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Although intestinal metabolism plays an important role in drug disposition, early predictions of human outcomes are challenging, in part because of limitations of available in vitro models. To address this, we have evaluated three in vitro models of human intestine (microsomes, permeabilized enterocytes, and cryopreserved intestinal mucosal epithelium) as tools to assess intestinal metabolism and estimate the fraction escaping gut metabolism (fg) in drug discovery. The models were tested with a chemically diverse set of 32 compounds, including substrates for oxidoreductive, hydrolytic, and conjugative enzymes. Liquid chromatography–high-resolution mass spectrometry was used to quantify substrate disappearance [intrinsic clearance (CLint)] and qualify metabolite formation (quantitative-qualitative bioanalysis). Fraction unbound in the incubation (fu,inc) was determined by rapid equilibrium dialysis. Measured in vitro results (CLint and fu,inc) were supplemented with literature data [passive Caco-2 apical to basolateral permeability, enterocyte blood flow, and intestinal surface area (A)] and combined using a midazolam-calibrated Qgut model to predict human fg values. All three models showed reliable CYP and UDP-glucuronosyltransferase activities, but enterocytes and mucosa may offer advantages for low-clearance compounds and alternative pathways (e.g., sulfation, hydrolases, and flavin-containing monooxigenases). Early predictions of human fg values were acceptable for the high-fg compounds (arbitrarily fg > 0.7). However, predictions of low- and moderate-fg values (arbitrarily fg Significance Statement We found that cellular models of the human gut (permeabilized enterocytes and cryopreserved intestinal mucosa) offer an alternative to and potential advantage over intestinal microsomes in studies of drug metabolism, particularly for low-clearance compounds and alternative pathways (e.g., sulfation, hydrolases, and flavin-containing monooxigenases). The predictivity of human fraction escaping gut metabolism for common CYP and UDP-glucuronosyltransferase substrates based on the Qgut model is still limited, however, and appropriate further evaluation is recommended.

Background: Although the liver is the primary organ of drug metabolism, the lungs also contain drug-metabolizing enzymes and may, therefore, contribute to the elimination of drugs. In this investigation, the Precision-cut Lung Slice (PCLS) technique was standardized with the aims of characterizing and comparing rat and human pulmonary drug metabolizing activity. Method: Due to the limited availability of human lung tissue, standardization of the PCLS method was performed with rat lung tissue. Pulmonary enzymatic activity was found to vary significantly with rat age and rat strain. The Dynamic Organ Culture (DOC) system was superior to well-plates for tissue incubations, while oxygen supply appeared to have a limited impact within the 4h incubation period used here. Results: The metabolism of a range of phase I and phase II probe substrates was assessed in rat and human lung preparations. Cytochrome P450 (CYP) activity was relatively low in both species, whereas phase II activity appeared to be more significant.Conclusion:PCLS is a promising tool for the investigation of pulmonary drug metabolism. The data indicates that pulmonary CYP activity is relatively low and that there are significant differences in enzyme activity between rat and human lung.

1. AFQ056 phenotyping results indicate that CYP1A1 is responsible for the formation of the oxidative metabolite, M3. In line with the predominant assumption that CYP1A1 is mainly expressed in extrahepatic tissues, only traces of M3 were detected in hepatic systems. The aim of this study was to investigate the pulmonary CYP1A1 mediated metabolism of AFQ056 in rat.2. Western blot analysis confirmed that CYP1A1 is expressed in rat lung albeit at low levels. M3 formation was clearly observed in recombinant rat CYP1A1, lung microsomes and lung tissue slices and was strongly inhibited by ketoconazole in the incubations. As CYP3A4 and CYP2C9 metabolites were only observed at trace levels, we concluded that the reduced M3 formation was due to CYP1A1 inhibition.3. AFQ056 lung clearance (CLlung) as estimated from in vitro data was predicted to be negligible (

Aldehyde oxidase (AO) catalyzes oxidations of azaheterocycles and aldehydes, amide hydrolysis, and diverse reductions. AO substrates are rare among marketed drugs, and many candidates failed due to poor pharmacokinetics, interspecies differences, and adverse effects. As most issues arise from complex and poorly understood AO biology, an effective solution is to stop or decrease AO metabolism. This perspective focuses on rational drug design approaches to modulate AO-mediated metabolism in drug discovery. AO biological aspects are also covered, as they are complementary to chemical design and important when selecting the experimental system for risk assessment. The authors' recommendation is an early consideration of AO-mediated metabolism supported by computational and in vitro experimental methods but not an automatic avoidance of AO structural flags, many of which are versatile and valuable building blocks. Preferably, consideration of AO-mediated metabolism should be part of the multiparametric drug optimization process, with the goal to improve overall drug-like properties.

