Peter D. Gorycki

Pyrimidinylimidazole inhibitors of CSBP/P38 kinase demonstrating decreased inhibition of hepatic cytochrome P450 enzymes

Pyrimidine analogs of the pyrimidinylimidazole class of CSBP/p38 kinase inhibitors were prepared in an effort to reduce the potent inhibition of hepatic cytochrome P450 observed for the pyridinyl compounds. The substitution of pyrimidin-4-yl, 2-methoxypyrimidin-4-yl, or 2-methylaminopyrimidin-4-yl for pyridin-4-yl effectively dissociates CSBP/p38 kinase from P450 inhibition for this series and furthermore achieves an increase in oral activity.

Bioavailability, metabolism and disposition of oral pazopanib in patients with advanced cancer

1. Pazopanib (Votrient) is an oral tyrosine kinase inhibitor that was recently approved for the treatment of renal cell carcinoma and soft tissue sarcoma. 2. In this two-part study, we investigated the metabolism, disposition of [14C]pazopanib, and the oral bioavailability of pazopanib tablets in patients with advanced cancer. 3. In part A, three men each received a single oral dose of [14C]pazopanib in suspension (400 mg, 70 µCi). Pazopanib was the predominant drug-related component in circulation. Two metabolites derived from hydroxylation and one from N-demethylation were also circulating, but were minor, each accounting for <5% of plasma radioactivity. Faecal elimination predominated, accounting for 82.2% of the administered radio-dose, with negligible renal elimination (2.6% of dose). Pazopanib was primarily excreted as the unchanged drug in faeces (67% of dose). 4. In part B, seven additional patients received a single intravenous administration of 5 mg pazopanib (day 1) followed by oral administration of 800 mg pazopanib tablet once daily for 26 days (days 3 or 5–28). In the three evaluable patients from part B, pazopanib had a slow plasma clearance and a small volume of distribution. The absolute oral bioavailability of the 800 mg pazopanib tablet ranged from 14% to 39%.

Metabolism and Disposition of Oral Dabrafenib in Cancer Patients: Proposed Participation of Aryl Nitrogen in Carbon-Carbon Bond Cleavage via Decarboxylation following Enzymatic Oxidation

A phase I study was conducted to assess the metabolism and excretion of [14C]dabrafenib (GSK2118436; N-{3-[5-(2-amino-4-pyrimidinyl)-2-(1,1-dimethylethyl)-1,3-thiazol-4-yl]-2-fluorophenyl}-2,6-difluorobenzene sulfonamide, methanesulfonate salt), a BRAF inhibitor, in four patients with BRAF V600 mutation–positive tumors after a single oral dose of 95 mg (80 µCi). Assessments included the following: 1) plasma concentrations of dabrafenib and metabolites using validated ultra-high-performance liquid chromatography—tandem mass spectrometry methods, 2) plasma and blood radioactivity, 3) urinary and fecal radioactivity, and 4) metabolite profiling. Results showed the mean total recovery of radioactivity was 93.8%, with the majority recovered in feces (71.1% of administered dose). Urinary excretion accounted for 22.7% of the dose, with no detection of parent drug in urine. Dabrafenib is metabolized primarily via oxidation of the t-butyl group to form hydroxy-dabrafenib. Hydroxy-dabrafenib undergoes further oxidation to carboxy-dabrafenib, which subsequently converts to desmethyl-dabrafenib via a pH-dependent decarboxylation. The half-lives for carboxy- and desmethyl-dabrafenib were longer than for parent and hydroxy-dabrafenib (18–20 vs. 5–6 hours). Based on area under the plasma concentration-time curve, dabrafenib, hydroxy-, carboxy-, and desmethyl-dabrafenib accounted for 11%, 8%, 54%, and 3% of the plasma radioactivity, respectively. These results demonstrate that the major route of elimination of dabrafenib is via oxidative metabolism (48% of the dose) and biliary excretion. Based on our understanding of the decarboxylation of carboxy-dabrafenib, a low pH-driven, nonenzymatic mechanism involving participation of the aryl nitrogen is proposed to allow prediction of metabolic oxidation and decarboxylation of drugs containing an aryl nitrogen positioned α to an alkyl (ethyl or t-butyl) side chain.

