Drug design structural alert: formation of trifluoroacetaldehyde through N-dealkylation is linked to testicular lesions in rat.

In the process of drug design, it is important to consider potential structural alerts that may lead to toxicosis. This work illustrates how using trifluoroethane as a part of a novel chemical entity led to cytochrome P450 - mediated N-dealkylation and the formation of trifluoroacetaldehyde, a known testicular toxicant, in exploratory safety studies in rats. Testicular toxicosis was noted microscopically in a dose-dependent manner as measured by testicular spermatocytic degeneration and necrosis and excessive intratubular cellular debris in the epididymis. This apparent toxic effect correlated well with the dose-dependent formation of trifluoroacetaldehyde, identified from in vitro rat liver microsome metabolism studies. A similar safety study performed with an N-tetrazole substitution in place of the N-trifluoroethane showed no evidence of testicular injury, implicating further the role of trifluoroacetaldehyde in the testicular lesion observed. These results highlight the relevance of early metabolic and safety testing in assessing potential structural alerts in drug design.

Discovery of an Oral Potent Selective Inhibitor of Hematopoietic Prostaglandin D Synthase (HPGDS).

Hematopoietic prostaglandin D synthase (HPGDS) is primarly expressed in mast cells, antigen-presenting cells, and Th-2 cells. HPGDS converts PGH2 into PGD2, a mediator thought to play a pivotal role in airway allergy and inflammatory processes. In this letter, we report the discovery of an orally potent and selective inhibitor of HPGDS that reduces the antigen-induced response in allergic sheep.

Absolute difference of hepatobiliary transporter multidrug resistance-associated protein (MRP2/Mrp2) in liver tissues and isolated hepatocytes from rat, dog, monkey, and human.

We previously reported that hepatobiliary transporter multidrug resistance-associated protein (MRP2/Mrp2) is considered to be the major cause of the interspecies differences detected by efflux of fluorescent substrates in isolated hepatocytes. In the present study, the interspecies differences of MRP2/Mrp2 were first evaluated by quantitative real-time polymerase chain reaction and Western blotting. The mRNA levels were able to distinguish the difference among species with a rank order comparable with the corresponding activities observed, whereas the extents of the differences remained unknown. The cross-reactions of MRP2/Mrp2 protein of different species with anti-human MRP2 polyclonal antibody were found by Western blotting. However, because of the unknown binding affinity of antibody to MRP2/Mrp2 protein across species and lack of purified MRP2/Mrp2 proteins for calibration, the immunoblotting assay was excluded from the absolute quantification of MRP2/Mrp2 protein for multiple species. By using our newly developed liquid chromatography-tandem mass spectrometry quantification method, we were able to measure the absolute amount of MRP2/Mrp2 in liver tissues and isolated hepatocytes across species. Freshly isolated hepatocytes conserved MRP2/Mrp2 protein levels that are comparable with those in the liver tissues. The amount of Mrp2 in rat liver was approximately 10-fold higher than that in other species. Moreover, a significant loss of Mrp2 protein in the membrane fraction of rat cryopreserved hepatocytes was observed. Thus, the absolute differences of MRP2/Mrp2 levels in various species were determined, for the first time, by direct quantification. The results could potentially fill the translational gaps of in vitro/in vivo or preclinical species to human extrapolation of hepatobiliary elimination mediated by MRP2/Mrp2.

Gamma-glutamyltranspeptidase-mediated degradation of diclofenac-S-acyl-glutathione in vitro and in vivo in rat.

Diclofenac, a nonsteroidal antiinflammatory drug, is known to be metabolized to chemically reactive intermediates that transacylate GSH forming diclofenac-S-acyl-glutathione (D-SG) in vivo in rat and in vitro in rat and human hepatocytes. Recently, it was reported that the treatment of rats with diclofenac led to a substantial decrease in the activity of hepatic gamma-glutamyltranspeptidase (gamma-GT), an extracellular canalicular membrane enzyme. Because studies have indicated that D-SG is a chemically reactive transacylating species that is excreted into rat bile, we propose that D-SG formed in the liver may be a substrate for, and potential inhibitor of, hepatic gamma-GT. The present experiments were performed to investigate the ability of D-SG to be a substrate for gamma-GT in vivo in rat and in vitro with commercially available gamma-GT enzyme. We also examined the ability of D-SG to inhibit gamma-GT in vitro. Thus, LC-MS/MS analysis of bile extracts from diclofenac-dosed rats (200 mg/kg, iv) showed the presence of the gamma-GT-mediated D-SG degradation product diclofenac-N-acyl-cysteinylglycine (D- N-CG), where a total of approximately 8 microg was excreted 6 h postadministration. When D-SG (100 microM) was incubated with gamma-GT (1 unit/mL), the GSH adduct was degraded in a linear time-dependent fashion where approximately 94 microM D- N-CG was formed after 20 min of incubation. Dialysis studies showed that inhibition of gamma-GT by D-SG was completely reversible. Further inhibition studies showed that D-SG is a competitive inhibitor of the gamma-GT enzyme. Results from theses studies indicate that D-SG is a substrate for gamma-GT; however, the conjugate may not contribute significantly to the decrease in gamma-GT activity reported to occur in vivo in rat.

