Delineating novel metabolic pathways of DPC 963, a non-nucleoside reverse transcriptase inhibitor, in rats. Characterization of glutathione conjugates of postulated oxirene and benzoquinone imine intermediates by LC/MS and LC/NMR

The metabolic activation of (S)-5,6-difluoro-4-cyclopropylethynyl-4-trifluoromethyl-3,4-dihydro-2(1H)-quinazolinone, DPC 963, in rats was investigated by identifying and characterizing the GSH and mercapturic acid conjugates excreted in the bile and urine, respectively. The structures of these adducts, which were unequivocally elucidated by LC/MS/MS and NMR experiments, revealed the existence of at least three distinct metabolic pathways leading to these products. One of the pathways, which has been described previously, involves the activation of the acetylene group after an initial hydroxylation on the methine carbon of the cyclopropyl ring. Metabolite M1 was demonstrated to be formed via this pathway after an enzymatic addition of GSH across the triple bond of the substituted acetylene. The second pathway, also previously described, leads to diastereoisomeric GSH adducts M3 and M4 after the formation of a highly reactive oxirene intermediate. This postulated oxirene subsequently rearranges to an alpha, beta-unsaturated cyclobutenyl ketone intermediate capable of undergoing a 1,4-Michael addition with a nucleophile such as GSH. In addition to these pathways, DPC 963 was found to undergo a metabolic activation previously undescribed for structural analogues of this compound. It is postulated that an oxidative defluorination mediated by cytochrome P450 leads to the formation of a putative benzoquinone imine intermediate which subsequently reacts with GSH to form two aromatic ring-substituted regioisomeric conjugates, M5 and M6. In addition to forming the GSH adducts, the benzoquinone imine was also found to be reduced to its unreactive hydroquinone metabolite, which was excreted as the glucuronide conjugate in rat bile. Studies with induced rat microsomes, cDNA-expressed rat P450 isozymes, and polyclonal antibodies against rat P450 clearly demonstrated that the rat P450s 3A1/3A2 were responsible for the formation of postulated oxirene and benzoquinone intermediates.

The species-dependent metabolism of efavirenz produces a nephrotoxic glutathione conjugate in rats

Efavirenz, a potent nonnucleoside reverse transcriptase inhibitor widely prescribed for the treatment of HIV infection, produces renal tubular epithelial cell necrosis in rats but not in cynomolgus monkeys or humans. This species selectivity in nephrotoxicity could result from differences in the production or processing of reactive metabolites, or both. A detailed comparison of the metabolites produced by rats, monkeys, and humans revealed that rats produce a unique glutathione adduct. The mechanism of formation and role of this glutathione adduct in the renal toxicity were investigated using both chemical and biochemical probes. Efavirenz was labeled at the methine position on the cyclopropyl ring with the stable isotope deuterium, effectively reducing the formation of the cyclopropanol metabolite, an obligate precursor to the glutathione adduct. This substitution markedly reduced both the incidence and severity of nephrotoxicity as measured histologically. Further processing of this glutathione adduct was also important in producing the lesion and was demonstrated by inhibiting gamma-glutamyltranspeptidase with acivicin pretreatment (10 mg/kg, IV) prior to dosing with efavirenz. Again, both the incidence and severity of the nephrotoxicity were reduced, such that four of nine rats given acivicin were without detectable lesions. These studies provide compelling evidence that a species-specific formation of glutathione conjugate(s) from efavirenz is involved in producing nephrotoxicity in rats. Mechanisms are proposed for the formation of reactive metabolites that could be responsible for the renal toxicity observed in rats.

Mass spectrometric and NMR characterization of metabolites of roxifiban, a potent and selective antagonist of the platelet glycoprotein IIb/IIIa receptor

1. The methyl ester prodrug roxifiban is an orally active, potent and selective antagonist of the platelet glycoprotein GPIIb/IIIa receptor and is being developed for the prevention and treatment of arterial thrombosis. 2. Roxifiban was rapidly hydrolyzed to the zwitterion XV459 in vivo and by liver slices from the rat, mouse and human and by intestinal cores from dog. XV459 was metabolized to only a small extent in vitro and in vivo. 3. Studies with rat and dog given radiolabelled roxifiban showed limited oral absorption with the majority of the radiolabel being excreted in faeces. After i.v. doses of 14C-roxifiban, most of the radioactivity was recovered in the urine of rat whereas the dog excreted significant amounts of radioactivity in bile and urine. 4. XV459 could be metabolized extrahepatically by dog gut flora to produce an isoxazoline ring-opened metabolite. In vitro hepatic metabolism of XV459 was mainly by hydroxylation at the prochiral and chiral centres of the isoxazoline ring. These hydroxylated metabolites were not detected in the urine and plasma of human volunteers administered roxifiban. 5. Initial LC/MS identification of metabolites was achieved by dosing the rat with an equimolar mixture of d0:d4 roxifiban and detecting isotopic clusters of pseudomolecular ions. Unequivocal characterization of these metabolites was achieved by LC/MS, LC/NMR and high-field NMR techniques using synthetic standards of the metabolites. 6. The synthesis of one hydroxylated metabolite enabled the assignment of the correct stereochemistry of the substituted hydroxyl group on the isoxazoline ring.

