Potent retro-inverso D-peptide for simultaneous targeting of angiogenic blood vasculature and tumor cells.

The application of tumor targeting ligands for the treatment of cancer holds the promise of enhanced efficacy and reduced toxicity. L-SP5 ((L)(SVSVGMKPSPRP)) is a peptide that recognizes tumor neovasculature but not normal blood vessels (Lee et al., Cancer Res. 2007, 67, 10958-65). The current report presents the design and application of D-SP5 ((D)(PRPSPKMGVSVS)), a novel retro-inverso analogue of L-SP5. Peptides D-SP5 and parental L-SP5 are shown to compete for the same target sites of a yet unknown cellular target and possess a dual-targeting bioactivity for both activated endothelial cells (HUVEC) and several tumor cell lines. Cellular uptake experiments showed superior in vitro targeting abilities of D-SP5 compared with L-SP5, such as enhanced internalization into stimulated HUVEC or KB, U87, and SGC tumor cells. A radioligand receptor binding assay revealed a higher cell affinity of D-SP5 in all tested cell lines, with K-d values for D-SP5 about 2-fold lower than for L-SP5. An up to 3-fold higher maximum binding capacity (B-max) to cells of D-SP5 was noted. Fluorescein-labeled D-SP5 upon intravenous administration displayed strong association with tumor endothelium. D-SP5-conjugated PEG-DSPE micelles displayed enhanced tumor homing (evidenced by near infrared in vivo imaging). When loaded with doxorubicin, D-SP5 micelles could markedly suppress tumor growth with higher efficacy than L-SP5 micelles both in vitro and in vivo in KB tumor xenografts. In summary, the data demonstrate that D-SP5 displays higher binding affinities toward tumor endothelium as well as tumor cells and enhanced tumor targeting capability in vitro and in vivo.