A Lundahl

Drug metabolism of CYP3A4, CYP2C9 and CYP2D6 substrates in pigs and humans

Pigs are becoming increasingly used as a test animal both in pharmacological and toxicological assessment of new drug compounds. For interspecies comparisons and predictions it is important to characterize the expression and function of membrane transport and enzymatic proteins in pigs, particularly at a mechanistic level which will make extrapolation of observation between pig and man to be made with more confidence. The major objective of this report was to increase the integrative knowledge of drug metabolism in pigs and to compare with corresponding data from human liver microsomes. This was done by using human substrates of CYP3A4 (verapamil and testosterone), CYP2C9 (diclofenac) and CYP2D6 (dextromethorphan). In addition, the mRNA expression of important drug metabolizing enzymes and carrier-mediated transporters were assessed in intestine and liver tissues from pigs. It was shown that CYP3A4 activity is quantitatively comparable between the two species but data suggest that qualitative differences may exist. Verapamil showed similar metabolism pattern as in humans and the CYP3A4 inhibitor ketoconazole was able to inhibit the depletion of both R- and S-verapamil. A correlation between individual pig CYP3A mRNA expression and in vivo hepatic extraction ratio was established which indicates that CYP3A is the major determinant factor in both pigs and humans. However, investigations of the metabolism of testosterone resulted in qualitative different metabolite pattern between pigs and humans. The metabolism of diclofenac was very low in pig liver microsomes and did not correlate to corresponding activity in human liver microsomes. In contrast dextromethorphan exhibited a very extensive and rapid metabolism in pig liver microsomes compared to human data. Together with previously determined gene expression data it confirms that CYP2D6 substrates will be very rapidly metabolized in pigs. The mRNA data increased the knowledge of the interindividual variability and the relative expression of different enzymes and transporters in pig intestine and liver. In conclusion, this study has increased the understanding of similarities and differences between pig and human biotransformation of drugs by providing new data for four different model compounds.

Enterohepatic Disposition of Rosuvastatin in Pigs and the Impact of Concomitant Dosing with Cyclosporine and Gemfibrozil

The hepatobiliary transport and local disposition of rosuvastatin in pig were investigated, along with the impact of concomitant dosing with two known multiple transport inhibitors; cyclosporine and gemfibrozil. Rosuvastatin (80 mg) was administered as an intrajejunal bolus dose in treatments I, II, and III (TI, TII, and TIII, respectively; n = 6 per treatment). Cyclosporine (300 mg) and gemfibrozil (600 mg) were administered in addition to the rosuvastatin dose in TII and TIII, respectively. Cyclosporine was administered as a 2-h intravenous infusion and gemfibrozil as an intrajejunal bolus dose. In treatment IV (TIV, n = 4) 5.9 mg of rosuvastatin was administered as an intravenous bolus dose. The study was conducted using a pig model, which enabled plasma sampling from the portal (VP), hepatic (VH), and femoral veins and bile from the common hepatic duct. The biliary recoveries of the administered rosuvastatin dose were 9.0 ± 3.5 and 35.7 ± 15.6% in TI and TIV, respectively. Rosuvastatin was highly transported into bile as shown by the median AUCbile/AUCVH ratio in TI of 1770 (1640–11,300). Gemfibrozil did not have an effect on the plasma pharmacokinetics of rosuvastatin, most likely because the unbound inhibitor concentrations did not exceed the reported IC50 values. However, cyclosporine significantly reduced the hepatic extraction of rosuvastatin (TI, 0.89 ± 0.06; TII, 0.46 ± 0.13) and increased the AUCVP and AUCVH by 1.6- and 9.1-fold, respectively. In addition, the biliary exposure and fe, bile were reduced by ≈50%. The strong effect of cyclosporine was in accordance with inhibition of sinusoidal uptake transporters, such as members of the organic anion-transporting polypeptide family, rather than canalicular transporters.

