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Am J Psychiatry. 2004 May;161(5):826-35.

Serotonin transporter occupancy of five selective serotonin reuptake inhibitors at different doses: an [11C]DASB positron emission tomography study.

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  • 1Clarke Site, Centre for Addiction and Mental Health and Department of Psychiatry, University of Toronto, 250 College St., Toronto, Ont. M5T 1R8, Canada. jmeyer@camhpet.on.ca



Minimum therapeutic doses of paroxetine and citalopram produce 80% occupancy for the serotonin (5-HT) transporter (5-HTT). The authors used [(11)C]DASB positron emission tomography to measure occupancies of three other selective serotonin reuptake inhibitors (SSRIs) at minimum therapeutic doses. The relationship between dose and occupancy was also investigated.


Striatal 5-HTT binding potential was measured in 77 subjects before and after 4 weeks of medication administration. Binding potential is proportional to the density of receptors not blocked by medication. Subjects received citalopram, fluoxetine, sertraline, paroxetine, or extended-release venlafaxine. Healthy subjects received subtherapeutic doses; subjects with mood and anxiety disorders received therapeutic doses. Percent reduction in 5-HTT binding potential for each medication and dose was calculated. To obtain test-retest data, binding potential was measured before and after 4 weeks in six additional healthy subjects.


Substantial occupancy occurred at subtherapeutic doses for all SSRIs. Compared to test-retest data, each drug at the minimum therapeutic dose had a significant effect on striatal 5-HTT binding potential. Mean occupancy at this dose was 76%-85%. At higher plasma SSRI concentrations, 5-HTT occupancy tended to increase above 80%. For each drug, as the dose (or plasma level) increased, occupancy increased nonlinearly, with a plateau for higher doses.


At tolerable doses, SSRIs have increasing occupancy with increasing plasma concentration or dose. Occupancy of 80% across five SSRIs occurs at minimum therapeutic doses. This suggests that 80% 5-HTT blockade is important for therapeutic effect. Occupancy should be measured during development of antidepressant compounds targeting the 5-HTT.

[PubMed - indexed for MEDLINE]
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