Fibroblast activation protein targeted therapy using [177Lu]FAPI-46 compared with [225Ac]FAPI-46 in a pancreatic cancer model

[68Ga]Ga-labeled FAPI Conjugated with Gly-Pro Sequence for PET Imaging of Malignant Tumors

PURPOSE

To improve tumor uptake and prolong tumor retention, a novel fibroblast activation protein (FAP) ligand based on a quinoline-based FAP inhibitor (FAPI) conjugated with the Gly-Pro sequence and 1,4,7,10-tetraazacyclododecane-N,N',N″,N‴-tetraacetic acid (DOTA) was radiolabeled with [68Ga]GaCl3 ([68Ga]Ga-DOTA-GPFAPI-04). Due to the tumor heterogeneity, this study aimed to further validate the preclinical value of [68Ga]Ga-DOTA-GPFAPI-04 PET imaging in tumor mice models with different FAP expression levels.

METHODS

[68Ga]Ga-DOTA-GPFAPI-04 was synthesized and its partition coefficient was measured. The stability of [68Ga]Ga-DOTA-GPFAPI-04 was tested in phosphate-buffered saline (PBS, pH 7.4) and fetal bovine serum (FBS). Small animal PET and semi-quantitative studies were conducted in Panc-1 and A549 xenograft tumor mice models compared with [68Ga]Ga-DOTA-FAPI-04. Immunofluorescent and immunohistochemical staining and western blot assay were performed to confirm FAP expression in xenograft tumors.

RESULTS

[68Ga]Ga-DOTA-GPFAPI-04 exhibited a radiochemical purity of > 99% and high stability in PBS and FBS. [68Ga]Ga-DOTA-GPFAPI-04 had higher hydrophilic property than [68Ga]Ga-DOTA-FAPI-04 (-4.09 ± 0.05 vs -3.45 ± 0.05). Small animal PET and semi-quantitative analysis revealed Panc-1 xenograft tumor displayed higher tumor uptake of [68Ga]Ga-DOTA-GPFAPI-04 and tumor-to-background ratios compared to A549 xenograft tumor, consistent with the results of immunofluorescence, immunohistochemistry, and western blot. Moreover, [68Ga]Ga-DOTA-GPFAPI-04 demonstrated higher tumor accumulation and longer tumor retention than [68Ga]Ga-DOTA-FAPI-04 in both Panc-1 and A549 xenograft tumors. Furthermore, the FAP-binding specificity of [68Ga]Ga-DOTA-GPFAPI-04 was confirmed in vivo by co-injection of unlabeled GPFAPI-04.

CONCLUSION

[68Ga]Ga-DOTA-GPFAPI-04 showed more favorable in vivo tumor imaging and longer tumor retention compared to [68Ga]Ga-DOTA-FAPI-04, which has high potential to be a promising PET probe for detecting FAP-positive tumors.

Design of a Fibroblast Activation Protein-Targeted Radiopharmaceutical Therapy with High Tumor-to-Healthy-Tissue Ratios

Because of upregulated expression on cancer-associated fibroblasts, fibroblast activation protein (FAP) has emerged as an attractive biomarker for the imaging and therapy of solid tumors. Although many FAP ligands have already been developed for radiopharmaceutical therapies (RPTs), most suffer from inadequate tumor uptake, insufficient tumor residence times, or off-target accumulation in healthy tissues, suggesting a need for further improvements. Methods: A new FAP-targeted RPT with a novel ligand (FAP8-PEG3-IP-DOTA) was designed by combining the desirable features of several previous ligand-targeted RPTs. Uptake and retention of [111In]In or [177Lu]Lu-FAP8-PEG3-IP-DOTA were assessed in KB, HT29, MDA-MB-231, and 4T1 murine tumor models by radioimaging or ex vivo biodistribution analyses. Radiotherapeutic potencies and gross toxicities were also investigated by monitoring tumor growth, body weight, and tissue damage in tumor-bearing mice. Results: FAP8-PEG3-IP-DOTA exhibited high affinity (half-maximal inhibitory concentration, 1.6 nM) and good selectivity for FAP relative to its closest homologs, prolyl oligopeptidase (half-maximal inhibitory concentration, ∼14.0 nM) and dipeptidyl peptidase-IV (half-maximal inhibitory concentration, ∼860 nM). SPECT/CT scans exhibited high retention in 2 different solid tumor models and minimal uptake in healthy tissues. Quantitative biodistribution analyses revealed tumor-to-healthy-tissue ratios of more than 5 times for all major organs, and live animal studies demonstrated 65%-93% suppression of tumor growth in all 4 models tested, with minimal or no evidence of systemic toxicity. Conclusion: We conclude that [177Lu]Lu-FAP8-PEG3-IP-DOTA constitutes a promising and safe RPT candidate for FAPα-targeted radionuclide therapy of solid tumors.

Covalent targeted radioligands potentiate radionuclide therapy

Targeted radionuclide therapy, in which radiopharmaceuticals deliver potent radionuclides to tumours for localized irradiation, has addressed unmet clinical needs and improved outcomes for patients with cancer1-4. A therapeutic radiopharmaceutical must achieve both sustainable tumour targeting and fast clearance from healthy tissue, which remains a major challenge5,6. A targeted ligation strategy that selectively fixes the radiopharmaceutical to the target protein in the tumour would be an ideal solution. Here we installed a sulfur (VI) fluoride exchange (SuFEx) chemistry-based linker on radiopharmaceuticals to prevent excessively fast tumour clearance. When the engineered radiopharmaceutical binds to the tumour-specific protein, the system undergoes a binding-to-ligation transition and readily conjugates to the tyrosine residues through the 'click' SuFEx reaction. The application of this strategy to a fibroblast activation protein (FAP) inhibitor (FAPI) triggered more than 80% covalent binding to the protein and almost no dissociation for six days. In mice, SuFEx-engineered FAPI showed 257% greater tumour uptake than did the original FAPI, and increased tumour retention by 13-fold. The uptake in healthy tissues was rapidly cleared. In a pilot imaging study, this strategy identified more tumour lesions in patients with cancer than did other methods. SuFEx-engineered FAPI also successfully achieved targeted β- and α-radionuclide therapy, causing nearly complete tumour regression in mice. Another SuFEx-engineered radioligand that targets prostate-specific membrane antigen (PSMA) also showed enhanced therapeutic efficacy. Considering the broad scope of proteins that can potentially be ligated to SuFEx warheads, it might be possible to adapt this strategy to other cancer targets.

Preclinical study and first-in-human imaging of [18F]FAP-2286, and comparison with 2-[18F]FDG PET/CT in various cancer patients

PURPOSE

Fibroblast-activated protein (FAP) is highly expressed in cancer-associated fibroblasts (CAFs) of many solid cancers, but low or absent in normal tissues. Our study aimed to develop a novel FAP-specific tracer, namely [18F]FAP-2286, and evaluated its performance in comparison with well-established agents such as [18F]FAPI-42 and [68Ga]Ga-FAP-2286 in preclinical research, as well as 2-[18F]FDG in pilot clinical study.

METHODS

[18F]FAP-2286 was manually synthesized in accordance with Good Manufacturing Practice (GMP). Subsequent investigations encompassed cell uptake, competitive binding affinity, internalization and efflux assays using HT-1080hFAP cell lines. PET imaging and biodistribution studies were conducted in HEK-293ThFAP, A549hFAP, HT-1080hFAP tumor-bearing mice as well as HEK-293T, A549 and HT-1080 control groups. Furthermore, clinical evaluation of [18F]FAP-2286 was performed in fifteen patients with various cancers compared to 2-[18F]FDG PET.

RESULTS

The radiolabeling yield of [18F]FAP-2286 was 30.53 ± 5.20%, with a radiochemical purity exceeding 97%. In cell assays, [18F]FAP-2286 showed specific uptake, high internalization fraction and low cellular efflux. Rapid tumor uptake and satisfactory tumor retention was observed on micro-PET imaging and cancer patients. Meanwhile, the clinical research demonstrated that [18F]FAP-2286 may represent an alternative for low glucose-metabolism malignant tumors PET imaging such as gastric cancers.

CONCLUSION

[18F]FAP-2286 showed superior imaging quality including rapid and high target uptake and satisfactory retention in both tumor-bearing mice and cancer patients. It may emerge as a promising candidate for early or delayed phase imaging and 2-[18F]FDG non-avid cancers PET scan.

The synergy of 177Lu-FAPI-46 with tyrosine kinase inhibitor in a sarcoma patient-derived xenograft mouse model

BACKGROUND

Given the heterogeneity and high mortality associated with metastatic soft tissue sarcoma, this study aims to evaluate the therapeutic efficacy of combining 177Lu-FAPI-46 with Pazopanib against this malignancy.

METHODS

Patient-derived xenograft (PDX)-bearing mice were randomly divided into three groups: the control group, the 177Lu-FAPI-46 monotherapy group, and the 177Lu-FAPI-46 combined with Pazopanib therapy group. Therapeutic efficacy was regularly monitored.

RESULTS

The microPET imaging showed a 0.84-fold decrease in the T/M ratio of 68Ga-FAPI-46 on day 7/8 post combination therapy, while the control group exhibited a 1.23-fold increase. Combination therapy significantly inhibited tumor proliferation, as evidenced by reduced Ki-67 and increased caspase 3 expressions. Notably, there was no significant body weight loss observed in any group.

CONCLUSION

This study successfully demonstrated the reduction in FAP expression and suppression of tumor volume in sarcoma PDX following the combination therapy of 177Lu-FAPI-46 with Pazopanib.

