The stimulator of interferon genes (STING) plays a vital role in innate immunity and is a promising anticancer target. Visualizing STING expression is essential for optimizing and guiding STING-targeted immunotherapies. This study aimed to design and develop macrocyclic peptide radiotracers for evaluating STING expression within the tumor microenvironment. A series of macrocyclic peptide-targeted STING radiotracers ( 68 GaGa-FMAD, 68 GaGa-FMTD, 68 GaGa-FMT2D, and 68 GaGa-FMT3D) were synthesized by modifying the amino acid sequences and linkers. The radiotracers exhibited high radiochemical yield, radiochemical purity, and molar activity. PET imaging of 68 GaGa-FMAD and 68 GaGa-FMTD showed suboptimal tumor uptake, indicating limited STING-targeting efficacy. 68 GaGa-FMT3D demonstrated tumor uptake comparable to that of 68 GaGa-FMT2D while exhibiting superior pharmacokinetic properties, as indicated by a higher tumor-to-muscle (T/M) ratio. The tumor uptake values of 68 GaGa-FMT2D and 68 GaGa-FMT3D significantly decreased after NNK-2 (STING-specific binding compound) preadministration, indicating STING-specific targeting. The competition assays demonstrated high affinities, with IC 50 values of 8.81 and 5.25 nmol/L for 68 GaGa-FMT2D and 68 GaGa-FMT3D, respectively. Furthermore, tumor uptake of 68 GaGa-FMT3D was significantly correlated with STING protein levels in the doxorubicin-treated model. Consequently, 68 GaGa-FMT3D, exhibiting high affinity for and specificity to STING, possesses favorable pharmacokinetics, serving as a promising tool for quantifying STING expression within the tumor microenvironment. Rational design of 68 Ga-labeled macrocyclic peptide for STING-targeted imaging in tumor microenvironment.
Zhang et al. (Sun,) studied this question.