Cytotoxicity assay: cell viability test

To assess cell viability, a colorimetric assay based on the MTS-PMS assay (Promega Corp., Madison, WI, USA) was adopted. CHO-K1 cells maintained in DMEM with 10 % FBS were seeded in 96-well plates (10⁴ cells/well, 100 μL per well) and maintained for 24 h at 37 °C with 5 % CO₂. Deionized water (5 μL/mL in culture medium) was used as the vehicle control (VC). After that, the cells were incubated with different concentrations of DOTATATE (0.5, 0.9, 1.8, 3.6 and 7.3 ng/mL) and UBI_29-41 (0.3, 0.6, 1.1, 2.3 and 4.5 ng/mL) for 24 h. The cell density was determined by adding 20 μL/well of MTS (2 mg/mL) (3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium and PMS (0.92 mg/mL) (phenazinemethosulfate) in a 20:1 ratio (v/v). After 2 h of incubation, the absorbance values were obtained by the spectrophotometric reading at 490 nm. Each concentration of the compound was tested in quadruplicate in three independent assays. Results were expressed as the percentage of viable cells, with 100 % referring to control cells. The test compound was not considered to be cytotoxic if the cell viability value was at least 90 % of the untreated or negative controls.