Safety of Fibroblast Activation Protein-Targeted Radionuclide Therapy by a Low-Dose Dosimetric Approach Using 177Lu-FAPI04

[68Ga]Ga-FAPI04 Outperforms [18F]FDG PET/CT for Detecting Nodal Metastasis of Head and Neck Squamous Cell Carcinoma: A Single-Center Experience

This study assesses fibroblast activated protein inhibitor (FAPI) targeted PET/CT imaging against [18F]FDG PET/CT (FDG PET) for detecting nodal involvement in head and neck squamous cell carcinoma (HNSCC), intending to improve diagnostic precision for metastatic lymph nodes and lay the groundwork for future investigations. Methods: Patients diagnosed with HNSCC were retrospectively enrolled. All patients underwent [68Ga]Ga-FAPI04 PET/CT (FAPI PET) and FDG PET within 6 d. Primary tumor, lymph nodes, and tracer uptake were visually and quantitatively compared. The metastatic lymph nodes were evaluated using patient-and lesion-based analyses, with biopsy or postoperative histopathological examination as the reference. Results: The cohort includes 24 patients (17 men, 7 women; mean age 60 ± 11.8 years) who underwent FDG and FAPI PET for preoperative diagnostic workup or restaging due to known recurrence of HNSCC. Lesions included 24 primary tumors, 54 cervical lymph nodes, and 5 metastases. Primary tumors exhibited significant uptake on both PET modalities (median maximum standardized uptake value [SUVmax]: FDG 19.4 ± 11.6, FAPI 16.9 ± 4.6), with no statistically significant difference (p > 0.5). For lymph nodes, FAPI and FDG PET showed median SUVmax of 9.18 ± 6.77 and 9.67 ± 6.5, respectively. The patient-based analysis found FDG PET sensitivity at 88.2% and FAPI PET at 94.1%, with FAPI PET specificity significantly higher (85.7% vs. 42.8% for FDG PET). Lesion-based analysis revealed FAPI PET sensitivity and specificity at 84.2% and 93.7%, respectively, contrasting FDG PET's at 81.5% and 25%, respectively. Conclusion: This study underscores the efficacy of FAPI PET in detecting primary tumors in HNSCC. Furthermore, FAPI PET shows improved specificity over FDG PET for metastatic lymph nodes advocating further investigations for integrating FAPI PET into HNSCC clinical protocols for its enhanced precision in detecting metastatic lymph nodes.

Efficient α and β- radionuclide therapy targeting fibroblast activation protein-α in an aggressive preclinical mouse tumour model

PURPOSE

Targeted radionuclide therapy (TRT) is a cancer treatment with relative therapeutic efficacy across various cancer types. We studied the therapeutic potential of TRT using fibroblast activation protein-α (FAP) targeting sdAbs (4AH29) labelled with 225Ac or 131I in immunocompetent mice in a human FAP (hFAP) expressing lung cancer mouse model. We further explored the combination of TRT with programmed cell death ligand 1 (PD-L1) immune checkpoint blockade (ICB).

METHODS

We studied the biodistribution and tumour uptake of [131I]I-GMIB-4AH29 and [225Ac]Ac-DOTA-4AH29 by ex vivo γ-counting. Therapeutic efficacy of [131I]I-GMIB-4AH29 and [225Ac]Ac-DOTA-4AH29 was evaluated in an immunocompetent mouse model. Flow cytometry analysis of tumours from [225Ac]Ac-DOTA-4AH29 treated mice was performed. Treatment with [225Ac]Ac-DOTA-4AH29 was repeated in combination with PD-L1 ICB.

RESULTS

The biodistribution showed high tumour uptake of [131I]I-GMIB-4AH29 with 3.5 ± 0.5% IA/g 1 h post-injection (p.i.) decreasing to 0.9 ± 0.1% IA/g after 24 h. Tumour uptake of [225Ac]Ac-DOTA-4AH29 was also relevant with 2.1 ± 0.5% IA/g 1 h p.i. with a less steep decrease to 1.7 ± 0.2% IA/g after 24 h. Survival was significantly improved after treatment with low and high doses [131I]I-GMIB-4AH29 or [225Ac]Ac-DOTA-4AH29 compared to vehicle solution. Moreover, we observed significantly higher PD-L1 expression in tumours of mice treated with [225Ac]Ac-DOTA-4AH29 compared to vehicle solution. Therefore, we combined high dose [225Ac]Ac-DOTA-4AH29 with PD-L1 ICB showing therapeutic synergy.

CONCLUSION

[225Ac]Ac-DOTA-4AH29 and [131I]I-GMIB-4AH29 exhibit high and persistent tumour targeting, translating into prolonged survival in mice bearing aggressive tumours. Moreover, we demonstrate that the combination of PD-L1 ICB with [225Ac]Ac-DOTA-4AH29 TRT enhances its therapeutic efficacy.

In vitro and in vivo analyses of eFAP: a novel FAP-targeting small molecule for radionuclide theranostics and other oncological interventions

BACKGROUND

Fibroblast activation protein (FAP), a transmembrane serine protease overexpressed by cancer-associated fibroblasts in the tumor stroma, is an interesting biomarker for targeted radionuclide theranostics. FAP-targeting radiotracers have demonstrated to be superior to [18F]FDG PET/CT in various solid cancers. However, these radiotracers have suboptimal tumor retention for targeted radionuclide therapy (TRT). We aimed to develop a novel FAP-targeting pharmacophore with improved pharmacokinetics by introducing a substitution at the 8-position of (4-quinolinoyl)-glycyl-2-cyanopyrrolidine, which allows for conjugation of a chelator, dye, or other payloads.

RESULTS

Here we showed the synthesis of DOTA-conjugated eFAP-6 and sulfo-Cyanine5-conjugated eFAP-7. After chemical characterization, the uptake and specificity of both tracers were determined on FAP-expressing cells. In vitro, [111In]In-eFAP-6 demonstrated a superior affinity and a more rapid, although slightly lower, peak uptake than gold standard [111In]In-FAPI-46. Confocal microscopy demonstrated a quick FAP-mediated internalization of eFAP-7. Studies with HT1080-huFAP xenografted mice confirmed a more rapid uptake of [177Lu]Lu-eFAP-6 vs. [177Lu]Lu-FAPI-46. However, tumor retention at 24 h post injection of [177Lu]Lu-eFAP-6 was lower than that of [177Lu]Lu-FAPI-46, hereby currently limiting its use for TRT.

CONCLUSION

The superior affinity and faster tumor accumulation of eFAP-6 over FAPI-46 makes it a suitable compound for radionuclide imaging. After further optimization, the eFAP series has great potential for various oncological interventions, including fluorescent-guided surgery and effective targeted radionuclide theranostics.

Feasibility and therapeutic potential of [177Lu]Lu-FAPI-2286 in patients with advanced metastatic sarcoma

INTRODUCTION

The unique expression pattern of fibroblast activation protein (FAP) in stromal and tumor cells, particularly in sarcomas, and its absence in normal tissues, have positioned it as a promising theragnostic approach for the detection and treatment of various cancer types. The objective of this prospective study is to assess the feasibility, safety, biodistribution, and therapeutic efficacy of [177Lu]Lu-FAPI-2286 in patients with advanced metastatic sarcoma.

PATIENTS AND METHODS

Eight patients with advanced metastatic sarcoma, who were unresectable or had experienced disease recurrence following conventional treatments, underwent PTRT (peptide-targeted radionuclide therapy) using [177Lu]Lu-FAPI-2286. Prior to the treatment, confirmation of tumor uptake was obtained through [68Ga]Ga-FAPI-2286 PET/CT.

