A highly innovative approach to ultrasound imaging involves the targeting and real-time, in vivo visualization of physiologic processes with molecular-specific imaging. Molecular-targeted microbubbles have been developed as an extension of CEUS
[23][24]. This could be of particular interest in the metastatic setting, where the response to systemic therapy is currently determined by the change in tumor volume. However, changes in tumor physiology occur sooner than measurable tumor volume changes, which might allow for earlier assessment of tumor progression, response to systemic therapy and, ultimately, therapeutic decision making. Moreover, the use of molecular imaging techniques might better characterize features relating to intrinsic disease biology, such as angiogenesis, potentially leading to more individualized treatment decision making
[25][26]. In a recent report, Rojas et al. studied ccRCC in a xenograft model of immunodeficient mice treated with the anti-angiogenic vascular endothelial growth factor receptor (VEGFR) tyrosine kinase inhibitor sunitinib
[24]. They administered a microbubble contrast agent targeted to VEGFR-2 and subsequently imaged the tumors with CEUS after 1 week of treatment. They reported changes in VEGFR-2 expression at that time, as determined on ultrasound molecular imaging in the sunitinib-treated group, as opposed to changes in tumor volume, which only became apparent after 3 weeks. Moreover, after 1 week, response to therapy was detected in 92% of cases with ultrasound molecular imaging, whereas the detection rate was only 40% with volume measurements. Likewise, Ingels et al. studied the potential of ultrasound molecular imaging to track the response to sunitinib in a ccRCC mice xenograft model
[23]. These mice, harboring ccRCC, were randomized between treatment with sunitinib and control and were injected with both non-targeted microbubbles and microbubbles targeting VEGFR-1 and follicle-stimulating hormone receptor (FSHR). Both the VEGFR-1 and FSHR signal enhancement were significantly lower in the sunitinib group at all times of treatment, while there was no significant difference between the two groups for the non-targeted microbubble ultrasound signal. Thus, they confirmed the potential of ultrasound molecular imaging for the longitudinal assessment of treatment response to sunitinib. However, despite its potential for serial monitoring of disease, as well as longitudinal assessment of disease response to systemic therapy, ultrasound molecular imaging is still in the very early phases of development and further research endeavors will determine whether these techniques can provide additional value in clinical practice.