Eunicea fusca and Pseudopterogorgia elisabethae as a resource for bioactive diterpenes: A journey through drug discovery, glycosylation chemistry, and chemical proteomics

摘要

Natural products have long been recognized for their medicinal value in traditional medicine and remain an important component of modern drug discovery. Known for their immense structural diversity, natural products are secondary metabolites that often exhibit unique pharmacological properties. In the marine habitat, alcyonacean octocorals are a rich source of diterpenes with promising therapeutic potential. For instance, Eunicea fusca contains fuscoside B, a diterpene arabinoside with potent topical anti-inflammatory activity. Notably, fuscoside B selectively inhibits 5-lipoxygenase and has a negligible effect on prostaglandin (PG) biosynthesis. Pseudopteroxazole, an amphilectane diterpene from the octocoral Pseudopterogorgia elisabethae, shows activity against Mycobacterium tuberculosis, the etiologic agent of tuberculosis.This thesis describes a variety of approaches to discover and develop diterpenes from alcyonacean corals. An investigation of E. fusca collected from Hillsboro Ledge, Florida has led to the discovery of eunicidiol, a topical anti-inflammatory diterpene. The structure of eunicidiol was elucidated by 1D and 2D NMR spectroscopy, while the absolute configuration was unambiguously assigned by the Mosher method. The anti-inflammatory activity of eunicidiol was evaluated by measuring its ability to reduce phorbol myristate acetate (PMA)-induced edema in a mouse ear model. Topical application of a 100 μg/ear dose of eunicidiol significantly reduced edema with activity that was superior to indomethacin, an anti-inflammatory drug serving as an industry benchmark.A library of novel fuscoside B analogues has been synthesized from fuscol and eunicol using a modified Koenigs-Knorr glycosylation approach. This semisynthesis is the first successful glycosylation of fuscol and has provided the eunicosides, a novel structural class of diterpene glycosides. In addition, glycoside mimics were prepared by introducing ethylene glycol (EG)-based substituents in place of the arabinose moiety. The library of compounds was evaluated for anti-inflammatory activity in the PMA-induced mouse ear edema assay, which demonstrated the remarkable influence of the carbohydrate substituent and its configuration on the in vivo efficacy of this glycoside class.Pseudopteroxazole has shown activity against resistant strains of M. tuberculosis as well as in the low-oxygen-recovery assay (LORA), a mycobacterial model of non-replicating persistence (NRP). To investigate the antitubercular mechanism of action of pseudopteroxazole, a collection of semisynthetic probes was prepared to identify putative protein targets by affinity chromatography. During the course of this synthesis, structure-activity relationships (SAR) were assessed in order to maintain high binding affinity to the target protein(s). This strategy has led to the identification of several putative targets, including (3R)-hydroxyacyl-ACP dehydratase subunit C (HadC). This result suggests that pseudopteroxazole may exert its antitubercular activity, at least in part, by inhibiting mycolic acid biosynthesis.