Discovery of Orally Bioavailable and Liver-Targeted Hypoxia-Inducible Factor Prolyl Hydroxylase (HIF-PHD) Inhibitors for the Treatment of Anemia

We report herein the design and synthesis of a series of orally active, liver-targeted hypoxia-inducible factor prolyl hydroxylase (HIF-PHD) inhibitors for the treatment of anemia. In order to mitigate the concerns for potential systemic side effects, we pursued liver-targeted HIF-PHD inhibitors relying on uptake via organic anion transporting polypeptides (OATPs). Starting from a systemic HIF-PHD inhibitor (1), medicinal chemistry efforts directed toward reducing permeability and, at the same time, maintaining oral absorption led to the synthesis of an array of structurally diverse hydroxypyridone analogues. Compound 28a was chosen for further profiling, because of its excellent in vitro profile and liver selectivity. This compound significantly increased hemoglobin levels in rats, following chronic QD oral administration, and displayed selectivity over systemic effects.

Discovery of a novel glucagon receptor antagonist N-[(4-{(1S)-1-[3-(3, 5-dichlorophenyl)-5-(6-methoxynaphthalen-2-yl)-1H-pyrazol-1-yl]ethyl}phenyl)carbonyl]-β-alanine (MK-0893) for the treatment of type II diabetes

A potent, selective glucagon receptor antagonist 9m, N-[(4-{(1S)-1-[3-(3,5-dichlorophenyl)-5-(6-methoxynaphthalen-2-yl)-1H-pyrazol-1-yl]ethyl}phenyl)carbonyl]-β-alanine, was discovered by optimization of a previously identified lead. Compound 9m is a reversible and competitive antagonist with high binding affinity (IC(50) of 6.6 nM) and functional cAMP activity (IC(50) of 15.7 nM). It is selective for glucagon receptor relative to other family B GPCRs, showing IC(50) values of 1020 nM for GIPR, 9200 nM for PAC1, and >10000 nM for GLP-1R, VPAC1, and VPAC2. Compound 9m blunted glucagon-induced glucose elevation in hGCGR mice and rhesus monkeys. It also lowered ambient glucose levels in both acute and chronic mouse models: in hGCGR ob/ob mice it reduced glucose (AUC 0-6 h) by 32% and 39% at 3 and 10 mpk single doses, respectively. In hGCGR mice on a high fat diet, compound 9m at 3, and 10 mpk po in feed lowered blood glucose levels by 89% and 94% at day 10, respectively, relative to the difference between the vehicle control and lean hGCGR mice. On the basis of its favorable biological and DMPK properties, compound 9m (MK-0893) was selected for further preclinical and clinical evaluations.

Differences in the metabolism and pharmacokinetics of two structurally similar PPAR agonists in dogs: involvement of taurine conjugation

1. The metabolism and pharmacokinetics of two structurally similar PPAR agonists, MRL-I, (2R)-7-[3-[2-chloro-4-(4-fluorophenoxy)phenoxy]propoxy]-2-ethyl-3,4-dihydro-2H-benzopyran-2-carboxylic acid, and MRL-II, (2R)-7-[3-[2-chloro-4-(2,2,2,-trifluoroethoxy)phenoxy]propoxy]-3,4-dihydro-2-methyl-2H-benzopyran-2-carboxylic acid, in dogs were investigated. 2. MRL-I was absorbed rapidly in dogs and exhibited linear pharmacokinetics over the dose range examined, 1-25mgkg(-1). In contrast, the pharmacokinetics of MRL-II were non-linear following both intravenous and oral administration. 3. The acyl glucuronide (AG) conjugate was the only radioactive component detected in bile from dogs dosed with [14C]MRL-I, whereas bile from dogs dosed with [14C]MRL-II contained varying amounts of both the AG and taurine conjugates. The percentages of the acyl glucuronide and taurine conjugates of [14C]MRL-II in dog bile were dose dependent. A higher percentage of radioactivity was associated with the taurine conjugate (about 41%) following intravenous administration at 0.2mgkg(-1) than at 0.9mgkg(-1) (about 14%) or oral administration at 5 mgkg(-1) (about 6%). The decrease in the percentage of radioactivity associated with the taurine conjugate at 0.9 mgkg(-1) was accompanied by a concomitant increase in the amount of the acyl glucuronide. 4. MRL-I, but not MRL-II, was subject to significant enterohepatic recirculation in dogs. Continuous collection of bile resulted in an 11-fold decrease in the terminal half-life of MRL-I in plasma (1.5 versus 16.6 h), and a 2.4-fold increase in its plasma clearance (4.0 versus 1.7 ml min(-1) kg(-1)) after intravenous administration at 1 mg kg(-1). 5. Collectively, the data suggest that the presence and subsequent saturation of the taurine conjugation pathway might have contributed to the non-linear pharmacokinetics of MRL-II in the dog.

Metabolism and excretion of the dipeptidyl peptidase 4 inhibitor [14C]sitagliptin in humans

The metabolism and excretion of [(14)C]sitagliptin, an orally active, potent and selective dipeptidyl peptidase 4 inhibitor, were investigated in humans after a single oral dose of 83 mg/193 muCi. Urine, feces, and plasma were collected at regular intervals for up to 7 days. The primary route of excretion of radioactivity was via the kidneys, with a mean value of 87% of the administered dose recovered in urine. Mean fecal excretion was 13% of the administered dose. Parent drug was the major radioactive component in plasma, urine, and feces, with only 16% of the dose excreted as metabolites (13% in urine and 3% in feces), indicating that sitagliptin was eliminated primarily by renal excretion. Approximately 74% of plasma AUC of total radioactivity was accounted for by parent drug. Six metabolites were detected at trace levels, each representing <1 to 7% of the radioactivity in plasma. These metabolites were the N-sulfate and N-carbamoyl glucuronic acid conjugates of parent drug, a mixture of hydroxylated derivatives, an ether glucuronide of a hydroxylated metabolite, and two metabolites formed by oxidative desaturation of the piperazine ring followed by cyclization. These metabolites were detected also in urine, at low levels. Metabolite profiles in feces were similar to those in urine and plasma, except that the glucuronides were not detected in feces. CYP3A4 was the major cytochrome P450 isozyme responsible for the limited oxidative metabolism of sitagliptin, with some minor contribution from CYP2C8.

Characterization of Two Cyclic Metabolites of Sitagliptin

Two novel metabolites of the dipeptidyl peptidase inhibitor sitagliptin (MK-0431, (2R)-4-oxo-4-[3-(trifluoromethyl)-5,6-dihydro[1,2,4]triazolo[4,3-a]pyrazin-7(8H)-yl]-1-(2,4,5-trifluorophenyl)-butan-2-amine), were identified after purification from dog urine. The metabolites (referred to as M2 and M5) were characterized by hydrogen/deuterium exchange tandem mass spectrometry and NMR spectroscopy nuclear Overhauser effect experiments as the cis and trans stereoisomers formed by cyclization of the primary amino group with the alpha carbon of the piperazine ring, following oxidative desaturation.

Disposition of the dipeptidyl peptidase 4 inhibitor sitagliptin in rats and dogs

The pharmacokinetics, metabolism, and excretion of sitagliptin [MK-0431; (2R)-4-oxo-4-[3-(trifluoromethyl)-5,6-dihydro[1,2,4]triazolo[4,3-a]pyrazin-7(8H)-yl]-1-(2,4,5-trifluorophenyl)butan-2-amine], a potent dipeptidyl peptidase 4 inhibitor, were evaluated in male Sprague-Dawley rats and beagle dogs. The plasma clearance and volume of distribution of sitagliptin were higher in rats (40-48 ml/min/kg, 7-9 l/kg) than in dogs ( approximately 9 ml/min/kg, approximately 3 l/kg), and its half-life was shorter in rats, approximately 2 h compared with approximately 4 h in dogs. Sitagliptin was absorbed rapidly after oral administration of a solution of the phosphate salt. The absolute oral bioavailability was high, and the pharmacokinetics were fairly dose-proportional. After administration of [(14)C]sitagliptin, parent drug was the major radioactive component in rat and dog plasma, urine, bile, and feces. Sitagliptin was eliminated primarily by renal excretion of parent drug; biliary excretion was an important pathway in rats, whereas metabolism was minimal in both species in vitro and in vivo. Approximately 10 to 16% of the radiolabeled dose was recovered in the rat and dog excreta as phase I and II metabolites, which were formed by N-sulfation, N-carbamoyl glucuronidation, hydroxylation of the triazolopiperazine ring, and oxidative desaturation of the piperazine ring followed by cyclization via the primary amine. The renal clearance of unbound drug in rats, 32 to 39 ml/min/kg, far exceeded the glomerular filtration rate, indicative of active renal elimination of parent drug.

