Clinical studies point toward a potential role of the serotonin transporter (SERT) binding as a predictor of clinical outcome in the treatment of depression. After long-term treatment with clinical doses of SSRIs the expected SERT occupancy is about 80%. Here, we were interested to investigate the relationship of SERT occupancy values between short- and longterm treatment.

To test if the SERT occupancy at steady-state can be predicted based on the single dose occupancy by escitalopram (S-citalopram) or citalopram (racemate of S-citalopram and R-citalopram).

18 patients with major depressive disorder received either escitalpram (10 mg/d) or citalopram (20 mg/d) in a double-blind, randomized, longitudinal study. They underwent three PET scans using the radioligand [11C]DASB: PET1 baseline, PET2 6 hours after first drug intake and PET3 after three weeks of daily oral treatment. Occupancy of SERT was quantified in six subcortical regions: thalamus, N. caudatus, putamen, mibrain, dorsal raphe and median raphe nuclei. Data was analyzed by means of multiple linear regression models corrected for baseline SERT availability values using SPSS 15.0.

Single dose occupancy of the SERT significantly predicted steady-state occupancy after three weeks in three regions: thalamus (r2 = 0.45, p = 0.009), N. caudatus (r2 = 0.4, p = 0.006) and putamen (r2 = 0.43, p = 0.005). Other regions did not show significant relationships.

In this study we demonstrated that single-dose occupancy in SERT rich regions such as thalamus, N. caudatus and the putamen could serve as reliable predictors for steady-state occupancy. However, a linear model failed to explain the relationship in regions known for serotonergic cell origin.