The effect of St. John's wort on the pharmacokinetics, metabolism and biliary excretion of finasteride and its metabolites in healthy men

The aim of this study was to investigate what the consequences of induced drug metabolism, caused by St. Johns wort (SJW, Hypericum perforatum) treatment, would have on the plasma, biliary and urinary pharmacokinetics of finasteride and its two previously identified phase I metabolites (hydroxy-finasteride and carboxy-finasteride). Twelve healthy men were administered 5mg finasteride directly to the intestine via a catheter with a multi-channel tubing system, Loc-I-Gut, before and after 14 days SJW (300mg b.i.d, hyperforin 4%) treatment. Bile samples were withdrawn via the Loc-I-Gut device from the proximal jejunum. LC-ESI-MS/MS was used to analyze finasteride and its metabolites in plasma, bile and urine. HPLC-UV was used to analyze hyperforin in plasma. The herbal treatment significantly reduced the peak plasma concentration (C(max)), the area under the plasma concentration-time curve (AUC(0-24h)) and the elimination half-life (t(1/2)) of finasteride. The geometric mean ratios (90% CI) were 0.42 (0.36-0.49), 0.66 (0.56-0.79) and 0.54 (0.48-0.61), respectively. Finasteride was excreted unchanged to a minor extent into bile and urine. Hydroxy-finasteride was not detected in plasma, bile or urine. Carboxy-finasteride was quantified in all three compartments and its plasma pharmacokinetics was significantly affected by SJW treatment. Hyperforin concentration in plasma was 21+/-7ng/ml approximately 12h after the last dose of the 14 days SJW treatment. In conclusion, SJW treatment for 2 weeks induced the metabolism of finasteride and caused a reduced plasma exposure of the drug. New knowledge was gained about the biliary and urinary excretion or the drug and its metabolites.

Identification of Finasteride Metabolites in Human Bile and Urine by High-Performance Liquid Chromatography/Tandem Mass Spectrometry

The objective of this study was to further investigate the metabolism of the 5α-reductase inhibitor, finasteride, and to identify previously unknown phase I and phase II metabolites in vitro and in vivo in human bile and urine. Healthy volunteers were given 5 mg of finasteride, directly to the intestine, and bile and urine were collected for 3 and 24 h, respectively. A single-pass perfusion technique, Loc-I-Gut, was used for drug administration and bile collection from the proximal jejunum, distal to papilla of Vater. Incubations with human liver microsomes/S9 fractions and different cofactors were performed with finasteride and the previously known metabolites, ω-hydroxy finasteride (M1) and finasteride-ω-oic acid (M3). Liquid chromatography coupled to triple quadrupole mass spectrometry (MS) with positive/negative electrospray ionization and ion trap with MSn measurements were used for structural investigations and identification of metabolites. Two hydroxy metabolites of finasteride, other than M1, and one intact hydroxy finasteride glucuronide were identified in vitro and in bile and urine. The glucuronide and at least one of the hydroxy metabolites were previously unidentified. M1 and M3 were glucuronidated in vitro by specific human UDP-glucuronosyltransferases, UGT1A4 and UGT1A3, respectively. M1 glucuronide was not identified in vivo, and M3 glucuronide, an acyl glucuronide, was present in low amounts in bile from a few individuals. In conclusion, previously undescribed metabolites were identified, in vitro and in human urine and bile. Bile collection using the Loc-I-Gut technique followed by sensitive mass spectrometry analysis led to the discovery of novel, both phase I and phase II, finasteride metabolites in human bile.

In Vivo Investigation in Pigs of Intestinal Absorption, Hepatobiliary Disposition, and Metabolism of the 5α-Reductase Inhibitor Finasteride and the Effects of Coadministered Ketoconazole

