DISPOSITION OF HALOPERIDOL AND REDUCED HALOPERIDOL PLASMA-LEVELS AFTER SINGLE-DOSE HALOPERIDOL DECANOATE ADMINISTRATION

A single dose of haloperidol decanoate 100 mg was administered to 15 schizophrenic patients. Blood samples were obtained prior to injection, 1 h, 3 h, 6 h, 8 h, 1 day, 2 days, 3 days, 4 days, 5 days, 6 days, one week, two weeks, three weeks and four weeks post-injection. Haloperidol and its reduced metabolite, reduced haloperidol, plasma levels were assayed by HPLC with electrochemical detection. The pharmacokinetic parameters of haloperidol were determined. The mean time of maximal (T-max) plasma levels for haloperidol was 5.73 +/- 0.80 days. The haloperidol plasma levels showed a biexponential decline with an elimination half-life of 15.78 +/- 5.90 days. Reduced haloperidol was rapidly formed from the haloperidol. The T-max of reduced haloperidol was 7.00 +/- 2.35 days. The mean ratio reduced haloperidol/haloperidol was 0.155 +/- 0.111. Since the T-max occurs at approximately six days, a weekly loading dose of haloperidol decanoate is feasible during the transition from oral to depot therapy.