Bioactive Compounds in the Chemical Defence of Marine Sponges Structure-Activity Relationships and Pharmacological Targets

Abstract: Marine invertebrates, in particular sponges, represent a source of a wide range of secondary metabolites, many of which have been attributed various defensive capabilities against environmental stress factors. In this thesis sponge-derived low-molecular peptide-like compounds and associated analogs are investigated for bioactivity and pharmacological targets.The compound bromobenzisoxazolone barettin (cyclo[(6-bromo-8-(6-bromo-benzioxazol -3(1H)-one)-8-hydroxy)tryptophan)]arginine) was isolated from the sponge Geodia barretti and its ability to inhibit larval settlement of the barnacle Balanus improvisus was determined. With an EC50 value of 15 nM, this compound’s antifouling effect was higher than those of the previously reported brominated dipeptides from Geodia barretti, i.e., barettin and 8,9-dihydrobarettin; moreover, this antifouling effect was demonstrated to be reversible. However, the compound lacked affinity for 5-HT1-7 receptors, whereas barettin possessed specific affinity to 5-HT2A, 5-HT2C and 5-HT4, while 8,9-dihydrobarettin interacted with 5-HT4. In an attempt to evaluate structure-activity relationships synthesized analogs with barettin and dipodazine scaffolds were investigated for antifouling activity. The analog benso[g]dipodazine, with an EC50 value of 34 nM, displayed the highest settlement inhibition.The studies of the structure-activity relationships of sponge-derived compounds were extended to cover analogs of agelasines and agelasimines originally isolated from sponges of the genus Agelas. Synthesized (+)-agelasine D and two structurally close analogs were investigated for cytotoxic and antibacterial activity. The profound cytotoxicity and broad spectrum antibacterial activity found prompted a further investigation of structure-activity relationships in 42 agelasine and agelasimine analogs and several characteristics that increased bioactivity were identified.In conclusion this work has produced new results regarding the potent bioactivity of compounds derived from the sponges Geodia barretti and Agelas spp. and increased SAR knowledge of the fouling inhibition, cytotoxicity and antimicrobial activity of these compounds.