**Olesya S. Malyarenko \*,**†**, Tatiana I. Imbs** † **and Svetlana P. Ermakova**

G.B. Elyakov Pacific Institute of Bioorganic Chemistry, Far-Eastern Branch of the Russian Academy of Sciences, 159, 100-let Vladivostoku Prospect, 690022 Vladivostok, Russia; tatyanaimbs@mail.ru (T.I.I.); ermakova@piboc.dvo.ru (S.P.E.)

**\*** Correspondence: malyarenko.os@gmail.com; Tel.: +7-423-231-07-05; Fax: +7-423-231-40-50

† These authors contributed equally to this work.

Academic Editors: Cédric Delattre and Luisa Tesoriere Received: 18 June 2020; Accepted: 12 July 2020; Published: 14 July 2020

**Abstract:** The anticancer and radiosensitizing effects of high-molecular-weight phlorethols **CcPh** (Mw = 2520 Da) isolated from the brown algae of *Costaria costata* on human colorectal carcinoma HCT 116 and HT-29 cells were investigated. Phlorethols **CcPh** possessed cytotoxic activity against HT-29 (IC50 = 92 μg/mL) and HCT 116 (IC50 = 94 μg/mL) cells. **CcPh** at non-toxic concentrations inhibited the colony formation in colon cancer cells and significantly enhanced their sensitivity to low non-toxic X-ray irradiation. The combinatory effect of radiation and **CcPh** was synergistic (Combination index < 0.7). Algal phlorethols might be prospective candidates as radiosensitizers to improve the scheme of radiotherapy.

**Keywords:** phlorotannins; phlorethols; anticancer activity; colorectal cancer; radiosensitizer; radiotherapy
