Evaluation of S-[ 11C]citalopram as a radioligand for in vivo labelling of 5-hydroxytryptamine uptake sites

The biologically active S-enantiomer of [ N-methyl- 11C]citalopram was evaluated as a radioligand for in vivo labelling of the 5-hydroxytryptamine uptake site in brain, using ex vivo tissue counting in rats and positron emission tomography in man. In rats, the maximal signal for total versus non-specific binding was approx. 2 at 60–120 min after radioligand injection. Subsequent studies in man failed to identify a specific signal over a 90 min scanning period, due to prolonged retention of non-specific label.