Discovery of ML326: The first sub-micromolar, selective M5 PAM

Article abstract of DOI:10.1016/j.bmcl.2013.03.032

This Letter describes the further chemical optimization of the M ₅ PAM MLPCN probes ML129 and ML172. A multi-dimensional iterative parallel synthesis effort quickly explored isatin replacements and a number of southern heterobiaryl variations with no improvement over ML129 and ML172. An HTS campaign identified several weak M₅ PAMs (M₅ EC ₅₀ >10 μM) with a structurally related isatin core that possessed a southern phenethyl ether linkage. While SAR within the HTS series was very shallow and unable to be optimized, grafting the phenethyl ether linkage onto the ML129/ML172 cores led to the first sub-micromolar M₅ PAM, ML326 (VU0467903), (human and rat M₅ EC₅₀s of 409 nM and 500 nM, respectively) with excellent mAChR selectivity (M ₁-M₄ EC₅₀s >30 μM) and a robust 20-fold leftward shift of the ACh CRC.