PI3K Inhibition Sensitized Cancerous Cells to Tumor Treating Fields (TTFields)

Purpose/Objective(s)

Tumor Treating Fields (TTFields) are alternating electric fields, approved for treatment of patients with newly diagnosed glioblastoma (GBM), recurrent GBM, or unresectable malignant pleural mesothelioma. Additional clinical trials are underway in other tumor types, including ovarian cancer, non-small cell lung carcinoma (NSCLC), and hepatocellular carcinoma (HCC). While TTFields therapy has been demonstrated to extend life, tumor progression eventually occurs in most patients. The current research aimed to identify molecular mechanisms involved in reduced cancer cellular sensitivity to TTFields, and targeting these potential pathways in order to re-sensitize the cells to TTFields.

Materials/Methods

Ovarian A2780, GBM U-87 MG, and NSCLC H1299 cells with reduced sensitivity to TTFields were generated by continuous long-term application of TTFields (7 or 13 days, depending on the cell line). Changes in signaling pathways in these cells were examined by Luminex multiplex assay, and specific pathway markers were validated by Western blot of the cell lysates as well as by immunohistochemistry in tumor sections from N1S1 HCC tumor-bearing rats treated with sham or TTFields. Next, TTFields were applied in conjunction with relevant inhibitors, followed by cell count measurements and western blot examinations. Finally, the concomitant application of TTFields with a relevant inhibitor was evaluated in mice orthotopically implanted with MOSE-L firefly luciferase (FFL) ovarian cancer cells. Tumor volume was measured at study end using the In Vivo Imaging System (IVIS) to detect the luciferin signal.

Results

Sensitivity of cancer cells to TTFields was decreased following continuous long-term application of TTFields. PI3K/AKT/mTOR signaling pathway was activated in these cells, with significant increases in phosphorylation levels of AKT and RPS6. This elevation was also observed in tumor sections from rats treated with TTFields. Treatment with PI3K inhibitors re-sensitized the cells to TTFields and downregulated the phosphorylation of AKT. In vivo, concomitant application of TTFields with the PI3K inhibitor alpelisib resulted in enhanced efficacy.

Conclusion

The current study demonstrated that the PI3K/AKT/mTOR signaling pathway is involved in reduced cancer cell sensitivity to long-term application of TTFields. Furthermore, re-sensitization to TTFields could be achieved with PI3K inhibitors, providing a rationale for further examining the potential benefit of TTFields concomitant with PI3K inhibitors.