Enhancement of cytotoxicity of antimicrobial peptide magainin II in tumor cells by bombesin-targeted delivery

Aim： To investigate whether the conjugation of magainin II （MG2）, an antimicrobial peptides （AMPs）, to the tumor-homing peptide bombesin could enhance its cytotoxicity in tumor cells. Methods： A magainin II-bombesin conjugate （MG2B） was constructed by attaching magainin II （MG2） to bombesin at its N-terminus. The peptides were synthesized using Fmoc-chemistry. The in vitro cytotoxicity of the peptide in cancer cells was quantitatively deter- mined using the CCK-8 cell counting kit. Moreover, the in vivo antitumor effect of the peptide was determined in tumor xenograft mod- els. Results： The IC5o of MG2B for cancer cells （10-15 pmol/L） was at least 10 times lower than the IC50 of unconjugated MG2 （125 μmol/L）. Moreover, the binding affinity of MG2B for cancer cells was higher than that of unconjugated MG2. in contrast, conjugation to a bombesin analog lacking the receptor-binding domain failed to increase the cytotoxicity of MG2, suggesting that bombesin conjugation enhances the cytotoxicity of MG2 in cancer cells through improved binding. Indeed, MG2B selectively induced cell death in cancer cells in vitro with the IC50 ranging from 10 to 15 pmol/L, which was about 6-10 times lower than the IC5o for normal cells. MG2B （20 mp=/kg per day, intratumorally injected for 5 d） also exhibited antitumor effects in mice bearing MCF-7 tumor grafts. The mean weights of tumor grafts in MG2B- and PBS-treated mice were 0.21±0.05 g and 0.59±0.12 g, respectively. Conclusion： The results suggest that conjugation of AMPs to bombesin might be an alternative approach for targeted cancer therapy.