263 Journal of Environmental Pathology, Toxicology and Oncology, 34(3):263–275 (2015) 0731-8898/15/$35.00 © 2015 by Begell House, Inc. Radioprotective Effect of Carvacrol Against X-Radiation–Induced Cellular Damage in Cultured Human Peripheral Blood Lymphocytes Sivaranjani Arivalagan, 1 Nisha Susan Thomas, 1 Thayalan Kuppusamy, 2 & Nalini Namasivayam 1, * 1 Department of Biochemistry and Biotechnology, Faculty of Science, Annamalai University, Annamalai Nagar, Tamil Nadu, India; and 2 Medical Physics Division, Dr. Kamakshi Memorial Hospital, Pallikaranai, Chennai, Tamil Nadu, India *Address correspondence to: Nalini Namasivayam, Department of Biochemistry and Biotechnology, Faculty of Science, Annamalai University, Annamalai Nagar 608 002, Tamil Nadu, India; Tel.: 91-4144-238343; Fax: +91-4144-239141; nalininam@yahoo.com. ABSTRACT: In the present study, we evaluated the radioprotective effect of carvacrol (CVC) against X-radiation– induced cellular damage in cultured human blood lymphocytes. By MTT assay, the LD 50 doses of CVC and X-radiation to lymphocytes were determined to be 100 µg/ml and 4 Gy, respectively. To explore the radioprotective effect of CVC, the cultured lymphocytes were treated with 100 µg/mL of CVC 30 min prior to 4 Gy irradiation. Subsequently, the radiation-induced damage was screened by micronuclei (MN) and dicentric chromosome (DC) frequencies and comet assay. The percentage of cell death was evaluated by acridine orange/ethidium bromide (AO/EB) staining. The radiation-induced oxidative stress was estimated by assessing the changes in the levels of enzymatic antioxidants and lipid peroxidation markers. Compared with the sham control, we observed increases in MN and DC frequencies, comet attributes, % cell death, and lipid peroxidation with a concomitant decrease in the antioxidant status of the lymphocytes treated with radiation alone. Pre-treatment of lymphocytes with CVC (100 µg/mL) altered those changes mediated by radiation. These results clearly indicate that CVC may be an effective radioprotector against X-radiation. It has the ability to scavenge the free radicals produced and to protect cells from radiation-induced cell damage. KEY WORDS: carvacrol, X-radiation, lymphocytes, micronuclei, oxidative stress I. INTRODUCTION Radiation can be electromagnetic (e.g., X-rays or gamma rays) or corpuscular (e.g., electrons, photons, neutrons, alpha particles, or heavy ions). Ionizing radiation is considered a ubiquitous environmental carcinogen that damages DNA directly by energy deposition or indirectly by the generation of reactive oxygen species (ROS) and free radicals. 1,2 Although ionizing radiation has been found to produce delete- rious effects on the biological systems, this property of ionizing radiation has been ingeniously exploited to an advantage in the treatment of various neoplastic disorders in humans. 3 Radiotherapy is the one of the most common therapeutic approaches to treating human cancers. At least 50% of all cancer patients receive radiotherapy at some stage during the course of their illness. Currently, radiation is used to treat localized solid tumors. 4 The cellular damage induced by ionizing radiation is predominantly mediated through the generation of ROS. Radiation-induced ROS generates double-strand breaks (DSBs) in the cellular DNA. 5 Most of the experiments to understand the molecular basis of DNA damage caused by X-radi- ation have been conducted using alkaline single-cell gel electrophoresis 6 and the cytokinesis-blocking micronucleus method. 7 Apart from DNA damage, lipid peroxidation (LPO) is also considered to be a critical event of the ROS-mediated radiation effect. 8 Lipid radicals (L•) are believed to be formed by the reaction of hydroxyl (•OH) radicals generated by ionizing radiation with unsaturated lipids (LHs), which can subsequently react with oxygen to form