Evaluation of absorption, distribution, metabolism, and excretion of [14C]-rucaparib, a poly(ADP-ribose) polymerase inhibitor, in patients with advanced solid tumors.

Rucaparib, a poly(ADP-ribose) polymerase inhibitor, is licensed for use in recurrent ovarian, fallopian tube, or primary peritoneal cancer. We characterized the absorption, distribution, metabolism, and elimination of rucaparib in 6 patients with advanced solid tumors following a single oral dose of [14C]-rucaparib 600 mg (≈140 µCi). Total radioactivity (TRA) in blood, plasma, urine, and feces was measured using liquid scintillation counting. Unchanged rucaparib concentrations in plasma were determined using validated liquid chromatography with tandem mass spectrometry. Maximum concentration (Cmax) of TRA and unchanged rucaparib in plasma was 880 ng Eq/mL and 428 ng/mL, respectively, at approximately 4 h post dose; terminal half-life was >25 h for both TRA and rucaparib. The plasma TRA-time profile was parallel to yet higher than that of rucaparib, suggesting the presence of metabolites in plasma. Mean blood:plasma ratio of radioactivity was 1.0 for Cmax and 0.8 for area under the concentration-time curve from time zero to infinity. Mean postdose recovery of TRA was 89.3% over 12 days (71.9% in feces; 17.4% in urine). Unchanged rucaparib and M324 (oxidative metabolite) were the major components in plasma, contributing to 64.0% and 18.6% of plasma radioactivity, respectively. Rucaparib and M324 were the major rucaparib-related components (each ≈7.6% of dose) in urine, whereas rucaparib was the predominant component (63.9% of dose) in feces. The high fecal recovery of unchanged rucaparib could be attributed to hepatic excretion and/or incomplete oral absorption. Overall, these data suggest that rucaparib is eliminated through multiple pathways, including metabolism and renal and biliary excretion.

Pharmacokinetics and Comprehensive Analysis of the Tissue Distribution of Eravacycline in Rabbits.

Eravacycline (7-fluoro-9-pyrrolidinoacetamido-6-demethyl-6-deoxytetracycline or TP-434) is a novel, fully synthetic broad-spectrum fluorocycline with potent activity against Gram-positive bacteria, anaerobes, and multidrug-resistant Enterobacteriaceae We characterized the plasma pharmacokinetics of eravacycline and conducted a comprehensive analysis of the eravacycline tissue distribution in rabbits after multiple-day dosing. For single-dose pharmacokinetic analysis, eravacycline was administered to New Zealand White (NZW) rabbits at 1, 2, 4, 8, and 10 mg/kg of body weight intravenously (i.v.) once a day (QD) (n = 20). For multidose pharmacokinetic analysis, eravacycline was administered at 0.5, 1, 2, and 4 mg/kg i.v. QD (n = 20) for 6 days. Eravacycline concentrations in plasma and tissues were analyzed by a liquid chromatography-tandem mass spectrometry assay. Mean areas under the concentration-time curves (AUCs) following a single eravacycline dose ranged from 5.39 µg · h/ml to 183.53 µg · h/ml. Within the multidose study, mean AUCs ranged from 2.53 µg · h/ml to 29.89 µg · h/ml. AUCs correlated linearly within the dosage range (r = 0.97; P = 0.0001). In the cardiopulmonary system, the concentrations were the highest in the lung, followed by the heart > pulmonary alveolar macrophages > bronchoalveolar lavage fluid; for the intra-abdominal system, the concentrations were the highest in bile, followed by the liver > gallbladder > spleen > pancreas; for the renal system, the concentrations were the highest in urine, followed by those in the renal cortex > renal medulla; for the musculoskeletal tissues, the concentrations were the highest in muscle psoas, followed by those in the bone marrow > adipose tissue; for the central nervous system, the concentrations were the highest in cerebrum, followed by those in the aqueous humor > cerebrospinal fluid > choroid > vitreous. The prostate and seminal vesicles demonstrated relatively high mean concentrations. The plasma pharmacokinetic profile of 0.5 to 4 mg/kg in NZW rabbits yields an exposure comparable to that in humans (1 or 2 mg/kg every 12 h) and demonstrates target tissue concentrations in most sites.

Alternate strategies to obtain mass balance without the use of radiolabeled compounds: application of quantitative fluorine (19F) nuclear magnetic resonance (NMR) spectroscopy in metabolism studies.

