ABSTRACT:Absorption, metabolism, and excretion of paliperidone, an atypical antipsychotic, was studied in five healthy male subjects after a single dose of 1 mg of [ 14 C]paliperidone oral solution (ϳ16 Ci/ subject). One week after dosing, 88.4 to 93.8% (mean 91.1%) of the administered radioactivity was excreted: 77.1 to 87.1% (mean 79.6%) in urine and 6.8 to 14.4% (mean 11.4%) in the feces. Paliperidone was the major circulating compound (97% of the area under the plasma concentration-time curve at 24 h). No metabolites could be detected in plasma. Renal excretion was the major route of elimination with 59% of the dose excreted unchanged in urine. About half of the renal excretion occurred by active secretion. Unchanged drug was not detected in feces. Four metabolic pathways were identified as being involved in the elimination of paliperidone, each of which accounted for up to a maximum of 6.5% of the biotransformation of the total dose. Biotransformation of the drug occurred through oxidative N-dealkylation (formation of the acid metabolite M1), monohydroxylation of the alicyclic ring (M9), alcohol dehydrogenation (formation of the ketone metabolite M12), and benzisoxazole scission (formation of M11), the latter in combination with glucuronidation (M16) or alicyclic hydroxylation (M10). Unchanged drug, M1, M9, M12, and M16 were detected in urine; M10 and M11 were detected in feces. The monohydroxylated metabolite M9 was solely present in urine samples of extensive CYP2D6 metabolizers, whereas M10, another metabolite monohydroxylated at the alicyclic ring system, was present in feces of poor metabolizers as well. In conclusion, paliperidone is not metabolized extensively and is primarily renally excreted.

ABSTRACT:Absorption, metabolism, and excretion of darunavir, an inhibitor of human immunodeficiency virus protease, was studied in eight healthy male subjects after a single oral dose of 400 mg of [ 14 C]-darunavir given alone (unboosted subjects) or with ritonavir [100 mg b.i.d. 2 days before and 7 days after darunavir administration (boosted subjects)]. Plasma exposure to darunavir was 11-fold higher in boosted subjects. Total recoveries of radioactivity in urine and feces were 93.9 and 93.5% of administered radioactivity in unboosted and boosted subjects, respectively. The most radioactivity was recovered in feces (81.7% in unboosted subjects and 79.5% in boosted subjects, compared with 12.2 and 13.9% recovered in urine, respectively). Darunavir was extensively metabolized in unboosted subjects, mainly by carbamate hydrolysis, isobutyl aliphatic hydroxylation, and aniline aromatic hydroxylation and to a lesser extent by benzylic aromatic hydroxylation and glucuronidation. Total excretion of unchanged darunavir accounted for 8.0% of the dose in unboosted subjects. Boosting with ritonavir resulted in significant inhibition of carbamate hydrolysis, isobutyl aliphatic hydroxylation, and aniline aromatic hydroxylation but had no effect on aromatic hydroxylation at the benzylic moiety, whereas excretion of glucuronide metabolites was markedly increased but still represented a minor pathway. Total excretion of unchanged darunavir accounted for 48.8% of the administered dose in boosted subjects as a result of the inhibition of darunavir metabolism by ritonavir. Unchanged darunavir in urine accounted for 1.2% of the administered dose in unboosted subjects and 7.7% in boosted subjects, indicating a low renal clearance. Darunavir administered alone or with ritonavir was well tolerated.Darunavir (TMC114, Prezista; Tibotec BVBA, Mechelen, Belgium) is an inhibitor of the human immunodeficiency virus (HIV) protease (Fig. 1). Its molecular formula is C 27 H 37 N 3 O 7 S ⅐ C 2 H 5 OH and molecular weight is 593. Darunavir is metabolized mainly by cytochrome P450 isozyme 3A (CYP3A) . As observed with other protease inhibitor (PIs) that are CYP3A4 substrates (Cooper et al., 2003;Zeldin and Petruschke, 2004), administration of darunavir with lowdose ritonavir as a pharmacokinetic booster results in clinically relevant increases in the systemic exposure to darunavir. Ritonavir is a potent CYP3A4 inhibitor, and inhibition of this isozyme in the intestinal tract and liver, where CYP3A4 exerts its effect on first-pass metabolism, reduces the metabolism of the parent drug, with a consequent increase in exposure to the unchanged drug.Darunavir is therefore administered in combination with low-dose ritonavir, and a dosing regimen of 600/100 mg b.i.d., used together with other antiretroviral agents, has been shown to be effective in decreasing the HIV-1 viral load in antiretroviral treatment-experienced adults, such as those with HIV-1 strains resistant to more than one PI (Clotet et al., 2007). On this basis, darunavir has received reg...