Enantioselective formation of ibuprofen-S-acyl-glutathione in vitro in incubations of ibuprofen with rat hepatocytes.

Ibuprofen is metabolized to chemically reactive ibuprofen-1- O-acyl-glucuronide (I-1- O-G) and ibuprofen- S-acyl-CoA (I-CoA) derivatives, which are proposed to mediate the formation of drug-protein adducts via the transacylation of protein nucleophiles. We examined the ability of ibuprofen to undergo enantioselective metabolism to ibuprofen- S-acyl-glutathione thioester (I-SG) in incubations with rat hepatocytes, where I-CoA formation is known to be highly enantioselective in favor of the (R)-(-)-ibuprofen isomer. We proposed that potential enantioselective transacylation of glutathione forming I-SG in favor of the (R)-(-)-isomer would reveal the importance of acyl-CoA formation, versus acyl glucuronidation, in the generation of reactive transacylating-type intermediates of the drug. Thus, when (R)-(-)- and (S)-(+)-ibuprofen (100 microM) were incubated with hepatocytes, the presence of I-CoA and I-SG was detected in incubation extracts by LC-MS/MS techniques. The formation of I-CoA and I-SG in hepatocyte incubations with (R)-(-)-ibuprofen was rapid and reached maximum concentrations of 2.6 microM and 1.3 nM, respectively, after 8-10 min of incubation. By contrast, incubations with (S)-(+)-ibuprofen resulted in 8% and 3.9% as much I-CoA and I-SG formation, respectively, compared to that in corresponding incubations with the (R)-(-)-isomer. Experiments with a pseudoracemic mixture of (R)-(-)-[3,3,3-(2)H3]- and (S)-(+)-ibuprofen showed that >99% of the I-SG detected in hepatocyte incubations contained deuterium and therefore was derived primarily from (R)-(-)-ibuprofen bioactivation. Inhibition of (R)-(-)-ibuprofen (10 microM) glucuronidation with (-)-borneol (100 microM) led to a 98% decrease in I-1-O-G formation; however, no decrease in I-SG production was observed. Coincubation with pivalic, valproic, or lauric acid (500 microM each) was shown to lead to a significant inhibition of I-CoA formation and a corresponding decrease in I-SG production. Results from these studies demonstrate that the reactive I-CoA derivative, and not the I-1-O-G metabolite, plays a central role in the transacylation of GSH in incubations with rat hepatocytes.

Identification of interspecies difference in efflux transporters of hepatocytes from dog, rat, monkey and human.

The large interspecies differences of hepatobiliary transport present a challenge for the allometric prediction of human biliary excretion for drug candidates primarily cleared via hepatobiliary secretion. In the present study, we determined the metabolic stabilities of common fluorescent substrates of hepatobiliary efflux transporters and developed a rapid efflux assay to determine the functional activities of MRP/Mrp, BCRP/Bcrp and P-gp in hepatocytes of four species. The specificities of transporter-mediated dye efflux were confirmed by selective transporter inhibitors. Among tested species, transporter-specific dye efflux kinetics was consistent between freshly isolated and cryopreserved hepatocytes. Hepatocyte elimination half-lives of MRP/Mrp substrates GS-MF and calcein were observed in the rank order of human>monkey>dog>rat. The fourfold higher MRP/Mrp substrate efflux rate of rat hepatocytes compared to human is likely due to the species-specific functional differences of MRP2/Mrp2 expressed on the canalicular membrane. We also observed efficient BCRP-mediated pheophorbide A (PhA) efflux by human and dog hepatocytes, while PhA extrusion in monkey and rat hepatocytes appeared limited. P-gp function measured by DiOC2(3) efflux was minimal in hepatocytes of all origins and no significant species differences were detected. Our results demonstrated marked differences in hepatocyte MRP/Mrp and BCRP/Bcrp activities across species, indicating that they may contribute to the species differences of in vivo hepatobiliary excretion. These results also suggest the potential utility of primary hepatocytes, either fresh or cryopreserved, as an in vitro model to predict interspecies differences in the biliary transport of MRP/Mrp and BCRP/Bcrp substrates.

Mechanistic studies on the bioactivation of diclofenac: identification of diclofenac-S-acyl-glutathione in vitro in incubations with rat and human hepatocytes.

