FAPIBODY · Next-Generation FAP Tracers: Single-Domain Antibody–FAPI Conjugates for Cancer Theranostics

This proposal aims to create next-generation molecular imaging probes for cancer by targeting the fibroblast activation protein (FAP), a prominent biomarker of cancer-associated fibroblasts. FAP small-molecule inhibitors (FAPIs) have recently shown great promise for PET imaging, enabling sensitive visualization of tumors across multiple cancer types. However, despite their clinical impact, current FAPIs suffer from rapid systemic clearance and limited tumor retention, restricting their diagnostic sensitivity and therapeutic applicability. To address these shortcomings, this project introduces an innovative multivalent strategy that combines the potency of FAPIs with the specificity of single-domain antibodies (sdAbs). The proposal is structured around four main objectives: (1) systematic identification and characterization of sdAbs with strong affinity, high residence time, stability, and favorable biophysical properties; (2) chemical synthesis and optimization of sdAb–FAPI conjugates with tailored linkers to maximize stability and functionality; (3) structural and mechanistic validation of binding interactions using cryo-electron microscopy (cryo-EM), providing molecular-level insights for rational probe design; and (4) radiolabeling of the most promising conjugates and preclinical validation in PET/SPECT imaging to benchmark their performance against reference FAP tracers. By combining approaches from , chemical biology, structural biology, enzymology and radiopharmacy, the proposal will deliver innovative FAP-targeted probes with enhanced tumor retention, improved imaging quality, and expanded theranostic potential. These outcomes will not only advance cancer diagnostics but may also open new opportunities for treating diseases characterized by FAP expression.