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

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 M-5 PAMs (M-5 EC50 >10 mu M) 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 M-5 PAM, ML326 (VU0467903), (human and rat M-5 EC(50)s of 409 nM and 500 nM, respectively) with excellent mAChR selectivity (M-1-M-4 EC(50)s >30 mu M) and a robust 20-fold leftward shift of the ACh CRC. (C) 2013 Elsevier Ltd. All rights reserved.