Fibroblast Activation Protein Inhibitor Tracers and Their Preclinical, Translational, and Clinical Status in China

Pharmacokinetics study of FT-FAPI, a novel multi-nuclide label-able FAP targeting tracer, in mice and healthy volunteers

PURPOSE

Fibroblast activation protein (FAP) has emerged as one of the most promising theranostic targets. FT-FAPI, a potential FAP-targeted probe with enhanced tumor-targeting ability using an organotrifluoroborate linker, was verified for the biological effect in cell and animal experiments in previous work. However, the differences in pharmacokinetic profiles, biodistribution and dosimetry of FT-FAPI under multiple nuclides labeling in animals and humans is unclear. In this study, we sought to explore the discrepancies in the performance of FT-FAPI after labeling with different radioisotopes and to compare 68Ga-FT-FAPI with 68Ga-FAPI-04 in healthy volunteers.

METHODS

Preparation and quality control of 18F-/68Ga-/177Lu-FT-FAPI injections were conducted. Biodistribution studies were performed in mice and pharmacokinetic analysis were performed using blood samples. 18F-FT-FAPI and 68Ga-FT-FAPI were initially tested in normal volunteers, respectively. Further, 68Ga-FT-FAPI was scanned at multiple time points for 1 h dynamic, 2 h and 3 h static imaging, and compared with 68Ga-FAPI-04 to clarify the distribution patterns and excretion parameters. Radiation dosimetry was estimated based on the uptake of the probes in organs of mice or humans, respectively.

RESULTS

18F-/68Ga-/177Lu-FT-FAPI showed high safety and tolerability in mice or humans. The pharmacokinetic characteristics of FT-FAPI in mice and humans best fit a two-compartment model. In mice, the results of biodistribution showed that the clearance half-life (T1/2β) of FT-FAPI varied with different labeled radionuclides, with 68Ga-FT-FAPI having the shortest T1/2β of 26.1 min. In HT-1080-FAP tumor-bearing mice, 177Lu-FT-FAPI showed higher tumor uptake and longer retention time than 177Lu-FAPI-04, which implies a higher radiation dose in the tumor. In healthy volunteers, both 18F-FAPI-04 and 68Ga-FT-FAPI were metabolized by the kidneys and excreted through the urinary system. The uptake of 68Ga-FT-FAPI in most normal tissues was similar to that of 68Ga-FAPI-04, being higher only in the sublingual gland, thyroid gland and pancreas. 68Ga-FT-FAPI had similar T1/2β (75.0 min vs. 77.2 min) and a slightly higher effective dose (ED) (12.4 ± 1.51 µSv·MBq-1 vs. 9.99 ± 1.85 µSv·MBq-1) compared to 68Ga-FAPI-04. Compared to 68Ga-FT-FAPI, 18F-FT-FAPI has slightly higher liver and bone uptake, especially in the delayed time points.

CONCLUSION

The pharmacokinetics of FT-FPAI varies depending on the labeled nuclide. Compared to 177Lu-FAPI-04, 177Lu-FT-FAPI showed higher tumor uptake and longer retention time. In healthy volunteers, 68Ga-FT-FAPI had lower renal uptake and higher sublingual gland, thyroid gland and pancreas uptake at 1 h p.i. than that of 68Ga-FAPI-04, with no significant differences in other organs. Further optimization will be conducted for the radiolabeling process of 18F-FT-FAPI to evaluate its diagnostic efficacy.

[68Ga]Ga‑FAPI PET/CT in the evaluation of Langerhans cell histiocytosis: comparison with [18F]FDG PET/CT

PURPOSE

We aimed to explore the value of [68Ga]Ga‑FAPI PET/CT for the evaluation of Langerhans cell histiocytosis (LCH) in comparison with [18F]FDG PET/CT.

METHODS

Thirty-two patients pathologically diagnosed with LCH were enrolled in this study. [68Ga]Ga‑FAPI and [18F]FDG PET/CT were performed within 1 week to identify disease extent and status. The detectability and intensity of the involved organs were compared between these two tracers.

