PENK and ovarian cancer: According to previous reports, [39,40], a wide variety of synthetic and natural opioids, including those specific for μ (DAMGO, endomorphin-1 and -2), δ (DPDPE) and κ (U69593) opioid receptors, showed that none of these compounds had any effect on growth of ovarian cancer cell, except for opioid growth factor (OGF), endogenous opioid peptide.