Comparison of [68Ga]Ga-DOTA-FAPI-04 and [18F] FDG PET/CT for the diagnosis of primary and metastatic lesions in patients with various types of cancer

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.

Fatty acid-conjugated radiopharmaceuticals for fibroblast activation protein-targeted radiotherapy

INTRODUCTION

Radiopharmaceuticals that target cancer-associated fibroblasts (CAFs) have become an increasingly attractive strategy for cancer theranostics. Recently, a series of fibroblast activation protein inhibitor (FAPI)-based radiopharmaceuticals have been successfully applied to the diagnosis of a variety of cancers and exhibited excellent tumor selectivity. Nevertheless, CAF-targeted radionuclide therapy encounters difficulties in cancer treatment, as the tumor uptake and retention of FAPIs are insufficient. To meet this challenge, we tried to conjugate albumin-binding moiety to FAPI molecule for prolonged circulation that may increase the accumulation and retention of radiopharmaceuticals in tumor.

METHODS

Two fatty acids, lauric acid (C12) and palmitic acid (C16), were conjugated to FAPI-04 to give two albumin-binding FAPI radiopharmaceuticals, denoted as FAPI-C12 and FAPI-C16, respectively. They had been radiolabeled with gallium-68, yttrium-86, and lutecium-177 for stability study, binding affinity assay, PET and SPECT imaging, biodistribution, and radionuclide therapy study to systematically evaluate their potential for CAF-targeted radionuclide therapy.

RESULTS

FAPI-C12 and FAPI-C16 showed high binding affinity to FAP with the IC₅₀ of 6.80 ± 0.58 nM and 5.06 ± 0.69 nM, respectively. They were stable in both saline and plasma. The tumor uptake of [⁶⁸Ga]Ga-FAPI-04 decreased by 56.9% until 30 h after treated with FAPI-C16 before, and the uptakes of [⁸⁶Y]Y-FAPI-C12 and [⁸⁶Y]Y-FAPI-C16 in HT-1080-FAP tumor were both much higher than that of HT-1080-Vehicle tumor which identified the high FAP specific of these two radiopharmaceuticals. Both FAPI-C12 and FAPI-C16 showed notably longer circulation and significantly enhanced tumor uptake than those of FAPI-04. [¹⁷⁷Lu]Lu-FAPI-C16 had the higher tumor uptake at both 24 h (11.22 ± 1.18%IA/g) and 72 h (6.50 ± 1.19%IA/g) than that of [¹⁷⁷Lu]Lu-FAPI-C12 (24 h, 7.54 ± 0.97%IA/g; 72 h, 2.62 ± 0.65%IA/g); both of them were much higher than [¹⁷⁷Lu]Lu-FAPI-04 with the value of 1.24 ± 0.54%IA/g at 24 h after injection. Significant tumor volume inhibition of [¹⁷⁷Lu]Lu-FAPI-C16 at the high activity of 29.6 MBq was observed, and the median survival was 28 days which was much longer than that of the [¹⁷⁷Lu]Lu-FAPI-04 treated group of which the median survival was only 10 days.

CONCLUSION

This proof-of-concept study validates the hypothesis that conjugation of albumin binders may shift the pharmacokinetics and enhance the tumor uptake of FAPI-based radiopharmaceuticals. This could be a general strategy to transform the diagnostic FAP-targeted radiopharmaceuticals into their therapeutic pairs.

Molecular Imaging of Tumor Microenvironment to Assess the Effects of Locoregional Treatment for Hepatocellular Carcinoma

Liver cancer is one of the leading causes of cancer deaths worldwide. Among all primary liver cancers, hepatocellular carcinoma (HCC) is the most common type, representing 75%‐85% of all primary liver cancer cases. Median survival following diagnosis of HCC is approximately 6 to 20 months due to late diagnosis in its course and few effective treatment options. Interventional therapy with minimal invasiveness is recognized as a promising treatment for HCC. However, due to the heterogeneity of HCC and the complexity of the tumor microenvironment, the long‐term efficacy of treatment for HCC remains a challenge in the clinic. Tumor microenvironment, including factors such as hypoxia, angiogenesis, low extracellular pH, interstitial fluid pressure, aerobic glycolysis, and various immune responses, has emerged as a key contributor to tumor residual and progression after locoregional treatment for HCC. New approaches to noninvasively assess the treatment response and assist in the clinical decision‐making process are therefore urgently needed. Molecular imaging tools enabling such an assessment may significantly advance clinical practice by allowing real‐time optimization of treatment protocols for the individual patient. This review discusses recent advances in the application of molecular imaging technologies for noninvasively assessing changes occurring in the microenvironment of HCC after locoregional treatment.