Nuclear magnetic resonance (NMR) spectroscopy is playing an increasingly important role in the quantitation of small and large molecules. Recently, we demonstrated that (1)H NMR could be used to quantitate drug metabolites isolated in submilligram quantities from biological sources. It was shown that these metabolites, once quantitated by NMR, were suitable to be used as reference standards in quantitative LC/MS-based assays, hence circumventing the need for radiolabeled material or synthetic standards to obtain plasma exposure estimates in humans and preclinical species. The quantitative capabilities of high-field NMR is further demonstrated in the current study by obtaining the mass balance of fluorinated compounds using (19)F-NMR. Two fluorinated compounds which were radio-labeled with carbon-14 on metabolically stable positions were dosed in rats and urine and feces collected. The mass balance of the compounds was obtained initially by counting the radioactivity present in each sample. Subsequently, the same sets of samples were analyzed by (19)F-NMR, and the concentrations determined by this method were compared with data obtained using radioactivity counting. It was shown that the two methods produced comparable values. To demonstrate the value of this analytical technique in drug discovery, a fluorinated compound was dosed intravenously in dogs and feces and urine collected. Initial profiling of samples showed that this compound was excreted mainly unchanged in feces, and hence, an estimate of mass balance was obtained using (19)F-NMR. The data obtained by this method was confirmed by additional quantitative studies using mass spectrometry. Hence cross-validations of the quantitative (19)F-NMR method by radioactivity counting and mass spectrometric analysis were demonstrated in this study. A strategy outlining the use of fluorinated compounds in conjunction with (19)F-NMR to understand their routes of excretion or mass balance in animals is proposed. These studies demonstrate that quantitative (19)F-NMR could be used as an alternate technique to obtain an estimate of the mass balance of fluorinated compounds, especially in early drug development where attrition of the compounds is high, and cost savings could be realized through the use of such a technique rather than employing radioactive compounds. The potential application of qNMR in conducting early human ADME studies with fluorinated compounds is also discussed.

Application of quantitative NMR in pharmacological evaluation of biologically generated metabolites: implications in drug discovery.

It is important to gain an understanding of the pharmacological activities of metabolite(s) of compounds in development, especially if they are found in systemic circulation in humans. Pharmacological evaluation of metabolites is normally conducted with synthetic standards, which become available during various stages of drug development. However, the synthesis of metabolite standards may be protracted, taking anywhere from several weeks to months to be completed. This often slows down early pharmacological evaluation of metabolites. Once a metabolite(s) is found to possess comparable (or greater) pharmacological activity than the parent compound, additional studies are performed to better understand the implications of circulating pharmacologically active metabolite(s). To conduct some of these studies as early as possible without slowing the progression of a compound in development is important, especially if critical go or no-go decisions impinge on the outcomes from these studies. Early pharmacological evaluation of significant metabolites is hereby proposed to be conducted in the drug discovery stage so that all pertinent studies and information can be gathered in a timely manner for decision-making. It is suggested that these major metabolites be isolated, either from biological or chemical sources, and quantified appropriately. For biologically generated metabolites, NMR is proposed as the tool of choice to quantitate these metabolites before their evaluation in pharmacological assays. For metabolites that have the same UV characteristics as the parent compound, quantitation can be conducted using UV spectroscopy instead of NMR. In this article, we propose a strategy that could be used to determine the pharmacological activities of metabolites isolated in submilligram quantities.

Characterization of HKI-272 covalent binding to human serum albumin.

