Colorectal cancer is the third most common malignancy and the fourth leading cause of death globally. The primary treatments for this cancer are surgery and radiotherapy that is used to prevent recurrence of the disease. However, local invasions and metastases are leading causes of failure of the colon cancer treatment. This is related to a complex effect of radiation on the tumor cells and tissue and the activation of signaling pathways that lead to radioresistance. Therefore, it is possible to overcome radioresistance of the tumor by using factors that cause radiosensitivity and improve the output of radiotherapy. Today, herbal remedies are used to prevent and even treat cancer. One of these herbal remedies is galbanic acid (GBA). Due to the anti-tumor properties of GBA on various cancer cell lines, it is expected that GBA causes radiosensitivity in HT-29 cells. We have used the Alamar Blue test to evaluate the toxic effects of GBA on HT-29 cells. The cells were cultured in 96-well plates and treated with various concentrations (5, 10, 20, and 40 g/mL GBA for 24, 48, and 72 h. In addition, the clonogenic assay was used to evaluate the effect of GBA on the radiation sensitivity of HT-29 cells. In this test, the cells were treated with only one drug concentration (10 g/mL) at radiation doses of 2, 4, and 6 Gy. The cells were seeded in a 6-well culture plate, and after eight days, the colonies were formed. These colonies were then fixed with methanol and stained with Giemsa stain. Based on these findings, it is established that GBA inhibits the growth of HT-29 cells. Moreover, it was found that the survival rate of cells decreases with increasing concentration of medication. However, the effect did not depend on the treatment duration. The IC50 values for 24, 48, and 72 h exposures were 17.13, 23.58, and 19.49 g/mL. According to the clonogenic assay results, there was no significant difference between the survival fraction of cells treated with radiation and combined therapy (radiation and medicine). Despite the observed toxicity of GBA to HT-29 cells, it did not influence the radiation sensitivity.

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