Disposition of glutathione conjugates in rats by a novel glutamic acid pathway: characterization of unique peptide conjugates by liquid chromatography/mass spectrometry and liquid chromatography/NMR

With the advent of liquid chromatography/mass spectrometry and liquid chromatography/NMR, it has become easier to characterize metabolites that were once difficult to isolate and identify. These techniques have enabled us to uncover the existence of an alternate pathway for the disposition of glutathione adducts of several structurally diverse compounds. Studies were carried out using acetaminophen as a model compound to investigate the role of the glutamic acid pathway in disposition of the glutathione adducts. Although the mercapturic acid pathway was the major route of degradation of the glutathione adducts, it was found that the conjugation of the glutathione, cysteinylglycine, and cysteine adducts of acetaminophen with the gamma-carboxylic acid of the glutamic acid was both interesting and novel. The coupling of the glutathione adduct and the products from the mercapturic acid pathway with the glutamic acid led to unusual peptide conjugates. The natures of these adducts were confirmed unequivocally by comparisons with synthetic standards. This pathway (addition of glutamic acids) led to larger peptides, in contrast to the mercapturic acid pathway, in which the glutathione adducts are broken down to smaller molecules. The enzyme responsible for the addition of glutamic acid to the different elements of the mercapturic acid pathway is currently unknown. It is postulated that the gamma-carboxylic acid is activated (perhaps by ATP) before enzymatic addition to the alpha-amino group of cysteine or glutamate takes place. The discovery of these peptide conjugates of acetaminophen represents a novel disposition of glutathione adducts of compounds. The formation of such conjugates may represent yet another pathway by which drugs could produce covalent binding via their reactive intermediates.

Identification and characterization of efavirenz metabolites by liquid chromatography/mass spectrometry and high field NMR: species differences in the metabolism of efavirenz

Efavirenz (Sustiva, Fig. 1) is a potent and specific inhibitor of HIV-1 reverse transcriptase approved for the treatment of HIV infection. To examine the potential differences in the metabolism among species, liquid chromatography/mass spectrometry profiles of efavirenz metabolites in urine of rats, guinea pigs, hamsters, cynomolgus monkeys, and humans were obtained and compared. The metabolites of efavirenz were isolated, and structures were determined unequivocally by mass spectral and NMR analyses. Efavirenz was metabolized extensively by all the species as evidenced by the excretion of none or trace quantities of parent compound in urine. Significant species differences in the metabolism of efavirenz were observed. The major metabolite excreted in the urine of all species was the O-glucuronide conjugate (M1) of the 8-hydroxylated metabolite. Efavirenz was also metabolized by direct conjugation with glucuronic acid, forming the N-glucuronide (M2) in all five species. The sulfate conjugate of 8-OH efavirenz (M3) was found in the urine of rats and cynomolgus monkeys but not in humans. In addition to the aromatic ring-hydroxylated products, metabolites with a hydroxylated cyclopropane ring (at C14) were also isolated. GSH-related products of efavirenz were identified in rats and guinea pigs. The cysteinylglycine adduct (M10), formed from the GSH adduct (M9), was found in significant quantities in only rat and guinea pig urine and was not detected in other species. In vitro metabolism studies were conducted to show that the GSH adduct was produced from the cyclopropanol intermediate (M11) in the presence of only rat liver and kidney subcellular fractions and was not formed by similar preparations from humans or cynomolgus monkeys. These studies indicated the existence of a specific glutathione-S-transferase in rats capable of metabolizing the cyclopropanol metabolite (M11) to the GSH adduct, M9. The biotransformation pathways of efavirenz in different species were proposed based on some of the in vitro results.

Liquid chromatography/mass spectrometry and high-field nuclear magnetic resonance characterization of novel mixed diconjugates of the non-nucleoside human immunodeficiency virus-1 reverse transcriptase inhibitor, efavirenz

Efavirenz (Sustiva) is a potent and specific inhibitor of the HIV-1 reverse transcriptase and is approved for the treatment of HIV infection. The metabolism of efavirenz in different species has been described previously. Efavirenz is primarily metabolized in rats to the glucuronide conjugate of 8-OH efavirenz. Electrospray ionization-liquid chromatography/mass spectrometry analyses of bile samples from rats dosed with either efavirenz or with 8-OH efavirenz revealed three polar metabolites, M9, M12, and M13, with pseudomolecular ions [M-H](-) at m/z 733, 602, and 749, respectively. The characteristic mass spectral fragmentation patterns obtained for metabolites M9 and M13 suggested that these were glutathione-sulfate diconjugates, and the presence of a glutathione moiety in metabolite M9 was confirmed by liquid chromatograpy/nuclear magnetic resonance (NMR) analysis of bile extracts. Metabolite M12 was characterized by liquid chromatography/mass spectrometry as a glucuronide-sulfate diconjugate. Unambiguous structures of M9, M12, and M13 were obtained from one-dimensional proton and carbon NMR as well as proton-proton (correlated spectroscopy, two-dimensional shift correlation), proton-carbon heteronuclear multiple quantum correlation, and long-range proton-carbon (heteronuclear multiple bond correlation) correlated two-dimensional NMR analyses of metabolites isolated from rat bile. The mass spectral and NMR analyses of M10, which was isolated from rat urine, suggested a cysteinylglycine-sulfate diconjugate. The isolation of these polar metabolites for further characterization by NMR was aided by mass spectral analyses of HPLC fractions and solid phase extraction extracts during the isolation steps. The complete characterization of these novel diconjugates demonstrates that further phase II metabolism of polar conjugates such as sulfates could take place in vivo.