The study was initiated as an observation of incomplete extraction recovery of N-(4-(3-chloro-4-(2-pyridinylmethoxy)anilino)-3-cyano-7-ethoxy-6-quinolyl)-4-(dimethylamino)-2-butenamide (HKI-272) from human plasma. The objective of this study was to 1) identify the binding site(s) of HKI-272 to human plasma protein(s); 2) characterize the nature of the binding; and 3) evaluate the potential reversibility of the covalent binding. After incubation of [(14)C]HKI-272 with human plasma, the mixture was directly injected on liquid chromatography/mass spectrometry (LC/MS), and an intact molecular mass of HKI-272 human serum albumin (HSA) adduct was determined to be 66,999 Da, which is 556 Da (molecular mass of HKI-272) larger than the measured molecular mass of HSA (66,443 Da). For peptide mapping, the incubation mixture was separated with SDS-polyacrylamide gel electrophoresis followed by tryptic digestion combined with LC/tandem MS. A radioactive peptide fragment, LDELRDEGKASSAK [amino acid (AA) residue 182-195 of albumin], was confirmed to covalently bind to HKI-272. In addition, after HCl hydrolysis, a radioactive HKI-272-lysine adduct was identified by LC/MS. After combining the results of tryptic digestion and HCl hydrolysis, the AA residue of Lys190 of HSA was confirmed to covalently bind to HKI-272. A standard HKI-272-lysine was synthesized and characterized by NMR. The data showed that the adduct was formed via Michael addition with the epsilon-amine of lysine attacking to the beta-carbon of the amide moiety of HKI-272. Furthermore, reversibility of the covalent binding of HKI-272 to HSA was shown when a gradual release of HKI-272 was observed from protein pellet of HKI-272-treated human plasma after resuspension in phosphate buffer, pH 7.4, at 37 degrees C for 18 h.

Bioactivation of lamotrigine in vivo in rat and in vitro in human liver microsomes, hepatocytes, and epidermal keratinocytes: characterization of thioether conjugates by liquid chromatography/mass spectrometry and high field nuclear magnetic resonance spectroscopy.

Previous studies suggested that lamotrigene (LTG) underwent bioactivation to a reactive aryl epoxide intermediate in rats. Nevertheless, definitive structures of these thioether conjugates, which are often needed to substantiate the mechanism of bioactivation and identity of reactive intermediate(s), were not fully established. In the present study, GSH, cysteinylglycine, and N-acetyl cysteine conjugates of LTG were isolated from bile of rats orally dosed with LTG (100 mg/kg), and their structures were fully elucidated by LC/MS and NMR. The definitive structural characterization of these metabolites provided evidence for the existence of a reactive aryl epoxide that was trapped as a GSH adduct. In vitro studies using various hepatic cellular and subcellular fractions obtained from human and rat were performed to demonstrate that LTG underwent bioactivation to form a GSH conjugate that was identical to the one initially characterized from in vivo studies. Human P450 2A6 and rat P450 2C11 appeared to be the primary enzymes activating LTG in human and rat liver microsomes, respectively. Interindividual variation in the bioactivation of LTG was demonstrated with 20 individual human liver microsomes. Furthermore, it was shown that human epidermal keratinocytes were capable of forming the same GSH conjugate, suggesting that LTG could be bioactivated in skin cells. The results from these studies suggest that LTG has the potential to undergo hepatic and nonhepatic bioactivation, leading to a reactive aryl epoxide intermediate in human. The bioactivation of LTG in epidermal cells provides a possible explanation for the idiosyncratic cutaneous reactions associated with LTG therapy.

Characterization of cytochrome P450-mediated bioactivation of a compound containing the chemical scaffold, 4,5-dihydropyrazole-1-carboxylic acid-(4-chlorophenyl amide), to a chemically reactive p-chlorophenyl isocyanate intermediate in human liver microsomes.

Compound A (Cmpd A) was previously reported to form p-chlorophenyl isocyanate (CPIC), which was trapped by GSH to yield S- (N- [p-chlorophenyl] carbamoyl) glutathione adduct (SCPG) in the presence of human liver microsomes. In this study, P450 3A4 and 2C9 were demonstrated to be the enzymes mediating the activation of Cmpd A to CPIC in human liver microsomes based on inhibitory and correlation studies. Enzyme kinetics studies indicated that P450 3A4 was the primary enzyme involved in the activation of Cmpd A. In silico P450 3A4 active site docking of Cmpd A exhibited a low energy pose that orientated the pyrazole ring proximate to the heme iron atom, in which the distance between the C-3 and potential activated oxygen species was shown to be 3.4 A. Quantum molecular calculations showed that the electron density on C-3 was relatively higher than those on C-4 and C-5. These measurements suggested that the C-3 of Cmpd A was the preferred site of oxidation and hence predisposed Cmpd A in forming CPIC as previously proposed. The in silico prediction was corroborated by studies with the C-3 substituted analogue (methyl at C-3), which showed minimal conversion to CPIC in human liver microsomes. These results demonstrated a pivotal role for P450 3A4 in bioactivating Cmpd A by oxidizing at C-3 of the pyrazoline, hence facilitating the CPIC formation. Evidence of the bioactivation to CPIC in vivo was obtained by liquid chromatography-mass spectrometry (LC/MS) analysis of urine samples from human subjects administered a structural analogue of Cmpd A. The presence of S-(N-[p-chlorophenyl] carbamoyl) N-acetyl l-cysteine (SCPAC) as well as p-chlorophenyl aniline (CPA) was unequivocally demonstrated in the urine samples. The chemical scaffold, 4,5-dihydropyrazole-1-carboxylic acid-[(4-chlorophenyl)-amide], was demonstrated to possess potential metabolic liability in forming a reactive intermediate, CPIC, in humans. Bioactivation to CPIC may cause undesirable side effects through its reactivity and subsequent conversion to CPA, an established rodent carcinogen.

