4.3. Cell Viability Assay

To test chemotherapeutic response, cells were plated in 96-well plates at 3000 cells per well. After 24 h, cells were treated with increasing concentrations of oxaliplatin, 5-fluorouracil or both components, and plates were further incubated for another 72 h before viability was measured. For the radiation experiments, cell viability was measured 144 h post-irradiation. When required, cells were treated with 10 µM of the ATR inhibitor VE-821 for 24 h before adding a combination of 5-fluorouracil and oxaliplatin (FOLFOX) or ionizing radiation. Viability was measured using the CellTiter96 AQueous One Solution Cell Proliferation Assay, also known as MTS (Promega, Madison, WI, USA). Absorbance was measured using the Epoch microplate reader (BioTek Instruments, Winooski, VT, USA). Each cellular viability was normalized to its corresponding non-treated counterpart. The MTT Cell Proliferation assay (Sigma-Aldrich) was used to assess viability after treatment with irinotecan, gemcitabine, and paclitaxel drugs. Absorbance was measured by SpectraMax M2 and analyzed using the software SoftMax Pro (Molecular Devices, Sunnyvale, CA, USA).