RESULTS

After four cycles of PTRT with [177Lu]Lu-FAPI-2286 (6660-7400 MBq), with a 6-8-week interval between each cycle, no grade 3 or 4 side effects were observed in the patients, and the treatment was well tolerated by all participants. The results demonstrated a 52.37% reduction in the average volume of the primary tumor, accompanied by a significant decrease in SUVmax and TBR of the metastatic lesions (29.67% and 43.66% respectively), especially in cases of lung metastasis. Furthermore, besides the improvement in physical capacity, there was a noticeable reduction in pain, an increase in overall survival, and enhanced satisfaction with the treatment reported by the patients.

CONCLUSION

[177Lu]Lu-FAPI-2286 PTRT, utilized for diverse cancer types, exhibited favorable tolerability in sarcoma patients, with minimal side effects, long-lasting retention of the radiopeptide within the tumor, and promising therapeutic effects. Preliminary findings of this prospective study need to be confirmed through further clinical trials.

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.

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.

From basic research to clinical application: targeting fibroblast activation protein for cancer diagnosis and treatment

PURPOSE

This study aims to review the multifaceted roles of a membrane protein named Fibroblast Activation Protein (FAP) expressed in tumor tissue, including its molecular functionalities, regulatory mechanisms governing its expression, prognostic significance, and its crucial role in cancer diagnosis and treatment.

METHODS

Articles that have uncovered the regulatory role of FAP in tumor, as well as its potential utility within clinical realms, spanning diagnosis to therapeutic intervention has been screened for a comprehensive review.

RESULTS

Our review reveals that FAP plays a pivotal role in solid tumor progression by undertaking a multitude of enzymatic and nonenzymatic roles within the tumor stroma. The exclusive presence of FAP within tumor tissues highlights its potential as a diagnostic marker and therapeutic target. The review also emphasizes the prognostic significance of FAP in predicting tumor progression and patient outcomes. Furthermore, the emerging strategies involving FAPI inhibitor (FAPI) in cancer research and clinical trials for PET/CT diagnosis are discussed. And targeted therapy utilizing FAP including FAPI, chimeric antigen receptor (CAR) T cell therapy, tumor vaccine, antibody-drug conjugates, bispecific T-cell engagers, FAP cleavable prodrugs, and drug delivery system are also introduced.

CONCLUSION

FAP's intricate interactions with tumor cells and the tumor microenvironment make it a promising target for diagnosis and treatment. Promising strategies such as FAPI offer potential avenues for accurate tumor diagnosis, while multiple therapeutic strategies highlight the prospects of FAP targeting treatments which needs further clinical evaluation.

Applications of FAPI PET/CT in the diagnosis and treatment of breast and the most common gynecologic malignancies: a literature review

The fibroblast activating protein (FAP) is expressed by some fibroblasts found in healthy tissues. However, FAP is overexpressed in more than 90% of epithelial tumors, including breast and gynecological tumors. As a result, the FAP ligand could be used as a target for diagnosis and treatment purposes. Positron emission tomography/computed tomography (PET/CT) is a hybrid imaging technique commonly used to locate and assess the tumor's molecular and metabolic functions. PET imaging involves the injection of a radiotracer that tends to accumulate more in metabolically active lesions such as cancer. Several radiotracers have been developed to target FAP in PET/CT imaging, such as the fibroblast-activation protein inhibitor (FAPI). These tracers bind to FAP with high specificity and affinity, allowing for the non-invasive detection and quantification of FAP expression in tumors. In this review, we discussed the applications of FAPI PET/CT in the diagnosis and treatment of breast and the most common gynecologic malignancies. Radiolabeled FAPI can improve the detection, staging, and assessment of treatment response in breast and the most common gynecologic malignancies, but the problem with normal hormone-responsive organs remains insurmountable. Compared to the diagnostic applications of FAPI, further research is needed for future therapeutic applications.

Diagnostic Accuracy of 68Ga-FAPI Versus 18F-FDG PET in Patients with Various Malignancies

To assess the diagnostic accuracy of 68Ga-labeled fibroblast activation protein inhibitor (FAPI) and 18F-labeled FDG PET for the detection of various tumors, we performed a head-to-head comparison of both imaging modalities across a range of tumor entities as part of our ongoing 68Ga-FAPI PET observational trial. Methods: The study included 115 patients with 8 tumor entities who received imaging with 68Ga-FAPI for tumor staging or restaging between October 2018 and March 2022. Of those, 103 patients received concomitant imaging with 68Ga-FAPI and 18F-FDG PET and had adequate lesion validation for accuracy analysis. Each scan was evaluated for the detection of primary tumor, lymph nodes, and visceral and bone metastases. True or false positivity and negativity to detected lesions was assigned on the basis of histopathology from biopsies or surgical excision, as well as imaging validation. Results: 68Ga-FAPI PET revealed higher accuracy than 18F-FDG PET in the detection of colorectal cancer (n = 14; per-patient, 85.7% vs. 78.6%; per-region, 95.6% vs. 91.1%) and prostate cancer (n = 22; per-patient, 100% vs. 90.9%; per-region, 96.4% vs. 92.7%). 68Ga-FAPI PET and 18F-FDG PET had comparable per-patient accuracy in detecting breast cancer (n = 16, 100% for both) and head and neck cancers (n = 10, 90% for both modalities). 68Ga-FAPI PET had lower per-patient accuracy than 18F-FDG PET in cancers of the bladder (n = 12, 75% vs. 100%) and kidney (n = 10, 80% vs. 90%), as well as lymphoma (n = 9, 88.9% vs. 100%) and myeloma (n = 10, 80% vs. 90%). Conclusion: 68Ga-FAPI PET demonstrated higher diagnostic accuracy than 18F-FDG PET in the diagnosis of colorectal cancer and prostate cancer, as well as comparable diagnostic performance for cancers of the breast and head and neck. Accuracy and impact on management will be further assessed in an ongoing prospective interventional trial (NCT05160051).

PET/CT FAPI: Procedure and evidence review in oncology

Neoplasms are composed of malignant tumor cells, which are surrounded by other non-tumor cellular elements, in what has been defined as the microenvironment or tumor stroma. Evidence on the importance of the tumor microenvironment has not stopped growing in recent years. It plays a central role in cell proliferation, tissue invasion, angiogenesis and cell migration. The paradigm is the family of new FAPI radiopharmaceuticals that show the density of the fibroblast activation protein (FAP) which is overexpressed in the cell membrane of activated cancer-associated fibroblasts (CAF), and its presence is related to poor prognosis. This educational document includes the procedure for performing PET/CT FAPI, biodistribution and the main potentially clinical applications in oncology to date.

The role of [68 Ga]Ga-FAPI-04 PET/CT in renal cell carcinoma: a preliminary study

PURPOSE

We aimed to investigate the role of [68 Ga]Ga-FAPI-04 PET/CT and uptake patterns of primary and metastatic lesions in patients with renal cell carcinoma (RCC).

METHODS

Twenty patients with a suspicious lesion considered primary renal malignancy or a history of RCC were included in our study. Two patients were excluded from further analyses due to other confirmed malignancies. Six patients were newly diagnosed, while the indication of 12 patients was restaging. All patients underwent [68 Ga]Ga-FAPI-04 and [18F]F-FDG PET/CT. SUVmax and tumor-to-background ratio (TBR) of primary (n = 7) and local recurrent lesions (n = 6) and lymph node (n = 26), lung (n = 32), bone (n = 5), and other metastases (n = 14) were compared between the two tracers.

