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3-[(3aR,4R,5S,7aS)-5-{(1R)-1-[3,5-bis(trifluoromethyl)phenyl]ethoxy}-4-(4-fluorophenyl)octahydro-2H-isoindol-2-yl]cyclopent-2-en-1-one (17) is a high affinity, brain-penetrant, hydroisoindoline-based neurokinin-1 (NK(1)) receptor antagonist with a long central duration of action in preclinical species and a minimal drug-drug interaction profile. Positron emission tomography (PET) studies in rhesus showed that this compound provides 90% NK(1) receptor blockade in rhesus brain at a plasma level of 67 nM, which is about 10-fold more potent than aprepitant, an NK(1) antagonist marketed for the prevention of chemotherapy-induced and postoperative nausea and vomiting (CINV and PONV). The synthesis of this enantiomerically pure compound containing five stereocenters includes a Diels-Alder condensation, one chiral separation of the cyclohexanol intermediate, an ether formation using a trichloroacetimidate intermediate, and bis-alkylation to form the cyclic amine.

Herein we describe the discovery and optimization of a new series of 2,3‐disubstituted and 2,3,6‐trisubstituted muscarinic acetylcholine receptor 4 (M4) positive allosteric modulators (PAMs). Iterative libraries enabled rapid exploration of one‐dimensional structure–activity relationships (SAR) and identification of potency‐enhancing heterocycle and N‐alkyl pyrazole substituents. Further optimization led to identification of the potent, receptor‐subtype‐selective, brain‐penetrant tool compound 24 (7‐[3‐[1‐[(1‐fluorocyclopentyl)methyl]pyrazol‐4‐yl]‐6‐methyl‐2‐pyridyl]‐3‐methoxycinnoline). It is efficacious in preclinical assays that are predictive of antipsychotic effects, producing dose‐dependent reversal of amphetamine‐induced hyperlocomotion in rats and mice, but not in M4 knockout mice. Cholinergic‐related adverse effects observed in rats treated with 24 at unbound plasma concentrations more than 3‐fold higher than an efficacious dose in the hyperlocomotion assay were fewer and less severe than those observed in rats treated with the nonselective M4 agonist xanomeline, suggesting a receptor‐subtype‐selective PAM has the potential for an improved safety profile.

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