What We Have Gained from Ibogaine: α3β4 Nicotinic Acetylcholine Receptor Inhibitors as Treatments for Substance Use Disorders

Single Amino Acid Substitution in Loop1 Switches the Selectivity of α-Conotoxin RegIIA towards the α7 Nicotinic Acetylcholine Receptor

α-Conotoxins are disulfide-rich peptides obtained from the venom of cone snails, which are considered potential molecular probes and drug leads for nAChR-related disorders. However, low specificity towards different nAChR subtypes restricts the further application of many α-conotoxins. In this work, a series of loop1 amino acid-substituted mutants of α-conotoxin RegIIA were synthesized, whose potency and selectivity were evaluated by an electrophysiological approach. The results showed that loop1 alanine scanning mutants [H5A]RegIIA and [P6A]RegIIA blocked rα7 nAChR with IC50s of 446 nM and 459 nM, respectively, while their inhibition against rα3β2 and rα3β4 subtypes was negligible, indicating the importance of the fifth and sixth amino acid residues for RegIIA's potency and selectivity. Then, second-generation mutants were designed and synthesized, among which the analogues [H5V]RegIIA and [H5S]RegIIA showed significantly improved selectivity and comparable potency towards rα7 nAChR compared with the native RegIIA. Overall, these findings provide deep insights into the structure-activity relationship of RegIIA, as well as revealing a unique perspective for the further modification and optimization of α-conotoxins and other active peptides.

Tabernanthalog and ibogainalog inhibit the α7 and α9α10 nicotinic acetylcholine receptors via different mechanisms and with higher potency than the GABAA receptor and CaV2.2 channel

In this study, we have investigated the pharmacological activity and structural interaction of two novel psychoplastogens, tabernanthalog (TBG) and ibogainalog (IBG) at heterologously-expressed rat (r) and human (h) nicotinic acetylcholine receptors (nAChRs), the rα1β2γ2L γ-aminobutyric acid type A receptor (GABAAR), and the human voltage-gated N-type calcium channel (CaV2.2 channel). Both compounds inhibited the nAChRs with the following receptor selectivity: α9α10 > α7 > α3β2 ≅ α3β4, indicating that β2/β4 subunits are relatively less important for their activity. The potencies of TBG and IBG were comparable at hα7 and hα9α10 subtypes, and comparable to their rat counterparts. TBG- and IBG-induced inhibition of rα7 was ACh concentration-independent and voltage-dependent, whereas rα9α10 inhibition was ACh concentration-dependent and voltage-independent, suggesting that they interact with the α7 ion channel pore and α9α10 orthosteric ligand binding site, respectively. These results were supported by molecular docking studies showing that at the α7 model TBG forms stable interactions with luminal rings at 9', 13', and 16', whereas IBG mostly interacts with the extracellular-transmembrane junction. In the α9α10 model, however, these compounds interacted with several residues from the principal (+) and complementary (-) sides in the transmitter binding site. Ibogaminalog (DM506) also interacted with a non-luminal site at α7, and one α9α10 orthosteric site. TBG and IBG inhibited the GABAAR and CaV2.2 channels with 10 to 30-fold lower potencies. In sum, we show that TBG and IBG inhibit the α7 and α9α10 nAChRs by noncompetitive and competitive mechanisms, respectively, and with higher potency than the GABAAR and CaV2.2 channel.

Evolution and diversification of carboxylesterase-like [4+2] cyclases in aspidosperma and iboga alkaloid biosynthesis

Monoterpene indole alkaloids (MIAs) are a large and diverse class of plant natural products, and their biosynthetic construction has been a subject of intensive study for many years. The enzymatic basis for the production of aspidosperma and iboga alkaloids, which are produced exclusively by members of the Apocynaceae plant family, has recently been discovered. Three carboxylesterase (CXE)-like enzymes from Catharanthus roseus and Tabernanthe iboga catalyze regio- and enantiodivergent [4+2] cycloaddition reactions to generate the aspidosperma (tabersonine synthase, TS) and iboga (coronaridine synthase, CorS; catharanthine synthase, CS) scaffolds from a common biosynthetic intermediate. Here, we use a combined phylogenetic and biochemical approach to investigate the evolution and functional diversification of these cyclase enzymes. Through ancestral sequence reconstruction, we provide evidence for initial evolution of TS from an ancestral CXE followed by emergence of CorS in two separate lineages, leading in turn to CS exclusively in the Catharanthus genus. This progression from aspidosperma to iboga alkaloid biosynthesis is consistent with the chemotaxonomic distribution of these MIAs. We subsequently generate and test a panel of chimeras based on the ancestral cyclases to probe the molecular basis for differential cyclization activity. Finally, we show through partial heterologous reconstitution of tabersonine biosynthesis using non-pathway enzymes how aspidosperma alkaloids could have first appeared as "underground metabolites" via recruitment of promiscuous enzymes from common protein families. Our results provide insight into the evolution of biosynthetic enzymes and how new secondary metabolic pathways can emerge through small but important sequence changes following co-option of preexisting enzymatic functions.

