Transforming growth factor-β (TGF-β) promotes cancer invasion and metastasis and is therefore a potential drug target for cancer treatment. Fresolimumab, which neutralizes all mammalian active isoforms of TGF-β, was radiolabeled with (89)Zr for PET to analyze TGF-β expression, antibody tumor uptake, and organ distribution.
Methods: (89)Zr was conjugated to fresolimumab using the chelator N-succinyldesferrioxamine-B-tetrafluorphenol. (89)Zr-fresolimumab was analyzed for conjugation ratio, aggregation, radiochemical purity, stability, and immunoreactivity. (89)Zr-fresolimumab tumor uptake and organ distribution were assessed using 3 protein doses (10, 50, and 100 μg) and compared with (111)In-IgG in a human TGF-β-transfected Chinese hamster ovary xenograft model, human breast cancer MDA-MB-231 xenograft, and metastatic model. Latent and active TGF-β1 expression was analyzed in tissue homogenates with enzyme-linked immunosorbent assay.
Results: (89)Zr was labeled to fresolimumab with high specific activity (>1 GBq/mg), high yield, and high purity. In vitro validation of (89)Zr-fresolimumab showed a fully preserved immunoreactivity and long (>1 wk) stability in solution and in human serum. In vivo validation showed an (89)Zr-fresolimumab distribution similar to IgG in most organs, except for a higher uptake in the liver in all mice and higher kidney uptake in the 10-μg group. (89)Zr-fresolimumab induced no toxicity in mice; it accumulated in primary tumors and metastases in a manner similar to IgG. Both latent and active TGF-β was detected in tumor homogenates, whereas only latent TGF-β could be detected in liver homogenates. Remarkably high (89)Zr-fresolimumab uptake was seen in sites of tumor ulceration and in scar tissue, processes in which TGF-β is known to be highly active.
Conclusion: Fresolimumab tumor uptake and organ distribution can be visualized and quantified with (89)Zr-fresolimumab PET. This technique will be used to guide further clinical development of fresolimumab and could possibly identify patients most likely to benefit.