In vitro activity: Linagliptin shows a potent inhibition effect against DPP-4 in vitro and a low affinity for hERG channel and M1 receptor (IC50 295 nM). Linagliptin acts as a competitive inhibitor with a Ki of 1 nM, and also shows 10,000-fold more selectivity for DPP-4 than DPP-8, DPP-9, amino-peptidases N and P, prolyloligopeptidase, trypsin, plasmin, and thrombin, and 90-fold more selectivity than fibroblast activation protein in vitro.

Kinase Assay: EDTA plasma (20 μL) is diluted with 30 μL of DPP-4 assay buffer (100 mM Tris and 100 mM NaCl, adjusted to pH 7.8 with HCl) and mixed with 50 μL of H-Ala-Pro-7-amido-4-trifluoromethylcoumarin. The 200 mM stock solution in dimethylformamide is diluted 1:1000 with water to yield a final concentration of 100 μM. The plate is incubated at room temperature for 10 min, and fluorescence in the wells is determined by using a Victor 1420 Multilabel Counter at an excitation wavelength of 405 nm and an emission wavelength of 535 nm. For the detection of DPP-4 activity in wound lysates, 100 μg of protein from the respective wound lysates are used instead of 20 μL of plasma. Active GLP-1 is also detected from 100 μg of respective wound tissue samples and analyzed by using the Mouse/Rat Total Active GLP-1 Assay Kit.

Cell Assay: A total of 4.0×107 keratinocytes per well are seeded into 24-well plates. After reaching 50% confluence, cells are starved for 24 h with DMEM. Proliferation of cells is assessed by using 1 μCi/mL of [3H]methyl-thymidine in DMEM in the presence of 10% fetal bovine serum and increasing concentrations of linagliptin (3, 30, 300, or 600 nM) for 24 h. Cells are then washed twice with phosphate-buffered saline and incubated in 5% trichloroacetic acid at 4°C for 30 min, and the DNA is solubilized in 0.5mol/LNaOH for 30 min at 37°C. Finally, [3H]thymidine incorporation is determined.