This letter describes the continued optimization of M5 NAM ML375 (VU0483253). While a valuable in vivo tool compound, ML375 has an excessively long elimination half-life in rat (t1/2 = 80 hours), which can be problematic in certain rodent addiction paradigms (e.g., reinstatement). Thus, we required an M5 NAM of comparable potency to ML375, but with a rat t1/2 of less than 4 hours. Steep SAR plagued this chemotype, and here we detail aniline replacements that offered some improvements over ML375, but failed to advance. Ultimately, incorporation of a single methyl group to the 9b-phenyl ring acted as a metabolic shunt, providing (S)-11 (VU6008667), an equipotent M5 NAM, with high CNS penetration, excellent selectivity versus M1–4 and the desired short half-life (t1/2 = 2.3 hours) in rat.

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 to 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.

This letter describes a ligand-based virtual screening campaign utilizing SAR data around the M5 NAMs, ML375 and VU6000181. Both QSAR and Shape scores were employed to virtually screen a 98,000-member compound library. Neither approach alone proved productive, but a consensus score of the two models identified a novel scaffold which proved to be a modestly selective, but weak inhibitor (VU0549108) of the M5 mAChR (M5 IC50 = 6.2 μM, M1-4 IC50s >10 μM) based on an unusual 8-((1,3,5-trimethyl-1H-pyrazol-4-yl)sulfonyl)-1-oxa-4-thia-8-azaspiro[4,5]decane scaffold. [3H]-NMS binding studies showed that VU0549108 interacts with the orthosteric site (Ki of 2.7 μM), but it is not clear if this is negative cooperativity or orthosteric binding. Interestingly, analogs synthesized around VU0549108 proved weak, and SAR was very steep. However, this campaign validated the approach and warranted further expansion to identify additional novel chemotypes.