Synthesis, preclinical evaluation, and a pilot clinical PET imaging study of 68Ga-labeled FAPI dimer

Design and preclinical evaluation of 99mTc-Labeled dimer FAPI-46 derivatives as potential tumor radiotracers

Fibroblast activation protein (FAP) is a crucial target for tumor diagnosis and treatment. FAP inhibitors (FAPIs) can selectively bind to FAP, and ligands with multiple targeting groups are anticipated to improve tumor-specific uptake. A dimeric FAPI ligand (L2) with high affinity for FAP was selected. Four hydrophilic 99mTc-labeled complexes ([99mTc]Tc-L2-TPPTS, [99mTc]Tc-L2-TPPMS, [99mTc]Tc-L2-PDA, and [99mTc]Tc-L2-NIC) were successfully prepared and exhibited good stability in vitro. Among them, [99mTc]Tc-L2-TPPTS and [99mTc]Tc-L2-PDA showed superior cellular uptake and specific binding to FAP. They displayed minimal nontarget uptake in normal mice and exhibited significant tumor uptake (22.01 ± 1.38 % ID/g and 26.58 ± 2.17 % ID/g at 1 h post-injection) with high specificity for FAP in U87MG tumor-bearing mice. SPECT/CT imaging experiments revealed specific accumulation of both complexes at the U87MG, PANC-1, and HT-1080-FAP tumor sites, suggesting their excellent specificity for FAP. In particular, [99mTc]Tc-L2-TPPTS has lower nontarget uptake in various tumor models and accelerated blood clearance. Additionally, an L2-TPPTS kit was successfully prepared providing convenient conditions for subsequent clinical transformation research.

Clinical Translation of a Dual-Integrin αvβ3- and CD13-Targeting PET Tracer

PURPOSE

Angiogenesis is essential in the development and progression of tumors. This study aimed to investigate the clinical application of 68 Ga-labeled heterodimeric peptide ( 68 Ga-HX01) targeting integrin αvβ3 and CD13 in tumor neovascularization.

PATIENTS AND METHODS

Six healthy volunteers were recruited to study the biodistribution, pharmacokinetics, and radiation of 68 Ga-HX01. Twelve patients with various malignancies were enrolled to seek the preliminary clinical value of 68 Ga-HX01. In healthy volunteers, SUVs of each major organ on 68 Ga-HX01 PET were measured. The clinical data, lesion numbers, and uptake were recorded in patients. The integrin αvβ3 and CD13 expression of the resected tumors was checked via immunohistochemistry staining.

RESULTS

With a mean injected dose of 167.98 ± 26.32 MBq, 68 Ga-HX01 was well tolerated and safe without side effects in 6 healthy volunteers. The radiation absorbed effective dose of 68 Ga-HX01 was 1.94 × 10 -2 mSv/MBq, and the urinary bladder wall held the highest absorbed effective dose (0.15 ± 5.87 × 10 -2 mSv/MBq). In 12 patients with various malignancies, 68 Ga-HX01 PET could clearly visualize the lesions from the surrounding tissues. The SUV max values in tumors were significantly higher than those in the surrounding tissues ( P < 0.05). A positive correlation trend between tumor SUV max and semiquantitative integrin αvβ3 and CD13 expression was determined ( P < 0.05).

CONCLUSIONS

For clinical use, 68 Ga-HX01 is safe with low radiation absorbed effective dose. It also indicates the efficiency of dual integrin αvβ3 and CD13-targeting PET radiotracer in tumor diagnosis, which may assist in patient prognosis and selecting eligible patients for antiangiogenic therapy.

FAP-targeted delivery of radioiodinated probes: A progressive albumin-driven strategy for tumor theranostics

Fibroblasts activated protein (FAP) appears to be a promising target for tumor theranostics. However, the development of radioiodinated probes for FAP has been slow. In this study, a progressive abumin-driven strategy was adopted to improve the FAP-targeted delivery of radioiodinated probes for tumor theranostics. A series of FAP-targeted probes (namely [131I]IPB-FAPI, [131I]IPB-FAPI-A1, [131I]IPB-FAPI-A3, [131I]FSDD3I) were synthesized by incorporating an albumin-binding moiety (4-(p-iodophenyl)butyric acid, 4-IPBA) labeled with radioiodine. The specificity and binding characteristics of the radiotracers to FAP and human serum albumin (HSA) were confirmed. SPECT imaging results showed that the [131I]FSDD3I had more prominent tumor retention property and superior target-to-nontarget ratio, which were consistent with the biodistribution results. As expected, the FAP-targeted therapy with 11.1 MBq [131I]FSDD3I significantly inhibited tumor growth. In conclusion, this proof-of-concept study employed a progressive design strategy to enhance pharmacokinetics of radioiodinated FAP-targeted probes. Among these radioiodinated FAPI probes, 131I-labeled FSDD3I ([131I]FSDD3I) emerged as a standout candidate with superior competitive advantages for application in radioiodine-guided internal irradiation therapy.

A novel PET tracer for noninvasive imaging the checkpoints expression of innate and adaptive immunity in tumors by simultaneously targeting CD24 and PD-L1

The success of tumor immunotherapy depends on the innate and adaptive immune responses, with CD24 and PD-L1 being key targets. DBP1 peptide is a novel bispecific D-peptide, targeting both CD24 and PD-L1 simultaneously. In this study, by radiolabeling DBP1 peptide, we developed a novel PET modality to noninvasively evaluate CD24 and PD-L1 expressions in tumors. To enhance the solubility of DBP1, a hydrophilic lysine was added into C-terminal residue of the peptide, which was then modified with a chelator NOTA to produce the radiotracer precursor NOTA-DBP1k. NOTA-DBP1k showed high affinity for CD24 (KD = 10.70 ± 0.70 nM) and PD-L1 (KD = 5.40 ± 0.61 nM). [68Ga]Ga-NOTA-DBP1k was synthesized with a high radiochemical yield (71 ± 3.0 %) and exhibited high hydrophilicity and stability. [68Ga]Ga-NOTA-DBP1k showed higher uptake in high CD24/PD-L1 expressed MCF-7 cells than that in low CD24/PD-L1 expressed U-87MG cells in vitro. In vivo, [68Ga]Ga-NOTA-DBP1k showed high uptake in MCF-7 tumors and had favorable tumor-to-background ratios by microPET imaging. On the contrary, low uptake was found in U-87MG tumors, which was significantly lower than that in MCF-7 tumors (0.42 ± 0.02 %ID/g vs. 1.01 ± 0.06 %ID/g, p < 0.05). The biodistribution study was consistent with the findings of microPET imaging results. These results demonstrated that [68Ga]Ga-NOTA-DBP1k can noninvasively image the CD24 and PD-L1 checkpoint expression of innate and adaptive immunity in tumors and may be helpful for guiding the CD24/PD-L1 dual-checkpoints blockage immunotherapy.

Evaluation of a Novel Gd-FAPI Dimer Molecular Probe Targeting Fibroblast Activation Protein for Imaging of Solid Tumors

Cancer-associated fibroblasts (CAFs) are essential components of the tumor microenvironment. Fibroblast activation protein (FAP) is overexpressed in CAFs. FAP-targeted molecular imaging agents, including the FAP inhibitors (FAPIs), have shown promising results in tumor diagnosis. We aimed to design a Gd-labeled FAPI Dimer, Gd-DOTA-Suc-Lys-(FAPI04)2, to optimize the pharmacokinetics and evaluate its potential capacity for targeting FAP-positive solid tumors in vivo. The Gd-labeled FAPI Dimer was successfully synthesized with exceeding 98% purity. Preclinical pharmacokinetics were determined in assessed FAP-positive U87 cell-derived xenografts and FAP-negative C6-derived xenografts using small-animal T1-weighted 7.0T MR imaging. The longitudinal correlation coefficient (r1) of the agent was 3.813 mM-1·S-1. The administration of the Gd-FAPI04 Dimer probe showed a notable enhancement of tumor contrast on T1-weighted whole-body MRI. At 10 and 30 minutes post-injection, the U87 subcutaneous tumor demonstrated significantly greater contrast enhancement than the C6 subcutaneous tumor (P <0.05). In vivo, the safety of the Gd-FAPI-04 Dimer probe was evaluated, which showed no tissue damage in vital organs like the heart, liver, spleen, lung, and kidneys, as indicated by unchanged morphology compared to a normal saline control group. The novel Gd-FAPI04 Dimer molecular probe, Gd-DOTA-Suc-Lys-(FAPI-04)2 specifically targeting FAP may serve as a safe and promising tool for the diagnostic imaging of solid tumors.

FAP-Targeted SPECT/CT and PET/CT Imaging for Breast Cancer Patients

ABSTRACT

Breast cancer presents a significant global health challenge, necessitating continued innovation in diagnostic and therapeutic approaches. Recent advances have led to the identification of cancer-associated fibroblasts, which are highly prevalent in breast cancers and express fibroblast activation proteins (FAPs), as critical targets. FAP-specific radiotracers, when used with PET/CT and SPECT/CT, have significant potential for improving early breast cancer detection, staging, treatment response monitoring, and therapeutic intervention. This review provides insight into FAP-targeted molecular imaging, exploring advanced techniques for protein status assessment, development of early-phase targeted therapies, and other emerging applications. The advent of FAP-targeted imaging stands to significantly enhance personalized oncologic care, leading to improved breast cancer management and overall patient outcomes.

