The five subtypes of the muscarinic acetylcholine receptor (mAChR [1][2][3][4][5] or M 1-5 ) are differentially expressed G protein-coupled receptors (GPCRs) important to a variety of physiological functions, including attention, learning and memory, pain, sleep, movement, gastrointestinal motility and cardiovascular regulation, among others. [1][2][3][4][5] Based on mounting data, M 1 and M 4 receptors are considered potential therapeutic targets for numerous CNS diseases and disorders such as Alzheimer's disease and schizophrenia. [6][7][8][9] However, due to high sequence conservation of the orthosteric binding site across subtypes, discovery of truly subtype-selective compounds has proven historically challenging. Indeed, M 2 -and M 3 -related side effects (e.g. GI disturbance, salivation, lacrimation and bradycardia) have contributed to failure in the clinical development of muscarinic agonists despite promising therapeutic efficacy. [6,7] Furthermore, deep biological insight into the specific roles of the mAChRs in both basic neurobiology and CNS pathologies has been hindered by the paucity of selective tools. The additional drug metabolism/pharmacokinetic (DMPK)-related challenges inherent to CNS drug discovery have also hampered progress in this area. Despite these hurdles, a number of novel subtype-selective and centrally penetrating muscarinic compounds, including agonists, antagonists, and potentiators, have recently emerged from functional cell-based screening approaches. [10][11][12][13][14][15] We previously reported the identification of a series of novel M 4 -selective potentiators (positive allosteric modulators or PAMs) that enhance receptor activation in response to acetylcholine (ACh) by an allosteric mechanism (Figure 1). [10,12] These compounds increase the potency of ACh at M4 but lack intrinsic agonist activity on their own. Initial optimization focused on improving the physiochemical properties of lead compound VU10010 (1), which possessed an EC 50 value of 400 nM and elicited a 47-fold leftward shift of an ACh concentration-response curve (CRC) by Ca 2+ mobilization assay in rat M 4 /G qi5 -expressing cells, but suffered from solubility issues and lack of brain penetration. [10,12] This limited effort produced two analogues, VU0152099 (2) and VU0152100 (3), which had similar potency and comparable efficacy to the parent compound i but were centrally penetrating and displayed in vivo activity in a rodent behavioral model predictive of antipsychotic efficacy.[10] These compounds were also devoid of ancillary pharmacological activity when evaluated against a large number of GPCRs, ion channels, and enzymes.[10] Despite the utility of compounds 2 and 3 for in vitro and in vivo pharmacological studies, we sought to further explore the SAR in this series with a more exhaustive optimization campaign by employing an iterative analogue library approach. The rationale for this effort stemmed in part from an initial limited lead optimization campaign, poor metabolic stability of compounds 2 and 3...