TY - JOUR T1 - Metabolism and Disposition of Oral Dabrafenib in Cancer Patients: Proposed Participation of Aryl Nitrogen in Carbon-Carbon Bond Cleavage via Decarboxylation following Enzymatic Oxidation JF - Drug Metabolism and Disposition JO - Drug Metab Dispos SP - 2215 LP - 2224 DO - 10.1124/dmd.113.053785 VL - 41 IS - 12 AU - David A. Bershas AU - Daniele Ouellet AU - Donna B. Mamaril-Fishman AU - Noelia Nebot AU - Stanley W. Carson AU - Samuel C. Blackman AU - Royce A. Morrison AU - Jerry L. Adams AU - Kristen E. Jurusik AU - Dana M. Knecht AU - Peter D. Gorycki AU - Lauren E. Richards-Peterson Y1 - 2013/12/01 UR - http://dmd.aspetjournals.org/content/41/12/2215.abstract N2 - 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. ER -