Development and Preclinical Evaluation of ⁶⁸ Ga-Labeled B7-H3-Specific Bicyclic Peptides as Immuno-PET Tracers for Oncology

B7-H3 (CD276) is an emerging target for cancer theranostics, highlighting the need for imaging probes capable of noninvasively quantifying B7-H3 expression in tumors. Here, we developed three 68 Ga-labeled B7-H3-targeting bicyclic peptide tracers with different PEG linker lengths. All tracers showed high radiochemical purity (>96%), favorable in vitro stability, and rapid blood clearance. Among them, [ 68 Ga]Ga-B7H3-FZ1 exhibited the highest affinity ( K D = 83.22 nM). Micro-PET/CT imaging demonstrated that tumor uptake of [ 68 Ga]Ga-B7H3-FZ1 correlated positively with B7-H3 expression across multiple tumor models. In H1299 tumors. B7-H3 overexpression increased uptake from 1.09 ± 0.18 to 3.50 ± 0.97%ID/g at 30 min postinjection, confirming target specificity. Biosafety studies indicated no obvious toxicity. These results support [ 68 Ga]Ga-B7H3-FZ1 as a promising PET tracer for noninvasive B7–H3 imaging.