RESULTS

We detected 90 lesions in 18 patients with varying FAPI and FDG uptake values on both PET/CT. The median TBR of FAPI-PET/CT of all lesions was higher than TBR of FDG-PET/CT with statistically significance (5.6 vs. 2.1, p < 0.001). In primary and recurrent lesions, the median SUVmax, TBR, and tumor volume on FAPI-PET/CT were higher than FDG-PET/CT. The median SUVmax of lung lesions on FAPI-PET/CT was statistical significantly higher than FDG-SUVmax (3.8 vs. 1.8, p = 0.02). The median of FAPI-SUVmax on primary lesions was lower in the early stage based on TNM compared to the advanced stage. FAPI-SUVmax in 49% of all lesions were SUVmax ≥ 6, and 13% were SUVmax ≥ 10. In patient-based analyses, seven patients (39%) had at least one lesion with FAPI-SUVmax ≥ 10; 12 patients (67%) had at least one lesion with FAPI-SUVmax ≥ 6.

CONCLUSION

This study showed the potential utility of [68 Ga]Ga-FAPI-04 PET/CT showing promising results in RCC. We have presumed that FAPI-PET/CT may be performed for complementary imaging modality providing prognosis and possibility of theranostic application in selected patients.

The inferior performance of [68Ga]Ga-FAPI-04 PET/CT as a diagnostic and theranostic biomarker in [177Lu]Lu-DOTATATE refractory well-differentiated neuroendocrine tumors

PURPOSE

We aimed to investigate the potential of [68Ga]Ga-FAPI-04 PET/CT as an alternative diagnostic and theranostic tool in well-differentiated NETs refractory to [177Lu]Lu-DOTATATE therapy.

METHODS

Patients who received at least two cycles of [177Lu]Lu-DOTATATE therapy for metastatic NETs and progressed under treatment were included. All patients had performed [68Ga]Ga-DOTATATE and [68Ga]Ga-FAPI-04 PET/CT within 3 weeks. The number of PET-positive lesions related to NETs and tumor sites was documented. Mann-Whitney U and chi-square tests were used to compare SUVmax levels of tracers and the number of detected metastases.

RESULTS

Twelve patients (7 male, 5 female) who met the eligibility criteria were included in the study. Ten patients had grade 1-2 NET of various origins, and two had paraganglioma and pheochromocytoma. One hundred ninety-eight of 230 lesions (86%) were SSTR positive with a median SUVmax of 16.6 (2.2-76.5), and 88 of 230 lesions (38.2%) were [68Ga]Ga-FAPI-04 positive with a median SUVmax of 5.1 (2.3-21). Median SUVmax level and detected number of tumors were significantly higher in [68Ga]Ga-DOTATATE PET/CT (p=<0.001). [68Ga]Ga-FAPI-04 PET/CT was completely (n:2) or almost completely (n:3) negative in 5 (42%) patients. Two (17%) patients had flip-flop SSTR/FAPI uptake in tumors. In four patients (33%), tumor uptake or the number of PET-positive lesions was inferior in [68Ga]Ga-FAPI-04 PET/CT. In only one patient (8%), tumor uptakes were higher in [68Ga]Ga-FAPI-04 PET/CT. Low-dose [177Lu]Lu-FAPI-46 dosimetry was performed on the FAPI-dominant patient; absorbed radiation doses per GBq were 1.26 Gy, 0.36 Gy, 0.32 Gy, and 0.2 Gy for kidneys, liver, spleen, and total body, respectively. The mean absorbed dose per GBq was 0.33 Gy for liver mass and 0.41 Gy for metastatic lymph nodes.

CONCLUSION

Our preliminary results demonstrated that [68Ga]Ga-FAPI-04 PET/CT mainly failed in well-differentiated NETs refractory to [177Lu]Lu-DOTATATE therapy and had a limited role as an alternative diagnostic or theranostic agent. Further investigations with a larger patient population are required to determine the impact of [68Ga]Ga-FAPI-04 PET/CT on NETs.

The effects of kilovoltage (kV) and milliampere seconds (mAs) values for SPECT attenuation correction: An anthropomorphic phantom study

The purpose of SPECT/CT imaging to be used in dosimetric calculations is to perform accurate attenuation correction by giving the patient the minimum radiation dose. in accordance with ALARA principles (as low as reasonably achievable), it is critical to limit the amount of radiation given to the patient as far as possible in medical fields where radiation therapy is applied. The goal of this study is to determine the optimum kV and mAs values needed to keep the radiation dose that can be given to the patient in CT scans at a minimum level for attenuation correction, as well as to determine the CT-induced radiation doses on patients with different kV and mAs values using an anthropomorphic phantom. Radionuclides 177Lu and 131I were used in SPECT imaging. For CT-based attenuation correction, images were taken with various values between 80 kV × 30 mAs and 140 kV × 300 mAs using the anthropomorphic phantom. After that attenuation correction was performed with the obtained CT images for SPECT images. Then, CT-derived radiation doses were calculated. Our results showed that 80 kV × 30 mAs values can be used in scans after radionuclide treatment for CT attenuation correction. It was determined that the patient was exposed to a radiation dose of 0.432 mSv as a result of the CT scanning with the full detector width area of gamma camera detector. Thus, scanning with these values (80 kV × 30 mAs) resulted in approximately 95% less radiation exposure (0,432 mSv) than the value (9,38 mSv) with scanning of the manufacturer's recommended value (120 kV × 200 mAs).

The Role and Application of Fibroblast Activating Protein

Fibroblast activation protein-α (FAP), a type-II transmembrane serine protease, is rarely expressed in normal tissues but highly abundant in pathological diseases, including fibrosis, arthritis, and cancer. Ever since its discovery, we have deciphered its structure and biological properties and continue to investigate its roles in various diseases while attempting to utilize it for targeted therapy. To date, no significant breakthroughs have been made in terms of efficacy. However, in recent years, several practical applications in the realm of imaging diagnosis have been discovered. Given its unique expression in a diverse array of pathological tissues, the fundamental biological characteristics of FAP render it a crucial target for disease diagnosis and immunotherapy. To obtain a more comprehensive understanding of the research progress of FAP, its biological characteristics, involvement in diseases, and recent targeted application research have been reviewed. Moreover, we explored its development trend in the direction of clinical diagnoses and treatment.