The overall aim of this detailed investigation of the pharmacokinetics (PK) and metabolism of finasteride in pigs was to improve understanding of in vivo PK for this drug and its metabolites. Specific aims were to examine the effects of ketoconazole coadministration on the PK in three plasma compartments (the portal, hepatic, and femoral veins), bile, and urine and to use these data to study in detail the intestinal absorption and the liver extraction ratio and apply a semiphysiological based PK model to the data. The pigs received an intrajejunal dose of finasteride (0.8 mg/kg) either alone (n = 5) or together with ketoconazole (10 mg/kg) (n = 5) or an intravenous dose (0.2 mg/kg) (n = 3). Plasma, bile, and urine (collected from 0 to 6 h) were analyzed with ultraperformance liquid chromatography-tandem mass spectrometry. Ketoconazole increased the bioavailability of finasteride from 0.36 ± 0.23 to 0.91 ± 0.1 (p < 0.05) and the terminal half-life from 1.6 ± 0.4 to 4.0 ± 1.1 h (p < 0.05). From deconvolution, it was found that the absorption rate from the intestine to the portal vein was rapid, and the product of the fraction absorbed and the fraction that escaped gut wall metabolism was high (fa · FG ∼1). Interestingly, the apparent absorption rate constant (ka) to the femoral vein was lower than that to the portal vein, probably because of binding and distribution within the liver. The liver extraction ratio was time-dependent and varied with the two routes of administration. After intrajejunal administration, from 1 to 6 h, the liver extraction ratio was significantly (p < 0.05) reduced by ketoconazole treatment from intermediate (0.41 ± 0.21) to low (0.21 ± 0.10).

Effects of Ketoconazole on the In Vivo Biotransformation and Hepatobiliary Transport of the Thrombin Inhibitor AZD0837 in Pigs

Ketoconazole has been shown in clinical trials to increase the plasma exposure of the oral anticoagulant prodrug AZD0837 [(2S)-N-{4- [(Z)-amino(methoxyimino)methyl]benzyl}-1-{(2R)-2-[3-chloro-5-(difluoromethoxy)phenyl]-2-hydroxyethanoyl}-azetidine-2-carboxamide] and its active metabolite, AR-H067637 [(2S)-N-{4-[amino(imino)methyl]benzyl}-1-{(2R)-2-[3-chloro-5-(difluoromethoxy)phenyl]-2-hydroxyethanoyl}-azetidine-2-carboxamide]. To investigate the biotransformation of AZD0837 and the effect of ketoconazole on this process, we used an experimental model in pigs that allows repeated sampling from three blood vessels, the bile duct, and a perfused intestinal segment. The pigs received AZD0837 (500 mg) given enterally either alone (n = 5) or together with single-dose ketoconazole (600 mg) (n = 6). The prodrug (n = 2) and its active metabolite (n = 2) were also administered intravenously to provide reference doses. The plasma data revealed considerable interindividual variation in the exposure of the prodrug, intermediate metabolite, and active metabolite. However, AR-H067637 was detected at very high concentrations in the bile with low variability (Aebile = 53 ± 6% of the enteral dose), showing that the compound had indeed been formed in all of the animals and efficiently transported into the bile canaliculi. Concomitant dosing with ketoconazole increased the area under the plasma concentration-time curve for AZD0837 (by 99%) and for AR-H067637 (by 51%). The effect on the prodrug most likely arose from inhibited CYP3A-mediated metabolism. Reduced metabolism also seemed to explain the increased plasma exposure of the active compound because ketoconazole prolonged the terminal half-life with no apparent effect on the extensive biliary excretion and biliary clearance. These in vivo results were supported by in vitro depletion experiments for AR-H067637 in pig liver microsomes with and without the addition of ketoconazole.

High-resolution mass spectrometric investigation of the phase I and II metabolites of finasteride in pig plasma, urine and bile.

Abstract 1. The metabolite profile of the 5α-reductase type II inhibitor finasteride has been studied in pig plasma, urine and bile using high-resolution mass spectrometry. The porcine biotransformation products were compared to those formed by human liver microsomes and to literature data of recently identified human in vivo metabolites. The objective of this study was to gain further evidence for the validity of using pigs for advanced, invasive drug–drug interaction studies that are not possible to perform in humans. 2. The use of high-resolution mass spectrometry with accurate mass measurements enabled identification of the metabolites by calculation of their elemental compositions as well as their fragmentation patterns. 3. There was an excellent match between the porcine and human metabolic profiles, corroborating the pig as a model of human drug metabolism. The glucuronides of the two recently described human hydroxylated metabolites MX and MY and the carboxylated metabolite M3 were identified as the major biotransformation products of finasteride in pig urine and bile. 4. Furthermore, the CYP enzymes involved in the formation of the hydroxylated metabolites were characterized. Human recombinant CYP3A4 could produce the two major hydroxylated metabolites MX and MY, whereas human recombinant CYP2D6 formed MY only.