Advances in Radioligand Theranostics in Oncology

Theranostics with radioligands (radiotheranostics) has played a pivotal role in oncology. Radiotheranostics explores the molecular targets expressed on tumor cells to target them for imaging and therapy. In this way, radiotheranostics entails non-invasive demonstration of the in vivo expression of a molecular target of interest through imaging followed by the administration of therapeutic radioligand targeting the tumor-expressed molecular target. Therefore, radiotheranostics ensures that only patients with a high likelihood of response are treated with a particular radiotheranostic agent, ensuring the delivery of personalized care to cancer patients. Within the last decades, a couple of radiotheranostics agents, including Lutetium-177 DOTATATE (177Lu-DOTATATE) and Lutetium-177 prostate-specific membrane antigen (177Lu-PSMA), were shown to prolong the survival of cancer patients compared to the current standard of care leading to the regulatory approval of these agents for routine use in oncology care. This recent string of successful approvals has broadened the interest in the development of different radiotheranostic agents and their investigation for clinical translation. In this work, we present an updated appraisal of the literature, reviewing the recent advances in the use of established radiotheranostic agents such as radioiodine for differentiated thyroid carcinoma and Iodine-131-labeled meta-iodobenzylguanidine therapy of tumors of the sympathoadrenal axis as well as the recently approved 177Lu-DOTATATE and 177Lu-PSMA for differentiated neuroendocrine tumors and advanced prostate cancer, respectively. We also discuss the radiotheranostic agents that have been comprehensively characterized in preclinical studies and have shown some clinical evidence supporting their safety and efficacy, especially those targeting fibroblast activation protein (FAP) and chemokine receptor 4 (CXCR4) and those still being investigated in preclinical studies such as those targeting poly (ADP-ribose) polymerase (PARP) and epidermal growth factor receptor 2.

Recent advances in the development of 225Ac- and 211At-labeled radioligands for radiotheranostics

Radiotheranostics utilizes a set of radioligands incorporating diagnostic or therapeutic radionuclides to achieve both diagnosis and therapy. Imaging probes using diagnostic radionuclides have been used for systemic cancer imaging. Integration of therapeutic radionuclides into the imaging probes serves as potent agents for radionuclide therapy. Among them, targeted alpha therapy (TAT) is a promising next-generation cancer therapy. The α-particles emitted by the radioligands used in TAT result in a high linear energy transfer over a short range, inducing substantial damage to nearby cells surrounding the binding site. Therefore, the key to successful cancer treatment with minimal side effects by TAT depends on the selective delivery of radioligands to their targets. Recently, TAT agents targeting biomolecules highly expressed in various cancer cells, such as sodium/iodide symporter, norepinephrine transporter, somatostatin receptor, αvβ3 integrin, prostate-specific membrane antigen, fibroblast-activation protein, and human epidermal growth factor receptor 2 have been developed and have made remarkable progress toward clinical application. In this review, we focus on two radionuclides, 225Ac and 211At, which are expected to have a wide range of applications in TAT. We also introduce recent fundamental and clinical studies of radiopharmaceuticals labeled with these radionuclides.

68Ga-DOTA-D-Alanine-BoroPro Radiotracer for Imaging of the Fibroblast Activation Protein in Malignant and Non-Malignant Diseases

Recently, we reported a new fibroblast activation protein (FAP) inhibitor radiopharmaceutical based on the 99mTc-((R)-1-((6-hydrazinylnicotinoyl)-D-alanyl) pyrrolidin-2-yl) boronic acid (99mTc-HYNIC-D-Alanine-BoroPro)(99mTc-HYNIC-iFAP) structure for tumor microenvironment SPECT imaging. This research aimed to synthesize 68Ga-[2,2',2″,2‴-(2-(4-(2-(5-(((S)-1-((S)-2-boronopyrrolidin-1-yl)-1-oxopropan-2-yl)carbamoyl)pyridin-2-yl)hydrazine-1-carbothioamido)benzyl)-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetrayl)tetraacetic acid] (68Ga-DOTA-D-Alanine-BoroPro)(68Ga-iFAP) as a novel radiotracer for PET imaging and evaluate its usefulness for FAP expression in malignant and non-malignant tissues. The coupling of p-SCN-benzene DOTA with HYNIC-iFAP was used for the chemical synthesis and further labeling with 68Ga. Radiochemical purity was verified by radio-HPLC. The specificity of 68Ga-iFAP was evaluated in HCT116 cells, in which FAP expression was verified by immunofluorescence and Western blot. Biodistribution and biokinetic studies were performed in murine models. 68Ga-iFAP uptake at the myocardial level was assessed in mice with induced infarction. First-in-human images of 68Ga-iFAP in healthy subjects and patients with myocardial infarction, glioblastoma, prostate cancer, and breast cancer were also obtained. DOTA-D-Alanine BoroPro was prepared with a chemical purity of 98% and was characterized by UPLC mass spectroscopy, FT-IR, and UV-vis. The 68Ga-iFAP was obtained with a radiochemical purity of >95%. In vitro and in vivo studies demonstrated 68Ga-iFAP-specific recognition for FAP, rapid renal elimination, and adequate visualization of the glioblastoma, breast tumor, prostate cancer, and myocardial infarction sites. The results of this research justify further dosimetry and clinical trials to establish the specificity and sensitivity of 68Ga-iFAP PET for FAP expression imaging.

Volumetric Parameters Derived from CXCR4-Directed PET/CT Predict Outcome in Patients with Gastrointestinal Neuroendocrine Carcinomas

BACKGROUND

Gastro-entero-pancreatic neuroendocrine carcinomas (GEP-NECs) are an aggressive subgroup of neuroendocrine neoplasms (NENs). In patients affected with NEN, there is a growing body of evidence that increased C-X-C motif chemokine receptor (CXCR4) expression is linked to decreasing overall survival (OS) in an ex-vivo setting. Thus, we aimed to determine whether the in-vivo-derived CXCR4-directed whole-body PET signal can also determine GEP-NEC patients with shorter OS.

METHODS

We retrospectively included 16 patients with histologically proven GEP-NEC, who underwent CXCR4-directed PET/CT for staging and therapy planning. We assessed maximum, peak, and mean standardized uptake values as well as whole-body tumor volume (TV) and total-lesion uptake (TLU = SUVmean × TV) using a semi-automatic segmentation tool with a 50% threshold. Association of PET-based biomarkers and OS or radiographic progression-free survival (rPFS; according to RECIST 1.1 criteria) was analyzed using univariable and multivariable cox regression.

RESULTS

Median OS and rPFS was 7.5 and 7 months, respectively. A significant correlation between TV and TLU was found for OS (TV: hazard ratio (HR) 1.007 95% confidence interval (CI) 1.000-1.014, p = 0.0309; TLU: HR 1.002 95% CI 1.000-1.003, p = 0.0350) and rPFS (TV: HR 1.010 95% CI 1.002-1.021; p = 0.0275; TLU: HR 1.002 95% CI 1.000-1.004, p = 0.0329), respectively. No significant correlation with OS or rPFS was found for non-volumetric parameters (p > 0.4). TV remained a significant predictive marker for OS and rPFS in multivariable analysis (OS: HR 1.012 95%, CI 1.003-1.022, p = 0.0084; rPFS: HR 1.009, 95% CI 0.9999-1.019, p = 0.0491), whereas TLU remained only prognostic for OS (HR 1.009, 95% CI 0.9999-1.019, p = 0.0194) but narrowly failed significance for rPFS (p = 0.0559).

CONCLUSION

In-vivo assessment of CXCR4 PET-derived volumetric parameters is predictive for outcome of patients with GEP-NEC and could be used as a risk stratification tool, which detects patients prone to early progression.

Theranostics - a sure cure for cancer after 100 years?

Cancer has remained a formidable challenge in medicine and has claimed an enormous number of lives worldwide. Theranostics, combining diagnostic methods with personalized therapeutic approaches, shows huge potential to advance the battle against cancer. This review aims to provide an overview of theranostics in oncology: exploring its history, current advances, challenges, and prospects. We present the fundamental evolution of theranostics from radiotherapeutics, cellular therapeutics, and nanotherapeutics, showcasing critical milestones in the last decade. From the early concept of targeted drug delivery to the emergence of personalized medicine, theranostics has benefited from advances in imaging technologies, molecular biology, and nanomedicine. Furthermore, we emphasize pertinent illustrations showcasing that revolutionary strategies in cancer management enhance diagnostic accuracy and provide targeted therapies customized for individual patients, thereby facilitating the implementation of personalized medicine. Finally, we describe future perspectives on current challenges, emerging topics, and advances in the field.

A pilot study of [68Ga]Ga-fibroblast activation protein inhibitor-04 PET/CT in renal cell carcinoma

OBJECTIVES

As a promising positron emission tomography (PET) tracer, [68Ga]Ga-fibroblast activation protein inhibitor-04([68Ga]Ga-FAPI-04) performs better than 2-[18F]fluoro-2-deoxy-d-glucose ([18F]FDG) at diagnosing primary and metastatic lesions in patients with various types of cancer. We investigated the utility of [68Ga]Ga-FAPI-04 PET/CT for the detection of primary and metastatic lesions in renal cell carcinoma (RCC). [18F]FDG PET/CT were used for comparison.

METHODS

Twenty-two patients with suspected RCC or recurrent RCC were enrolled in our study. Among these patients, 14 were newly diagnosed with RCC, 3 had recurrent RCC, and 5 were excluded from further analysis due to having benign renal tumours. Seventeen patients with RCC underwent [68Ga]Ga-FAPI-04 PET/CT, and 6 of them also received [18F]FDG PET/CT. The positive detection rates were calculated and compared with those in patients who underwent both scans.

RESULTS

Data from 17 patients with RCC (median age: 60.5 years, interquartile range [IQR]: 54-70 years) were evaluated. The positive detection rate of [68Ga]Ga-FAPI-04 PET/CT for RCC was 64.7% (11/17). Lymph node metastases (n = 44), lung metastasis (n = 1), and bone metastasis (n = 1) were detected. Six patients with RCC underwent [68Ga]Ga-FAPI-04 and [18F]FDG PET/CT. [68Ga]Ga-FAPI-04 PET/CT showed a higher positive detection rate than [18F]FDG PET/CT in detecting RCC (83.3% [5/6] vs. 50% [3/6], P = 0.545). Additionally, [68Ga]Ga-FAPI-04 PET/CT has higher SUVmax (3.20 [IQR: 2.91-5.80 vs. 2.71 [IQR: 2.13-3.10], P = 0.116) and tumour-to-background ratio (TBR) values (1.60 [IQR: 1.33-3.67] vs. 0.86 [0.48-1.21], P = 0.028) than [18F]FDG PET/CT.

