Dipeptide-based inhibitors with C-substituted (alkyl or aminoalkyl) alpha-amino acids in the P2 position and born-norleucine (boro-Nle) in the P1 position were synthesized. Relative to born-proline, boro-Nle as a P1 residue was shown able to significantly dial out DPP4, FAP, DPP8, and DPP9 activity. Dab-boro-Nle (4g) proved to be the most selective and potent DPP7 inhibitor with a DPP7 IC50 value of 480 pM. (c) 2005 Elsevier Ltd. All rights reserved.