Absorption, Metabolism, and Excretion of [14C]MK-0767 (2-Methoxy-5-(2,4-dioxo-5-thiazolidinyl)-N-[[4-(trifluoromethyl)phenyl] methyl]benzamide) in Humans

MK-0767 (KRP-297; 2-methoxy-5-(2,4-dioxo-5-thiazolidinyl)-N-[[4-(trifluoromethyl)phenyl] methyl]benzamide) is a thiazolidinedione (TZD)-containing dual agonist of the peroxisome proliferator-activated receptors alpha and gamma that has been studied as a potential treatment for patients with type 2 diabetes. The metabolism and excretion of [14C]MK-0767 were evaluated in six human volunteers after a 5-mg (200 microCi) oral dose. Excretion of 14C radioactivity was found to be nearly equal into the urine (approximately 50%) and feces (approximately 40%). Elimination of [14C]MK-0767 was primarily by metabolism, with minimal excretion of parent compound into the urine (<0.5% of dose) and feces (approximately 14% of the dose). [14C]MK-0767 was the major circulating compound-related entity (>96% of radioactivity) through 48 h postdose. It was also found that approximately 91% of the total radioactivity area under the curve was due to intact MK-0767. Several minor metabolites were detected in plasma (<1% of radioactivity, each), formed by cleavage of the TZD ring and subsequent S-methylation and oxidation. All the metabolites excreted into urine were formed by TZD cleavage, whereas the major metabolite in feces was the O-demethylated derivative of MK-0767.

SPECIES DIFFERENCES IN METABOLISM AND PHARMACOKINETICS OF A SPHINGOSINE-1-PHOSPHATE RECEPTOR AGONIST IN RATS AND DOGS: FORMATION OF A UNIQUE GLUTATHIONE ADDUCT IN THE RAT

The pharmacokinetics and metabolism of 1-(4-((4-phenyl-5-trifluoromethyl-2-thienyl)methoxy)benzyl)azetidine-3-carboxylic acid (MRL-A), a selective agonist for the sphingosine-1-phosphate 1 (S1P1) receptor, were investigated in rats and dogs. In both species, more than 50% of the dose was excreted in bile. Specific to the rat, and observed in bile, were a taurine conjugate of MRL-A and a glucuronide conjugate of an azetidine lactam metabolite. In dogs, a smaller portion of the dose (54% of administered dose) was excreted intact in bile, and the major metabolites detected were an azetidine N-oxide of MRL-A and an acylglucuronide of an N-dealkylation product. This latter metabolite was also observed in rat bile. Stereoselective formation of the N-oxide isomer was observed in dogs, whereas the rat produced comparable amounts of both isomers. The formation of a unique glutathione adduct was observed in rat bile, which was proposed to occur via N-dealkylation, followed by reduction of the putative aldehyde product to form the alcohol, and dehydration of the alcohol to generate a reactive quinone methide intermediate. Incubation of a synthetic standard of this alcohol in rat microsomes fortified with reduced glutathione or rat hepatocytes resulted in formation of this unique glutathione adduct.

Mechanistic studies on the metabolic scission of thiazolidinedione derivatives to acyclic thiols

Thiazolidinedione (TZD) derivatives have been reported to undergo metabolic activation of the TZD ring to produce reactive intermediates. In the case of troglitazone, it was proposed that a P450-mediated S-oxidation leads to TZD ring scission and the formation of a sulfenic acid intermediate, which may be trapped as a GSH conjugate. In the present study, we employed a model compound {denoted MRL-A, (+/-)-5-[(2,4-dioxothiazolidin-5-yl)methyl]-2-methoxy-N-[[(4-trifluoromethoxy)phenyl]methyl]benzamide} to investigate the mechanism of TZD ring scission. When MRL-A was incubated with monkey liver microsomes (or recombinant P450 3A4 and NADPH-P450 reductase) in the presence of NADPH and oxygen, the major products of TZD ring scission were the free thiol metabolite (M2) and its dimer (M3). Furthermore, a GSH conjugate of M2 (M4) also was formed when the incubation mixture was supplemented with GSH. Experiments with isolated M2 suggested that this metabolite was unstable and underwent spontaneous autooxidation to M3. A qualitatively similar metabolite profile was observed when MRL-A was incubated with recombinant P450 3A4 and cumene hydroperoxide. Because an oxygen atom is transferred to MRL-A under these conditions, these data suggested that S-oxidation alone may result in TZD ring scission and formation of M2 via a sulfenic acid intermediate. Also, because the latter incubation mixture did not contain any reducing agents, the formation of M2 may have occurred due to disproportionation of the sulfenic acid. When NADPH was added to the incubation mixture containing P450 3A4 and cumene hydroperoxide, the formation of M3 increased, suggesting that the sulfenic acid was reduced to M2 by NADPH and subsequently underwent dimerization to yield M3 (vide supra). When NADPH was replaced by GSH, the formation of M4 increased, consistent with reduction of the sulfenic acid by GSH. In summary, these results suggest that the TZD ring in MRL-A is activated by an initial P450-mediated S-oxidation step followed by spontaneous scission of the TZD ring to a putative sulfenic acid intermediate; the latter species then undergoes reduction to the free thiol by GSH, NADPH, and/or disproportionation. Finally, the thiol may dimerize to the corresponding disulfide or, in the presence of S-adenosylmethionine, form the stable S-methyl derivative.

SPECIES DIFFERENCES IN THE ELIMINATION OF A PPAR AGONIST HIGHLIGHTED BY OXIDATIVE METABOLISM OF ITS ACYL GLUCURONIDE

A species difference was observed in the excretion pathway of 2-[[5,7-dipropyl-3-(trifluoromethyl)-1,2-benzisoxazol-6-yl]oxy]-2-methylpropanoic acid (MRL-C), an alpha-weighted dual peroxisome proliferator-activated receptor alpha/gamma agonist. After intravenous or oral administration of [14C]MRL-C to rats and dogs, radioactivity was excreted mainly into the bile as the acyl glucuronide metabolite of the parent compound. In contrast, when [14C]MRL-C was administered to monkeys, radioactivity was excreted into both the bile and the urine as the acyl glucuronide metabolite, together with several oxidative metabolites and their ether or acyl glucuronides. Incubations in hepatocytes from rats, dogs, monkeys, and humans showed the formation of the acyl glucuronide of the parent compound as the major metabolite in all species. The acyl glucuronide and several hydroxylated products, some which were glucuronidated at the carboxylic acid moiety, were observed in incubations of MRL-C with NADPH- and uridine 5'-diphosphoglucuronic acid-fortified liver microsomes. However, metabolism was more extensive in the monkey microsomes than in those from the other species. When the acyl glucuronide metabolite of MRL-C was incubated with NADPH-fortified liver microsomes, in the presence of saccharo-1,4-lactone, it underwent extensive oxidative metabolism in the monkey but considerably less in the rat, dog, and human liver microsomes. Collectively, these data suggested that the oxidative metabolism of the acyl glucuronide might have contributed to the observed in vivo species differences in the metabolism and excretion of MRL-C.