CONCLUSIONS

These findings suggest that [68Ga]Ga-FAPI-04 PET/CT has potential value in RCC diagnosis. Further studies are warranted to validate these results.

ADVANCES IN KNOWLEDGE

Clinical utility of [68Ga]Ga-FAPI-04 in RCC remains unclear, and there are not many similar studies in the literature. We evaluated the role of [68Ga]Ga-FAPI-04 in diagnosing RCC.

Preclinical Pharmacokinetics and Biodistribution of LR004, a Novel Antiepidermal Growth Factor Receptor Monoclonal Antibody

LR004 is a novel chimeric (human/mouse) monoclonal antibody developed for the treatment of advanced colorectal carcinoma with detectable epidermal growth factor receptor (EGFR) expression. We aimed to investigate the preclinical pharmacokinetics (PK) and in vivo biodistribution of LR004. The PK profiles of LR004 were initially established in rhesus monkeys. Subsequently, 125I radionuclide-labeled LR004 was developed and the biodistribution, autoradiography, and NanoSPECT/CT of 125I-LR004 in xenograft mice bearing A431 tumors were examined. The PK data revealed a prolonged half-life and nonlinear PK characteristics of LR004 within the dose range of 6-54 mg/kg. The radiochemical purity of 125I-LR004 was approximately 98.54%, and iodination of LR004 did not affect its specific binding activity to the EGFR antigen. In a classical biodistribution study, 125I-LR004 exhibited higher uptake in highly perfused organs than in poorly perfused organs. Prolonged retention properties of 125I-LR004 in tumors were observed at 4 and 10 days. Autoradiography and NanoSPECT/CT confirmed the sustained retention of 125I-LR004 at the tumor site in xenograft mice. These findings demonstrated the adequate tumor targeting capabilities of 125I-LR004 in EGFR-positive tumors, which may improve dosing strategies and future drug development.

Comparison of Nuclear Medicine Therapeutics Targeting PSMA among Alpha-Emitting Nuclides

Currently, targeted alpha therapy (TAT) is a new therapy involving the administration of a therapeutic drug that combines a substance of α-emitting nuclides that kill cancer cells and a drug that selectively accumulates in cancer cells. It is known to be effective against cancers that are difficult to treat with existing methods, such as cancer cells that are widely spread throughout the whole body, and there are high expectations for its early clinical implementation. The nuclides for TAT, including 149Tb, 211At, 212/213Bi, 212Pb (for 212Bi), 223Ra, 225Ac, 226/227Th, and 230U, are known. However, some nuclides encounter problems with labeling methods and lack sufficient preclinical and clinical data. We labeled the compounds targeting prostate specific membrane antigen (PSMA) with 211At and 225Ac. PSMA is a molecule that has attracted attention as a theranostic target for prostate cancer, and several targeted radioligands have already shown therapeutic effects in patients. The results showed that 211At, which has a much shorter half-life, is no less cytotoxic than 225Ac. In 211At labeling, our group has also developed an original method (Shirakami Reaction). We have succeeded in obtaining a highly purified labeled product in a short timeframe using this method.

Preclinical Development of PNT6555, a Boronic Acid-Based, Fibroblast Activation Protein-α (FAP)-Targeted Radiotheranostic for Imaging and Treatment of FAP-Positive Tumors

The overexpression of fibroblast activation protein-α (FAP) in solid cancers relative to levels in normal tissues has led to its recognition as a target for delivering agents directly to tumors. Radiolabeled quinoline-based FAP ligands have established clinical feasibility for tumor imaging, but their therapeutic potential is limited due to suboptimal tumor retention, which has prompted the search for alternative pharmacophores. One such pharmacophore is the boronic acid derivative N-(pyridine-4-carbonyl)-d-Ala-boroPro, a potent and selective FAP inhibitor (FAPI). In this study, the diagnostic and therapeutic (theranostic) potential of N-(pyridine-4-carbonyl)-d-Ala-boroPro-based metal-chelating DOTA-FAPIs was evaluated. Methods: Three DOTA-FAPIs, PNT6555, PNT6952, and PNT6522, were synthesized and characterized with respect to potency and selectivity toward soluble and cell membrane FAP; cellular uptake of the Lu-chelated analogs; biodistribution and pharmacokinetics in mice xenografted with human embryonic kidney cell-derived tumors expressing mouse FAP; the diagnostic potential of 68Ga-chelated DOTA-FAPIs by direct organ assay and small-animal PET; the antitumor activity of 177Lu-, 225Ac-, or 161Tb-chelated analogs using human embryonic kidney cell-derived tumors expressing mouse FAP; and the tumor-selective delivery of 177Lu-chelated DOTA-FAPIs via direct organ assay and SPECT. Results: DOTA-FAPIs and their natGa and natLu chelates exhibited potent inhibition of human and mouse sources of FAP and greatly reduced activity toward closely related prolyl endopeptidase and dipeptidyl peptidase 4. 68Ga-PNT6555 and 68Ga-PNT6952 showed rapid renal clearance and continuous accumulation in tumors, resulting in tumor-selective exposure at 60 min after administration. 177Lu-PNT6555 was distinguished from 177Lu-PNT6952 and 177Lu-PNT6522 by significantly higher tumor accumulation over 168 h. In therapeutic studies, all 3 177Lu-DOTA-FAPIs exhibited significant antitumor activity at well-tolerated doses, with 177Lu-PNT6555 producing the greatest tumor growth delay and animal survival. 225Ac-PNT6555 and 161Tb-PNT6555 were similarly efficacious, producing 80% and 100% survival at optimal doses, respectively. Conclusion: PNT6555 has potential for clinical translation as a theranostic agent in FAP-positive cancer.

Cancer-associated fibroblasts in neoadjuvant setting for solid cancers

Therapeutic approaches for cancer have become increasingly diverse in recent times. A comprehensive understanding of the tumor microenvironment (TME) holds great potential for enhancing the precision of tumor therapies. Neoadjuvant therapy offers the possibility of alleviating patient symptoms and improving overall quality of life. Additionally, it may facilitate the reduction of inoperable tumors and prevent potential preoperative micrometastases. Within the TME, cancer-associated fibroblasts (CAFs) play a prominent role as they generate various elements that contribute to tumor progression. Particularly, extracellular matrix (ECM) produced by CAFs prevents immune cell infiltration into the TME, hampers drug penetration, and diminishes therapeutic efficacy. Therefore, this review provides a summary of the heterogeneity and interactions of CAFs within the TME, with a specific focus on the influence of neoadjuvant therapy on the microenvironment, particularly CAFs. Finally, we propose several potential and promising therapeutic strategies targeting CAFs, which may efficiently eliminate CAFs to decrease stroma density and impair their functions.

Gallium68-Labeled Fibroblast Activation Protein Inhibitor (68Ga-FAPI) PET/CT as an Alternative to Fluoro18-Fluorodeoxyglucose (18F-FDG) PET/CT: Discussion in a Case of Metastatic Adenocarcinoma of Pancreas

Positron emission tomography (PET) is an integral part of the imaging of solid tumors in today's oncology practice. The most commonly used PET radiotracer is 18F-Fluorodeoxyglucose (18F-FDG). FDG PET has imaging characteristics of a high tumor-to-background uptake ratio and is used in the detection of primary as well as metastatic sites. However, a significant pitfall is its inability to differentiate between neoplastic and infective lesions. To address this concern, many PET radiotracers have been developed and tried over time, a promising one being radiolabelled fibroblast activation protein inhibitor (FAPI). Fibroblast-activated protein (FAP) is a type II transmembrane glycoprotein expressed by cancer-associated fibroblasts (CAFs); it forms a significant component of the tumor stroma. Since there is over-expression of CAF in the majority of malignancies, it is a potential target for molecular imaging using PET. Several radiolabeled FAP inhibitors have been developed for PET imaging of malignancies and have also been used in theranostic applications.

Fibroblast Activation Protein-Targeted Radioligand Therapy with 177Lu-EB-FAPI for Metastatic Radioiodine-Refractory Thyroid Cancer: First-in-Human, Dose-Escalation Study

PURPOSE

Fibroblast activation protein (FAP) is a promising target for tumor treatment. In this study, we aimed to investigate the safety and efficacy of the albumin binder-conjugated FAP-targeted radiopharmaceutical, 177Lu-EB-FAPI (177Lu-LNC1004), in patients with metastatic radioiodine-refractory thyroid cancer (mRAIR-TC).

PATIENTS AND METHODS

This open-label, non-randomized, first-in-human, dose-escalation, investigator-initiated trial had a 3+3 design and involved a 6-week 177Lu-LNC1004 treatment cycle in patients with mRAIR-TC at 2.22 GBq initially, with subsequent cohorts receiving an incremental 50% dose increase until dose-limiting toxicity (DLT) was observed.

RESULTS

177Lu-LNC1004 administration was well tolerated, with no life-threatening adverse events observed. No patients experienced DLT in Group A (2.22 GBq/cycle). One patient experienced grade 4 thrombocytopenia in Group B (3.33 GBq/cycle); hence, another three patients were enrolled, none of whom experienced DLT. Two patients experienced grade 3 and 4 hematotoxicity in Group C (4.99 GBq/cycle). The mean whole-body effective dose was 0.17 ± 0.04 mSv/MBq. Intense 177Lu-LNC1004 uptake and prolonged tumor retention resulted in high mean absorbed tumor doses (8.50 ± 12.36 Gy/GBq). The mean effective half-lives for the whole-body and tumor lesions were 90.20 ± 7.68 and 92.46 ± 9.66 hours, respectively. According to RECIST, partial response, stable disease, and progressive disease were observed in 3 (25%), 7 (58%), and 2 (17%) patients, respectively. The objective response and disease control rates were 25% and 83%, respectively.

