Structural modifications of the aminopyridine P(1)(') group of imidazole acetic acid based TAFIa inhibitors led to the discovery of the aminocyclopentyl analog 28, a 1 nM TAFIa inhibitor with CLT(50) functional activity of 14 nM but without selectivity against CPB. While not as active, aminobutyl derivative 27 provided an improved 6.7-fold selectivity for TAFIa versus CPB.