Regulatory Guidance documents ICH Q3A (R2) and ICH Q3B (R2) state that "impurities that are also significant metabolites present in animal and/or human studies are generally considered qualified". However, no guidance is provided regarding data requirements for qualification, nor is a definition of the term "significant metabolite" provided. An opportunity is provided to define those categories and potentially avoid separate toxicity studies to qualify impurities. This can reduce cost, animal use and time, and avoid delays in drug development progression. If the concentration or amount of a metabolite, in animals or human, is similar to that of the known, structurally identical impurity (arising from the administered test material), the qualification of the impurity on the grounds of it also being a metabolite is justified. We propose two complementary approaches to support conclusions to this effect: 1) demonstrate that the impurity is formed by metabolism in animals and/or man, based preferably on plasma exposures or, alternatively, amounts excreted in urine, and, where appropriate, 2) show that animal exposure to (or amount of) the impurity/metabolite is equal or greater in animals than in humans. An important factor of both assessments is the maximum theoretical concentration (or amount) (MTC or MTA) of the impurity/metabolite achievable from the administered dose and recommendations on the estimation of the MTC and MTA are presented.

The expression of flavin-containing monooxygenase (FMO) varies extensively between human and commonly used preclinical species such as rat and mouse. The aim of this study was to investigate the pulmonary FMO activity in rat using benzydamine. Furthermore, the contribution of rat lung to the clearance of benzydamine was investigated using an in vivo pulmonary extraction model.Benzydamine N-oxygenation was observed in lung microsomes and lung slices. Thermal inactivation of FMO and CYP inhibition suggested that rat pulmonary N-oxygenation is predominantly FMO mediated while any contribution from CYPs is negligible.The predicted lung clearance (CLlung) estimated from microsomes and slices was 16 ± 0.6 and 2.1 ± 0.3 mL/min/kg, respectively. The results from in vivo pulmonary extraction indicated no pulmonary extraction following intravenous and intra-arterial dosing to rats. Interestingly, the predicted CLlung using rat lung microsomes corresponded to approximately 35% of rat CLliver suggesting that ...

RO5263397 [(S)-4-(3-fluoro-2-methyl-phenyl)-4,5-dihydro-oxazol-2-ylamine], a new compound that showed promising results in animal models of schizophrenia, is mainly metabolized in humans by N-glucuronidation. Enzyme studies, using the (then) available commercial uridine 5'-diphosphate-glucuronosyltransferases (UGTs), suggested that UGT1A4 is responsible for its conjugation. In the first clinical trial, in which RO5263397 was administered orally to healthy human volunteers, a 136-fold above-average systemic exposure to the parent compound was found in one of the participants. Further administration in this trial identified two more such poor metabolizers, all three of African origin. Additional in vitro studies with recombinant UGTs showed that the contribution of UGT2B10 to RO5263397 glucuronidation is much higher than UGT1A4 at clinically relevant concentrations. DNA sequencing in all of these poor metabolizers identified a previously uncharacterized splice site mutation that prevents assembly of full-length UGT2B10 mRNA and thus functional UGT2B10 protein expression. Further DNA database analyses revealed the UGT2B10 splice site mutation to be highly frequent in individuals of African origin (45%), moderately frequent in Asians (8%) and almost unrepresented in Caucasians (1%). A prospective study using hepatocytes from 20 individual African donors demonstrated a100-fold lower intrinsic clearance of RO5263397 in cells homozygous for the splice site variant allele. Our results highlight the need to include UGT2B10 when screening the human UGTs for the enzymes involved in the glucuronidation of a new compound, particularly when there is a possibility of N-glucuronidation. Moreover, this study demonstrates the importance of considering different ethnicities during drug development.

The presence of bovine serum albumin (BSA) largely modulates the enzyme kinetics parameters of the human UDP-glucuronosyltransferase (UGT) 1A9, increasing both the apparent aglycone substrate affinity of the enzyme and its limiting reaction velocity (Drug Metab Dispos 39:2117-2129, 2011). For a better understanding of the BSA effects and an examination of whether its presence changes the catalytic mechanism, we have studied the enzyme kinetics of 4-methylumbelliferone glucuronidation by UGT1A9 in the presence and absence of 0.1% BSA, using bisubstrate enzyme kinetic experiments, in both the forward and reverse directions, as well as product and dead-end inhibition. The combined results strongly suggest that the reaction mechanism of UGT1A9, and presumably other human UGTs as well, involves the formation of a compulsory-order ternary-complex, with UDP-α-d-glucuronic acid (UDPGA) as the first binding substrate. Based on the enzyme kinetic parameters measured for the forward and reverse reactions, the equilibrium constant of the overall reaction was calculated (Keq = 574) and the relative magnitudes of the reaction rate constants were elucidated. The inclusion of BSA in the bisubstrate kinetic experiments quantitatively changed the apparent enzyme kinetic parameters, presumably by removing internal inhibitors that bind to the binary enzyme-UDPGA (E-UDPGA) complex, as well as to the ternary E-UDPGA-aglycone complex. Nevertheless, the underlying compulsory-order ternary-complex mechanism with UDPGA binding first is the same in both the absence and presence of BSA. The results offer a novel understanding of UGT enzyme kinetic mechanism and BSA effects.