CONCLUSIONS

FAP-targeted radioligand therapy with 177Lu-LNC1004 at 3.33 GBq/cycle was well tolerated in patients with advanced mRAIR-TC, with high radiation dose delivery to the tumor lesions, encouraging therapeutic efficacy, and acceptable side effects.

68Ga-Labeled Fibroblast Activation Protein Inhibitor (68Ga-FAPI) PET for Pancreatic Adenocarcinoma: Data from the 68Ga-FAPI PET Observational Trial

The fibroblast activation protein (FAP) is highly expressed on carcinoma-associated fibroblasts in the stroma of pancreatic cancer and thus is a promising target for imaging and therapy. Preliminary data on PET imaging with radiolabeled FAP inhibitors (FAPIs) demonstrate superior tumor detection. Here we assess the accuracy of FAP-directed PET in patients with pancreatic cancer. Methods: Of 64 patients with suspected or proven pancreatic cancer, 62 (97%) were included in the data analysis of the 68Ga-FAPI PET observational trial (NCT04571086). All of these patients underwent contrast-enhanced CT, and 38 patients additionally underwent 18F-FDG PET. The primary study endpoint was the association of 68Ga-FAPI PET uptake intensity and histopathologic FAP expression. Secondary endpoints were detection rate, diagnostic performance, interreader reproducibility, and change in management. Datasets were interpreted by 2 masked readers. Results: The primary endpoint was met: The association between 68Ga-FAPI SUVmax and histopathologic FAP expression was significant (Spearman r, 0.48; P = 0.04). For histopathology-validated lesions, 68Ga-FAPI PET showed high sensitivity and positive predictive values (PPVs) on per-patient (sensitivity, 100%; PPV, 96.3%) and per-region (sensitivity, 100%; PPV, 97.0%) bases. In a head-to-head comparison versus 18F-FDG or contrast-enhanced CT, 68Ga-FAPI detected more tumor on a per-lesion (84.7% vs. 46.5% vs. 52.9%), per-patient (97.4% vs. 73.7% vs. 92.1%), or per-region (32.6% vs. 18.8% vs. 23.7%) basis, respectively. 68Ga-FAPI PET readers showed substantial overall agreement on the basis of the Fleiss κ: primary κ, 0.77 (range, 0.66-0.88). Minor and major changes in clinical management occurred in 5 patients (8.4%) after 68Ga-FAPI PET. Conclusion: We confirmed an association of 68Ga-FAPI PET SUVmax and histopathologic FAP expression in pancreatic cancer patients. Additionally, we found high detection rate and diagnostic accuracy, superior to those of 18F-FDG PET/CT. 68Ga-FAPI might become a powerful diagnostic tool for pancreatic cancer work-up.

Tumor-Targeted Interleukin 2 Boosts the Anticancer Activity of FAP-Directed Radioligand Therapeutics

We studied the antitumor efficacy of a combination of 177Lu-labeled radioligand therapeutics targeting the fibroblast activation protein (FAP) (OncoFAP and BiOncoFAP) with the antibody-cytokine fusion protein L19-interleukin 2 (L19-IL2) providing targeted delivery of interleukin 2 to tumors. Methods: The biodistribution of 177Lu-OncoFAP and 177Lu-BiOncoFAP at different molar amounts (3 vs. 250 nmol/kg) of injected ligand was studied via SPECT/CT in mice bearing subcutaneous HT-1080.hFAP tumors, and self-absorbed tumor and organ doses were calculated. The in vivo anticancer effect of 5 MBq of the radiolabeled preparations was evaluated as monotherapy or in combination with L19-IL2 in subcutaneously implanted HT-1080.hFAP and SK-RC-52.hFAP tumors. Tumor samples from animals treated with 177Lu-BiOncoFAP, L19-IL2, or both were analyzed by mass spectrometry-based proteomics to identify therapeutic signatures on cellular and stromal markers of cancer and on immunomodulatory targets. Results: 177Lu-BiOncoFAP led to a significantly higher self-absorbed dose in FAP-positive tumors (0.293 ± 0.123 Gy/MBq) than did 177Lu-OncoFAP (0.157 ± 0.047 Gy/MBq, P = 0.01) and demonstrated favorable tumor-to-organ ratios at high molar amounts of injected ligand. Administration of L19-IL2 or 177Lu-BiOncoFAP as single agents led to cancer cures in only a limited number of treated animals. In 177Lu-BiOncoFAP-plus-L19-IL2 combination therapy, complete remissions were observed in all injected mice (7/7 complete remissions for the HT-1080.hFAP model, and 4/4 complete remissions for the SK-RC-52.hFAP model), suggesting therapeutic synergy. Proteomic studies revealed a mechanism of action based on the activation of natural killer cells, with a significant enhancement of the expression of granzymes and perforin 1 in the tumor microenvironment after combination treatment. Conclusion: The combination of OncoFAP-based radioligand therapeutics with concurrent targeting of interleukin 2 shows synergistic anticancer effects in the treatment of FAP-positive tumors. This experimental finding should be corroborated by future clinical studies.

Gallium-68-labeled fibroblast activation protein inhibitor-46 PET in patients with resectable or borderline resectable pancreatic ductal adenocarcinoma: A phase 2, multicenter, single arm, open label non-randomized study protocol

BACKGROUND

Pancreatic ductal adenocarcinoma (PDAC) is a lethal disease prone to widespread metastatic dissemination and characterized by a desmoplastic stroma that contributes to poor outcomes. Fibroblast activation protein (FAP)-expressing Cancer-Associated Fibroblasts (CAFs) are crucial components of the tumor stroma, influencing carcinogenesis, fibrosis, tumor growth, metastases, and treatment resistance. Non-invasive tools to profile CAF identity and function are essential for overcoming CAF-mediated therapy resistance, developing innovative targeted therapies, and improved patient outcomes. We present the design of a multicenter phase 2 study (clinicaltrials.gov identifier NCT05262855) of [68Ga]FAPI-46 PET to image FAP-expressing CAFs in resectable or borderline resectable PDAC.

METHODS

We will enroll up to 60 adult treatment-naïve patients with confirmed PDAC. These patients will be eligible for curative surgical resection, either without prior treatment (Cohort 1) or after neoadjuvant therapy (NAT) (Cohort 2). A baseline PET scan will be conducted from the vertex to mid-thighs approximately 15 minutes after administering 5 mCi (±2) of [68Ga]FAPI-46 intravenously. Cohort 2 patients will undergo an additional PET after completing NAT but before surgery. Histopathology and FAP immunohistochemistry (IHC) of initial diagnostic biopsy and resected tumor samples will serve as the truth standards. Primary objective is to assess the sensitivity, specificity, and accuracy of [68Ga]FAPI-46 PET for detecting FAP-expressing CAFs. Secondary objectives will assess predictive values and safety profile validation. Exploratory objectives are comparison of diagnostic performance of [68Ga]FAPI-46 PET to standard-of-care imaging, and comparison of pre- versus post-NAT [68Ga]FAPI-46 PET in Cohort 2.

CONCLUSION

To facilitate the clinical translation of [68Ga]FAPI-46 in PDAC, the current study seeks to implement a coherent strategy to mitigate risks and increase the probability of meeting FDA requirements and stakeholder expectations. The findings from this study could potentially serve as a foundation for a New Drug Application to the FDA.

TRIAL REGISTRATION

@ClinicalTrials.gov identifier NCT05262855.

Effective Treatment of SSTR2-Positive Small Cell Lung Cancer Using 211At-Containing Targeted α-Particle Therapy Agent Which Promotes Endogenous Antitumor Immune Response

Small cell lung cancer (SCLC) is a neuroendocrine tumor with a high degree of malignancy. Due to limited treatment options, patients with SCLC have a poor prognosis. We have found, however, that intravenously administered octreotide (Oct) armed with astatine-211 ([211At]SAB-Oct) is effective against a somatostatin receptor 2 (SSTR2)-positive SCLC tumor in SCLC tumor-bearing BALB/c nude mice. In biodistribution analysis, [211At]SAB-Oct achieved the highest concentration in the SCLC tumors up to 3 h after injection as time proceeded. A single intravenous injection of [211At]SAB-Oct (370 kBq) was sufficient to suppress SSTR2-positive SCLC tumor growth in treated mice by inducing DNA double-strand breaks. Additionally, a multitreatment course (370 kBq followed by twice doses of 370 kBq for a total of 1110 kBq) inhibited the growth of the tumor compared to the untreated control group without significant off-target toxicity. Surprisingly, we found that [211At]SAB-Oct could up-regulate the expressions of calreticulin and major histocompatibility complex I (MHC-I) on the tumor cell membrane surface, suggesting that α-particle internal irradiation may activate an endogenous antitumor immune response through the regulation of immune cells in the tumor microenvironment, which could synergically enhance the efficacy of immunotherapy. We conclude that [211At]SAB-Oct is a potential new therapeutic option for SSTR2-positive SCLC.

Comparing the clinical value of baseline [68 Ga]Ga-FAPI-04 PET/CT and [18F]F-FDG PET/CT in pancreatic ductal adenocarcinoma: additional prognostic value of the distal pancreatitis

PURPOSE

Anatomical and molecular staging strategies are needed for the personalized treatment of localized pancreatic ductal adenocarcinoma (PDAC). This study evaluated the performance of [68 Ga]Ga-FAPI-04 and [18F]F-FDG PET/CT on the disease staging and prognostic value of patients with localized PDAC on contrast-enhanced (CE)-CT images.

METHODS

Patients with suspected localized PDAC on CE-CT were recruited for static [68 Ga]Ga-FAPI-04 and 18[F]F-FDG and PET/CT, and select patients underwent simultaneous 60-min dynamic 68 Ga-FAPI-04 PET/CT. The diagnostic and staging performances of the static PET/CT results were evaluated by delineating regions of interest in the primary tumor, whole pancreas, and distal pancreas in both types of scans and then evaluating correlations between the PET/CT findings and clinicopathological characteristics. Furthermore, Kaplan-Meier and hazard ratio (log-rank) methods were used to evaluate the prognostic value of the combined dynamic [68 Ga]Ga-FAPI-04 and static [18F]F-FDG PET/CT method.