Nuclear magnetic resonance spectroscopy as a quantitative tool to determine the concentrations of biologically produced metabolites: implications in metabolites in safety testing.

Nuclear magnetic resonance (NMR) spectroscopy has traditionally been considered as an indispensable tool in elucidating structures of metabolites. With the advent of Fourier transform (FT) spectrometers, along with improvements in software and hardware (such as high-field magnets, cryoprobes, versatile pulse sequences, and solvent suppression techniques), NMR is increasingly being considered as a critical quantitative tool, despite its lower sensitivity as compared to mass spectrometry. A specific quantitative application of NMR is in determining the concentrations of biologically isolated metabolites, which could potentially be used as reference standards for further quantitative work by liquid chromatography/mass spectrometry. With the recent demands from regulatory agencies on quantitative information on metabolites, it is proposed that NMR will play a significant role in strategies aimed at addressing metabolite coverage in toxicological species. Traditionally, biologically isolated metabolites have not been considered as a way of generating "reference standards" for further quantitative work. However, because of the recent FDA guidance on safety testing of metabolites, one has to consider means of authenticating and quantitating biologically or nonbiologically generated metabolites. 1H NMR is being proposed as the method of choice, as it is able to be used as both a qualitative and a quantitative tool, hence allowing structure determination, purity check, and quantitative measurement of the isolated metabolite. In this publication, the application of NMR as a powerful and robust analytical technique in determining the concentrations of in vitro or in vivo isolated metabolites is discussed. Furthermore, to demonstrate the reliability and accuracy of metabolite concentrations determined by NMR, validation and cross-validation with gravimetric and mass spectrometric methods were conducted.

Obtaining exposures of metabolites in preclinical species through plasma pooling and quantitative NMR: addressing metabolites in safety testing (MIST) guidance without using radiolabeled compounds and chemically synthesized metabolite standards.

The recent guidance on "Safety Testing of Drug Metabolites" issued by the U.S. Food and Drug Administration, Center for Drug Evaluation and Research (CDER) has highlighted the importance of identifying and characterizing drug metabolites as early as possible in drug discovery and development. Furthermore, upon identifying significant circulating metabolites in human plasma, it has become important to demonstrate that these metabolites are present at an equal or greater exposure level (area under the curve, AUC) in any one of the preclinical species used in safety testing. Frequently, synthetic standards of metabolites are not available, and hence, obtaining their AUC values can be a challenge. In this report, we demonstrate how combinations of nuclear magnetic resonance (NMR) spectroscopy, liquid chromatography/ultraviolet/mass spectrometry (LC/UV/MS), and plasma pooling methods were used to obtain reliable AUC values of metabolites present in the plasma of preclinical species from short-term safety studies. Plasma pooling methods were compared to the traditional approaches of obtaining quantitative information on the levels of circulating metabolites in preclinical species. The exposure values obtained via sample pooling were comparable to those obtained by traditional methods of analyzing samples individually. In the absence of synthetic chemical standards, calculations of AUC values of metabolites, using either sample pooling or traditional approaches, were achieved through the use of UV detectors. In cases where the UV properties of metabolites were significantly different from their parent compounds, NMR was used as a quantitative tool to obtain exposure values. NMR was found to be useful in quantitating biologically produced metabolites, which could subsequently be used as reference compounds for further quantitative studies. The limitations of UV detectors to obtain exposure estimates are discussed. A practical solution is presented that will enable us to obtain a quantitative assessment of metabolite exposure in humans and coverage in toxicology species, hence, circumventing the use of radiolabeled compounds or authentic chemically synthesized standards of metabolites.

