Extensive SAR studies of the P3 capping group led to the discovery of a series of potent inhibitors with sultam and cyclic sulfonyl urea moieties as the P3 capping. The bicyclic thiophene-sultam or phenyl-sultam cappings were selected for further SAR development. Modification at the P3 side chain determined that the tert-butyl group was the best choice at that position. Optimization of P1 residue significantly improved potency and selectivity. ...