The marketed long-acting M3 antagonists for treatment of chronic obstructive pulmonary disease have inappropriate plasma stability (either overstable or excessively unstable), which causes substantial systemic exposure or poor patient compliance. To discover novel M3 antagonists with proper plasma stability, we synthesized and biologically evaluated a series of chiral quaternary ammonium salts of pyrrolidinol esters, which were designed by structural recombination of the marketed M3 antagonists. As a result, two novel potent M3 antagonists, (R/S)-3-[2-hydroxy-2,2-di(thiophen-2-yl)acetoxy]-1,1-dimethylpyrrolidinium bromides (1 a: Ki =0.16 nm, IC50 =0.38 nm, t1/2 =9.34 min; 1 b: Ki =0.32 nm, IC50 =1.01 nm, t1/2 =19.2 min) with proper plasma stability were identified, which (particularly 1 a) hold great promise as clinical drug candidates to overcome the drawbacks caused by the inappropriate stability of the currently marketed M3 antagonists. In addition, structure-activity relationship studies revealed that the R configuration of the pyrrolidinyl C3 atom was clearly better than the S configuration.
Keywords: ammonium salts; antagonists; nitrogen heterocycles; stability; structure-activity relationships.
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