RESULTS

We included 49 patients with histologically confirmed PDAC adenocarcinomas; 32 underwent 60-min dynamic [68 Ga]Ga-FAPI-04 PET/CT imaging simultaneously. The static [68 Ga]Ga-FAPI-04 method had significantly higher accuracy and uptake values than the static [18F]F-FDG method for primary PDAC lesions, metastatic lymph nodes, and distal metastases. Furthermore, 18.4% and 10.2% of the patients' stages changed after using the [68 Ga]Ga-FAPI-04 and [18F]F-FDG PET/CT methodologies, respectively, compared to the CE-CT-designated stage. The Ki values obtained from dynamic [68 Ga]Ga-FAPI-04 PET/CT did not differ between PDAC and distal obstructive pancreatitis lesions. Pathologically enlarged tumor size, poor differentiation, and perineural invasion were associated with increased [68 Ga]Ga-FAPI-04 uptake but not with [18F]F-FDG uptake. The preoperative prognostic performance of [68 Ga]Ga-FAPI-04 was better than that of [18F]F-FDG. Interestingly, combined [68 Ga]Ga-FAPI-04 and [18F]F-FDG uptake results in the whole pancreas could further stratify patients based on their postoperative prognosis.

CONCLUSION

6[68 Ga]Ga-FAPI-04 PET/CT was more sensitive and accurate than [18F]F-FDG PET/CT for tumor, node, and metastasis staging of PDAC identified on CE-CT. Additionally, [68 Ga]Ga-FAPI-04 uptake was significantly associated with pathologically aggressive tumor features. Combined [68 Ga]Ga-FAPI-04 and [18F]F-FDG PET/CT findings improved the prognostic value, potentially providing a non-invasive guide for clinical management. Finally, increased fibroblast activity in PDAC-induced obstructive pancreatitis may be associated with poor patient survival rates.

From FDG and beyond: the evolving potential of nuclear medicine

The radiopharmaceutical 2-[fluorine-18]fluoro-2-deoxy-D-glucose (FDG) has been dominantly used in positron emission tomography (PET) scans for over 20 years, and due to its vast utility its applications have expanded and are continuing to expand into oncology, neurology, cardiology, and infectious/inflammatory diseases. More recently, the addition of artificial intelligence (AI) has enhanced nuclear medicine diagnosis and imaging with FDG-PET, and new radiopharmaceuticals such as prostate-specific membrane antigen (PSMA) and fibroblast activation protein inhibitor (FAPI) have emerged. Nuclear medicine therapy using agents such as [177Lu]-dotatate surpasses conventional treatments in terms of efficacy and side effects. This article reviews recently established evidence of FDG and non-FDG drugs and anticipates the future trajectory of nuclear medicine.

Recent Pre-Clinical Advancements in Nuclear Medicine: Pioneering the Path to a Limitless Future

The theranostic approach in oncology holds significant importance in personalized medicine and stands as an exciting field of molecular medicine. Significant achievements have been made in this field in recent decades, particularly in treating neuroendocrine tumors using 177-Lu-radiolabeled somatostatin analogs and, more recently, in addressing prostate cancer through prostate-specific-membrane-antigen targeted radionuclide therapy. The promising clinical results obtained in these indications paved the way for the further development of this approach. With the continuous discovery of new molecular players in tumorigenesis, the development of novel radiopharmaceuticals, and the potential combination of theranostics agents with immunotherapy, nuclear medicine is poised for significant advancements. The strategy of theranostics in oncology can be categorized into (1) repurposing nuclear medicine agents for other indications, (2) improving existing radiopharmaceuticals, and (3) developing new theranostics agents for tumor-specific antigens. In this review, we provide an overview of theranostic development and shed light on its potential integration into combined treatment strategies.

An antibody-radionuclide conjugate targets fibroblast activation protein for cancer therapy

PURPOSE

Fibroblast activation protein is one of the most attractive targets for tumor diagnosis and therapy. There have been many successful clinical translations with small molecules and peptides, yet only a few anti-FAP antibody diagnostic or therapeutic agents have been reported. Antibodies often feature good tumor selectivity and long tumor retention, which may be a better-match with therapeutic radionuclides (e.g.,177Lu, 225Ac) for cancer therapy. Here we report a 177Lu-labeled anti-FAP antibody, PKU525, as a therapeutic radiopharmaceutical for FAP-targeted radiotherapy.

METHODS

The anti-FAP antibody is produced as a derivative of sibrotuzumab. The pharmacokinetics and blocking study are performed with 89Zr-labeled antibody by PET imaging. The conjugation strategies have been screened and tested with SPECT imaging through 177Lu-labeling. The biodistribution and radiotherapy studies are performed on 177Lu-labeled anti-FAP antibody in NU/NU mice-bearing HT-1080-FAP tumors.

RESULTS

A multiple time-point PET imaging study shows that the tumor accumulation of [89Zr]Zr-DFO-PKU525 is intense, selective, and relatively rapid. The time activity curve indicates that the tumor uptake continually increases until reaches the highest uptake (SUVmax = 18.4 ± 2.3, n = 4) at 192 h, then gradually declines. Radioactivity rapidly cleared from the blood, liver, and other major organs, resulting in high tumor-to-background ratios. An in vivo blocking experiment suggests that [89Zr]Zr-DFO-PKU525 is FAP-specific and the uptake in FAP-negative tumors is almost negligible. Ex vivo biodistribution study shows that the tumor uptake of [177Lu]Lu-DOTA-NCS-PKU525 is 23.04 ± 5.11% ID/g, 33.2 ± 6.36% ID/g, 19.87 ± 6.84% ID/g and 19.02 ± 5.90% ID/g at 24 h, 96 h, 168 h, and 240 h after injection (n = 5), which is corroborated with the PET imaging. In therapeutic assays, multiple doses of [177Lu]Lu-DOTA-NCS-PKU525 have been tested in tumor-bearing mice, and the data suggests that 3.7 MBq may be sufficient to completely suppress the tumor growth in mice without showing observable side effects.

CONCLUSION

A FAP-targeted antibody-radionuclide conjugate was developed and evaluated in vitro and in vivo. Its tumor accumulation is rapid and high with a clean background. It remarkably suppresses the tumors in mice while the side effect is almost negligible, showing that it is promising for further clinical translational studies.

Potential Value of FAPI PET/CT in the Detection and Treatment of Fibrosing Mediastinitis: Preclinical and Pilot Clinical Investigation

Fibrosing mediastinitis (FM) is a rare proliferative disease within the mediastinum that leads to pulmonary hypertension, which has been regarded as a major cause of death. This study aims to evaluate the potential value of fibroblast activation protein inhibitor (FAPI)-PET/CT in the integration of diagnosis and treatment of FM through targeting FAPI in fibrosis rats and provide a theoretical basis for clinical management of FM patients. By performing a 18F-FAPI PET/CT scan, the presence of FAPI-avid in the fibrotic lesion was determined. Through a fibrosis rat model, 18F-FAPI-74 was used for lesion imaging and 177Lu-FAPI-46 was utilized to investigate the potential therapeutic effect on FM in vivo. In addition, biodistribution analysis and radiation dosimetry were carried out. With the 177Lu-FAPI-46 pharmacokinetic data of rats as the input, the estimated dose for female adults was computed, which can provide some useful information for the safe application of radiolabeled FAPI in the detection and treatment of FM in patients. Then, major findings on the use of FAPI PET/CT and SPECT/CT in FM were presented. 18F-FAPI-74 showed a high-level uptake in FM lesions of patients (SUVmax 7.94 ± 0.26), which was also observed in fibrosis rats (SUVmax 2.11 ± 0.23). Consistently, SPECT/CT imaging of fibrosis rats also revealed that 177Lu-FAPI-46-avid was active for up to 60 h in fibrotic lesions. In addition to this robust diagnostic performance, a possible therapeutic impact was evaluated as well. It turned out that no spontaneous healing of lesions was observed in the control group, whereas there was complete healing on day 9, day 11, and day 14 in the 30, 100, and 300 MBq groups, respectively. With a significant difference in the free of event rate in the Kaplan-Meier curve among four groups (P < 0.001), a dose of 300 MBq displayed the best therapeutic effect, and no obvious damage was observed in the kidney. Furthermore, organ-absorbed doses and an effective dose (0.4320 mSv/MBq) of 177Lu-FAPI-46 presumed for patients were assumed to give a preliminary indication of its safe use in clinical practice. In conclusion, 18F-FAPI-46 PET/CT can be a potentially valuable tool for the diagnosis of FM. Of note, 177Lu-FAPI-46 may be a novel and safe radiolabeled reagent for the integration of diagnosis and treatment of FM.

Developments in 177Lu-based radiopharmaceutical therapy and dosimetry

177Lu is a radioisotope that has become increasingly popular as a therapeutic agent for treating various conditions, including neuroendocrine tumors and metastatic prostate cancer. 177Lu-tagged radioligands are molecules precisely designed to target and bind to specific receptors or proteins characteristic of targeted cancer. This review paper will present an overview of the available 177Lu-labelled radioligands currently used to treat patients. Based on recurring, active, and completed clinical trials and other available literature, we evaluate current status, interests, and developments in assessing patient-specific dosimetry, which will define the future of this particular treatment modality. In addition, we will discuss the challenges and opportunities of the existing dosimetry standards to measure and calculate the radiation dose delivered to patients, which is essential for ensuring treatments' safety and efficacy. Finally, this article intends to provide an overview of the current state of 177Lu- tagged radioligand therapy and highlight the areas where further research can improve patient treatment outcomes.