Application of liquid chromatography/mass spectrometry in accelerating the identification of human liver cytochrome P450 isoforms involved in the metabolism of iloperidone

Iloperidone, [1-[4-[3-[4-(6-fluoro-1, 2-benzisoxazol-3-yl)-1-piperidinyl]propoxy]-3-methoxyphenyl]eth anone, 1, is currently undergoing clinical trials as a potential antipsychotic agent. The metabolism of iloperidone was studied in human liver microsomes to define the metabolic pathways and to identify the cytochrome P450 (CYP) isoforms responsible for the formation of major iloperidone metabolites. Iloperidone was extensively metabolized in vitro via hydroxylation, reduction and O-demethylation to produce 1-[4-[3-[4-(6-fluoro-1, 2-benzisoxazol-3-yl)-1-piperidinyl]propoxy]-3-methoxyphenyl]-2- hydrox yethanone, 4; 4-[3-[4-(6-fluoro-1, 2-benzisoxazol-3-yl)-1-piperidinyl]propoxy]-3-methoxy-alpha-met hylben zene methanol, 3, and 1-[4-[3-[4-(6-fluoro-1, 2-benzisoxazol3-yl)-1-piperidinyl]propoxy]-3-hydroxyphenyl]etha none, 2, respectively, in decreasing order of abundance. The major in vitro metabolite, 4, present in trace quantities in urine, was postulated to be either eliminated in bile as a conjugate or further metabolized to a phenol, 4-[3-[4-(6-fluoro-1, 2-benzoisoxazol-3-yl)-piperidin1-yl]propoxy]-3-methoxyphenol , 5. The formation of the three major in vitro metabolites 2, 3 and 4 was NADPH dependent. The major circulating and urinary metabolite in humans dosed with 1 was metabolite 3. The mean apparent Km and Vmax for formation of 2 by human liver microsomes was 7.4 +/- 3.0 microM and 0.0343 +/- 0.0134 nmol min-1 mg-1, respectively. The mean apparent Km and Vmax for 3 was 101.2 +/- 34.7 microM and 0.1414 +/- 0.0346 nmol min-1 mg-1, respectively. The mean apparent Km and Vmax for 4 was 39.7 +/- 10.8 microM and 0.1372 +/- 0.056 nmol min-1 mg-1, respectively. The CYP isoenzymes responsible for the formation of metabolites 2, 3 and 4 were determined by using selective chemical inhibitors and by correlation studies. Metabolites 2 and 4 were formed by CYP3A4 and by the polymorphic CYP2D6 respectively. Metabolite 3 is postulated to be produced mainly by a cytosolic enzyme(s), although CYP3A, CYP1A2 and CYP2E1 isozymes were shown to be involved in its formation as well. The power of liquid chromatography/mass spectrometry in greatly accelerating the process of identifying the human liver CYP isoforms involved in the metabolism of iloperidone was demonstrated in this study. Liquid chromatography/mass spectrometry was used in the initial studies to confirm the identities of the metabolites. This was followed by accurate and reliable quantitation of individual metabolites present in biological extracts by operating the mass spectrometer in the selected ion monitoring mode.

Metabolism of an atypical antipsychotic agent, 3-[4-[4-(6-fluorobenzo[b]thien-3-yl)-1-piperazinyl]butyl]-2, 5,5-trimethyl-4-thiazolidinone (HP236)

Metabolism of an atypical antipsychotic agent, 3-[4-[4-(6-fluorobenzo[b]thien-3-yl)-1-piperazinyl]butyl]-2, 5,5-trimethyl-4-thiazolidinone (HP236) by rats is described. HP236 was extensively metabolized both in vitro and in vivo. Metabolites were identified using electrospray interface-LC/MS (ESI-LC/MS) and 1H NMR spectroscopy. Protonated molecular ions, MH+, were observed for all the metabolites of HP236 using ESI-LC/MS. Tandem MS was performed on these quasimolecular ions to provide structural information. Structures of metabolites were confirmed by chromatographic and spectroscopic comparisons to synthetic standards. Metabolic pathways for HP236 both in vivo and in vitro included: a) cleavage of the thiazolidinone ring structure to give N-acetyl-N-[4-[4-(6-fluorobenzo[b] thien-3-yl)-1-piperazinyl]butyl]-2-methyl-propanamide, N-[4-[4-(6-fluorobenzo[b] thien-3-yl)-1-piperazinyl]-butyl]-2-methyl-propanamide, and N-[4-[4-(6-fluorobenzo[b] thien-3-yl)-1-piperazinyl]butyl]-acetamide; b) oxidation of the sulfide to give a mixture of sulfoxide diastereoisomers, 3-[4-[4-(6-fluorobenzo[b]thien-3-yl)-1-piperazinyl]butyl]-1-oxo-2, 5,5-trimethyl-4-thiazolidinone and a sulfone, 1,1-dioxo-3-[4-[4-(6-fluorobenzo[b]thien-3-yl)-1-piperazinyl]butyl ]-2, 5,5-trimethyl-4-thiazolidinone; and c) N-dealkylation at the piperazine ring to produce 3-(4-(1-piperazinyl]butyl]-2, 5,5-trimethyl-4-thiazolidinone and 6-fluoro-3-(1-piperazinyl)benzo[b]thiophene. Metabolites found circulating in the plasma of rats dosed with HP236 were identical to those produced in vitro using rat liver microsomes or S9 fractions; however, LC/MS analysis of rat urine extract showed that the metabolite profile was different than that obtained from plasma or from in vitro extracts. The metabolite resulting from the cleavage of the thiazolidinone ring, N-acetyl-N-[4-[4-(6-fluorobenzo[b] thien-3-yl)-1-piperazinyl]-butyl]-2-methyl-propanamide, was the major circulating metabolite in the plasma of rats dosed with HP236. Results from in vitro studies showed that the metabolite was also produced by incubating the sulfoxides and sulfone analogs of HP236 with rat liver microsomes. A mechanism for the formation of N-acetyl-N-[4-[4-(6-fluorobenzo [b]thien-3-yl)-1-piperazinyl]butyl]-2-methyl-propanamide from HP236 is proposed.