Metabolism and Disposition of Eltrombopag, an Oral, Nonpeptide Thrombopoietin Receptor Agonist, in Healthy Human Subjects

The metabolism and disposition of eltrombopag, the first-in-class small molecule human thrombopoietin receptor agonist, were studied in six healthy men after a single oral administration of a solution dose of [14C]eltrombopag (75 mg, 100 μCi). Eltrombopag was well tolerated. The drug was quickly absorbed and was the predominant circulating component in plasma (accounting for 63% of the total plasma radioactivity). A mono-oxygenation metabolite (M1) and acyl glucuronides (M2) of eltrombopag were minor circulating components. The predominant route of elimination of radioactivity was fecal (58.9%). Feces contained approximately 20% of dose as glutathione-related conjugates (M5, M6, and M7) and another 20% as unchanged eltrombopag. The glutathione conjugates were probably detoxification products of a p-imine methide intermediate formed by metabolism of M1, which arises through cytochrome P450-dependent processes. Low levels of covalently bound drug-related intermediates to plasma proteins, which could result from the reaction of the imine methide or acyl glucuronide conjugates with proteins, were detected. The bound material contributes to the longer plasma elimination half-life of radioactivity. Renal elimination of conjugates of hydrazine cleavage metabolites (mostly as M3 and M4) accounted for 31% of the radiodose, with no unchanged eltrombopag detected in urine.

SB-242235, a selective inhibitor of p38 mitogen-activated protein kinase. II: In vitro and in vivo metabolism studies and pharmacokinetic extrapolation to man

1. Inhibition of p38 MAP kinase has been investigated extensively as a potential therapy for cytokine-mediated diseases such as autoimmune and inflammatory diseases. SB-242235 (1-(4-piperidinyl)-4-(4-fluorophenyl)-5-(2-methoxy-4-pyrimidinyl) imidazole) is a potent and selective p38 MAP kinase inhibitor; the preclinical pharmacokinetics of SB-242235 have been described previously. The present studies were conducted to describe the in vitro metabolic rates and routes of SB-242235 metabolism, to characterize its in vivo preclinical metabolism, and to use these data to aid in the prediction of the pharmacokinetic behaviour of SB-242235 in man. 2. SB-242235 was metabolically stable in rat, dog, monkey and human hepatic microsomes, isolated hepatocytes and liver slices in vitro. The in vivo preclinical metabolism studies were consistent with the in vitro findings; SB-242235 was minimally metabolized, and was primarily excreted unchanged in the urine (45 and 67% of the administered dose in the rat and monkey, respectively). 3. Allometric scaling using various correction factors predicted that SB-242235 would have low clearance in man with a predicted half-life ranging from 11.5 to 18.7h. This prediction was consistent with the observed mean half-life of 16.4 h in the first-in-man study for SB-242235. An allometric scaling method with a correction for interspecies differences in glomerular filtration rate provided the most accurate prediction of the pharmacokinetic behaviour of SB-242235 in humans, although the clinical data also highlight potential difficulties in conducting prospective allometry.

Metabolism of ( 14 C)GSK977779 in Rats and Its Implication with the Observed Covalent Binding

GSK977779 is a potent HM74a agonist evaluated for the treatment of dyslipidemia. The disposition and metabolism of [14C]GSK977779 (67.6 μmol/kg p.o.) was studied in male and female rats. The compound was well absorbed and its primary route of elimination was in the feces. Based on metabolite profiling of plasma extracts and urine and bile samples, it was demonstrated that GSK977779 was extensively metabolized in the rat by N-dealkylation, mono- and dioxygenation, reductive and oxidative cleavage of the 1,2,4-oxadiazole ring, and conjugative pathways. After plasma extraction high amounts of nonextractable radioactivity were observed, which were more pronounced in female rats. Size-exclusion chromatography and SDS gel electrophoresis indicated that the majority of the nonextractable radioactivity was covalently bound to plasma proteins. Solubilization of the plasma protein pellet followed by high-performance liquid chromatography and mass spectrometry suggested that a carboxylic acid metabolite derived from oxadiazole ring cleavage may be responsible for the observed covalent binding of the radioactivity to rat plasma proteins.

