Synthesis and autoradiographic evaluation of a novel high-affinity Tc-99m ligand for the 5-HT2A receptor

The synthesis and in vitro autoradiography of a novel Tc-99m ligand with subnanomolar affinity to the 5-HT_2A receptor is reported. The complex combines the 4-(4-fluoro)-benzoyl piperidine portion derived from the 5-HT_2A receptor antagonist ketanserin with a neutral oxotechnetium(V) chelate in form of a mixed ligand “3+1” unit containing the SNS/S donor set. The analogous rhenium compound has been synthesized as a surrogate for the Tc-99m complex for use in receptor binding assays and for complete structural characterization. In competition experiments the Tc-99 complex as well as its Re analogue display subnanomolar affinity toward the 5-HT_2A receptor (K_i 0.44 nM for Tc, 0.25 nM for Re). The subnanomolar 5-HT_2A receptor binding of the Re complex was confirmed by functional in vitro antagonism of contractile effects evoked by 5-HT in rat arterial tissue. Re 1 inhibited 5-HT-induced, 5-HT_2A receptor-mediated contractions of isolated rat tail artery in a competitive fashion and possessed nanomolar affinity ( pA₂=9.08, pA₂ representing the negative decadic logarithm of the Re 1/5-HT_2A-receptor dissociation constant [mol/L]). Like ketanserin, Re 1 displayed moderate affinity for adrenergic _1D ( pA₂=8.23) and histamine H₁ receptors ( pA₂=8.00), and was >600-fold up to 10,700-fold less active at several neurotransmitter receptor subtypes. In vitro autoradiographic studies clearly indicate the accumulation of the Tc-99m compound in 5-HT_2A-receptor-rich areas of the brain. This enrichment can be blocked by 5-HT_2A receptor antagonists such as mianserin and ketanserin and is therefore specific.