Pharmacological evaluation of novel Alzheimer's disease therapeutics: acetylcholinesterase inhibitors related to galanthamine

Acetylcholinesterase (AChE) inhibitors from several chemical classes have been tested for the symptomatic treatment of Alzheimer's disease; however, the therapeutic success of these compounds has been limited. Recently, another AChE inhibitor, galanthamine hydrobromide (GAL), has shown increased clinical efficacy and safety. Using biochemical, behavioral and pharmacokinetic analyses, this report compares GAL with two of its analogs, 6-O-acetyl-6-O-demethylgalanthamine hydrochloride (P11012) and 6-O-demethyl-6-O[(adamantan-1-yl)-carbonyl]galanthamine hydrochloride (P11149), for their therapeutic potential. P11012 and P11149 were found to be potent, competitive and selective inhibitors of AChE, demonstrating central cholinergic activity, behavioral efficacy and safety. P11012 and P11149, though pharmacokinetic analyses, were shown to act as pro-drugs, yielding significant levels of 6-O-demethylgalanthamine. In vitro, 6-O-demethylgalanthamine was 10- to 20-fold more potent than GAL as an inhibitor of AChE, and it demonstrated greater selectivity for inhibition of AChE vs. butyrylcholinesterase. Like GAL, both P11012 and P11149 showed central cholinergic activity biochemically, by significantly inhibiting rat brain AChE; physiologically, by causing hypothermia; and behaviorally, by attenuating scopolamine-induced deficits in passive avoidance. In addition, GAL, P11012 and P11149 enhanced step-down passive avoidance, another measure of behavioral efficacy. By comparing efficacious doses with primary overt effects, P11012 and P11149 had better oral therapeutic indices than GAL. Oral pharmacokinetic analyses of GAL, P11012 and P11149 revealed differences. Although P11012 and P11149 exhibited similar area under the curve values, 191149 had slower, lower and more sustained concentration maximum levels. P11012 and GAL rapidly reached their concentration maximums, but GAL, in brain had the highest area under the curve and concentration maximum. Because of its composite profile, including duration of action, oral therapeutic index and pharmacokinetics, P11149 is considered the better therapeutic candidate for the treatment of Alzheimer's disease.

Application of hyphenated LC/NMR and LC/MS techniques in rapid identification of in vitro and in vivo metabolites of iloperidone

Iloperidone, 1(-)[4(-)[3(-)[4-(6-fluro-1,2-benzisoxazol-3-yl)-1- piperidinyl]propoxy]-3-methoxyphenyl]ethanone, is currently undergoing clinical trials as a potential antipsychotic agent. Iloperidone was found to be extensively metabolized to a number of metabolites by rats, dogs, and humans. LC/MS/MS was used to characterize and identify metabolites of iloperidone present in complex biological mixtures obtained from all three species. Identification of some of the unknown metabolites in rat bile was achieved successfully by combination of LC/NMR and LC/MS with a minimum amount of sample cleanup. The utility of coupling a semipreparative HPLC to LC/MS instrument for further characterization of collected metabolites was demonstrated. It was shown that iloperidone was metabolized by O-dealkylation processes to yield 6-fluoro-3(-)[1-(3-hydroxypropyl)-4-piperidinyl]-1,2-benzisoxazole and 1(-)[4(-)[3(-)[4-(6-fluoro-1,2-benzisoxazol-3-yl)-1- piperidinyl]propoxy]-2-hydroxyphenyl]ethanone. Oxidative N-dealkylation led to the formation of 6-fluoro-3-(4-piperidinyl)-1,2-benzisoxazole and a secondary metabolite, 3(-)[(4-acetyl-2-methoxy)phenoxy]propionic acid. Iloperidone was reduced to produce 4(-)[3(-)[4-(6-fluoro-1,2-benzisoxazol-3-yl)-1-piperidinyl]- propoxy]-3-methoxy-alpha-methylbenzenemethanol as the major metabolite in humans and rats. Hydroxylation of iloperidone produced 1(-)[4(-)[3(-)[4-(6-fluoro-1,2-benzisoxazol-3-yl)-1- piperidinyl]propoxy]-2-hydroxy-5-methoxyphenyl]ethanone and 1(-)[4(-)[3(-)[4-(6-fluoro-1,2-benzisoxazol-3-yl)-1-piperidinyl]-3 -methoxyphenyl]propoxy]-2-hydroxyethanone, the later of which was found to be the principal metabolite in dogs. The identities of all these metabolites were established by comparing the LC/MS retention times and mass spectral data with synthetic standards.