In vitro metabolism of MK-0767 [(+/-)-5-[(2,4-dioxothiazolidin-5-yl)methyl]-2-methoxy-N-[[(4-trifluoromethyl)-phenyl] methyl]benzamide], a peroxisome proliferator-activated receptor alpha/gamma agonist. II. Identification of metabolites by liquid chromatography-tandem mass spectrometry

The in vitro metabolism of MK-0767 [(+/-)-5-[(2,4-dioxothiazolidin-5-yl) methyl]-2-methoxy-N-[[(4-trifluoromethyl)-phenyl] methyl]benzamide], a novel 2,4-thiazolidinedione (TZD)-containing peroxisome proliferator-activated receptor alpha/gamma agonist, was studied in rat, dog, monkey, and human liver microsomes and hepatocytes, as well as in recombinant human CYP3A4-containing microsomes. Twenty-two metabolites (some at trace levels) were detected by liquid chromatography-tandem mass spectrometry analysis. All appeared to be phase I metabolites except for a glucuronide conjugate of a hydroxylated metabolite that was detected at trace levels. A constant neutral loss scan experiment performed on a triple quadrupole mass spectrometer proved to be very useful for resolving the metabolites from endogenous compounds. It was observed that the initial site of metabolism of MK-0767 was at the TZD ring leading to two major metabolites, namely the 5-hydroxy-TZD metabolite (M24) and the mercapto metabolite (M22). The latter was formed via the cleavage of the TZD ring with the elimination of the carbonyl adjacent to the sulfur atom. The structure of M24 was established by accurate mass measurements and NMR analysis. This hydroxy-TZD metabolite might represent an important precursor for a group of metabolites formed by TZD ring opening and subsequent loss of the sulfur moiety. The mercapto metabolite, on the other hand, is probably the key precursor for the TZD ring-opened metabolites with retention of the sulfur, even though the detailed mechanism of the ring scission remains to be characterized. From these studies, it was concluded that the TZD ring was the major site of metabolism of MK-0767. All the metabolites produced in vitro from human preparations were detected in the corresponding preparations from the nonclinical species.

In vitro metabolism of MK-0767 [(+/-)-5-[(2,4-dioxothiazolidin-5-yl)methyl]-2-methoxy-N-[[(4-trifluoromethyl) phenyl]methyl]benzamide], a peroxisome proliferator-activated receptor alpha/gamma agonist. I. Role of cytochrome P450, methyltransferases, flavin monooxygenases, and esterases

The metabolism of MK-0767, (+/-)-5-[(2,4-dioxothiazolidin-5-yl)methyl]-2-methoxy-N-[[(4-trifluoromethyl) phenyl]methyl]benzamide, a thiazolidinedione (TZD)-containing peroxisome proliferator-activated receptor alpha/gamma agonist, was studied in liver microsomes and hepatocytes from humans and rat, dog, and rhesus monkey, to characterize the enzyme(s) involved in its metabolism. The major site of metabolism is the TZD ring, which underwent opening catalyzed by CYP3A4 to give the mercapto derivative, M22. Other metabolites formed in NADPH-fortified liver microsomes included the TZD-5-OH derivative (M24), also catalyzed by CYP3A4, and the O-desmethyl derivative (M28), whose formation was catalyzed by CYP2C9 and CYP2C19. Metabolite profiles from hepatocyte incubations were different from those generated with NADPH-fortified microsomal incubations. In addition to M22, M24, and M28, hepatocytes generated several S-methylated metabolites, including the methyl mercapto (M25), the methyl sulfoxide amide (M16), and the methyl sulfone amide (M20) metabolites. Addition of the methyl donor, S-adenosyl methionine, in addition to NADPH, to microsomal incubations enhanced the turnover and resulted in metabolite profiles similar to those in hepatocyte incubations. Collectively, these results indicated that methyltransferases played a major role in the metabolism of MK-0767. Using enzyme-specific inhibitors, it was concluded that microsomal thiol methyltransferases play a more important role than the cytosolic thiopurine methyltransferase. Baculovirus-expressed human flavin-containing monooxygenase 3, as well as CYP3A4, oxidized M25 to M16, whereas further oxidation of M16 to M20 was catalyzed mainly by CYP3A4. Esterases were involved in the formation of the methyl sulfone carboxylic acids, minor metabolites detected in hepatocytes.

Differences in the pharmacokinetics of peroxisome proliferator-activated receptor agonists in genetically obese Zucker and sprague-dawley rats: implications of decreased glucuronidation in obese Zucker rats

Genetically obese Zucker rats exhibit symptoms similar to those of obese patients with insulin-resistance or Type II diabetes; therefore, they have been used as a genetic model to study obesity, as well as a pharmacological model for the discovery of new drugs for the treatment of Type II diabetes and hyperlipidemia. In the present study, we compared the pharmacokinetics of two novel peroxisome proliferator-activated receptor (PPAR) agonists, MRL-I [(2R)-7-[3-[2-chloro-4-(4-fluorophenoxy)phenoxy]propoxy]-2-ethyl-3,4-dihydro-2H-benzopyran-2-carboxylic acid] and MRL-II [(2R)-7-[3-[2-chloro-4-(2,2,2-trifluoroethoxy)phenoxy]propoxy]-3,4-dihydro-2-methyl-2H-benzopyran-2-carboxylic acid], in obese Zucker and lean Sprague-Dawley rats following a single intravenous administration. The plasma clearance of both MRL-I and MRL-II was significantly lower in obese Zucker rats (4- and 2-fold, respectively) compared with Sprague-Dawley rats, but without any significant change in the volume of distribution, which resulted in a dramatic increase in the half-life (7- and 3-fold, respectively). The reversible in vitro plasma protein binding of [(14)C]MRL-I and [(14)C]MRL-II was comparable in the two strains, approximately 96% bound. The expression levels of uridine diphosphate-glucuronosyltransferases 1A1, 1A6, 2B1, and CYP2C11 and 3A1 mRNA in liver were lower (30-50%) in Zucker compared with Sprague-Dawley rats, as were the liver glutathione S-transferases (70%), quinone reductase (30%), organic anion-transporting protein 2 (80%), and multidrug resistance-associated protein 2 (Mrp2) (50%) mRNA levels. However, Mrp3 mRNA levels were similar in both strains. Consistent with these observations, the intrinsic clearance (CL(int)), calculated from the V(max)/K(m) of glucuronidation of [(14)C]MRL-I and [(14)C]MRL-II in liver microsomes, was approximately 2-fold lower in obese Zucker rats; the K(m) values were comparable in the two strains for both compounds. In conclusion, differences in the pharmacokinetics of two novel PPAR agonists, both cleared, predominantly, by conjugation, were evident in genetically obese Zucker rats compared with Sprague-Dawley rats. These differences were consistent with changes in the mRNA levels of hepatic drug-metabolizing enzymes and transporters. This information should be considered when comparing pharmacokinetic and efficacious doses in the obese Zucker rats, used as a pharmacological model, with those in Sprague-Dawley rats, which are used widely for drug metabolism and toxicology studies.

Metabolism and disposition of gemfibrozil in Wistar and multidrug resistance-associated protein 2-deficient TR- rats

1. The roles of multidrug resistance-associated protein (Mrp) 2 deficiency and Mrp3 up-regulation were evaluated on the metabolism and disposition of gemfibrozil. 2. Results from in vitro studies in microsomes showed that the hepatic intrinsic clearance (CLint) for the oxidative metabolism of gemfibrozil was slightly higher (1.5-fold) in male TR- rats, which are deficient in Mrp2, than in wild-type Wistar rats, whereas CLint for glucuronidation was similar in both strains. 3. The biliary excretion of intravenously administered [14C]gemfibrozil was significantly impaired in TR-) rats compared with Wistar rats (22 versus 93% of the dose excreted as the acyl glucuronides over 72 h). Additionally, the extent of urinary excretion of radioactivity was much higher in TR- than in Wistar rats (78 versus 2.6% of the dose). 4. There were complex time-dependent changes in the total radioactivity levels and metabolite profiles in plasma, liver and kidney, some of which appeared to be related to the up-regulation of Mrp3. 5. Overall, it was demonstrated that alterations in the expression of the transporters Mrp2 and Mrp3 significantly affected the excretion as well as the secondary metabolism and distribution of [14C]gemfibrozil.

