Synthesis, Pharmacological Characterization, and Binding Mode Analysis of 8-Hydroxy-Tetrahydroisoquinolines as 5-HT7 Receptor Inverse Agonists

The serotonin 7 receptor (5-HT7R) regulates various processes in the central nervous system, including mood, learning, and circadian rhythm control, among others. Receptor activation can lead to activation of the Gαs protein and a subsequent increase of intracellular cyclic adenosine monophosphate (cAMP). Receptor interaction with inverse agonists results in a decrease of basal cAMP levels and therefore a downstream effect of reduced neuronal excitability and neurotransmission. Recently, pellotine (1a), a Lophophora alkaloid, was unexpectedly shown to be an inverse agonist of the 5-HT7R. Therefore, we evaluated close analogs of compound 1a, both naturally occurring and synthetic analogs, as inverse agonists of the 5-HT7R. Functional evaluation in a GloSensor cAMP assay revealed a preference for an 8-hydroxy-6,7-dimethoxy substitution pattern over 6,7,8-trimethoxy analogs or 8-hydroxy-6,7-methylenedioxy analogs. This was supported by molecular dynamics simulations, where the 8-hydroxy substitution allowed more robust interaction with the 5-HT7R, which correlated with inverse agonism efficacy. Additionally, N-methylation (as in 1a) improved the potency of the evaluated analogs. In this series, the most potent inverse agonist was anhalidine (1b) (EC50 = 219 nM, Emax = -95.4%), which lacks the 1-methyl, compared to pellotine (1a), and showed a 2-fold higher functional potency. Altogether, these results provide key insights for the further development of potent low molecular weight inverse agonists of the 5-HT7R.