Validation of a rat in vivo [3H]M100907 binding assay to determine a translatable measure of 5-HT2A receptor occupancy

An in vivo binding assay is characterized for [3H]M100907 binding to rat brain, as a measure of 5-HT2A receptor occupancy. Dose–response analyses were performed for various 5-HT2A antagonist reference agents, providing receptor occupancy ED50 values in conjunction with plasma and brain concentration levels. Ketanserin and M100907 yielded dose-dependent increases in 5-HT2A receptor occupancy with ED50s of 0.316 mg/kg and 0.100 mg/kg, respectively. The atypical antipsychotics risperidone, olanzapine, and clozapine dose-dependently inhibited in vivo [3H]M100907 binding with ED50 values of 0.051, 0.144, and 1.17 mg/kg, respectively. In contrast, the typical antipsychotic haloperidol exhibited only 20.1% receptor occupancy at 10 mg/kg despite producing dose-dependent increases in plasma and brain exposure levels. The novel psychopharmacologic agent asenapine dose-dependently occupied 5-HT2A receptors in rat brain with an ED50 of 0.011 mg/kg, demonstrating higher 5-HT2A receptor potency compared with the other atypical antipsychotics tested. This enhanced potency was supported by a lower plasma exposure EC50 of 0.477 ng/ml, compared with risperidone (1.57 ng/ml) and olanzapine (7.81 ng/ml) and was confirmed in time course studies. The validated [3H]M100907 rat in vivo binding assay allows for preclinical measurement of 5-HT2A receptor occupancy, providing essential data for understanding the pharmacological profile of novel antipsychotic agents. Additionally, the corresponding plasma and brain drug exposure data analyses provides a valuable data set for 5-HT2A reference agents by enabling direct comparison with any complementary studies performed in rats, thus providing a foundation for predictive pharmacokinetic/pharmacodynamic models and, importantly, allowing for translation to human receptor occupancy studies using [11C]M100907 positron emission tomography.