Kinase experiment:

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 experiment:

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℃ for 30 min, and the DNA is solubilized in 0.5mol/LNaOH for 30 min at 37℃. Finally, [3H]thymidine incorporation is determined.

Animal experiment:

Each experimental group (vehicle or linagliptin treatment) consists of 10 individual ob/ob mice (n=10). Animals are treated orally once a day (8:00 AM) by gastrogavage using vehicle (1% methylcellulose) or linagliptin (3 mg/kg body weight in 1% methylcellulose) beginning 2 days (day–2) before wounding. After wounding, animals are subsequently treated once a day throughout the 10-day healing period.

BI1356 is a potent and competitive DPP-4 inhibitor, which exhibited DPP-4 inhibiting activity in several independent tests with IC50 values of 0.4, 0.5, 0.9, and 1.1nM.[1]
DPP-4 is an N-terminal dipeptidyl exopeptidase existing as a membrane-bound protein and also as a soluble protein in plasma. It plays a major role in the degradation of incretins such as GLP-1 which is of great importance in the process of glucose metabolism. Under physiological conditions, GLP-1 is truncated by DPP-4 rapidly, which is located on the capillary endothelium proximal to the L-cells where GLP-1 is secreted in the ileum. GLP-1 could sensitize β-cells to glucose stimulation,consequently increasing intracellular cAMP concentrations in β-cells and accelerating and augmenting insulin response to absorb glucose. By being measured at the substrate at the binding site, BI1356 inhibits the DPP-4 enzyme.[1]
DPP-4 was extracted from confluent Caco-2 cells to determine the inhibition activity of BI1356. Assays were performed by mixing the inhibitor solution with substrates and the Caco-2 cell extract, which illustrated BI1356 inhibited DPP-4 with IC 50 values of 0.4, 0.5, 0.9, and 1.1 nM. BI1356 also possesses a very significant selectivity for DPP-4 relative to other dipeptidyl peptidases aminopeptidases N and P, prolyloligopeptidase, and the proteases trypsin, plasmin, and thrombin.[1]
An in vivo evaluation showed that BI1356 dose-dependently inhibited the DPP-4 enzyme in plasma within 30 min of administration. Separate doses ranging from 1 to 10 mg/kg achieved significant inhibition activity of DPP-4, which also showed persistent DPP-4 inhibition activity. The ED50 value for inhibition of plasma DPP-4 activity was calculated to be 0.9 mg/kg 24 h post dose. In a clinical study,  BI1356 produced a remarkable, clinically meaningful and persistent improvement in glycaemic control, in accordance with enhanced parameters of β-cell function.  Patients treated with BI1356 were more likely to achieve a reduction in HbA1c of ≥ 0.5% comparing control.[1,2]
References:
1.Thomas L, Eckhardt M, Langkopf E, et al. (R)-8-(3-amino-piperidin-1-yl)-7-but-2-ynyl-3-methyl-1-(4-methyl-quinazolin-2-ylmethyl)-3, 7-dihydro-purine-2, 6-dione (BI 1356), a novel  xanthine-based dipeptidyl peptidase 4 inhibitor, has a superior potency and longer duration of action compared with other dipeptidyl peptidase-4 inhibitors[J]. Journal of Pharmacology and Experimental Therapeutics, 2008, 325(1): 175-182.
2.Del Prato S, Barnett A H, Huisman H, et al. Effect of linagliptin monotherapy on glycaemic control and markers of β‐cell function in patients with inadequately controlled type 2 diabetes: a randomized controlled trial[J]. Diabetes, Obesity and Metabolism, 2011, 13(3): 258-267.