Determining the Multivalent Effects of d-Peptide-Based Radiotracers

Dextrorotary (d) peptides, composed of d-amino acids, are hyper-resistant to proteolytic hydrolysis, making them valuable ligands with excellent in vivo stability for radiopharmaceutical development. Multimerization is a well-established strategy for enhancing the in vivo performance of l-peptide-based radiopharmaceuticals. However, the effect of multimerization on the in vivo fate of d-peptide-based radiopharmaceuticals remains largely unexplored. Here, we synthesized the d-peptide DPA, which targets PD-L1, along with its dimer (DP2) and trimer (DP3). PET/CT imaging and ex vivo biodistribution studies were performed to delineate the pharmacokinetics and target interactions of [⁶⁸Ga]DPA, [⁶⁸Ga]DP2, and [⁶⁸Ga]DP3 in both normal and tumor-bearing mice. Our results revealed that tumor uptake and kidney retention increased with higher valency ([⁶⁸Ga]DP3 > [⁶⁸Ga]DP2 > [⁶⁸Ga]DPA). No significant differences were observed in the liver, heart, lung, spleen, intestine, or bone among the three radiotracers. Interestingly, a significant reduction of radioactivity in the bloodstream was detected for the [⁶⁸Ga]DP3-treated group compared to the other two groups. Data analysis revealed that chiral configuration of amino acids and the linking chemistry used in multimerization are the two dominant factors in the in vivo fate of d-peptide multimers. These findings indicate that d-peptide multimerization exerts a distinct influence on in vivo profiles compared to l-peptide multimerization. This study deepens our understanding of how mirror-imaged peptides/proteins interact with the living systems, paving the way for the development of radiopharmaceuticals that harness d-peptides as targeting moieties.