A Novel 68Ga-Labeled 2-Azabicyclo[3.1.0]Hexane-3-Carbonitrile-Based Fibroblast Activation Protein-Targeted Tracer for Cancer Imaging With Positron Emission Tomography

Most of the reported small molecule-based fibroblast activation protein (FAP)-targeted radioligands are derived from UAMC1110 and contain a 4-difluoro-2-cyanopyrrolidine moiety. In this study, we investigated the effect of replacing the 4-difluoro-2-cyanopyrrolidine moiety of [68Ga]Ga-FAPI-04 with 2-azabicyclo[3.1.0]hexane-3-carbonitrile on the in vitro/vivo FAP-targeting capability. The newly derived 68Ga-labeled FAP-targeted tracer, [68Ga]Ga-JC02076, was obtained in 43.5 ± 10.4% decay-corrected radiochemical yield within 33.5 ± 5.8 min (n = 4). The radiochemical purity and molar activity were 97.2 ± 3.4% and 411.6 ± 232.5 GBq/μmol, respectively. Ga-JC02076 showed good binding affinity for FAP (IC50 = 29.7 ± 3.5 nM). Most importantly, [68Ga]Ga-JC02076 enabled clear visualization of HEK293T:hFAP tumor xenografts in PET images and had good tumor uptake (7.17 ± 2.19 %ID/g) and excellent tumor-to-bone (17.3 ± 6.99) and tumor-to-muscle (32.3 ± 12.5) uptake ratios at 1 h post-injection. Our data suggest that N-(4-quinolinoyl)-Gly-(2-azabicyclo[3.1.0]hexane-3-carbonitrile) is a promising pharmacophore for the design of FAP-targeted tracers.

Improved FAPI-radiopharmaceutical pharmacokinetics from the perspectives of a dose escalation study

PURPOSE

This study explores the use of fibroblast activation protein inhibitors (FAPI) targeting radiopharmaceuticals as a new approach for pan-cancer treatment, focusing on key factors affecting their effectiveness. We hypothesized that adjusting the administered radiotracer dose one could enhance the tumor-to-background ratios.

METHODS

In a dose-escalation study with PC3 xenografts, all radiotracers were administered at doses between 10 and 1500 pmol, followed by biodistribution and PET/CT imaging. Their selectivity towards FAP, PREP, and DDP4, along with their stability in vivo, was assessed by biodistribution and metabolite analysis, respectively. Organ FAP expression was quantified using qPCR, and circulating FAP (sFAP) levels were measured in mouse and human blood samples via ELISA. Proof-of-principle human studies were also conducted.

RESULTS

Increasing the dose from 10 to 600 pmol significantly reduced blood uptake and enhanced tumor uptake, optimizing their in vivo performance. All radiotracers showed peak efficacy at 350-600 pmol, with altered pharmacokinetics beyond 600 pmol. Biodistribution studies validated the in vivo selectivity of all radiotracers towards FAP, even in the presence of PREP and DPP4 inhibitors, while they demonstrated remarkable stability in vivo. FAP expression was confirmed in various organs, with sFAP quantified in both healthy mice and humans. Human studies with [68Ga]Ga-DOTA.SA.FAPI revealed reduced off-target uptake (e.g., pancreas, salivary glands, heart), aligning with the preclinical findings.

CONCLUSION

The study highlights the crucial need for precise FAPI-radiotracer dosing, optimizing PET imaging, reducing radiation exposure, and enhancing treatment by accounting for FAP biology and sFAP's influence on pharmacokinetics.

Comparative Study of Dimeric Fibroblast Activation Protein-Targeting Radioligands Labeled with Fluorine-18, Copper-64, and Gallium-68

Fibroblast activation protein inhibitors (FAPIs) labeled with gallium-68 and lutetium-177 show potential for use in the diagnosis and treatment of various cancers expressing FAP. However, 177Lu-labeled FAPIs often exhibit short tumor retention time, limiting their therapeutic applications. To improve tumor retention, we synthesized three radiolabeled dimeric FAPIs, [18F]1, [64Cu]2, and [68Ga]3. These were prepared by chelating Al[18F]F to 1,4,7-triazacyclononane-1,4,7-triacetic acid (NOTA)-l-glutamic acid (E)-(FAPI)2 and copper-64 or gallium-68 to 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA)-E-(FAPI)2. NOTA-E-(FAPI)2 and DOTA-E-(FAPI)2 showed higher binding affinities for FAP compared with that of FAPI-04 (IC50 = 0.47 and 0.16 nM vs 0.89 nM, respectively). All radioligands were synthesized in high decay-corrected radiochemical yields (59-96%) and were stable in fetal bovine serum and phosphate-buffered saline. The more hydrophilic radioligand, [68Ga]3, was selected for cellular uptake studies, which confirmed FAP-specific uptake. Positron emission tomography imaging and ex vivo biodistribution studies in U87MG tumor-bearing mice revealed high tumor uptake of all three radioligands, with significant blocking observed after preinjection of FAPI-04. [64Cu]2 and [68Ga]3 exhibited favorable in vivo pharmacokinetics compared to those of [18F]1. Notably, [68Ga]3 showed lower normal organ uptake than did the other two radioligands, and moreover, it exhibited higher, more prolonged tumor uptake than its monomeric counterpart [68Ga]Ga-FAPI-04 over a 3 h period, suggesting its potential as a promising FAP-specific theranostic radioligand.

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.

Design, preclinical evaluation, and first-in-human PET study of [68Ga]Ga-PSFA-01: a PSMA/FAP heterobivalent tracer

PURPOSE

Prostate cancer (PCa), characterized by tumor heterogeneity, may exhibit low or absent prostate-specific membrane antigen (PSMA) expression in cancerous lesions, limiting the detection sensitivity of monospecific probes. Given that fibroblast activation protein (FAP) is frequently overexpressed in the tumor microenvironment (TME), we developed a PSMA/FAP dual-targeting tracer to address this limitation.

METHODS

The precursor (PSFA-01) was synthesized by coupling a quinolone-based FAP-targeting scaffold and EuK with HBED-CC via amide bonds. The dual-receptor-binding affinity and cell uptake of PSFA-01 and [natGa]Ga-PSFA-01 was evaluated in vitro. Micro-PET/CT imaging was performed on 22Rv1 and U87MG tumor-bearing mice. The feasibility of [68Ga]Ga-PSFA-01 PET/CT in a clinical setting was evaluated in a metastatic prostate cancer patient, and the results were compared with those of [68Ga]Ga-FAPI-04 and [68Ga]Ga-PSMA-11 PET/CT.

RESULTS

PSFA-01 and [natGa]Ga-PSFA-01 showed high affinity for both FAP and PSMA proteins (Ki = 0.14-1.02 nM). On micro-PET/CT imaging, the 22Rv1 tumor uptake of [68Ga]Ga-PSFA-01 (SUVmax = 3.89 ± 0.47) was higher than that of [68Ga]Ga-PSMA-11 (SUVmax = 2.96 ± 0.48). The U87MG tumor uptake of [68Ga]Ga-PSFA-01 was significantly higher (SUVmax = 7.29 ± 1.13) than [68Ga]Ga-FAPI-04 (SUVmax = 0.28 ± 0.12), showing tumor to muscle ratio as 12.68 ± 1.93 at 1 h p.i. On clinical trial, the primary tumor and metastatic lesions were distinctly identified by [68Ga]Ga-PSFA-01 (21 lesions), demonstrating superior performance compared to [68Ga]Ga-FAPI-04 (3 lesions) and [68Ga]Ga-PSMA-11 (13 lesions) in terms of lesion count and specificity.

CONCLUSIONS

[68Ga]Ga-PSFA-01 exhibited satisfactory PSMA and FAP dual-receptor-targeting properties both in vitro and in vivo. This study highlights the clinical feasibility of [68Ga]Ga-PSFA-01 PET/CT for detecting metastatic tumors of prostate cancer more sensitively compared to monomeric [68Ga]Ga-PSMA-11 and [68Ga]Ga-FAPI-04, which also suggests that a PSMA/FAP dual-targeted radionuclide therapy could potentially overcome challenges related to tumor heterogeneity and insufficient PSMA expression in PCa.

TRIAL REGISTRATION

Clinical trial registry NCT06387381, Registered 1 May 2024.

Advances in Preclinical Research of Theranostic Radiopharmaceuticals in Nuclear Medicine

Theranostics of nuclear medicine refers to the combination of radionuclide imaging and internal irradiation therapy, which is currently a research hotspot and an important direction for the future development of nuclear medicine. Radiopharmaceutical is a vital component of nuclear medicine and serves as one of the fundamental pillars of molecular imaging and precision medicine. At present, a variety of radiopharmaceuticals have been developed for various targets such as fibroblast activation protein (FAP), prostate-specific membrane antigen (PSMA), somatostatin receptor 2 (SSTR2), C-X-C motif chemokine receptor 4 (CXCR4), human epidermal growth factor-2 (HER2), and integrin αvβ3, and some of them have been successfully applied in clinical practice. The radiopharmaceutical with theranostic function plays an important role in the diagnosis, treatment, efficacy evaluation, and prognosis prediction of cancers and is the key to realize the personalized treatment of tumors. This Review summarizes the preclinical research progress of theranostic radiopharmaceuticals toward the above targets in the field of nuclear medicine and discusses the prospects and development directions of radiopharmaceuticals in the future.

FAP-targeted radioligand therapy with 68Ga/177Lu-DOTA-2P(FAPI)2 enhance immunogenicity and synergize with PD-L1 inhibitors for improved antitumor efficacy

BACKGROUND

Fibroblast activation protein (FAP)-targeted radioligand therapy, with immunomodulatory effects, has shown efficacy in both preclinical and clinical studies. We recently reported on a novel dimeric FAP-targeting radiopharmaceutical, 68Ga/177Lu-DOTA-2P(FAPI)2, which demonstrated increased tumor uptake and prolonged retention in various cancers. However, further exploration is required to understand the therapeutic efficacy and underlying mechanisms of combining 68Ga/177Lu-DOTA-2P(FAPI)2 radioligand therapy with PD-1/PD-L1 immunotherapy.

METHODS

Regarding the change in PD-L1 expression and DNA double-strand breaks induced by radiopharmaceuticals, CT26-FAP tumor cells were incubated with 68Ga and 177Lu labeled DOTA-2P(FAPI)2, respectively. Monotherapy with 68Ga-DOTA-2P(FAPI)2, 177Lu-DOTA-2P(FAPI)2, and PD-L1 immunotherapy as well as combination therapy (68Ga/177Lu-DOTA-2P(FAPI)2 and PD-L1 immunotherapy) were tested and evaluated to evaluate in vivo antitumor efficacy. Furthermore, immunohistochemical staining and single-cell RNA sequencing were used to analyze changes in the tumor microenvironment (TME) and elucidate the underlying mechanisms of action of this combination therapy.

