Cancer

Contrast Ultrasound for non-invasive Imaging of Solid Tumors

Microbubble ultrasound contrast agents have been used for qualitative assessment of solid lesions in a clinical setting for several years. The development of targeted microbubbles has enabled the use of this technology for molecular imaging in the context of experimental research. Ultrasound is unique among the various imaging modalities available for preclinical use in the cancer field in that it is able to evaluate tumor anatomy, in addition to molecular and blood pool imaging, with the same scanner. Ultrasound imaging does not require ionizing radiation, and contrast exams can typically be completed in a matter of minutes.

Imaging Tumor Perfusion with Contrast Ultrasound

The size of Targeson’s microbubble contrast agents (1-8 um) renders them purely intravascular flow tracers. Agents are typically administered intravascularly as a bolus, and several quantitative parameters (such as the amplitude of the contrast enhancement and the time taken to peak enhancement or clearance) have been shown to be reproducible measurements of tumor perfusion.

Figure 1. Contrast ultrasound imaging of subcutaneous tumors in mice. Leftmost panel shows B-mode scan of human bladder cancer tumor (MB49) on flank of mouse (tumor circled in blue). Subsequent panels are representative contrast scans of three tumors 15s after administration of Targestar®-P microbubble contrast agent. Regions of tumor that are not perfused (highlighted by red circle in second panel) are easily discriminated from those receiving blood flow. Images were acquired with Sequoia (Siemens) at 14 MHz.

Molecular Imaging of Tumor Angiogenesis with Contrast Ultrasound

Techniques for molecular imaging of angiogenesis have applicability to both basic science research and drug development, with particular importance to the field of cancer research. Several molecular markers of angiogenesis are known to be expressed on the vascular endothelial cells lining tumor vessels, but not healthy vessels. When expressed on the luminal surface, these markers thus available for molecular imaging by ultrasound. In particular, the vascular endothelial growth factor receptor-2 (VEGFR2) and alpha-v beta-3 integrin have shown utility as targets for ultrasound molecular imaging in several tumor models. VEGFR2 is a key imaging biomarker of angiogenesis, and has particular relevance in the field of anti-angiogenic therapy. Targeson’s Visistar VEGFR2 is a microbubble coated with a VEGFR2-binding protein that enables specific detection of VEGFR2 expression on angiogenic tumor endothelium. Like VEGFR2, the alpha-v beta-3 (αVβ3) integrin has been shown to be over-expressed on angiogenic endothelium. Targeson’s Visistar Integrin contains a cyclic RGD-containing peptide covalently conjugated to the surface of the microbubble. The high copy number of the ligand enables binding of the microbubble to alpha-v integrins. Visistar Integrin has been validated for imaging alpha-v beta-3 in mouse models of prostate (PC-3), bladder (MB-49), bowel (MC-38) and breast (Met-1) cancer (Figure 2). Because the integrin-targeting ligand on Visistar Integrin is a small molecule and not an antibody, Visistar Integrin and is expected to work in multiple models of angiogenesis across species.

Figure 2. Imaging tumor angiogenesis with Visistar VEGFR2 and Visistar Integrin. Left: Visistar VEGFR2 accumulation in a subcutaneous mouse model of colon cancer (line MC-38). Middle: Visistar Integrin accumulation in an orthotopic mouse model of breast cancer (line Met-1). Bottom panel on both columns shows representative image after administration of control microbubbles. Right: quantitated contrast signal within Met-1 tumors (n=5 mice). Image Credits: K. Ferrara and X. Hu (University of California, Davis); S. Klibanov and R. Price (University of Virginia).