Neovascular Targeting with Cyclic RGD Peptide (cRGDf-ACHA) to Enhance Delivery of Radioimmunotherapy

Radioimmunotherapy (RIT) has been hampered by delivery of only a small fraction of the administered dose of radiolabeled MAb to tumor. A strategy for creating and controlling tumor vascular permeability would enable more effective RIT. The αvβ3 integrin receptor is an appealing target for strategies designed to enhance permeability of tumor vessels because it is highly and preferentially expressed in most tumors. In human tumor mouse models, apoptosis of neovascular endothelial cells has been demonstrated after treatment with αvβ3 antagonists. Since this apoptotic effect could transiently increase permeability of tumor blood vessels, radiolabeled antibodies (MAb) circulating during this period would have increased access to extravascular tumor. To determine if this hypothesis was correct, a pharmacokinetic study of an immunospecific MAb given after an αvβ3 antagonist was performed in nude mice bearing human breast cancer xenografts. The αvβ3 antagonist, cyclic RGD pentapeptide (c-RGDf-ACHA; cyclo arginine glycine aspartic acid D-phenylalanine -1 amino cyclohexane carboxylic acid), inhibits αvβ3 binding to its vitronectin ligand at nanomolar levels. Cyclic RGD peptide (250 μg i.p.) given 1 hour before ¹¹¹In-ChL6 MAb resulted in a 40 - 50% increase in tumor uptake (concentration), when compared to the control tumor uptake, of MAb 24 hours after administration. When cyclic RGD peptide was given as a continuous infusion (17.5 ug/hr) for 1 or 24 hours before ¹¹¹In-ChL6, tumor uptake of ¹¹¹In-ChL6 was increased less, and. these data were not statistically different from the control data. There were no differences for any of the groups in the concentrations of ¹¹¹In-ChL6 in normal organs or blood when compared to the control group. The results suggest that cyclic RGD peptide provided a temporary, selective increase in tumor vascular permeability, that allowed a larger fraction of the ¹¹¹In-ChL6 to accumulate in the tumor.