Improving Tumor-to-Background Contrast Through Hydrophilic Tetrazines: The Construction of 18F Labeled PET Agents Targeting Non-Small Cell Lung Carcinoma

Bioorthogonal reactions have been widely used in biomedical field. 18 F-TCO/Tetrazine ligation is the most reactive radiolabelled inverse electron demand Diels-Alder reactions, but its application had been limited due to modest contrast ratios of the resulting conjugates. Here we describe the use of hydrophilic tetrazines to improve tumor to background contrast of neurotensin receptor targeted PET agents. PET agents were constructed using a rapid Diels-Alder reaction of the radiolabeled trans- cyclooctene ( 18 F-sTCO) with neurotensin (NT) conjugates of a 3,6-diaryltetrazine, 3-methyl-6-aryltetrazine, and a derivative of 3,6-di-(2-hydroxyethyl)tetrazine. Although cell binding assay demonstrated all agents have comparable binding affinity, the conjugate derived from 3,6-di-(2-hydroxyethyl)tetrazine demonstrated the highest tumor to muscle contrast, followed by conjugates of the 3-methyl-6-aryltetrazine and 3,6-diaryltetrazine. Hydrophilic tetrazines lead to improved tumor to background contrast in PET probe construction.