oxygenation pathways of capravirine were assigned. In addition, the percentage contribution of a precursor metabolite to the formation of each of its sequential metabolites (called sequential contribution) and the percentage contribution of a sequential metabolite formed from each of its precursor metabolites (called precursor contribution) were determined. An advantage of this system is that the sequential metabolism of each isolated (14)C metabolite can be monitored selectively by radioactivity in the presence of all relevant metabolic components (i.e., nonlabeled parent and its other metabolites). This methodology should be applicable to mechanistic studies of other compounds involving complicated sequential metabolic reactions when radiolabeled materials are available.

Strategies for dealing with metabolite elucidation in drug discovery and development

Structural information on metabolites can be a considerable asset for enhancing and streamlining the process of developing new drug candidates. Modern approaches that generate and use metabolite structural information can accelerate the drug discovery and development process by eliminating potentially harmful candidates earlier in the process and improving the safety of new drugs. This review examines the relative merits of current and potential strategies for dealing with metabolite characterization.

Liquid chromatography/atmospheric pressure ionization-mass spectrometry in drug metabolism studies

The study of the metabolic fate of drugs is an essential and important part of the drug development process. The analysis of metabolites is a challenging task and several different analytical methods have been used in these studies. However, after the introduction of the atmospheric pressure ionization (API) technique, electrospray and atmospheric pressure chemical ionization, liquid chromatography/mass spectrometry (LC/MS) has become an important and widely used method in the analysis of metabolites owing to its superior specificity, sensitivity and efficiency. In this paper the feasibility of LC/API-MS techniques in the identification, structure characterization and quantitation of drug metabolites is reviewed. Sample preparation, LC techniques, isotope labeling, suitability of different MS techniques, such as tandem mass spectrometry, and high-resolution MS in drug metabolite analysis, are summarized and discussed. Automation of data acquisition and interpretation, special techniques and possible future trends are also the topics of the review.