18F- or 177Lu-labeled bivalent ligand of fibroblast activation protein with high tumor uptake and retention

PurposeFibroblast activation protein (FAP) has become a promising cancer-related target for diagnosis and therapy. The aim of this study was to develop a bivalent FAP ligand for both diagnostic PET imaging and endoradiotherapy.MethodsWe synthesized a bivalent FAP ligand (ND-bisFAP) and labeled it with 18F or 177Lu. FAP-positive A549-FAP cells were used to study competitive binding to FAP, cellular internalization, and efflux properties in vitro. Micro-PET imaging with [18F]AlF-ND-bisFAPI was conducted in mice bearing A549-FAP or U87MG tumors. Biodistribution and therapeutic efficacy of [177Lu]Lu-ND-bisFAPI were conducted in mice bearing A549-FAP tumors.ResultsThe FAP binding affinity of ND-bisFAPI is 0.25 ± 0.05 nM, eightfold higher in potency than the monomeric DOTA-FAPI-04 (IC50 = 2.0 ± 0.18 nM). In A549-FAP cells, ND-bisFAPI showed specific uptake, a high internalized fraction, and slow cellular efflux. Compared to the monomeric [18F]AlF-FAPI-42, micro-PET imaging with [18F]AlF-ND-bisFAPI showed higher specific tumor uptake and retention for at least 6 h. Biodistribution studies showed that [177Lu]Lu-ND-bisFAPI had higher tumor uptake than [177Lu]Lu-FAPI-04 at the 24, 72, 120, and 168 h time points (all P < 0.01). [177Lu]Lu-ND-bisFAPI delivered fourfold higher radiation than [177Lu]Lu-FAPI-04 to A549-FAP tumors. For the endoradiotherapy study, 37 MBq of [177Lu]Lu-ND-bisFAPI significantly reduced tumor growth compared to the same dose of [177Lu]Lu-FAPI-04. Half of the dose of [177Lu]Lu-ND-bisFAPI (18.5 MBq) has comparable median survival as 37 MBq of [177Lu]Lu-FAPI-04 (37 vs 36 days).ConclusionThe novel bivalent FAP ligand was developed as a theranostic radiopharmaceutical and showed promising properties including higher tumor uptake and retention compared to the established radioligands [18F]AlF-FAPI-42 and [177Lu]Lu-FAPI-04. Preliminary experiments with 18F- or 177Lu-labeled ND-bisFAPI showed promising imaging properties and favorable anti-tumor responses.