Preclinical Identification Of Tumor-Draining Lymph Nodes Using a Multimodal Non-invasive In vivo Imaging Approach

Resection of the tumor-draining lymph -node (TDLN) represents a standard method to identify metastasis for several malignancies. Interestingly, recent preclinical studies indicate that TDLN resection diminishes the efficacy of immune checkpoint inhibitor-based cancer immunotherapies. Thus, accurate preclinical identification of TDLNs is pivotal to uncovering the underlying immunological mechanisms. Therefore, we validated preclinically, and clinically available non-invasive in vivo imaging approaches for precise TDLN identification.

For visualization of the lymphatic drainage into the TDLNs by non-invasive in vivo optical imaging, we injected the optical imaging contrast agents Patent Blue V (582.7 g mol^-1) and IRDye® 800CW polyethylene glycol (PEG; 25,000-60,000 g mol^-1), subcutaneously (s.c.) in close proximity to MC38 adenocarcinomas at the right flank of experimental mice. For determination of the lymphatic drainage and the glucose metabolism in TDLNs by non-invasive in vivo PET/magnetic resonance imaging (PET/MRI), we injected the positron emission tomography (PET) tracer (2-deoxy-2[¹⁸F]fluoro-D-glucose (¹⁸F-FDG) [181.1 g mol^-1]) in a similar manner. For ex vivo cross-correlation, we isolated TDLNs and contralateral nontumor-draining lymph nodes (NTDLNs) and performed optical imaging, biodistribution, and autoradiography analysis.

The clinically well-established Patent Blue V was superior for intraoperative macroscopic identification of the TDLNs compared with IRDye® 800CW PEG but was not sensitive enough for non-invasive in vivo detection by optical imaging. Ex vivo Patent Blue V biodistribution analysis clearly identified the right accessory axillary and the proper axillary lymph node (LN) as TDLNs, whereas ex vivo IRDye® 800CW PEG completely failed. In contrast, functional non-invasive in vivo ¹⁸F-FDG PET/MRI identified a significantly elevated uptake exclusively within the ipsilateral accessory axillary TDLN of experimental mice and was able to differentiate between the accessory axillary and the proper LN. Ex vivo biodistribution and autoradiography confirmed our in vivo ¹⁸F-FDG PET/MRI results.

When taken together, our results demonstrate the feasibility of ¹⁸F-FDG-PET/MRI as a valid method for non-invasive in vivo, intraoperative, and ex vivo identification of the lymphatic drainage and glucose metabolism within the TDLNs. In addition, using Patent Blue V provides additive value for the macroscopic localization of the lymphatic drainage both visually and by ex vivo optical imaging analysis. Thus, both methods are valuable, easy to implement, and cost-effective for preclinical identification of the TDLN.