Disulfide-constrained peptides ML-YSD-07 and ML-PD-03 enable PET imaging of cancer-associated fibroblasts via LRRC15 binding.

The tumor microenvironment (TME) plays a critical role in cancer progression and therapeutic resistance. Cancer-associated fibroblasts (CAFs) are key components of the TME, influencing tumor growth, metastasis, and immune evasion. Leucine-rich-repeat-containing protein 15 (LRRC15) is a promising biomarker, selectively expressed on CAFs, offering a target for non-invasive imaging. Current imaging methods often lack the specificity to accurately visualize CAF populations, hindering precise staging and monitoring of treatment response.

Researchers developed disulfide-constrained peptides (DCPs) using a combinatorial library approach, assisted by machine learning. They identified two lead candidates, ML-YSD-07 and ML-PD-03, and evaluated their binding affinity for murine LRRC15 (muLRRC15). The peptides' localization specificity was assessed on muLRRC15-expressing fibroblasts. For in vivo studies, PET imaging was performed using 18F-radiolabeled ML-YSD-07 in a murine pancreatic cancer model, chosen for its high enrichment of LRRC15-expressing CAFs. Additionally, crystal structures of apo-muLRRC15 and ML-YSD-07-bound muLRRC15 were determined to elucidate the binding mechanism.

The developed disulfide-constrained peptides, ML-YSD-07 and ML-PD-03, demonstrated subnanomolar affinities for muLRRC15. These peptides specifically localized onto muLRRC15-expressing fibroblasts in vitro. In a murine pancreatic cancer model, PET imaging with 18F-radiolabeled ML-YSD-07 exhibited specific tumor accumulation, indicating successful targeting of LRRC15-expressing CAFs. This specific accumulation highlights the potential for accurate visualization of the CAF-rich tumor microenvironment. Crystal structures of apo-muLRRC15 and ML-YSD-07-bound muLRRC15 revealed that the DCPs evolved to adopt a distinct binding conformation, efficiently interacting with a flat epitope on muLRRC15.