Molecular Ultrasound Imaging Using a Targeted Contrast Agent for Assessing Early Tumor Response to Antiangiogenic Therapy

Cancer accounts for nearly 1 in every 4 deaths in the United States and affects nearly 12 million people.1 Cancerous masses, or tumors, cannot survive in the body beyond 1 to 2 mm unless they are vascularized.2,3 Angiogenesis, the development of new capillary blood vessels, has been shown to be directly associated with malignancy and is necessary for cancer to survive and progress. Proper staging of cancer is critical in determining the appropriate method of therapy as well as accurately assessing prognosis. Drug treatments for cancer encompass two main categories: antimitotic and targeted. Antimitotic drugs, or chemotherapeutics, act by killing cells that are rapidly dividing. Targeted therapy, most commonly associated with antibody therapy, is more specific and uses monoclonal antibodies to target malignant cells. Antiangiogenic drugs, such as bevacizumab, are targeted vascular-disrupting agents that attempt to stop new blood vessel formation and may stop or retard the growth and spread of tumors. These antiangiogenic therapies have been used clinically in conjunction with chemotherapy to help treat cancer such as colon, breast, lung, and brain cancer.4 Specifically, bevacizumab has been proven in preclinical animal models to be effective within 7 days of treatment on both magnetic resonance5 and ultrasound (US)6,7 imaging. Ultrasound contrast agents are subcapillary-sized, gas-filled microbubbles small enough to pass through microvasculature.8,9 Contrast-enhanced US is the application of microbubbles to traditional US imaging, originally designed to improve visualization of echocardiograms.10 Contrast-enhanced US imaging has since been explored for numerous preclinical and clinical opportunities, such as detection and cancer staging. Other applications of microbubbles currently being explored include increasing drug delivery through microbubble-mediated US therapy and evaluation of the tumor response to therapy.11–13 Different cancers and their endothelial cells overexpress certain receptor proteins, which allows the application of microbubbles to be expanded to include the use of tumor vasculature marker targeting to provide added visualization of angiogenesis and vessel assessment.14–17 Molecular US imaging includes active microbubble targeting to single or multiple receptors to provide additional enhanced visualization for cancer detection, disease staging, or receptor profile analysis.15,18–20 A triple-targeted microbubble allows increased visualization of angiogenesis through increased receptor binding.13 Targeting microbubbles to P-selectin, vascular endothelial growth factor receptor 2 (VEGFR2), and αvβ3-integrin angiogenic molecular markers has been shownto effectively increase visualization of tumor vasculature by 60% over single-targeted strategies and 40% of dualtargeted strategies in preclinical breast cancer models.13 Other groups have also shown the advantages of using targeted microbubble strategies through enhanced visualization and assessment of tumor vascularity compared to traditional contrast-enhanced US.21–23 Molecular US imaging has potential to be used to develop targeted personalized diagnostics and therapy.24 In this study, we explored molecular US imaging of a triple-targeted microbubble for assessing the early tumor response to antiangiogenic therapy.