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

Abstract

Fibroblast 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. We 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. The 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). The 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.