Pyridine containing M(1) positive allosteric modulators with reduced plasma protein binding

The pyridazine heterocycle in molecular recognition and drug discovery

The pyridazine ring is endowed with unique physicochemical properties, characterized by weak basicity, a high dipole moment that subtends π-π stacking interactions and robust, dual hydrogen-bonding capacity that can be of importance in drug-target interactions. These properties contribute to unique applications in molecular recognition while the inherent polarity, low cytochrome P450 inhibitory effects and potential to reduce interaction of a molecule with the cardiac hERG potassium channel add additional value in drug discovery and development. The recent approvals of the gonadotropin-releasing hormone receptor antagonist relugolix (24) and the allosteric tyrosine kinase 2 inhibitor deucravacitinib (25) represent the first examples of FDA-approved drugs that incorporate a pyridazine ring. In this review, the properties of the pyridazine ring are summarized in comparison to the other azines and its potential in drug discovery is illustrated through vignettes that explore applications that take advantage of the inherent physicochemical properties as an approach to solving challenges associated with candidate optimization.

Graphical Abstract

Drug Design Targeting the Muscarinic Receptors and the Implications in Central Nervous System Disorders

There is substantial evidence that cholinergic system function impairment plays a significant role in many central nervous system (CNS) disorders. During the past three decades, muscarinic receptors (mAChRs) have been implicated in various pathologies and have been prominent targets of drug-design efforts. However, due to the high sequence homology of the orthosteric binding site, many drug candidates resulted in limited clinical success. Although several advances in treating peripheral pathologies have been achieved, targeting CNS pathologies remains challenging for researchers. Nevertheless, significant progress has been made in recent years to develop functionally selective orthosteric and allosteric ligands targeting the mAChRs with limited side effect profiles. This review highlights past efforts and focuses on recent advances in drug design targeting these receptors for Alzheimer’s disease (AD), schizophrenia (SZ), and depression.

Further optimization of the M1 PAM VU0453595: Discovery of novel heterobicyclic core motifs with improved CNS penetration

This Letter describes the continued chemical optimization of the VU0453595 series of M1 positive allosteric modulators (PAMs). By surveying alternative 5,6- and 6,6-heterobicylic cores for the 6,7-dihydro-5H-pyrrolo[3,4-b]pyridine-5-one core of VU453595, we found new cores that engendered not only comparable or improved M1 PAM potency, but significantly improved CNS distribution (Kps 0.3-3.1). Moreover, this campaign provided fundamentally distinct M1 PAM chemotypes, greatly expanding the available structural diversity for this valuable CNS target, devoid of hydrogen-bond donors.

Discovery and SAR of muscarinic receptor subtype 1 (M1) allosteric activators from a molecular libraries high throughput screen. Part 1: 2,5-dibenzyl-2H-pyrazolo[4,3-c]quinolin-3(5H)-ones as positive allosteric modulators

Results from a 2012 high-throughput screen of the NIH Molecular Libraries Small Molecule Repository (MLSMR) against the human muscarinic receptor subtype 1 (M1) for positive allosteric modulators is reported. A content-rich screen utilizing an intracellular calcium mobilization triple-addition protocol allowed for assessment of all three modes of pharmacology at M1, including agonist, positive allosteric modulator, and antagonist activities in a single screening platform. We disclose a dibenzyl-2H-pyrazolo[4,3-c]quinolin-3(5H)-one hit (DBPQ, CID 915409) and examine N-benzyl pharmacophore/SAR relationships versus previously reported quinolin-3(5H)-ones and isatins, including ML137. SAR and consideration of recently reported crystal structures, homology modeling, and structure-function relationships using point mutations suggests a shared binding mode orientation at the putative common allosteric binding site directed by the pendant N-benzyl substructure.

