... Tetrahedron Letters. Volume 50, Issue 2, 14 January 2009, Pages 212–215. ... 23 μM) delivered two analogs, 22 and 24, with over a 6-fold increase in M1 inhibitory activity (M1 IC 50 = 3.99 μM and 6.64 μM, respectively) while maintaining selectivity versus M2–M5 (IC 50 >> 50 μM ...