Haloperidol inhibition of monoamine oxidase in vivo and in vitro.

Haloperidol was found to inhibit monoamine oxidase (MAO) activity in sonicated platelets by 50% (IC50) at a concentration of 10(-4) M. Preincubation of the sonicated platelets with haloperidol before the assay did not shift the dose-response curve. When cultured human skin fibroblast MAO was assayed along with haloperidol, MAO activity was only slightly affected (estimated IC50 = 3 X 10(-2) M), even with 1 hr of preincubation. When fibroblasts were cultured with medium containing haloperidol, however, the haloperidol IC50 for MAO activity was 3 X 10(-7) M after 3 days, 2 X 10(-8) M after 7 days, and 3 X 10(-9) M after 14 days. We conclude that haloperidol alters MAO enzyme activity acutely in vitro and that the inhibition increases at lower concentrations with chronic treatment in vivo.