Hyaluronan conjugated with IR-783

The primary function of the lymphatic system is to drain ~10% of the interstitial fluid from small capillaries to lymphatic vessels through lymph nodes (LNs) and finally to the venous system (1-5). LNs form a natural filter for lymphatic drainage and prevent the possible migration of cancer cells from the lymphatic system into the rest of the body. As the first LN that receives lymph drainage from a tumor bed, the sentinel LN is very likely to contain cancer cells if the primary tumor has spread the lymphatic system (1, 2). The lymphatic system is complex, and its detection and mapping remain challenging (1, 2, 6, 7). However, with advances of new imaging agents and techniques, imaging and mapping of both the lymphatic vessels and the LNs are now possible with x-ray–computed tomography, ultrasound, nuclear medicine, and magnetic resonance imaging (7-9). There is increasing evidence that tumor cells induce lymphangiogenesis in tumor-draining LNs in experimental models of cancer (10, 11) as well as in LNs of patients with metastatic melanoma and breast cancer (12, 13). Therefore, LN lymphangiogenesis might provide a target to image the early stages of tumor metastasis. In addition, chronic inflamed tissues produce vascular endothelial growth factor to induce lymphangiogenesis in draining LNs. Hyaluronan (HA) is an abundant extracellular matrix glycosaminoglycan in skin and mesenchymal tissues, where it facilitates cell migration during inflammation, wound healing, metastasis, and embryonic morphogenesis (14, 15). CD44 is an integral cell membrane glycoprotein on leukocytes and has an important role in matrix adhesion, lymphocyte activation, and leukocyte LN homing (16). CD44 binds HA to induce extravasation of lymphoid cells. Lymphatic vessel endothelial hyaluronan receptor-1 (LYVE-1) has a 41% homology to CD44, and LYVE-1 expression is largely restricted to endothelial cells of lymphatic vessels and splenic sinusoidal endothelial cells. Expression is undetectable on lymphocytes, hematopoietic cells, or vascular endothelial cells. Among various near-infrared agents, only indocyanine green (ICG), with absorption at 780 nm and emission at 820 nm, is approved by the United States Food and Drug Administration for clinical applications in angiography, blood flow evaluation, and liver function assessment (17). Sharma et al. (18) conjugated HA with near-infrared dye IR-783 to form IR783-HA for imaging of the lymphatic vasculature in pigs.