N-Substituted calothrixin B derivatives inhibited the proliferation of HL-60 promyelocytic leukemia cells

Calothrixin B, whose structure consists of a pentacyclic indolo[3,2-j]phenanthridine framework, was originally isolated as a secondary metabolite from Calothrix cyanobacteria and was found to exhibit potent antiproliferative activity. In this study, treatment with 100 μM of calothrixin B did not inhibit the proliferation of HL-60 promyelocytic leukemia cells, the result indicated that calothrixin B could possess weaker activity against leukemia cells. In a previous study of the structure-antiproliferative activity relationships of calothrixin B analogs, the tetracyclic 5H-pyrido[4,3-b]carbazole-4,11(6H)-dione structure, which does not include the benzene ring found in calothrixin B, was reported to be necessary for antiproliferative activity; however, the induction of substituents on the indole nitrogen atom of calothrixin B decreased the antiproliferative activity of the compound. To develop calothrixin B analogs with good antiproliferative activity against both HL-60 and HCT-116 cells, we synthesized various N-substituted calothrixin B analogs and assessed their activity. Compared with calothrixin B, N-OMe calothrixin B displayed almost the same or slightly stronger antiproliferative activity against HCT-116 cells, whereas the N-MOM analog demonstrated slightly weaker activity against these cells. These two analogs also inhibited proliferation of HL-60 cells, whereas calothrixin B did not exhibit antiproliferative activity toward these cells. Among calothrixin B analogs, N-OMe and N-MOM calothrixins B are cytotoxic against HCT-116 cells and not the lack of the effect on HL-60 cells.