Non-amines, drugs without an amine nitrogen, potently block serotonin transport: novel antidepressant candidates?

The serotonin transporter (SERT) is a principal site of action of therapeutic antidepressants in the brain. Without exception, these inhibitors of serotonin transport contain an amine nitrogen in their structure. We previously demonstrated that novel compounds without an amine nitrogen in their structure (non-amines), blocked dopamine transport in cells transfected with the human dopamine transporter. The present study investigated whether, in the absence of an amine nitrogen, certain non-amines bind selectively to the SERT and block the transport of serotonin. At 10 microM concentration, select non-amines displayed no, or little, affinity for 9 serotonin, 5 dopamine, 7 adrenergic, 5 muscarinic cholinergic, 3 opiate and histamine receptors. The affinities of non-amines for [(3)H]citalopram binding sites on the SERT and their potencies for blocking [(3)H]serotonin transport were measured in cloned human SERT stably or transiently expressed in HEK-293. Whether oxa- or carba-based, non-amines bound to [(3)H]citalopram-labeled sites and blocked [(3)H]serotonin transport in the low nanomolar range, at values equal to or higher than those of some conventional antidepressants. A non-amine, O-1809, was 99-fold more selective for the serotonin over the dopamine transporter. As substituents on the aromatic ring of non-amines confer high affinity for the SERT, we investigated the hypothesis that aromatic-aromatic interactions may contribute significantly to non-amine/transporter association. A SERT mutant was produced in which a highly conserved aromatic amino acid, phenylalanine 548, was replaced by an alanine (F548A). Although the affinities of several non-amines were unchanged in the mutant SERT, the affinity of imipramine was decreased, revealing possible differences in amine and non-amine binding domains on the SERT. The similar affinities of non-amines and conventional antidepressant drugs for the SERT support the view that an amine nitrogen is not essential for drugs to block serotonin transport with high affinity. Non-amines open avenues for developing a new generation of antidepressants.