Evaluation of biodistribution and anti-tumor effect of a dimeric RGD peptide–paclitaxel conjugate in mice with breast cancer

Targeting drugs to receptors involved in tumor angiogenesis has been demonstrated as a novel and promising approach to improve cancer treatment. In this study, we evaluated the anti-tumor efficacy of a dimeric RGD peptide–paclitaxel conjugate (RGD2–PTX) in an orthotopic MDA-MB-435 breast cancer model.

To assess the effect of conjugation and the presence of drug moiety on the MDA-MB-435 tumor and normal tissue uptake, the biodistribution of ³H–RGD2–PTX was compared with that of ³H–PTX. The treatment effect of RGD2–PTX and RGD2+PTX was measured by tumor size, ¹⁸F-FDG/PET, ¹⁸F-FLT/PET, and postmortem histopathology.

By comparing the biodistribution of ³H–RGD2–PTX and ³H–PTX, we found that ³H–RGD2–PTX had higher initial tumor exposure dose and prolonged tumor retention than ³H–PTX. Metronomic low-dose treatment of breast cancer indicated that RGD2–PTX is significantly more effective than PTX+RGD2 combination and solvent control. Although in vivo ¹⁸F-FLT/PET imaging and ex vivo Ki67 staining indicated little effect of the PTX-based drug on cell proliferation, ¹⁸F-FDG/PET imaging showed significantly reduced tumor metabolism in the RGD2–PTX-treated mice versus those treated with RGD2+PTX and solvent control. Terminal uridine deoxynucleotidyl transferase dUTP nick-end labeling (TUNEL) staining also showed that RGD2–PTX treatment also had significantly higher cell apoptosis ratio than the other two groups. Moreover, the microvessel density was significantly reduced after RGD2–PTX treatment as determined by CD31 staining.

Our results demonstrate that integrin-targeted delivery of paclitaxel allows preferential cytotoxicity to integrin-expressing tumor cells and tumor vasculature. The targeted delivery strategies developed in this study may also be applied to other chemotherapeutics for selective tumor killing.