68Ga-FAPI-46 PET/MR Detects Recurrent Cholangiocarcinoma and Intraductal Papillary Mucinous Neoplasm in a Patient Showing Increasing CEA with Negative 18F-FDG PET/CT and Conventional CT

A patient with intrahepatic cholangiocarcinoma who underwent hepatic resection with completed neoadjuvant chemotherapy presented with increased CEA levels. Previous whole abdominal and chest CT scan revealed no evidence of local recurrence or metastasis. ⁶⁸ Ga-FAPI-46 PET showed significantly higher tumor-to-background contrast of recurrent tumor and nodal metastasis, which were undetectable in the FDG PET or conventional CT scan. These findings changed patient management. Larger studies with histopathological correlation and comparisons with other imaging modalities are required to validate the diagnostic performance. Moreover, a cystic lesion with FAPI uptake at the neck to the proximal body of the pancreas without FDG uptake is also incidentally noted. Differential diagnoses include sided branch IPMN and serous cystadenoma.

Comparison of 18F-AlF-NOTA-FAPI and 18F-FDG Imaging in a Patient With Gastric Signet-Ring Cell Carcinoma

ABSTRACT

Several case reports have shown fibroblast activation protein inhibitor (FAPI) imaging to be superior to 18F-FDG imaging in the delineation of primary gastric signet-ring cell carcinoma lesions. In this case, 18F-AlF-NOTA-FAPI PET/CT showed more metastatic lesions that had more increased activity than 18F-FDG PET/CT. However, neither 18F-AlF-NOTA-FAPI nor 18F-FDG imaging revealed any abnormal uptake in the primary gastric lesion, which was subsequently demonstrated to be the primary lesion by pathology.

68Ga-FAPI PET/CT Imaging of Graves Ophthalmopathy in a Patient With Esophageal Cancer

ABSTRACT

68Ga-FAPI PET/CT has been used in the assessment of a variety of malignancies. In addition, FAPI uptake in nonmalignant diseases has also been reported. Herein, we present a case of Graves ophthalmopathy detected by 68Ga-FAPI PET/CT.

68Ga-FAPI PET/CT Versus 18F-FDG PET/CT for Detecting Metastatic Lesions in a Case of Radioiodine-Refractory Differentiated Thyroid Cancer

ABSTRACT

18F-FDG PET/CT is a useful imaging modality for detecting disease recurrence and metastases in patients with radioiodine-refractory differentiated thyroid cancer (RAIR-DTC). However, the sensitivity of 18F-FDG PET/CT in RAIR-DTC is not very satisfactory. 68Ga-FAPI is a promising PET tracer that allowed imaging of various types of cancer. In this case, 68Ga-FAPI showed a far better signal-to-background ratio than 18F-FDG for detecting the RAIR-DTC metastatic lesions, especially for detecting the small pulmonary metastases. Therefore, 68Ga-FAPI may be a promising alternative to 18F-FDG for detecting tumor recurrence and metastases in RAIR-DTC. It could also be used for guiding FAP-targeted radionuclide therapy.

