**1. Introduction**

Fucoidans found in brown algae are documented for their broad spectrum of bioactive properties, including anticancer, anticoagulant, antioxidant, anti-inflammatory, and immunomodulatory activities beneficial for the pharmaceutical, cosmeceutical, nutraceutical, and functional food industries [1]. The structure of fucoidan comprises of alternating units of L-fucose with mannose, glucose, galactose, and xylose monosaccharides with substituted sulfate groups. The primary linkage between α-l-fucopyranosyl residues is the (1 →3) and (1 →4) with sulfate groups substituted at either C2, C4, or both positions [1,2]. However, di fferences in fucoidan structure have been observed in di fferent brown algae and remain ambiguous for their structures are highly heterogeneous. Typically, the structural characterization of fucoidan is accomplished by analyzing the monosaccharides composition, their connectivity, and substituted functional groups (methylation analysis, FTIR, and NMR) [3]. The publication by Bilan et al. (2013) reveals some significant findings regarding the fucoidan structure of *S. polycystum* harvested from Vietnam, which is comprised of 3-linked α-l-fucopyranose 4-sulfate residues [4]. Apart from the species specificity, fucoidan structure could vary depending on environmental factors and stress conditions. The structure of fucoidan (molecular weight, monosaccharide composition, and their sequence, degree, and substitution of sulfate groups) is a critical factor that determines its biofunctional properties. Li et al. (2008) provide a detailed review of up to date understanding regarding the structure-activity relationship of fucoidans [5]. By far, the anticancer properties of fucoidan have been found to increase with the increasing degree of sulfation and with reduction of molecular weight up to a certain limit.

Cancer has become a burden and a leading cause of death throughout the world. The number of cancer incidents in 2012 reached approximately 14.1 million, among which 8.2 million deaths have been reported [6]. The rapid growth of cancer incidents is predicted to be due to the increasing population and modernizing lifestyle patterns. Consumption of foods with artificial coloring, flavoring, and preservatives, use of tobacco, physical inactivity, exposure to ionizing radiation, and carcinogenic substances are among the major factors that contribute to the development of cancer [7]. Though a vast number of anticancer drugs are available, continuing the search for new cancer medicine is a prevailing task as many available anticancer agents cause serious side e ffects on normal tissues. The lack of selectivity is one of the major drawbacks in many of these drugs except for a limited number of anticancer medications such as "vemurafenib" that show their e ffects on melanoma cells with V600E BRAF mutation and monoclonal antibodies that specifically target cancer antigens [8]. The heterogeneous nature of cancers and multidrug resistance causes the development of anticancer drugs a challenging task [9]. Fucoidans, by far, have been recognized as a biocompatible substance that indicates positive biological responses. A recent study by Silchenko et al. (2017) describes the anticancer activity of fucoidan and sulfated fucooligosaccharides from *Sargassum horneri,* whereas the fucoidans are reported to suppress the colony formation of DLD-1 (colorectal adenocarcinoma) cells [10]. Fucoidans mediate anticancer functions not only by regulating intracellular mechanisms of cancer cells such as apoptosis, but also the activity of immune cells such as lymphocytes and natural killer cells [8,11]. Apart from the direct chemotherapeutic e ffects, fucoidan rich polysaccharides have shown radioprotective e ffects that make them a useful substance in cancer-radiotherapy [12]. The objective of the current study was to explore the anticancer activity of fucoidan rich polysaccharides of untapped *S. polycystum* harvested from the tropical island, Sri Lanka, and to explore its structural properties. Recently Palanisamy et al. (2017) have reported the anticancer potential of a fucoidan extract from *S. polycystum* on MCF-7 cells [13]. The present study so far is the first to report modified enzyme assisted extraction of fucoidan enriched polysaccharides from *S. polycystum*, anion exchange chromatographic purification, and detailed information of their anticancer properties.
