Allen N. Jones

Metabolism and Excretion of Anacetrapib, a Novel Inhibitor of the Cholesteryl Ester Transfer Protein, in Humans

Anacetrapib is a novel cholesteryl ester transfer protein inhibitor being developed for the treatment of primary hypercholesterolemia and mixed dyslipidemia. The absorption, distribution, metabolism, and excretion of anacetrapib were investigated in an open-label study in which six healthy male subjects received a single oral dose of 150 mg and 165 μCi of [14C]anacetrapib. Plasma, urine, and fecal samples were collected at predetermined times for up to 14 days postdose and were analyzed for total radioactivity, the parent compound, and metabolites. The majority of the administered radioactivity (87%) was eliminated by fecal excretion, with negligible amounts present in urine (0.1%). The peak level of radioactivity in plasma (∼2 μM equivalents of [14C]anacetrapib) was achieved ∼4 h postdose. The parent compound was the major radioactive component (79–94% of total radioactivity) in both plasma and feces. Three oxidative metabolites, M1, M2, and M3, were detected in plasma and feces and were identified as the O-demethylated species (M1) and two secondary hydroxylated derivatives of M1 (M2 and M3). Each metabolite was detected at low levels, representing ≤14% of the radioactivity in plasma or fecal samples. In vitro data indicated that anacetrapib is metabolized mainly by CYP3A4 to form M1, M2, and M3. Overall, these data, along with those from other preclinical and clinical studies, indicate that anacetrapib probably exhibits a low-to-moderate degree of oral absorption in humans and the absorbed fraction of the dose is eliminated largely via CYP3A4-catalyzed oxidative metabolism, followed by excretion of metabolites by the biliary-fecal route.

Absorption, Metabolism, and Excretion of [14C]MK-0524, a Prostaglandin D2 Receptor Antagonist, in Humans

[(3R)-4-(4-Chlorobenzyl)-7-fluoro-5-(methylsulfonyl)-1,2,3,4-tetrahydrocyclopentaindol-3-yl]acetic acid (MK-0524) is a potent orally active human prostaglandin D2 receptor 1 antagonist that is currently under development for the prevention of niacin-induced flushing. The metabolism and excretion of [14C]MK-0524 in humans were investigated in six healthy human volunteers following a single p.o. dose of 40 mg (202 μCi). [14C]MK-0524 was absorbed rapidly, with plasma Cmax achieved 1 to 1.5 h postdose. The major route of excretion of radioactivity was via the feces, with 68% of the administered dose recovered in feces. Urinary excretion averaged 22% of the administered dose, for a total excretion recovery of ∼90%. The majority of the dose was excreted within 96 h following dosing. Parent compound was the primary radioactive component circulating in plasma, comprising 42 to 72% of the total radioactivity in plasma for up to 12 h. The only other radioactive component detected in plasma was M2, the acyl glucuronic acid conjugate of the parent compound. The major radioactive component in urine was M2, representing 64% of the total radioactivity. Minor metabolites included hydroxylated epimers (M1/M4) and their glucuronic acid conjugates, which occurred in the urine as urea adducts, formed presumably during storage of samples. Fecal radioactivity profiles mainly comprised the parent compound, originating from unabsorbed parent and/or hydrolyzed glucuronic acid conjugate of the parent compound. Therefore, in humans, MK-0524 was eliminated primarily via metabolism to the acyl glucuronic acid conjugate, followed by excretion of the conjugate into bile and eventually into feces.

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 α and γ 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 μCi) oral dose. Excretion of 14C radioactivity was found to be nearly equal into the urine (∼50%) and feces (∼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 (∼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 ∼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.

Synthesis, stability, and radiolytic decomposition of carbon‐14 labeled MK0677

MK0677 is an orally active growth hormone secretagogue. The crystallized carbon-14 labeled material was found to undergo radiolytic decomposition via a peroxide intermediate which resulted in loss of the benzyl group. The rate was diminished when the tracer was crystallized from nitrogen-degassed solvents. Storage stability was best in aqueous ethanol.

Protocols of In Vitro Protein Covalent Binding Studies in Liver

Xenobiotics, including therapeutic agents, can produce a variety of beneficial, as well as adverse, effects in mammals. One potential source of drug-mediated toxicity stems from metabolic activation of the parent compound, typically catalyzed by one or more members of the cytochrome P450 family of enzymes. The resulting electrophile, if not quenched by low molecular weight endogenous nucleophiles, can form covalent adducts to cellular proteins, potentially resulting in enzyme inactivation, cell death, or formation of an immunogenic species. The toxicological consequences of exposure to such reactive intermediates range from mild inflammation to organ failure, anaphylaxis, and death. At Merck Research Laboratories, the potential of drug candidates to bind covalently to proteins is evaluated at the lead optimization stage of drug discovery by incubating a radiolabeled analog of the compound in question with liver microsomal preparations (under oxidative conditions) or whole cells (full cellular metabolic capability), typically derived from rat and human liver. A semi-automated method based on the Brandel Harvester technique then is used to measure the formation of covalent adducts of the test compound to liver proteins. This assay is viewed as an important component of drug discovery programs, since the findings are employed to guide specific efforts to abrogate bioactivation issues through informed structural modification of lead compounds.

Metabolism and excretion of [14C]taranabant, a cannabinoid-1 inverse agonist, in humans

Taranabant (N-[(1S,2S)-3-(4-Chlorophenyl)-2-(3-cyanophenyl)-1-methylpropyl]-2-methyl-2-{[5-(trifluoromethyl)pyridin-2-yl]oxy}propanamide or MK-0364) is an orally active inverse agonist of the cannabinoid 1 (CB-1) receptor that was under development for the management of obesity. The metabolism and excretion of taranabant were investigated following a single oral dose of 5 mg/201 μCi [14C]taranabant to six healthy male subjects. The overall excretion recovery of the administered radioactivity was nearly quantitative (∼ 92%), with the majority of the dose (∼ 87%) excreted into faeces and a much smaller fraction (∼ 5%) into urine. Taranabant was absorbed rapidly, with Cmax of radioactivity attained at 1–2-h postdose. The parent compound and its monohydroxylated metabolite, M1, were the major radioactive components circulating in plasma and comprised ∼ 12–24% and 33–42%, respectively, of the plasma radioactivity for up to 48 h. A second monohydroxylated metabolite, designated as M1a, represented ∼ 10–12% of the radioactivity in the 2- and 8-h postdose plasma profiles. Metabolite profiles of the faeces samples consisted mainly of the (unabsorbed) parent compound and multiple diastereomeric carboxylic acid derivatives derived from oxidation of the geminal methyl group of the parent compound and of the hydroxylated metabolite/s. These data suggest that, similar to rats and monkeys, taranabant is primarily eliminated in humans via oxidative metabolism and excretion of metabolites via the biliary/faecal route.