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George A Doss

Publications and source records attributed to George A Doss.

At least 19 recordsLinked to original sources
PubMed2006

Highly constrained bicyclic VLA-4 antagonists.

VLA-4 is implicated in several inflammatory and autoimmune disease states. A series of cyclic beta-amino acids (beta-aa) was studied as VLA-4 antagonists. Binding affinity was highly dependent on the dihedral angle (phi) between the amino and the carboxyl termini of the beta-aa. Compound 5 m where the beta-aa is embedded in a bicycle possesses the most preferred phi (120 degrees). It is a potent and bioavailable VLA-4 antagonist (VCAM-Ig alpha4beta1 IC50 = 54 nM, rat po F = 49%).

Biological Availability↗
PubMed2006

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.

Administration, Oral↗
PubMed2006

Short synthesis of octosyl nucleosides.

[reaction: see text] Commercial 1,2:5,6-di-O-isopropylidene-alpha-d-allofuranose was converted to a protected bicyclic octosyl acid thioglycoside donor by a 10-step sequence that features an intramolecular ester enolate alkylation. Glycosylation of N-benzoyladenine and methyl uridine-5-carboxylate followed by deprotection gave the respective nucleosides "octosyl adenine" and octosyl acid A.

Adenine↗
PubMed2006

Addressing metabolic activation as an integral component of drug design.

Formation of reactive intermediates by metabolism of xenobiotics represents a potential liability in drug discovery and development. Although it is difficult, if not impossible, to predict toxicities of drug candidates accurately, it is prudent to try to minimize bioactivation liabilities as early as possible in the stage of drug discovery and lead optimization. Measurement of covalent binding to liver microsomal proteins in the presence and the absence of NADPH, as well as the use of trapping agents such as glutathione or cyanide ions to provide structural information on reactive intermediates, have been used routinely to screen drug candidates. These in vitro experiments are often supplemented with in vivo covalent binding data in rats. The resulting data are not only used to eliminate potentially risky compounds, but, more importantly, they provide invaluable information to direct the Medicinal Chemistry group efforts to design analogs with less propensity to undergo bioactivation. Select case histories are presented in which this approach was successfully applied at Merck.

Animals↗
PubMed2005

Evidence for the bioactivation of zomepirac and tolmetin by an oxidative pathway: identification of glutathione adducts in vitro in human liver microsomes and in vivo in rats.

Although zomepirac (ZP) and tolmetin (TM) induce anaphylactic reactions and form reactive acyl glucuronides, a direct link between the two events remains obscure. We report herein that, in addition to acyl glucuronidation, both drugs are subject to oxidative bioactivation. Following incubations of ZP with human liver microsomes fortified with NADPH and glutathione (GSH), a metabolite with an MH+ ion at m/z 597 was detected by LC/MS/MS. On the basis of collision-induced dissociation and NMR evidence, the structure of this metabolite was determined to be 5-[4'-chlorobenzoyl]-1,4-dimethyl-3-glutathionylpyrrole-2-acetic acid (ZP-SG), suggesting that the pyrrole moiety of ZP had undergone oxidation to an epoxide intermediate, followed by addition of GSH and loss of the elements of H2O to yield the observed conjugate. The oxidative bioactivation of ZP most likely is catalyzed by cytochrome P450 (P450) 3A4, since the formation of ZP-SG was reduced to approximately 10% of control values following pretreatment of human liver microsomes with ketoconazole or with an inhibitory anti-P450 3A4 IgG. A similar GSH adduct, namely 5-[4'-methylbenzoyl]-1-methyl-3-glutathionylpyrrole-2-acetic acid (TM-SG), was identified when TM was incubated with human liver microsomal preparations. The relevance of these in vitro findings to the in vivo situation was established through the detection of the same thiol adducts in rats treated with ZP and TM, respectively. Taken together, these data suggest that, in addition to the formation of acyl glucuronides, oxidative metabolism of ZP and TM affords reactive species that may haptenize proteins and thereby contribute to the drug-mediated anaphylactic reactions.

Animals↗
PubMed2005

Dipeptidyl peptidase IV inhibitors derived from beta-aminoacylpiperidines bearing a fused thiazole, oxazole, isoxazole, or pyrazole.

A series of beta-aminoacylpiperidines bearing various fused five-membered heterocyclic rings was synthesized as dipeptidyl peptidase IV inhibitors. Potent and relatively selective inhibition could be obtained, depending on choice of heterocycle, regioisomerism, and substitution. In particular, one analog (74, DPP-IV IC50=26 nM) exhibited good oral bioavailability and acceptable half-life in the rat, albeit with rather high clearance.

Dipeptidyl Peptidase 4↗
PubMed2005

Synthesis and activity of 4,5-diarylimidazoles as human CB1 receptor inverse agonists.