Screening and Preclinical Evaluation of Novel Radiolabeled Anti-Fibroblast Activation Protein-α Recombinant Antibodies

Background: Fibroblast activation protein-α (FAPα) is selectively overexpressed in tumor-associated fibroblasts in more than 90% of epithelial tumors, and may be a good target for anticancer treatment, for example, using an anti-FAPα recombinant antibody (rAb) labeled with radionuclides. In the present report, the radiolabeling and preclinical evaluation of novel anti-FAPα rAbs were investigated. Materials and Methods: Two novel anti-FAPα VHHs (AMS002-1 and AMS002-2) with high binding affinity to FAPα were selected from an antibody phage library. The anti-FAPα VHHs were then fused with the Fc fragment of human IgG4 to create two VHH-Fc rAbs. The VHH-Fc rAbs were radiolabeled with 89Zr and 177Lu. The radiolabeled products were evaluated by radioligand-binding assays using FAPα-expressing cells. The biodistribution and tumor-targeting properties were investigated by small-animal PET/CT. AMS002-1-Fc, which showed promising tumor-targeting properties in 89Zr-microPET imaging, was radiolabeled with 177Lu for efficacy study on HT1080 tumor-bearing mice and monitored with SPECT/CT imaging. Results: The two VHH-Fc rAbs with good affinity with KD values in low nanomolar range were identified. Both PET/CT imaging with 89Zr-AMS002-1-Fc rAb and SPECT/CT imaging with 177Lu-AMS002-1-Fc rAb demonstrated highest tumor uptakes at 72 h p.i. and long tumor retention in the preclinical models. Furthermore, ex vivo biodistribution analysis revealed high tumor uptake of 89Zr-AMS002-1-Fc at 48 h p.i. with the value of 6.91% ± 2.08% ID/g. Finally, radioimmunotherapy with 177Lu-AMS002-1-Fc rAb delayed the tumor growth without significant weight loss in mice with HT1080 xenografts. The tumor size of untreated control group was 2.59 times larger compared with the treatment group with 177Lu-AMS002-1-Fc at day 29. Conclusion: 89Zr/177Lu-AMS002-1-Fc represent a pair of promising radiopharmaceuticals for theranostics on FAPα-expressing tumors.

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.

Automated Synthesis of [68Ga]Ga-FAPI-46 on a Scintomics GRP Synthesizer

[68Ga]Ga-FAPI-46 is a radiolabelled fibroblast activation protein inhibitor that selectively binds to fibroblast activation protein (FAP), which is overexpressed by cancer-associated fibroblasts (CAFs) in the tumour microenvironment. In recent years, radiolabelled FAP inhibitors (FAPIs) are becoming increasingly important in cancer diagnostics and also for targeted radionuclide therapy. Because of the increasing demand for radiolabelled FAPIs, automating the synthesis of these compounds is of great interest. In this work, we present a newly programmed automatic synthesis process of [68Ga]Ga-FAPI-46 on a Scintomics GRP module using two Galli Ad generators as a radionuclide source. Dedicated cassettes for the labelling of 68Ga-peptides were used without any modifications. The generators were connected via a three-way valve to the module and eluted automatically over a strong cation exchange (SCX) cartridge by using the vacuum pump of the synthesis module, eliminating the need to transfer the eluates into a separate vial. After a reaction step in HEPES buffer, the compound was purified by solid-phase extraction (SPE) over a Sep-Pak Light C18 cartridge. The evaluation of 10 routine syntheses of [68Ga]Ga-FAPI-46 resulted in a radiochemical yield of 72.6 ± 4.9%. The radiochemical purity was 97.6 ± 0.3%, and the amount of free gallium-68 and colloid was <2%. The final product fulfilled the quality criteria, which were adapted from relevant monographs of the European Pharmacopoeia (Ph. Eur.). This work presents the successful preparation of multiple doses of [68Ga]Ga-FAPI-46 in a GMP-compliant automated process for clinical use.

Theranostics in breast cancer

Introduction

Breast cancer is a complex disease and constitutes the leading cause of cancer in women globally. Conventional treatment modalities include surgery, chemotherapy, radiation therapy, and hormonal therapy; all of these have their limitations and often result in significant side effects or toxicity. Targeted radionuclide therapy based on a theranostic approach has been successfully applied in several malignancies, such as prostate cancer, thyroid cancer, and neuro-endocrine tumours. Several studies have also highlighted the potential of theranostic applications in breast cancer.

Aim

This review aims to provide an overview of the most promising current and future theranostic approaches in breast cancer.

Discussion

The discussion includes pre-clinical as well as clinical data on some of the most successful targets used to date. Examples of potential theranostic approaches include those targeting the Human epidermal growth factor receptor 2 (HER2) expression, angiogenesis, aspects of the tumour microenvironment, Gastrin-releasing peptide receptor (GRPR), Prostate-specific membrane antigen (PSMA) and Chemokine receptor 4 (CXCR-4) expression. Several challenges to widespread clinical implementation remain, which include regulatory approval, access to the various radiopharmaceuticals and imaging technology, cost-effectiveness, and the absence of robust clinical data.

Conclusion

Theranostic approaches have the potential to greatly improve diagnosis, treatment, and outcomes for patients with breast cancer. More research is needed to fully explore the potential of such approaches and to identify the best potential targets, considering feasibility, costs, efficacy, side effects and outcomes.

Utility of 68 Ga-FAPI-04 PET/CT in Adenoid Cystic Carcinoma Compared With 18 F-FDG PET/CT : Two Case Reports

ABSTRACT

68 Ga-fibroblast activation protein inhibitor (FAPI) PET/CT is an emerging imaging modality with high sensitivity and high tumor-to-background ratio in various cancers including in the head and neck regions. The authors present 2 cases of adenoid cystic carcinoma who underwent 68 Ga-FAPI-04 and 18 F-FDG PET/CT. Locoregional recurrence has been detected more precisely in the first case with 68 Ga-FAPI-04. In the second case, 68 Ga-FAPI-04 outperformed 18 F-FDG in the number of lesions and demonstrated intense FAP uptake on widespread metastases, which could provide a treatment option as a theranostic concept. These cases highlight that 68 Ga-FAPI-04 PET/CT may be useful for detecting local recurrence and metastases and help select patients for radionuclide treatments targeting cancer-associated fibroblasts.

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.

An anthropomorphic body phantom for the determination of calibration factor in radionuclide treatment dosimetry

The aim of this study is to create an inhomogeneous human-like phantom, whose attenuation and scattering effects are similar to the human body, as an alternative to the homogeneous phantoms traditionally used in calibration factor (CF) determination. The phantom was designed to include the thorax, abdomen and upper pelvis regions sized to represent a 75-kg male with a body mass index of 25. Measurements using Lu-177 with 50- and 100-mL lesion volumes were performed using inhomogeneous anthropomorphic body phantom (ABP) and homogeneous NEMA PET body phantom. There was a difference of 5.7% of Calibration Factor including attenuation and scatter effect between ABP and NEMA PET body phantom. Because it better reflects the attenuation and scatter effect, it is recommended to use a human-like inhomogeneous phantom for determination of CF instead of a homogeneous phantom.

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.

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.

FAPI-Negative FDG-Avid Metastatic Relapse of Breast Cancer on PET/CT

ABSTRACT

A 69-year-old woman with breast cancer underwent 18F-FDG PET/CT because of the increased CA-15-3 level. 18F-FDG PET/CT showed multiple hypermetabolic lymph nodes (LNs) in the neck and mediastinum. The patient was also referred for 68Ga-fibroblast activation protein inhibitor (FAPI) 04 PET/CT for further evaluation. However, 18F-FDG-avid LNs were FAPI-negative on 68Ga-FAPI-04 PET/CT. Supraclavicular LN biopsy confirmed the metastasis of breast cancer. Recent reports have focused on the potential of FAPI PET imaging in breast cancer; however, this case highlights that false-negative 68Ga-FAPI-04-PET/CT findings should also be considered while evaluating metastatic spread.

68 Ga-FAPI-04 PET/CT and 18 F-FDG PET/CT in Metastatic Papillary Renal Cell Cancer

ABSTRACT

Papillary renal cell cancer is a rare malignancy with limited treatment options in the advanced stage of the disease. We present the case of a 62-year-old man with progressive left-sided papillary renal cell carcinoma who underwent 68 Ga-FAPI (fibroblast activated protein inhibitor)-04 and 18 F-FDG PET/CT imaging. 68 Ga-FAPI-04 PET/CT demonstrated variable FAP expression in all metastatic lesions detected by 18 F-FDG PET/CT, including multiple lymph nodes, bone, and thyroid. This case highlights that FAP-targeted imaging can be a promising modality for diagnostic and theranostic use in papillary renal cell carcinoma.

