PDE5A and neoplasm: The present studies were designed to build upon our work with [pemetrexed + sildenafil], [sorafenib + sildenafil] and [sorafenib + pemetrexed] and were to determine whether PDE5 inhibitors such as sildenafil (Viagra®) enhance the anti-tumor effects of [pemetrexed + sorafenib] in non-small cell lung cancer (NSCLC).