Advances in PET/CT Imaging for Breast Cancer

One out of eight women will be affected by breast cancer during her lifetime. Imaging plays a key role in breast cancer detection and management, providing physicians with information about tumor location, heterogeneity, and dissemination. In this review, we describe the latest advances in PET/CT imaging of breast cancer, including novel applications of 18F-FDG PET/CT and the development and testing of new agents for primary and metastatic breast tumor imaging and therapy. Ultimately, these radiopharmaceuticals may guide personalized approaches to optimize treatment based on the patient's specific tumor profile, and may become a new standard of care. In addition, they may enhance the assessment of treatment efficacy and lead to improved outcomes for patients with a breast cancer diagnosis.

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

PURPOSE

A series of radiotracers targeting fibroblast activation protein (FAP) with great pharmacokinetics have been developed for cancer diagnosis and therapy. Nevertheless, the use of dominant PET tracers, gallium-68-labeled FAPI derivatives, was limited by the short nuclide half-life and production scale, and the therapeutic tracers exhibited rapid clearance and insufficient tumor retention. In this study, we developed a FAP targeting ligand, LuFL, containing organosilicon-based fluoride acceptor (SiFA) and DOTAGA chelator, capable of labeling fluorine-18 and lutetium-177 in one molecular with simple and highly efficient labeling procedure, to achieve cancer theranostics.

METHODS

The precursor LuFL (20) and [natLu]Lu-LuFL (21) were successfully synthesized and labeled with fluorine-18 and lutetium-177 using a simple procedure. A series of cellular assays were performed to characterize the binding affinity and FAP specificity. PET imaging, SPECT imaging, and biodistribution studies were conducted to evaluate pharmacokinetics in HT-1080-FAP tumor-bearing nude mice. A comparison study of [177Lu]Lu-LuFL ([177Lu]21) and [177Lu]Lu-FAPI-04 was carried out in HT-1080-FAP xenografts to determine the cancer therapeutic efficacy.

RESULTS

LuFL (20) and [natLu]Lu-LuFL (21) demonstrated excellent binding affinity towards FAP (IC50: 2.29 ± 1.12 nM and 2.53 ± 1.87 nM), compared to that of FAPI-04 (IC50: 6.69 ± 0.88 nM). In vitro cellular studies showed that 18F-/177Lu-labeled 21 displayed high specific uptake and internalization in HT-1080-FAP cells. Micro-PET, SPECT imaging and biodistribution studies with [18F]/[177Lu]21 revealed higher tumor uptake and longer tumor retention than those of [68 Ga]/[177Lu]Ga/Lu-FAPI-04. The radionuclide therapy studies showed significantly greater inhibition of tumor growth for the [177Lu]21 group, than for the control group and the [177Lu]Lu-FAPI-04 group.

CONCLUSION

The novel FAPI-based radiotracer containing SiFA and DOTAGA was developed as a theranostics radiopharmaceutical with simple and short labeling process, and showed promising properties including higher cellular uptake, better FAP binding affinity, higher tumor uptake and prolong retention compared to FAPI-04. Preliminary experiments with 18F- and 177Lu-labeled 21 showed promising tumor imaging properties and favorable anti-tumor efficacy.

Organotrifluoroborate enhances tumor targeting of fibroblast activation protein inhibitors for targeted radionuclide therapy

PURPOSE

Fibroblast activation protein (FAP) is a pan-cancer target and now the state-of-the-art to develop radiopharmaceuticals. FAP inhibitors have been of great success in developing imaging tracers. Yet, the overly rapid clearance cannot match with the long half-lives of regular therapeutic radionuclides. Though strategies that aim to elongate the circulation of FAPIs are being developed, here we describe an innovation that uses α-emitters of short half-lives (e.g., 213Bi) to pair the rapid pharmacokinetics of FAPIs.

METHODS

An organotrifluoroborate linker is engineered to FAPIs to give two advantages: (1) selectively increases tumor uptake and retention; (2) facile 18F-radiolabeling for positron emission tomography to guide radiotherapy with α-emitters, which can hardly be traced in general.

RESULTS

The organotrifluoroborate linker helps to improve the internalization in cancer cells, resulting in notably higher tumor uptake while the background is clean. In FAP-expressed tumor-bearing mice, this FAPI labeled with 213Bi, a short half-life α-emitter, exhibits almost complete suppression to tumor growth while the side effect is negligible. Additional data shows that this strategy is generally applicable to guide other α-emitters, such as 212Bi, 212Pb, and 149Tb.

CONCLUSION

The organotrifluoroborate linker may be of importance to optimize FAP-targeted radiopharmaceuticals, and the short half-lived α-emitters may be of choice for the rapid-cleared small molecule-based radiopharmaceuticals.

Clinical Advances and Perspectives in Targeted Radionuclide Therapy

Targeted radionuclide therapy has become increasingly prominent as a nuclear medicine subspecialty. For many decades, treatment with radionuclides has been mainly restricted to the use of iodine-131 in thyroid disorders. Currently, radiopharmaceuticals, consisting of a radionuclide coupled to a vector that binds to a desired biological target with high specificity, are being developed. The objective is to be as selective as possible at the tumor level, while limiting the dose received at the healthy tissue level. In recent years, a better understanding of molecular mechanisms of cancer, as well as the appearance of innovative targeting agents (antibodies, peptides, and small molecules) and the availability of new radioisotopes, have enabled considerable advances in the field of vectorized internal radiotherapy with a better therapeutic efficacy, radiation safety and personalized treatments. For instance, targeting the tumor microenvironment, instead of the cancer cells, now appears particularly attractive. Several radiopharmaceuticals for therapeutic targeting have shown clinical value in several types of tumors and have been or will soon be approved and authorized for clinical use. Following their clinical and commercial success, research in that domain is particularly growing, with the clinical pipeline appearing as a promising target. This review aims to provide an overview of current research on targeting radionuclide therapy.

Fibroblast Activation Protein Inhibitor-Based Radionuclide Therapies: Current Status and Future Directions

Metastatic malignancies have limited management strategies and variable treatment responses. Cancer cells develop beside and depend on the complex tumor microenvironment. Cancer-associated fibroblasts, with their complex interaction with tumor and immune cells, are involved in various steps of tumorigenesis, such as growth, invasion, metastasis, and treatment resistance. Prooncogenic cancer-associated fibroblasts emerged as attractive therapeutic targets. However, clinical trials have achieved suboptimal success. Fibroblast activation protein (FAP) inhibitor-based molecular imaging has shown encouraging results in cancer diagnosis, making them innovative targets for FAP inhibitor-based radionuclide therapies. This review summarizes the results of preclinical and clinical FAP-based radionuclide therapies. We will describe advances and FAP molecule modification in this novel therapy, as well as its dosimetry, safety profile, and efficacy. This summary may guide future research directions and optimize clinical decision-making in this emerging field.

Role of 68 Ga-FAPI PET/CT in Assessing Hepatobiliary Malignancies : A Prospective Pilot Study

INTRODUCTION AND AIM

Preliminary studies showed good expression of fibroblast activating protein inhibitor (FAPI) in hepatocellular carcinoma (HCC) and cholangiocarcinoma (CC). Our aims were to study the diagnostic performance of 68 Ga-FAPI PET/CT in diagnosing the primary hepatobiliary malignancies and to compare its performance with 18 F-FDG PET/CT.

PATIENTS AND METHODS

Patients suspected to have HCC and CC were recruited prospectively. FDG and FAPI PET/CT studies were completed within 1 week. Final diagnosis of malignancy was achieved by tissue diagnosis (either histopathological examination or fine-needle aspiration cytology) and radiological correlation from conventional modalities. Results were compared with final diagnosis and expressed as sensitivity, specificity, positive predictive value, negative predictive value, and diagnostic accuracy.

RESULTS

Forty-one patients were included. Thirty-one were positive for malignancy and 10 were negative. Fifteen were metastatic. Of 31, 18 were CC and 6 were HCC. For overall diagnosis of the primary disease, FAPI PET/CT performed exceptionally compared with FDG PET/CT with sensitivity, specificity, and accuracy of 96.77%, 90%, and 95.12%, respectively, versus 51.61%, 100%, and 63.41% for FDG PET/CT. FAPI PET/CT clearly outperformed FDG PET/CT for the evaluation of CC with sensitivity, specificity, and accuracy of 94.4%, 100%, and 95.24%, respectively, whereas for FDG PET/CT sensitivity, specificity, and accuracy were 50%, 100%, and 57.14%, respectively. Diagnostic accuracy of FAPI PET/CT was 61.54% for metastatic HCC compared with 84.62% for FDG PET/CT.

CONCLUSIONS

Our study highlights the potential role of FAPI-PET/CT in evaluating CC. It also ascertains its usefulness in the cases of mucinous adenocarcinoma. Although it showed a higher lesion detection rate than FDG in primary HCC, its diagnostic performance in the metastatic setting is questionable.

EDB-FN targeted probes for the surgical navigation, radionuclide imaging, and therapy of thyroid cancer

PURPOSE

Extradomain B of fibronectin (EDB-FN) is a promising diagnostic and therapeutic biomarker for thyroid cancer (TC). Here, we identified a high-affinity EDB-FN targeted peptide named EDBp (AVRTSAD) and developed three EDBp-based probes, Cy5-PEG4-EDBp(Cy5-EDBp), [18F]-NOTA-PEG4-EDBp([18F]-EDBp), and [177Lu]-DOTA-PEG4-EDBp ([177Lu]-EDBp), for the surgical navigation, radionuclide imaging, and therapy of TC.

METHODS

Based on the previously identified EDB-FN targeted peptide ZD2, the optimized EDB-FN targeted peptide EDBp was identified by using the alanine scan strategy. Three EDBp-based probes, Cy5-EDBp, [18F]-EDBp, and [177Lu]-EDBp, were developed for fluorescence imaging, positron emission tomography (PET) imaging, and radiotherapy in TC tumor-bearing mice, respectively. Additionally, [18F]-EDBp was evaluated in two TC patients.