Picogram determination of iloperidone in human plasma by solid-phase extraction and by high-performance liquid chromatography-selected-ion monitoring electrospray mass spectrometry

A very sensitive liquid chromatographic-mass spectrometric (LC-MS) method has been developed to quantitate iloperidone, 1, and its principal metabolite, 4-[3-[4-(6-fluoro-1,2- benzisoxazol-3-yl)-1-piperidinyl]propoxy]-3-methoxy-alpha- methylbenzenemethanol, 2, in human plasma. Iloperidone is currently used in clinical trials for the treatment of psychoses. The analytes were extracted from human plasma using mixed-mode Bond-Elut Certify cartridges and quantitated using selected-ion monitoring electrospray ionization mass spectrometery (SIM-ES-MS). No interference was observed from endogenous compounds following the extraction of plasma samples from six different human subjects. The limit of quantitation for 1 and 2 was 250 pg/ml of plasma. The standard curves were linear over a working range of 250 pg to 20 ng/ml. Absolute recoveries from plasma ranged from 82 to 101% and 73 to 97% for 1 and 2, respectively. Overall intra-day precision ranged from 0 to 9% and 0.9 to 12.5% for 1 and 2, respectively. The method was found to be rugged and very reliable due to the high specificity of SIM-ES-MS. The results obtained from this study confirm the application of solid-phase extraction combined with SIM-ES-MS in quantitating basic drugs in small quantities in biological extracts.

Determination of metoclopramide and two of its metabolites using a sensitive and selective gas chromatographic-mass spectrometric assay

A modified gas chromatographic-mass spectrometric (GC-MS) assay has been developed to quantitate metoclopramide (MCP) and two of its metabolites [monodeethylated-MCP (mdMCP), dideethylated-MCP (ddMCP)] in the plasma, bile and urine of sheep. The heptafluorobutyryl derivatives of the compounds were formed and quantitated using electron-impact ionization in the selected-ion monitoring mode (MCP, m/z 86, 380; mdMCP, m/z 380 and ddMCP, m/z 380). No interference was observed from endogenous compounds following the extraction of various biological fluids obtained from non-pregnant sheep. Sample preparation has been simplified and the method is more selective and sensitive (2 fold) than our previous assay using electron-capture detection. The limit of quantitation for MCP, mdMCP and ddMCP was 1 ng/ml in plasma, urine and bile, requiring 0.5 ml of sample. This represents 2.5 pg of the analytes at the detector. The standard curves were linear over a working range of 1-40 ng/ml. Absolute recoveries in plasma ranged from 76.5-94.7%, 79.2-96.8%, 80.3-102.2% for MCP, mdMCP and ddMCP, respectively. In urine, recoveries ranged from 56.5-87.8%, 61.5-87.5%, 62.6-90.2% for MCP, mdMCP and ddMCP, respectively. Recoveries in bile ranged from 83.5-100.9%, 78.5-90.5%, 66.9-79.2% for MCP, mdMCP and ddMCP, respectively. Overall intra-day precision ranged from 2.9% for MCP in plasma to 12.6% for mdMCP in bile. Overall inter-day precision ranged from 5.9% for MCP in urine to 14.9% for ddMCP in bile. Bias was the greatest at the 1 ng/ml concentration in all biological fluids ranging from a low of 2.4% for mdMCP in plasma to a high of 11.9% for ddMCP in urine. Applicability of the assay for pharmacokinetic studies of MCP, mdMCP and ddMCP in the plasma and urine of a non-pregnant ewe is demonstrated.

Characterization of metabolites of xylazine produced in vivo and in vitro by LC/MS/MS and by GC/MS

The metabolic fate of xylazine, 2-(2,6-dimethylphenylamino)-5,6-dihydro-4H-1,3-thiazine, in horses is described. The major metabolites identified in the hydrolyzed horse urine were 2-(4'-hydroxy-2',6'-dimethylphenylamino)-5,6-dihydro-4H-1,3-thiazi ne, 2-(3'-hydroxy-2',6'-dimethylphenylamino)-5,6-dihydro-4H-1,3-thiazi ne, N-(2,6-dimethylphenyl)thiourea, and 2-(2',6'-dimethylphenylamino)-4-oxo-5,6-dihydro-1,3-thiazine. These metabolites were also produced by incubating xylazine with rat liver microsomes. The major metabolite produced in vitro by rat liver preparations was found to be the ring opened N-(2,6-dimethylphenyl)thiourea. The identities of these metabolites were confirmed by spectroscopic comparisons with synthetic standards. Phenolic metabolic standards were synthesized efficiently by the use of Fenton's reagent. This reagent was used to monohydroxylate multiply substituted aromatic ring systems. LC/MS/MS, with an atmospheric pressure chemical ionization source, was found to be particularly useful in confirming the presence of phenolic metabolites in hydrolyzed equine urine and microsomal extracts. These phenolic metabolites could not be analyzed by GC/MS even after derivatization with silylating agents. The advantage of LC/MS/MS was that no or little sample preparation of urine or microsomal extract was necessary prior to the analysis. A mechanism is also proposed for the formation of the major metabolite, N-(2,6-dimethylphenyl)thiourea, from xylazine.