68Ga-DOTA-FAPI-04 PET/CT as a Promising Tool for Differentiating Ovarian Physiological Uptake: Preliminary Experience of Comparative Analysis With 18F-FDG

Objectives: This study aimed to investigate the physiological distribution characteristics of ⁶⁸Ga-DOTA-FAPI-04 in the ovary, and assess the feasibility of early diagnosis of primary ovarian disease with ⁶⁸Ga-DOTA-FAPI-04 PET/CT. Methods: We retrospectively analyzed the data of patients who received ¹⁸F-FDG and ⁶⁸Ga-DOTA-FAPI-04 PET/CT scanning in the Nuclear Medicine Department of our hospital within 3 days from September 2020 to January 2021. We selected the data in which ovaries showed abnormal FDG activity. Patients with abnormal ovarian FDG uptake with focus confirmed by pathological biopsy or clinical follow-up as pathological changes were excluded. The uptake of tracers (¹⁸F-FDG and ⁶⁸Ga-FAPI) was semi-quantitatively analyzed. Results: This study included 14 patients (average age was 38.6). Physiological ovarian uptake was mostly unilateral, and there was no significant difference in SUVmax between the left and right sides (FDGt = 0.272, FAPIt = 0.592). The ovary SUVmax of FDG (4.89 ± 1.84) was statistically significantly higher than that of FAPI (1.53 ± 0.37). The Le/Li ratio on FDG is 3.38 ± 1.81, TBR is 5.81 ± 1.98, while the Le/Li ratio on FAPI is 3.57 ± 1.26, TBR is 0.94 ± 0.19. Conclusion: Our research shows that ovarian functional or pathological changes can be manifested as FDG avid, while ⁶⁸Ga-DOTA-FAPI-04 has no physiological accumulation in the ovary and is not affected by the menstrual cycle. Therefore, ⁶⁸Ga-DOTA-FAPI-04 has unique advantages in the diagnosis of ovarian diseases, and can identify them early and accurately.

68Ga-Labeled GX1 Dimer: A Novel Probe for PET/Cerenkov Imaging Targeting Gastric Cancer

Purpose

To synthesize the dimer of GX1 and identify whether its affinity and targeting are better than those of GX1. To prepare ⁶⁸Ga-DOTA-KEK-(GX1)₂ and to apply it to PET and Cerenkov imaging of gastric cancer.

Methods

⁶⁸Ga-DOTA-KEK-(GX1)₂ was prepared, and the labeling yield and stability were determined. Its specificity and affinity were verified using an in vitro cell binding assay and competitive inhibition test, cell immunofluorescence, and cell uptake and efflux study. Its tumor-targeting ability was determined by nano PET/CT and Cerenkov imaging, standardized uptake value (SUV), signal-to-background ratio (SBR) quantification, and a biodistribution study in tumor-bearing nude mice.

Results

⁶⁸Ga-DOTA-KEK-(GX1)₂ was successfully prepared, and the labeling yield was more than 97%. It existed stably for 90 min in serum. The binding of ⁶⁸Ga-DOTA-KEK-(GX1)₂ to cocultured HUVECs (Co-HUVECs) was higher than that to human umbilical vein endothelial cells (HUVECs), BGC823 cells, and GES cells. It was also higher than that of ⁶⁸Ga-DOTA-GX1, indicating that the dimer did improve the specificity and affinity of GX1. The binding of KEK-(GX1)₂ to Co-HUVECs was significantly higher than that of GX1. Additionally, the uptake of ⁶⁸Ga-DOTA-KEK-(GX1)₂ by Co-HUVECs was higher than that of ⁶⁸Ga-DOTA-GX1 and reached a maximum at 60 min. Nano PET/CT and Cerenkov imaging showed that the tumor imaging of the nude mice injected with ⁶⁸Ga-DOTA-KEK-(GX1)₂ was clear, and the SUV and SBR value of the tumor sites were significantly higher than those of the nude mice injected with ⁶⁸Ga-DOTA-GX1, indicating that the probe had better targeting in vivo. Finally, the biodistribution showed quantitatively that when organs such as the kidney and liver metabolized rapidly, the radioactivity of the tumor site of the nude mice injected with ⁶⁸Ga-DOTA-KEK-(GX1)₂ decreased relatively slowly. At the same time, the percentage of injected dose per gram (%ID/g) of the tumor site was higher than that of other normal organs except the liver and kidney at 60 min, which indicated that the tumor had good absorption of the probe.

Conclusion

GX1 was modified successfully, and the in vivo and in vitro properties of the GX1 dimer were significantly better than those of GX1. The imaging probe, ⁶⁸Ga-DOTA-KEK-(GX1)₂, was successfully prepared, which provides a candidate probe for PET and Cerenkov diagnosis of gastric cancer.