Structure-activity relationship studies directed toward the optimization of 4,5-diarylimidazole-2-carboxamide analogs as human CB1 receptor inverse agonists resulted in the discovery of the two amide derivatives 24a and b (hCB1 IC50 = 6.1 and 4.0 nM) which also demonstrated efficacy in overnight feeding studies in the rat for reduction in both food intake and overall body weight.

Animals↗
PubMed2005

Estrogen receptor ligands. 12. Synthesis of the major metabolites of an ERalpha-selective, dihydrobenzoxathiin antagonist for osteoporosis.

[reaction: see text] During the course of drug metabolism studies, a major metabolite of compound 1 was detected in rhesus monkeys and assigned structure 4. The intriguing biotransformation of 1 leading to 4 was confirmed by a 19-step total synthesis starting from resorcinol (11), the key feature of which was the construction of the oxygen bridge utilizing a phenolic oxidation and trapping sequence. In addition, the synthesis of a related metabolite (5) is described.

Animals↗
PubMed2005

Potent Kv1.3 inhibitors from correolide-modification of the C18 position.

Kv1.3, the voltage-gated potassium channel in human T cells, represents a new target for treating immunosuppression and autoimmune diseases. Correolide (1), a pentacyclic natural product, is a potent and selective Kv1.3 channel blocker. Simplification of correolide via removal of its E-ring generates enone 4, whose modification produced a new series of tetracyclic Kv1.3 blockers. The structure-activity relationship for this class of compounds in two functional assays, Rb_Kv and human T cell proliferation, is presented herein. The most potent analog 43 is 15-fold more potent than correolide as inhibitor of human T cell proliferation.

Biological Assay↗
PubMed2005

Identification of a new source of interference leached from polypropylene tubes in mass-selective analysis.

An interference leached from polypropylene tubes was identified to be a sulfoxide oxidative product of didodecyl 3,3'-thiodipropionate (DDTDP) that is used to prevent oxidative degradation of synthetic polymers. A sulfone oxidative product of DDTDP leached from the polypropylene tubes was also observed. The interfering compounds were isolated by LC and characterized using time-of-flight mass spectrometry and NMR. Authentic sulfoxide and sulfone products of DDTDP were also prepared by reacting DDTDP with hydrogen peroxide reaching an unequivocal structural assignment. In conclusion, when analytes of interest are solubilized in predominantly organic solvents and kept in polypropylene containers, the possibility of contamination from leached chemicals should be taken into account.

Journal Article↗
PubMed2005

In vitro bioactivation of dihydrobenzoxathiin selective estrogen receptor modulators by cytochrome P450 3A4 in human liver microsomes: formation of reactive iminium and quinone type metabolites.

Estrogens and selective estrogen receptor modulators (SERMs) are prescribed widely in the clinic to alleviate symptoms in postmenopausal women, and they are metabolized to reactive intermediates, which may elicit adverse effects. As part of our efforts to develop safer SERMs, in vitro covalent protein binding of (2S,3R)-(+)-3-(4-hydroxyphenyl)-2-[4-(2-piperidin-1-ylethoxy)phenyl]-2,3-dihydro-1,4-benzoxathiin-6-ol (I) was evaluated. Radioactivity from [3H]I became covalently bound to proteins in a fashion that was both time- and NADPH-dependent in human liver microsomes and reached a value of 1106 pmol equiv/mg protein following a 45 min incubation. At least three pathways are involved in the bioactivation of I, namely, oxidative cleavage of the dihydrobenzoxathiin moiety to give a hydroquinone/para-benzoquinone redox couple, hydroxylation at position 5 or 7 of the benzoxathiin moiety leading to an o-quinone intermediate, and metabolism of the piperidine ring to give an iminium ion. The latter reactive intermediate was identified as its bis-cyano adduct when human liver microsomal incubations were performed in the presence of sodium cyanide. Structural modification of I, including a replacement of the piperidine with a pyrrolidine group, led to (2S,3R)-(+)-3-(3-hydroxyphenyl)-2-[4-(2-pyrrolidin-1-ylethoxy)phenyl]-2,3-dihydro-1,4-benzoxathiin-6-ol (II), which did not form a reactive iminium ion. Following the incubation of II with human liver microsomes, covalent binding to proteins was reduced (461 pmol equiv/mg protein), the residual level of binding apparently due to the formation of a rearranged biphenyl quinone type metabolite. Studies with inhibitory antibodies and chemical inhibitors showed that P450 3A4 was the primary enzyme responsible for oxidative bioactivation of I and II in human liver microsomes. These studies thus demonstrated that gaining an understanding of bioactivation mechanisms may be exploited in terms of guiding structural modifications of drug candidates to minimize covalent protein binding and, hopefully, to lower the potential for drug-mediated adverse effects.

Benzoquinones↗
PubMed2005

Metabolic activation of a 1,3-disubstituted piperazine derivative: evidence for a novel ring contraction to an imidazoline.

