Potencies of haloperidol metabolites as inhibitors of the human noradrenaline, dopamine and serotonin transporters in transfected COS-7 cells

Abstract.

Extrapyramidal symptoms, such as tardive dyskinesia, often develop in patients on long-term treatment with haloperidol. It has been proposed that these symptoms could be caused by neurotoxic effects of haloperidol metabolites following uptake by monoamine transporters, in an analogous mechanism to the neurotoxic effect of MPP⁺ (1-methyl-4-phenylpyridinium) metabolised from MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine). In this study, the hypothesis was partially investigated by determining the potencies of haloperidol and reduced haloperidol and the corresponding pyridinium and tetrahydropyridine metabolites, compared with MPP⁺ and MPTP, as inhibitors of the noradrenaline transporter (NAT), dopamine transporter (DAT) and 5-HT transporter (SERT).

Two days after COS-7 cells were transiently transfected with the cDNA for the human NAT, DAT or SERT (Lipofectamine method), the cells were incubated with 10 nM [³H]noradrenaline, dopamine or 5-HT, respectively, for 2 min at 37°C, in the absence or presence of various concentrations of the eight compounds or a specific uptake inhibitor (NAT: nisoxetine 1 µM; DAT: GBR 12909 1 µM; SERT: citalopram 10 µM). Specific amine uptake (fmol/mg protein) was calculated as the difference in uptake in the absence and presence of the specific uptake inhibitor. K _i values were calculated for the eight compounds for inhibition of NAT, DAT and SERT.

Haloperidol, its five metabolites and MPP⁺ and MPTP all inhibited NAT, DAT and SERT. For the pyridinium and tetrahydropyridine metabolites of haloperidol, there were not marked differences between their potencies as inhibitors between each other for NAT or DAT or between NAT and DAT, with all of the K _i values in the range of 5.8–16 µM. However, there were more marked differences for SERT, with all but one of the metabolites showing selectivity for inhibition of SERT relative to NAT and DAT. Haloperidol and reduced haloperidol had similar inhibitory potencies for all three transporters, and were clearly less potent than the other haloperidol metabolites only for inhibition of SERT. The lack of correlation between the inhibitory potencies of the haloperidol metabolites and their structural analogues, MPTP and MPP⁺, suggests that they are not likely to cause neurotoxicity by a mechanism analogous to that of the latter neurotoxin.