Thieno[2,3‐c]isoquinolines: A Novel Chemotype of Antiproliferative Agents Inducing Cellular Apoptosis While Targeting the G2/M Phase and Tubulin

In the era of modern synthetic methodology and advanced bio-evaluation techniques and considering the notorious history of hepatocellular carcinoma (HCC), hopeful expectations regarding novel bioactive chemotypes have grown dramatically. Among the widely versatile motifs in drug discovery studies are isoquinoline and thieno[2,3-b]pyridine. Herein, the molecular merging of both motifs evoked thieno[2,3-c]isoquinoline as a novel antiproliferative chemotype being hardly studied against HCC. Accordingly, compound series 4, 5, 7 and 8 were synthesized and bioevaluated against the HepG2 cell line. The role of C7-Ac/C8-OH substituents, C8-C9 unsaturation, 1H-pyrrol-1-yl ring closure at C1-NH2 and C6-Ph p-halo-substitution were biologically studied and successfully furnished the lead 5b while showing safe profile against Vero cells. Further, flow cytometric and Annexin V-FITC/PI apoptotic bio-investigations of 5b unveiled remarkable cell cycle arrest at the G2/M phase besides a 60-fold increase in apoptosis. The use of a DFT conformational study followed by Molecular docking and molecular mechanics/generalized born surface area scoring evoked potential tubulin-targeting activity of 5b at colchicine-binding site, which was confirmed by experimental evidence (Tub Inhib IC50  = 71 µM vs. 14 µM for colchicine). Accordingly, preserving C7-acetyl and optimizing halogen position while preserving [6S,7R]-stereochemistry is crucial for optimum binding to colchicine binding site of tubulin.