Sentinel node mapping with thermoresponsive magnetic nanoparticles in rats

Background: In this study, we investigated the ability of magnetic resonance imaging (MRI) after interstitial administration of thermoresponsive magnetic nanoparticles to detect the sentinel lymph node (SLN). Materials and Methods: Postcontrast MRI scans were acquired following subcutaneous injection of thermoresponsive magnetic nanoparticles into the thoracic wall of rats. The signal-to-noise ratio of axillary lymph nodes was calculated to assess whether the SLN could be detected by MRI. In a second experiment, after injecting thermoresponsive magnetic nanoparticles, i.e., Therma-Max 36, Therma-Max 42, Therma-Max 55, and Ferridex, into the subserosa of the cecum of rats, the injection sites, the SLNs, and the distant lymph nodes were resected and examined histologically in order to determine which nanoparticles, if any, were specifically retained in the SLN. Results: MRI showed that the signal-to-noise ratio of axillary SLNs was significantly lower 24 h after injection of Therma-Max 42 than on the precontrast images (P < 0.05). Histologic evaluation revealed that Therma-Max 36 aggregated at body temperature and did not migrate to the SLN. Therma-Max 42, on the other hand, aggregated, and the particles became large enough to be retained in the SLNs. Therma-Max 55 and Ferridex did not aggregate, and they both migrated to the SLNs and the distant lymph nodes. Conclusions: The results of this study showed that thermoresponsive magnetic nanoparticles could be targeted to the SLN by adjusting the temperature at which they aggregate, and that they could be used as a contrast agent for SLN mapping by MRI.