Abstract 3205: Structural analysis of cardiac glycosides to determine the basis for tumoristatic activity
Authors: Kaushik Vivek , Kaulkarni Yogesh , Wright Clayton , O'Doherty George , Azad Neelam , Iyer Anand 
Published: May 2015

Cardiac glycosides (cardenolides) such as digitoxin are routinely used in the clinic to treat arrhythmias; recent studies however have delved into their potential as anti-cancer agents. One major concern with the use of cardenolides is their narrow therapeutic index, which precludes their use in oncology due to the higher doses required to achieve tumoristatic effects. However, synthesis of chemical analogs of cardenolides with much higher anti-tumor efficacy whilst maintaining the pathway-specificities of their parent compounds is highly desirable.

Our group has previously developed a patented palladium-catalyzed de novo synthesis methodology for synthesis of complex carbohydrates, which has allowed us to synthesize a library of digitoxin analogues. One novel non-natural monosaccharide derivative (β-D-digitoxin or MonoD) in particular demonstrates strong cytotoxic effects on a wide variety of tumors as assessed by measuring activity against the NCI panel of 60 cancer cell lines. More importantly, MonoD exerts at least a five-fold higher tumoristatic effect as compared to digitoxin. As opposed to digitoxin that has three sugar moieties (trisaccharide) attached to a steroid domain, MonoD is a monosaccharide with only one sugar unit. This suggests that the length of the sugar side chain may play a critical role in determining anti-cancer efficacy of cardenolides. Furthermore, our preliminary work suggests that the nature of substituents on both the carbohydrate and steroid portions of digitoxin affects its efficacy. In order to further delineate the basis for such differential effects, we selected three naturally occurring cardenolides that have similar structures to that of MonoD with variations in either the sugar region, the steroid unit or both - Convallatoxin (CV), Peruvoside (PV) and Cymarin (CM) - and assessed for tumoristatic effects.

Our preliminary data shows that differences in functional groups can have direct effects on the efficacy of cardenolides in exerting tumoristatic effects. All the selected cardenolides demonstrated potent inhibition of cell viability as assessed by availability of functional ATP using Cell Titer Glo®. Confounding however was the fact that unlike naturally occurring cardenolides, increasing amounts of MonoD led to increased availability of ATP. However, apoptosis assays clearly indicated that MonoD did lead to cell death. We will be pursuing further structural modification analysis to determine the underlying cause of such differential effects. A SAR study to delineate specific variations in the substituents in both the sugar and the steroid unit of cardenolides may help access compounds that have higher anti-tumorigenic potential against cancers.

Citation Format: Vivek Kaushik, Yogesh Kaulkarni, Clayton Wright, George O'Doherty, Neelam Azad, Anand Iyer. Structural analysis of cardiac glycosides to determine the basis for tumoristatic activity. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 3205. doi:10.1158/1538-7445.AM2014-3205