Inhibition of metastasis of lewis lung carcinoma by a synthetic peptide within growth factor‐like domain of urokinase in the experimental and spontaneous metastasis model

Four synthetic peptides (residues 20‐30 and 17‐34) within the growth factor‐like domain (GFD) of murine and human urokinase‐type plasminogen activator (uPA) were examined to determine whether they inhibit production of experimental and spontaneous lung metastasis by murine Lewis lung carcinoma (3LL) cells. In an in vivo experimental metastasis assay, which determines mainly the later steps of the metastatic migration process (extravasation from the bloodstream and then growth into pulmonary tumor), none of the peptides introduced by i.v. single co‐injection into syngeneic C57B1/6 mice inhibited pulmonary metastasis, when 3LL cells were pre‐incubated with the peptides followed by i.v. co‐injection of the peptide and cells. In addition, none of the peptides, when injected i.p. daily for 7 days after i.v. tumor cell inoculation, reduced the number of lung tumor colonies. In a second in vivo assay that measures metastasis from a primary tumor (spontaneous metastasis model), multiple i.p. injections of the mouse peptide 17‐34 for 7 days after s.c. tumor cell inoculation significantly inhibited metastatic lung tumor colonization in a dose‐dependent manner, whereas human peptide 17‐34 had no effect. Mouse and human peptide 20‐30 had no effect either. The inhibition of lung metastasis was not due to direct antitumor effects of mouse peptide 17‐34. Our results indicate that occupation of uPA receptors on 3LL cells by the enzymatically inactive mouse peptide 17‐34 or prevention of rebinding of uPA synthesized by tumor cells to their receptor specifically reduced tumor cell invasion and formation of metastasis and that uPA may regulate more efficiently the mechanism involved in the entry of tumor cells into vascular circulation than extravasation during the metastatic process.