DEVELOPMENT AND APPLICATION OF A SPECIFIC AND SENSITIVE RADIOIMMUNOASSAY FOR TRIHEXYPHENIDYL TO A PHARMACOKINETIC STUDY IN HUMANS

A radioimmunoassay (RIA) for trihexyphenidyl was developed through the use of a bovine thyroglobulin conjugate of trihexyphenidyl hemisuccin ate. Immunization of New Zealand white rabbits with this drug-protein conjugate yielded antisera, for which the antibody titer and specifici ty were evaluated. An antiserum that had the highest titer and minimal cross-reactivities to major metabolites of trihexyphenidyl, such as t rihexyphenidyl N-oxide (2%), hydroxytrihexyphenidyl (1%), and the anti psychotic drugs fluphenazine (<1%), flupenthixol (<1%), chlorpromazine (<1%), and haloperidol (<1%), was selected for development of a RIA. The described RIA enables the quantitation of 7.8 pg of trihexyphenidy l in 200 mu L of human plasma with a mean coefficient of variation of <6% across the range of the standard curve. Assay specificity was furt her demonstrated by comparison of results obtained directly and after selective extraction of trihexyphenidyl from replicate samples. This R IA procedure was applied to the analysis of steady state plasma sample s obtained from patients undergoing treatment with trihexyphenidyl (2- 8 mg) and plasma samples obtained from eight healthy male volunteers a fter administration of a single 4 mg oral dose of the drug. The result s of the latter single dose studies demonstrated that the mean +/-SD f or the peak concentration (C-max), the time to C-max (T-max), the rate of absorption (K-a), and the area under the curve from 0 to 72 h (AUC (0-72)) were found to be 7.15 +/- 2.58 ng/mL, 1.32 +/- 0.58 h, 2.07 +/ - 0.93 1/h, and 201 +/- 71 ng h/mL, respectively. The human subjects h ad a biphasic plot of mean plasma concentration Versus time consisting of an initially rapid distribution phase (T-alpha 1/2 = 5.33 +/- 3.23 h) and a later slower elimination phase (T-beta 1/2 = 32.7 +/- 6.35 h ).