In this open-label, single-center study, eight healthy men each received a single 500-mg dose of Doripenem is a new parenteral carbapenem antibiotic with broad-spectrum activity against gram-negative and gram-positive pathogens, including strains resistant to multiple antibiotic classes (1, 6, 7). It is indicated for adults in the treatment of complicated intra-abdominal infections and complicated urinary tract infections, including pyelonephritis. Doripenem exhibits in vitro activity against contemporary strains of gram-negative bacteria that are often responsible for serious, hospital-acquired infections, including Pseudomonas aeruginosa and extended-spectrum betalactamase-and AmpC beta-lactamase-producing Enterobacteriaceae (10-12, 15, 19). In addition, doripenem is less likely than meropenem or imipenem to select for carbapenem-resistant strains of Pseudomonas aeruginosa (17). Because carbapenems produce time-dependent bactericidal activity, the ability to deliver doripenem via prolonged infusion increases the time that drug concentrations are likely to remain above the MIC for the infecting pathogen (2, 5). This may be of critical importance for difficult-to-treat pathogens that are not susceptible to other carbapenems or for which the MICs are near the susceptibility limits of the drug.The present study was designed to characterize the disposition, metabolism, and excretion of doripenem in healthy men following a single 500-mg dose administered as a 1-h intravenous infusion, which is the standard doripenem dose indicated for treatment of subjects with serious bacterial infections. On the basis of clinical-trial data, the proposed dose of doripenem for treatment of moderate to severe infections is 500 mg administered by a 1-h or 4-h infusion (C. Lucasti, A. Jasovich, O. Umeh, J. Jiang, and K. Kaniga, presented at the 17th European Congress of Clinical Microbiology and Infectious Diseases, 2007; O. Malafaia, O. Umeh, and J. Jang, presented at the 46th Interscience Conference on Antimicrobial Agents and Chemotherapy, 2006; K. Naber, R. Redman, P. Kotey, L. Lorens, and K. Kaniga, presented at the 17th European Congress of Clinical Microbiology and Infectious Diseases, 2007). MATERIALS AND METHODSThe study protocol was reviewed and approved by an independent ethics committee. The study was conducted in accordance with the ethical principles of the Declaration of Helsinki and in compliance with good clinical practices and all applicable regulatory requirements. All subjects participating in the study provided written informed consent.Subjects. Eight healthy men characterized by a screening physical examination, medical history, vital signs, 12-lead electrocardiogram, laboratory testing, and normal renal function were enrolled. The study cohort had a median age of 20.5 years (range, 18 to 45 years), a median weight of 87.5 kg (range, 55 to 93 kg), and a median body mass index of 22.9 kg/m 2 (range, 19 to 28 kg/m 2 ); all were Caucasian. Eligible patients had not smoked for at least 6 months and agreed to refrain ...

Cell-based in vitro models are invaluable tools in elucidating the pharmacokinetic profile of a drug candidate during its drug discovery and development process. As biotransformation is one of the key determinants of a drug's disposition in the body, many in vitro models to study drug metabolism have been established, and others are still being developed and validated. This review is aimed at providing the reader with a concise overview of the characteristics and optimal application of established and emerging in vitro cell-based models to study human drug metabolism and induction of drug metabolising enzymes in the liver. The strengths and weaknesses of liver-derived models, such as primary hepatocytes, either freshly isolated or cryopreserved, and from adult or fetal donors, precision-cut liver slices, and cell lines, including immortalised cells, reporter cell lines, hepatocarcinoma-derived cell lines and recombinant cell lines, are discussed. Relevant cell culture configuration aspects as well as other models such as stem cell-derived hepatocyte-like cells and humanised animal models are also reviewed. The status of model development, their acceptance by health authorities and recommendations for the most appropriate use of the models are presented.

