18F-labeled ultrasmall nanoparticle probe for long-term PET tracking and real-time quantitative analysis of transplanted T lymphocytes in situ based on bioorthogonal reaction

The real-time, accurate understanding of the biodistribution of transplanted therapeutic cells is critical for their development and application. Positron emission tomography (PET) imaging has the potential to realize the noninvasive and quantitative evaluation of the spatial-temporal distribution of transplanted cells due to its unlimited tissue-penetration depth and quantitative capabilities, but its utility is limited by various shortcomings of existing imaging strategies or probes, such as the short half-life of the commonly used radioisotope fluorine-18 (¹⁸F) and the interference of high nonspecific background signals to the exact localization and quantification of target cells. Here, we describe a new strategy for the long-term PET tracking and real-time quantitative analysis of transplanted cells based on a tetrazine-bearing, ¹⁸F-labeled ultrasmall rare-earth nanoparticle probe (Tz-[¹⁸F]NaGdF₄) capable of labeling trans-cyclooctene (TCO) pretagged T lymphocytes via a bioorthogonal reaction at any point after their transplantation. The ultrasmall Tz-[¹⁸F]NaGdF₄ is readily prepared and characterized by rapid renal clearance and low nonspecific accumulation in major organs. The specific reaction between the Tz-[¹⁸F]NaGdF₄ probe and pretagged cells under physiological conditions enabled the specific PET imaging of target cells with low background in situ, and the measured PET image-derived standardized uptake value linearly and positively correlated with the number of pretagged T lymphocytes retained in tissues, ensuring their accurate quantification based on real-time PET signals. Therefore, the bioorthogonal reaction-based Tz-[¹⁸F]NaGdF₄-PET represents a useful tool for the long-term tracking and real-time quantification of transplanted cells in vivo.