AUTHOR=Arevalo Gary E. , Frank Michelle K. , Decker Katelin S. , Theodoraki Maria A. , Theodorakis Emmanuel A. 

TITLE=Gambogic acid: Multi-gram scale isolation, stereochemical erosion toward epi-gambogic acid and biological profile

JOURNAL=Frontiers in Natural Products

VOLUME=1

YEAR=2023

URL=https://www.frontiersin.org/journals/natural-products/articles/10.3389/fntpr.2022.1018765

DOI=10.3389/fntpr.2022.1018765

ISSN=2813-2602

ABSTRACT=<p><bold>Introduction:</bold> Extracted from gamboge resin, gambogic acid (<bold>GBA</bold>) is a natural product that displays a complex caged xanthone structure and exhibits promising antitumor properties. However, efforts to advance this compound to clinical applications have been thwarted by its limited availability that in turn, restricts its pharmacological optimization.</p><p><bold>Methods:</bold> We report here an efficient method that allows multigram scale isolation of <bold>GBA</bold> in greater than 97% diastereomeric purity from various sources of commercially available gamboge. The overall process includes: (a) isolation of organic components from the resin; (b) separation of <bold>GBA</bold> from the organic components via crystallization as its pyridinium salt; and (c) acidification of the salt to isolate the free <bold>GBA</bold>.</p><p><bold>Results and Discussion:</bold> We found that <bold>GBA</bold> is susceptible to epimerization at the C2 center that produces <italic>epi</italic>-gambogic acid (<bold><italic>epi</italic>-GBA</bold>), a common contaminant of all commercial sources of this compound. Mechanistic studies indicate that this epimerization proceeds <italic>via</italic> an <italic>ortho</italic>-quinone methide intermediate. Although the observed stereochemical erosion accounts for the chemical fragility of <bold>GBA</bold>, it does not significantly affect its biological activity especially as it relates to cancer cell cytotoxicity. Specifically, we measured similar levels of cytotoxicity for either pure <bold>GBA</bold> or an equilibrated mixture of <bold>GBA</bold>/ <bold><italic>epi</italic>-GBA</bold> in MBA-MB-231 cells with IC<sub>50</sub> values at submicromolar concentration and induction of apoptosis after 12 hours of incubation. The results validate the pharmacological promise of gambogic acid and, combined with the multigram-scale isolation, should enable drug design and development studies.</p>