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Following a single oral or intravenous administration of the R,R(+) and S,S(-) (14)C-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 micro g. 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 micro g. 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 (R(2) = 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.