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.