MRP2 is an efflux transporter that is expressed mainly in the canalicular membrane of hepatocytes, where it expels polar and ionic compounds into the bile. MRP2 is also present in the apical membrane of enterocytes and epithelial cells of proximal tubules of the kidney. Inhibition of MRP2 transport can lead to the accumulation of metabolites and other MRP2 substrates up to toxic levels in these cells. The transport properties of MRP2 are frequently studied with the vesicular transport assay. The assay identifies compounds that interact with MRP2 by measuring the effect of a compound on the transport of a radioactively labeled or fluorescent probe. We have compared the effect of eight selected test compounds (quercetin, disopyramide, paracetamol, indomethacin, diclofenac, estrone-3-sulfate, budesonide, and thioridazine) on the MRP2-mediated transport of three commonly used probes: 5(6)-carboxy-2,7-dichlorofluorescein, leukotriene C4 and estradiol-17-β-d-glucuronide (E217βG). Five of the test compounds had different probe-dependent effects on the MRP2-mediated transport, suggesting differences in the transport mechanism of the probes. Our results underline the complexity of substrate recognition by these efflux transporters and the difficulties in directly comparing results obtained with different assays, especially when different probes are used.

The human UDP-glucuronosyltransferase (UGT) enzymes UGT1A9 and UGT2B7 play important roles in the hepatic glucuronidation of many drugs. The presence of bovine serum albumin (BSA) during in vitro assays was recently reported to lower the K(m) values of both these UGTs for their aglycone substrates without affecting the corresponding V(max) values. Nonetheless, using the specific substrates entacapone and zidovudine (AZT) for UGT1A9 and UGT2B7, respectively, and using an improved ultrafiltration method for measuring drug binding to BSA and to biological membranes, we found that the presence of BSA during the glucuronidation reaction leads to a large increase in the V(max) value of UGT1A9, in addition to lowering its K(m) value. On the other hand, in the case of UGT2B7, our results agree with the previously described effect of BSA, namely lowering the K(m) value without a large effect on the enzyme's V(max) value. The unexpected BSA effect on UGT1A9 was independent of the expression system because it was found in a recombinant enzyme that was expressed in baculovirus-infected insect cells as well as in the native enzyme in human liver microsomes. Moreover, the effect of BSA on the kinetics of 4-methylumbelliferone glucuronidation by recombinant UGT1A9 was similar to its effect on entacapone glucuronidation. Contrary to the aglycone substrates, the effect of BSA on the apparent K(m) of UGT1A9 for the cosubstrate UDP-α-D-glucuronic acid was nonsignificant. Our findings call for further investigations of the BSA effects on different UGTs and the inhibitors that it may remove.

A small library of 3- and 4-pyridyl-substituted pyrroles was prepared from N-acylated amino acids. Nicotinoyl or isonicotinoyl chloride was used for the N-acylation of benzyl esters of amino acids. Debenzylation by palladium-catalyzed hydrogenation gave N-acylated amino acids. Dehydration of the acylated amino acids gave cyclic intermediates, munchnones or azlactones, which were treated in situ with alkynes in 1,3-dipolar cycloadditions. The starting materials were prepared in a parallel fashion, and microwave irradiation was used to facilitate the cycloaddition reactions. The regiochemistry of the cycloaddition was studied.

Although skin is the largest organ of the human body, cutaneous drug metabolism is often overlooked, and existing experimental models are insufficiently validated. This proof-of-concept study investigated phase II biotransformation of 11 test substrates in fresh full-thickness human skin explants, a model containing all skin cell types. Results show that skin explants have significant capacity for glucuronidation, sulfation, N-acetylation, catechol methylation, and glutathione conjugation. Novel skin metabolites were identified, including acyl glucuronides of indomethacin and diclofenac, glucuronides of 17β-estradiol, N-acetylprocainamide, and methoxy derivatives of 4-nitrocatechol and 2,3-dihydroxynaphthalene. Measured activities for 10 μM substrate incubations spanned a 1000-fold: from the highest 4.758 pmol·mg skin(-1)·h(-1) for p-toluidine N-acetylation to the lowest 0.006 pmol·mg skin(-1)·h(-1) for 17β-estradiol 17-glucuronidation. Interindividual variability was 1.4- to 13.0-fold, the highest being 4-methylumbelliferone and diclofenac glucuronidation. Reaction rates were generally linear up to 4 hours, although 24-hour incubations enabled detection of metabolites in trace amounts. All reactions were unaffected by the inclusion of cosubstrates, and freezing of the fresh skin led to loss of glucuronidation activity. The predicted whole-skin intrinsic metabolic clearances were significantly lower compared with corresponding whole-liver intrinsic clearances, suggesting a relatively limited contribution of the skin to the body's total systemic phase II enzyme-mediated metabolic clearance. Nevertheless, the fresh full-thickness skin explants represent a suitable model to study cutaneous phase II metabolism not only in drug elimination but also in toxicity, as formation of acyl glucuronides and sulfate conjugates could play a role in skin adverse reactions.