RESULTS

The binding affinity of EDBp to the EDB fragment protein (Kd = 14.4 ± 1.4 nM, n = 3) was approximately 336-fold greater than that of the ZD2 (Kd = 4839.7 ± 361.7 nM, n = 3). Fluorescence imaging with Cy5-EDBp facilitated the complete removal of TC tumors. [18F]-EDBp PET imaging clearly delineated TC tumors, with high tumor uptake (16.43 ± 1.008%ID/g, n = 6, at 1-h postinjection). Radiotherapy with [177Lu]-EDBp inhibited tumor growth and prolonged survival in TC tumor-bearing mice (survival time of different treatment groups: saline vs. EDBp vs. ABRAXANE vs. [177Lu]-EDBp = 8.00 d vs. 8.00 d vs. 11.67 d vs. 22.33 d, ***p < 0.001). Importantly, the first-in-human evaluation of [18F]-EDBp demonstrated that it had specific targeting properties (SUVmax value of 3.6) and safety.

CONCLUSION

Cy5-EDBp, [18F]-EDBp, and [177Lu]-EDBp are promising candidates for the surgical navigation, radionuclide imaging, and radionuclide therapy of TC, respectively.

Fibroblast activation protein-targeted radionuclide therapy: background, opportunities, and challenges of first (pre)clinical studies

INTRODUCTION

Fibroblast activation protein (FAP) is highly overexpressed in stromal tissue of various cancers. While FAP has been recognized as a potential diagnostic or therapeutic cancer target for decades, the surge of radiolabeled FAP-targeting molecules has the potential to revolutionize its perspective. It is presently hypothesized that FAP targeted radioligand therapy (TRT) may become a novel treatment for various types of cancer. To date, several preclinical and case series have been reported on FAP TRT using varying compounds and showing effective and tolerant results in advanced cancer patients. Here, we review the current (pre)clinical data on FAP TRT and discuss its perspective towards broader clinical implementation.  METHODS: A PubMed search was performed to identify all FAP tracers used for TRT. Both preclinical and clinical studies were included if they reported on dosimetry, treatment response or adverse events. The last search was performed on July 22 2022. In addition, a database search was performed on clinical trial registries (date 15th of July 2022) to search for prospective trials on FAP TRT.

RESULTS

In total, 35 papers were identified that were related to FAP TRT. This resulted in the inclusion of the following tracers for review: FAPI-04, FAPI-46, FAP-2286, SA.FAP, ND-bisFAPI, PNT6555, TEFAPI-06/07, FAPI-C12/C16, and FSDD.

CONCLUSION

To date, data was reported on more than 100 patients that were treated with different FAP targeted radionuclide therapies such as [177Lu]Lu-FAPI-04, [90Y]Y-FAPI-46, [177Lu]Lu-FAP-2286, [177Lu]Lu-DOTA.SA.FAPI and [177Lu]Lu-DOTAGA.(SA.FAPi)2. In these studies, FAP targeted radionuclide therapy has resulted in objective responses in difficult to treat end stage cancer patients with manageable adverse events. Although no prospective data is yet available, these early data encourages further research.

Evaluation of Astatine-211-Labeled Fibroblast Activation Protein Inhibitor (FAPI): Comparison of Different Linkers with Polyethylene Glycol and Piperazine

Fibroblast activation proteins (FAP) are overexpressed in the tumor stroma and have received attention as target molecules for radionuclide therapy. The FAP inhibitor (FAPI) is used as a probe to deliver nuclides to cancer tissues. In this study, we designed and synthesized four novel ²¹¹At-FAPI(s) possessing polyethylene glycol (PEG) linkers between the FAP-targeting and ²¹¹At-attaching moieties. ²¹¹At-FAPI(s) and piperazine (PIP) linker FAPI exhibited distinct FAP selectivity and uptake in FAPII-overexpressing HEK293 cells and the lung cancer cell line A549. The complexity of the PEG linker did not significantly affect selectivity. The efficiencies of both linkers were almost the same. Comparing the two nuclides, ²¹¹At was superior to ¹³¹I in tumor accumulation. In the mouse model, the antitumor effects of the PEG and PIP linkers were almost the same. Most of the currently synthesized FAPI(s) contain PIP linkers; however, in our study, we found that PEG linkers exhibit equivalent performance. If the PIP linker is inconvenient, a PEG linker is expected to be an alternative.

Implant Imaging: Perspectives of Nuclear Imaging in Implant, Biomaterial, and Stem Cell Research

Until now, very few efforts have been made to specifically trace, monitor, and visualize implantations, artificial organs, and bioengineered scaffolds for tissue engineering in vivo. While mainly X-Ray, CT, and MRI methods have been used for this purpose, the applications of more sensitive, quantitative, specific, radiotracer-based nuclear imaging techniques remain a challenge. As the need for biomaterials increases, so does the need for research tools to evaluate host responses. PET (positron emission tomography) and SPECT (single photon emission computer tomography) techniques are promising tools for the clinical translation of such regenerative medicine and tissue engineering efforts. These tracer-based methods offer unique and inevitable support, providing specific, quantitative, visual, non-invasive feedback on implanted biomaterials, devices, or transplanted cells. PET and SPECT can improve and accelerate these studies through biocompatibility, inertivity, and immune-response evaluations over long investigational periods at high sensitivities with low limits of detection. The wide range of radiopharmaceuticals, the newly developed specific bacteria, and the inflammation of specific or fibrosis-specific tracers as well as labeled individual nanomaterials can represent new, valuable tools for implant research. This review aims to summarize the opportunities of nuclear-imaging-supported implant research, including bone, fibrosis, bacteria, nanoparticle, and cell imaging, as well as the latest cutting-edge pretargeting methods.

Clinical Translation of Targeted α-Therapy: An Evolution or a Revolution?

The field of radioligand therapy has advanced greatly in recent years, driven largely by β-emitting therapies targeting somatostatin receptor-expressing tumors and the prostate-specific membrane antigen. Now, more clinical trials are under way to evaluate α-emitting targeted therapies as potential next-generation theranostics with even higher efficacy due to their high linear energy and short range in human tissues. In this review, we summarize the important studies ranging from the first Food and Drug Administration-approved α-therapy, ²²³Ra-dichloride, for treatment of bone metastases in castration-resistant prostate cancer, including concepts in clinical translation such as targeted α-peptide receptor radiotherapy and ²²⁵Ac-PSMA-617 for treatment of prostate cancer, innovative therapeutic models evaluating new targets, and combination therapies. Targeted α-therapy is one of the most promising fields in novel targeted cancer therapy, with several early- and late-stage clinical trials for neuroendocrine tumors and metastatic prostate cancer already in progress, along with significant interest and investment in additional early-phase studies. Together, these studies will help us understand the short- and long-term toxicity of targeted α-therapy and potentially identify suitable therapeutic combination partners.

Synthesis and Preclinical Evaluation of a Novel FAPI-04 Dimer for Cancer Theranostics

Overexpression of fibroblast activation protein (FAP) in cancer-associated fibroblasts in a wide variety of tumors enables a highly selective targeting strategy using FAP inhibitors (FAPIs). Quinoline-based FAPIs labeled with radionuclides have been widely developed for tumor-targeted nuclear medicine imaging. However, the short retention time of FAPIs at the tumor site limits their application in radionuclide therapy. In this study, a novel FAPI-04 dimer was synthesized and labeled with radionuclides to prolong the retention time in tumors for imaging and therapy. To prepare the FAPI-04 dimer complex, DOTA-Suc-Lys-(FAPI-04)₂, we used Fmoc-Lys(Boc)-OH as the linker to conjugate two FAPI-04 structures by an amide reaction. The resulting product was further modified by DOTA groups to allow for conjugation with radioactive metals. Both [⁶⁸Ga]Ga-(FAPI-04)₂ and [¹⁷⁷Lu]Lu-(FAPI-04)₂ showed a radiochemical purity of >99% and remained stable in vitro. In vivo, micro-PET images of SKOV3, A431, and H1299 xenografts revealed that the tumor uptake of [⁶⁸Ga]Ga-(FAPI-04)₂ was about twice that of [⁶⁸Ga]Ga-FAPI-04 and that the accumulation of [⁶⁸Ga]Ga-(FAPI-04)₂ at the tumor site did not significantly decrease even 3h after injection. The tumor-abdomen ratio of [⁶⁸Ga]Ga-(FAPI-04)₂ images was significantly higher than that of [¹⁸F]F-FDG images. For radionuclide therapy, [¹⁷⁷Lu]Lu-(FAPI-04)₂ effectively retarded tumor growth and displayed good tolerance. In conclusion, the DOTA-Suc-Lys-(FAPI-04)₂ design enhanced its uptake in FAP-expressing tumors, improved its retention time at the tumor site, and produced high-contrast imaging in xenografts after radionuclide labeling. Furthermore, it showed a noticeable antitumor effect. DOTA-Suc-Lys-(FAPI-04)₂ provides a new approach for applying FAPI derivatives in tumor theranostics.

Fibroblasts as Turned Agents in Cancer Progression

Differentiated epithelial cells reside in the homeostatic microenvironment of the native organ stroma. The stroma supports their normal function, their G0 differentiated state, and their expansion/contraction through the various stages of the life cycle and physiologic functions of the host. When malignant transformation begins, the microenvironment tries to suppress and eliminate the transformed cells, while cancer cells, in turn, try to resist these suppressive efforts. The tumor microenvironment encompasses a large variety of cell types recruited by the tumor to perform different functions, among which fibroblasts are the most abundant. The dynamics of the mutual relationship change as the sides undertake an epic battle for control of the other. In the process, the cancer “wounds” the microenvironment through a variety of mechanisms and attracts distant mesenchymal stem cells to change their function from one attempting to suppress the cancer, to one that supports its growth, survival, and metastasis. Analogous reciprocal interactions occur as well between disseminated cancer cells and the metastatic microenvironment, where the microenvironment attempts to eliminate cancer cells or suppress their proliferation. However, the altered microenvironmental cells acquire novel characteristics that support malignant progression. Investigations have attempted to use these traits as targets of novel therapeutic approaches.

