**1. Introduction**

Radiation therapy is traditionally considered one of the most effective methods for influencing tumors [1]. The standard methods of radiation therapy are limited because any attempt to increase the total irradiation dose is associated with a high risk of severe post-radiation damage [2]. This necessitates new approaches to realize the main objective of radiation therapy, which is to increase the radiosensitivity of cancer cells. An increase in the effect of ionizing radiation can be achieved using radiosensitizers even without increasing the total irradiation dose [3,4].

Brown algae are an attractive source of biologically active compounds for researchers. They serve as raw materials for various therapeutic and prophylactic preparations, since they contain a number of bioactive substances, such as polysaccharides, mannitol, vitamins, macro- and microelements, polyphenols, iodine-containing organic compounds, and polyunsaturated fatty acids

Phlorotannins are a type of phenolic metabolite found in brown algae. This group of phenolic compounds contains a large number of hydroxyl groups and chelates divalent metal ions; they have polymer structures and are highly soluble in water and firmly bound to proteins, polysaccharides, and other biopolymers. The monomer unit of phlorotannins is phloroglucinol (1,3,5-trihydroxybenzene). Based on the type of monomer bond, phlorotannins can be divided into four classes: fuhalols and phlorethols (ether bond), fucols (phenyl bond), fucophlorethols (ether and phenyl bonds), and eckols and carmalols (dibenzodioxin bond). Within each class, the binding of monomers to each other can occur at different positions of the phloroglucinol ring, which leads to the formation of structural isomers in addition to conformational ones. One species of algae is often known to produce phlorotannins with different structures and different degrees of polymerization [5].

We have previously shown that the aqueous ethanol extract of *Costaria costata* containing abundant phlorotannins possesses antitumor potential [6]. Moreover, phlorotannins from the same species of brown algae were found to be effective inhibitors of fucoidanase (enzyme hydrolyzed fucoidan) [7]. Since phlorotannins from brown algae are considered to have different types of biological activities and up to a certain concentration do not exert any toxic effect [8–10], they are of research interest as perspective bioactive agents for cancer therapy.

In the present work, we checked the hypothesis whether phlorotannins from *C. costata* inhibited cell viability and colony formation of human colorectal cancer cells as well as sensitized HT-29 and HCT 116 cells to X-ray radiation.