Human aldehyde oxidase (AO) is a molybdoflavoenzyme that commonly oxidizes azaheterocycles in therapeutic drugs. Although high metabolic clearance by AO resulted in several drug failures, existing in vitro-in vivo correlations are often poor and the extrahepatic role of AO practically unknown. This study investigated enzymatic activity of AO in fresh human skin, the largest organ of the body, frequently exposed to therapeutic drugs and xenobiotics. Fresh, full-thickness human skin was obtained from 13 individual donors and assayed with two specific AO substrates: carbazeran and zoniporide. Human skin explants from all donors metabolized carbazeran to 4-hydroxycarbazeran and zoniporide to 2-oxo-zoniporide. Average rates of carbazeran and zoniporide hydroxylations were 1.301 and 0.164 pmol⋅mg skin(-1)⋅h(-1), resulting in 13 and 2% substrate turnover, respectively, after 24 hours of incubation with 10 μM substrate. Hydroxylation activities for the two substrates were significantly correlated (r(2) = 0.769), with interindividual variability ranging from 3-fold (zoniporide) to 6-fold (carbazeran). Inclusion of hydralazine, an irreversible inhibitor of AO, resulted in concentration-dependent decrease of hydroxylation activities, exceeding 90% inhibition of carbazeran 4-hydroxylation at 100 μM inhibitor. Reaction rates were linear up to 4 hours and well described by Michaelis-Menten enzyme kinetics. Comparison of carbazeran and zoniporide hydroxylation with rates of triclosan glucuronidation and sulfation and p-toluidine N-acetylation showed that cutaneous AO activity is comparable to tested phase II metabolic reactions, indicating a significant role of AO in cutaneous drug metabolism. To our best knowledge, this is the first report of AO enzymatic activity in human skin.

We have examined the glucuronidation of psilocin, a hallucinogenic indole alkaloid, by the 19 recombinant human UDP-glucuronosyltransferases (UGTs) of subfamilies 1A, 2A, and 2B. The glucuronidation of 4-hydroxyindole, a related indole that lacks the N,N-dimethylaminoethyl side chain, was studied as well. UGT1A10 exhibited the highest psilocin glucuronidation activity, whereas the activities of UGTs 1A9, 1A8, 1A7, and 1A6 were significantly lower. On the other hand, UGT1A6 was by far the most active enzyme mediating 4-hydroxyindole glucuronidation, whereas the activities of UGTs 1A7-1A10 toward 4-hydroxyindole resembled their respective psilocin glucuronidation rates. Psilocin glucuronidation by UGT1A10 followed Michaelis-Menten kinetics in which psilocin is a low-affinity high-turnover substrate (K(m) = 3.8 mM; V(max) = 2.5 nmol/min/mg). The kinetics of psilocin glucuronidation by UGT1A9 was more complex and may be best described by biphasic kinetics with both intermediate (K(m1) = 1.0 mM) and very low affinity components. The glucuronidation of 4-hydroxyindole by UGT1A6 exhibited higher affinity (K(m) = 178 microM) and strong substrate inhibition. Experiments with human liver and intestinal microsomes (HLM and HIM, respectively) revealed similar psilocin glucuronidation activity in both samples, but a much higher 4-hydroxyindole glucuronidation rate was found in HLM versus HIM. The expression levels of UGTs 1A6-1A10 in different tissues were studied by quantitative real-time-PCR, and the results, together with the activity assays findings, suggest that whereas psilocin may be subjected to extensive glucuronidation by UGT1A10 in the small intestine, UGT1A9 is likely the main contributor to its glucuronidation once it has been absorbed into the circulation.

A small library of 3- and 4-pyridyl-substituted pyrroles was prepared from N-acylated amino acids. Nicotinoyl or isonicotinoyl chloride was used for the N-acylation of benzyl esters of amino acids. Debenzylation by palladium-catalyzed hydrogenation gave N-acylated amino acids. Dehydration of the acylated amino acids gave cyclic intermediates, munchnones or azlactones, which were treated in situ with alkynes in 1,3-dipolar cycloadditions. The starting materials were prepared in a parallel fashion, and microwave irradiation was used to facilitate the cycloaddition reactions. The regiochemistry of the cycloaddition was studied.