Assessing the Optimal Time Point for the Measurement of Extrastriatal Serotonin Transporter Binding with 123I-FP-CIT SPECT in Healthy, Male Subjects

I-123-N-omega-fluoropropyl-2 beta-carboxymethoxy-3 beta-(4-iodophenyl) nortropane (I-123-FP-CIT) is commonly used to assess the dopamine transporter in the striatum. However, recent studies suggest that this tracer may be used also to assess binding to monoamine transporters in the midbrain or diencephalon, which may reflect predominantly serotonin transporter (SEAT) binding. However, it is still unclear at what time point after injection SPECT should be performed for optimal assessment of SERT with I-123-FP-CIT. Therefore, we examined the time course of extrastriatal I-123-FP-CIT binding. Methods: Nineteen healthy, male subjects were included, and SPECT images were acquired up to 3 h after I-123-FP-CIT injection. Region-of-interest analysis was performed, and specific-to-nonspecific binding ratios were calculated. Results: Specific-to-nonspecific I-123-FP-CIT binding ratios in the midbrain and diencephalon were significantly higher 2 h after injection than 1 h after injection and remained stable between 2 and 3 h after injection. Conclusion: The optimal time frame for assessing I-123-FP-CIT binding to extrastriatal SEAT is between 2 and 3 h after injection of the tracer.