Design and Efficient Synthesis of RalA Inhibitors Containing the Dihydro‐α‐carboline Scaffold

Ras-related protein RalA is a member of the Ras small GTPases superfamily. Its activation plays an important role in regulating tumor initiation, invasion, migration, and metastasis. In this study, we designed a new type of RalA inhibitor containing a dihydro-alpha-carboline scaffold. The structurally new dihydro-alpha-carboline derivatives could be efficiently synthesized in good yields through a newly developed three-component [3+2+1] cyclization reaction. Evaluation of the biological activity showed that some of the dihydro-alpha-carboline derivatives can inhibit RalA/B and proliferative activities of NSCLC cell lines. The 4-(pyridin-3-yl)-dihydro-alpha-carboline compound (3 o) was found to be the most potent derivative, with IC50 values of 0.43 +/- 0.03, 0.64 +/- 0.07, 0.93 +/- 0.10, and 1.54 +/- 0.15 mu M against A549, H1299, H460, and H1975 cells, respectively. Mechanism investigation suggested that 3 o inhibits the RalA/B activation of A549, down-regulates Bcl-2, stimulates cytochrome c and PARP cleavage, and induces cell apoptosis. A molecular docking study revealed that 3 o can form stable hydrogen bonds with residues of RalA. Moreover, amide-pi and alkyl-pi interactions also contributed to the affinity between 3 o and RalA.