MB243 (a 1,3-disubstituted piperazine) is a new, potent, and selective melanocortin receptor subtype-4 agonist with potential application in the treatment of obesity and/or erectile dysfunction. MB243 was observed to covalently bind extensively to liver microsomal proteins from rats and humans. In the presence of glutathione, two thioether adducts were detected in liver microsomal incubations by radiochromatography and LC/MS/MS analysis. These adducts were also formed when bile duct-cannulated rats were dosed with MB243. The two adducts were isolated, and their structures were determined by accurate mass MS/MS and NMR analyses. The proposed structures resulted from a novel contraction of the piperazine ring to yield a substituted imidazoline. A mechanism is proposed, which involves an initial six electron oxidation of the piperazine ring to form a reactive intermediate, which is trapped by glutathione. Hydrolysis of the glutamic acid residue followed by internal aminolysis by the cysteine amino group resulted in opening of the piperazine ring, which is followed by ring closure to an imidazoline. The resulting cysteinyl-glycine conjugate underwent subsequent hydrolysis of the glycine residue. Understanding of the mechanism of bioactivation led to the design of MB243 analogues that exhibited reduced covalent protein binding.

Animals↗
PubMed2005

Zafirlukast metabolism by cytochrome P450 3A4 produces an electrophilic alpha,beta-unsaturated iminium species that results in the selective mechanism-based inactivation of the enzyme.

Zafirlukast is a leukotriene antagonist indicated for the treatment of mild to moderate asthma, but the drug has been associated with occasional idiosyncratic hepatotoxicity. Structurally, zafirlukast is similar to 3-methylindole because it contains an N-methylindole moiety that has a 3-alkyl substituent on the indole ring. The results presented here describe the metabolic activation of zafirlukast via a similar mechanism to that described for 3-methylindole. NADP(H)-dependent biotransformation of zafirlukast by hepatic microsomes from rats and humans afforded a reactive metabolite, which was detected as its GSH adduct. Mass spectrometry and NMR data indicated that the GSH adduct was formed by the addition of GSH to the methylene carbon between the indole- and methoxy-substituted phenyl rings of zafirlukast. The formation of this reactive metabolite in human liver microsomes was shown to be exclusively catalyzed by CYP3A enzymes. Evidence for in vivo metabolic activation of zafirlukast was obtained when the same GSH adduct was detected in bile of rats given an iv or oral dose of the drug. On the basis of results with model peroxidases and of the structures of product alcohols from incubations containing H2(18)O, it appeared that zafirlukast underwent dehydrogenation by two sequential one-electron oxidations. In addition, zafirlukast proved to be a mechanism-based inhibitor of CYP3A4 activity in human liver microsomes and in microsomes containing cDNA-expressed CYP3A4. The enzyme inhibitory property of zafirlukast was selective for this enzyme among all of the P450 enzymes that were tested in human liver microsomes. The inactivation was characterized by a K(I) of 13.4 microM and k(inact) of 0.026 min(-1). In summary, zafirlukast dehydrogenation to an electrophilic alpha,beta-unsaturated iminium intermediate may be associated with idiosyncratic hepatotoxicity and/or cause drug-drug interactions through inactivation of CYP3A4.

Animals↗
PubMed2004

Glucuronidation, oxidative metabolism, and bioactivation of enterolactone in rhesus monkeys.

Enterolactone (ENL) and enterodiol (END) are mammalian lignans derived from the plant lignans matairesionol (MAT), secoisolariciresinol (SECO), and other dietary precursors. ENL was found to undergo extensive glucuronidation with rhesus liver microsomes to form O-glucuronides at both phenolic hydroxy groups. In addition to glucuronidation, ENL was found to be a good substrate for oxidative metabolism. The major products had a m/z of 313 or 295 by LC-MS analysis in negative ion mode and were determined to be products of monohydroxylation of ENL. The m/z 295 products were the result of a dehydration of the m/z 313 in the MS source. Addition of N-acetylcysteine (NAC) to the NADPH incubations resulted in a decrease of at least 2 major monohydroxylated products and the formation of a major and several minor new products with a m/z of 474. The major adduct was isolated, purified for NMR, and confirmed to be the NAC adduct of the ENL catechol. Incubations of ENL with liver microsomes containing UDPGA, NADPH, and NAC resulted in the formation of ENL-glucuronides; no NAC adducts were detected by LC-MS. Incubations of ENL with human and rhesus hepatocytes resulted in several metabolites. The major metabolites in hepatocytes were the glucuronic acid conjugates; minor amounts of the sulfate conjugate(s) and monohydroxylated products were also detected by LC-MS. Glutathione or other thiol adducts were not detected in hepatocytes. Conclusion. The high efficiency and specificity for the glucuronidation of ENL decrease its potential toxicity via CYP450 bioactivation.

4-Butyrolactone↗
PubMed2004

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.

Animals↗