Mechanistic evaluation of the combination effect of paclitaxel (Pac) and bevacizumab (Bev) in breast cancer xenografts

Abstract

11007 Background: Chemosensitization is the ability to augment the effects of standard chemotherapy. One of the possible mechanisms for chemosensitization is improving intratumoral chemotherapy concentrations by increasing total or regional delivery of chemotherapy to the tumor. This mechanism potentially leads to a decreased tumor interstitial fluid pressure (IFP). This study evaluates the interaction between tumor IFP, intra-tumor drug accumulation and blood vessel perfusion. Methods: GILM2 human breast cancer (triple negative phenotype) cells were injected into the mammary fat pad of female nude mice. Mice were randomized to 4 arms: Control, Pac 24 mg/Kg, Bev 10 mg/Kg, and the same dose for the combination Pac + Bev, twice a week for 3 weeks. Tumor IFP was assessed using an ultraminiature catheter-tip technique (Ozerdem 2005). Concentration of Pac in tumor tissue was measured using liquid chromatography mass spectrometry (LC/MS) and blood vessel perfusion was assessed by dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI). Ultrastructural morphology changes were assessed by electron microscopy (EM). Results: Treatment started 35 days after implantation; tumor tissue concentration of Pac was significantly increased when combined with Bev (P = 0.03). In mice treated with the combination of Pac + Bev, IFP values decreased more gradually, with sustained and significant reduction at the end of the treatment (P = 0.0068; mean SEM day 0: 28,44 ± 6,1; mean SEM day 21: 7,41 ± 2,0). The IFP changes in the other 3 experimental arms did not reach statistical significance. The combination of Pac and Bev produced substantial changes to the proportions of endothelial cell/pericytes and increased apoptosis in tumor cells by 75% (detected by immunostaining for cleaved-caspase 3). Conclusions: The combination of Pac and Bev produces significant tumor reduction and increases three-fold the concentration of Pac in the tumor. This intra-tumor augmentation of Pac was correlated with the decrease of tumor IFP, which was independent of the action of Bev alone. Results of DCE-MRI and EM analyses of the treated tumors will also be presented. No significant financial relationships to disclose.