Prothipendyl: detection and elimination in the horse--a case report

The azaphenothiazine neuroleptic prothipendyl (Dominal) is suspected to be administered illegally at low doses to race-horses to improve their performance. Since for this species pharmacokinetic data of the drug are missing we studied its elimination from blood and urine in a standard-bred mare. At a low (subtherapeutic) dose (i.v., 0.24 mg/kg) the horse is described to be less excited while locomotor activity and attention remain unaffected. In contrast, sedation and ataxia are brought about at 1 mg/kg (therapeutic dose). Identification of prothipendyl given i.v. at subtherapeutic doses was achieved in blood only by means of gas chromatography-mass spectrometry (GC-MS), while the neuroleptic was found both in blood and urine upon 1 mg/kg. Quantification of the neuroleptic was carried out by virtue of triflupromazine as internal standard with the MS operating in the selected ion monitoring (SIM) mode. Under these conditions, the detection limit was 10 ng/ml body fluid. Disappearance of prothipendyl from blood was determined in the horse studied from terminal elimination process, yielding a t1/2 of 2.4 h. The results suggest that for detection of prothipendyl in the horse--in contrast to phenothiazine neuroleptics--screening of blood is preferred over urine as the drug was not recovered in urine after administration of subtherapeutic doses.