Molecular and behavioral pharmacology of novel 2-aminotetralin derivatives targeting serotonin G protein-coupled receptors

Abstract

Modulation of serotonin (5-hydroxytryptamine, 5-HT) receptor activity is an effective strategy for treating neuropsychiatric disorders. However, selectively targeting therapeutically relevant 5-HT receptors (5-HTRs) over receptors associated with adverse events is a formidable task. Our lab has shown that 4-phenyl-2-aminotetralin (4-PAT) derivatives are potent modulators of 5-HT2A and 5-HT2CRs, both important neurotherapeutic targets, but obtaining selectivity over highly homologous 5-HT2B and histamine H1Rs is challenging. In this work, we performed affinity, signal transduction, molecular modeling, and mutagenesis studies using 42 novel 4-PAT derivatives to understand structure-activity relationships at 5-HT2-type and H1Rs. We identified novel selective 5-HT2A/5-HT2CR inverse agonists and compared them to the antipsychotic drug pimavanserin (a selective 5-HT2A/5-HT2CR inverse agonist) in vitro and in mouse models of 5-HT2R engagement and sedative liability, i.e., head twitch response and locomotor activity. We found that most 4-PAT diastereomers in the (2S,4R)-configuration bound non-selectively to 5-HT2A, 5-HT2C, and H1Rs, with >100-fold selectivity over 5-HT2BRs. In contrast, diastereomers in the (2R,4R)-configuration bound preferentially to 5-HT2A over 5-HT2CRs, with >100-fold selectivity over 5-HT2B and H1Rs. Mutagenesis results suggested that G2385.42 and V2355.39 in 5-HT2ARs (conserved in 5-HT2CRs) are important for high affinity binding and selectivity, whereas interactions with T1945.42 and W1584.56 are important for binding to H1Rs. The 4-PAT-type ligand (2S,4R)-2k, a potent and selective 5-HT2A/5-HT2CR inverse agonist, had activity like pimavanserin in the mouse head twitch response assay and was unique in not suppressing locomotor activity. We provide evidence the novel 4-PAT chemotype can yield selective 5-HT2A/5-HT2CR inverse agonists for antipsychotic drug development by optimizing ligand-receptor interactions in transmembrane domains 4 and 5, and chirality can be exploited to attain selectivity over H1Rs to help circumvent sedative effects. Work from our group has also identified the 5-substituted-2-aminotetralin (5-SAT) chemotype, which is structurally related to 4-PATs through a shared 2-aminotetralin scaffold, as a platform to generate dual 5-HT1A/5-HT7R modulators. Our lab previously reported an orally active 5-SAT analog, (S)-5a, that acts as a potent partial agonist at 5-HT1A and 5-HT7Rs, reduces repetitive behaviors and increases social approach behavior in multiple mouse models of autism spectrum disorder (ASD). However, it is unknown how the activity of (S)-5a activity at 5-HT1A and 5-HT7Rs contributes to its preclinical efficacy. Therefore, we characterized equilibrium and kinetic binding parameters and performed molecular modeling studies to understand how (S)-5a engages its high affinity targets. We also characterized the affinity as well as the functional potency and efficacy on adenylyl cyclase signaling for a series of structurally related 5-SATs at 5-HT1A and 5-HT7Rs to identify novel analogs which might have similar or improved preclinical efficacy as (S)-5a. Three key analogs were identified which exhibit comparable affinity and high efficacy agonist activity to (S)-5a at 5-HT1ARs but differ in their functional activity at 5-HT7Rs (i.e., neutral antagonism, partial agonism, or inverse agonism). We also discovered a highly potent full agonist of 5-HT1ARs which exhibits ~40-fold selectivity to bind 5-HT1A over 5-HT7Rs. Comparative assessment of these four analogs in vivo were performed in mice using the head twitch response assay, and locomotor activity in an open field to evaluate their ability to modulate central serotonergic activity and motor activity. Our results show that the four lead ligands were behaviorally active at relatively low doses (≤ 1 mg*kg-1) in the head twitch response assay, likely, through activation of 5-HT1ARs, and, did not cause locomotor suppression or behavioral symptoms that model serotonin syndrome. Our findings identify the 5-SAT chemotype as a novel class of serotonergic ligands warranting further investigation for ASD drug development. With consideration to the polygenic nature of neuropsychiatric disorders, novel 2-aminotetralin derivatives were assessed for their affinity and functional activity at a variety of class A GPCRs. Ligands did not reveal 'red-flag' activities for CNS drug development.--Author's abstract