Isolation and characterization of carbinolamide and phenolic glucuronide conjugates of (+-)-N-methyl-N-(1-methyl-3,3-diphenylpropyl) formamide and N-formylmethamphetamine by FAB/MS, LC/MS/MS, and NMR

The metabolic disposition of (+-)-N-methyl-N-(1-methyl-3,3- diphenyl-propyl)formamide, especially with regard to the formation of water soluble glucuronides, is described. The glucuronide conjugates, (+-)-N-hydroxymethyl-N-(1-methyl-3,3-diphenylpropyl)formamide glucuronide, (+-)-N-methyl-N-[1-methyl-3-(4'-hydroxyphenyl)-3-phenylpropyl]formamide glucuronide, and (+-)-N-methyl-N-[1-methyl-3-(4'-hydroxy-3'-methoxyphenyl)-3- phenylpropyl]formamide glucuronide were isolated from the bile of rats dosed with the parent compound. These conjugates were characterized spectroscopically by 1H-NMR, FAB/MS, and LC/MS/MS. Because it is becoming more common to isolate the intact glucuronide conjugates of xenobiotics, we investigated some common mass spectral fragmentation patterns of these conjugates, especially by LC/MS/MS. The fragmentation patterns for each of the conjugates were obtained under MS/MS conditions and compared. Specifically, the fragmentation patterns of phenolic glucuronide and an aliphatic O-glucuronide, in particular a carbinolamide glucuronide, were investigated. The data obtained from these studies was used to predict the nature of glucuronide conjugates obtained from rats dosed with the formamide analog, N-formylmethamphetamine. This is the first spectroscopic characterization of an intact carbinolamide glucuronide conjugate isolated from the bile of rats.

Formation and reversibility of S-linked conjugates of N-(1-methyl-3,3-diphenylpropyl)isocyanate, an in vivo metabolite of N-(1-methyl-3,3-diphenylpropyl)formamaide, in rats

The metabolic disposition of N-(1-methyl-3,3-diphenylpropyl) formamide was studied in rats. The water-soluble metabolites, N-acetyl-S-[N-(1-methyl-3,3-diphenylpropylcarbamoyl)]cysteine and S-[N-(1-methyl-3,3-diphenylpropylcarbamoyl)]glutathione, were identified in urine and bile, respectively, of rats doses with the secondary formamide. The structures of these metabolites were confirmed by comparison with synthetic standards and by using liquid chromatography mass spectrometry and fast atom bombardment mass spectrometry. Synthetic standards of these metabolites were obtained by reacting the N-(1-methyl-3,3-diphenylpropyl)isocyanate with glutathione or N-acetylcysteine in methanolic solutions. The isocyanate was obtained in high yield by reacting 1-methyl-3,3-diphenylpropylamine with trichloromethyl chloroformate. The S-linked conjugates released the isocyanate in mild alkali, but were stable under acidic conditions. The released isocyanate was characterized by comparison with the synthetic standard using GC/MS and HPLC. A mechanism is proposed for the base-catalyzed elimination of the isocyanate from the thiol conjugates.

Pathways of gallopamil metabolism. Regiochemistry and enantioselectivity of the N-dealkylation processes

The N-dealkylation pathway for the metabolism of pseudoracemic gallopamil was studied in the presence of rat and human liver microsomes and in vivo in rats and man. Metabolites were characterized by comparison of their GC/MS retention times and fragmentation patterns with those of authentic compounds. In the presence of rat liver microsomes, N-dealkylation accounted for about 90% of the observed oxidative metabolism, affording a 4:1 ratio of norgallopamil (2) and, N-methyl-N-[2-methyl-3-cyano-3-(3,4,5-trimethoxyphenyl)-6-hexyl] amine (3), and about 1% of N-methyl-N-(3,4-dimethoxyphenethyl)amine (4). Secondary amines 2 and 3 arose enantioselectively from S-(-)-gallopamil, the S/R ratios being 1.36 and 1.71, respectively. The alcohols, 3,4-dimethoxyphenylethanol (6) and 2-methyl-3-cyano-3-(3,4,5-trimethoxyphenyl)-6-hexanol (8), were formed from the respective intermediate aldehydes 5 and 7, probably non-enzymatically, under the reductive conditions (NADPH) of the microsomal incubations. Incubation of gallopamil with 9,000g supernatant fraction of rat liver led to carboxylic acid metabolites arising from oxidative metabolism of the aldehydes. 3,4-Dimethoxyphenylacetic acid (12) and 4-(3,4,5-trimethoxyphenyl)-5-methyl-4-cyanohexanoic acid (11) were formed in a 3:5:1 ratio. In the presence of human liver microsomes, formation of 2 also predominated over formation of 3, with alcohols 6 and 8 being produced as well. However, 4 was not observed. Consistently, the N-dealkylation process provided slightly more R than S products with the S/R ratio being 0.7-0.9 for metabolites 2, 3, 6, and 8. The amines formed from N-dealkylation were also observed as urinary metabolites in a human subject after a single oral dose of pseudoracemic gallopamil.(ABSTRACT TRUNCATED AT 250 WORDS)

