The First Structure–Activity Relationship Studies for Designer Receptors Exclusively Activated by Designer Drugs

Abstract

Over the past decade, two independent technologies have emerged and been widely adopted by the neuroscience community for remotely controlling neuronal activity: optogenetics which utilize engineered channelrhodopsin and other opsins, and chemogenetics which utilize engineered G protein-coupled receptors (Designer Receptors Exclusively Activated by Designer Drugs (DREADDs)) and other orthologous ligand–receptor pairs. Using directed molecular evolution, two types of DREADDs derived from human muscarinic acetylcholine receptors have been developed: hM3Dq which activates neuronal firing, and hM4Di which inhibits neuronal firing. Importantly, these DREADDs were not activated by the native ligand acetylcholine (ACh), but selectively activated by clozapine N-oxide (CNO), a pharmacologically inert ligand. CNO has been used extensively in rodent models to activate DREADDs, and although CNO is not subject to significant metabolic transformation in mice, a small fraction of CNO is apparently metabolized to clozapine in humans and guinea pigs, lessening the translational potential of DREADDs. To effectively translate the DREADD technology, the next generation of DREADD agonists are needed and a thorough understanding of structure–activity relationships (SARs) of DREADDs is required for developing such ligands. We therefore conducted the first SAR studies of hM3Dq. We explored multiple regions of the scaffold represented by CNO, identified interesting SAR trends, and discovered several compounds that are very potent hM3Dq agonists but do not activate the native human M3 receptor (hM3). We also discovered that the approved drug perlapine is a novel hM3Dq agonist with >10 000-fold selectivity for hM3Dq over hM3.

DREADDs for Neuroscientists

“…Because of the potential for back-metabolism of CNO to clozapine and other clozapine metabolites in non-rodent species—including the pharmacologically diverse compound N-desmethyl-clozapine (NDMC) (Davies et al, 2005)—we have developed new non-CNO chemical actuators (Chen et al, 2015). The first of these—Compound 21 (Figure 3B)—has minimal off-target activity and exquisite selectivity for activating hM3Dq versus muscarinic and other GPCRs (Chen et al, 2015).…”

“…The first of these—Compound 21 (Figure 3B)—has minimal off-target activity and exquisite selectivity for activating hM3Dq versus muscarinic and other GPCRs (Chen et al, 2015). Preliminary studies indicate that Compound 21 has equivalent potency in studies in vivo when compared with CNO (unpublished data).…”

“…Perlapine has >10,000-fold selectivity for activating hM3Dq versus muscarinic receptors with an EC 50 at hM3Dq of 2.8 nM (Chen et al, 2015). Given perlapine’s modest affinity for some biogenic amine receptors (e.g., 5-HT2A, 5-HT6, 5-HT7, and D4) (Davies et al, 2005; Roth et al, 1992, 1994, 1995), it is essential to test perlapine at the lowest possible dose in animals not expressing DREADDs before embarking on studies involving DREADDs.…”

“…Additionally the availability of salvinorin B analogues which are water soluble—as salvinorin B is typically dissolved in dimethylsulfoxide—would also be useful. The development of these sorts of tool compounds could be achieved via conventional medicinal chemistry approaches (Chen et al, 2015) and by new technologies developed by my lab and my collaborators. These new chemical biology technologies allow for the design and validation of novel drug-like molecules using a combination of in silicio and in vitro approaches (Keiser et al, 2009; Besnard et al, 2012; Huang et al, 2015c; Kroeze et al, 2015).…”

Practical Strategies and Concepts in GPCR Allosteric Modulator Discovery: Recent Advances with Metabotropic Glutamate Receptors

“…1–12 As X-ray crystal structures and cocrystals with orthosteric and/or allosteric ligands are increasingly available, GPCR ligand optimization is embracing structure-based drug design and subsequently providing the next generation of homology models for in silico screening. 10 Advances in molecular pharmacology and screening have propelled the discovery and development of allosteric modulators, biased ligands, and designer receptors exclusively activated by designer drugs (DREADDs), 19,20 while deepening our conceptual understanding and utilization of signal bias along with divergent ligand profiles targeting GPCR heterodimers. 10 Of all of these advances, allosteric modulation is the front runner, with allosteric ligands reported across the four major GPCR families (families A, B, C, and F) that overcome major limitations and liabilities of their orthosteric congeners (nondrug-like properties, limited CNS exposure, peptidic ligands, subtype and/or GPCR-nome selectivity, desensitization, down-regulation).…”