RESULTS

Our findings indicated that FAP-targeting radiopharmaceuticals can induce DNA double-strand breaks and upregulate PD-L1 expression, with 177Lu-DOTA-2P(FAPI)2 proving to be more effective than 68Ga-DOTA-2P(FAPI)2. Both 68Ga-DOTA-2P(FAPI)2 and 177Lu-DOTA-2P(FAPI)2 radiopharmaceuticals significantly improved therapeutic outcomes when combined with anti-PD-L1 monoclonal antibody (αPD-L1 mAb). Notably, the combination of 177Lu-DOTA-2P(FAPI)2 with αPD-L1 mAb immunotherapy eliminated tumors in mouse models. Mice treated with this regimen not only exhibited exceptional responses to the initial immune checkpoint inhibitor therapy but also showed 100% tumor rejection on subsequent tumor cell re-inoculation. Further mechanistic studies have shown that 177Lu-DOTA-2P(FAPI)2 combined with αPD-L1 mAb can reprogram the TME, enhancing antitumor intercellular communication, which activates antitumor-related intercellular contacts such as FasL-Fas interactions between T cells and NK cells with tumor cells and increasing the proportion of infiltrating CD8+ T-cells while reducing regulatory T cells and inhibiting tumor progression. Our research also demonstrates that mature neutrophils play a role in enhancing the efficacy of the combined therapy, as shown in neutrophil-blocking experiments.

CONCLUSIONS

Our study robustly advocates for use of FAP-targeting radiopharmaceuticals, particularly 177Lu-DOTA-2P(FAPI)2, alongside immunotherapy in treating FAP-positive tumors. This combination therapy transforms the TME and enables a translatable approach to increasing the sensitivity to PD-1/PD-L1 immunotherapy, leading to improved complete remission rates and extended overall survival.

Development of [68Ga]Ga/[177Lu]Lu-DOTA-NI-FAPI-04 Containing a Nitroimidazole Moiety as New FAPI Radiotracers with Improved Tumor Uptake and Retention

Fibroblast activation protein (FAP), which is overexpressed in cancer-associated fibroblasts (CAFs), represents a promising target for cancer diagnosis and therapy. Hypoxia is a common feature of solid tumors. A bivalent agent, DOTA-NI-FAPI-04 (1), was developed by incorporating hypoxia-sensitive nitroimidazole (NI) into the FAP-targeting agent FAPI-04. Compound 1 exhibited a strong FAP binding affinity with an IC50 of 7.44 nM. Radiolabeled [68Ga]Ga-1 and [177Lu]Lu-1 demonstrated enhanced in vitro cell uptake. In vivo positron emission tomography/computed tomography (PET/CT) imaging showed that [68Ga]Ga-1 displayed significantly higher specific uptake and retention in U87MG tumor-bearing mice compared to [68Ga]Ga-FAPI-04 (SUVavg: 7.87 vs 1.99% ID/mL at 120 min). Biodistribution studies confirmed superior tumor uptake of [68Ga]Ga-1 (48.15 vs 5.72% ID/g at 120 min). Similarly, [177Lu]Lu-1 exhibited higher tumor uptake than [177Lu]Lu-FAPI-04 (50.75 vs 20.48% ID/g at 120 min). These preliminary results suggest that a nitroimidazole-containing bivalent-targeting agent, [68Ga]Ga/[177Lu]Lu-1, is a promising candidate for tumor theranostics.

Radiopharmaceuticals and their applications in medicine

Radiopharmaceuticals involve the local delivery of radionuclides to targeted lesions for the diagnosis and treatment of multiple diseases. Radiopharmaceutical therapy, which directly causes systematic and irreparable damage to targeted cells, has attracted increasing attention in the treatment of refractory diseases that are not sensitive to current therapies. As the Food and Drug Administration (FDA) approvals of [177Lu]Lu-DOTA-TATE, [177Lu]Lu-PSMA-617 and their complementary diagnostic agents, namely, [68Ga]Ga-DOTA-TATE and [68Ga]Ga-PSMA-11, targeted radiopharmaceutical-based theranostics (radiotheranostics) are being increasingly implemented in clinical practice in oncology, which lead to a new era of radiopharmaceuticals. The new generation of radiopharmaceuticals utilizes a targeting vector to achieve the accurate delivery of radionuclides to lesions and avoid off-target deposition, making it possible to improve the efficiency and biosafety of tumour diagnosis and therapy. Numerous studies have focused on developing novel radiopharmaceuticals targeting a broader range of disease targets, demonstrating remarkable in vivo performance. These include high tumor uptake, prolonged retention time, and favorable pharmacokinetic properties that align with clinical standards. While radiotheranostics have been widely applied in tumor diagnosis and therapy, their applications are now expanding to neurodegenerative diseases, cardiovascular diseases, and inflammation. Furthermore, radiotheranostic-empowered precision medicine is revolutionizing the cancer treatment paradigm. Diagnostic radiopharmaceuticals play a pivotal role in patient stratification and treatment planning, leading to improved therapeutic outcomes in targeted radionuclide therapy. This review offers a comprehensive overview of the evolution of radiopharmaceuticals, including both FDA-approved and clinically investigated agents, and explores the mechanisms of cell death induced by radiopharmaceuticals. It emphasizes the significance and future prospects of theranostic-based radiopharmaceuticals in advancing precision medicine.

Current status of FAP-directed cancer theranostics: a bibliometric analysis

Fibroblast activation protein (FAP) is a key molecule in the field of oncology, with significant impacts on tumor diagnosis and treatment. Importantly, it has paved the way for the development of radiotracers for quinoline-based FAP inhibitors (FAPIs), which are currently among the most promising radiotracers for PET imaging in cancer. We performed a bibliometric analysis of scientific publications related to FAP and FAPI-based radiotracers, which included the quantification and visualization of current research trends and prospects based on various bibliometric indicators. In our survey of FAP-related studies in the Web of Science Core Collection databases, R and VOSviewer were used for visualization and bibliometric analyses based on country, institute, author, journal, and keywords. We also examined the methodology, radionuclide type, imaging instruments, and major diseases associated with studies on FAPI-based radiotracers. The results revealed 2,664 FAP-related publications from 1992 to the present. Germany, the USA, and China dominated paper publications, multinational collaborations, and societal impacts on FAP research. Southwest Medical University was the most productive institute, while Haberkorn Uwe authored the most cited papers and the highest H-index. The European Journal of Nuclear Medicine and Molecular Imaging and the Journal of Nuclear Medicine were the most influential periodicals. Keywords "FAP", "68Ga-FAPI", and "PET/CT" emerged as the most significant in this field. This study may help elucidate current research trends, hotspots, and directions for future research.

Fibroblast activation protein inhibitors positron emission tomography/computed tomography: Review of the literature

Positron emission tomography/computed tomography (PET/CT) with radiolabeled fibroblast activation protein inhibitors (FAPI) is an increasingly relevant molecular diagnostic image in oncology given the high expression of FAP in cancer associated fibroblast, being present in almost 90% of the epithelial carcinomas, which allows imaging with excellent diagnostic performance and can also become a therapeutic strategy. This review summarizes the literature on FAPI-PET/CT for the cancer evaluation and compares it in some scenarios with the ¹⁸F-Fluorodeoxyglucose PET/CT.

Pharmacokinetic Positron Emission Tomography Imaging of an Optimized CD38-Targeted 68Ga-Labeled Peptide in Multiple Myeloma: A Pilot Study

Multiple myeloma (MM) is an incurable disease characterized by its clinical and prognostic heterogeneity. Despite conventional chemotherapy and autologous hematopoietic stem cell transplantation, the management of relapsed and refractory MM disease poses significant challenges, both medically and socioeconomically. CD38, highly expressed on the surface of MM cells, serves as a distinct tumor biological target in MM. Peptides offer advantages over antibodies, enabling precise tumor imaging and facilitating early tumor diagnosis and dynamic immunotherapy monitoring. In this study, we developed PF381, a CD38-targeted peptide, and investigated its role in diagnosis, biodistribution, and dosimetry through 68Ga-labeling for preclinical evaluation in tumor-bearing models. We screened a microchip-based combinatorial chemistry peptide library to obtain the amino acid sequence of PF381. Affinity for human CD38 was evaluated by SPRi. PF381 was conjugated with DOTA for radiolabeling with 68Ga, and the complex was characterized by HPLC. PET imaging was performed in murine tumor models after the administration of [68Ga]Ga-DOTA-PF381. Biodistribution analysis compared CD38-positive H929 and CD38-negative U266 tumors, and human radiation dosimetry was estimated. Tumor sections were stained for CD38 expression. SPRi showed that PF381 had a high affinity for CD38 with a KD of 2.49 × 10-8 M. HPLC measured a radiolabeling efficiency of 78.45 ± 7.91% for [68Ga]Ga-DOTA-PF381, with >98% radiochemical purity. PET imaging revealed rapid and persistent accumulation of radioactivity in CD38-positive H929 tumors, contrasting with negligible uptake in CD38-negative U266 tumors. Biodistribution confirmed higher uptake in H929 tumors (0.75 ± 0.03%ID/g) vs U266 (0.26 ± 0.08%ID/g, P < 0.001). The kidney received the highest radiation dose (3.57 × 10-02 mSv/MBq), with an effective dose of 1.41 × 10-02 mSv/MBq. Immunofluorescence imaging supported PET and biodistribution findings. We developed a novel peptide targeting CD38 and proved that 68Ga-labeled PF381 had rapid targeting and good tumor penetration capabilities. Therefore, 68Ga-labeled PF381 could achieve high sensitivity in vivo imaging for CD38-positive hematological malignancies.

Synthesis and Evaluation of 99mTc-Labeled l-Aspartic Acid as a EuK Polymer Linker for Targeting PSMA to a Novel SPECT Tumor Tracer

The development of novel tracers targeting prostate-specific membrane antigen (PSMA) has great potential for improving the diagnosis and treatment of prostate cancer (PCa). This study aimed to improve the absolute tumor uptake and tumor-to-background ratios (TBRs) of this novel PSMA tracer by increasing the number of pharmacophores, Glu-urea-Lys (EuK), that specifically bind to PSMA. We successfully synthesized four radioligands and prepared a total of 12 stable radiotracers by coordinating 99mTc with various coligands. [99mTc]Tc-EUKD-EDDA showed the best pharmacokinetic properties both in vitro and in vivo. It effectively increased the absolute uptake in tumors and resulted in good tumor retention. Rapid clearance in nontarget organs resulted in high TBRs. High-contrast SPECT/CT images were obtained within 2-6 h after injection, suggesting that [99mTc]Tc-EUKD-EDDA has great application potential in time-lapse imaging of PCa, which is important for improving the diagnostic accuracy of PCa in clinical practice.

