The bioanalytical results obtained from the PK/PD study are shown in Figure . As can be seen based on a mouse cell EC50 of ~150 ng/mL and brain tissue binding of 94.8%, free drug exposures exceeding the mouse cell EC50 were observed at 10 hours in the plasma, cerebellum and CSF. A significant increase of D-serine levels compared to vehicle was also observed in both the plasma and cerebellum at all three time points measured (Figure ). In addition, a parallel study was run to assess the receptor occupancy (RO) in the cerebellum with the Takeda tracer compound PGM019260 following the protocol published in Neurochemistry Research 2017 . As shown in Figure , the study confirmed significant RO of compound 37 in the PK/PD study, as projected by PK modeling (Figure ). The results of the PK/PD study are summarized in Table . Based upon the data, the PK/PD study with compound 37 has successfully demonstrated the pharmacological potential of hDAO inhibitors from the DHP dione chemical series. In parallel with the PK/PD study, compound 37 was also assessed in a catalepsy model using the same dosing regimen (150 mg/kg p.o. BID, Q4hr) that had generated the positive response in the PK/PD study. During this study, plasma samples were taken and used to assess the prolactin levels at 6 hours post the first dose, which was predicted to be around Cmax. As shown in Figure , no catalepsy or increase in prolactin levels was observed in this study. Plasma and brain concentrations of 37 were determined indicating that levels were similar to those achieved in the PK/PD study (data not shown). This study confirms that 37 is well tolerated in vivo at exposure levels required to evoke the desired PD responses.