Design and Synthesis of γ- and δ-Lactam M Positive Allosteric Modulators (PAMs): Convulsion and Cholinergic Toxicity of an M-Selective PAM with Weak Agonist Activity.

Recent data demonstrated that activation of the muscarinic M receptor by a subtype-selective positive allosteric modulator (PAM) contributes to the gastrointestinal (GI) and cardiovascular (CV) cholinergic adverse events (AEs) previously attributed to M and M activation. These studies were conducted using PAMs that also exhibited allosteric agonist activity, leaving open the possibility that direct activation by allosteric agonism, rather than allosteric modulation, could be responsible for the adverse effects. This article describes the design and synthesis of lactam-derived M PAMs that address this hypothesis. The lead molecule from this series, compound 1 (PF-06827443), is a potent, low-clearance, orally bioavailable, and CNS-penetrant M-selective PAM with minimal agonist activity. Compound 1 was tested in dose escalation studies in rats and dogs and was found to induce cholinergic AEs and convulsion at therapeutic indices similar to previous compounds with more agonist activity. These findings provide preliminary evidence that positive allosteric modulation of M is sufficient to elicit cholinergic AEs.