Synthesis of α3β4 Nicotinic Acetylcholine Receptor Modulators Derived from Aristoquinoline That Reduce Reinstatement of Cocaine-Seeking Behavior

Growing evidence suggests that inhibition of the α3β4 nicotinic acetylcholine receptor (nAChR) represents a promising therapeutic strategy to treat cocaine use disorder. Recently, aristoquinoline (1), an alkaloid from Aristotelia chilensis, was identified as an α3β4-selective nAChR inhibitor. Here, we prepared 22 derivatives of 1 and evaluated their ability to inhibit the α3β4 nAChR. These studies revealed structure-activity trends and several compounds with increased potency compared to 1 with few off-target liabilities. Additional mechanistic studies indicated that these compounds inhibit the α3β4 nAChR noncompetitively, but do not act as channel blockers, suggesting they are negative allosteric modulators. Finally, using a cocaine-primed reinstatement paradigm, we demonstrated that 1 significantly attenuates drug-seeking behavior in an animal model of cocaine relapse. The results from these studies further support a role for the α3β4 nAChR in the addictive properties of cocaine and highlight the possible utility of aristoquinoline derivatives in treating cocaine use disorder.

Total Synthesis of Ervaoffine J and K

Herein, we describe the total synthesis of ervaoffine J & K from a central intermediate. Ervaoffine J was synthesized in eight steps in 14 % yield. Our strategy features an aerobic Winterfeldt oxidation to introduce the 4-quinolone moiety. Ervaoffine K was produced in ten steps and 10 % yield. The synthesis leveraged (bromodifluoromethyl)-trimethylsilane to induce a regioselective von Braun-type C-N bond fragmentation. This C-N bond cleavage unveiled the tetrasubstituted all-syn cyclohexane core of ervaoffine K and enabled the completion of its synthesis.

Advances in small molecule selective ligands for heteromeric nicotinic acetylcholine receptors

The study of nicotinic acetylcholine receptors (nAChRs) has significantly progressed in the last decade, due to a) the improved techniques available for structural studies; b) the identification of ligands interacting at orthosteric and allosteric recognition sites on the nAChR proteins, able to tune channel conformational states; c) the better functional characterization of receptor subtypes/subunits and their therapeutic potential; d) the availability of novel pharmacological agents able to activate or block nicotinic-mediated cholinergic responses with subtype or stoichiometry selectivity. The copious literature on nAChRs is related to the pharmacological profile of new, promising subtype selective derivatives as well as the encouraging preclinical and early clinical evaluation of known ligands. However, recently approved therapeutic derivatives are still missing, and examples of ligands discontinued in advanced CNS clinical trials include drug candidates acting at both neuronal homomeric and heteromeric receptors. In this review, we have selected heteromeric nAChRs as the target and comment on literature reports of the past five years dealing with the discovery of new small molecule ligands or the advanced pharmacological/preclinical investigation of more promising compounds. The results obtained with bifunctional nicotinic ligands and a light-activated ligand as well as the applications of promising radiopharmaceuticals for heteromeric subtypes are also discussed.

DM506 (3-Methyl-1,2,3,4,5,6-hexahydroazepino[4,5-b]indole fumarate), a Novel Derivative of Ibogamine, Inhibits α7 and α9α10 Nicotinic Acetylcholine Receptors by Different Allosteric Mechanisms

The main objective of this study was to determine the pharmacological activity and molecular mechanism of action of DM506 (3-methyl-1,2,3,4,5,6-hexahydroazepino[4,5-b]indole fumarate), a novel ibogamine derivative, at different nicotinic acetylcholine receptor (nAChR) subtypes. The functional results showed that DM506 neither activates nor potentiates but inhibits ACh-evoked currents at each rat nAChR subtype in a non-competitive manner. The receptor selectivity for DM506 inhibition follows the sequence: α9α10 (IC50 = 5.1 ± 0.3 μM) ≅ α7β2 (5.6 ± 0.2 μM) ∼ α7 (6.4 ± 0.5 μM) > α6/α3β2β3 (25 ± 1 μM) > α4β2 (62 ± 4 μM) ≅ α3β4 (70 ± 5 μM). No significance differences in DM506 potency were observed between rat and human α7 and α9α10 nAChRs. These results also indicated that the β2 subunit is not involved or is less relevant in the activity of DM506 at the α7β2 nAChR. DM506 inhibits the α7 and α9α10 nAChRs in a voltage-dependent and voltage-independent manner, respectively. Molecular docking and molecular dynamics studies showed that DM506 forms stable interactions with a putative site located in the α7 cytoplasmic domain and with two intersubunit sites in the extracellular-transmembrane junction of the α9α10 nAChR, one located in the α10(+)/α10(─) interface and another in the α10(+)/α9(─) interface. This study shows for the first time that DM506 inhibits both α9α10 and α7 nAChR subtypes by novel allosteric mechanisms likely involving modulation of the extracellular-transmembrane domain junction and cytoplasmic domain, respectively, but not by direct competitive antagonism or open channel block.