Characterization of sentinel lymph nodes using dual-targeted microbubbles

The purpose of this ongoing study is to evaluate the ability of molecular ultrasound imaging to detect the metastatic involvement in the sentinel lymph node (SLN) of melanoma. The SLN is the first node along the lymphatic channel from the primary tumor and accurate characterization of the SLN helps in the clinical management of melanoma. To date, eight swine (3–7 kg; Sinclair BioResources, Columbia, MO, USA) with naturally occurring melanoma have been studied. Contrast enhanced ultrasound on a S3000 scanner with a 9L4 probe (Siemens Healthineers, Mountain View, CA, USA) was used for imaging. Dual-targeted microbubbles were created with Targestar SA (Targeson, San Diego, CA, USA) labeled with P-selectin and αVβ3-integrin. IgG labeled Targestar SA was used as control. A two stage imaging approach was used to first identify and then characterize SLNs. First, 0.25 ml of Sonazoid (GE Healthcare, Oslo, Norway) was injected around the tumor and SLNs were identified. Next, dual-targeted and control microbubbles were injected IV with a 30 min interval between injections. Agents were allowed to circulate for 4 min to enable binding. Then 2 sets of image clips were collected before and after a high-power destruction sequence. The mean intensity difference in pre- and post- destruction images was calculated as a relative bubble retention measure. Some non-SLNs were also imaged in this fashion as control. All examined nodes were dissected and histologically examined for the presence of metastatic involvement. A total of 30 lymph nodes (16 SLNs and 14 non-SLNs) were analyzed with 10 SLNs showing metastatic involvement greater than 5%. All non-SLNs were benign. The mean intensity of the dual-targeted bubbles was significantly higher than that of IgG control bubbles in the metastatic SLNs (19.23 ± 23.95 AU vs. 0.20 ± 0.26 AU; p=0.03). Benign nodes did not show significant difference in the mean intensity of the dual-targeted and control bubbles (0.85 ± 1.81 AU vs. 0.03 ± 0.26 AU; p=0.07). Additionally, the mean intensity of dual-targeted bubbles for metastatic nodes was significantly different from that of benign nodes (p=0.002), while control bubbles did not differentiate between metastatic and benign nodes (p=0.09). The results indicate that dual-targeted microbubbles labeled with P-selectin and αVβ3-integrin can help to characterize SLNs.