Tefludazine

Neuroleptic drug-induced acute extra-pyramidal syndromes are one of the major reasons why patients discontinue their antipsychotic medicines. The typical (e.g., haloperidol) neuroleptic drug produces acute extrapyramidal symptoms in the majority of patients, whereas the atypical (clozapine) neuroleptic produces only minimal motor system side effects. Serotonin S2 antagonists often reduce or prevent catalepsy in rodents, but the limited number of studies in nonhuman primates have produced conflicting results. The hypothesis of a high serotonin S2/dopamine D2 antagonism ratio as a mechanism underlying atypical neuroleptic effects in preventing acute extrapyramidal syndromes deserves further evaluation in nonhuman primate models because extrapyramidal symptoms in monkeys closely resemble those in patients. Cebus monkeys (22-28 years old) were tested with compounds that ranged from low to high S2/D2 antagonism ratios. These were haloperidol, fluphenazine, clopenthixol, melperone, tefludazine, setoperone, risperidone, and clozapine. A saline control was included with a wide dose range of each of these drugs that was tested in a once-weekly, blindly-scored random drug administration schedule. Dystonia was scored on four different symptoms by an experienced rater who was blind to drug dosage. All the compounds, with the exception of clozapine, produced clinically indistinguishable dose-related dystonia. The only difference was the dose at which dystonia appeared. In contrast to rodent studies, these nonhuman primate investigations with drugs, spanning a wide range of S2/D2 antagonism ratios, produced clinically similar extrapyramidal symptoms. Thus, adding an S2 antagonism component to neuroleptics does not appear to provide an explanation for the motor side effect profile of atypical neuroleptics, or a method for designing neuroleptic drugs that will be free of extrapyramidal symptoms.

Neuroleptic drug-induced acute extrapyramidal syndromes (EPS) are the major reasons why patients discontinue their antipsychotic medicines. Serotonin S2 antagonists prevent catalepsy in rodents but the effects in nonhuman primates have received only minimal study and deserve further evaluation as potential "non-neuroleptic neuroleptics" (antipsychotic effects free of acute EPS). Twenty Cebus monkeys (22 to 28 yrs. old) were tested with compounds that ranged from high to very low D2/S2 ratios. These were haloperidol, clopenthixol, tefludazine, and setoperone, all tested in the dosage range of .01 to .25 mg/kg and compared with saline i.m. once weekly in a random schedule. Dystonia was scored on four different symptoms by an experienced rater blind to drug dosage. All four active compounds produced clinically indistinguishable, dose related dystonia with very similar dose thresholds. In contrast to rodent studies these nonhuman primate investigations with drugs of widely differing D2/S2 antagonism ratios produced clinically similar EPS. Thus adding an S2 antagonism component to neuroleptics appears not to provide a unique approach to neuroleptic therapy which will be free of acute EPS.