Evaluation of the EPR Effect in the CAM-Model by Molecular Imaging with MRI and PET Using (89)Zr-Labeled HSA

SIMPLE SUMMARY: The CAM model is a promising alternative to murine models in terms of the 3Rs principles. However, its value for the noninvasive assessment of the biodistribution and accumulation of radiolabeled macromolecules by PET and MR imaging needs further evaluation. Thus, we analyzed the biodistribution and nonspecific tumor accumulation based on the EPR effect of (89)Zr-labeled human serum albumin by PET and MRI in the xenograft CAM model. The results were correlated to in vivo results from the xenografted mouse model as the standard of reference. In both models, the xenografted TZM-bl and PC-3 tumors were visualized by PET imaging after 24 h. Furthermore, no significant differences were detected concerning the influx kinetics of (89)Zr-labeled albumin into the two tumors. Therefore, the chicken model is a potential alternative to the animal model for initial PET studies on the characteristics of EPR-dependent target accumulation of radiolabeled macromolecules. ABSTRACT: Mouse models are commonly used to study the biodistribution of novel radioligands, but alternative models corresponding to the 3Rs principles, such as the chorioallantoic membrane (CAM) model, are highly required. While there are promising data from the CAM model regarding target-specific radiolabeled compounds, its utility for assessing macromolecule biodistribution and analyzing the EPR effect remains to demonstrated. Using (89)Zr-labeled human serum albumin, the accumulation of nontarget-specific macromolecules in CAM and mouse xenograft models was studied using PET and MRI. Therefore, the radioligand [(89)Zr]Zr-DFO-HSA was analyzed in both chicken embryos (n = 5) and SCID mice (n = 4), each with TZM-bl and PC-3 tumor entities. Dynamic PET and anatomical MRI, as well as ex vivo biodistribution analyses, were performed to assess ligand distribution over 24 h. Histological staining and autoradiography verified the intratumoral accumulation. The tumors were successfully visualized for CAM and mouse models by PET, and the albumin influx from the blood into the respective tumors did not differ significantly. The accumulation and retention of HSA in tumors due to the EPR effect was demonstrated for both models. These results highlight that the CAM model is a potential alternative to the mouse model for initial studies with novel radiolabeled macromolecules with respect to the 3Rs principles.