Synthesis and biological activity of bisindole derivatives as novel MARK4 inhibitors

Bisindole alkaloids are well-known bioactive natural products presenting numerous pharmacological properties. Following the indolocarbazole biosynthetic pathway a series of bisindole derivatives were synthesised and their activity against MARK4 kinase was investigated. The binding of bisindoles on MARK4 was measured with both fluorescence quenching and ITC. Further, enzyme inhibition assays revealed that the tested compounds are MARK4 inhibitors with IC50 values in the low micromolar range. The inhibitory activity of bisindoles 20 and 33 was tested against four additional kinases. The data showed that 20 and 33 have a relatively low selectivity against MARK4 as they also moderately inhibit other kinases such as PDK3. Most of the synthesized compounds were subjected to molecular docking studies to rationalise the molecular interactions within the catalytic site of MARK4. The antiproliferative activity of derivatives 18, 26, 33 and 20 was evaluated against A549, MCF7 and OVCAR-3 cancer cells. Among the tested compounds, 20 displayed the best cytotoxic activity against OVCAR-3 cells with GI50 = 7 ± 0.5 μΜ, TGI = 10 ± 0.8 μΜ and IC50 = 20 ± 1.2 μΜ and A549 cancer cells with GI50 = 4 ± 0.52 μΜ, TGI = 12.5 ± 0.8 μΜ and IC50 32.5 ± 2.4 μM. In addition, compounds 18 and 26 induce apoptosis in A549 cells and deviated the cells from early to late apoptotic events. Overall, the new bisindole derivatives possess significant antiproliferative properties, yet a non-selective cytotoxic effect was observed on normal MRC5 cells. Our observations support the suitability of bisindole scaffold for the design of potent MARK4 inhibitors, but to improve the drugability further optimisation and structural modifications are necessary.