Application of in vitro Drug Metabolism Studies in Chemical Structure Optimization for the Treatment of Fibrodysplasia Ossificans Progressiva (FOP).

Currently no approved treatment exists for fibrodysplasia ossificans progressiva (FOP) patients, and disease progression results in severe restriction of joint function and premature mortality. LDN-193189 has been demonstrated to be efficacious in a mouse FOP disease model after oral administration. To support species selection for drug safety evaluation and to guide structure optimization for back-up compounds, in vitro metabolism of LDN-193189 was investigated in liver microsome and cytosol fractions of mouse, rat, dog, rabbit, monkey and human. Metabolism studies included analysis of reactive intermediate formation using glutathione and potassium cyanide (KCN) and analysis of non-P450 mediated metabolites in cytosol fractions of various species. Metabolite profiles and metabolic soft spots of LDN-193189 were elucidated using LC/UV and mass spectral techniques. The in vitro metabolism of LDN-193189 was significantly dependent on aldehyde oxidase, with formation of the major NIH-Q55 metabolite. The piperazinyl moiety of LDN-193189 was liable to NADPH-dependent metabolism which generated reactive iminium intermediates, as confirmed through KCN trapping experiments, and aniline metabolites (M337 and M380), which brought up potential drug safety concerns. Subsequently, strategies were employed to avoid metabolic liabilities leading to the synthesis of Compounds 1, 2, and 3. This study demonstrated the importance of metabolite identification for the discovery of novel and safe drug candidates for the treatment of FOP and helped medicinal chemists steer away from potential metabolic liabilities.

Application of stable isotope-labeled compounds in metabolism and in metabolism-mediated toxicity studies.

Stable isotope-labeled compounds have been synthesized and utilized by scientists from various areas of biomedical research during the last several decades. Compounds labeled with stable isotopes, such as deuterium and carbon-13, have been used effectively by drug metabolism scientists and toxicologists to gain better understanding of drugs' disposition and their potential role in target organ toxicities. The combination of stable isotope-labeling techniques with mass spectrometry and nuclear magnetic resonance (NMR) spectroscopy, which allows rapid acquisition and interpretation of data, has promoted greater use of these stable isotope-labeled compounds in absorption, distribution, metabolism, and excretion (ADME) studies. Examples of the use of stable isotope-labeled compounds in elucidating structures of metabolites and delineating complex metabolic pathways are presented in this review. The application of labeled compounds in mechanistic toxicity studies will be discussed by providing an example of how strategic placement of a deuterium atom in a drug molecule mitigated specific-specific renal toxicity. Other examples from the literature demonstrating the application of stable isotope-labeled compounds in understanding metabolism-mediated toxicities are presented. Furthermore, an example of how a stable isotope-labeled compound was utilized to better understand some of the gene changes in toxicogenomic studies is discussed. The interpretation of large sets of data produced from toxicogenomics studies can be a challenge. One approach that could be used to simplify interpretation of the data, especially from studies designed to link gene changes with the formation of reactive metabolites thought to be responsible for toxicities, is through the use of stable isotope-labeled compounds. This is a relatively unexplored territory and needs to be further investigated. The employment of analytical techniques, especially mass spectrometry and NMR, used in conjunction with stable isotope-labeled compounds to establish and understand mechanistic link between reactive metabolite formation, genomic, and proteomic changes and onset of toxicity is proposed. The use of stable isotope-labeled compounds in early human ADME studies as a way of identifying and possibly quantifying all drug-related components present in systemic circulation is suggested.

Bioactivation of a dihydropyrazole-1-carboxylic acid-(4-chlorophenyl amide) scaffold to a putative p-chlorophenyl isocyanate in rat liver microsomes and in vivo in rats.