Diclofenac, a nonsteroidal anti-inflammatory drug, is metabolized to diclofenac-1-O-acyl glucuronide (D-1-O-G), a chemically reactive conjugate that has been implicated as playing a role in the idiosyncratic hepatoxicity associated with its use. The present studies investigated the ability of diclofenac to be metabolized to diclofenac-S-acyl-glutathione thioester (D-SG) in vitro in incubations with rat and human hepatocytes and whether its formation is dependent on a transacylation-type reaction between D-1-O-G and glutathione. When diclofenac (100 microM) was incubated with hepatocytes, D-SG was detected in both rat and human incubation extracts by a sensitive LC-MS/MS technique. The initial formation rate of D-SG in rat and human hepatocyte incubations was rapid and reached maximum concentrations of 1 and 0.8 nM, respectively, after 4 min of incubation. By contrast, during incubations with rat hepatocytes, the formation of D-1-O-G increased over 30 min of incubation, reaching a maximum concentration of 14.6 microM. Co-incubation of diclofenac (50 microM) with (-)-borneol (400 microM), an inhibitor of glucuronidation, led to a 94% decrease in D-1-O-G formation, although no significant decrease in D-SG production was observed. Together, these results indicate that diclofenac becomes metabolically activated in vitro in rat and human hepatocytes to reactive acylating derivatives that transacylate glutathione forming D-SG, but which is not solely dependent on transacylation by the D-1-O-G metabolite. From these results, it is proposed that reactive acylating metabolites of diclofenac, besides D-1-O-G, may be significant in the protein acylation that occurs in vivo and therefore also be important with regard to the mechanism(s) of diclofenac-mediated idiosyncratic hepatotoxicity.

Studies on the chemical reactivity of diclofenac acyl glucuronide with glutathione: identification of diclofenac-S-acyl-glutathione in rat bile.

Diclofenac, a nonsteroidal anti-inflammatory drug, is metabolized to a reactive acyl glucuronide that has been proposed to mediate toxic adverse drug reactions associated with its use. In the present study, we examined the ability of diclofenac acyl glucuronide (D-1-O-G) to transacylate glutathione (GSH) in vitro in buffer and in vivo in rats. Thus, in vitro reactions of D-1-O-G (100 microM) with GSH (10 mM) at pH 7.4 and 37 degrees C showed a linear time-dependent formation of diclofenac-S-acyl-glutathione (D-SG, 3 microM/h) through 60 min of incubation, reaching a maximum of 3.7 microM after 2 h of incubation. The major reaction that occurred was acyl migration of D-1-O-G (t1/2, 54 min) to less reactive isomers. The D-SG thioester product was shown to be unstable by degrading primarily to 1-(2,6-dichlorophenyl)indolin-2-one and by hydrolysis to diclofenac. After administration of diclofenac to rats (200 mg/kg), bile was collected and analyzed for D-SG by liquid chromatography-tandem mass spectrometry. Results indicated the presence of D-SG, which was confirmed by coelution with synthetic standard and by its tandem mass spectrum. When the reactivity of D-SG (100 microM) was compared with D-1-O-G (100 microM) in vitro in reactions with N-acetylcysteine (NAC, 10 mM), results showed the quantitative reaction of D-SG with NAC after 30 min of incubation, whereas only approximately 1% of D-1-O-G reacted to form diclofenac-S-acyl-NAC at the same time point. Results from these studies indicate that GSH reacts with D-1-O-G in vitro, and presumably in vivo, to form D-SG, and that the product D-SG thioester is chemically more reactive in transacylation-type reactions than the D-1-O-G metabolite.

Identification of zomepirac-S-acyl-glutathione in vitro in incubations with rat hepatocytes and in vivo in rat bile.

Zomepirac (ZP), a nonsteroidal anti-inflammatory drug that was withdrawn from use, is metabolized to zomepirac-1-O-acyl-glucuronide (ZP-1-O-G), a chemically reactive conjugate that has been implicated in the toxicity of the drug. In the present studies, we investigated the ability of ZP to become bioactivated to reactive metabolites that transacylate glutathione (GSH) forming ZP-S-acyl-glutathione thioester (ZP-SG) in vitro and in vivo in rat. When ZP (100 microM) was incubated with rat hepatocytes, ZP-SG was detected in incubation extracts by a sensitive selected reaction monitoring liquid chromatography/tandem mass spectrometry (LC/MS-MS) technique. The initial formation rate of ZP-SG was rapid and reached a maximum concentration of 0.24 +/- 0.03 nM after 4 min of incubation, then decreased, in a fairly linear fashion, to 0.07 +/- 0.03 nM after 60 min of incubation. The product ZP-SG (1 microM) was shown to be unstable by undergoing rapid hydrolysis (apparent half-life approximately 0.8 min) in incubations with rat hepatocytes. After administration of ZP to a male Sprague-Dawley rat (100 mg/kg i.p.), bile was collected and analyzed for ZP-SG by LC/MS-MS. Results indicated the presence of ZP-SG in bile (6.7 microg excreted after 6 h of collection), which was confirmed by coelution with synthetic standard and by its tandem mass spectrum. Together, these results demonstrate that ZP becomes metabolically activated in vitro in rat hepatocytes and in vivo in rat to reactive acylating derivative(s), such as ZP-1-O-G, that transacylate GSH forming ZP-SG. Finally, we propose that ZP-SG thioester could be used as a marker derivative for mechanistic studies on the bioactivation of the drug.