BIOACTIVATION AND IRREVERSIBLE BINDING OF THE COGNITION ACTIVATOR TACRINE USING HUMAN AND RAT-LIVER MICROSOMAL PREPARATIONS - SPECIES-DIFFERENCE

Tacrine's [1,2,3,4-tetrahydro-9-acridinamine monohydrochloride monohyd rate, (THA)] metabolic fate was examined using human and rat liver mic rosomal preparations. Following 1-hr incubations with human microsomes , [C-14]THA (0.4 muM) was extensively metabolized to 1-hydroxyTHA with trace amounts of 2-, 4-, and 7-hydroxyTHA also produced. Poor recover y of radioactivity in the postreaction incubates suggested association of THA-derived radioactivity with precipitated microsomal protein. Af ter exhaustive extraction, 0.034, 0.145, 0.126, and 0.012 nmol eq boun d/mg protein/60 min of THA-derived radioactivity was bound to human li ver preparations H109, H111, H116, and H118, respectively. Preparation s H109 and H118 were lower in P4501A2 content and catalytic activity a s compared with preparations H111 and H116. Incubations of equimolar [ C-14]1-hydroxyTHA with human liver microsomes also resulted in binding to protein, although to a lesser extent than observed with THA. [C-14 ]THA (0.4 muM) was incubated for 1 hr with rat liver microsomes (1 muM P-450) prepared from noninduced (N), phenobarbital (PB), isoniazid (I ), and 3-methylcholanthrene (3-MC)-pretreated animals. In all incubati ons, 1-hydroxyTHA was the major biotransformation product detected. Af ter exhaustive extraction, 0.048, 0.054, 0.049, and 0.153 nmol eq/mg p rotein/60 min of THA-derived radioactivity was bound to microsomal pro tein from N, PB, I, and 3-MC pretreated rats. Increased binding with 3 -MC induced rat liver preparations suggests the involvement of the P-4 50 1A subfamily in THA bioactivation. Glutathione (5 mM) coincubation inhibited the irreversible binding of THA-derived radioactivity in bot h human and 3-MC-induced rat liver preparations, whereas human epoxide hydrase (100 mug/incubate) had a relative minor effect. A mechanism i s proposed involving a putative quinone methide(s) intermediate in the bioactivation and irreversible binding of THA. A species difference i n THA-derived irreversible binding exists between human and noninduced rat liver microsomes, suggesting that the rat is a poor model for stu dying the underlying mechanism(s) of THA-induced elevations in liver m arker enzymes found in clinical investigations.