Synthesis and pharmacological evaluation of analogues of benzyl quinolone carboxylic acid (BQCA) designed to bind irreversibly to an allosteric site of the M ₁ muscarinic acetylcholine receptor

Activation of the M1 muscarinic acetylcholine receptor (mAChR) is a prospective treatment for alleviating cognitive decline experienced in central nervous system (CNS) disorders. Current therapeutics indiscriminately enhance the activity of the endogenous neurotransmitter ACh, leading to side effects. BQCA is a positive allosteric modulator and allosteric agonist at the M1 mAChR that has high subtype selectivity and is a promising template from which to generate higher affinity, more pharmacokinetically viable drug candidates. However, to efficiently guide rational drug design, the binding site of BQCA needs to be conclusively elucidated. We report the synthesis and pharmacological validation of BQCA analogues designed to bind irreversibly to the M1 mAChR. One analogue in particular, 11, can serve as a useful structural probe to confirm the location of the BQCA binding site; ideally, by co-crystallization with the M1 mAChR. Furthermore, this ligand may also be used as a pharmacological tool with a range of applications.

Discovery of naphthyl-fused 5-membered lactams as a new class of m1 positive allosteric modulators

Selective activation of the M1 muscarinic receptor via positive allosteric modulation represents an original approach to treat the cognitive decline in patients with Alzheimer's disease. A series of naphthyl-fused 5-membered lactams were identified as a new class of M1 positive allosteric modulators and were found to possess good potency and in vivo efficacy.

Development of M1 mAChR allosteric and bitopic ligands: prospective therapeutics for the treatment of cognitive deficits

Since the cholinergic hypothesis of memory dysfunction was first reported, extensive research efforts have focused on elucidating the mechanisms by which this intricate system contributes to the regulation of processes such as learning, memory, and higher executive function. Several cholinergic therapeutic targets for the treatment of cognitive deficits, psychotic symptoms, and the underlying pathophysiology of neurodegenerative disorders, such as Alzheimer's disease and schizophrenia, have since emerged. Clinically approved drugs now exist for some of these targets; however, they all may be considered suboptimal therapeutics in that they produce undesirable off-target activity leading to side effects, fail to address the wide variety of symptoms and underlying pathophysiology that characterize these disorders, and/or afford little to no therapeutic effect in subsets of patient populations. A promising target for which there are presently no approved therapies is the M1 muscarinic acetylcholine receptor (M1 mAChR). Despite avid investigation, development of agents that selectively activate this receptor via the orthosteric site has been hampered by the high sequence homology of the binding site between the five muscarinic receptor subtypes and the wide distribution of this receptor family in both the central nervous system (CNS) and the periphery. Hence, a plethora of ligands targeting less structurally conserved allosteric sites of the M1 mAChR have been investigated. This Review aims to explain the rationale behind allosterically targeting the M1 mAChR, comprehensively summarize and critically evaluate the M1 mAChR allosteric ligand literature to date, highlight the challenges inherent in allosteric ligand investigation that are impeding their clinical advancement, and discuss potential methods for resolving these issues.

Synthesis and pharmacological profiling of analogues of benzyl quinolone carboxylic acid (BQCA) as allosteric modulators of the M1 muscarinic receptor

Established therapy in Alzheimer's disease involves potentiation of the endogenous orthosteric ligand, acetylcholine, at the M1 muscarinic receptors found in higher concentrations in the cortex and hippocampus. Adverse effects, due to indiscriminate activation of other muscarinic receptor subtypes, are common. M1 muscarinic positive allosteric modulators/allosteric agonists such as BQCA offer an attractive solution, being exquisitely M1-selective over other muscarinic subtypes. A common difficulty with allosteric ligands is interpreting SAR, based on composite potency values derived in the presence of fixed concentration of agonist. In reality these values encompass multiple pharmacological parameters, each potentially and differentially sensitive to structural modification of the ligand. We report novel BQCA analogues which appear to augment ligand affinity for the receptor (pK(B)), intrinsic efficacy (τB), and both binding (α) and functional (β) cooperativity with acetylcholine. Ultimately, development of such enriched SAR surrounding allosteric modulators will provide insight into their mode of action.

The muscarinic M1 receptor positive allosteric modulator PQCA improves cognitive measures in rat, cynomolgus macaque, and rhesus macaque

RATIONALE

The current standards of care for Alzheimer's disease, acetylcholinesterase inhibitors, have limited efficacy due to a host of mechanism-related side effects arising from indiscriminate activation of muscarinic and nicotinic receptors. The M1 muscarinic receptor is predominantly expressed in the brain in regions involved in cognition, and therefore selective activation of the M1 receptor would be expected to boost cognitive performance with reduced risk of peripheral side effects.