Comparative Study of [18F]AlF-LNC1007, [18F]FDG, and [18F]AlF-NOTA-FAPI-04 PET/CT in Breast Cancer Diagnosis: A Methodological Exploration and Analytical Insight

Objective: To compare the diagnostic value of [18F]AlF-LNC1007, [18F]FDG, and [18F]AlF-NOTA-FAPI-04 PET/CT in breast cancer. Methods: 33 patients with highly suspected or already diagnosed but untreated breast cancer were enrolled in the study and underwent [18F]AlF-LNC1007 (30 patients), [18F]FDG (22 patients), and [18F]AlF-NOTA-FAPI-04 (8 patients) PET/CT. Quantitative measurements included the SUVmax and tumor-to-background ratio (TBR) for all lesions and background tissues. The Chi-square test was used for intergroup diagnostic efficacy, and the Wilcoxon test was used for intergroup SUVmax or TBR. Diagnostic efficacy for lymph node metastasis was evaluated using receiver operating characteristic (ROC) analysis. Results: Compared to [18F]FDG, [18F]AlF-LNC1007 had a higher positive predictive value (100% vs 91%, P = 0.0004) in lymph node metastases (42 vs 46) and higher sensitivity (100 vs 76%, P = 0.0003) in bone metastases (33 vs 25) but lower sensitivity (93 vs 100%, P = 0.001) in liver metastases. Apart from liver metastases, [18F]AlF-LNC1007 PET/CT had higher SUVmax in primary tumor and other metastases, with no statistical difference in TBR. Compared to [18F]AlF-NOTA-FAPI-04 PET/CT, [18F]AlF-LNC1007 had less false-positive and a higher positive predictive value in bone metastases (99 vs 95%, P = 0.0003) but had lower SUVmax(P < 0.01) in all primary and metastases lesions. The TBR difference between [18F]AlF-LNC1007 and [18F]AlF-NOTA-FAPI-04 was statistically significant only in bone metastases (5.97 vs 5.02, P = 0.001). The comparison of lymph node detection efficacy between [18F]AlF-LNC1007 and [18F]FDG PET/CT showed significant differences in SUVmax cutoff values for diagnosing lymph node metastases (2.62 vs 3.90), sensitivity (95.2% vs 66.67), and specificity (100% vs 85.00) (all P < 0.001). Conclusion: [18F]AlF-LNC1007 demonstrated superior efficacy compared to [18F]FDG and [18F]AlF-NOTA-FAPI-04 and higher uptake than [18F]FDG in primary tumor, lymph node and bone metastases, and higher TBR than [18F]AlF-NOTA-FAPI-04, especially in bone metastases. [18F]AlF-LNC1007 also showed high specificity in differentiating inflammatory and metastatic lymph nodes.

Development of a homotrimeric PSMA radioligand based on the NOTI chelating platform

BACKGROUND

The NOTI chelating scaffold can readily be derivatized for bioconjugation without impacting its metal complexation/radiolabeling properties making it an attractive building block for the development of multimeric/-valent radiopharmaceuticals. The objective of the study was to further explore the potential of the NOTI chelating platform by preparing and characterizing homotrimeric PSMA radioconjugates in order to identify a suitable candidate for clinical translation.

RESULTS

Altogether, three PSMA conjugates based on the NOTI-TVA scaffold with different spacer entities between the chelating unit and the Glu-CO-Lys PSMA binding motif were readily prepared by solid phase-peptide chemistry. Cell experiments allowed the identification of the homotrimeric conjugate 9 comprising NaI-Amc spacer with high PSMA binding affinity (IC50 = 5.9 nM) and high PSMA-specific internalization (17.8 ± 2.5%) compared to the clinically used radiotracer [68Ga]Ga-PSMA-11 with a IC50 of 18.5 nM and 5.2 ± 0.2% cell internalization, respectively. All 68Ga-labeled trimeric conjugates showed high metabolic stability in vitro with [68Ga]Ga-9 exhibiting high binding to human serum proteins (> 95%). Small-animal PET imaging revealed a specific tumor uptake of 16.0 ± 1.3% IA g-1 and a kidney uptake of 67.8 ± 8.4% IA g-1 for [68Ga]Ga-9. Clinical PET imaging allowed identification of all lesions detected by [68Ga]Ga-PSMA-11 together with a prolonged blood circulation as well as a significantly lower kidney and higher liver uptake of [68Ga]Ga-9 compared to [68Ga]Ga-PSMA-11.

CONCLUSIONS

Trimerization of the Glu-CO-Lys binding motif for conjugate 9 resulted in a ~ threefold higher binding affinity and cellular uptake as well as in an altered biodistribution profile compared to the control [68Ga]Ga-PSMA-11 due to its intrinsic high binding to serum proteins. To fully elucidate its biodistribution, future studies in combination with long-lived radionuclides, such as 64Cu, are warranted. Its prolonged biological half-life and favorable tumor-to-kidney ratio make this homotrimeric conjugate also a potential candidate for future radiotherapeutic applications in combination with therapeutic radionuclides such as 67Cu.

Preclinical Study of a Dual-Target Molecular Probe Labeled with 68Ga Targeting SSTR2 and FAP

OBJECTIVE

Currently, 68Ga-labeled somatostatin analogs (SSAs) are the most commonly used imaging agents for patients with neuroendocrine tumors (NETs) in clinical practice, demonstrating good results in tumor diagnosis. For applications in peptide receptor radionuclide therapy (PRRT), targeted drugs should have high tumor uptake and prolonged tumor retention time. To enhance the uptake and retention of tracers in NETs, our goal is to design a 68Ga-labeled heterodimer for optimizing pharmacokinetics and assess whether this form is more efficacious than its monomeric equivalents.

METHODS

Using the somatostatin analog TATE and quinoline-based compound FAPI-46 as raw materials, we designed and synthesized 68Ga-labeled TATE-46. The labeling efficiency and stability were verified by Radio-HPLC. The receptor binding properties and tumor targeting were examined both in vitro and in vivo by using NCI-H727 (SSTR2/FAP, positive) and Mc38 (SSTR2/FAP, negative) cell lines and tumor-bearing mouse models. Preclinical evaluation was performed through cell uptake, pharmacokinetics, Micro PET, and biodistribution studies, and the results were compared with [68Ga]Ga-DOTA-TATE and [68Ga]Ga -FAPI-46. Immunohistochemistry and HE staining were performed on tumor tissues from tumor-bearing mice for further validation.

RESULTS

[68Ga]Ga-TATE-46 showed comparable SSTR2 and FAP targeting ability to monomeric TATE and FAPI-46 in cell uptake and PET imaging studies. [68Ga]Ga-TATE-46 exhibited significantly higher uptake in NCI-H727 (SSTR2/FAP, positive) tumors compared to [68Ga]Ga-DOTA-TATE (p < 0.001) and [68Ga]Ga-FAPI-46 (p < 0.001). No increased uptake of [68Ga]Ga-TATE-46 was observed in MC38 tumors (SSTR2/FAP, negative). Additionally, excess DOTA-TATE and/or unlabeled FAPI-46 significantly blocked the uptake of [68Ga]Ga-TATE-46 in NCI-H727 tumors (p < 0.001), confirming its dual-receptor targeting characteristics. The ex vivo biodistribution, immunofluorescence and immunohistochemistry results were in line with the in vivo imaging findings.

CONCLUSION

Compared with 68Ga-labeled FAPI-46 and DOTA-TATE mono-specific tracers, the dual-target tracer [68Ga]Ga-TATE-46 improves tumor uptake, extends tumor retention, and enhances pharmacokinetics. It is an effective probe for non-invasive detection of tumors expressing FAP and SSTR2, and it is worth further studying its application in the expression of sstr2 and FAP-related tumors.

211At radiolabeled APBA-FAPI for enhanced targeted-alpha therapy of glioma

Fibroblast activation protein-α (FAPα) is highly expressed in tumor-associated cells and has become one of the most attractive targeting sites in cancer diagnosis and therapy. To ameliorate the rapid metabolism of FAPα inhibitor (FAPI), here, a multifunctional binding agent was introduced to simultaneously achieve 211At radiolabeling and tumor retention prolongation of corresponding radiolabeled drug. 211At-APBA-FAPI was successfully synthesized by conjugating 211At with the designed FAPI carrier in satisfactory radiochemical yield (>60 %). 211At-APBA-FAPI exhibited excellent in vitro stability, significant tumor affinity and specific killing effect on FAPα-positive U87MG cells. Molecular docking reveals that FAPI decorated with albumin binder can bind with FAPα protein via multiple intermolecular interactions with a considerable binding energy of -9.66 kcal/mol 211At-APBA-FAPI exhibits good targeting in murine xenograft models, showing obviously longer tumor retention than previously-reported radioastatinated compound. As a result, 211At-APBA-FAPI presents pronounced therapeutic effect with ignorable normal organs/tissues biotoxicity. All these indicate that introducing a multifunctional binding agent can effectively enhance the availability of FAPI for 211At conjugation and tumoricidal effect, providing vital hints for the translation of targeted-alpha therapy based on radiolabeled FAPI derivatives.

