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Objectives: To characterise clinically translatable long-circulating (BR38) and VEGFR2-targeted (BR55) microbubbles (MB) and to assess their ability to discriminate breast cancer models with different aggressiveness.Methods: The circulation characteristics of BR38 and BR55 were investigated in healthy mice. The relative blood volume (rBV) of MDA-MB-231 (n = 5) or MCF-7 (n = 6) tumours was determined using BR38. In the same tumours in-vivo binding specificity of BR55 was tested and VEGFR2 expression assessed. Data validation included quantitative immunohistological analysis.Results: BR38 had a longer blood half-life than BR55 (>600 s vs. 218 s). BR38-enhanced ultrasound showed greater vascularisation in MDA-MB-231 tumours (p = 0.022), which was in line with immunohistology (p = 0.033). In-vivo competitive binding experiments proved the specificity of BR55 to VEGFR2 (p = 0.027). Binding of BR55 was significantly higher in MDA-MB-231 than in MCF-7 tumours (p = 0.049), which corresponded with the VEGFR2 levels found histologically (p = 0.015). However, differences became smaller when normalising the levels of BR55 to the rBV.Conclusions: BR38 and BR55 are well suited to characterising and distinguishing breast cancers with different angiogenesis and aggressiveness. Long-circulating BR38 MB allow extensive 3-dimensional examinations of larger or several organs. BR55 accumulation faithfully reflects the VEGFR2 status in tumours and depicts even small differences in angiogenesis. [ABSTRACT FROM AUTHOR]