Tetrazine s-TCO ligation: A simple approach to improve probe’s blood circulation?

Objectives During the last decade, numerous peptide-based agents have been developed and evaluated as PET imaging agents. However, one of the limitations associated with peptide probes is the short half-life in vivo. In this study, we introduce conformationally strained trans-cyclooctene-tetrazine-PEG (sTCO-Tz-PEG) system to increase probe’s circulation half-life in vivo.

Methods 18F-labeled sTCO was synthesized from a tosylate precursor. Tetrazine ligation was carried out with a cyclic RGD-tetrazine or NT-tetrazine analogue. The resulting imaging probe was subjected to micro PET studies in human U87MG and AsPC1 tumor bearing mice.

Results 18F-sTCO was synthesized with an isolation yield of 9.3+/- 2.4%. The sTCO tetrazine ligation can be done with good radiochemical yield and high purity. The tumor uptake of 18F-sTCO-Tz-PEG-RGD probe increased from 5.3+/- 0.2%ID/g at 1 h p.i. to 8.9+/- 0.5%ID/g at 4h p.i. The 18F-sTCO-Tz-PEG-NT agents have a tumor uptake of 4.3+/- 0.1%ID/g at 1 h p.i. and maintained at 4.4+/- 0.2% ID/g even at 6.5 h p.i. (compared with ~0.63%ID/g tumor uptake at 2h p.i. for our previously reported 18F-NT analog).

Conclusions A sTCO-Tz-PEG system was developed that demonstrated significantly increased in vivo blood circulation. The new platform hold the great potential to convert fast clearing peptides into PET probes with relatively long circulation time in vivo, which may improve the absolute tumor uptake.

• Download figure
• Open in new tab
• Download powerpoint