A radiohybrid theranostics ligand labeled with fluorine-18 and lutetium-177 for fibroblast activation protein-targeted imaging and radionuclide therapy

Abstract

Purpose A series of radiotracers targeting fibroblast activation protein (FAP) with great pharmacokinetics have been developed for cancer diagnosis and therapy. Nevertheless, the use of dominant PET tracers, gallium-68–labeled FAPI derivatives, was limited by the short nuclide half-life and production scale, and the therapeutic tracers exhibited rapid clearance and insufficient tumor retention. In this study, we developed a FAP targeting ligand, LuFL, containing organosilicon-based fluoride acceptor (SiFA) and DOTAGA chelator, capable of labeling fluorine-18 and lutetium-177 in one molecular with simple and highly efficient labeling procedure, to achieve cancer theranostics. Methods The precursor LuFL (20) and [natLu]Lu-LuFL (21) were successfully synthesized and labeled with fluorine-18 and lutetium-177 using a simple procedure. A series of cellular assays were performed to characterize the binding affinity and FAP specificity. PET imaging, SPECT imaging, and biodistribution studies were conducted to evaluate pharmacokinetics in HT-1080-FAP tumor-bearing nude mice. A comparison study of [177Lu]Lu-LuFL ([177Lu]21) and [177Lu]Lu-FAPI-04 was carried out in HT-1080-FAP xenografts to determine the cancer therapeutic efficacy. Results LuFL (20) and [natLu]Lu-LuFL (21) demonstrated excellent binding affinity towards FAP (IC50: 2.29 ± 1.12 nM and 2.53 ± 1.87 nM), compared to that of FAPI-04 (IC50: 6.69 ± 0.88 nM). In vitro cellular studies showed that 18F-/177Lu-labeled 21 displayed high specific uptake and internalization in HT-1080-FAP cells. Micro-PET, SPECT imaging and biodistribution studies with [18F]/[177Lu]21 revealed higher tumor uptake and longer tumor retention than those of [68 Ga]/[177Lu]Ga/Lu-FAPI-04. The radionuclide therapy studies showed significantly greater inhibition of tumor growth for the [177Lu]21 group, than for the control group and the [177Lu]Lu-FAPI-04 group. Conclusion The novel FAPI-based radiotracer containing SiFA and DOTAGA was developed as a theranostics radiopharmaceutical with simple and short labeling process, and showed promising properties including higher cellular uptake, better FAP binding affinity, higher tumor uptake and prolong retention compared to FAPI-04. Preliminary experiments with 18F- and 177Lu-labeled 21 showed promising tumor imaging properties and favorable anti-tumor efficacy.

Synthesis and Evaluation of [18F]SiFA-Conjugated Ligands for Fibroblast Activation Protein Imaging

“…In this study, we synthesized radioligands using a 18 F/ 19 F exchange reaction via SiFA conjugated to (Gla) n -FAPI ( n = 1 and 2) (Scheme ). Because of the highly lipophilic properties of SiFAs, polar functional groups are introduced into SiFA-conjugated biomolecules. − These polar groups can include hydrophilic chelators, acidic amino acids, carbohydrates, poly­(ethylene glycol) groups, and permanently positively charged groups. ,,− In this study, we introduced one and two Gla residues for the synthesis of ligands 1 and 2 (Schemes and ), respectively, because the Gla residue has one additional carboxylic acid compared to that of l -glutamic acid. In the 18 F/ 19 F exchange reactions with these ligands, the azeotropic distillation step was deleted because it could result in variable radiochemical yields and an increased radiolabeling time.…”

“…One of the 18 F/ 19 F exchange reactions involves the Si– 18 F labeling method, in which ligands containing a di- tert -butylfluorosilylphenyl group can be used as precursors for the isotope exchange reaction to provide stable silicon fluoride acceptor ([ 18 F]­SiFA)-conjugated ligands. − However, the addition of hydrophilic functionalities to [ 18 F]­SiFA-conjugated ligands is required because of the lipophilic nature of the di- tert -butylfluorosilylphenyl group. Recently, [ 18 F]­SiFA-conjugated biomolecules have been developed, and the most successful radioligand, [ 18 F]­SiTATE, is currently in clinical use for somatostatin receptor imaging. − …”

Simple and Versatile Pharmacokinetic Model for Radioligand Therapy

“…8 legend). However, for radioligands that bind to FAP-positive CAFs, the internalization ratio appears to be considerably lower [4, 5]. Accordingly, the retention time in tumor tissue is largely limited by radioligand dissociation from extracellular FAP.…”

“…Based on this reasoning, the larger of the exponential rate constants 𝛽 3 and 𝜅 𝑠 reported in Table 1 and Table 2 𝜅 ̃𝑒 for radioligands FAPI-04, Ga-3-3, Ga-6-3, Ga-8-1. [4]).…”

“…8 legend). However, for radioligands that bind to FAP-positive CAFs, the internalization ratio appears to be considerably lower [4,5] As can be gathered from equation (I) the effective clearance rate 𝜅 𝑒 can be lowered by reducing either 𝜅 𝑃𝐾 or 𝐾 𝐷 . Modification of radioligand chemistry affects both 𝜅 𝑃𝐾 and 𝐾 𝐷 .…”