Compound I (4,5-dihydropyrazole-1,5-dicarboxylic acid-1-[(4-chlorophenyl)-amide] 5-[(2-oxo-2 H-[1,3']bipyridinyl-6'-yl)-amide] was found to undergo metabolic activation in rat liver microsomes in the presence of NADPH. A reactive intermediate, postulated to be p-chlorophenyl isocyanate (CPIC), was trapped by GSH in vitro and characterized by liquid chromatography tandem mass spectrometry (LC/MS/MS). Subsequently, the structure of the GSH conjugate was confirmed by a comparison with a synthetic standard. The GSH conjugate was also found in the bile of rats that received an oral dose (10 mg/kg) of compound I. Further analyses of rat bile and urine using online electrochemical derivatization coupled to LC/MS demonstrated the presence of p-chlorophenyl aniline (CPA), a hydrolytic product of the intermediate isocyanate. This provided further evidence for the potential existence of CPIC. Approximately 7% of the dose was accounted by the products of CPIC, which included the GSH conjugate and CPA excreted in bile and urine. Multiple rat cytochrome P450 enzymes, including P450 1A, P450 2C, and P450 3A, appeared to be responsible for the activation of compound I to CPIC. The activation kinetics of compound I to CPIC in male rat liver microsomes exhibited a biphasic profile, indicative of at least two contributing P450 enzymes. One enzyme showed a small value of K m at 42 microM and a low V max of 66 pmol min (-1) mg (-1), while the other exhibited a large value of K m at 148 microM and a high V max of 1200 pmol min (-1) mg (-1). The formation of a putative CPIC intermediate, a carbamoylating species known to be capable of covalent binding to macromolecules, suggests a potential liability associated with the compound, particularly the dihydropyrazole-1-carboxylic acid-(4-chlorophenyl amide) scaffold, which appears to be responsible for the generation of CPIC. The mechanism of bioactivation to the putative CPIC is postulated to involve an initial P450-mediated hydroxylation of the pyrazoline at the 3 position followed by subsequent decomposition to CPIC. This mechanistic insight into the bioactivation allowed for the development of a rational structural modification strategy to mitigate or minimize the reactive metabolite formation. One of the approaches included the introduction of a metabolically stable substituent with electron-donating character at the 3 position of pyrazoline to block CPIC formation.

Inhibition of hepatobiliary transport as a predictive method for clinical hepatotoxicity of nefazodone.

Treatment with the antidepressant nefazodone has been associated with clinical idiosyncratic hepatotoxicty. Using membranes expressing human bile salt export pump (BSEP), human sandwich hepatocytes, and intact rats, we compared nefazodone and its marketed analogs, buspirone and trazodone. We found that nefazodone caused a strong inhibition of BSEP (IC(50) = 9 microM), inhibition of taurocholate efflux in human hepatocytes (IC(50) = 14 microM), and a transient increase in rat serum bile acids 1 h after oral drug administration. Buspirone or trazodone had no effect on biliary transport system. Nefazodone produced time- and concentration-dependent toxicity in human hepatocytes with IC(50) = 18 microM and 30 microM measured by inhibition of protein synthesis after 6 h and 24 h incubation, respectively. Toxicity was correlated with the amount of unmetabolized nefazodone. Partial recovery in toxicity by 24 h has been associated with metabolism of nefazodone to sulfate and glucuronide conjugates. The saturation of nefazodone metabolism resulted in sustained decrease in protein synthesis and cell death at 50 microM. The toxicity was not observed with buspirone or trazodone. Addition of 1-aminobenzotriazole (ABT), an inhibitor of CYP450, resulted in enhancement of nefazodone toxicity at 10 microM and was associated with accumulation of unmetabolized nefazodone. In human liver microsomes, ABT also prevented metabolism of nefazodone and formation of glutathione conjugates. We suggest that inhibition of bile acid transport by nefazodone is an indicator of potential hepatotoxicity. Our findings are consistent with the clinical experience and suggest that described methodology can be applied in the selection of nonhepatotoxic drug candidates.

A comprehensive listing of bioactivation pathways of organic functional groups.

The occurrence of idiosyncratic adverse drug reactions during late clinical trials or after a drug has been released can lead to a severe restriction in its use and even in its withdrawal. Metabolic activation of relatively inert functional groups to reactive electrophilic intermediates is considered to be an obligatory event in the etiology of many drug-induced adverse reactions. Therefore, a thorough examination of the biochemical reactivity of functional groups/structural motifs in all new drug candidates is essential from a safety standpoint. A major theme attempted in this review is the comprehensive cataloging of all of the known bioactivation pathways of functional groups or structural motifs commonly utilized in drug design efforts. Potential strategies in the detection of reactive intermediates in biochemical systems are also discussed. The intention of this review is not to "black list" functional groups or to immediately discard compounds based on their potential to form reactive metabolites, but rather to serve as a resource describing the structural diversity of these functionalities as well as experimental approaches that could be taken to evaluate whether a "structural alert" in a new drug candidate undergoes bioactivation to reactive metabolites.

