(68)Ga-TRAP-(RGD)(3) Hybrid Imaging for the In Vivo Monitoring of α(v)ß(3)-Integrin Expression as Biomarker of Anti-Angiogenic Therapy Effects in Experimental Breast Cancer

OBJECTIVES: To investigate (68)Ga-TRAP-(RGD)(3) hybrid imaging for the in vivo monitoring of α(v)ß(3)-integrin expression as biomarker of anti-angiogenic therapy effects in experimental breast cancer. MATERIALS AND METHODS: Human breast cancer (MDA-MB-231) xenografts were implanted orthotopically into the mammary fat pads of n = 25 SCID mice. Transmission/emission scans (53 min to 90 min after i.v. injection of 20 MBq (68)Ga-TRAP-(RGD)(3)) were performed on a dedicated small animal PET before (day 0, baseline) and after (day 7, follow-up) a 1-week therapy with the VEGF antibody bevacizumab or placebo (imaging cohort n = 13; therapy n = 7, control n = 6). The target-to-background ratio (TBR, VOImax(tumor)/VOImean(muscle)) served as semiquantitative measure of tumor radiotracer uptake. Unenhanced CT data sets were subsequently acquired for anatomic coregistration and morphology-based tumor response assessments (CT volumetry). The imaging results were validated by multiparametric ex vivo immunohistochemistry (α(v)ß(3)-integrin, microvascular density–CD31, proliferation–Ki-67, apoptosis–TUNEL) conducted in a dedicated immunohistochemistry cohort (n = 12). RESULTS: (68)Ga-TRAP-(RGD)(3) binding was significantly reduced under VEGF inhibition and decreased in all bevacizumab-treated animals (ΔTBR(follow-up/baseline): therapy -1.07±0.83, control +0.32±1.01, p = 0.022). No intergroup difference in tumor volume development between day 0 and day 7 was observed (Δvolume(therapy) 134±77 μL, Δvolume(control) 132±56 μL, p = 1.000). Immunohistochemistry revealed a significant reduction of α(v)ß(3)-integrin expression (308±135 vs. 635±325, p = 0.03), microvascular density (CD31, 168±108 vs. 432±70, p = 0.002), proliferation (Ki-67, 5,195±1,002 vs. 7,574±418, p = 0.004) and significantly higher apoptosis (TUNEL, 14,432±1,974 vs. 3,776±1,378, p = 0.002) in the therapy compared to the control group. CONCLUSIONS: (68)Ga-TRAP-(RGD)(3) hybrid imaging allows for the in vivo assessment of α(v)ß(3)-integrin expression as biomarker of anti-angiogenic therapy effects in experimental breast cancer.