Quetiapine Extended-Release Versus Immediate-Release Formulation: A Positron Emission Tomography Study

Objective: The pharmacokinetic and pharmacodynamic profile of the immediate-release (IR) formulation of quetiapine is characterized by a rapid peak in plasma level and striatal dopamine D₂ receptor occupancy, followed by a rapid decrease to baseline levels, necessitating the use of twice-daily dosing. An extended-release (XR) formulation of quetiapine is currently being developed to achieve similar efficacy using a once-daily dosing regimen. We compared the central D₂ receptor binding between the IR and XR formulations.

Method: In this open-label, crossover positron emission tomography study using [¹¹C]-raclopride, we compared the central D₂ receptor binding potential at expected peak and trough plasma levels using equivalent daily doses of the IR and XR formulations (300, 600, and 800 mg/day) in 12 subjects. Data were collected from April 2002 to May 2003.

Results: The mean plasma level of quetiapine at trough was significantly lower than that at peak for all dose groups of both formulations except for IR 300 and 800 mg (all p values < .05), while the mean plasma level did not differ significantly between formulations at trough and peak. The mean occupancy at peak was significantly higher than that at trough for all dose groups other than IR 800 mg/day (all p values < .05) and did not differ significantly between formulations at trough and peak.

Conclusion: Once-daily dosing of the XR formulation gives peak and trough plasma levels and central D₂ receptor occupancy comparable to twice-daily dosing of the IR formulation. These data should be considered while determining equivalent doses, as well as switching strategies.