In vitro effect of haloperidol, chlorpromazine, imipramine and lithium on the erythrocyte catechol-O-methyltransferase.

Haloperidol, chlorpromazine, imipramine and lithium in concentrations similar to the average therapeutic plasma levels did not exert any in vitro inhibition on the erythrocyte catechol-O-methyltransferase (COMT). The inhibitory effects of haloperidol, chlorpromazine and imipramine were noted first at concentrations 1000-10 000 times their respective average therapeutic plasma levels. Unlike the former three psychotropic drugs, lithium exerted an in vitro inhibition already at a concentration 40 times the average therapeutic plasma level and that in a competitive way. The inhibitory effect of lithium could be neutralized by increasing the magnesium concentration, likewise in a competitive way.