Somatostatin receptor imaging with [68Ga]Ga-DOTATATE positron emission tomography/computed tomography (PET/CT) in patients with nasopharyngeal carcinoma

PURPOSE

To explore the feasibility of [⁶⁸Ga]Ga-DOTATATE positron emission tomography/computed tomography (PET/CT) in patients with non-keratinizing nasopharyngeal carcinoma (NPC) and to evaluate whether [⁶⁸Ga]Ga-DOTATATE PET/CT could be used for non-invasive determination of somatostatin receptor 2 (SSTR2) expression in NPC.

METHODS

This prospective study included patients with NPC who underwent [⁶⁸Ga]Ga-DOTATATE PET/CT between February and May 2021. The [⁶⁸Ga]Ga-DOTATATE and [¹⁸F]FDG uptakes in primary and metastatic NPC lesions were calculated and compared, and the [⁶⁸Ga]Ga-DOTATATE uptake between SSTR2 score groups was analysed.

RESULTS

A total of 36 participants (25 patients, initial staging; 11 patients, recurrence detection) were included; 33 patients also underwent [¹⁸F]FDG PET/CT for staging/restaging as a part of their routine diagnostic workup. [⁶⁸Ga]Ga-DOTATATE PET/CT showed an intense tracer uptake in primary and metastatic NPC lesions. The radiotracer uptake was higher with [⁶⁸Ga]Ga-DOTATATE than with [¹⁸F]FDG PET in primary NPC lesions (SUVmax: 12.03 vs. 10.07, P = 0.048; tumour-to-brain ratio: 36.16 vs. 0.86, P < 0.001) and regional lymph node metastases (median SUVmax: 9.11 vs. 6.12, P < 0.001) and comparable in bone and visceral metastases. Importantly, most NPC lesions showed intense SSTR2 expression (85.7%), which was strongly correlated with the [⁶⁸Ga]Ga-DOTATATE uptake. The SUVmax of SSTR2-negative lesions was significantly lower than that of SSTR2-positive lesions (SUVmax: 4.95 vs. 12.61, P = 0.013).

CONCLUSION

[⁶⁸Ga]Ga-DOTATATE PET/CT is a promising imaging modality for detecting primary and metastatic NPC, with favourable image contrast and comparable diagnostic efficacy when compared to [¹⁸F]FDG PET/CT. An intense SSTR2 expression was observed in most NPCs, and this expression was significantly correlated with the [⁶⁸Ga]Ga-DOTATATE uptake.

Head-to-Head Comparison of Fibroblast Activation Protein Inhibitors (FAPI) Radiotracers versus [18F]F-FDG in Oncology: A Systematic Review

Several recent studies comparing radiolabeled fibroblast activation protein inhibitors (FAPI) and fluorine-18 fluorodeoxyglucose ([18F]F-FDG) as positron emission tomography (PET) radiotracers in oncology have been published. The aim of this systematic review is to perform an updated evidence-based summary about the comparison of these PET radiotracers in oncology to better address further research in this setting. Studies or subsets of studies comparing radiolabeled FAPI and [18F]F-FDG as PET radiotracers in oncology were eligible for inclusion in this systematic review. A systematic literature search of PubMed/MEDLINE and Cochrane library databases was performed until August 2021. Literature data about the comparison of [18F]F-FDG and radiolabeled FAPI are rapidly increasing. Overall, taking into account radiotracer uptake and tumor-to-background uptake ratio, compared to [18F]F-FDG PET, an equal or higher detection of primary tumors and/or metastatic lesions was usually demonstrated by using radiolabeled FAPI PET. In particular, the cancer entities with better detection rate of tumor lesions by using radiolabeled FAPI PET, compared to [18F]F-FDG PET, were gastrointestinal tumors, liver tumors, breast cancer and nasopharyngeal carcinoma. Further comparison studies are needed to better evaluate the best field of application of radiolabeled FAPI PET.