OBJECTIVES

Here we investigated whether the selective M1 muscarinic receptor positive allosteric modulator, PQCA, improves cognitive performance and cerebral blood flow.

RESULTS

PQCA attenuated a scopolamine-induced deficit in novel object recognition in rat, self-ordered spatial search in cynomolgus macaque, and the object retrieval detour task in rhesus macaque. Beneficial effects in each of these assays and species were observed at similar plasma drug concentrations. Furthermore, at similar drug concentrations that were effective in the behavioral studies, PQCA increased blood flow in the frontal cortex of mice, providing a translational biomarker that could be used to guide dose selection for clinical studies.

CONCLUSIONS

These findings provide a framework for appropriately testing an M1 selective compound in patients with Alzheimer's disease.

Further exploration of M₁ allosteric agonists: subtle structural changes abolish M₁ allosteric agonism and result in pan-mAChR orthosteric antagonism

This letter describes the further exploration of two series of M(1) allosteric agonists, TBPB and VU0357017, previously reported from our lab. Within the TPBP scaffold, either electronic or steric perturbations to the central piperidine ring led to a loss of selective M(1) allosteric agonism and afforded pan-mAChR antagonism, which was demonstrated to be mediated via the orthosteric site. Additional SAR around a related M(1) allosteric agonist family (VU0357017) identified similar, subtle 'molecular switches' that modulated modes of pharmacology from allosteric agonism to pan-mAChR orthosteric antagonism. Therefore, all of these ligands are best classified as bi-topic ligands that possess high affinity binding at an allosteric site to engender selective M(1) activation, but all bind, at higher concentrations, to the orthosteric ACh site, leading to non-selective orthosteric site binding and mAChR antagonism.

Identification of amides as carboxylic Acid surrogates for quinolizidinone-based m1 positive allosteric modulators

Selective activation of the M1 muscarinic receptor via positive allosteric modulation represents an approach to treat the cognitive decline in patients with Alzheimer's disease. A series of amides were examined as a replacement for the carboxylic acid moiety in a class of quinolizidinone carboxylic acid M1 muscarinic receptor positive allosteric modulators, and leading pyran 4o and cyclohexane 5c were found to possess good potency and in vivo efficacy.

Quinolizidinone carboxylic acids as CNS penetrant, selective m1 allosteric muscarinic receptor modulators

Positive allosteric modulation of the M1 muscarinic receptor represents an approach to treat the cognitive decline in patients with Alzheimer's disease. Replacement of a quinolone ring system in a quinolone carboxylic acid series of M1 modulators with a quinolizidinone bearing a basic amine linkage led to a series of compounds with higher free fraction, enhanced CNS exposure, and improved efficacy in rodent in vivo models of cognition.

Allosteric activators of muscarinic receptors as novel approaches for treatment of CNS disorders

Muscarinic acetylcholine receptors (mAChRs) represent exciting therapeutic targets for the treatment of multiple CNS disorders. The high degree of conservation of amino acids comprising the orthosteric acetylcholine (ACh) binding site between individual mAChR subtypes has hindered the development of subtype-selective compounds that bind to this site. As a result, many academic and industry researchers are now focusing on developing allosteric activators of mAChRs including both positive allosteric modulators (PAMs) and allosteric agonists. In the past 10 years major advances have been achieved in the discovery of allosteric ligands that possess much greater selectivity for individual mAChR subtypes when compared to previously developed orthosteric agents. These novel allosteric modulators of mAChRs may provide therapeutic potential for treatment of a number of CNS disorders such as Alzheimer's disease and schizophrenia.

N-heterocyclic derived M1 positive allosteric modulators

Replacement of a phenyl ring with N-linked heterocycles in a series of quinolone carboxylic acid M1 positive allosteric modulators was investigated. In particular, a pyrazole derivative exhibited improvements in potency, free fraction, and CNS exposure.

Parallel synthesis of N-biaryl quinolone carboxylic acids as selective M(1) positive allosteric modulators

An iterative analog library synthesis approach was employed in the exploration of a quinolone carboxylic acid series of selective M(1) positive allosteric modulators, and strategies for improving potency and plasma free fraction were identified.