Toxins of Ascidians and Their Therapeutic Potential

Aspects relating to the toxic ascidian diversity; toxic compounds, and chemical classes of ascidian toxins; cytotoxic ascidians; and the therapeutic potential of ascidian toxins are presented in this chapter. Ascidians or sea-squirts (Phylum, Chordata; Sub-phylum; Tunicata; Class, Ascidiacea) which are commonly called urochordates/tunicates are exclusively marine. There are about 3000 species of ascidians. They are solitary or colonial and are found distributed in all depths of the seas of the world. Ascidians, along with invertebrates such as sponges, cnidarians, and bryozoans, are known to produce a rich variety of secondary metabolites including cyclic peptides and depsipeptides and aromatic alkaloids that are produced by the endosymbiotic micro-organisms and not produced by the ascidian themselves. Most of the metabolites of these ascidians are considered to be toxins with diverse activities, and they have the potential for development as therapeutic drugs for a wide variety of diseases. It is worth mentioning here that two ascidian toxins have been marketed for cancer treatment. Among them, Ecteinascidin (ET-743, trabectedin) which has been derived from Ecteinascidia turbinata is FDA approved and marketed under the trade name Yondelis ® ; and Aplidin ® (dehyrodidemnin B, plitidepsin) sourced from Aplidium albicans has attained orphan drug status. Both are marketed by PharmaMar (Madrid, Spain) ( Watters, 2018 ; Santhanam et al., 2019 b). The species of ascidians possessing therapeutic properties are shown in the following tables.