Comparison Length of Linker in Compound for Nuclear Medicine Targeting Fibroblast Activation Protein as Molecular Target

Novel nuclear medicine therapeutics are being developed by labeling medium-molecular-weight compounds with short-lived alpha-emitting radionuclides. Fibroblast activation protein α (FAPα) is recognized as a highly useful molecular target, and its inhibitor, FAPI, is a compound capable of theranostics, both therapeutic and diagnostic, for cancer treatment. In this study, we compared the functions of two compounds that target FAPα: 211At-FAPI1 and 211At-FAPI2. First, in vitro screening procedures are generally accepted because of the low endogenous expression of FAPα. We suggest the usefulness of this 3D culture system for in vitro screening. Second, when FAPIs are used therapeutically, the expected therapeutic effects are often not achieved. Therefore, we compared the accumulation and excretion in tumor tissues and the anti-tumor effects based on the length of the linker in the compounds. The compounds were rapidly labeled using the Shirakami reaction. Doubling the linker length increased tumor retention. Additionally, the excretion pathway was altered, suggesting a potential reduction in toxicity. Although no significant differences were observed in the anti-tumor effects of 211At-FAPI1 and 211At-FAPI2, it was confirmed that the linker length affects the biological half-life.

Preliminary Study of Radionuclide-Labeled MerTK-Targeting PET Imaging Agents for the Diagnosis of Melanoma

MerTK PET imaging holds potential as a promising approach for assessing tumor aggressiveness and monitoring treatment response. In this study, we synthesized a series of 18F- and 68Ga-labeled tracers derived from MerTK inhibitors for detection of MerTK expression. Among the synthesized agents, the dimeric compounds [68Ga]10 and [68Ga]12 demonstrated good in vivo and in vitro stability, high affinities to the MerTK receptor, and good MerTK-targeting specificity. Notably, [68Ga]10 exhibited a tumor uptake of 2.6 ± 0.2%ID/g at 1 h p. i. in B16F10 tumor-bearing mice, nearly tripling the uptake of its monomeric counterpart [68Ga]3. A similar enhancement was observed with [68Ga]12 compared to its monomeric analog [68Ga]6. Additionally, [18F]14 achieved a tumor uptake of 7.6 ± 0.5%ID/g at 2 h p. i., outperforming the previously reported [18F]15. Biodistribution analysis further validated the results, highlighting their potential for clinical investigation.

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.

Recent Clinical Implications of FAPI: Imaging and Therapy

ABSTRACT

The fibroblast activation protein (FAP) is a biomarker that is selectively overexpressed on cancer-associated fibroblasts (CAFs) in various types of tumoral tissues and some nonmalignant diseases, including fibrosis, arthritis, cardiovascular, and metabolic diseases. FAP plays a critical role in tumor microenvironment through facilitating proliferation, invasion, angiogenesis, immunosuppression, and drug resistance. Recent studies reveal that FAP might be regarded as a promising target for cancer diagnosis and treatment. FAP-targeted imaging modalities, especially PET, have shown high sensitivity and specificity in detecting FAP-expressing tumors. FAP-targeted imaging can potentially enhance tumor detection, staging, and monitoring of treatment response, and facilitate the development of personalized treatment strategies. This study provides a comprehensive view of FAP and its function in the pathophysiology of cancer and nonmalignant diseases. It also will discuss the characteristics of radiolabeled FAP inhibitors, particularly those based on small molecules, their recent clinical implications in imaging and therapy, and the associated clinical challenges with them. In addition, we present the results of imaging and biodistribution radiotracer 68 Ga-FAPI-46 in patients with nonmalignant diseases, including interstitial lung disease, primary biliary cirrhosis, and myocardial infarction, who were referred to our department. Our results show that cardiac FAP-targeted imaging can provide a novel potential biomarker for managing left ventricle remodeling. Moreover, this study has been organized and presented in a manner that offers a comprehensive overview of the current status and prospects of FAPI inhibitors in the diagnosis and treatment of diseases.

Construction of in-situ self-assembled agent for NIR/PET dual-modal imaging and photodynamic therapy for hepatocellular cancer

Hepatocellular cancer (HCC) remained a life-threatening carcinoma. Agents for HCC imaging and therapy were expected to possess different intratumoral retention time. To construct an agent with different intratumoral retention time when applied for tumor imaging or therapy remained great values. A lasialoglycoprotein receptor (ASGPR) targeted lactobionic acid derivative (LABO) was constructed for fluorescent imaging and photodynamic therapy of HCC. 18F labeled LABO (18F-LABO) was developed for PET imaging of HCC. LABO and 18F-LABO showed similar molecular structure. LABO exhibited characteristic of viscosity and concentration-induced intratumoral in-situ self-assembly to expand the intratumoral retention. LABO was non-fluorescent at free stage, but emitted NIR fluorescence and generated irradiation-induced ROS after self-assembly for fluorescent imaging and photodynamic therapy. ASGPR specificity of LABO and 18F-LABO was confirmed using HepG2 cell. Biodistribution and fluorescent imaging confirmed the different tumor retention time of LABO and 18F-LABO when used for photodynamic therapy and PET imaging. PET imaging and photodynamic therapy were performed on HepG2 tumor bearing mice, which revealed that 18F-LABO/LABO could specifically accumulated in the HepG2 tumor for tumor location/inhibition. LABO/18F-LABO with excellent HCC specificity but different intratumoral behaviors showed great values for the PET/NIR imaging and photodynamic therapy for HCC.

Preclinical Evaluation of 177Lu-OncoFAP-23, a Multivalent FAP-Targeted Radiopharmaceutical Therapeutic for Solid Tumors

Fibroblast activation protein (FAP) is abundantly expressed in the stroma of most human solid tumors. Clinical-stage radiolabeled FAP ligands are increasingly used as tools for the detection of various cancer lesions. To unleash the full therapeutic potential of FAP-targeting agents, ligands need to remain at the tumor site for several days after administration. We recently described the discovery of OncoFAP, a high-affinity small organic ligand of FAP with a rapid accumulation in tumors and low uptake in healthy tissues in cancer patients. Trimerization of OncoFAP provided a derivative (named TriOncoFAP, or OncoFAP-23) with improved FAP affinity. In this work, we evaluated the tissue biodistribution profile and the therapeutic performance of OncoFAP-23 in tumor-bearing mice. Methods: OncoFAP-23 was radiolabeled with the theranostic radionuclide 177Lu. Preclinical experiments were conducted on mice bearing SK-RC-52.hFAP (BALB/c nude mice) or CT-26.hFAP (BALB/c mice) tumors. 177Lu-OncoFAP and 177Lu-FAP-2286 were included in the biodistribution study as controls. Toxicologic evaluation was performed on Wistar rats and CD1 mice by injecting high doses of OncoFAP-23 or its cold-labeled counterpart, respectively. Results: 177Lu-OncoFAP-23 emerged for its best-in-class biodistribution profile, high and prolonged tumor uptake (i.e., ∼16 percentage injected dose/g at 96 h), and low accumulation in healthy organs, which correlates well with its potent single-agent anticancer activity at low levels of administered radioactivity. Combination treatment with the tumor-targeted interleukin 2 (L19-IL2, a clinical-stage immunocytokine) further expands the therapeutic window of 177Lu-OncoFAP-23 by potentiating its in vivo antitumor activity. Proteomics studies revealed a potent tumor-directed immune response on treatment with the combination. OncoFAP-23 and natLu-OncoFAP-23 exhibited a favorable toxicologic profile, without showing any side effects or signs of toxicity. Conclusion: OncoFAP-23 presents enhanced tumor uptake and tumor retention and low accumulation in healthy organs, findings that correspond to a strongly improved in vivo antitumor efficacy. The data presented in this work support the clinical development of 177Lu-OncoFAP-23 for the treatment of FAP-positive solid tumors.

Preclinical evaluation and first-in-human study of [18F]AlF-FAP-NUR for PET imaging cancer-associated fibroblasts

BACKGROUND

Fibroblast activation protein (FAP) has gained attention as a promising molecular target with potential utility for cancer diagnosis and therapy. [68Ga]Ga-labeled FAP-targeting peptides have been successfully applied to positron emission tomography (PET) imaging of various tumor types. To meet the applicable demand for peptide-based FAP tracers with high patient throughput, we herein report the radiosynthesis, preclinical evaluation, and the first-in-human imaging of a novel [18F]F-labeled FAP-targeting peptide.

RESULTS

[18F]AlF-FAP-NUR was automatedly prepared within 45 min with a non-decay corrected radiochemical yield of 18.73 ± 4.25% (n = 3). Compared to [68Ga]Ga-FAP-2286, the [18F]F-labeled peptide demonstrated more rapid, higher levels of cellular uptake and internalization, and lower levels of cellular efflux in HT1080-FAP cells. Micro-PET imaging and biodistribution studies conducted on xenograft mice models revealed a similar distribution pattern between the two tracers. However, [18F]AlF-FAP-NUR demonstrated significantly higher tumor-specific uptake resulting in improved Tumor-Background Ratios (TBRs). In the patients, a significant accumulation of [18F]AlF-FAP-NUR was found in the primary tumor. High uptake of the tracer within the bladder indicated that its major route of excretion was through urine.

CONCLUSIONS

Based on the physical imaging properties and longer half-life of [18F]F, [18F]AlF-FAP-NUR exhibited promising characteristics such as enhanced tumor-specific accumulation and elevated TBRs, which made it a viable candidate for further clinical investigation.

TRIAL REGISTRATION

www.Chictr.org.cn , ChiCTR2300076976 Retrospectively registered 25 October 2023. at, URL: https://www.chictr.org.cn/showproj.html?proj=206753 .

Development of an FAP-Targeted PET Probe Based on a Novel Quinolinium Molecular Scaffold

Fibroblast activation protein (FAP) has recently gained significant attention as a promising tumor biomarker for both diagnosis and therapeutic applications. A series of radiopharmaceuticals based on fibroblast activation protein inhibitors (FAPIs) have been developed and translated into the clinic. Though some of them such as radiolabeled FAPI-04 probes have achieved favorable in vivo imaging performance, further improvement is still highly desired for obtaining radiopharmaceuticals with a high theranostics potential. In this study, we innovatively designed an FAPI ligand SMIC-3002 by changing the core quinoline motif of FAPI-04 to the quinolinium scaffold. The engineered molecule was further radiolabeled with 68Ga to generate a positron emission tomography (PET) probe, [68Ga]Ga-SMIC-3002, which was then evaluated in vitro and in vivo. [68Ga]Ga-SMIC-3002 demonstrated high in vitro stability, nanomolar affinity for FAP (8 nM for protein, 23 nM for U87MG cells), and specific uptake in FAP-expressing tumors, with a tumor/muscle ratio of 19.1 and a tumor uptake of 1.48 ± 0.03 ID/g% at 0.5 h in U87MG tumor-bearing mice. In summary, the quinolinium scaffold can be successfully used for the development of the FAP-targeted tracer. [68Ga]Ga-SMIC-3002 not only shows high potential for clinical translation but also offers insights into designing a new generation of FAPI tracers.