225Ac-Labeled Somatostatin Analogs in the Management of Neuroendocrine Tumors: From Radiochemistry to Clinic

The widespread use of peptide receptor radionuclide therapy (PRRT) represents a major therapeutic breakthrough in nuclear medicine, particularly since the introduction of ¹⁷⁷Lu-radiolabeled somatostatin analogs. These radiopharmaceuticals have especially improved progression-free survival and quality of life in patients with inoperable metastatic gastroenteropancreatic neuroendocrine tumors expressing somatostatin receptors. In the case of aggressive or resistant disease, the use of somatostatin derivatives radiolabeled with an alpha-emitter could provide a promising alternative. Among the currently available alpha-emitting radioelements, actinium-225 has emerged as the most suitable candidate, especially regarding its physical and radiochemical properties. Nevertheless, preclinical and clinical studies on these radiopharmaceuticals are still few and heterogeneous, despite the growing momentum for their future use on a larger scale. In this context, this report provides a comprehensive and extensive overview of the development of ²²⁵Ac-labeled somatostatin analogs; particular emphasis is placed on the challenges associated with the production of ²²⁵Ac, its physical and radiochemical properties, as well as the place of ²²⁵Ac-DOTATOC and ²²⁵Ac-DOTATATE in the management of patients with advanced metastatic neuroendocrine tumors.

Fibroblast Activation Protein Inhibitor Theranostics

Fibroblast activation protein (FAP) was first reported in 1986. However, FAP is not expressed in normal fibroblasts, normal or malignant epithelial cells, or the stroma of benign epithelial tumors. FAP is a cell membrane-bound serine peptidase overexpressed on the surface of cancer-associated fibroblasts and, as such, is a novel target for molecular imaging of several tumors. FAP inhibitors (FAPI) are potential theranostic molecular probes for various cancers. A tumor model expressing FAP was used to verify or confirm the usefulness of FAPI experimentally.

Fibroblast Activation Protein Inhibitor (FAPI)-Based Theranostics-Where We Are at and Where We Are Heading: A Systematic Review

Cancer is the leading cause of death around the globe, followed by heart disease and stroke, with the highest mortality to this day. We have reached great levels of understanding of how these various types of cancer operate at a cellular level and this has brought us to what we call "precision medicine" where every diagnostic examination and the therapeutic procedure is tailored to the patient. FAPI is among the new tracers that can be used to assess and treat many types of cancer. The aim of this review was to gather all the known literature on FAPI theranostics. A MEDLINE search was conducted on four web libraries, PUBMED, Cochrane, Scopus, and Web of Sciences. All of the available articles that included both diagnoses and therapy with FAPI tracers were collected and put through the CASP (Critical Appraisal Skills Programme) questionnaire for systematic reviewing. A total of 8 records were deemed suitable for CASP review, ranging from 2018 to November 2022. These studies were put through the CASP diagnostic checklist, in order to assess the goal of the study, diagnostic and reference tests, results, descriptions of the patient sample, and future applications. Sample sizes were heterogeneous, both for size as well as for tumor type. Only one author studied a single type of cancer with FAPI tracers. Progression of disease was the most common outcome, and no relevant collateral effects were noted. Although FAPI theranostics is still in its infancy and lacks solid grounds to be brought into clinical practice, it does not show any collateral effects that prohibit administration to patients, thus far, and has good tolerability profiles.

Advancements of molecular imaging and radiomics in pancreatic carcinoma

Despite the recent progress of medical technology in the diagnosis and treatment of tumors, pancreatic carcinoma remains one of the most malignant tumors, with extremely poor prognosis partly due to the difficulty in early and accurate imaging evaluation. This paper focuses on the research progress of magnetic resonance imaging, nuclear medicine molecular imaging and radiomics in the diagnosis of pancreatic carcinoma. We also briefly described the achievements of our team in this field, to facilitate future research and explore new technologies to optimize diagnosis of pancreatic carcinoma.

Synthesis and Preclinical Evaluation of Three Novel 68Ga-Labeled Bispecific PSMA/FAP-Targeting Tracers for Prostate Cancer Imaging

Tumor heterogeneity limits the efficacy and reliability of monospecific radiopharmaceuticals in prostate cancer diagnosis and therapy. To overcome this limitation and improve lesion detection sensitivity, we developed and evaluated three bispecific radiotracers that can target both prostate-specific membrane antigen (PSMA) and fibroblast activation protein (FAP), which are the two key proteins overexpressed in prostate cancer. Three FAP-targeting ligands with various linker lengths were synthesized through multistep organic synthesis, and then connected to the PSMA-targeting motif. IC₅₀(PSMA) and IC₅₀(FAP) values of Ga-complexed bispecific ligands, Ga-AV01017, Ga-AV01030, and Ga-AV01038 were 25.2-71.6 and 1.25-2.74 nM, respectively. The uptake values in PSMA-expressing LNCaP tumor xenografts were 4.38 ± 0.55, 5.17 ± 0.51, and 4.25 ± 0.86 %ID/g for [⁶⁸Ga]Ga-AV01017, [⁶⁸Ga]Ga-AV01030, and [⁶⁸Ga]Ga-AV01038, respectively, which were lower than the monospecific PSMA-targeting tracer [⁶⁸Ga]Ga-HTK03041 (23.1 ± 6.11 %ID/g). The uptake values in FAP-expressing HEK293T:hFAP tumor xenografts were 2.99 ± 0.37, 3.69 ± 0.81, 3.64 ± 0.83 %ID/g for [⁶⁸Ga]Ga-AV01017, [⁶⁸Ga]Ga-AV01030, and [⁶⁸Ga]Ga-AV01038, respectively, which were also lower than the monospecific FAP-targeting tracer, [⁶⁸Ga]Ga-FAPI-04 (12.5 ± 2.00 %ID/g). We observed that the bispecific tracers had prolonged blood retention, in which tracers with a longer linker tend to have a higher blood uptake and lower tumor uptake. Further investigations are needed to optimize the linker selection to generate promising bispecific PSMA/FAP-targeting tracers for prostate cancer imaging.

Recent Trends in Diagnostic Biomarkers of Tumor Microenvironment

The tumor microenvironment (TME) play critical roles in tumor survival, progression, and metastasis and can be considered potential targets for molecular imaging of cancer. The targeting agents for imaging of TME components (e.g., fibroblasts, mesenchymal stromal cells, immune cells, extracellular matrix, blood vessels) provide a promising strategy to target these biomarkers for the early diagnosis of cancers. Moreover, various cancer types have similar tumor immune microenvironment (TIME) features that targeting those biomarkers and offer clinically translatable molecular imaging of cancers. In this review, we categorize and summarize the components in TME which have been targeted for molecular imaging. Moreover, this review updated the recent progress in targeted imaging of TIME biological molecules by various modalities for the early detection of cancer.

A Dimeric FAP-Targeting Small-Molecule Radioconjugate with High and Prolonged Tumor Uptake

Imaging procedures based on small-molecule radioconjugates targeting fibroblast activation protein (FAP) have recently emerged as a powerful tool for the diagnosis of a wide variety of tumors. However, the therapeutic potential of radiolabeled FAP-targeting agents is limited by their short residence time in neoplastic lesions. In this work, we present the development and in vivo characterization of BiOncoFAP, a new dimeric FAP-binding motif with an extended tumor residence time and favorable tumor-to-organ ratio. Methods: The binding properties of BiOncoFAP and its monovalent OncoFAP analog were assayed against recombinant human FAP. Preclinical experiments with ¹⁷⁷Lu-OncoFAP-DOTAGA (¹⁷⁷Lu-OncoFAP) and ¹⁷⁷Lu-BiOncoFAP-DOTAGA (¹⁷⁷Lu-BiOncoFAP) were performed on mice bearing FAP-positive HT-1080 tumors. Results: OncoFAP and BiOncoFAP displayed comparable subnanomolar dissociation constants toward recombinant human FAP in solution, but the bivalent BiOncoFAP bound more avidly to the target immobilized on solid supports. In a comparative biodistribution study, ¹⁷⁷Lu-BiOncoFAP exhibited a more stable and prolonged tumor uptake than ¹⁷⁷Lu-OncoFAP (∼20 vs. ∼4 percentage injected dose/g, respectively, at 24 h after injection). Notably, ¹⁷⁷Lu-BiOncoFAP showed favorable tumor-to-organ ratios with low kidney uptake. Both ¹⁷⁷Lu-OncoFAP and ¹⁷⁷Lu-BiOncoFAP displayed potent antitumor efficacy when administered at therapeutic doses to tumor-bearing mice. Conclusion: ¹⁷⁷Lu-BiOncoFAP is a promising candidate for radioligand therapy of cancer, with favorable in vivo tumor-to-organ ratios, a long tumor residence time, and potent anticancer efficacy.

Performance and Prospects of [68Ga]Ga-FAPI PET/CT Scans in Lung Cancer

Fibroblast activation protein (FAP) could be a promising target for tumor imaging and therapy, as it is expressed in >90% of epithelial cancers. A high level of FAP-expression might be associated with worse prognosis in several cancer types, including lung cancer. FAPI binds this protein and allows for labelling to Gallium-68, as well as several therapeutic radiopharmaceuticals. As FAP is only expressed at insignificant levels in adult normal tissue, FAPI provides a highly specific tumor-marker for many epithelial cancers. In this review, current information on the use of [68Ga]Ga-FAPI PET/CT in lung cancer is presented. [68Ga]Ga-FAPI shows a high uptake (standardized uptake value = SUVmax) and tumor-to-background ratio (TBR) in primary lung cancer lesions, as well as in metastatic lesions of other tumor types located in the lung and in lung cancer metastases located throughout the body. Where a comparison was made to [18F]FDG PET/CT, [68Ga]Ga-FAPI showed a similar or higher SUVmax and TBR. In brain and bone metastases, [68Ga]Ga-FAPI PET/CT outperformed [18F]FDG PET/CT. In addition to this strong diagnostic performance, a possible prognostic value of [68Ga]Ga-FAPI PET/CT in lung cancer is proposed.