αvß3-Integrin–Targeted Magnetic Resonance Imaging for the Assessment of Early Antiangiogenic Therapy Effects in Orthotopic Breast Cancer Xenografts

OBJECTIVESThe aim of this study was to investigate magnetic resonance imaging (MRI) with αvß3-integrin–targeted ultrasmall superparamagnetic iron oxide nanoparticles (RGD-USPIO) for the in vivo monitoring of early antiangiogenic therapy effects in experimental breast cancer. MATERIALS AND METHODSOrthotopic human breast cancer (MDA-MB-231) xenograft-bearing severe combined immunodeficiency mice were imaged before and after a 1-week therapy with the vascular endothelial growth factor receptor-antibody bevacizumab or placebo (n = 10 per group, daily intraperitoneal injections of bevacizumab or a volume-equivalent placebo solution, respectively) on a clinical 3 T scanner (Magnetom Skyra; Siemens Healthcare, Erlangen, Germany) before and 60 minutes after the intravenous injection of RGD-USPIO (P04000; Guerbet, Villepinte, France). R2 relaxometry employing a T2-weighted spin-echo sequence with 4 echo times (echo time, 20/40/60/80 milliseconds; repetition time, 3800 milliseconds; matrix, 128 × 128; field of view, 50 × 50; slice thickness, 1.2 mm; time to acquisition, 25 minutes) was used as semiquantitative measure to determine RGD-USPIO endothelial binding. In addition, the T2-weighted images were used to perform volumetric tumor response assessments. Imaging results were validated by ex vivo multiparametric immunohistochemistry with regard to αvß3-integrin expression, microvascular density (CD31), proliferation (Ki-67), and apoptosis (TUNEL). RESULTSRGD-USPIO endothelial binding was significantly reduced after vascular endothelial growth factor inhibition, compared with the control group in which an increased endothelial binding was detected ([INCREMENT]R2Therapy = −0.80 ± 0.78 s; [INCREMENT]R2Control = +0.27 ± 0.59 s; P = 0.002). Correspondingly, immunohistochemistry revealed a significantly lower αvß3-integrin expression (91 ± 30 vs 357 ± 72; P < 0.001), microvascular density (CD31, 109 ± 46 vs 440 ± 208; P < 0.001), tumor cell proliferation (Ki-67, 4040 ± 1373 vs 6530 ± 1217; P < 0.001), as well as significantly higher apoptosis (TUNEL, 11186 ± 4387 vs 4017 ± 1191; P = 0.004) in the therapy compared with the control group. Contrary to the changes in αvß3-integrin expression detected by RGD-USPIO MRI, morphology-based tumor response assessments did not show a significant intergroup difference in tumor volume development over the course of the experiment (ΔVolTherapy +71 ± 40 μL vs ΔVolControl +125 ± 81 μL; P > 0.05). CONCLUSIONSRGD-USPIO MRI allows for the no