Isolation and Bioassay of Linear Veraguamides from a Marine Cyanobacterium (Okeania sp.)

Abstract

Marine cyanobacteria have gained momentum in recent years as a source of novel bioactive small molecules. This paper describes the structure elucidation and pharmacological evaluation of two new, veraguamide O (1) and veraguamide P (2), and one known, veraguamide C(3), analogs isolated from a cyanobacterial collection made in the Las Perlas Archipelago of Panama. We hypothesized that these compounds would be cytotoxic in cancer cell lines. The compounds were screened against HEK-293, estrogen receptor positive (MCF-7), and triple-negative breast cancer (MDA-MB-231) cells as well as against a broad panel of membrane bound receptors. The planar structures were determined based on NMR and MS data along with comparison to previously isolated veraguamide analogs. Phylogenetic analysis of the collection suggests it to be an Okeania sp., a similar species to the cyanobacterium reported to produce other veraguamides. Veraguamide O shows no cytotoxicity (greater than 100 μM) against ER positive cells (MCF-7) with 13 μM IC₅₀ against MDA-MB-231 TNBC cells. Interestingly, these compounds show affinity for the sigma2/TMEM-97 receptor making them potential leads for development of non-toxic sigma 2 targeting ligands.

Figure 1:

Structures of isolated veraguamides O (1), P (2), and C (3).

Figure 2.

COSY and TOCSY connectivity and selected HMBC correlations for 1.

Figure 3.

Photos of marine cyanobacteria DUQ0008 and phylogeny. (A) Macroscopic field picture of sample DUQ0008 before collection. (B) Confocal microscopy image of DUQ0008 at 10X magnification with FITC (487 nm), TRITC (560nm) and Cy5 (637 nm) channels. Scale bar = 200 μm. (C) Transmitted light image of DUQ0008. Scale bar = 200 μm. (D) Phylogenetic tree showing the closest related species to DUQ0008 based off 16S rRNA sequencing with comparison to natural product producing cyanobacteria from the Okeania genus. Compounds produced by species are indicated if available; accession numbers are in brackets; numbers next to tree show percent of trees showing alignment in 600 replicates.

Figure 4.

Veraguamides O and C selectively reduce viability in estrogen receptor positive and TNBC cells. Isolated veraguamides O or C activity (A) in the estrogen positive breast cancer cell line, MCF-7 and (B) in the TNBC cell line, MDA-MB-231. Data represent the mean value +/− SE of three different experiments. (C) Veraguamide C has no effect on cell viability in HEK-293 cells. *P<0.05, **P<0.01, ***P<0.001; **** p<0.0001 vs control group determined by one-way ANOVA with the Bonferroni post hoc test.

Scheme 1.

Partial hydrolysis of veraguamides O (1) and P (2).