Trabectedin (YONDELIS) is a potent anticancer agent which was recently approved in Europe for the treatment of soft tissue sarcoma. The drug is currently also in clinical development for the treatment of ovarian carcinoma. In vitro experiments were conducted to investigate the hepatic metabolism of [(14)C]trabectedin in Cynomolgus monkey and human liver subcellular fractions. The biotransformation of trabectedin was qualitatively similar in 12,000 x g supernatants of both species, and all human metabolites were also produced by the monkey. The trabectedin metabolites were identified by QTOF mass spectrometry, and HPLC co-chromatography with reference compounds. Trabectedin was metabolized via different biotransformation pathways. Most of the metabolic conversions occurred at the trabectedin A domain including mono-oxidation and di-oxidation, carboxylic acid formation with and without additional oxidation, and demethylation either without (N-demethylation to ET-729) or with additional mono-, di- or tri-oxidation. Another metabolite resulted from O-demethylation at the trabectedin C subunit, and in addition, aliphatic ring opening of the methylene dioxybridge at the B domain was detected. Overall, demethylation and oxidation played a major role in phase I metabolism of the drug. Human cDNA expressed CYPs 1A2, 2A6, 2B6, 2C8, 2C9, 2C18, 2D6, 2E1, 3A4 and 3A5 in E. coli membranes, but not CYP1B1, 2C19, and 4A11 were able to metabolize [(14)C]trabectedin. Experiments with chemical inhibitors and CYP inhibitory antibodies indicated that, at therapeutic levels, CYP3A4 is the main human CYP isoform involved in trabectedin's hepatic metabolism. In monkey and human liver microsomes, trabectedin was not substantially metabolized by glucuronidation.

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A radioassay for specific determination of the rates of UDP-glucuronic acid-dependent conversion of bilirubin into the two isomeric (C-8, C-12) bilirubin monoglucuronides and bilirubin diglucuronide is described and illustrated by its application to rat liver microsomes. The method is based on measurement of the relative amounts of radiolabel in unesterified bilirubin and its mono- and di-esterified reaction products after incubation with [14C]bilirubin as substrate. This analysis is performed by the alkaline-methanolysis procedure, combined with one of two t.l.c. systems developed in order to enhance the sensitivity, accuracy and precision of the radioassay. Results for rates of total bilirubin glucuronide formation obtained with the new assay and the standard enzyme assay based on the ethyl anthranilate diazo-method were identical. However, the sensitivity of the latter technique is approx. 10-fold lower than that of the radioassay.

A study was made of the binding of 7-ethoxy-, 7-n-propoxy-and 7-n-pentoxy-coumarin to cytochrome P-4502 reconstituted into large unilamellar liposomes composed of a mixture of egg L-a-phosphatidylcholine, egg phosphatidylethanolamine and dipalmitoyl phosphatidic acid (2:1:0.06, by weight). The apparent spectral dissociation constants QPP-increased linearly with increasing proteoliposomal concentration. When both cytochrome P-4502 and NADPH:cytochrome P-450 reductase were reconstituted into liposomes, the apparent Michaelis constants KmPP for 0-dealkylation of 7-methoxy-, 7-ethoxy-and 7-n-propoxy-coumarin showed a similar dependence on the proteoliposomal concentration. The results were in accordance with models for kinetic or equilibrium processes in biphasic systems containing membrane-bound catalytic or acceptor sites, in which a linear solute partition in the bilayer membrane is postulated. The methyl, ethyl and n-propyl ether were readily dealkylated. However, the O-dealkylation rate of 7-nbutoxycoumarin was low and became very small for longer alkyl ethers. Both the effective dissociation constants and effective Michaelis constants decreased with elongation of the alkyl side chain of the coumarins. From plots of the apparent dissociation constants and apparent Michaelis constants against the lipid volume fraction of the proteoliposomes, the membrane partition coefficients for several homologues were calculated. When protein-free liposomes were added to 7-n-alkoxycoumarin solutions, the fluorescence intensity of the coumarins decreased and eventually became negligible in the presence of an excess of liposomal material. On the assumption that the overall fluorescence can be ascribed exclusively to the fraction of 7-n-alkoxycoumarin molecules present in the aqueous phase, partition coefficients for liposomal accumulation of the test compounds could be determined directly. For several coumarin ethers, comparable values were derived for the membrane partition coefficients from binding, kinetic and fluorescence intensity measurements. The change in free energy per methylene group of the 7-n-alkoxycoumarins for partitioning between n-octanol and buffer was significantly different from the value for liposome partitioning.