Efficacy and Safety Evaluation of 177Lu-FAP-2286 in the Treatment of Advanced Lung Cancer

PURPOSE

The aim of this study was to evaluate the efficacy and safety of peptide-targeted radionuclide therapy (PTRT) with 177Lu-FAP-2286 in advanced lung cancer.

PATIENTS AND METHODS

This single-center prospective study included 9 patients diagnosed with advanced lung cancer. These patients met the inclusion criteria and received PTRT with 177Lu-FAP-2286. Short-term efficacy was assessed using RECIST 1.1 and PERCIST 1.0 criteria. Long-term efficacy was evaluated through overall survival, progression-free survival (PFS), overall response rate, EORTC QLQ-C30 v3.0, Eastern Cooperative Oncology Group, and Karnofsky Performance Status. Toxicity response was assessed using CTCAE v5.0.

RESULTS

The results based on RECIST 1.1 and PERCIST 1.0 criteria were comparable, with 44% of patients showing a partial metabolic response, 33.3% with stable metabolic disease, and 22.22% with progressive metabolic disease. The highest metabolic response after treatment reached 66.89%, and the overall response rate could reach 77.78%. In the long-term efficacy assessment, the median overall survival and PFS were 10 months and 6 months, respectively. The 2 patients with the lowest PFS (3 months) started PTRT relatively late. EORTC QLQ-C30 v3.0, Eastern Cooperative Oncology Group, and Karnofsky Performance Status scores showed that the overall health status, symptom response, and quality of life of patients improved after 177Lu-FAP-2286 treatment. The most noticeable improvements in clinical symptoms were dyspnea and cancer-related pain. No grade III/IV toxicity events were observed during follow-up period, and fibrinogen decreased significantly after treatment.

CONCLUSIONS

177Lu-FAP-2286 has the potential to be a viable PTRT option for patients with advanced lung cancer.

Magnetic Resonance Imaging of Fibroblast Activation Protein Using a Targeted Gadolinium-Based Contrast Agent

The aim of this study was to synthesize a quinoline-based MRI contrast agent, Gd-DOTA-FAPI04, and assess its capacity for targeting fibroblast activation protein (FAP)-positive tumors in vivo. Gd-DOTA-FAPI04 was synthesized by attaching a 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) complex of gadolinium(III) to FAP inhibitor FAPI04. The longitudinal relaxation time (T1) of the contrast agent was measured using a Siemens Prisma 3.0T MR system, and the CCK-8 assay was performed to evaluate its potential cytotoxicity. Male nude mice bearing tumors grown from FAP-expressing fibrosarcoma cells were divided into experimental (n = 4) and control (n = 4) groups, and T1-weighted image enhancement was measured at different times (0, 10, 30, 60, 90, and 120 min) postinjection of Gd-DOTA-FAPI04. The control group received an additional preinjection of excess FAPI04. FAP expression in tumor tissue was investigated by using immunohistochemistry with an anti-FAP antibody. The longitudinal relaxivities of gadodiamide and Gd-DOTA-FAPI04 were measured to be 3.734 mM-1 s-1 and 5.323 mM-1 s-1, respectively. The CCK-8 assay demonstrated that Gd-DOTA-FAPI04 has minimal toxicity to cultured human fibrosarcoma cells. In vivo MRI showed that peak accumulation of Gd-DOTA-FAPI04 in FAP-expressing tumors occurred 1 h postinjection and could be blocked by preinjection of excess FAPI04. Immunohistochemical analysis of harvested tumor tissue supported the above findings. Gd-DOTA-FAPI04 is a promising contrast agent for in vivo imaging of FAP.

Generic semi-automated radiofluorination strategy for single domain antibodies: [18F]FB-labelled single domain antibodies for PET imaging of fibroblast activation protein-α or folate receptor-α overexpression in cancer

BACKGROUND

Radiofluorination of single domain antibodies (sdAbs) via N-succinimidyl-4-[18F]fluorobenzoate ([18F]SFB) has shown to be a promising strategy in the development of sdAb-based PET tracers. While automation of the prosthetic group (PG) [18F]SFB production, has been successfully reported, no practical method for large scale sdAb labelling has been reported. Therefore, we optimized and automated the PG production, enabling a subsequently efficient manual conjugation reaction to an anti-fibroblast activation protein (FAP)-α sdAb (4AH29) and an anti-folate receptor (FR)-α sdAb (2BD42). Both the alpha isoform of FAP and the FR are established tumour markers. FAP-α is known to be overexpressed mainly by cancer-associated fibroblasts in breast, ovarian, and other cancers, while its expression in normal tissues is low or undetectable. FR-α has an elevated expression in epithelial cancers, such as ovarian, brain and lung cancers. Non-invasive imaging techniques, such as PET-imaging, using tracers targeting specific tumour markers can provide molecular information over both the tumour and its environment, which aides in the diagnosis, therapy selection and assessment of the cancer treatment.

RESULTS

[18F]SFB was synthesized using a fully automated three-step, one-pot reaction. The total procedure time was 54 min and results in [18F]SFB with a RCP > 90% and a RCY d.c. of 44 ± 4% (n = 13). The manual conjugation reaction after purification produced [18F]FB-sdAbs with a RCP > 95%, an end of synthesis activity > 600 MBq and an apparent molar activity > 10 GBq/µmol. Overall RCY d.c., corrected to the trapping of [18F]F- on the QMA, were 9% (n = 1) and 5 ± 2% (n = 3) for [18F]FB-2BD42 and [18F]FB-4AH29, respectively.

CONCLUSION

[18F]SFB synthesis was successfully automated and upscaled on a Trasis AllInOne module. The anti-hFAP-α and anti-hFR-α sdAbs were radiofluorinated, yielding similar RCYs d.c. and RCPs, showing the potential of this method as a generic radiofluorination strategy for sdAbs. The radiofluorinated sdAbs showed a favourable biodistribution pattern and are attractive for further characterization as new PET tracers for FAP-α and FR-α imaging.

Targeted Radionuclide Therapy in Glioblastoma

Despite the development of various novel therapies, glioblastoma (GBM) remains a devastating disease, with a median survival of less than 15 months. Recently, targeted radionuclide therapy has shown significant progress in treating solid tumors, with the approval of Lutathera for neuroendocrine tumors and Pluvicto for prostate cancer by the US Food and Drug Administration (FDA) and the European Medicines Agency (EMA). This achievement has shed light on the potential of targeted radionuclide therapy for other solid tumors, including GBM. This review presents the current status of targeted radionuclide therapy in GBM, highlighting the commonly used therapeutic radionuclides emitting alpha, beta particles, and Auger electrons that could induce potent molecular and cellular damage to treat GBM. We then explore a range of targeting vectors, including small molecules, peptides, and antibodies, which selectively target antigen-expressing tumor cells with minimal or no binding to healthy tissues. Considering that radiopharmaceuticals for GBM are often administered locoregionally to bypass the blood-brain barrier (BBB), we review prominent delivery methods such as convection-enhanced delivery, local implantation, and stereotactic injections. Finally, we address the challenges of this therapeutic approach for GBM and propose potential solutions.

Clinical translation of a novel FAPI dimer [68Ga]Ga-LNC1013

Fibroblast activation protein (FAP) has emerged as a highly promising target for cancer diagnostic imaging and targeted radionuclide therapy. To exploit the therapeutic potential of suitably radiolabeled FAP inhibitors (FAPIs), this study presents the design and synthesis of a series of FAPI dimers to increase tumor uptake and retention. Preclinical evaluation and a pilot clinical PET imaging study were conducted to screen the lead compound with the potential for radionuclide therapy.

METHODS

Three new FAPI dimers were synthesized by linking two quinoline-based FAPIs with different spacers. The in vitro binding affinity and preclinical small animal PET imaging of the compounds were compared with their monomeric counterparts, FAPI-04 and FAPI-46. The lead compound, [68Ga]Ga -LNC1013, was then evaluated in a pilot clinical PET imaging study involving seven patients with gastrointestinal cancer.

RESULTS

The three newly synthesized FAPI homodimers had high binding affinity and specificity in vitro and in vivo. Small animal PET imaging and biodistribution studies showed that [68Ga]Ga-LNC1013 had persistent tumor retention for at least 4 h, also higher uptake than the other two dimers and the monomer counterparts, making it the lead compound to enter clinical investigation. In the pilot clinical PET imaging study, seven patients were enrolled. The effective dose of [68Ga]Ga-LNC1013 was 8.24E-03 mSv/MBq. The human biodistribution of [68Ga]Ga-LNC1013 demonstrated prominent tumor uptake and good tumor-to-background contrast. [68Ga]Ga-LNC1013 PET imaging showed potential in capturing primary and metastatic lesions and outperforming 18F-FDG PET in detecting pancreatic and esophageal cancers. The SUVmax for lesions with [68Ga]Ga-FAPI-46 decreased over time, whereas [68Ga]Ga-LNC1013 exhibited persistently high tumor uptake from 1 to 4 h post-injection.

CONCLUSION

Dimerization is an effective strategy to produce FAPI derivatives with favorable tumor uptake, long tumor retention, and imaging contrast over its monomeric counterpart. We demonstrated that [68Ga]Ga-LNC1013, the lead compound without any piperazine moiety, had superior diagnostic potential over [68Ga]Ga-FAPI-46 and 18F-FDG, suggesting the future potential of LNC1013 for radioligand therapy of FAP-positive cancers.

