Exposure-Efficacy Analysis and Dopamine D2 Receptor Occupancy in Adults with Schizophrenia after Treatment with the Monthly Intramuscular Injectable Risperidone ISM

Dopamine D2 receptor occupancy (D2RO) significantly influences the clinical effectiveness and safety of many antipsychotic drugs. Maintaining a D2RO range of 65%-80% provides the best antipsychotic effects while minimizing adverse reactions. Data from a Phase III trial were used to establish an exposure–response relationship for monthly intramuscular Risperidone ISM (75 and 100 mg) or placebo administered to adults with schizophrenia. Pharmacodynamic analysis was based on an E_max model for Positive and Negative Syndrome Scale (PANSS) developed in NONMEM. Plasma concentrations of the active moiety were derived using a previously developed population pharmacokinetic model, which was used for D2RO simulations in conjunction with a published E_max model. The optimal D2RO range (65%-80%) was reached for the median within hours following the first injection of both Risperidone ISM doses. At steady state, median D2RO for both doses remained above 65% throughout the 28-day dosing period and demonstrated lower variability than oral risperidone. PANSS response did not differ significantly between dose groups, most likely because active moiety concentrations had already reached the plateau of the concentration–response relationship. The pharmacokinetic/pharmacodynamic analysis showed a profound placebo effect (−11.7%), and an additional maximal drug effect (−6.6%) resulting in a total PANSS improvement over time of −18.3%. Pharmacokinetic/pharmacodynamic modeling quantified a PANSS improvement over time after Risperidone ISM administration. The response was not significantly different in either dose group, likely because D2RO was already above the proposed efficacy threshold (65%) within 1 h after the first Risperidone ISM injection and remained above this level following repeated administrations.