An 125I-labeled octavalent peptide fluorescent nanoprobe for tumor-homing imaging in vivo

Targeting radiopeptides are promising agents for radio-theranostics. However, in vivo evaluation of their targeting specificity is often obscured by their short biologic half-lives and low binding affinities. Here, we report an approach to efficiently examine targeting radiopeptides with a new class of octavalent peptide fluorescent nanoprobe (Octa-FNP) platform, which is composed of candidate targeting peptides and a tetrameric far-red fluorescent protein (tfRFP) scaffold. To shed light on this process, 125I-Octa-FNP, 125I-tfRFP and 125I-peptide were synthesized, and their targeting functionalities were compared. Both fluorescence imaging and radioactive quantification results confirmed that 125I-Octa-FNP had a significantly higher cellular binding capability than 125I-tfRFP. In vivo biodistribution studies show that at 6 h post-injection, 125I-Octa-FNP had 2-fold and 30-fold higher tumor uptake than that of 125I-tfRFP and 125I-peptide, respectively. Moreover, γ-imaging at 24 h post-injection revealed a remarkable accumulation of 125I-Octa-FNP in the tumor while maintaining an extremely low background contrast, which was further confirmed by immunofluorescence analysis. These data suggested that, as an engineered and multivalent platform, Octa-FNP could enhance the tumor targeting of a designed peptide and provide excellent contrast radioimaging, making it a valuable tool for the evaluation of the targeting ability of specifically designed radiopeptides for cancer theranostics.