RESULTS

Thirty patients had active disease while two had non-active disease. In patients with active disease, the most commonly involved organ was bone (27/30), and [68Ga]Ga-FAPI PET/CT detected more osseous lesions (106/106) than [18F]FDG PET/CT (52/106). [68Ga]Ga-FAPI also identified liver, skin, and salivary gland involvement, which were often missed by [18F]FDG. Although both tracers detected thymus and pituitary gland involvement, [68Ga]Ga-FAPI demonstrated higher image contrast and more diagnostic confidence. Lymph node involvement, however, was not visualized by [68Ga]Ga-FAPI. Due to the superior sensitivity of [68Ga]Ga-FAPI, approximately 30% (10/30) of patients experienced reclassification in disease status or subtype. Furthermore, [68Ga]Ga‑FAPI appeared to be advantageous in response assessment.

CONCLUSION

[68Ga]Ga-FAPI PET/CT outperforms [18F]FDG PET/CT in detecting osseous and extra-nodal lesions in LCH, providing a valuable tool for precise disease evaluation and treatment planning.

99mTc-Labeled Quinolone-Based Novel Skeletal Tracers for Tumor Visualization through Fibroblast Activation Protein

Fibroblast activation protein (FAP) has emerged as a prominent target for tumor diagnosis. Quinoline-based FAP PET tracers demonstrated clinical feasibility. However, there is a relative scarcity of clinical studies on 99mTc-labeled FAP SPECT tracers. The existing quinoline-derived 99mTc-FAPI tracer exhibits relatively low tumor uptake and suboptimal pharmacokinetic properties, which restrict its clinical application. Consequently, it is necessary to alter the pharmacophores to improve its druggability. In this study, a novel quinolone-based pharmacophore was developed by utilizing scaffold hopping and conformational constrained strategies. Serial screening and preclinical evaluations were conducted. The 99mTc-FAPI-YQ3 showed extremely high tumor uptake and excellent pharmacokinetic properties. Additionally, 99mTc-FAPI-YQ3 demonstrated reliable safety characteristics and clinical efficacy on four different oncology patients. In conclusion, 99mTc-FAPI-YQ3 was a promising radiotracer for FAP-targeted cancer diagnosis, shedding light on substantially advancing SPECT molecular imaging.

Semiquantitative muscle parameters derived from FAPI and FDG PET/CT in evaluating sarcopenia among patients with malignant tumors

BACKGROUND

The objective of this study is to explore and compare the potential utility of fibroblast activation protein inhibitor (FAPI) and fluorodeoxyglucose PET/computed tomography (CT) in assessing sarcopenia among patients with malignant tumors.

METHODS

A retrospective analysis was conducted on 127 patients with histologically confirmed malignant tumors who underwent both 18 F/ 68 Ga-FAPI and fluorine-18-fluorodeoxyglucose ( 18 F-FDG) PET/CT scans. Clinical characteristics and PET/CT parameters of maximum and mean standard uptake value (SUV max and SUV mean ) of muscle at the 3 rd lumbar (L3) level were reviewed. Skeletal muscle area at the L3 level was measured, and skeletal muscle index was calculated to determine sarcopenia. The association between sarcopenia and PET/CT parameters was analyzed.

RESULTS

The incidence of sarcopenia was 41.7% among these 127 patients. Higher age, male, lower BMI, lower SUV max and SUV mean of muscle from 18 F/ 68 Ga-FAPI PET/CT, and lower SUV max of muscle from 18 F-FDG PET/CT were correlated with a higher prevalence of sarcopenia ( P  < 0.05). Besides, no significant differences in SUV max and SUV mean of muscle were noted between 18 F-FAPI and 68 Ga-FAPI groups. The best cutoff value of SUV max of muscle from 18 F/ 68 Ga-FAPI PET/CT was 1.17, yielding the area under the curve (AUC) of 0.764 and sensitivity and specificity of 74.3% and 71.7%, while the optimal cutoff value of SUV max of muscle from 18 F-FDG PET/CT was 0.76, with an AUC of 0.642 and sensitivity and specificity of 36.5% and 86.8%, respectively.

CONCLUSION

Patients with sarcopenia exhibit decreased muscle uptake of FAPI and fluorodeoxyglucose. FAPI PET/CT emerges as a more valuable tool for sarcopenia assessment in patients with malignant tumors compared to fluorodeoxyglucose PET/CT.

