Background: In vivo determination of serotonin transporter (5-HTT) occupancy by selective serotonin reuptake inhibitors (SSRI) using positron emission tomography (PET) can aid in determination of dosing. Previous studies used chronic SSRI administration that may down-regulate 5-HTT and used the cerebellum as reference region despite measurable 5-HTT. We examine the reference region and occupancy after acute sertraline dosing.
Methods: We conducted autoradiography of human postmortem cerebellum to determine an optimal reference region. We quantified 5-HTT binding using [(11)C]DASB and arterial input functions in 17 healthy volunteers. Baseline PET scans were followed by a scan 4-6 days after 25 mg to 100mg of daily sertraline. Several modeling methods and outcome measures were assessed.
Results: Cerebellar gray matter is the optimal reference region. Occupation of 5-HTT sites saturates at low plasma sertraline levels (K(D) = 1.9 ng/ml) with maximal occupancies of 106.8 +/- 8.3% across all brain regions. There is a weak correlation between oral sertraline and plasma sertraline. Occupancy measures vary based on the reference region and outcome measure used.
Conclusions: Occupancy studies and postmortem autoradiography can help define the optimal reference region. Reference tissue modeling using the optimal reference region returns the same occupancy measures as those determined using an arterial input function.