A simple approach toward the synthesis of the marine sponge derived pigment fascaplysin was used to obtain the marine alkaloids 3-bromofascaplysin and 3,10-dibromofascaplysin. These compounds were used for first syntheses of the alkaloids 14-bromoreticulatate and 14-bromoreticulatine. Preliminary bioassays showed that 14-bromoreticulatine has a selective antibiotic (to Pseudomonas aeruginosa) activity and reveals cytotoxicity toward human melanoma, colon, and prostate cancer cells. 3,10-Dibromofascaplysin was able to target metabolic activity of the prostate cancer cells, without disrupting cell membrane’s integrity and had a wide therapeutic window amongst the fascaplysin alkaloids.

Marine alkaloid fascaplysin and its derivatives are known to exhibit promising anticancer properties in vitro and in vivo. However, toxicity of these molecules to non-cancer cells was identified as a main limitation for their clinical use. Here, for the very first time, we synthesized a library of fascaplysin derivatives covering all possible substituent introduction sites, i.e., cycles A, C and E of the 12H-pyrido[1-2-a:3,4-b’]diindole system. Their selectivity towards human prostate cancer versus non-cancer cells, as well as the effects on cellular metabolism, membrane integrity, cell cycle progression, apoptosis induction and their ability to intercalate into DNA were investigated. A pronounced selectivity for cancer cells was observed for the family of di- and trisubstituted halogen derivatives (modification of cycles A and E), while a modification of cycle C resulted in a stronger activity in therapy-resistant PC-3 cells. Among others, 3,10-dibromofascaplysin exhibited the highest selectivity, presumably due to the cytostatic effects executed via the targeting of cellular metabolism. Moreover, an introduction of radical substituents at C-9, C-10 or C-10 plus C-3 resulted in a notable reduction in DNA intercalating activity and improved selectivity. Taken together, our research contributes to understanding the structure–activity relationships of fascaplysin alkaloids and defines further directions of the structural optimization.

Fascaplysin is a marine alkaloid which is considered to be a lead drug candidate due to its diverse and potent biological activity. As an anticancer agent, fascaplysin holds a great potential due to the multiple targets affected by this alkaloid in cancer cells, including inhibition of cyclin-dependent kinase 4 (CDK4) and induction of intrinsic apoptosis. At the same time, the studies on structural optimization are hampered by its rather high toxicity, mainly caused by DNA intercalation. In addition, the number of methods for the syntheses of its derivatives is limited. In the current study, we report a new two-step method of synthesis of fascaplysin derivatives based on low temperature UV quaternization for the synthesis of thermolabile 9-benzyloxyfascaplysin and 6-tert-butylfascaplysin. 9-Benzyloxyfascaplysin was used as the starting compound to obtain 9-hydroxyfascaplysin. However, the latter was found to be chemically highly unstable. 6-tert-Butylfascaplysin revealed a significant decrease in DNA intercalation when compared to fascaplysin, while cytotoxicity was only slightly reduced. Therefore, the impact of DNA intercalation for the cytotoxic effects of fascaplysin and its derivatives needs to be questioned.

Based on the results of our own preliminary studies, the derivative of the marine alkaloid fascaplysin containing a phenyl substituent at C-9 was selected to evaluate the therapeutic potential in vivo and in vitro. It was shown that this compound has outstandingly high antimicrobial activity against Gram-positive bacteria, including antibiotic-resistant strains in vitro. The presence of a substituent at C-9 of the framework is of fundamental importance, since its replacement to neighboring positions leads to a sharp decrease in the selectivity of the antibacterial action, which indicates the presence of a specific therapeutic target in bacterial cells. On a model of the acute bacterial sepsis in mice, it was shown that the lead compound was more effective than the reference antibiotic vancomycin seven out of nine times. However, ED₅₀ value for 9-phenylfascaplysin (7) was similar for the unsubstituted fascaplysin (1) in vivo, despite the former being significantly more active than the latter in vitro. Similarly, assessments of the anticancer activity of compound 7 against various variants of Ehrlich carcinoma in mice demonstrated its substantial efficacy. To conduct a structure–activity relationship (SAR) analysis and searches of new candidate compounds, we synthesized a series of analogs of 9-phenylfascaplysin with varying aryl substituents. However, these modifications led to the reduced aqueous solubility of fascaplysin derivatives or caused a loss of their antibacterial activity. As a result, further research is required to explore new avenues for enhancing its pharmacokinetic characteristics, the modification of the heterocyclic framework, and optimizing of treatment regimens to harness the remarkable antimicrobial potential of fascaplysin for practical usage.

For the first time, a comparative analysis has been conducted to elucidate the biosynthesis of three families of natural products—staurosporines/rebeccamycins, cladoniamides, and fascaplysins. Based on the available data, a well-founded hypothesis was formed that these metabolites arise through a shared biosynthetic pathway. A comparative evaluation of biological activity profiles and molecular mechanisms of action of the major representatives of these alkaloid families and their derivatives shows that, despite an apparent similarity between the activity spectra of indolo[2,3-a]pyrrolo[3,4-c]carbazoles and fascaplysins, they operate through different mechanisms. The biological effects of fascaplysin are driven primarily by the induction of metabolic stress rather than by the inhibition of DNA topoisomerase I or of a broad-spectrum protein kinases. The successful optimization of natural indolo[2,3-a]pyrrolo[3,4-c]carbazoles—compounds with initially poorer pharmacokinetic properties than those of fascaplysin—to drug-like candidates underscores the substantial pharmaceutical potential of the fascaplysin scaffold. Several existing fascaplysin derivatives, after the improvement of their pharmacokinetic characteristics, may serve as promising leads for the development of a new class of antibiotics.