Structural modifications of the aminopyridine P1′ group of imidazole acetic acid based TAFIa inhibitors led to the discovery of the aminocyclopentyl analog 28, a 1nM TAFIa inhibitor with CLT50 functional activity of 14nM but without selectivity against CPB. While not as active, aminobutyl derivative 27 provided an improved 6.7-fold selectivity for TAFIa versus CPB.