Preclinical Evaluation of 68Ga/177Lu-Labeled FAP-Targeted Peptide for Tumor Radiopharmaceutical Imaging and Therapy

Fibroblast activation protein (FAP) has been considered a promising target for tumor imaging and therapy. This study designed a novel peptide, FAP-HXN, specifically targeting FAP and exhibiting significant potential as a radionuclide-labeled theranostic agent. Preclinical studies were conducted to evaluate the potency, selectivity, and efficacy of FAP-HXN. Methods: FAP-HXN was synthesized and characterized for selectivity and specificity toward FAP. Cellular uptake of the radiolabeled FAP-HXN in human embryonic kidney (HEK)-293-FAP cells with high expressions of FAP was evaluated. The diagnostic and therapeutic potential of 68Ga- and 177Lu-labeled radioligands was evaluated in HEK-293-FAP tumor-bearing mice compared with the FAP-targeting peptide FAP-2286. Results: FAP-HXN demonstrated high binding ability to human and mouse sources of FAP. Moreover, the in vivo studies confirmed the high affinity and specificity of radiolabeled FAP-HXN. Small-animal PET imaging demonstrated that [68Ga]Ga-FAP-HXN had continuous tumor uptake in FAP-positive tumors after administration compared with [68Ga]Ga-FAP-2286. In the therapeutic experiments, [177Lu]Lu-FAP-HXN showed significant antitumor activity in HEK-293-FAP xenografts at well-tolerated doses, which also exhibited longer tumor retention and better tumor growth inhibition compared with [177Lu]Lu-FAP-2286. Conclusion: The preclinical studies revealed that radiolabeled FAP-HXN had high tumor uptake, prolonged retention, and significant anticancer efficacy in HEK-293-FAP xenografts. FAP-HXN shows promising potential as a novel theranostic radioligand for FAP-positive tumors.

Current Status and Perspectives of Novel Radiopharmaceuticals with Heterologous Dual-targeted Functions: 2013-2023

Radiotracers provide molecular- and cellular-level information in a noninvasive manner and have become important tools for precision medicine. In particular, the successful clinical application of radioligand therapeutic (RLT) has further strengthened the role of nuclear medicine in clinical treatment. The complicated microenvironment of the lesion has rendered traditional single-targeted radiopharmaceuticals incapable of fully meeting the requirements. The design and development of dual-targeted and multitargeted radiopharmaceuticals have rapidly emerged. In recent years, significant progress has been made in the development of heterologous dual-targeted radiopharmaceuticals. This perspective aims to provide a comprehensive overview of the recent progress in these heterologous dual-targeted radiopharmaceuticals, with a special focus on the design of ligand structures, pharmacological properties, and preclinical and clinical evaluation. Furthermore, future directions are discussed from this perspective.

Exploring the value of FAP-targeted PET/CT in differentiating breast cancer molecular subtypes: a preliminary study

PURPOSE

This prospective study aims to evaluate the value of [18F]AlF-NOTA-fibroblast activation protein inhibitor (FAPI)-04 positron emission tomography-computed tomography (PET/CT) in predicting molecular subtypes of breast cancer.

METHODS

The study consecutively recruited patients suspected of having breast cancer from a single center who were prospectively enrolled from July 2023 to May 2024 and underwent [18F]AlF-NOTA-FAPI-04 PET/CT. This study compared the differences in tracer uptake among breast cancers with different adverse prognostic factors and molecular subtypes. The classification performance for each molecular subtype of breast cancer was assessed using a receiver operating characteristic (ROC) curve.