Phenobarbital and phenytoin increased acetaminophen hepatotoxicity due to inhibition of UDP-glucuronosyltransferases in cultured human hepatocytes.

Here we present a preclinical model to assess drug-drug interactions due to inhibition of glucuronidation. Treatment with the antiepileptics phenobarbital (PB) or phenytoin (PH) has been associated with increased incidence of acetaminophen (APAP) hepatotoxicity in patients. In human hepatocytes, we found that the toxicity of APAP (5 mM) was increased by simultaneous treatment with phenobarbital (2 mM) or phenytoin (0.2 mM). In contrast, pretreatment with PB for 48 h prior to APAP treatment did not increase APAP toxicity unless both drugs were present simultaneously. Cells treated with APAP in combination with PB or PH experienced decreases in protein synthesis as early as 1 h, ultrastructural changes by 24 h, and release of liver enzymes by 48 h. Toxicity correlated with inhibition of APAP glucuronidation. PB or PH also inhibited APAP glucuronidation in rat and human liver microsomes and expressed human UGT1A6, 1A9, and 2B15. As with intact hepatocytes, PB and PH were neither hydroxylated nor glucuronidated, suggesting the direct inhibition of UGTs. Our findings suggest that, in multiple drug therapy, an inhibitory complex between UGT and one of the drugs can lead to decreased glucuronidation and increased systemic exposure and toxicity of a coadministered drug.

Liquid chromatography/mass spectrometry determination of endogenous plasma acetyl and palmitoyl carnitines as potential biomarkers of beta-oxidation in mice.

A robust bioanalytical method capable of measuring acetyl and palmitoyl carnitines was developed and validated. Application of hydrophilic interaction chromatography (HILIC) enabled retention of these highly polar and difficult to analyze compounds on a silica HPLC column. The chromatography was conducted with a high percentage of an organic component in the mobile phase, allowing high sensitivity for the pre-existing positively charged quaternary ammonium ions by electrospray ionization mass spectrometry. Successful application of the method to reliably quantify naturally occurring acyl carnitines in mouse plasma depended on the use of corresponding deuterated analogues. The specificity of the method, achieved through the use of stable isotope labeled compounds in combination with a mass spectral multiple reaction monitoring technique, permitted a non-invasive assessment of the overall change in the levels of these acyl carnitines in the plasma of intact animals administered peroxisome proliferator activated receptor (PPAR) agents. These acyl carnitines, as carriers of the corresponding long-chain fatty acids for transport into mitochondria, can be employed as potential biomarkers for significant alteration in the beta-oxidation process in an intact animal.

Transport, metabolism, and hepatotoxicity of flutamide, drug-drug interaction with acetaminophen involving phase I and phase II metabolites.

Treatment with flutamide has been associated with clinical hepatotoxicty. The toxicity, metabolism,and transport of flutamide were investigated using cultured human hepatocytes. Flutamide and its major metabolite, 2-hydroxyflutamide, caused an inhibition of taurocholate efflux in human hepatocytes with an IC50=75 microM and 110 microM, respectively. Treatment of hepatocytes with flutamide or 2-hydroxyflutamide for 24 h resulted in time- and concentration-dependent toxicity as assessed by inhibition of protein synthesis. Toxicity was greater after 1 h than after 24 h of treatment. Recovery in inhibition of protein synthesis by 24 h was attributed to the decreased presence of flutamide due to its metabolism. Flutamide was metabolized by hepatocytes to several metabolites, and formation of reactive intermediates of flutamide, as evidenced by the presence of glutathione-related adducts, was observed. Inhibition of flutamide metabolism by 1-aminobenzotriazole (ABT) resulted in enhancement of flutamide toxicity, which was associated with sustained levels of nonmetabolized drug. ABT also prevented the formation of reactive intermediates of flutamide. There was an additive toxicity when cells were treated with a combination of flutamide and 2-hydroxyflutamide. Simultaneous treatment with flutamide and acetaminophen (APAP) resulted in additive to synergistic toxic effects. Flutamide and APAP were found to have significant effects on each other's metabolism. Flutamide inhibited glucuronidation and sulfation of APAP, resulting in greater amounts of APAP available for bioactivation. APAP inhibited the hydroxylation of flutamide, and subsequent sulfation and acetylation of 4-nitro-3-(trifluoromethyl) aniline, a metabolite of flutamide. In summary, we suggest that inhibition of bile acid efflux by flutamide and its 2-hydroxy metabolite may play a role in flutamide-induced liver injury. Both flutamide and 2-hydroxyflutamide are responsible for cytotoxicity if not metabolized. The data also suggest a possible drug-drug interaction between flutamide and APAP, resulting in inhibition of flutamide metabolism and increased APAP bioactivation and toxicity.

