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• First disclosure of VU6005806/AZN-00016130, an M 4 PAM profiled as a putative candidate. • Novel SAR findings based on novel variations to the C3 position on the pyridazine core. • Minimum effective dose of 0.56 mg/kg po in rat AHL. 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. [ABSTRACT FROM AUTHOR]

This letter describes progress towards an M 4 PAM preclinical candidate inspired by an unexpected aldehyde oxidase (AO) metabolite of a novel, CNS penetrant thieno[2,3- c ]pyridine core to an equipotent, non-CNS penetrant thieno[2,3- c ]pyrdin-7(6 H)-one core. Medicinal chemistry design efforts yielded two novel tricyclic cores that enhanced M 4 PAM potency, regained CNS penetration, displayed favorable DMPK properties and afforded robust in vivo efficacy in reversing amphetamine-induced hyperlocomotion in rats. [ABSTRACT FROM AUTHOR]

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]

[Display omitted] This Letter details our efforts to develop novel tricyclic M 4 PAM scaffolds with improved pharmacological properties. This endeavor involved a "tie-back" strategy to replace the 3-amino-4,6-dimethylthieno[2,3- b ]pyridine-2-carboxamide core which lead to the discovery of two novel tricyclic cores: a 7,9-dimethylpyrido[3′,2′:4,5]thieno[3,2-d]pyrimidine core and 2,4-dimethylthieno[2,3- b :5,4-c′]dipyridine core. Both tricyclic cores displayed low nanomolar potency against the human M 4 receptor. [ABSTRACT FROM AUTHOR]

• Discovery of novel 2,3-dimethylimidazo[1,2- a ]pyrazine-6-carboxamide-based M 4 PAMs. • Broad survey of diverse heterocyclic cores. • Balanced human and rat M 4 PAM potency with excellent CNS penetration. This Letter details our efforts to develop new M 4 PAM scaffolds with improved pharmacological properties. This endeavor involved replacing the 3,4-dimethylpyridazine core with two novel cores: a 2,3-dimethyl-2 H -indazole-5-carboxamide core or a 1-methyl-1 H -benzo[d][1,2,3]triazole-6-carboxamide core. Due to shallow SAR, these cores were further evolved into two unique tricyclic cores: an 8,9-dimethyl-8H-pyrazolo[3,4- h ]quinazoline core and an 1-methyl-1 H -[1,2,3]triazolo[4,5- h ]quinazoline core. Both tricyclic cores displayed low nanomolar potency against both human and rat M 4. [ABSTRACT FROM AUTHOR]

• Discovery of two novel tricyclic-based M 4 PAMs. • Utility of scaffold hopping to improve potency/properties/DMPK. • Balanced human and rat M 4 PAM potency. • Rare 8,9-dimethyl-8 H -pyrazolo[3,4- h ]quinazoline and 1-methyl-1 H -[1,2,3]triazolo[4,5- h ]quinazoline cores. This Letter details our efforts to discover structurally unique M 4 PAMs containing 5,6-heteroaryl ring systems. In an attempt to improve the DMPK profiles of the 2,3-dimethyl-2H-indazole-5-carboxamide and 1-methyl-1 H -benzo[ d ][1,2,3]triazole-6-carboxamide cores, we investigated a plethora of core replacements. This exercise identified a novel 2,3-dimethylimidazo[1,2- a ]pyrazine-6-carboxamide core that provided improved M 4 PAM activity and CNS penetration. [ABSTRACT FROM AUTHOR]

• Novel 2.4-dimehtylcinnoline-based M 4 PAMs. • Utility of scaffold hopping to improve properties/DMPK. • Balanced human and rat M 4 PAM potency. This Letter details our efforts to replace the 2,4-dimethylquinoline carboxamide core of our previous M 4 PAM series, which suffered from high predicted hepatic clearance and protein binding. A scaffold hopping exercise identified a novel 3,4-dimethylcinnoline carboxamide core that provided good M 4 PAM activity and improved clearance and protein binding profiles. [ABSTRACT FROM AUTHOR]