Ligand-specific differential regulation of 5-hydroxytryptamine receptors: functional selectivity in serotonergic signaling

Translating biased agonists from molecules to medications: Serotonin 5-HT1A receptor functional selectivity for CNS disorders

Biased agonism (or "functional selectivity") at G-protein-coupled receptors has attracted rapidly increasing interest as a means to improve discovery of more efficacious and safer pharmacotherapeutics. However, most studies are limited to in vitro tests of cellular signaling and few biased agonists have progressed to in vivo testing. As concerns 5-HT_1A receptors, which exert a major control of serotonergic signaling in diverse CNS regions, study of biased agonism has previously been limited by the poor target selectivity and/or partial agonism of classically available ligands. However, a new generation of highly selective, efficacious and druggable agonists has advanced the study of biased agonism at this receptor and created new therapeutic opportunities. These novel agonists show differential properties for G-protein signaling, cellular signaling (particularly pERK), electrophysiological effects, neurotransmitter release, neuroimaging by PET and pharmacoMRI, and behavioral tests of mood, motor activity and side effects. Overall, NLX-101 (a.k.a. F15599) exhibits preferential activation of cortical and brain stem 5-HT_1A receptors, whereas NLX-112 (a.k.a. befiradol or F13640) shows prominent activation of 5-HT_1A autoreceptors in Raphe nuclei and in regions associated with motor control. Accordingly, NLX-101 is potently active in rodent models of depression and respiratory control, whereas NLX-112 shows promising activity in models of Parkinson's disease across several species - rat, marmoset and macaque. Moreover, NLX-112 has also been labeled with ¹⁸F to produce the first agonist PET radiopharmaceutical (known as [¹⁸F]-F13640) for investigation of the active state of 5-HT_1A receptors in rodent, primate and human. The structure-functional activity relationships of biased agonists have been investigated by receptor modeling and novel compounds have been identified which exhibit increased affinity at 5-HT_1A receptors and new profiles of cellular signaling bias, notably for β-arrestin recruitment versus pERK. Taken together, the data suggest that 5-HT_1A receptor biased agonists constitute potentially superior pharmacological agents for treatment of CNS disorders involving serotonergic mechanisms.

Drug repurposing for Dravet syndrome in scn1Lab-/- mutant zebrafish

Dravet syndrome (DS) is a severe genetic epileptic encephalopathy with onset during the first year of life. Zebrafish models recapitulating human diseases are often used as drug discovery platforms, but also for drug repurposing testing. It was recently shown that pharmacological modulation of three serotonergic (5-HT) receptors (5-HT_1D , 5-HT_2C , 5-HT_2A ) exerts antiseizure effects in a zebrafish scn1Lab^-/- mutant model of DS. Using the zebrafish DS model, our aim was to examine the possibility of repurposing efavirenz (EFA), lisuride (LIS), and rizatriptan (RIZA), marketed medicines with a 5-HT on- or off-target profile, as antiepileptic drugs for DS. To examine whether these compounds have a broader antiseizure profile, they were tested in pentylenetetrazol and ethyl ketopentenoate (EKP) zebrafish models. Pharmacological effects were assessed by locomotor behavior, local field potential brain recordings, and bioluminescence. EFA was active in all models, whereas LIS was selectively active in the zebrafish DS model. Mainly, a poor response was observed to RIZA. Taken together, our preclinical results show that LIS could be a potential candidate for DS treatment. EFA was also active in the EKP model, characterized by a high level of treatment resistance, and hence these data are potentially important for future treatment of drug-resistant epilepsy.

Arylpiperazine agonists of the serotonin 5-HT1A receptor preferentially activate cAMP signaling versus recruitment of β-arrestin-2

G protein-coupled receptors (GPCRs) mediate biological signal transduction through complex molecular pathways. Therapeutic effects of GPCR-directed drugs are typically accompanied by unwanted side effects, owing in part to the parallel engagement of multiple signaling mechanisms. The discovery of drugs that are 'functionally selective' towards therapeutic effects, based on their selective control of cellular responses through a given GPCR, is thus a major goal in pharmacology today. In the present study, we show that several arylpiperazine ligands of the serotonin 5-HT1A receptor (5-HT1AR) preferentially activate 3',5'-cyclic adenosine monophosphate (cAMP) signaling versus β-arrestin-2 recruitment. The pharmacology of these compounds is thus qualitatively different from the endogenous agonist serotonin, indicating functional selectivity of 5-HT1AR-mediated response pathways. Preliminary evidence suggests that phosphorylation of extracellular signal-regulated kinases 1/2 (ERK1/2) downstream of 5-HT1AR is a substrate of functionally selective signaling by partial agonists. We propose that the compounds described in the present study are useful starting points for the development of signaling pathway-selective drugs targeting 5-HT1AR.