IDENTIFICATION AND METABOLISM OF A NOVEL DIHYDROHYDROXY-S-GLUTATHIONYL CONJUGATE OF A PEROXISOME PROLIFERATOR-ACTIVATED RECEPTOR AGONIST, MK-0767 [(±)-5-[(2,4-DIOXOTHIAZOLIDIN-5-YL)METHYL]-2-METHOXY-N-[[(4-TRIFLUOROMETHYL) PHENYL]METHYL]BENZAMIDE], IN RATS

MK-0767 [(+/-)-5-[(2,4-dioxothiazolidin-5-yl)methyl]-2-methoxy-N-[[(4-trifluoromethyl)phenyl]methyl]benzamide] is a novel thiazolidinedione-containing peroxisome proliferator-activated receptor alpha/gamma agonist. In rats dosed orally with [14C]MK-0767, a dihydrohydroxy-S-glutathionyl conjugate of the parent compound was identified in the bile using liquid chromatography-mass spectometry and 1H NMR techniques. The formation of the conjugate likely proceeded via an arene oxide intermediate. The corresponding cysteinylglycine and cysteinyl conjugates likely formed from the further metabolism of the dihydrohydroxy-S-glutathionyl conjugate also were detected in rat bile. The dihydrohydroxy-S-glutathionyl conjugate was formed in vitro following the incubation of MK-0767 and glutathione with rat, dog, or monkey liver microsomes, and its formation was NADPH-dependent; however, this conjugate was not detected in human liver microsomal incubations. When incubated with rat intestinal contents, the dihydrohydroxy-S-glutathionyl conjugate was reduced to the parent compound (MK-0767), suggesting the involvement of intestinal microflora in its metabolism. There was no reduction of the conjugate by rat intestinal cytosol.

Simultaneous determination of MK-0767 and seven metabolites in rat urine using liquid chromatography/tandem mass spectrometry

MK-0767, 5-[2,4-dioxothiazolidin-5-yl)methyl]-2-methoxy-N-[[(4-trifluoromethyl)phenyl]methyl]benzamide (I, Table 1), is a dual peroxisome proliferator-activated receptor (PPAR) alpha/gamma agonist previously studied for the treatment of type 2 diabetes and dyslipidemia. To support further toxicological studies in one of the animal species used in chronic testing of I, a liquid chromatography/tandem mass spectrometry (LC/MS/MS) method for the simultaneous quantification of I and seven metabolites in rat urine was developed and validated. In this method, urine samples were diluted with acetonitrile/methanol (50:50, v/v) and injected directly onto the column of an LC system. Detection was achieved by MS/MS using a turbo ion spray probe monitoring precursor --> product ion combinations in selected reaction monitoring (SRM) mode. The linear range for I and three metabolites was 0.8-800 ng/mL, and 8-8000 ng/mL for four other metabolites found to be present in urine at higher concentrations than I. Intra-day and inter-day variation using this method were < or = 13.0%. The method exhibited good linearity, reproducibility, specificity and sufficient sensitivity when used for the analysis of rat urine samples. Concentrations of I and its major metabolites in rat urine were determined in samples collected between 0-24 h after dosing on the last day of administration of nine daily oral doses to three male (1000 mg/kg/day) and three female (300 mg/kg/day) Sprague-Dawley rats. The urinary concentrations of I and its metabolites were similar in male and female rats. The average concentrations of I were 0.51 and 0.33 microg/mL in male and female rats, respectively. Concentrations of four of the seven metabolites quantified were 6- to 45-fold higher than those of I. The most abundant metabolite, with concentrations of 24.2 and 13.3 microg/mL in male and female rat urine, respectively, was a methyl sulfoxide derivative formed by oxidative cleavage of the thiazolidinedione ring, followed by S-methylation and oxidation of the sulfide intermediate.

Induction of hepatic phase II drug-metabolizing enzymes by 1,7-phenanthroline in rats is accompanied by induction of MRP3

The purpose of the present study was to evaluate the effect of 1,7-phenanthroline (PH), which has been proposed to be a selective phase II enzyme inducer, on the gene expression of xenobiotic transporters, as well as hepatic and renal drug-metabolizing enzymes. After oral administration of PH for 3 days to male Sprague-Dawley rats, mRNA levels in liver (75 and 150 mg/kg doses) and kidney (75 mg/kg dose only) were determined using real-time quantitative polymerase chain reaction. At 150 mg/kg/day, PH treatment resulted in significant increases in hepatic mRNA levels of Mrp3 (36-fold), UGT1A6 (20-fold), UGT2B1 (4-fold), and quinone reductase (QR, 5-fold), compared with the vehicle-treated group. Similar increases in Mrp3 (99-fold), UGT1A6 (17-fold), UGT2B1 (3-fold), and QR (11-fold) mRNA levels were observed in the liver after PH treatment of rats at 75 mg/kg/day. In contrast, the expression levels of CYP2C11 and Oatp2 were decreased by approximately 80 and 50%, respectively. In addition, PH (75 mg/kg/day) elicited statistically significant changes in renal gene expression of CYP3A1, UGT1A6, QR, and Mrp3, but the magnitude of renal Mrp3 induction was less than 2-fold over control. Although PH is known to modulate hepatic glucuronidation in vivo, these data indicated that PH induced mRNA levels of the efflux transporter, Mrp3, which may also affect the disposition of xenobiotics.

Identification of novel metabolites of pioglitazone in rat and dog

1. Four new metabolites of pioglitazone were identified by liquid chromatography-mass spectrometry (LC-MS/MS) as being formed by hydroxylation (M-VII and M-VIII), opening of the thiazolidinedione ring (M-X) and by desaturation of the terminal ethyl side chain or tether ethoxy moiety (M-IX), respectively. The structure of one of the hydroxylated metabolites (M-VII) was confirmed by chemical modification using the Jones reaction. 2. Oxidative cleavage of the thiazolidinedione ring is a novel pathway not previously reported for pioglitazone. 3. The hydroxylated M-VII was detected in incubations with rat, dog and human liver and kidney microsomes, and in plasma from rats and dogs dosed orally with [(3)H]pioglitazone. 4. The carboxylic acid derivative of M-VII (M-V) and its taurine conjugate were the major radioactive components in dog bile.

Modification of the pyridine moiety of non-peptidyl indole GnRH receptor antagonists

The synthesis of a number of indole GnRH antagonists is described. Oxidation of the pyridine ring nitrogen, combined with alkylation at the two position, led to a compound with an excellent in vitro activity profile as well as oral bioavailability in both rats and dogs.

2-Arylindoles as gonadotropin releasing hormone (GnRH) antagonists: optimization of the tryptamine side chain

A series of 2-arylindoles containing novel heteroaromatic substituents on the tryptamine tether, based on compound 1, was prepared and evaluated for their ability to act as gonadotropin releasing hormone (GnRH) antagonists. Successful modifications of 1 included chain length variation (reduction) and replacement of the pyridine with heteroaromatic groups. These alterations culminated in the discovery of compound 27kk which had excellent in vitro potency and oral efficacy in rodents.

Orally bioavailable, indole-based nonpeptide GnRH receptor antagonists with high potency and functional activity

Stereospecific introduction of a methyl group to the indole-3-side chain enhanced activity in our tryptamine-derived series of GnRH receptor antagonists. Further improvements were achieved by variation of the bicyclic amino moiety of the tertiary amide and by adjustment of the tether length to a pyridine or pyridone terminus. These modifications culminated in analogue 24, which had oral activity in a rat model and acceptable oral bioavailability and half-life in dogs and monkeys.

Parallel extraction columns and parallel analytical columns coupled with liquid chromatography/tandem mass spectrometry for on-line simultaneous quantification of a drug candidate and its six metabolites in dog plasma

A method with parallel extraction columns and parallel analytical columns (PEC-PAC) for on-line high-flow liquid chromatography/tandem mass spectrometry (LC/MS/MS) was developed and validated for simultaneous quantification of a drug candidate and its six metabolites in dog plasma. Two on-line extraction columns were used in parallel for sample extraction and two analytical columns were used in parallel for separation and analysis. The plasma samples, after addition of an internal standard solution, were directly injected onto the PEC-PAC system for purification and analysis. This method allowed the use of one of the extraction columns for analyte purification while the other was being equilibrated. Similarly, one of the analytical columns was employed to separate the analytes while the other was undergoing equilibration. Therefore, the time needed for re-conditioning both extraction and analytical columns was not added to the total analysis time, which resulted in a shorter run time and higher throughput. Moreover, the on-line column extraction LC/MS/MS method made it possible to extract and analyze all seven analytes simultaneously with good precision and accuracy despite their chemical class diversity that included primary, secondary and tertiary amines, an alcohol, an aldehyde and a carboxylic acid. The method was validated with the standard curve ranging from 5.00 to 5000 ng/mL. The intra- and inter-day precision was no more than 8% CV and the assay accuracy was between 95 and 107%.

