The antimetastatic activity of enkephalin-like peptides.

[Met5]Enkephalin and [Leu5]enkephalin, two naturally occurring opioid-like peptides were found to decrease the number of spontaneous pulmonary metastases in C57BL/6J mice implanted in the foot pad with the B16-BL6 melanoma. The number of such metastatic nodules were reduced more than 2-fold. This effect was noted after several different injection schedules. In addition to their effect on metastasis the growth rate of the primary tumor was also decreased by these compounds.