Discovery of Mandelalide E and Determinants of Cytotoxicity for the Mandelalide Series

Abstract

Recollection of the tunicate source of the mandelalides has provided new and known analogues that have facilitated expanded analyses of the disputed cancer cytotoxicity of mandelalide A following a number of recent reported total syntheses. Using newly characterized mandelalide E, reisolated natural mandelalides B and C, and synthetic mandelalide A, the cytotoxicity of the mandelalides is demonstrated to be strongly influenced by compound glycosylation and assay cell density. Glycosylated mandelalides reduced the viability of human cancer cells cultured at a high starting density with a rank order of potency A > B ≫ E, yet display dramatically reduced cytotoxic efficacy against low density cultures.

New Mandelalides Expand a Macrolide Series of Mitochondrial Inhibitors

“…These analogues displayed high nanomolar to low micromolar activity (Figure 6; Table 1), in agreement with our proposal 10 that butyrylation of the monosaccharide abrogates mandelalide cytotoxicity. Interestingly, NCI-H460 lung cancer cells appeared more sensitive to these B-type mandelalides, in particular 3- O -butyrylated mandelalides 9 and 10 , which differ only in 21- O -acetylation of 10 .…”

“…26 Together these results extend our earlier observations that low density (glycolytic) HeLa cell cultures are less sensitive to 1 than high density (more oxidative) cultures. 10 …”

“…While the influence of macrocycle glycosylation for biological activity was established previously, 10 the isolation of a suite of mandelalides with varying acylation around both the monosaccharide and the macrocycle offered the opportunity to investigate the theory that nonspecific acylation of the primary mandelalide biosynthetic products comprises a potential detoxification mechanism carried out by co-existing microbes, the host tunicate and/or the mandelalide-producing bacterium, for self-resistance and/or cellular transport. Towards this end, the cytotoxic potential of all new analogues was tested against human HeLa and NCI-H460 lung carcinoma cells grouped according to macrocycle type and relative to at least one other macrocycle type.…”

“…This result for 7 , which differs from 5 only in an additional 3- O -butyryl substituent is consistent with the low micromolar cytotoxicity obtained previously for 5 . 10 For the C-type aglycones, some role of 24- O -butyrylation is implied when considering that 3 (3- O -butyryl substituent only) displayed weak (HeLa) to no (NCI-H460) activity at the concentrations tested (EC 50 > 3 µM), whereas 4 (3, 24- O -dibutyryl substituents) revealed sub to low micromolar activity (EC 50 660 nM, HeLa; 1.7 µM, NCI-H460). Aglycone 8 , which possesses 3- O -hexanoyl and 24- O -butyryl substituents, was the most active of the aglycones against HeLa cells (EC 50 330 nM), although considerably less active against NCI-H460 cells (EC 50 2.6 µM).…”

“…10 The re-isolation of 1–3 and identification of new mandelalide E ( 5 ) permitted further evaluation that demonstrated a dramatic loss of activity for the aglycone 3 relative to glycosylated 1 and 2 , and 100-fold loss in activity when the saccharide hydroxyl groups at C-3′ and C-4′ are esterified as in 5 . 10 These insights into the mechanistic basis for mandelalide selectivity are expanded here with the discovery that cytotoxic mandelalides inhibit mitochondrial function and induce apoptotic cell death, in a manner consistent with metabolic inhibition of the mammalian ATP synthase complex. ATP synthase has long been known as a target of natural products, including phenolic compounds from plants and antimicrobial cationic peptides from animals, 11 as well as the macrocyclic polyketide oligomycins and apoptolidins.…”