Disposition of L-732,531, a potent immunosuppressant, in rats and baboons

L-732,531 is a semi-synthetic analog of the macrolide tacrolimus (Prograf(R)). Like tacrolimus, L-732,531 is a potent immunosuppressant. In this study, its absorption, distribution, metabolism, and excretion were studied in rats and baboons. In rats, its blood and plasma levels were similar, whereas in baboons, its blood levels were, on average, twice as high as those in plasma. This was consistent with the in vitro blood-to-plasma ratio of L-732, 531, which in these two species, as well as in humans, was much lower than that of tacrolimus and showed a minimal concentration dependence. After iv administration to rats, the blood and plasma clearance of L-732,531 decreased from approximately 60 ml/min/kg at 0.2 mg/kg to 30 ml/min/kg when dosed at 1 and 3 mg/kg. After oral administration, plasma area under the concentration vs. time curve (AUC) and maximal plasma concentration (Cmax) increased more than proportionally to the dose. At 1, 5, and 15 mg/kg, plasma AUC was 29, 466, and 2832 ng.hr/ml, respectively, and Cmax was 10, 129, and 304 ng/ml, respectively. Bioavailability, although compromised by nonlinear kinetics, was estimated to be between 8% and 18%. In baboons, the clearance of L-732,531 was lower than that in rats, especially when calculated from blood concentrations (12 ml/min/kg at 0.2 mg/kg and 8 ml/min/kg at 1 mg/kg). After oral dosing, baboon plasma AUC and Cmax were much lower than those in rats, but as in rats, they increased more than proportionally with increasing doses. The bioavailability of L-732,531 in baboons was estimated at 3%, 9%, and 24% when animals were dosed at 5, 15, and 26 mg/kg po, respectively. After oral administration of [3H]L-732,531 at 5 mg/kg, approximately 32% of the radioactivity was recovered in bile and urine of rats, compared with 9% in baboons. High-performance liquid chromatography profiles of rat and baboon plasma, bile, urine, and feces indicated that L-732,531 was metabolized extensively to a complex mixture of products. Some intact parent drug was observed in feces of orally dosed animals, indicating incomplete absorption. In vitro, L-732,531 was metabolized more extensively by baboon liver microsomes than rat or human microsomes. Its metabolism in human liver microsomes was shown to be catalyzed primarily by cytochrome P450 3A isozymes.

Separation of the four stereoisomers of a potent inhibitor (L-694,458) of human leukocyte elastase and its determination in human plasma using achiral/chiral chromatography with column switching

A stereoselective method based on high-performance liquid chromatography (HPLC) and ultraviolet detection at 235 nm for the separation of the four possible stereoisomers of compound 1 ((S-(R*,S*))-2-(4-((4-methylpiperazin-1-yl)carbonyl)phenoxy)-3,3-d iethyl-N-(1-(3,4-(methylenedioxy)phenyl)butyl)-4-oxo-1-azetidinecarbo xamide, L-694,458), a potent, selective, and orally active human leukocyte elastase (HLE) inhibitor, in human and monkey plasma has been developed. The molecule of 1 contains two chiral centers and is being developed as a single stereoisomer with the absolute configuration S and R in positions 'a' and 'b', respectively. Although the baseline separation of each of the two pairs of enantiomers (SS/RR and SR/RS) was achieved on a single chiral column (Chiralcel OD-H) using hexane methyl-t-butyl ether (MTBE)-methanol 80/10/10, (v/v/v) as a mobile phase (alpha(RS,SR) = 2.03, alpha(RR,SS) = 4.97), only partial separation of RS from RR was observed under these conditions (k'RS = 3.32, k'RR = 3.08). Baseline separation of all four stereoisomers from each other and from endogenous plasma components required the initial chromatography of the two diastereomeric racemates (SS/RR and SR/RS) on the achiral silica column (50 x 4.6 mm, 5 microm), followed by column switching and further separation of the stereoisomers on a Chiralcel OD-H column (250 x 4.6 mm, 5 microm) using isopropanol (IPA)-hexane diethylamine (DEA), 65/35/0.3, (v/v/v) on both columns as a mobile phase. The drug was extracted from basified (pH 11) plasma (1 ml) using liquid liquid extraction with MTBE. After evaporation of the extract to dryness, the residue was reconstituted in the mobile phase (200 microl) and part of the extract (125 microl) was injected into the HPLC system. Using this method, it was demonstrated that after oral dosing of monkeys at 40 mg kg(-1) with 1 the only stereoisomer detected in the post-dose plasma samples was the starting material 1, and no inversion of the configuration at positions 'a' and 'b' of 1 had occurred in vivo. Based on this observation, a non-chiral assay for 1 in human plasma was also developed. The method was validated in the concentration range 10-500 ng ml(-1) with the assay precision (expressed as the coefficient of variation, CV) better than 9% and assay accuracy in the range of 95-107% of the nominal concentrations at all concentrations within the standard curve range. The total run time in the non-chiral assay was 12 min. The details of both chiral and non-chiral methods are provided.

Pharmacokinetics and disposition of the oxytocin receptor antagonist L-368,899 in rats and dogs

L-368,899 is a potent, orally-active oxytocin antagonist that completed phase I clinical trials for the prevention of preterm labor. The pharmacokinetics and disposition of L-368,899 were studied in rats (female and male) and dogs (female), the two species used in the toxicology studies. L-368,899 exhibited similar pharmacokinetics in rats and dogs. After iv dosing at 1, 2.5, and 10 mg/kg, the compound had a t1/2 of approximately 2 hr and plasma clearance between 23 and 36 ml/min/kg at all doses and in both species. The exception was female rats at the 10 mg/kg dose where plasma clearance decreased to 18 ml/min/kg. The Vdss was between 2.0 and 2.6 liters/kg for rats and 3.4 to 4.9 liters/kg for dogs. After oral doing, L-368,899 was rapidly absorbed. Mean Cmax values were achieved at <1 hr at the low doses (25 mg/kg in rats and 5 mg/kg in dogs) and between 1 and 4 hr at the higher doses (100 mg/kg in rats and 33 mg/kg in dogs). In bile duct-cannulated female rats, approximately 70% of a radioactive 28 mg/kg dose was recovered in bile and urine within 72 hr post dose. Plasma drug concentrations were higher in female than in male rats especially at the 25 mg/kg dose, where mean AUC values were 4.5-fold higher in the females. In both rats and dogs, plasma drug levels increased more than proportionally with increasing oral dose. In female rats, the mean AUC increased by approximately 8-fold between 25 and 100 mg/kg, while in female dogs, the mean AUC at the 33 mg/kg dose was 12-fold higher than that at 5 mg/kg. Oral bioavailability was estimated at 14% and 18% for the 5 mg/kg dose in female and male rats, respectively, 41% for the 25 mg/kg dose in male rats and 17% and 41%, respectively, for the 5 and 33 mg/kg doses in dogs. Owing to nonlinear kinetics, bioavailability could not be calculated for the other oral doses. L-368,899 was metabolized extensively in both species after iv and oral dosing, with <10% of the dose excreted unchanged. The main route of elimination was via the feces, which contained >70% of the radioactive dose by 48 hr, primarily as metabolites. The gender and dose dependence of the pharmacokinetics of L-389,899 in rats were attributed to gender differences in metabolizing capacity and saturation of hepatic metabolism, respectively. This conclusion was based primarily on results from experiments comparing the rate of in vitro metabolism of L-368,899 in liver microsomes, which showed that the Vmax and KM values for L-368,899 were 4-fold lower in female than in male rat liver microsomes.