A Case with 68Ga-FAPI Positive and 18F-FDG Negative Breast Cancer

A female patient diagnosed of infiltrative breast carcinoma using tru-cut biopsy underwent ¹⁸flourine-fluorodeoxyglucose (¹⁸F-FDG) positron emission tomography/computed tomography (PET/CT) for staging. The tumor was located in the superior external quadrant of the right breast, and did not exhibit pathological uptake in ¹⁸F-FDG PET/CT. Later, gallium-⁶⁸ (⁶⁸Ga) fibroblast activation protein-specific inhibitor (FAPI)-04 PET/CT imaging was performed and the primary tumor showed intense radiotracer accumulation. This presumes that ⁶⁸Ga-FAPI PET/CT imaging is superior to ¹⁸F-FDG imaging in detecting the primary tumor in breast cancer, thereby suggesting the replacement of FAPI by ¹⁸F-FDG in breast-cancer staging in the future.

New Frontiers in Cancer Imaging and Therapy Based on Radiolabeled Fibroblast Activation Protein Inhibitors: A Rational Review and Current Progress

Over the past decade, the tumor microenvironment (TME) has become a new paradigm of cancer diagnosis and therapy due to its unique biological features, mainly the interconnection between cancer and stromal cells. Within the TME, cancer-associated fibroblasts (CAFs) demonstrate as one of the most critical stromal cells that regulate tumor cell growth, progression, immunosuppression, and metastasis. CAFs are identified by various biomarkers that are expressed on their surfaces, such as fibroblast activation protein (FAP), which could be utilized as a useful target for diagnostic imaging and treatment. One of the advantages of targeting FAP-expressing CAFs is the absence of FAP expression in quiescent fibroblasts, leading to a controlled targetability of diagnostic and therapeutic compounds to the malignant tumor stromal area using radiolabeled FAP-based ligands. FAP-based radiopharmaceuticals have been investigated strenuously for the visualization of malignancies and delivery of theranostic radiopharmaceuticals to the TME. This review provides an overview of the state of the art in TME compositions, particularly CAFs and FAP, and their roles in cancer biology. Moreover, relevant reports on radiolabeled FAP inhibitors until the year 2021 are highlighted-as well as the current limitations, challenges, and requirements for those radiolabeled FAP inhibitors in clinical translation.

Comparison of the Relative Diagnostic Performance of [68Ga]Ga-DOTA-FAPI-04 and [18F]FDG PET/CT for the Detection of Bone Metastasis in Patients With Different Cancers

Purpose

The present retrospective analysis sought to compare the relative diagnostic efficacy of [⁶⁸Ga]Ga-DOTA-FAPI-04 to that of [¹⁸F]FDG PET/CT as a means of detecting bone metastases in patients with a range of cancer types.

Materials

In total, 30 patients with bone metastases associated with different underlying malignancies were retrospectively enrolled. All patients had undergone [⁶⁸Ga]Ga-DOTA-FAPI-04 and [¹⁸F]FDG PET/CT, and the McNemar test was used to compare the relative diagnostic performance of these two imaging modalities. The maximum standard uptake value (SUVmax) was used to quantify radiotracer uptake by metastatic lesions, with the relative uptake associated with these two imaging strategies being compared via the Mann-Whitney U test. The cohort was further respectively divided into two (osteolytic and osteoblastic bone metastases) and three clinical subgroups (lung cancer, thyroid cancer, and liver cancer).

Results

[⁶⁸Ga]Ga-DOTA-FAPI-04 PET/CT was found to be significantly more sensitive as a means of diagnosing bone metastases relative to [¹⁸F]FDG PET/CT ([109/109] 100% vs [89/109] 81.7%; P< 0.01), consistent with the significantly increased uptake of [⁶⁸Ga]Ga-DOTA-FAPI-04 by these metastatic lesions relative to that of [¹⁸F]FDG (n=109, median SUVmax, 9.1 vs. 4.5; P< 0.01). [⁶⁸Ga]Ga-DOTA-FAPI-04 accumulation was significantly higher than that of [¹⁸F]FDG in both osteolytic (n=66, median SUVmax, 10.6 vs 6.1; P < 0.01), and osteoblastic metastases (n=43, median SUVmax, 7.7 vs 3.7; P < 0.01). [⁶⁸Ga]Ga-DOTA-FAPI-04 uptakes were significantly higher than that of [¹⁸F]FDG in bone metastases from lung cancer (n = 62, median SUVmax, 10.7 vs 5.2; P < 0.01), thyroid cancer (n = 18, median SUVmax, 5.65 vs 2.1; P < 0.01) and liver cancer (n = 12, median SUVmax, 5.65 vs 3.05; P < 0.01). However, [⁶⁸Ga]Ga-DOTA-FAPI-04 detected 10 false-positive lesions, while only 5 false-positive were visualized by [¹⁸F]FDG PET/CT.