68 Ga-FAPI04 Versus 68 Ga-DOTATATE PET/CT in a Patient With Multiple Meningioma

ABSTRACT

We present the 68 Ga-DOTATATE and 68 Ga-FAPI (fibroblast activation protein inhibitor) PET/CT findings of a 61-year-old man diagnosed with atypical World Health Organization grade II multiple meningiomas. The patient has been stable for 2 years following multiple surgeries and external radiotherapy because of recurring disease until he recently described frequent headaches, and a follow-up examination confirmed new meningioma lesions on MRI. However, the patient was inoperable and was referred for 68 Ga-DOTATATE PET/CT to determine eligibility for salvage peptide receptor radionuclide therapy. He also underwent fibroblast activation protein-targeted imaging using 68 Ga-FAPI04 PET/CT, which revealed heterogeneous, low to mild fibroblast activation protein expression across multiple meningioma lesions.

Evaluation of 18F-FAPI-04 Imaging in Assessing the Therapeutic Response of Rheumatoid Arthritis

PURPOSE

Fibroblast activating protein (FAP) is highly expressed in the synovial tissues of rheumatoid arthritis (RA) patients. The aim of this study was to determine the feasibility of PET imaging with an Al[¹⁸F] F-NOTA-labeled FAP inhibitor 04(¹⁸F-FAPI-04) for the evaluation of arthritic progression and therapeutic response in experimental arthritis.

METHODS

Fibroblast-like synoviocytes (FLSs) were obtained from patients with RA or osteoarthritis (OA), and the relationship between ¹⁸F-FAPI-04 uptake and the inflammatory activity of RA FLSs was investigated. Collagen-induce arthritis (CIA) mice models were established and treated with methotrexate (MTX) or etanercept (ETC). Then, PET imaging was performed 24 h following ¹⁸F-FAPI-04 injection. The imaging results were compared by assessing macroscopic arthritis scores and histological staining.

RESULTS

¹⁸F-FAPI-04 uptake was obvious in RA FLSs that characterizing FAP activation. The higher the uptake of ¹⁸F-FAPI-04, the more severity of the inflammatory phenotype in RA FLS. Furthermore, the uptake of ¹⁸F-FAPI-04 in inflamed joints could be found even before the deformity of the parental joints could be observed by histological examination. Both MTX and ETC were effective in inhibiting the progression of arthritis in CIA mice was confirmed by macroscopic, histological, and radiographic pathology scores. Importantly, ¹⁸F-FAPI-04 uptake declined accordingly in CIA models following MTX and ETC treatment.

CONCLUSIONS

These findings suggest that PET imaging of ¹⁸F-FAPI-04 can be used to monitor treatment response in RA, and is more sensitive in disease speculation than macroscopic arthritis scoring.

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.

18F-FDG and 68Ga-FAPI-04 PET/CT in Evaluating Clear-Cell Endometrial Cancer

ABSTRACT

A 60-year-old woman diagnosed with clear cell endometrial carcinoma received neoadjuvant chemotherapy. However, the patient was inoperable and referred to our clinic to evaluate the extent of the disease. The patient underwent 18FDG PET/CT and 68Ga-FAPI (fibroblast activated protein inhibitor)-04 PET/CT scans. 68Ga-FAPI-04 PET/CT outperformed 18FDG PET/CT demonstrating higher uptake in the metastatic lesions.

Lesion Quantification Accuracy of Digital 90Y PET Imaging in the Context of Dosimetry in Systemic Fibroblast Activation Protein Inhibitor Radionuclide Therapy

Therapy with ⁹⁰Y-labeled fibroblast activation protein inhibitors (⁹⁰Y-FAPIs) was recently introduced as a novel treatment concept for patients with solid tumors. Lesion and organ-at-risk dosimetry is part of assessing treatment efficacy and safety and requires reliable quantification of tissue uptake. As ⁹⁰Y quantification is limited by the low internal positron-electron pair conversion rate, the increased effective sensitivity of digital silicon photomultiplier-based PET/CT systems might increase quantification accuracy and, consequently, allow for dosimetry in ⁹⁰Y-FAPI therapy. The aim of this study was to explore the conditions for reliable lesion image quantification in ⁹⁰Y-FAPI radionuclide therapy using a digital PET/CT system. Methods: Two tumor phantoms were filled with ⁹⁰Y solution using different sphere activity concentrations and a constant signal-to-background ratio of 40. The minimum detectable activity concentration was determined, and its dependence on acquisition time (15 vs. 30 min per bed position) and smoothing levels (all-pass vs. 5-mm gaussian filter) was investigated. Quantification accuracy was evaluated at various activity concentrations to estimate the minimum quantifiable activity concentration using contour-based and oversized volume-of-interest-based quantification approaches. A ±20% deviation range between image-derived and true activity concentrations was regarded as acceptable. Tumor dosimetry for 3 patients treated with ⁹⁰Y-FAPI is presented to project the phantom results to clinical scenarios. Results: For a lesion size of 40 mm and a clinical acquisition time of 15 min, both minimum detectable and minimum quantifiable activity concentrations were 0.12 MBq/mL. For lesion sizes of greater than or equal to 30 mm, accurate quantification was feasible for detectable lesions. Only for the smallest 10-mm sphere, the minimum detectable and minimum quantifiable activity concentrations differ substantially (0.43 vs. 1.97 MBq/mL). No notable differences between the 2 quantification approaches were observed. For the investigated tumors, absorbed dose estimates with reliable accuracy were achievable. Conclusion: For lesion sizes and activity concentrations that are expected to be observed in patients treated with ⁹⁰Y-FAPI, quantification with reasonable accuracy is possible. Further dosimetry studies are needed to thoroughly investigate the efficacy and safety of ⁹⁰Y-FAPI therapy.

Clinical Pharmacology of Radiotheranostics in Oncology

The combined use of diagnostic and therapeutic radioligands with the same molecular target, also known as theranostics, enables accurate patient selection, targeted therapy, and prediction of treatment response. Radioiodine, bone-seeking radioligands and norepinephrine analogs have been used for many years for diagnostic imaging and radioligand therapy of thyroid carcinoma, bone metastases, pheochromocytoma, paraganglioma, and neuroblastoma, respectively. In recent years, radiolabeled somatostatin analogs and prostate-specific membrane antigen ligands have shown clinical efficacy in the treatment of neuroendocrine tumors and prostate cancer, respectively. Several candidate compounds are targeting novel theranostic targets such as fibroblast activation protein, C-X-C chemokine receptor 4, and gastrin-releasing peptide receptor. In addition, several strategies to improve efficacy of radioligand therapy are being evaluated, including dosimetry-based dose optimization, multireceptor targeting, upregulation of target receptors, radiosensitization, pharmacogenomics, and radiation genomics. Design and evaluation of novel radioligands and optimization of dose and dose schedules, within the complex context of individualized multimodal cancer treatment, requires a multidisciplinary approach that includes clinical pharmacology. Significant increases in the use of these radiopharmaceuticals in routine oncological practice can be expected, which will have major impact on patient care as well as (radio)pharmacy utilization.

