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Structure-based ligand design toward a selective M3R antagonist. (A) Spatial orientation of QNB ring B and the nonconserved Phe181 in the second extracellular loop (ECL2) of M2R. (B) Structural modifications 1, 2, and 3 confer an up-righting and rotation of ring B, as well as steric interactions with Phe181 for compound 6b (105-fold selectivity for M3 over M2).