Conclusion

[⁶⁸Ga]Ga-DOTA-FAPI-04 PET/CT exhibits excellent diagnostic performance as a means of detecting bone metastases, and is superior to [¹⁸F]FDG PET/CT in this diagnostic context. Furthermore, [⁶⁸Ga]Ga-DOTA-FAPI-04 tracer uptake levels are higher than those of [¹⁸F]FDG for most bone metastases. However, owing to the potential for false-positive bone lesions, it is critical that physicians interpret all CT findings with caution to ensure diagnostic accuracy.

Incidental Detection of Sinonasal Inverted Papilloma With 68Ga-FAPI PET/CT in a Patient With Esophageal Cancer

ABSTRACT

A 68-year-old man with newly diagnosed esophageal cancer was included in a clinical trial of 68Ga-FAPI PET/CT study in tumors (ChiCTR2100044131). Increased tracer uptake was noted in esophageal cancer. In addition, sinonasal inverted papilloma also revealed increased FAPI activity.

68Ga-FAPI PET/CT Versus 18F-FDG PET/CT for the Evaluation of Disease Activity in Takayasu Arteritis

ABSTRACT

Evaluation of disease activity in Takayasu arteritis (TA) is challenging because clinical, biological, and imaging data do not always correlate. In this case, a 19-year-old woman with TA underwent 18F-FDG PET/CT for evaluating disease activity. However, no abnormal 18F-FDG uptake was observed in the large arteries. Follow-up with 68Ga-FAPI PET/CT showed intense activity in the mildly thickened walls of bilateral carotid, subclavian arteries, and thoracic and abdominal aorta. This case presented an example where 68Ga-FAPI outperformed 18F-FDG for detecting active inflammation of TA. Therefore, 68Ga-FAPI PET/CT may allow monitoring of disease activity and facilitate selecting the optimal therapy for TA patients.

Follicular Thyroid Adenoma Showing Avid Uptake on 68Ga-DOTA-FAPI-04 PET/CT

ABSTRACT

We describe the case of a 48-year-old man suspected to have recurrent colon carcinoma who underwent 68Ga-DOTA-FAPI-04 PET/CT, revealing the presence of an incidentaloma with intense FAPI uptake in the left thyroid lobe. Based on PET/CT findings, malignancy was suspected. Biopsy confirmed the diagnosis of follicular thyroid adenoma with fibrosis and calcification. This case emphasizes the need to increase awareness of this benign lesion in the differential diagnosis of incidental thyroid nodules with high FAPI activity.

Two Tumors, One Target: Preliminary Experience With 90Y-FAPI Therapy in a Patient With Metastasized Breast and Colorectal Cancer

ABSTRACT

We report a patient with breast cancer (BC) diagnosed in 2009 with metachronous lymph node, liver, and bone metastases. In 2017, colorectal cancer with peritoneal metastases was additionally diagnosed and treated with 8 cycles of capecitabine due to its antitumor activity against both malignancies. At progression of both diseases, FAPI PET/CT demonstrated positive tumor targeting in BC-related metastases and colorectal cancer-related metastases. The patient received an experimental therapy with 90Y-FAPI46. Although there was similar tracer uptake in the PET/CT, the radioligand therapy resulted in mixed response with disappearance of peritoneal metastases but minor efficacy treating the BC-related metastases.