Pathways of gallopamil metabolism. Regiochemistry and enantioselectivity of the O-demethylation processes

The oxidative O-demethylation of pseudoracemic gallopamil by rat and human liver microsomes was studied. By comparison of GC/MS retention times and fragmentation patterns with data from authentic standards, the four possible regioisomeric monophenolic metabolites, 2-(4-hydroxy-3,5-dimethoxyphenyl)-2-isopropyl-5-[(3,4- dimethoxyphenethyl)methylamino]-valeronitrile (2), 2-(5-hydroxy-3,4-dimethoxyphenyl)-2-isopropyl-5-[(3,4- dimethoxyphenethyl)methylamino]valeronitrile (3), 2-(3,4,5-trimethoxyphenyl)-2-isopropyl-5-[(4-hydroxy-3-methoxyphenethyl) -methylamino]valeronitrile (4), and 2-(3,4,5-trimethoxyphenyl)-2-isopropyl-5-[(3-hydroxy-4- methoxyphenethyl)methylamino]valeronitrile (5), were characterized. Rat liver microsomal oxidation produced all four regioisomeric monophenols which accounted for only 10% of the oxidative metabolism, the remaining 90% being N-dealkylation metabolites. Preference for metabolism of the O-methyl ethers at p-positions on each of the aromatic ring systems was noted, with more O-demethylation of the O-methyl ethers on the aromatic ring adjacent to the chiral center than on the aromatic ring in the short side chain. Significant enantio-selectivity was noted, the S/R ratios being 2.26, 1.97, 1.87 and 1.30 for formation of 2, 3, 4 and 5, respectively. Biliary excretion of the O-demethylated metabolites as conjugates, cleaved by beta-glucuronidase, was observed in rats after administration of pseudoracemic gallopamil. Significant stereoselectivity was noted, S/R ratios being 0.62, 1.61, 1.49 and 2.19 for 2, 3, 4 and 5, respectively. Human liver microsomal oxidation produced more p- than m-O-demethylation, with 4 less than 5, and 2 less than 3, but quantitatively the pathway is a minor one compared to N-dealkylation.(ABSTRACT TRUNCATED AT 250 WORDS)

Synthesis and identification of the N-glucuronides of norgallopamil and norverapamil, unusual metabolites of gallopamil and verapamil

N-Glucuronides of norgallopamil and norverapamil were found as biliary metabolites after administering the corresponding tertiary amines, gallopamil and verapamil, to rats. The structures of these unusual metabolites were established by comparison with spectral data of synthesized authentic standards and by enzymic hydrolysis of the conjugates. The N-glucuronide standards were synthesized by coupling the secondary amines to either glucuronic acid or to methyl tetra-O-acetyl-beta-D-glucopyranuronate. On i.p. dosing of rats with gallopamil or verapamil, 13 and 2% of the dose, respectively, appeared in the bile as the N-glucuronide of the secondary amine metabolite over an 8-hr period. Administration of norgallopamil resulted in approximately 25% of the dose being excreted as N-glucuronide conjugate in the bile. Substantially more of the S- than R-enantiomer of both gallopamil and verapamil was converted to the corresponding secondary amine N-glucuronide. The observed high S/R ratios suggest enantio-selectivity in this pathway could contribute to the observed stereoselectivity in other routes of metabolism of the parent tertiary amines.

Biotransformation of aliphatic formamides: metabolites of (+-)-N-methyl-N-(1-methyl-3,3-diphenylpropyl) formamide in rats

