Liposomal Arg-Gly-Asp analogs effectively inhibit metastatic B16 melanoma colonization in murine lungs

Analogs of a synthetic peptide having the L-arginine-L-glycine-L-aspartic acid (RGD) sequence have been found to decrease metastatic colonization. To enhance the metastasis-suppressing efficacy of these analogs, we sought to stabilize these analogs and to prolong their circulation time by incorporating them into a liposomal formulation. Various structures of RGD analogs grafted to hydrophobic groups were synthesized and then incorporated into liposomes. Liposomes composed of distearoylphosphatidylcholine, cholesterol, dipalmitoylphosphatidylglycerol and appropriate RGD analogs were injected intravenously along with B16BL6 murine melanoma cells into mice. Liposomal RGD (0.6 mumol of the analog equivalent to ca. 200 micrograms RGD peptides) inhibited lung colonization up to 76%. This dose is an order of magnitude lower than that for comparable inhibition reported for free RGD. Multi-dose administration of liposomal RGD (0.15 mumol of the analog) also inhibited the spontaneous lung metastasis of cells from a primary tumor site of B16BL6 cells subcutaneously implanted into the footpad of mice. Taken together, our data indicate that liposomal RGD may serve as a useful anti-metastatic agent.