Synthesis and Characterization of a Novel PET Tracer for Noninvasive Evaluation of FGL1 Status in Tumors

Fibrinogen-like protein 1 (FGL1) is a potential novel immune checkpoint target for malignant tumor diagnosis and therapy. Accurate detection of FGL1 levels in tumors via noninvasive PET imaging might be beneficial for managing the disease. To achieve this, multiple FGL1-targeting peptides (FGLP) were designed, and a promising candidate, 68Ga-NOTA-FGLP2, was identified through a high-throughput screening approach using microPET imaging of 68Ga-labeled peptides. Subsequent in vitro cell experiments showed that uptake values of 68Ga-NOTA-FGLP2 in FGL1 positive Huh7 tumor cells were significantly higher than those in FGL1 negative U87 MG tumor cells. Further microPET imaging showed that the Huh7 xenografts were clearly visualized with a favorable contrast. ROI analysis showed that the uptake values of the tracer in Huh7 xenografts were 2.63 ± 0.07% ID/g at 30 min p.i.. After treatment with an excess of unlabeled FGLP2, the tumor uptake significantly decreased to 0.54 ± 0.05% ID/g at 30 min p.i.. Moreover, the uptake in U87 MG xenografts was 0.44 ± 0.06% ID/g at the same time point. The tracer was excreted mainly through the renal system. 18F-FDG PET imaging was also performed in mice bearing Huh7 and U87 MG xenografts, respectively. However, there was no significant difference in the uptake between the tumors with different FGL1 expressions. Preclinical data indicated that 68Ga-NOTA-FGLP2 might be a suitable radiotracer for in vivo noninvasive visualization of tumors with abundant expression of FGL1. Further investigation of 68Ga-NOTA-FGLP2 for tumor diagnosis and therapy is undergoing.

Tailoring Fibroblast-Activation Protein Targeting for Theranostics: A Comparative Preclinical Evaluation of the 68Ga- and 177Lu-Labeled Monomeric and Dimeric Fibroblast-Activation Protein Inhibitors DOTA.SA.FAPi and DOTAGA.(SA.FAPi)2

BACKGROUND

FAP radiopharmaceuticals show promise for cancer diagnosis; however, their limited tumor residency hinders treatment. This study compared two FAPi derivatives, DOTA.SA.FAPi and DOTAGA.(SA.FAPi)2, labeled with gallium-68 and lutetium-177, aiming to determine an optimum combination for creating theranostic pairs.

METHODS

The radiotracers were studied for lipophilicity, binding to human serum proteins, and binding to human cancer-associated fibroblasts (CAFs) in vitro, including saturation and internalization/externalization studies. PET/SPECT/CT and biodistribution studies were conducted in PC3 and U87MG xenografts for [68Ga]Ga-DOTA.SA.FAPi and [68Ga]Ga-DOTAGA.(SA.FAPi)2. [177Lu]Lu-DOTA.SA.FAPi and [177Lu]Lu-DOTAGA.(SA.FAPi)2, were evaluated in PC3 xenografts. Biodistribution studies of [68Ga]Ga-DOTA.SA.FAPi were performed in healthy male and female mice.

RESULTS

All radiotracers exhibited strong binding to FAP. Their internalization rate was fast while only [177Lu]Lu-DOTAGA.(SA.FAPi)2 was retained longer in CAFs. [68Ga]Ga-DOTAGA.(SA.FAPi)2 and [177Lu]Lu-DOTAGA.(SA.FAPi)2 displayed elevated lipophilicity and affinity for human serum proteins compared to [68Ga]Ga-DOTA.SA.FAPi and [177Lu]Lu-DOTA.SA.FAPi. In vivo studies revealed slower washout of [68Ga]Ga-DOTAGA.(SA.FAPi)2 within 3 h compared to [68Ga]Ga-DOTA.SA.FAPi. The tumor-to-tissue ratios of [68Ga]Ga-DOTAGA.(SA.FAPi)2 versus [68Ga]Ga-DOTA.SA.FAPi did not exhibit any significant differences. [177Lu]Lu-DOTAGA.(SA.FAPi)2 maintained a significant tumor uptake even after 96 h p.i. compared to [177Lu]Lu-DOTA.SA.FAPi.

CONCLUSIONS

Dimeric compounds hold promise for therapy, while monomers are better suited for diagnostics. Finding the right combination is essential for effective disease management.

Rational Design and Pharmacomodulation of 18F-Labeled Biotin/FAPI-Conjugated Heterodimers

Due to the complex heterogeneity in different cancer types, the heterodimeric strategy has been intensively practiced to improve the effectiveness of tumor diagnostics. In this study, we developed a series of novel 18F-labeled biotin/FAPI-conjugated heterobivalent radioligands ([18F]AlF-NSFB, [18F]AlF-NSFBP2, and [18F]AlF-NSFBP4), synergistically targeting both fibroblast activation protein (FAP) and biotin receptor (BR), to enhance specific tumor uptake and retention. The in vitro and in vivo biological properties of these dual-targeting tracers were evaluated, with a particular focus on positron emission tomography imaging in A549 and HT1080-FAP tumor-bearing mice. Notably, in comparison to the corresponding FAP-targeted monomer [18F]AlF-NSF, biotin/FAPI-conjugated heterodimers exhibited a high uptake in tumor and prolong retention. In conclusion, as a proof-of-concept study, the findings validated the superiority of biotin/FAPI-conjugated heterodimers and the positive influence of biotin and linker on pharmacokinetics of radioligands. Within them, the bispecific [18F]AlF-NSFBP4 holds significant promise as a candidate for further clinical translational studies.

Rational Design and Comparison of Novel 99mTc-Labeled FAPI Dimers for Visualization of Multiple Tumor Types

Recognizing the significance of SPECT in nuclear medicine and the pivotal role of fibroblast activation protein (FAP) in cancer diagnosis and therapy, this study focuses on the development of 99mTc-labeled dimeric HF2 with high tumor uptake and image contrast. The dimeric HF2 was synthesized and radiolabeled with 99mTc in one pot using various coligands (tricine, TPPTS, EDDA, and TPPMS) to yield [99mTc]Tc-TPPTS-HF2, [99mTc]Tc-EDDA-HF2, and [99mTc]Tc-TPPMS-HF2 dimers. SPECT imaging results indicated that [99mTc]Tc-TPPTS-HF2 exhibited higher tumor uptake and tumor-to-normal tissue (T/NT) ratio than [99mTc]Tc-EDDA-HF2 and [99mTc]Tc-TPPMS-HF2. Notably, [99mTc]Tc-TPPTS-HF2 exhibited remarkable tumor accumulation and retention in HT-1080-FAP and U87-MG tumor-bearing mice, thereby surpassing the monomeric [99mTc]Tc-TPPTS-HF. Moreover, [99mTc]Tc-TPPTS-HF2 achieved acceptable T/NT ratios in the hepatocellular carcinoma patient-derived xenograft (HCC-PDX) model, which provided identifiable contrast and imaging quality. In conclusion, this study presents proof-of-concept research on 99mTc-labeled FAP inhibitor dimers for the visualization of multiple tumor types. Among these candidate compounds, [99mTc]Tc-TPPTS-HF2 showed excellent clinical potential, thereby enriching the SPECT tracer toolbox.

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

Quinoline-based fibroblast activation protein (FAP) inhibitors (FAPIs) have recently emerged as a focal point in global nuclear medicine, underscored by their promising applications in cancer theranostics and the diagnosis of various nononcological conditions. This review offers an in-depth summary of the existing literature on the evolution and use of FAPI tracers in China, tracing their journey from preclinical to clinical research. Moreover, this review also assesses the diagnostic accuracy of FAPI PET for the most common cancers in China, analyzes its impact on oncologic management paradigms, and investigates the potential of FAP-targeted radionuclide therapy in patients with advanced or metastatic cancer. This review also summarizes studies using FAPI PET for nononcologic disorders in China. Thus, this qualitative overview presents a snapshot of China's engagement with FAPI tracers, aiming to guide future research endeavors.

Peptidic heterodimer-based radiotracer targeting fibroblast activation protein and integrin αvβ3

PURPOSE

Several studies have demonstrated the advantages of heterodimers over their corresponding monomers due to the multivalency effect. This effect leads to an increased number of effective targeted receptors and, consequently, improved tumor uptake. Fibroblast activation protein (FAP) and integrin αvβ3 are found to be overexpressed in different components of the tumor microenvironment. In our pursuit of enhancing tumor uptake and retention, we designed and developed a novel peptidic heterodimer that synergistically targets both FAP and integrin αvβ3.

METHODS

FAP-RGD was synthesized from FAP-2286 and c(RGDfK) through a multi-step organic synthesis. The dual receptor binding property of 68Ga-FAP-RGD was investigated by cell uptake and competitive binding assays. Preclinical pharmacokinetics were determined in HT1080-FAP and U87MG tumor models using micro-positron emission tomography/computed tomography (micro-PET/CT) and biodistribution studies. The antitumor efficacy of 177Lu-FAP-RGD was assessed in U87MG tumor models. The radiation exposure and clinical diagnostic performance of 68 Ga-FAP-RGD were evaluated in healthy volunteers and cancer patients.

RESULTS

Bi-specific radiotracer 68Ga-FAP-RGD exhibited high binding affinity for both FAP and integrin αvβ3. In comparison to 68Ga-FAP-2286 and 68Ga-RGDfK, 68Ga-FAP-RGD displayed enhanced tumor uptake and longer tumor retention time in preclinical models. 177Lu-FAP-RGD could efficiently suppress the growth of U87MG tumor in vivo when applied at an activity of 18.5 and 29.6 MBq. The effective dose of 68Ga-FAP-RGD was 1.06 × 10-2 mSv/MBq. 68Ga-FAP-RGD demonstrated low background activity and stable accumulation in most neoplastic lesions up to 3 h.

CONCLUSION

Taking the advantages of multivalency effect, the bi-specific radiotracer 68Ga-FAP-RGD showed superior tumor uptake and retention compared to its corresponding monomers. Preclinical studies with 68Ga- or 177Lu-labeled FAP-RGD showed favorable image contrast and effective antitumor responses. Despite the excellent performance of 68Ga-FAP-RGD in clinical diagnosis, experimental efforts are currently underway to optimize the structure of FAP-RGD to increase its potential for clinical application in endoradiotherapy.