Investigations of Hydrazine Cleavage of Eltrombopag in Humans

After oral administration to humans, eltrombopag undergoes extensive cleavage of its hydrazine linkage to metabolites, which are exclusively eliminated in urine. In vitro, the cleavage pathway was not detected in systems using cytochrome P450 enzymes, renal or hepatic microsomes, or hepatocytes but was readily evident after anaerobic incubation with rodent cecal contents or human fecal homogenate. Antibiotic treatment in vitro and in vivo inhibited eltrombopag cleavage, further indicating that cleavage is via gut microbes. Antibiotic treatment did not alter the systemic exposure of eltrombopag in mice. Oral and intravenous pharmacokinetic characterization in the mice with one of the cleavage products indicated that it was readily absorbed, conjugated, and eliminated in urine, consistent with its fate after oral administration of eltrombopag. Variation in this microbial pathway, for example by antibiotic cotherapy, is unlikely to be clinically significant.

Investigation of metabolism and disposition of GSK1322322, a peptidase deformylase inhibitor, in healthy humans using the entero-test for biliary sampling.

GSK1322322 (N-((R)-2-(cyclopentylmethyl)-3-(2-(5-fluoro-6-((S)-hexahydropyrazino[2,1-c][1,4]oxazin-8(1H)-yl)-2-methylpyrimidin-4-yl)hydrazinyl)-3-oxopropyl)-N-hydroxy-formamide) is an antibiotic in development by GlaxoSmithKline. In this study, we investigated the metabolism and disposition of [14C]GSK1322322 in healthy humans and demonstrated the utility of the Entero-Test in a human radiolabel study. We successfully collected bile from five men using this easy-to-use device after single i.v. (1000 mg) or oral administration (1200 mg in a solution) of [14C]GSK1322322. GSK1322322 had low plasma clearance (23.6 liters/hour) with a terminal elimination half-life of ∼4 hours after i.v. administration. After oral administration, GSK1322322 was readily and almost completely absorbed (time of maximal concentration of 0.5 hour; bioavailability 97%). GSK1322322 predominated in the systemic circulation (>64% of total plasma radioactivity). An O-glucuronide of GSK1322322 (M9) circulated at levels between 10% and 15% of plasma radioactivity and was pharmacologically inactive. Humans eliminated the radioactive dose in urine and feces at equal proportions after both i.v. and oral doses (∼45%–48% each). Urine contained mostly unchanged GSK1322322, accounting for 30% of the dose. Bile contained mostly M9, indicating that glucuronidation was likely a major pathway in humans (up to 30% of total dose). In contrast, M9 was found in low amounts in feces, indicating its instability in the gastrointestinal tract. Therefore, without the Entero-Test bile data, the contribution of glucuronidation would have been notably underestimated. An unusual N-dehydroxylated metabolite (a secondary amide) of GSK1322322 was observed primarily in the feces and was most likely formed by gut microbes.

First-in-Human Studies for a Selective RET Tyrosine Kinase Inhibitor, GSK3179106, to Investigate the Safety, Tolerability, and Pharmacokinetics in Healthy Volunteers

Rearranged during transfection (RET), a neuronal growth factor receptor tyrosine kinase, regulates the development of the sympathetic, parasympathetic, motor, and sensory neurons in the enteric nervous system. GSK3179106 is a RET kinase inhibitor that was administered in double‐blind, randomized, placebo‐controlled single‐dose and repeat‐dose studies in healthy subjects to investigate its pharmacokinetics and safety/tolerability. In the single‐dose study (n = 16), GSK3179106 was dosed from 10 mg to 800 mg, including a food effect arm. In the repeat‐dose study (n = 46), GSK3179106 was dosed for 14 days with food once daily (QD) from 5 mg to 100 mg and twice daily (BID) at 100 mg and 200 mg. With single fasted doses, bioavailability was low and less than dose proportional. A significant food effect was observed with a 100‐mg QD dose. Drug exposure after QD and BID repeat dosing with food showed dose dependency up to 100 mg but was not dose proportional. There were no significant differences in exposure between 100‐mg and 200‐mg BID doses of GSK3179106. Accumulation was observed with both QD and BID dosing. Single doses up to 800 mg and repeat doses up to 400 mg were well tolerated with no safety concerns in healthy subjects.