Novel Indolocarbazole Derivative 12-(α-L-arabinopyranosyl)indolo[2,3-a]pyrrolo[3,4-c]carbazole-5,7-dione Is a Preferred c-Myc Guanine Quadruplex Ligand

The indolocarbazole derivative 12-(α-L-arabinopyranosyl)indolo[2,3-a]pyrrolo[3,4-c]carbazole-5,7-dione (AIC) has demonstrated a high potency (at nanomolar to submicromolar concentrations) towards the NCI panel of human tumor cell lines and transplanted tumors. Intercalation into the DNA double helix has been identified as an important prerequisite for AIC cytotoxicity. In this study, we provide evidence for preferential binding to the G-quadruplex derived from the c-Myc oncogene promoter (Pu18 d(AG(3)TG(4))(2); G-c-Myc). The association constant for AIC:G-c-Myc complex was ~100 times and 10 times greater than the respective values for the complexes AIC:c-Myc duplex and AIC:telomeric d(TTAGGG)(4) G-quadruplex. The concentrations at which AIC formed complexes with G-c-Myc were close to those that attenuated the steady-state level of the c-Myc mRNA in the human HCT116 colon carcinoma cell line. We suggest that preferential binding of AIC to G-c-Myc rather than to the c-Myc duplex might favor the quadruplex formation in the cells, thereby contributing to downregulation of the c-Myc expression by AIC.