PENK and cancer: One proposed mechanism of action for its efficacy in cancer therapy involves the blockage of the opioid growth factor ([Met5]-enkephalin, OGF) and its receptor (OGFr), a regulatory axis suggested to inhibit cell proliferation in human cancer and normal cells [22,23] through the modulation of the G1/S cell cycle phase via cyclin-dependent kinase inhibitory pathways [24,25,26].