Orally active inhibitors of human leukocyte elastase. II. Disposition of L-694,458 in rats and rhesus monkeys

The disposition of L-694,458, a potent monocyclic beta-lactam inhibitor of human leukocyte elastase, was studied in male Sprague-Dawley rats and rhesus monkeys. After iv dosing, L-694,458 exhibited similar pharmacokinetic parameters in rats and rhesus monkeys. The mean values for its plasma clearance, terminal half-life, and volume of distribution at steady state were 27 ml/min/kg, 1.8 hr, and 4.0 liters/kg in rats and 34 ml/min/kg, 2.3 hr, and 5 liters/kg in rhesus monkeys. The bioavailability of a 10 mg/kg oral dose was higher in rats (65%) than in rhesus monkeys (39%). In both species, concentrations of L-694,458 in plasma increased more than proportionally when the oral dose was increased from 10 mg/kg to 40 mg/kg. In monkeys a protracted plasma concentration-time profile was observed at 40 mg/kg, characterized by a delayed T(max) (8-24 hr) and a long terminal half-life (6 hr). [3H]L-694,458 was well absorbed after oral dosing to rats at 10 mg/kg, as indicated by the high recovery of radioactivity in bile (83%) and urine (6%) of bile duct-cannulated rats. Only approximately 5% or less of the radioactivity in bile, urine, and feces was a result of intact L-694,458, indicating that the compound was being eliminated by metabolism, followed by excretion of the metabolites in feces, via bile. Demethylenation of the methylenedioxyphenyl group resulting in the catechol was the primary metabolic pathway in human and rhesus monkey liver microsomes. In rat liver microsomes, the major metabolite was the N-oxide of the methyl-substituted piperazine nitrogen. In rats dosed iv and orally with [3H]L-694,458, concentrations of radioactivity were highest in the lung (the primary target tissue), adrenals, and liver. L-694,458 was unstable in rat blood and plasma, degrading via a pathway believed to be catalyzed by B-esterases and to involve cleavage of the beta-lactam ring and loss of the methylpiperazine phenoxy group. In vitro studies indicated that in human liver, L-694,458 was metabolized by CYP3A and 2C isozymes, and in both monkey and human liver microsomes the compound acted as an inhibitor of testosterone 6beta-hydroxylation.

Orally active inhibitors of human leukocyte elastase. III. Identification and characterization of metabolites of L-694,458 by liquid chromatography-tandem mass spectrometry

The in vitro and in vivo metabolism of N-[1(R)-(1,3-benzodioxol-5-yl)butyl]-3,3-diethyl-2(S)-[4-[(4-methy l-1-piperazinyl)carbonyl]phenoxy]-4-oxo-1-azetidinecarboxamide (L-694,458) was studied in male Sprague-Dawley rats and rhesus monkeys. Analysis by LC-MS/MS and NMR revealed that the major metabolite generated in incubations with rat liver microsomes resulted from N-oxidation of the piperazine group, while the major metabolite generated in monkey liver microsomes was the catechol that resulted from O-dealkylation of the methylenedioxyphenyl group. Other metabolites observed in these incubations include the piperazine N-desmethyl, several monohydroxylated derivatives of the parent compound, and three products that resulted from cleavage of the beta-lactam ring. Incubations of parent compound with rat hepatocytes in culture generated two major metabolites that resulted from cleavage of the piperazine ring with the loss of an ethylene group from one side of the ring; one of these metabolites retained the piperazine N-methyl group, while the other did not. The metabolite profiles in vivo were similar to those observed in vitro, but they were much more complex owing to secondary and, in some cases, tertiary biotransformations of many of the primary metabolites. Bile obtained from orally dosed rats contained more than 40 parent-related components, and many of these metabolites had arisen from piperazine ring cleavage.

Orally active inhibitors of human leukocyte elastase. I. Disposition of L-683,845 in rats and rhesus monkeys

L-683,845 is an orally active inhibitor of human leukocyte elastase. Its disposition was studied in rats and rhesus monkeys after dosing with a 3H- or 14C-labeled compound intravenously at 5 mg/kg and orally at 10 mg/kg. L-683,845 exhibited different pharmacokinetics in these two species. In rats, L-683,845 was well-absorbed after oral dosing, with a maximum concentration of 6 microg/ml at 2 hr and bioavailability of approximately 100%. After intravenous dosing, it was cleared slowly at approximately 3 ml/min/kg, with a terminal half-life of approximately 7 hr and a volume of distribution at steady-state of 1 liter/kg. After both intravenous and oral dosing, L-683,845 comprised 50-95% of plasma radioactivity. About 75% of the intravenous and 87% of the oral dose were recovered in the feces as parent and/or conjugates, with the remaining fraction recovered in the urine as polar components. In rhesus monkeys, maximum concentration after oral dosing was only 0.25 microg/ml, and bioavailability was 50%. Plasma clearance was 8-fold higher, at 23 ml/min/kg, and volume of distribution at steady-state larger, at 2 liters/kg, than in rats. The terminal half-life of L-683,845 could not be determined accurately after intravenous dosing, but seemed to be long in orally dosed animals, approximately 13 hr. Intact L-683,845 was a minor component in plasma comprising only approximately 20% of the radioactivity at most time points. Moreover, persistent levels of radioactivity were detected in plasma and urine of rhesus monkeys even at 1-month postdose, and > or = 25% of the radioactivity in plasma was irreversibly bound to proteins at the later time points. Recovery of the radioactivity was incomplete, with only 77% of the intravenous and 43% of the oral dose recovered over a 4-day period. L-683,845-derived radioactivity distributed to all major rat tissues, with highest levels in the liver followed by the small intestine, adrenals, kidneys, and lungs. Radioactivity concentrations in the liver were high even at 24 hr, 22.7 microg eq/g. A large portion of the intravenous dose was recovered in the small intestine, approximately 40% at 2 hr, indicating rapid and extensive biliary excretion. L-683,845 was metabolized primarily to the acyl glucuronide, which was very unstable in rat plasma, and was subject to hydrolysis to L-683,845 and rearrangement. The glucuronide and L-683,845 were degraded in rat plasma by opening the beta-lactam ring and loss of the C4 substituent followed by decarboxylation to give an olefin and/or decomposition to the monosubstituted urea. Based on inhibition by organophosphorus compounds, it is speculated that their degradation is catalyzed by a type B esterase.

The role of drug metabolism in drug discovery: a case study in the selection of an oxytocin receptor antagonist for development

Drug discovery is a process involving multiple disciplines and interests. During the research phase of drug discovery, usually a large number of compounds are evaluated for biological activity and toxicological potential in animal species. Various types of problems with respect to pharmacodynamics, pharmacokinetics, and toxicity are commonly encountered at this stage. Drug metabolism, as a discipline participating in a drug discovery team, can play an important role in identifying factors underlying the problems, facilitate the optimal selection of compounds for further development, provide information on metabolites for possible improvement in drug design, and contribute to the identification of the appropriate animal species for subsequent toxicity testing. During the process of evaluating oxytocin receptor antagonists for further development for treatment of preterm labor, in vivo and in vitro drug metabolism studies conducted in rats, dogs, and monkeys contributed to the selection of L-368,899 as the development candidate on the basis of pharmacokinetic and metabolism observations. The presence of active N-demethylated metabolites of two other equipotent compounds in rats and dogs was found to be the major factor responsible for the discrepancy between oral bioavailability and efficacies observed for these 2 compounds. For L-368,899, a compound that demonstrated 20-40% oral bioavailability in rats, dogs, and chimpanzees, extensive first-pass metabolism rather than absorption was determined as the major factor responsible for the poor bioavailability (< 1%) in rhesus monkeys. In vitro metabolism studies with hepatic microsomes from rats, dogs, monkeys, and humans substantiated the conclusion that the rate of hepatic metabolism of L-368,899 in monkeys is faster than in the other species.