Novel CDK19-Targeted Radiotracers: A Potential Design Strategy to Improve the Pharmacokinetics and Tumor Uptake

Cyclin-dependent kinase 19 (CDK19) is overexpressed in prostate cancer, making it an attractive target for both imaging and therapy. Since little is known about the optimized approach for radioligands of nuclear proteins, linker optimization strategies were used to improve pharmacokinetics and tumor absorption, including the adjustment of the length, flexibility/rigidity, and hydrophilicity/lipophilicity of linkers. Molecular docking was conducted for virtual screening and followed by IC50 determination. Both BALB/c mice and P-16 xenografts were used for tissue distribution and PET/CT imaging. The ligand 68Ga-10c demonstrated high absorption in tumor 5 min after injection and sustains long-term imaging within 3 h. Furthermore, 68Ga-10c exhibited slow clearance within the tumor and was predominantly metabolized in both the liver and kidneys, showing the potential to alleviate metabolic pressure and enhance tissue safety. Therefore, the linker optimization strategy is well suited for CDK19 and provides a reference for the radioactive ligands of other nuclear targets.

Recent Progress in Peptide-Based Molecular Probes for Disease Bioimaging

Recent strides in molecular pathology have unveiled distinctive alterations at the molecular level throughout the onset and progression of diseases. Enhancing the in vivo visualization of these biomarkers is crucial for advancing disease classification, staging, and treatment strategies. Peptide-based molecular probes (PMPs) have emerged as versatile tools due to their exceptional ability to discern these molecular changes with unparalleled specificity and precision. In this Perspective, we first summarize the methodologies for crafting innovative functional peptides, emphasizing recent advancements in both peptide library technologies and computer-assisted peptide design approaches. Furthermore, we offer an overview of the latest advances in PMPs within the realm of biological imaging, showcasing their varied applications in diagnostic and therapeutic modalities. We also briefly address current challenges and potential future directions in this dynamic field.

Advances in the Development of Positron Emission Tomography Tracers for Improved Detection of Differentiated Thyroid Cancer

Thyroid cancer poses a significant challenge in clinical management, necessitating precise diagnostic tools and treatment strategies for optimal patient outcomes. This review explores the evolving field of radiotracers in the diagnosis and management of thyroid cancer, focusing on prostate-specific membrane antigen (PSMA)-based radiotracers, fibroblast activation protein inhibitor (FAPI)-based radiotracers, Arg-Gly-Asp (RGD)-based radiotracers, and 18F-tetrafluoroborate (18F-TFB). PSMA-based radiotracers, initially developed for prostate cancer imaging, have shown promise in detecting thyroid cancer lesions; however, their detection rate is lower than 18F-FDG PET/CT. FAPI-based radiotracers, targeting fibroblast activation protein highly expressed in tumors, offer potential in the detection of lymph nodes and radioiodine-resistant metastases. RGD-based radiotracers, binding to integrin αvβ3 found on tumor cells and angiogenic blood vessels, demonstrate diagnostic accuracy in detecting radioiodine-resistant thyroid cancer metastases. 18F-TFB emerges as a promising PET tracer for imaging of lymph node metastases and recurrent DTC, offering advantages over traditional methods. Overall, these radiotracers show promise in enhancing diagnostic accuracy, patient stratification, and treatment selection in differentiated thyroid cancer, warranting further research and clinical validation. Given the promising staging capabilities of 18F-TFB and the efficacy of FAP-targeting tracers in advanced, potentially dedifferentiated cases, continued investigation in these domains is justified.

Development, preclinical evaluation and preliminary dosimetry profiling of SB03178, a first-of-its-kind benzo[h]quinoline-based fibroblast activation protein-α-targeted radiotheranostic for cancer imaging and therapy

Fibroblast activation protein-α (FAP) is a marker of cancer-associated fibroblasts (CAFs) that constitute a significant portion of most carcinomas. Since it plays a critical role in tumor growth and metastasis, its timely detection to identify tumor lesions in early developmental stages using targeted radiopharmaceuticals has gained significant impetus. In the present work, two novel FAP-targeted precursors SB03178 and SB04033 comprising of an atypical benzo[h]quinoline construct were synthesized and either chelated to diagnostic radionuclide gallium-68 or therapeutic radionuclide lutetium-177, with ≥90% radiochemical purities and 22-76% decay-corrected radiochemical yields. natGa-labeled complexes displayed dose-dependent FAP inhibition, with binding potency of natGa-SB03178 being ∼17 times higher than natGa-SB04033. To evaluate their pharmacokinetic profiles, PET imaging and ex vivo biodistribution analyses were executed in FAP-overexpressing HEK293T:hFAP tumor-bearing mice. While both tracers displayed clear tumor visualization that was primarily FAP-arbitrated, with negligible uptake in most peripheral tissues, [68Ga]Ga-SB03178 demonstrated higher tumor uptake and superior tumor-to-background contrast ratios than [68Ga]Ga-SB04033. 177Lu-labeled SB03178 was subjected to tumor retention studies, mouse dosimetry profiling and mouse-to-human dose extrapolations also using the HEK293T:hFAP tumor model. [177Lu]Lu-SB03178 exhibited a combination of high and sustained tumor uptake, with excellent tumor-to-critical organ uptake ratios resulting in a high radiation absorbed dose to the tumor and a low estimated whole-body dose to humans. Our preliminary findings are considerably encouraging to support clinical development of [68Ga]Ga-/[177Lu]Lu-SB03178 theranostic pair for use in a vast majority of FAP-overexpressing neoplasms, particularly carcinomas.

Design, Preclinical Evaluation, and Clinical Translation of 68Ga-FAPI-LM3, a Heterobivalent Molecule for PET Imaging of Nasopharyngeal Carcinoma

Extensive research has been conducted on radiolabeled fibroblast activation protein (FAP) inhibitors (FAPIs) and p-Cl-Phe-cyclo(d-Cys-Tyr-d-4-amino-Phe(carbamoyl)-Lys-Thr-Cys)d-Tyr-NH2 (LM3) peptides for imaging of FAP and somatostatin receptor 2 (SSTR2)-positive tumors. In this study, we designed and synthesized a FAPI-LM3 heterobivalent molecule radiolabeled with 68Ga and evaluated its effectiveness in both tumor xenografts and patients with nasopharyngeal carcinoma (NPC). Methods: The synthesis of FAPI-LM3 was based on the structures of FAPI-46 and LM3. After radiolabeling with 68Ga, its dual-receptor-binding affinity was evaluated in vitro and in vivo. Preclinical studies, including small-animal PET and biodistribution evaluation, were conducted on HT-1080-FAP and HT-1080-SSTR2 tumor xenografts. The feasibility of 68Ga-FAPI-LM3 PET/CT in a clinical setting was evaluated in patients with NPC, and the results were compared with those of 18F-FDG. Results: 68Ga-FAPI-LM3 showed high affinity for both FAP and SSTR2. The tumor uptake of 68Ga-FAPI-LM3 was significantly higher than that of 68Ga-FAPI-46 and 68Ga-DOTA-LM3 in HT-1080-FAP-plus-HT-1080-SSTR2 tumor xenografts. In a clinical study involving 6 NPC patients, 68Ga-FAPI-LM3 PET/CT showed significantly higher uptake than did 18F-FDG in primary and metastatic lesions, leading to enhanced lesion detectability and tumor delineation. Conclusion: 68Ga-FAPI-LM3 exhibited FAPI and SSTR2 dual-receptor-targeting properties both in vitro and in vivo, resulting in improved tumor uptake and retention compared with that observed with monomeric 68Ga-FAPI and 68Ga-DOTA-LM3. This study highlights the clinical feasibility of 68Ga-FAPI-LM3 PET/CT for NPC imaging.

Synthesis and Preclinical Evaluation of Novel 99mTc-Labeled FAPI-46 Derivatives with Significant Tumor Uptake and Improved Tumor-to-Nontarget Ratios

Fibroblast activation protein (FAP), which is expressed on the cell membranes of fibroblasts in most solid tumors, has become an important target for tumor diagnosis and treatment. However, previously reported 99mTc-labeled FAPI-04 complexes have high blood uptake, limiting their use in the clinic. In this work, six 99mTc-labeled FAPI-46 derivatives with different linkers (different amino acids, peptides, or polyethylene glycol) were prepared and evaluated. They had good in vitro stability, hydrophilicity, and good specificity for FAP. The biodistribution and MicroSPECT images revealed that they all had high specific tumor uptake for FAP, and their blood uptake was significantly decreased. Among them, [99mTc]Tc-6-1 exhibited the highest target-to-nontarget ratios (tumor/blood: 6.06 ± 1.19; tumor/muscle: 10.26 ± 0.44) and good tumor uptake (16.15 ± 0.83%ID/g), which also had significantly high affinity for FAP, good in vivo stability, and safety. Therefore, [99mTc]Tc-6-1 holds great potential as a promising molecular tracer for FAP tumor imaging.

177Lu-Labeled Bivalent Ligands of Prostate-Specific Membrane Antigen for Endoradiotherapy of Prostate Cancer

Recently, we developed a bivalent prostate-specific membrane antigen (PSMA) radioligand ([18F]AlF-Bi-PSMA), which showed higher tumor uptake and retention in PSMA-positive mouse models than the clinically used radioligands, [68Ga]Ga-PSMA-11 and [18F]PSMA-1007. Here, we developed two 177Lu-labeled bivalent PSMA ligands with (DOTA-Alb-Bi-PSMA) or without an albumin-binding motif (DOTA-Bi-PSMA) to enhance radiotherapeutic efficacy with minimal toxicity. The results demonstrated that both 177Lu-labeled bivalent radioligands showed good stability, high binding affinity, and PSMA-targeting specificity in vitro. Compared with [177Lu]Lu-PSMA-617, both [177Lu]Lu-Bi-PSMA and [177Lu]Lu-Alb-Bi-PSMA showed a higher area under the curve (AUC) of tumor accumulation and superior therapeutic efficacy. However, [177Lu]Lu-Alb-Bi-PSMA exhibited a dose-dependent increase in acute damage to kidneys. In terms of the radionuclide therapy efficacy and side effects, [177Lu]Lu-Bi-PSMA exhibited well-balanced action with high tumor-to-organs AUC ratios, resulting in remarkable therapeutic efficacy and negligible side effects. These promising results warrant further investigations to achieve the clinical translation of [177Lu]Lu-Bi-PSMA.