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.