Synthesis of Aminoacyl-2-Cyanopyrrolidine Compounds and Their Inhibitory Effect on the DPP-4 Enzyme

Diabetes mellitus is a major global health concern. DPP-4 (dipeptidyl peptidase-4) inhibitors containing a 2-cyanopyrrolidine scaffold are in clinical use; however, systematic evaluation of amides derived from amino acids adjacent to the N-2 position of GLP-1 (glucagon-like peptide-1) has not been reported.

A total of eight 2-cyanopyrrolidine derivatives, grouped into three series, were synthesized from N-Boc-protected amino acids via condensation with 2-cyanopyrrolidine followed by deprotection. The compounds were characterized by nuclear magnetic resonance (NMR), elemental analysis, and mass spectrometry (MS). DPP-4 inhibitory activity was evaluated in vitro using a DPP-IV Glo™ assay, with sitagliptin as a control. IC₅₀ (Half Maximal Inhibitory Concentration) values were determined from three independent replicates (n = 3) and calculated by nonlinear regression.

Compounds 4d, 4g, and 4h exhibited excellent DPP-4 inhibition, with IC₅₀ values of 1.36 ± 0.09, 5.24 ± 0.31, and 4.83 ± 0.22 nM, respectively. Compound 4d (histidine) was more potent than sitagliptin (6.13 ± 0.42 nM). The bis-cyanopyrrolidine derivatives 4g and 4h also showed strong activity. The remaining compounds (4a-4c, 4e, 4f) displayed moderate inhibition (IC₅₀ 112-181 nM).

Aminoacyl-2-cyanopyrrolidine derivatives bearing an imidazole or bis-cyanopyrrolidine structure are potent DPP-4 inhibitors. These findings highlight the importance of neighboring GLP-1 amino acid residues (positions 1 and 3) in inhibitor design. Further cellular and in vivo studies are warranted to evaluate the therapeutic potential of these compounds.