In vitro metabolism of FK-506 in rat, rabbit, and human liver microsomes: identification of a major metabolite and of cytochrome P450 3A as the major enzymes responsible for its metabolism

The metabolism of the immunosuppressant FK-506 was shown to be catalyzed primarily by cytochrome P450 isozymes of the P450 3A subfamily. Antibodies against rat P450 3A inhibited FK-506 metabolism by 82% in rat liver microsomes and by 35-56% in liver microsomes from humans, dexamethasone-induced rats, and erythromycin-induced rabbits. Poor species cross-reactivity of the antibodies, metabolic switching, and/or some metabolism by P450 isozymes other than P450 3A may be responsible for the incomplete inhibition observed. Besides anti-rat P450 3A, antibodies against rat P450 1A also appeared to have some inhibitory effect implicating these particular cytochrome P450 isozymes as having a minor role in FK-506 metabolism. The formation of 13-desmethyl FK-506, identified here as a major metabolite of FK-506 in all types of microsomes examined, was inhibited completely by anti-P450 3A in liver microsomes from dexamethasone-induced rats and erythromycin-induced rabbits but only partially in human and control rat liver microsomes.

Effects of the immunosuppressant FK-506 and its analog FK-520 on hepatic and renal cytochrome P450 mixed-function oxidase

The novel immunosuppressant FK-506 and its analog FK-520 were found to inhibit the hepatic microsomal mixed-function oxidase system in male Sprague-Dawley rats. At 5 and 10 mg/kg/day, s.c., for 6 days they caused 30-80% decreases in cytochrome P450 levels, NADPH-cytochrome P450 reductase, and benzphetamine N-demethylase activities. The metabolism of FK-506 itself was inhibited by 50%. FK-506 and FK-520 had a minimal effect on the renal cytochrome P450 levels unlike cyclosporin A which produced a 67% increase after six daily 25 mg/kg doses. A single dose of FK-506 (25 mg/kg, s.c.) had a minimal effect on the hepatic or renal metabolizing enzyme system. In vitro, addition of FK-506 and FK-520 to human and control rat liver microsomes resulted in a concentration-dependent inhibition of benzphetamine N-demethylation (10-20% at 50 microM, 60-75% at 250 microM). We suggest that in view of its potential to inhibit hepatic cytochrome P450-dependent mixed-function oxidase, resulting in the inhibition of its own metabolism, FK-506 should be administered with caution to transplant patients.

Localization of the heme-binding protein in the cytoplasm and of a heme-binding protein-like immunoreactive protein in the nucleus of rat liver parenchymal cells: immunocytochemical evidence of the subcellular distribution corroborated by radioimmunoassay and immunoblotting

The abundant heme-binding protein of the liver, probably identical with Z-protein or liver fatty acid-binding protein, has an apparent molecular weight of 14,000 Da and is presumably involved in the intracellular transport of a variety of compounds. The cellular and subcellular distribution of HBP in the liver was studied in adult male and female rats by postembedding immunocytochemistry using the protein A-gold technique. By light microscopic examination heme-binding protein is present exclusively in parenchymal cells and not found in the sinusoidal lining cells or other cells in portal tracts. Immunoreactivity for heme-binding protein is uniformly strong throughout the liver lobule in female rats but is markedly reduced in the pericentral region in male animals. By immunoelectron microscopy heme-binding protein immunoreactivity is localized in cytoplasm and nuclear matrix. The mitochondria and peroxisomes and the secretory apparatus are free of the label. In nuclei, gold labeling is confined to the interchromatin region (euchromatin) and nucleoli; condensed chromatin (heterochromatin) and nucleolus-associated chromatin are negative. The subcellular localization was substantiated by radioimmunoassay and immunoblotting of nuclear and cytosolic fractions. Immunoblotting shows that the heme-binding protein-like immunoreactive protein in the nucleus has a slightly larger molecular weight than that in the cytoplasm.

Protein-mediated efflux of heme from isolated rat liver mitochondria

Proteins are required for the efflux of heme from mitochondria and liposomes. The efflux from liposomes is independent of the heme-binding affinity of the protein (Biochem. 23:3715, 1984). We tested whether heme-binding proteins increase efflux of newly synthesized heme from structurally and functionally intact rat liver mitochondria. Mitochondria whose heme was labeled with 14C-delta-aminolevulinic acid, were incubated in the presence of glutathione transferases (GSTs), serum albumin (RSA) or heme-binding protein (HBP), all from the rat. HBP caused a 6-8 fold increase in efflux of newly synthesized heme as compared to that effected by RSA or GSTs. This result indicates that heme efflux from intact mitochondria, unlike that from liposomes, depends on the type of protein present and that HBP may specifically facilitate heme efflux from mitochondria.

Inverse relationship between total glutathione S-transferase content and bile acid release in isolated hepatocytes from untreated, phenobarbital pretreated and hypothyroid rats

The role of cytosolic anion binding proteins (glutathione S-transferases) in the hepatic transport of bile acids remains controversial. To investigate whether increased levels of the hepatocyte total glutathione S-transferase content were associated with changes in the release of bile acids from the hepatocyte, we measured the rate of release of radioactive bile acids in isolated hepatocytes from thyroidectomized, phenobarbital pretreated and untreated rats. The isolated hepatocytes were preincubated with either 14C-cholic acid or 14C-taurocholic acid, and the release rate of radiolabeled bile acids was determined. Hepatocyte total glutathione S-transferase content was measured by rocket immunoelectrophoresis. The release rate of the radiolabeled bile acids was significantly (P less than 0.005) decreased in both hypothyroid and phenobarbital pretreated hepatocytes. The levels of total glutathione S-transferase content were significantly (P less than 0.001) increased in the hepatocytes from both hypothyroid and phenobarbital pretreated animals. Our findings reveal a striking inverse relationship between the total glutathione S-transferase content of the hepatocyte and the release rate of radiolabeled bile acids in isolated hepatocytes from two independent animal models. These observations support the hypothesis that cytosolic anion binding proteins (glutathione S-transferases) may influence the net flux across the hepatocyte plasma membrane largely by limiting efflux.

Interaction of hemopexin, albumin and liver fatty acid-binding protein with protoporphyrin

Equilibrium constants for the binding of protoporphyrin to serum albumin and hemopexin and liver cytosolic fatty acid-binding protein of the rat were determined fluorometrically. The experimental equilibrium constant [10(6) M-1 (mean +/- S.D.)] values were 8.4 +/- 1.3, 10.0 +/- 2.4 and 34.0 +/- 3.0 for albumin, hemopexin and liver fatty acid-binding protein, respectively. Statistical analysis showed the equilibrium constant of binding of protoporphyrin to liver fatty acid-binding protein to be significantly (p less than 0.01) higher than that to albumin and hemopexin. The data suggest that in patients with erythropoietic protoporphyria an equilibrium gradient may exist which favors the uptake by hepatocytes of plasma protoporphyrin as a result of its greater affinity for intracellular liver fatty acid-binding protein.

Modulation of hepatic heme-binding Z protein in mice by the porphyrogenic carcinogens griseofulvin and hexachlorobenzene

Some reports link human hepatic porphyria with a risk of hepatocellular carcinoma. Hepatic protoporphyria and uroporphyria were induced in mice by feeding griseofulvin and hexachlorobenzene (HCB), respectively. These chemicals also cause liver cancer. Hepatic immunoreactive cytosolic levels of heme-binding Z protein (HBP) were reduced by 81% (griseofulvin) and 55% (HCB). In contrast, both treatments caused a greater than 4-fold increase in the immunoreactive levels of glutathione S-transferase isozymes (GST) which like HBP also bind heme. Unlike in vitro studies in the presence of porphyrins, no cross-linking of HBP was observed in vivo.

Oxidative effects of heme and porphyrins on proteins and lipids

Heme and porphyrins catalyze the formation of various reactive oxygen species under widely different conditions. Porphyrins are potent photosensitizers capable of transferring the energy of their excited state to oxygen, forming 1O2. Heme, by virtue of its iron content, is not a photosensitizer, but it can react with H2O2 forming reactive oxygen intermediates whose nature depends on the oxidation state of the iron. Ferric heme gives rise to a porphyrin cation radical, while ferrous heme catalyzes the formation of OH.. The reactive species formed by heme and porphyrins oxidize susceptible functional groups on protein and lipid components of membranes as well as serum and cytosolic proteins. Oxidation of membrane-bound proteins occurs independently of lipid peroxidation and it is characterized by extensive cross-linking, which takes place subsequent to amino acid oxidation. Serum and cytosolic proteins that bind and may transport heme and porphyrins are of special interest since they can modulate their toxicity by affecting their availability and reactivity. In addition, these proteins are particularly susceptible to oxidation, especially by heme, due to their proximity to the oxidizing species that are formed.