Application of in-line liquid chromatography-accurate radioisotope counting-mass spectrometry (LC-ARC-MS) to evaluate metabolic profile of [3H]-mefenamic acid in rat plasma.

Profiling of rat plasma using a highly sensitive LC-ARC-MS technique showed that [(3)H] mefenamic acid was metabolized to several products, including a sulfate conjugate and a hydroxylated analogue as major metabolites. This technique of detecting low levels of radioactivity in plasma was superior to previously used methods, such as beta-RAM detectors.

Determination of degradation pathways and kinetics of acyl glucuronides by NMR spectroscopy.

Acyl glucuronides have been implicated in the toxicity of many xenobiotics and marketed drugs. These toxicities are hypothesized to be a consequence of covalent binding of the reactive forms of the acyl glucuronide to proteins. Reactive intermediates of the acyl glucuronide arise from the migration of the aglycone leading to other positional and stereoisomers under physiological conditions. In order to screen for the potential liabilities of these metabolites during the early phase of pharmaceutical development, an NMR method based on the disappearance of the anomeric resonance of the O-1-acyl glucuronide was used to monitor the degradation kinetics of 11 structurally diverse acyl glucuronides, including those produced from the known nonsteroidal anti-inflammatory drugs (NSAIDs). The acyl glucuronides were either chemically synthesized or were isolated from biological matrices (bile, urine, and liver microsomal extracts). The half-lives attained utilizing this method were found to be comparable to those reported in the literature. NMR analysis also enabled the delineation of the two possible pathways of degradation: acyl migration and hydrolytic cleavage. The previously characterized 1H resonances of acyl migrated products are quite distinguishable from those that arise from hydrolysis. The NMR method described here could be used to rank order acyl glucuronide forming discovery compounds based on the potential reactivity of the conjugates and their routes of decomposition under physiological conditions. Furthermore, we have shown that in vitro systems such as liver microsomal preparations can be used to generate sufficient quantities of acyl glucuronides from early discovery compounds for NMR characterization. This is particularly important, as we often have limited supply of early discovery compounds to conduct in vivo studies to generate sufficient quantities of acyl glucuronides for further characterization.

Species-specific, P450- and sulfotransferase-mediated novel ring contraction of a naphthyridine-N-oxide compound in cynomolgus monkey.

BMS-A78277 (1) is a 5,10-dihydrobenzo[beta][1,8]naphthyridine-N-oxide compound that resides in a class of novel non-nucleoside reverse transcriptase inhibitors (NNRTIs), displaying improved activity against clinically relevant mutants of HIV-1 and possessing pharmacokinetic profiles amenable to once-daily dosing. In the course of investigating the nonclinical metabolism of 1, a circulating metabolite specific to the cynomolgus monkey was identified and subsequently characterized as the carboxyindole metabolite 2. The present investigation describes the biotransformation of this NNRTI in cynomolgus monkey, one which results in a net ring contraction of 1. The use of mass spectrometry and high field NMR analysis aided in the structural characterization of metabolite 2, the source of which originated from the urine and bile of cynomolgus monkeys receiving oral doses of 1. Preparation of a synthetic standard of 2 not only provided ultimate structural confirmation but also afforded ample material for biological testing. The metabolism of 1 was investigated in monkey hepatocytes and hepatic subcellular fractions. While microsomes were incapable of generating metabolite 2, incubation of 1 in monkey S9 fractions as well as hepatocytes resulted in measurable levels of the carboxyindole metabolite. Consequently, incubation of 1 in monkey hepatocytes, which were suspended in media containing (18)O-labeled water, resulted in the incorporation of (18)O into the carboxyindole metabolite, 2. These data implicate a mechanism involving the bioactivation of 1 to an electrophilic intermediate that upon hydrolysis undergoes a concerted ring contraction, resulting in the formation of 2. Previously confined to discussions regarding the metabolism of natural products and select aliphatic heterocycles, the present investigation extends the discussion of metabolism-mediated ring contraction to aromatics such as the present naphthyridine compound, 1.