RESULTS

Fifty-three participants (mean age, 51 ± 11 years; 52 females) were evaluated. Breast cancer lesions with adverse prognostic factors showed higher tracer uptake. The five different molecular subtypes exhibited varying levels of uptake. The luminal A and luminal B (HER2-negative) subtypes had relatively low uptake, while the luminal B (HER2-positive), HER2-positive, and triple-negative subtypes had relatively high uptake. ROC analysis identified the max standardized uptake value (SUVmax) as a significant classifier (AUC = 0.912, P = 0.0005) for the luminal A subtype, with 100% sensitivity and 83% specificity. For predicting the luminal B (HER2-negative) subtype, SUVmax had an AUC of 0.770 (P = 0.0015). SUVmax, with an AUC of 0.781 (P = 0.003), was used to identify the triple-negative subtype tumors, resulting in a sensitivity of 100% and specificity of 51%. Lastly, the ROC curve showed the cut-off 15.40 (AUC = 0.921, P < 0.0001) could classify luminal A & luminal B (HER2-negative), and luminal B (HER2-positive) & HER2-positive & triple-negative, yielding a sensitivity of 94% and specificity of 79%.

CONCLUSION

The uptake of [18F]AlF-NOTA-FAPI-04 is significantly correlated with the molecular subtypes of breast cancer, and [18F]AlF-NOTA-FAPI-04 PET/CT is a potential tool for noninvasive identification of luminal A subtypes and guidance of FAP-targeted therapies.

[68Ga]Ga-FAPI PET/CT in brain tumors: comparison with [18F]F-FDG PET/CT

Purpose

To investigate the feasibility of [68Ga]Ga-FAPI PET/CT in brain tumor imaging and to compare it with [18F]F-FDG PET/CT.

Methods

25 patients with MRI-suspected brain tumors were included in the study. They underwent whole body [18F]F-FDG PET/CT and [68Ga]Ga-FAPI PET/CT and brain scans. The target-to-background ratio (TBR) of brain tumors was calculated with the background of surrounding normal brain tissues uptake. The SUVmax and TBR of [18F]F-FDG PET/CT and [68Ga]Ga-FAPI PET/CT were compared. Additionally, the correlation between the uptake of the tracer by lesions with the greatest diameter of the lesion, the breadth of the oedema band, and the enhancement scores of the MRI enhancement scans was analyzed.

Result

[68Ga]Ga-FAPI PET/CT was superior to [18F]F-FDG PET/CT for lesion detection, especially for brain metastases. Among gliomas, only high-grade gliomas uptake [68Ga]Ga-FAPI. Compared with [18F]F-FDG PET/CT, [68Ga]Ga-FAPI PET/CT had a lower SUVmax but a significantly better TBR. On [68Ga]Ga-FAPI PET/CT, the TBR may be associated with brain tumor blood-brain barrier disruption.

Conclusions

[68Ga]Ga-FAPI PET/CT is a promising imaging tool for the assessment of brain tumors. Lack of physiological uptake of [68Ga]Ga-FAPI in normal brain parenchyma results in high TBR values, leading to better visualization of lesions and contributing to subsequent targeted therapy studies.

Advances in knowledge

Clinical utility of [68Ga]Ga-FAPI PET/CT in brain tumors remains unclear, and there aren't many similar studies in the literature. We evaluated the role of [68Ga]Ga-FAPI PET/CT in diagnosing brain tumors.

Radiotracers for Molecular Imaging of Angiotensin-Converting Enzyme 2

Angiotensin-converting enzymes (ACE) are well-known for their roles in both blood pressure regulation via the renin-angiotensin system as well as functions in fertility, immunity, hematopoiesis, and many others. The two main isoforms of ACE include ACE and ACE-2 (ACE2). Both isoforms have similar structures and mediate numerous effects on the cardiovascular system. Most remarkably, ACE2 serves as an entry receptor for SARS-CoV-2. Understanding the interaction between the virus and ACE2 is vital to combating the disease and preventing a similar pandemic in the future. Noninvasive imaging techniques such as positron emission tomography and single photon emission computed tomography could noninvasively and quantitatively assess in vivo ACE2 expression levels. ACE2-targeted imaging can be used as a valuable tool to better understand the mechanism of the infection process and the potential roles of ACE2 in homeostasis and related diseases. Together, this information can aid in the identification of potential therapeutic drugs for infectious diseases, cancer, and many ACE2-related diseases. The present review summarized the state-of-the-art radiotracers for ACE2 imaging, including their chemical design, pharmacological properties, radiochemistry, as well as preclinical and human molecular imaging findings. We also discussed the advantages and limitations of the currently developed ACE2-specific radiotracers.