The in vivo biliary and urinary metabolites of (+-)-N-methyl-N-(1-methyl-3,3-diphenylpropyl) formamide (1) from male Wistar rats have been characterized by gas chromatography/mass spectrometry. In urine, non-conjugated metabolites included 1,1-diphenyl-3-butanone (4) and 3-methylamino-1,1,diphenylbutane (7). beta-Glucuronidase liberated 4, 1,1-diphenyl-3-butanol (5), 1,1-diphenyl-3-butanone oxime (6), N-hydroxymethyl-N-(1-methyl-3, 3-diphenylpropyl) formamide (3), 1-(4-hydroxyphenyl)-1-phenyl-3-butanone (11), 1-(4-hydroxyphenyl)-1-phenyl-3-butanone oxime (12), N-methyl-N-(1-methyl-3-(4-hydroxyphenyl)-3-phenylpropyl) formamide (8), 1-(4-hydroxy-3-methoxyphenyl)-1-phenyl-3-butanone (16); 1-(4-hydroxy-3-methoxyphenyl)-1-phenyl-3-butanol (17), 1-(4-hydroxy-3-methoxyphenyl)-1-phenyl-3-butanone oxime (18), N-(1-methyl-3-(4-hydroxy-3-methoxyphenyl)-3-phenylpropyl) formamide (14) and N-methyl-N-(1-methyl-3-(4-hydroxy-3-methoxyphenyl)-3-phenylpropyl) formamide (13). Most of the carbinolamide (3) decomposed in the gas chromatograph inlet to N-(1-methyl-3,3-diphenylpropyl) formamide (2) unless stabilized as a trimethylsilyl (TMS) derivative. In bile, compounds 1, 2, 3, 5, 6, 11, 12 and 16 were present as non-conjugated metabolites. beta-Glucuronidase also liberated N-(1-methyl-3-(4-hydroxyphenyl-3-phenylpropyl) formamide (9), and all of the previously listed compounds except 7. Trimethylsilylation of the conjugated bile fraction revealed the presence of an additional two compounds: N-hydroxymethyl-N-(1-methyl-3-(4-hydroxyphenyl)-3-phenylpropyl) formamide (10) and N-hydroxymethyl-N-(1-methyl-3-(4-hydroxy-3-methoxyphenyl)-3-phenylpropyl ) formamide (15). A stable carbinolamide metabolite standard was synthesized and the mass spectral fragmentations of its TMS derivative studied by tandem mass spectroscopy. This is the first report on stable carbinolamide metabolites of high-molecular-weight formamides.

The pharmacokinetics and tissue distribution of gomphoside in Wistar rats

The excretion and tissue distribution of [3H]-gomphoside was studied after i.p. and i.v. administration of the cardiac glycoside (1 micrograms/g) to male Wistar rats. Following an intraperitoneal dosage of [3H]-gomphoside, most of the radioactivity (greater than 80%) had been excreted from the body by the end of 48 hours. Biliary excretion played a major role in elimination of [3H]-gomphoside with 90 +/- 15% of radioactivity being collected in 24 hours. Renal excretion formed a minor route of elimination of the cardiac glycoside; only 6 +/- 2% being excreted over 6 days. The distribution of radioactivity to tissues after an intravenous dose was rapid; most of the dose was located in the liver (32%), and the skeletal muscle (31%) 3 minutes after injection. The pharmacokinetics of [3H]-gomphoside could be described by a two-compartment open model with an average elimination half-life of 3.7 hours, and a large volume of distribution (2.3 +/- 0.3 ml/g body weight) characteristic of the commonly used cardiac glycosides (1).

In vitro metabolism of the cardiotonic steroids gomphogenin and calactin

The metabolism of gomphogenin and calactin was studied in vitro using respectively microsomes and the S9 fraction of homogenates from rat liver. These two substrates were previously shown to be in vitro and in vivo metabolites of gomphoside, a cardiotonic steroid belonging to a class of 5 alpha-cardenolide glycosides with doubly-linked hexosulose sugars. Structures of new metabolites were elucidated using 400 MHz 1H-NMR and chemical ionization mass spectrometry, while known compounds were identified by direct comparison. The major metabolite isolated from gomphogenin (2 alpha-hydroxyuzarigenin) metabolism was the oxidation product 2-oxo-uzarigenin which was further oxidized metabolically to 4 alpha-hydroxy-2-oxo-uzarigenin. Other metabolites were 2 alpha-hydroxyuzarigenone and its reduction product 3-epigomphogenin. Calactin was oxidized in vitro to 10-carboxyl-19-norgomphoside, the predominant metabolite, and underwent cleavage of the doubly-linked sugar to yield calotropagenin.

In vivo and in vitro metabolism of gomphoside, a cardiotonic steroid with doubly-linked sugar

The metabolism of gomphoside, a cardiotonic steroid glycoside with doubly-linked 4,6-dideoxyhexosulose sugar was studied in vivo in rats, and in vitro using rat liver microsomes. The biliary excretion of metabolites, following intraperitoneal administrative of [3H]gomphoside, was rapid with 68% of radioactivity being collected over 8 h. The metabolites in the bile were principally a water-soluble glucuronide conjugate of gomphoside, and a small amount of chloroform-soluble metabolites. Conversion of [3H]gomphoside to metabolites by microsomes at 37 degrees C reached a maximum of 16% under optimum conditions, producing the same set of metabolites as those in the chloroform-soluble fraction of the bile. The major chloroform-soluble metabolite was the aglycone of gomphoside, viz. gomphogenin or 2 alpha,3 beta, 14-trihydroxy-5 alpha-card-20(22)-enolide. The other major component was recovered gomphoside. Other metabolites were calactin, calotropin, and 2 alpha-hydroxyuzarigenin 3-(4,6-dideoxy-beta-D-arabino-hexopyranoside). Another metabolite, which is a new cardenolide was shown to be 3-epi-gomphogenin or 2 alpha,3 alpha, 14-trihydroxy-5 alpha-card-20(22)-enolide. Gomphoside glucuronide was shown spectroscopically to have the glucuronide residue attached to position 3' of the hexosulose sugar. It was cleaved by beta-D-glucuronidase to gomphoside, and is thus gomphoside 3'-beta-D-glucuronide. The metabolic transformations of gomphoside are summarized in Fig. 5.