Fibroblast heterogeneity in solid tumors: From single cell analysis to whole-body imaging

Cancer-Associated Fibroblasts (CAFs) represent the most prominent component of the tumor microenvironment (TME). Recent studies demonstrated that CAF are heterogeneous and composed of different subpopulations exerting distinct functions in cancer. CAF populations differentially modulate various aspects of tumor growth, including cancer cell proliferation, extra-cellular matrix remodeling, metastatic dissemination, immunosuppression and resistance to treatment. Among other markers, the Fibroblast Activation Protein (FAP) led to the identification of a specific CAF subpopulation involved in metastatic spread and immunosuppression. Expression of FAP at the surface of CAF is detected in many different cancer types of poor prognosis. Thus, FAP recently appears as an appealing target for therapeutic and molecular imaging applications. In that context, ⁶⁸Ga-labeled radiopharmaceutical-FAP-inhibitors (FAPI) have been recently developed and validated for quantitatively mapping FAP expression over the whole-body using Positron Emission Tomography (PET/CT). In this review, we describe the main current knowledge on CAF subpopulations and their distinct functions in solid tumors, as well as the promising diagnostic and therapeutic implications of radionuclides targeting FAP.

Imaging-guided targeted radionuclide tumor therapy: From concept to clinical translation

Since the first introduction of sodium iodide I-131 for use with thyroid patients almost 80 years ago, more than 50 radiopharmaceuticals have reached the markets for a wide range of diseases, especially cancers. The nuclear medicine paradigm also shifts from solely molecular imaging or radionuclide therapy to imaging-guided radionuclide therapy, which is deemed a vital component of precision cancer therapy and an emerging medical modality for personalized medicine. The imaging-guided radionuclide therapy highlights the systematic integration of targeted nuclear diagnostics and radionuclide therapeutics. Regarding this, nuclear imaging serves to "visualize" the lesions and guide the therapeutic strategy, followed by administration of a precise patient specific dose of radiotherapeutics for treatment according to the absorbed dose to different organs and tumors calculated by dosimetry tools, and finally repeated imaging to predict the prognosis. This strategy leads to significantly enhanced therapeutic efficacy, improved patient outcomes, and manageable adverse events. In this review, we provide an overview of imaging-guided targeted radionuclide therapy for different tumors such as advanced prostate cancer and neuroendocrine tumors, with a focus on development of new radioligands and their preclinical and clinical results, and further discuss about challenges and future perspectives.

From Automated Synthesis to In Vivo Application in Multiple Types of Cancer-Clinical Results with [68Ga]Ga-DATA5m.SA.FAPi

Radiolabeled FAPI (fibroblast activation protein inhibitors) recently gained attention as widely applicable imaging and potential therapeutic compounds targeting CAF (cancer-associated fibroblasts) or DAF (disease-associated fibroblasts in benign disorders). Moreover, the use of FAPI has distinct advantages compared to FDG (e.g., increased sensitivity in regions with high glucose metabolism, no need for fasting, and rapid imaging). In this study, we wanted to evaluate the radiochemical synthesis and the clinical properties of the new CAF-targeting tracer [68Ga]Ga-DATA5m.SA.FAPi. The compound consists of a (radio)chemically easy to use hybrid chelate DATA.SA, which can be labeled at low temperatures, making it an interesting molecule for 'instant kit-type' labeling, and a squaric acid moiety that provides distinct advantages for synthesis and radiolabeling. Our work demonstrates that automatic synthesis of the FAP inhibitor [68Ga]Ga-DATA5m.SA.FAPi is feasible and reproducible, providing convenient access to this new hybrid chelator-based tracer. Our studies demonstrated the diagnostic usability of [68Ga]Ga-DATA5m.SA.FAPi for the unambiguous detection of cancer-associated fibroblasts of various carcinomas and their metastases (NSCLC, liposarcoma, parotid tumors, prostate cancer, and pancreas adenocarcinoma), while physiological uptake in brain, liver, intestine, bone, and lungs was very low.

Radiosynthesis and First Preclinical Evaluation of the Novel 11C-Labeled FAP Inhibitor 11C-FAPI: A Comparative Study of 11C-FAPIs and (68Ga) Ga-DOTA-FAPI-04 in a High–FAP-Expression Mouse Model

Purpose: ⁶⁸Ga-labeled fibroblast activation protein inhibitors, such as [⁶⁸Ga]Ga-DOTA-FAPI-04 and [⁶⁸Ga]Ga-DOTA-FAPI-46, have been successfully applied in positron emission tomography imaging of various tumor types. To broaden the PET tracers of different positron nuclides for imaging studies of FAP-dependent diseases, we herein report the radiosynthesis and preclinical evaluation of two ¹¹C-labeled FAP inhibitors, ¹¹C-RJ1101 and ¹¹C-RJ1102. Methods: Two phenolic hydroxyl precursors based on a quinoline amide core coupled with a 2-cyanopyrrolidine moiety were coupled with [¹¹C]CH₃I to synthesize ¹¹C-RJ1101 and ¹¹C-RJ1102. In vivo small-animal PET and biological distribution studies of ¹¹C-RJ1101 and ¹¹C-RJ1102 compared to [⁶⁸Ga]Ga-DOTA-FAPI-04 were conducted in nude mice bearing U87MG tumor xenografts at 30, 60, and 90min, respectively. Results: ¹¹C-RJ1101 and ¹¹C-RJ1102 were synthesized in over 15% radiochemical yields, with specific activities of 67 GBq/μmol and 34 GBq/μmol, respectively, at the end of synthesis and radiochemical purities greater than 99%. In U87MG tumor xenograft PET studies, the three tracers experienced higher specific uptake at the tumor site. However, because of significant differences in metabolism and clearance, [⁶⁸Ga]Ga-DOTA-FAPI-04 experienced high uptake in the kidney, whereas ¹¹C-RJ1101 and ¹¹C-RJ1102 showed high uptake in the liver and intestine. Biodistribution studies revealed significant hepatobiliary excretion of ¹¹C-RJ1101 and ¹¹C-RJ1102. 11C-RJ1102 showed higher specific tumor uptake in U87MG xenografts (1.71 ± 0.08% injected dose per Gram of tissue [ID/g]) than ¹¹C-RJ1101 (1.34 ± 0.10%ID/g) and [⁶⁸Ga]Ga-DOTA-FAPI-04 (1.29 ± 0.04%ID/g) after 30 min p. i. In orthotopic glioma models, the uptake values were 0.07 ± 0.03% ([⁶⁸Ga]Ga-DOTA-FAPI-04) and 0.16 ± 0.03% (¹¹C-RJ1102), respectively. Conclusion: ¹¹C-RJ1101 and ¹¹C-RJ1102 are interesting candidates for translation to the clinic, taking advantage of the shorter half-life and physical imaging properties of C-11.

