Preclinical Evaluation and a Pilot Clinical Positron Emission Tomography Imaging Study of [68Ga]Ga-FAPI-FUSCC-II

Based on small molecules: development and application of fibroblast activation protein inhibitors radiopharmaceutical in tumor precision therapy

The discovery of biomarkers for malignant tumors is driving the development of new radiopharmaceuticals in nuclear medicine. The development and optimization of novel radiopharmaceuticals to occupy an increasingly important role in tumor diagnosis and treatment. In recent years, fibroblast activation protein (FAP) has gained attention as a promising tumor target due to its widespread expression across various tumors. FAP inhibitor (FAPI) radiopharmaceuticals are considered to be the most promising to be developed for targeting FAP due to their rapid and specific tumor targeting. This review briefly outlines the developmental history of FAP-targeted small-molecule enzyme activity inhibitors, highlighting the effective role of targeting molecules, linkers, and certain functional groups in the delivery of radioisotopes to cancerous tissues. These development strategies will serve as a reference for the further development and application of relevant radiopharmaceuticals. This review also delineates the progress on clinical FAPI as a radioisotope delivery vehicle for the targeted radioligand therapy of tumors and introduces the latest combination therapy involving FAPI radiopharmaceutical for tumor treatment. The findings provide novel therapeutic insights into the targeted radioligand therapy of tumors.

Preclinical and First-In-Human Imaging of Novel [18F]F-FAPI-FUSCC-07 Tracer: Comparative Prospective Study with [18F]F-FAPI-42 and [18F]F-FAPI-74

This study aimed to develop and evaluate a novel fibroblast activation protein (FAP)-specific tracer, fluorine-18-labeled fibroblast activation protein inhibitor-FUSCC-07 ([18F]F-FAPI-FUSCC-07), for use in both preclinical and clinical settings. Preclinical evaluations were conducted to assess the stability and partition coefficient of [18F]F-FAPI-FUSCC-07. Experiments involving human glioma U87MG cells demonstrated its cellular uptake and inhibitory properties. Further investigations included biodistribution analysis and micropositron emission tomography/computed tomography (PET/CT) imaging in U87MG tumor-bearing mice, which revealed strong tumor uptake and prolonged retention. In the clinical setting, [18F]F-FAPI-FUSCC-07 was compared directly with [18F]F-FAPI-42 and [18F]F-FAPI-74 to evaluate its performance in imaging various cancers. By expanding the patient cohort, the study provided a more comprehensive assessment of tracer uptake in lesions. The findings demonstrated that [18F]F-FAPI-FUSCC-07 exhibited high stability in phosphate-buffered saline and fetal bovine serum, as well as hydrophilic properties. Clinical imaging results indicated significantly higher tumor uptake and improved target-to-blood pool ratios compared to the other tracers. Moreover, PET imaging of patients with diverse cancers showed that [18F]F-FAPI-FUSCC-07 consistently provided superior image contrast in most cases. These results represent the first clinical evidence supporting the feasibility of [18F]F-FAPI-FUSCC-07 for imaging across multiple tumor types. The study highlights its potential as a promising tracer for FAPI PET imaging, offering enhanced diagnostic precision and broader applicability in oncology.

Development of ibuprofen-modified fibroblast activation protein radioligands to improve cancer therapy

FAP-targeting radioligands are used in cancer diagnosis and therapy, but their effectiveness is limited by poor tumor uptake and retention. This study aimed to develop new radioligands using an optimized amino acid linker and ibuprofen for better pharmacokinetics. Three novel quinoline-based FAP ligands with an ibuprofen moiety were synthesized and radiolabeled with gallium-68 and lutetium-177. The synthesized FAP ligands FAPI-Ibu1, 2, 3 showed high binding affinity for FAP, with IC50 values of 1.17 ± 0.09, 0.29 ± 0.06, and 0.78 ± 0.12 nM, respectively. 177Lu-labeled FAP ligands showed stability in vitro and demonstrated significant binding to human plasma proteins as well as FAP specificity. PET imaging and biodistribution studies of 68Ga- or 177Lu-labeled FAPI-Ibu1, 2, 3 revealed improved tumor accumulation and retention. Dosimetry calculation showed that [177Lu]Lu-FAPI-Ibu3 delivered a 9.9-fold higher absorbed dose to tumor than [177Lu]Lu-FAPI-04, but only 2.6-fold higher absorbed dose to kidneys leading to 3.8-fold improvement in the tumor-to-kidney absorbed dose ratios. In the endoradiotherapy study, 18.5 MBq of [177Lu]Lu-FAPI-Ibu3 resulted in longer median survival than the equivalent dose of [177Lu]Lu-FAPI-04 (22 vs 16 days). Three ibuprofen-modified FAP radioligands significantly improved tumor uptake, retention, and growth suppression compared to [177Lu]Lu-FAPI-04, with [177Lu]Lu-FAPI-Ibu3 emerging as the most promising candidate for further clinical translational studies.

Elucidating the linagliptin and fibroblast activation protein binding mechanism through molecular dynamics and binding free energy analysis

Fibroblast activation protein (FAP) is highly expressed in solid tumors and may be a potential diagnostic and therapeutic target in solid cancers. Linagliptin inhibits FAP; however, the interaction mechanism between linagliptin and FAP remains unclear. In this study, the binding free energy for linagliptin with human FAP was estimated at -13.66 kcal/mol, and the dissociation constant was 243 nM based on surface plasmon resonance analyses. E203, E204, and Y656 formed hydrogen bonds with ammonium. Y625 formed an unstable hydrogen bond with the carbonyl group. W623 and Y541 interacted with the quinazoline and pyrimidine-2,4-dione rings, respectively, via π-π interactions. The butyne group formed hydrophobic interactions with residues V650, Y653, Y656, and Y660. ZINC000299754517 and ZINC000299754576 were identified as potential FAP inhibitors. The R1 and R4 regions of linagliptin could be optimized to increase its FAP binding affinity. These findings can guide linagliptin structural optimization to improve its FAP binding affinity.

Development of fibroblast activation protein-α radiopharmaceuticals: Recent advances and perspectives

Fibroblast activation protein-α (FAP) has emerged as a promising target in the field of radiopharmaceuticals due to its selective expression in cancer-associated fibroblasts (CAFs) and other pathological conditions involving fibrosis and inflammation. Recent advancements have focused on developing FAP-specific radioligands for diagnostic imaging and targeted radionuclide therapy. This perspective summarized the latest progress in FAP radiopharmaceutical development, highlighting novel radioligands, preclinical evaluations, and potential clinical applications. Additionally, we analyzed the advantages and existing problems of targeted FAP radiopharmaceuticals, and discussed the key breakthrough directions of this target, so as to improve the development and conversion of FAP-targeted radiopharmaceuticals.