Evidence that phospholipase C is involved in the antitumour action of NSC768313, a new thieno[2,3-b]pyridine derivative

Jóhannes Reynisson, Jagdish K. Jaiswal, David Barker, Stacey A. N. D’mello, William A. Denny, Bruce C. Baguley, Euphemia Y. Leung

The thieno[2,3-b]pyridines were discovered by virtual high throughput screening as potential inhibitors of phospholipase C (PLC) isoforms and showed potent growth inhibitory effects in National Cancer Institute's human tumour cell line panel (NCI60). The mechanism of the anti-proliferative activity of thieno[2,3-b]pyridines is explored here. We aimed to investigate the basis for the anti-proliferative activity of these thieno[2,3-b]pyridines and to determine whether the cellular inhibition was related to their inhibition of PLC. Four breast cancer cell lines were used to assess the anti-proliferative effects (IC50 values) of six representative thieno[2,3-b]pyridines. The most potent compound (derivative 3; NSC768313), was further studied in MDA-MB-231 cells. DNA damage was examined by γH2AX expression level, and cell cycle arrest by flow cytometry. Cell morphology was examined by tubulin antibody staining. The growth inhibitory effect of combination treatment with derivative 3 and paclitaxel (tubulin inhibitor), doxorubicin (topoisomerase II inhibitor) or camptothecin (topoisomerase I inhibitor) was evaluated. A preliminary mouse toxicity assay was used to evaluate the pharmacological properties. Addition of the thieno[2,3-b]pyridine derivative 3 to the MDA-MB-231 cells induced G2/M growth inhibition, cell cycle arrest in G2-phase, membrane blebbing and the formation of multinucleated cells. It did not induce DNA damage, mitotic arrest or changes in calcium ion flux. Combination of derivative 3 with paclitaxel showed a high degree of synergy, while combinations with doxorubicin and camptothecin showed only additive effects. A mouse pharmacokinetic study of derivative 3 showed that after intraperitoneal injection of a single does (10 mg/Kg), the Cmax was 0.087 μmol/L and the half-life was 4.11 h. The results are consistent with a mechanism in which thieno[2,3-b]pyridine derivatives interact with PLC isoforms (possibly PLC-δ), which in turn affect the cellular dynamics of tubulin-β, inducing cell cycle arrest in G2-phase. We conclude that these compounds have novelty because of their PLC target and may have utility in combination with mitotic poisons for cancer treatment.