Development of Fibroblast Activation Protein Inhibitor-Based Dimeric Radiotracers with Improved Tumor Retention and Antitumor Efficacy

Fibroblast activation protein (FAP), a fundamental component of the tumor stroma, is overexpressed in cancer-associated fibroblasts (CAFs). As a promising theranostic probe, we evaluated whether the FAP inhibitor (FAPI) dimer (DOTA-2P[FAPI]₂) is more effective than its monomeric analogs for FAP-targeted radionuclide therapy. [⁶⁸Ga]Ga/[¹⁷⁷Lu]Lu-DOTA-2P(FAPI)₂ were assayed in a stability study, small-animal positron emission tomography (PET) and single-photon emission computed tomography (SPECT), biodistribution, and radionuclide therapy to comprehensively evaluate their preclinical pharmacokinetics. The pharmacokinetics of [⁶⁸Ga]Ga-DOTA-2P(FAPI)₂ and [¹⁷⁷Lu]Lu-DOTA-2P(FAPI)₂ were determined in FAP-positive hepatocellular carcinoma patient-derived xenografts (PDXs) and HT-1080-FAP cell-derived xenografts (CDXs). [⁶⁸Ga]Ga-DOTA-2P(FAPI)₂ and [¹⁷⁷Lu]Lu-DOTA-2P(FAPI)₂ were stable in phosphate-buffered saline for 4 h. The tumor retention of [⁶⁸Ga]Ga-DOTA-2P(FAPI)₂ was better than that of [⁶⁸Ga]Ga-FAPI-46 in HT-1080-FAP CDXs, while healthy organs showed low tracer uptake and fast body clearance. In single-photon emission computed tomography, [¹⁷⁷Lu]Lu-DOTA-2P(FAPI)₂ showed a higher uptake and longer retention for tumors in both PDXs and CDXs from 1-48 h. [¹⁷⁷Lu]Lu-DOTA-2P(FAPI)₂ showed the best inhibition of tumor growth in PDXs and CDXs. DOTA-2P(FAPI)₂ has increased tumor uptake and retention properties compared to FAPI-46, which significantly improves the use of FAPI-based vectors for PET imaging and radionuclide therapy. [¹⁷⁷Lu]Lu-DOTA-2P(FAPI)₂ may be safe and effective for the treatment of FAP-positive malignant tumors.

Radionuclide imaging and therapy directed towards the tumor microenvironment: a multi-cancer approach for personalized medicine

Targeted radionuclide theranostics is becoming more and more prominent in clinical oncology. Currently, most nuclear medicine compounds researched for cancer theranostics are directed towards targets expressed in only a small subset of cancer types, limiting clinical applicability. The identification of cancer-specific targets that are (more) universally expressed will allow more cancer patients to benefit from these personalized nuclear medicine–based interventions. A tumor is not merely a collection of cancer cells, it also comprises supporting stromal cells embedded in an altered extracellular matrix (ECM), together forming the tumor microenvironment (TME). Since the TME is less genetically unstable than cancer cells, and TME phenotypes can be shared between cancer types, it offers targets that are more universally expressed. The TME is characterized by the presence of altered processes such as hypoxia, acidity, and increased metabolism. Next to the ECM, the TME consists of cancer-associated fibroblasts (CAFs), macrophages, endothelial cells forming the neo-vasculature, immune cells, and cancer-associated adipocytes (CAAs). Radioligands directed at the altered processes, the ECM, and the cellular components of the TME have been developed and evaluated in preclinical and clinical studies for targeted radionuclide imaging and/or therapy. In this review, we provide an overview of the TME targets and their corresponding radioligands. In addition, we discuss what developments are needed to further explore the TME as a target for radionuclide theranostics, with the hopes of stimulating the development of novel TME radioligands with multi-cancer, or in some cases even pan-cancer, application.

Research progress on the role of fibroblast activation protein in diagnosis and treatment of cancer

Fibroblast activation protein (FAP) is a type II transmembrane protein, which is over-expressed in cancer-associated fibroblasts (CAFs). CAFs are tumor stromal cells that constitute a major component of cancer volume and are reportedly related to tumorigenesis, angiogenesis, metastasis, promotion of drug resistance and induction of tumor immunity. FAP is widely acknowledged as the signature protein of CAFs. At present, FAP inhibitors (FAPI) have achieved ideal results in tumor PET/computed tomography (CT) imaging. Theoretically, FAP-targeted drugs can inhibit tumor progression. Nonetheless, no satisfactory therapeutic effect has been observed so far, which has impeded their implementation in clinical practice. In this review, we describe the characteristics of FAP and its role in the occurrence and development of cancer. We also highlight the potential value of targeting FAP to improve current diagnostic and therapeutic approaches.

Fibroblast Activation Protein Inhibitor Imaging and Therapy in a Patient With Multiple Endocrine Neoplasia Type 2A Syndrome

ABSTRACT

A patient with multiple endocrine neoplasia type 2A syndrome who had exhausted several surgeries and radiotherapy was referred to nuclear medicine department for theranostic approaches. [68Ga]-DOTATATE PET/CT and [131I]I-mIBG SPECT/CT were performed, but the degree of uptake was insufficient for using the treatment companion of these tracers. Finally, 1 year later, [68Ga]-FAPI-46 PET/CT showed progressive disease with metastases to the lung, liver, bone, and lymph nodes with intense [68Ga]-FAPI-46 uptake. Treatment with [177Lu]Lu-FAPI-46 was done, and the patient tolerated treatment and showed evidence of clinical improvement following therapy.

Could Fibroblast Activation Protein (FAP)-Specific Radioligands Be Considered as Pan-Tumor Agents?

Background

Cancer-associated fibroblasts (CAFs) can strongly modulate the response to therapy of malignant tumor cells, facilitating their continuous proliferation and invading behaviors. In this context, several efforts were made in identifying the fibroblast activation protein (FAP) as a CAF recognizer and in designing FAP-specific PET radiotracers (as 68Ga-FAPI) along with FAP-specific therapeutic radioligands. Herein, we review different clinical studies using the various FAP-specific radioligands as novel theranostic agents in a wide range of oncologic and nononcologic indications.

Methods

A comprehensive systematic search was conducted on the PubMed and Scopus databases to find relevant published articles concerning the FAP-specific PET imaging as well as the FAP-specific radionuclide therapy in patients with oncologic and nononcologic indications. The enrolled studies were dichotomized into oncologic and nononcologic categories, and the required data were extracted by precisely reviewing the whole text of each eligible study. A meta-analysis was also performed comparing the detection rates of 68Ga-FAPI vs. 18F-FDG PET/CT using odds ratio (OR) and risk difference as outcome measures.

Results

Of the initial 364 relevant papers, 49 eligible articles (1479 patients) and 55 case reports were enrolled in our systematic review. These studies observed high radiolabeled FAPI avidity as early as 10 minutes after administration in primary sites of various malignant tumors. Based on the meta-analysis which was done on the reported detection rates of the 68Ga-FAPI and 18F-FDG PET/CT scans, the highest OR belonged to the primary lesion detection rate of gastrointestinal tumors (OR = 32.079, 95% CI: 4.001-257.212; p = 0.001) with low heterogeneity (I2 = 0%). The corresponding value of the nodal metastases belonged to hepatobiliary tumors (OR = 11.609, 95% CI: 1.888-71.365; p = 0.008) with low heterogeneity (I2 = 0%). For distant metastases, the highest estimated OR belonged to nasopharyngeal carcinomas (OR = 77.451, 95% CI: 7.323-819.201; p < 0.001) with low heterogeneity (I2 = 0%).

Conclusions

The outperformance of 68Ga-FAPI PET/CT over 18F-FDG PET/CT in identifying certain primary tumors as well as in detecting their metastatic lesions may open indications for evaluation of cases with inconclusive 18F-FDG PET/CT findings. What needs to be emphasized is that the false-positive results might be problematic and must be taken into account in 68Ga-FAPI PET/CT interpretation. More clarification on the role of FAPI radioligands in oncologic imaging, radionuclide therapy, and radiotherapy treatment planning is therefore required.

