Influence of the antiparkinsonian drugs on the plasma level of neuroleptics.

The interaction between various neuroleptics and antiparkinsonian drugs was analyzed by measuring the neuroleptic plasma level before and after withdrawal of antiparkinsonian drugs. The population completing the study consisted of 32 chronic schizophrenics treated with chlorpromazine (8), levomepromazine (14), thioridazine (6), or haloperidol (4). Twenty-five were also receiving benztropine; 4, trihexyphenidyl; and 3, procyclidine. During the first 4 weeks patients remained on neuroleptics and antiparkinsonians, the latter being withdrawn during the 5th week, and the neuroleptics alone being administered during 16 following weeks. The plasma level of neuroleptics was assayed by gas liquid chromatography, once weekly in the morning at two different times. The analysis of variance showed a significant difference in neuroleptic plasma level when patients took neuroleptics only versus the period they had received neuroleptics and antiparkinsonians. The multiple comparison based on Studentized range Q0-05 revealed a significant progressive increase of neuroleptic plasma level during 12 weeks after withdrawal of antiparkinsonian drugs after which a plateau was reached. The hypothetical mechanisms of action of antiparkinsonians on neuroleptic plasma level are discussed.