Abstract

Following a single oral or intravenous administration of the R,R(+) andS,S(−) ¹⁴C-pseudoracemate of (±)-(1′R*,3R*)-3-phenyl-1-[(1′,2′,3′,4′-tetrahydro-5′,6′-methylene-dioxy-1′-naphthalenyl) methyl] pyrrolidine methanesulfonate (ABT-200/I) to dogs, a total of six (R,R[+]) and eight (S,S[−]) metabolites were identified by high-pressure liquid chromatography/mass spectral techniques. Greater than 99% of the dose was eliminated as metabolites indicating that the clearance of I was predominantly metabolic. The catechol was the major excreted metabolite (fecal), whereas the urine and bile predominantly contained metabolites resulting from secondary biotransformation of the catechol via O-methylation, glucuronidation, and sulfation. After a single 12 mg/kg oral dose of racemic I to dogs, the mean area under the plasma curve (AUC_0–24h) averaged 4.55 μg · h/ml, with an apparent plasma clearance value of 2.70 l/h · kg. After 14 daily doses, the apparent plasma clearance was 3.5-fold lower (0.78 l/h · kg) and the AUC_0–24h about 4-fold higher (18.58 μg · h/ml). Isolation of liver microsomes from these animals indicated that a cytochrome P450 (P450)-metabolite complex (MI complex) was formed in the liver after both single and multiple dosing. The mean concentration of the MI complex 24 h after a single dose averaged 31 pmol/mg of microsomal protein, whereas the amount in the animals given multiple doses of I averaged 163 pmol/mg. There was a positive correlation (R² = 0.993) between the plasma AUC for I and the amount of the MI complex found in the liver of each animal. Formation of the MI complex was demonstrated in vitro in control dog liver microsomes, was NADPH-dependent, and was dissociated from P450 with 2-methylbenzimidazole. In vitro preincubation studies indicated that I was a mechanism-based inhibitor and that formation of the complex inhibited catechol formation. These results demonstrate that the liver P450s that metabolize I form an inhibitory MI complex after in vivo administration in dogs. Formation of the complex increases during multiple dosing and inhibits the enzymes from further metabolism of I resulting in nonlinear pharmacokinetics.