Methods of reducing the viability of cancer cells, preventing cancer cells of a subject from developing resistance to TTFields, and restoring sensitivity of cancer cells to TTFields by recommending or prescribing a PTGER3 inhibitor to a subject and applying an alternating electric field to the cancer cells are provided. In some instances, sensitivity of cancer cells to TTFields can be restored with one or more PTGER3 inhibitors (e.g., NSAIDs, cox2 inhibitors).