DDDR-25. Targeting glioma stem cells with a fungal metabolite ophiobolin A: synthesis and evaluation of C21-ketones

Abstract Glioblastoma (GBM) tumors consist of ecosystems with diverse neoplastic populations, including glioma stem cells (GSCs). The latter are believed to give rise to intratumoral heterogeneity, plasticity and resistance to treatment, contributing to poor prognosis and recurrence of GBM patients. Studies have revealed that deregulation of apoptotic signal transduction pathways are among the most prominent mechanisms driving chemotherapy resistance in GSCs and they constitute an important focus for the development of therapeutics. However, studies of small molecule effects in GSCs have revealed the induction of cell death pathways that are not dependent on apoptosis, such as ferroptosis, autophagic cell death, paraptosis, programmed necrosis and cuproptosis. We have previously reported that ophiobolin A, a fungal metabolite from the Bipolaris species, exhibited promising activity against GBM cells, displaying such GSC markers such as nestin and CD44, through induction of paraptosis. The development of ophiobolin A as anti-GBM agent, requires unraveling of the structure-activity relationship and as part of this effort we focused here on the C21-aldehyde functionality. The aldehyde group may represent a liability for example through metabolic oxidation to a carboxylic acid. We developed chemistry through which ophiobolin A was reacted with Grignard reagents in THF at 0 oC to afford C21-secondary alcohols. The latter were oxidized with PDC to the desired C21-ketones: C21-Me, Et, Pr, and i-Pr ketones were prepared. These C21-ketones were evaluated in a panel of patient-derived GSCs and GBM cell lines, but found to have inferior potencies compared to the natural C21-aldehyde-containing natural product. It can be concluded that increasing the steric bulk at C21 from a hydrogen to a larger alkyl group has a detrimental effect on cytotoxic potency. Further work will involve synthesizing C21-derivatives that are not significantly larger in size but differ substantially in their electronic contributions.