Interconversions between haloperidol and reduced haloperidol in schizophrenic patients and guinea pigs: A steady-state study

Plasma concentrations of haloperidol (HAL) and reduced haloperidol (RHAL) were measured in 8 schizophrenic patients, neuroleptic-free for at least 4 weeks, after repeated oral administrations of 10 mg HAL and R.HAL. Each agent was given for 10 days with a 2-week washout period between the two compounds. HAL and RHAL were interconverted in all subjects. Plasma RHAL/HAL ratios at steady state during HAL treatment were significantly greater than the HAL/RHAL ratios after repeated reduced HAL administrations (0.51 ± 0.12 vs. 0.16 ± 0.04 SD, p < 0.0005). This result suggests that the interconversions between HAL and RHAL are apparently not equivalent in humans. A negative correlation was found between RHAL/HAL ratios after HAL administrations and HAL/RHAL ratios after RHAL administrations (r = – 0.82, p < 0.05). Repeated injections of HAL or RHAL at low (0.1 mg/kg) or high (1.0 mg/ kg) doses were conducted in guinea pigs. Drug concentrations in striatum and plasma were measured. Both RHAI./HAL ratios after HAL injections and HAL RHAL ratios after RHAL injections were dose- and time-dependent. High doses and repeated injections produced greater RHAL/HAL ratios after HAL and smaller HAL/RHAL ratios after RHAL than those observed with low doses and single injections, respectively. Compared with the results obtained from schizophrenic patients, the conversion from HAL to RHAL in guinea pigs was greater than that in humans, but the back conversions appeared to be similar between the guinea pigs and humans. Based upon the dose-dependent increase in RHAL/HAL ratios, a hypothesis of the therapeutic window effect for HAL treatment is proposed.