Decreased cytosolic levels of the heme binding Z protein in rat hepatocyte nodules and hepatocellular carcinomas

Hepatocyte nodules and hepatocellular carcinomas were induced in male Fischer rats using the resistant hepatocyte model. The immunoreactive cytosolic levels of the heme-binding Z protein (HBP) were reduced by 56% (P less than 0.001; 2-tailed t-test) in early hepatocyte nodules (25 weeks) and hepatocellular carcinomas (10-12 months). This finding is in accordance with the previously reported reduced heme content of hepatocyte nodules and is consistent with the postulated role for HBP in intracellular heme transport and distribution. The immunoreactive levels of the glutathione S-transferase isozymes (GST) which like HBP bind heme, were elevated 2-fold (P less than 0.01) in early and late hepatocyte nodules and were unchanged in hepatocellular carcinomas.

The influence of heme-binding proteins in heme-catalyzed oxidations

We show here that heme-binding proteins may enhance, decrease, or completely inhibit heme-catalyzed oxidations and that in doing so the proteins themselves may be oxidized depending upon their relative affinities for heme and the nature of their interactions with this metalloporphyrin. That release of iron from heme was not responsible for the catalytic effect is indicated by the observation that heme induced more peroxidation of rat liver microsomal lipid in the presence of H2O2 than iron and that iron release is very low under the conditions employed. Hemopexin, which binds heme with high affinity, completely inhibited heme-catalyzed lipid peroxidation at concentrations slightly higher than that of heme, suggesting a unique role for this acute phase protein in antioxidant defense mechanisms. The protein itself was not oxidized, presumably because the putative bis-histidyl heme-hemopexin complex cannot interact with H2O2. Rat and human albumin and rat glutathione S-transferases (GST), proteins with moderate affinities for heme, decreased heme-catalyzed lipid peroxidation in a dose-dependent manner but were subject to oxidation. The GST were crosslinked forming a nondisulfide covalently linked subunit dimer as well as products of higher molecular weight whereas the oxidation products of the albumins had molecular weights only slightly higher than those of the native proteins. The changes in the electrophoretic patterns of GST and albumin were accompanied by a decrease in their tryptophan fluorescence and the formation of bityrosine-like products. Proteins with lower affinities for heme, such as bovine albumin and rat liver fatty acid-binding protein (L-FABP), enhanced lipid peroxidation at all concentrations tested. While bovine albumin was modified, L-FABP was not crosslinked nor were its tyrosine residues oxidized. Thus, the susceptibility of a protein to heme-mediated oxidative damage would appear to be determined by factors such as its affinity for heme, the nature of the amino acids in the vicinity of the bound catalyst and the availability of a free coordination site on the iron.

Effects of porphyrins on proteins of cytosol and plasma. In vitro photo-oxidation and cross-linking of proteins by naturally occurring and synthetic porphyrins

We examined the photodynamic effects of porphyrins, known photosensitizers, on proteins of cytosol and plasma that bind them and are implicated in their transport. Their susceptibility to photodecomposition by porphyrins was found to be higher than that of proteins with low or no affinity for tetrapyrroles. Inhibition of porphyrin binding by the addition of equimolar amounts of heme had no effect, indicating that protein photodecomposition may be induced, in part, by free or nonspecifically bound porphyrins. HBP, a heme-binding Z protein of liver cytosol, exhibited the highest susceptibility of all proteins tested, including glutathione S-transferases, albumin, hemopexin, and apotransferrin. HBP was extensively photo-oxidized, as evidenced by a decrease in its antigenicity and electrophoretic mobility, and it was cross-linked by naturally occurring porphyrins as well as by the synthetic tin-protoporphyrin and hematoporphyrin derivative. The water-soluble singlet oxygen scavengers L-histidine (50 mmol/L) and sodium azide (100 mmol/L) completely prevented the photodynamic effects of uroporphyrin (100 mumol/L) on HBP. Hydroxyl radical scavengers such as manitol and benzoate were partially effective, whereas water-insoluble singlet oxygen scavengers such as beta-carotene were totally ineffective. Preferential inhibition of cross-linking over other photodynamic effects of uroporphyrin was consistent with previous reports that cross-linking occurs subsequently to amino acid oxidation.

Cardiovascular responses in healthy subjects to a novel oral dopamine agonist, fenoldopam

Two double-blind randomized crossover studies were carried out in healthy male subjects to determine the cardiovascular effects of fenoldopam and to assess whether they could be modified by metoclopramide. Blood pressure (BP) and heart rate (HR) were measured supine and upright, before and for up to 4 h after dosing. Single oral doses of fenoldopam 50, 100 or 150 mg or placebo were administered in one study. In the other study, subjects received 20 mg metoclopramide or placebo intravenously, 20 min before an oral dose of 100 mg fenoldopam or placebo. Fenoldopam produced decreases in diastolic BP and increases in HR indicative of an orally active systemic arteriolar vasodilator. Mean peak responses occurred 30-60 min after dosing and values had returned to pretreatment levels within 3-4 h. The decrease in diastolic BP was partially attenuated by pretreatment with metoclopramide. Thus, at least in part, the fall in BP following fenoldopam could be due to an interaction with dopamine receptors.

A protein of the Z class of liver cytosolic proteins in the rat that preferentially binds heme

A low molecular weight protein purified from rat liver cytosol was observed to bind heme with an affinity higher than that for other organic anions. Purification was achieved by two procedures, one employing affinity chromatography on oleic acid-agarose, and the other using sequential ion-exchange and gel filtration chromatography after initial removal of aprotinin-sensitive proteases. Removal rather than inhibition of proteases improved the yield four times. Both procedures produced a stable protein. The purified protein binds heme with a higher affinity (Kd 0.15 microM) than any other organic anion tested including other (metallo)porphyrins, bilirubin, and oleic acid. Based on its molecular weight, amino acid composition, immunological properties, and the increase of its tissue levels in response to the administration of hypolipidemic agents, the protein was identified as being related to proteins of the Z class, whose members include fatty acid binding protein and sterol carrier protein. Like other Z proteins, our protein exhibits several forms on electrofocusing, but differs from fatty acid-binding protein and sterol carrier protein in that its major form exhibits a pI of 7.4. In view of its distinct isoelectric focusing pattern, its higher affinity for heme than for oleic acid, and its apparent inability to bind cholesterol and steroids, we cannot identify this protein as any of the above-mentioned proteins of the Z class. Consequently we have provisionally designated it heme-binding protein.

Purification and characterization of protein Z from rabbit liver cytosol

Protein Z was purified from rabbit liver cytosol by affinity chromatography on oleic acid-agarose and preparative sodium dodecyl sulfate-polyacrylamide gel electrophoresis. After removal of sodium dodecyl sulfate, the renatured protein was found to bind heme and bilirubin with a Kd of approximately 1 microM which produced large red shifts in their absorption spectra. On isoelectric focusing, rabbit protein Z exhibited two main bands with pI around 6.0.

Carbonic anhydrase activity in mitochondria from rat liver

An 18O exchange method has been used to determine the location of carbonic anhydrase in mitochondria from rat liver and to examine the role of this enzyme in the kinetics of CO2 in resting and respiring mitochondria. Using digitonin subfractionation, we have determined that a substantial fraction, 40 to 60%, of the carbonic anhydrase activity in the mitochondrion from rat liver is located in the space between the inner and outer membranes; the remaining activity was found in the matrix with no detectable activity in the sedimented membranes. The total catalytic CO2 hydration activity measured in intact mitochondria from rat liver was about 1% of that found in an equal volume of rat erythrocytes. The apparent permeability constant representing the barrier for the diffusion of HCO3(-) from external solution to intramitochondrial carbonic anhydrase, 9 X 10(-5) cm s-1, is near in magnitude to the permeability constant for the diffusion of HCO3(-) across the rat erythrocyte membrane, 4 X 10(-4) cm s-2. Calcium-induced respiratory jumps were shown to cause changes in the rate of 18O exchange between CO2 and H2O that were consistent with a net uptake of CO2 by the mitochondria.