Efficient construction of PET/fluorescence probe based on sarcophagine cage: An opportunity to integrate diagnosis with treatment

Objectives The objective of this study was to develop a hetero-functional chelator, BaAn(Boc)Sar from sarcophagine cage as a general platform for dual-modality probe construction.

Methods The sarcophagine cage was conjugated with a carboxyl moiety and a Boc-protected aniline moiety to give BaAn(Boc)Sar chelator. To evaluate this novel chelator, we site-specifically conjugated RGD dimer, and Cy5.5 onto BaAn(Boc)Sar, which was subsequently labeled with ⁶⁴Cu to make ⁶⁴Cu-BaAnSar-RGD₂-Cy5.5. The resulting probe was tested in U87MG tumor bearing nude mice by microPET and fluorescence imaging.

Results BaAn(Boc)Sar could be modified with RGD dimer and Cy5.5 in high chemical yield. ⁶⁴Cu-BaAnSar-RGD₂-Cy5.5 was obtained in almost quantitative yield after mixing ⁶⁴Cu and BaAnSar-RGD₂-Cy5.5 in ammonium acetate buffer at 37 °C for 10 min. MicroPET analysis shows the U87MG tumor uptake is 6.41 ± 0.28, 6.51 ± 1.45, and 5.92 ± 1.57 %ID/g at 1, 4, and 20 h post injection, respectively. Fluorescence imaging study also demonstrated that the probe preferentially localized in U87MG tumor at all the time points examined. Furthermore, as a proof of principle imaging and surgery study, this novel probe was successfully used in visual-guided surgery for removal of U87MG tumor.

Conclusions We have successfully developed a BaAn(Boc)Sar cage for PET/optical probe construction. The favorable ⁶⁴Cu labeling property of cage-like Sarcophagine, plus the hetero functional groups on each side, made this BaAn(Boc)Sar chelator very attractive for dual modality imaging probe construction. The PET/optical tumor-targeting probes described here not only allow direct comparison between PET and fluorescence imaging, but also integrate the non-invasive imaging with image-guided surgery. The BaAn(Boc)Sar chelator developed herein has the potential to efficiently construct various tumor targeted dual modality probes for both basic research and clinical applications