Inhibition of monoamine oxidases by haloperidol and its metabolites: pharmacological implications for the chemotherapy of schizophrenia.

The effect of haloperidol and its metabolites on human platelet monoamine oxidase B (MAO-B) and human placenta monoamine oxidase A (MAO-A) in vitro has been investigated. We found that 4-(4-chlorophenyl)-1-[4-(4-fluorophenyl)-4-oxobutyl]-pyridinium (HP+), 4-(4-chlorophenyl)-1-[4-(4-fluorophenyl)-4-oxobutyl]-1,2,3,6- tetrahydropyridine (HTP) and 4-chlorophenyl-1,2,3,6-tetrahydropyridine (CPTP) are potent inhibitors of MAO. HP+ appeared to be a reversible, uncompetitive and selective MAO-B inhibitor with a Ki of 0.83 microM. HTP was found to be an irreversible, uncompetitive and selective MAO-B inhibitor (Ki of 1.84 microM). CPTP inhibits both MAO-A and MAO-B. Some other haloperidol metabolites, i.e. 4-(4-chlorophenyl)-4-hydroxypyridine (CPHP), 4-(4-chlorophenyl)-1-[4-(4-fluorophenyl)-4-oxobutyl]-1,2,3,6- tetrahydropyridine N-oxide (HTPNO) and reduced haloperidol (RHAL), do not inhibit MAO to any appreciable degree at concentrations up to 100 microM. The results suggest that haloperidol metabolites may contribute to the reduction of platelet MAO-B activity in schizophrenic patients undergoing neuroleptic chemotherapy. An examination of the literature reveals that schizophrenic patients with low platelet MAO activity exhibit a strong association with the use of haloperidol. Other possible pharmacological implications of the inhibition of MAO activity are discussed.