The objective is to investigate the mechanism of the inhibitory effect of GRGDS (GlyArgGly-AspSer) pentapeptide on pulmonary colonization by murine melanoma (B16F10) cell and to assess the possible therapeutic potential of this molecule in the mouse model system. Since this peptide sequence corresponds to the cellbinding domain of the adhesive glycoprotein fibronectin, our working hypothesis for the peptidemediated inhibition of experimental metastasis is that it results from interference with cellular adhesive function. We will identify which adhesive molecule is inhibited by the pentapeptide by examining the relationship between the effects of GRGDS and related peptides on lung colonization and adhesion of B16F10 melanoma cells to substrata composed of fibronectin, laminin, collagen, vitronectin, and fibrinogen using classical in vitro assays. Identification of the site of pentapeptide interference will be further evaluated in similar in vivo and in vitro studies by utilizing the MCF7 human breast carcinoma cell line that attaches specifically to laminin. We will also examine the possibility that GRGDS decreases lung colonization potential of B16 melanoma cells by inhibition of platelet aggregation or emboli formation using thrombocytopenic animals to see if the peptide inhibitory effect is still manifested. To determine whether GRGDS is effective against tumor cells that metastasize other organs, the M5076 reticulum sarcoma cells that metastasize preferentially to the viscera and liver will be tested in experiments similar to those employed for the B16F10 melanoma. To investigate the possible relevance of interruption of cellular adhesive interactions in cancer therapy, we will examine the clearance rate of intravenously injected radiolabeled active peptides (GRGDS and others). If, as expected, these peptides are rapidly cleared from the blood vascular system, we will explore the effectiveness of their gradual release from subcutaneous implants of biodegradable methyl cellulose pellets against a tumor cell line (B16BL6) that spontaneously metastasizes the lung from a subcutaneous primary tumor. As alternative approaches to maintaining a constant plasma level of peptide sufficient to abrogate metastasis, we will (i) explore the use of implantable miniosmotic pumps, and/or (ii) modify the physical structure of GRGDS to reduce its excretion rate. This study is important for understanding the biology and biochemistry of tumor cell metastasis and may have clinical utility in a prophylacticalic or adjuvant settings.
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