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This letter describes progress towards an M 4 PAM preclinical candidate that resulted in the discovery of VU6005806/AZN-00016130. While the thieno[2,3-c]pyridazine core has been a consistent feature of key M 4 PAMs, no work had previously been reported with respect to alternate functionality at the C3 position of the pyridazine ring. Here, we detail new chemistry and analogs that explored this region, and quickly led to VU6005806/AZN-00016130, which was profiled as a putative candidate. While, the β-amino carboxamide moiety engendered solubility limited absorption in higher species precluding advancement (or requiring extensive pharmaceutical sciences formulation), VU6005806/AZN-00016130 represents a new, high quality preclinical in vivo probe., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

Both historical clinical and recent preclinical data suggest that the M 1 muscarinic acetylcholine receptor is an exciting target for the treatment of Alzheimer's disease and the cognitive and negative symptom clusters in schizophrenia; however, early drug discovery efforts targeting the orthosteric binding site have failed to afford selective M 1 activation. Efforts then shifted to focus on selective activation of M 1 via either allosteric agonists or positive allosteric modulators (PAMs). While M 1 PAMs have robust efficacy in rodent models, some chemotypes can induce cholinergic adverse effects (AEs) that could limit their clinical utility. Here, we report studies aimed at understanding the subtle structural and pharmacological nuances that differentiate efficacy from adverse effect liability within an indole-based series of M 1 ago-PAMs. Our data demonstrate that closely related M 1 PAMs can display striking differences in their in vivo activities, especially their propensities to induce adverse effects. We report the discovery of a novel PAM in this series that is devoid of observable adverse effect liability. Interestingly, the molecular pharmacology profile of this novel PAM is similar to that of a representative M 1 PAM that induces severe AEs. For instance, both compounds are potent ago-PAMs that demonstrate significant interaction with the orthosteric site (either bitopic or negative cooperativity). However, there are subtle differences in efficacies of the compounds at potentiating M 1 responses, agonist potencies, and abilities to induce receptor internalization. While these differences may contribute to the differential in vivo profiles of these compounds, the in vitro differences are relatively subtle and highlight the complexities of allosteric modulators and the need to focus on in vivo phenotypic screening to identify safe and effective M 1 PAMs.

Graphical abstract Highlights • First description of the role of the β-aminocarboxamide moiety within M 4 PAM scaffolds. • Novel des -amino and tricyclic variant M 4 PAMs. • The role of the IMHB motif was established, and could be re-enforced by tricycles. Abstract This letter describes a focused exercise to explore the role of the β-amino carboxamide moiety found in all of the first generation M 4 PAMs and question if the NH 2 group served solely to stabilize an intramolecular hydrogen bond (IMHB) and enforce planarity. To address this issue (and to potentially find a substitute for the β-amino carboxamide that engendered P-gp and contributed to solubility liabilities), we removed the NH 2 , generating des -amino congeners and surveyed other functional groups in the β-position. These modifications led to weak M 4 PAMs with poor DMPK properties. Cyclization of the β-amino carboxamide moiety by virtue of a pyrazole ring re-enforced the IMHB, led to potent (and patented) M 4 PAMs, many as potent as the classical bicyclic β-amino carboxamide analogs, but with significant CYP1A2 inhibition. Overall, this exercise indicated that the β-amino carboxamide moiety most likely facilitates an IMHB, and is essential for M 4 PAM activity within classical bicyclic M 4 PAM scaffolds. [ABSTRACT FROM AUTHOR]

Herein we describe the continued optimization of M 4 positive allosteric modulators (PAMs) within the 5-amino-thieno[2,3- c ]pyridazine series of compounds. In this letter, we disclose our studies on tertiary amides derived from substituted azetidines. This series provided excellent CNS penetration, which had been challenging to consistently achieve in other amide series. Efforts to mitigate high clearance, aided by metabolic softspot analysis, were unsuccessful and precluded this series from further consideration as a preclinical candidate. In the course of this study, we found that potassium tetrafluoroborate salts could be engaged in a tosyl hydrazone reductive cross coupling reaction, a previously unreported transformation, which expands the synthetic utility of the methodology. [ABSTRACT FROM AUTHOR]

This Letter details the discovery and subsequent optimization of a novel M 4 PAM scaffold based on an 6-fluoro-4-(piperidin-1-yl)quinoline-3-carbonitrile core, which represents a distinct departure from the classical M 4 PAM chemotypes. Optimized compounds in this series demonstrated improved M 4 PAM potency on both human and rat M 4 (4 to 5-fold relative to HTS hit), and displayed attractive physicochemical and DMPK profiles, including good CNS penetration (rat brain:plasma K p = 5.3, K p,uu = 2.4; MDCK-MDR1 (P-gp) ER = 1.1). [ABSTRACT FROM AUTHOR]

This letter describes the chemical optimization of a novel series of M 4 positive allosteric modulators (PAMs) based on a 5-amino-thieno[2,3- c ]pyridazine core, developed via iterative parallel synthesis, and culminating in the highly utilized rodent in vivo tool compound, VU0467154 ( 5 ). This is the first report of the optimization campaign (SAR and DMPK profiling) that led to the discovery of VU0467154, and details all of the challenges faced in allosteric modulator programs (steep SAR, species differences in PAM pharmacology and subtle structural changes affecting CNS penetration). [ABSTRACT FROM AUTHOR]

This Letter details our efforts to replace the 3-amino moiety, an essential pharmacophore for M 4 PAM activity in most M 4 PAMs to date, within the thieno[2,3- b ]pyridine core, as the β-amino carboxamide motif has been shown to engender poor solubility, varying degrees of P-gp efflux and represents a structural alert. A scaffold hopping exercise identified a novel 2,4-dimethylquinoline carboxamide core that provided M 4 PAM activity and good CNS penetration without an amino moiety. In addition, MacMillan photoredox catalysis chemistry was essential for construction of the 2,4-dimethylquinoline core. [ABSTRACT FROM AUTHOR]

Preclinical evidence in support of the potential utility of mGlu5 NAMs for the treatment of a variety of psychiatric and neurodegenerative disorders is extensive, and multiple such molecules have entered clinical trials. Despite some promising results from clinical studies, no small molecule mGlu5 NAM has yet to reach market. Here we present the discovery and evaluation of N-(5-fluoropyridin-2-yl)-6-methyl-4-(pyrimidin-5-yloxy)picolinamide (27, VU0424238), a compound selected for clinical evaluation. Compound 27 is more than 900-fold selective for mGlu5 versus the other mGlu receptors, and binding studies established a Ki value of 4.4 nM at a known allosteric binding site. Compound 27 had a clearance of 19.3 and 15.5 mL/min/kg in rats and cynomolgus monkeys, respectively. Imaging studies using a known mGlu5 PET ligand demonstrated 50% receptor occupancy at an oral dose of 0.8 mg/kg in rats and an intravenous dose of 0.06 mg/kg in baboons. [ABSTRACT FROM AUTHOR]