Evans blue-modified radiolabeled fibroblast activation protein inhibitor as long-acting cancer therapeutics

Rationale: Fibroblast activation protein (FAP) targeted molecular imaging radiotracers have shown promising preclinical and clinical results in tumor diagnosis. However, rapid clearance and inadequate tumor retention of these molecules have hindered them for further clinical translation in cancer therapy. In this study, we aimed to develop a series of albumin binder-truncated Evans blue (EB) modified FAP targeted radiotracers, and optimize the pharmacokinetic (PK) characteristics to overcome the existing limitations in order to apply in the radionuclide therapy of cancer.

Methods: A series of compounds with the general structure of EB-FAPI-Bn were synthesized based on a FAP inhibitor (FAPI) variant (FAPI-02) and radiolabeled with ¹⁷⁷LuCl₃. To verify the binding affinity and FAP targeting specificity of these tracers in vitro, U87MG cell uptake and competition assays were performed. Preclinical PK was evaluated in U87MG tumor-bearing mice using SPECT imaging and biodistribution studies. The lead compound EB-FAPI-B1 was selected and cancer therapeutic efficacy of ¹⁷⁷Lu-EB-FAPI-B1 was assessed in U87MG tumor-bearing mice.

Results:¹⁷⁷Lu-EB-FAPI-B1, B2, B3, B4 were stable in PBS (pH 7.4) and saline for at least 24 h. EB-FAPI-B1 showed high binding affinity (IC₅₀ = 16.5 nM) to FAP in vitro, which was comparable with that of FAPI-02 (IC₅₀ = 10.9 nM). SPECT imaging and biodistribution studies of ¹⁷⁷Lu-EB-FAPI-B1, B2, B3, B4 have proved their prominently improved tumor accumulation and retention at 96 h post-injection, especially for ¹⁷⁷Lu-EB-FAPI-B1, high tumor uptake and low background signal make it the optimal compound. Compared to the saline group, noteworthy tumor growth inhibitions of ¹⁷⁷Lu-EB-FAPI-B1 have been observed after administration of different dosages.

Conclusion: In this study, several EB modified FAPI-02 related radiopharmaceuticals have been synthesized successfully and evaluated. High binding affinity and FAP targeting specificity were identified in vitro and in vivo. Remarkably enhanced tumor uptake and retention of EB-FAPI-B1 were found over the unmodified FAPI-02. ¹⁷⁷Lu-EB-FAPI-B1 showed remarkable tumor growth suppression in U87MG tumor model with negligible side effects, indicating that ¹⁷⁷Lu-EB-FAPI-B1 is promising for clinical application and transformation.

Intratumoral Heterogeneity in a Patient With Metastatic Thymic Carcinoma on 18F-FDG, 68Ga-DOTATATE, and 68Ga-FAPI04 PET/CT

ABSTRACT

Herein, we present a 72-year-old woman with metastatic thymic carcinoma, referred for salvage peptide receptor radionuclide therapy with 177Lu-DOTATATE after completing all treatment options according to current clinical practice and guidelines. However, the patient was not eligible for 177Lu-DOTATATE peptide receptor radionuclide therapy and underwent 68Ga-FAPI04 PET/CT to assess the potential for a FAP-targeted therapy.

Non-conventional and Investigational PET Radiotracers for Breast Cancer: A Systematic Review

Breast cancer is one of the most common malignancies in women, with high morbidity and mortality rates. In breast cancer, the use of novel radiopharmaceuticals in nuclear medicine can improve the accuracy of diagnosis and staging, refine surveillance strategies and accuracy in choosing personalized treatment approaches, including radioligand therapy. Nuclear medicine thus shows great promise for improving the quality of life of breast cancer patients by allowing non-invasive assessment of the diverse and complex biological processes underlying the development of breast cancer and its evolution under therapy. This review aims to describe molecular probes currently in clinical use as well as those under investigation holding great promise for personalized medicine and precision oncology in breast cancer.

Radioligands Targeting Fibroblast Activation Protein (FAP)

Simple Summary

FAP-targeted radiotracers, recently introduced in cancer treatment, accumulate in Cancer-Associated Fibroblasts (CAFs). CAFs are present in tumor lesions but do not correspond to genuine cancer cells, although they behave in an abnormal and disease-promoting manner. One of their characteristic features, the expression of the surface protein FAP, can be utilized to discriminate between cancerous and healthy tissues. By the choice of an appropriate radionuclide, FAP-targeted tracers can be used for imaging or therapy in many cancer types. Therefore, the first successful application of FAP-targeted imaging has led to an enormous and growing interest in nuclear medicine and radiopharmacy.

Abstract

Targeting fibroblast activation protein (FAP) in cancer-associated fibroblasts (CAFs) has attracted significant attention in nuclear medicine. Since these cells are present in most cancerous tissues and FAP is rarely expressed in healthy tissues, anti-FAP tracers have a potential as pan-tumor agents. Compared to the standard tumor tracer [¹⁸F]FDG, these tracers show better tumor-to-background ratios (TBR) in many indications. Unlike [¹⁸F]FDG, FAP-targeted tracers do not require exhausting preparations, such as dietary restrictions on the part of the patient, and offer the possibility of radioligand therapy (RLT) in a theragnostic approach. Although a radiolabeled antibody was clinically investigated as early as the 1990s, the breakthrough event for FAP-targeting in nuclear medicine was the introduction and clinical application of the so-called FAPI-tracers in 2018. From then, the development and application of FAP-targeted tracers became hot topics for the radiopharmaceutical and nuclear medicine community, and attracted the interest of pharmaceutical companies. The aim of this review is to provide a comprehensive overview of the development of FAP-targeted radiopharmaceuticals and their application in nuclear medicine.

Fibroblast-Activated Protein Inhibitor PET/CT: Cancer Diagnosis and Management

Fibroblast activation protein (FAP), overexpressed on cancer-associated fibroblasts (CAFs), is a novel target for molecular imaging of various tumors. Recently, the development of several small-molecule FAP inhibitors for radiolabeling with 68Ga has resulted in the emergence of studies evaluating its clinical role in cancer imaging. Preliminary findings have demonstrated that, in contrast to radiotracers taking advantage of cancer-specific targets such as PSMA and DOTATATE, FAPs as a target are the most promising that can compete with 18FDG in terms of widespread indications. They also have the potential to overcome the shortcomings of 18FDG, particularly false-positive uptake due to inflammatory or infectious processes, low sensitivity in certain cancer types, and radiotherapy planning. In addition, the attractive theranostic properties may facilitate the treatment of many refractory cancers. This review summarizes the current FAP variants and related clinical studies, focusing on radiopharmacy, dosimetry, and diagnostic and theranostic applications.

NSCLC Biomarkers to Predict Response to Immunotherapy with Checkpoint Inhibitors (ICI): From the Cells to In Vivo Images

Lung cancer remains the leading cause of cancer-related death, and it is usually diagnosed in advanced stages (stage III or IV). Recently, the availability of targeted strategies and of immunotherapy with checkpoint inhibitors (ICI) has favorably changed patient prognosis. Treatment outcome is closely related to tumor biology and interaction with the tumor immune microenvironment (TME). While the response in molecular targeted therapies relies on the presence of specific genetic alterations in tumor cells, accurate ICI biomarkers of response are lacking, and clinical outcome likely depends on multiple factors that are both host and tumor-related. This paper is an overview of the ongoing research on predictive factors both from in vitro/ex vivo analysis (ranging from conventional pathology to molecular biology) and in vivo analysis, where molecular imaging is showing an exponential growth and use due to technological advancements and to the new bioinformatics approaches applied to image analyses that allow the recovery of specific features in specific tumor subclones.