The Added Value of 68Ga-FAPI-04 PET/CT in Patients with Head and Neck Cancer of Unknown Primary with 18F-FDG Negative Findings

¹⁸F-fluorodeoxyglucose (¹⁸F-FDG) positron emission tomography/computed tomography (PET/CT) plays an important role in locating of primary tumor for patients with head and neck cancer of unknown primary (HNCUP). Nevertheless, it can be challenging to locate the primary malignancy in ¹⁸F-FDG-PET/CT scan in some cases. As ⁶⁸Ga-radiolabeled fibroblast activation protein inhibitor (FAPI) PET/CT has promising results in detecting different tumor entities, our study aimed to evaluate the performance of ⁶⁸Ga-FAPI-PET/CT for detecting the primary tumor in HNCUP patients with negative ¹⁸F-FDG findings. Methods: A total of eighteen patients (16 males and 2 females; median age, 55 years; range, 24-72 years) with negative ¹⁸F-FDG findings were enrolled in this study. All patients underwent ¹⁸F-FDG and ⁶⁸Ga-FAPI-PET/CT within one week. Biopsy and histopathological examinations were done in the sites with positive ⁶⁸Ga-FAPI-PET/CT findings. Results: ⁶⁸Ga-FAPI-PET/CT detected the primary tumor in 7 out of 18 patients (38.89%). Among the 7 patients, in respect of the primary tumor sites, 1 was in nasopharynx, 2 were in palatine tonsil, 2 were in submandibular gland, and 2 were in hypopharynx. The primary tumors showed moderate to intensive uptake of FAPI (mean SUV_max, 8.79; range, 2.60-16.50) and excellent tumor-to-contralateral normal tissue ratio (mean SUV_max ratio, 4.50; range, 2.17-8.21). In lesion-based analysis, a total of 65 lymph nodes and 17 bone metastatic lesions were identified. The mean SUV_max of lymph node metastases were 9.05 ± 5.29 for FDG and 9.08 ± 4.69 for FAPI (P = 0.975); as for bone metastases, the mean SUV_max were 8.11 ± 3.00 for FDG and 6.96 ± 5.87 for FAPI, respectively (P = 0.478). The mean tumor-to-background ratio (TBR) values of lymph node and bone metastases were 10.65 ± 6.59 vs. 12.80 ± 8.11 (P = 0.100) and 9.08 ± 3.35 vs. 9.14 ± 8.40 (P = 0.976), respectively. Conclusion: We presented first evidence of diagnostic role of ⁶⁸Ga-FAPI-PET/CT in HNCUP, and our study demonstrated that ⁶⁸Ga-FAPI-PET/CT had the potential to improve the detection rate of primary tumor in HNCUP patients with negative FDG findings. Moreover, ⁶⁸Ga-FAPI had similar performance in assessing metastases with ¹⁸F-FDG.

FAP and FAPI-PET/CT in Malignant and Non-Malignant Diseases: A Perfect Symbiosis?

A fibroblast activation protein (FAP) is an atypical type II transmembrane serine protease with both endopeptidase and post-proline dipeptidyl peptidase activity. FAP is overexpressed in cancer-associated fibroblasts (CAFs), which are found in most epithelial tumors. CAFs have been implicated in promoting tumor cell invasion, angiogenesis and growth and their presence correlates with a poor prognosis. However, FAP can generally be found during the remodeling of the extracellular matrix and therefore can be detected in wound healing and benign diseases. For instance, chronic inflammation, arthritis, fibrosis and ischemic heart tissue after a myocardial infarction are FAP-positive diseases. Therefore, quinoline-based FAP inhibitors (FAPIs) bind with a high affinity not only to tumors but also to a variety of benign pathologic processes. When these inhibitors are radiolabeled with positron emitting radioisotopes, they provide new diagnostic and prognostic tools as well as insights into the role of the microenvironment in a disease. In this respect, they deliver additional information beyond what is afforded by conventional FDG PET scans that typically report on glucose uptake. Thus, FAP ligands are considered to be highly promising novel tracers that offer a new diagnostic and theranostic potential in a variety of diseases.

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

Cancer-associated fibroblasts (CAFs) are crucial components of the tumor microenvironment. Fibroblast activation protein (FAP) is overexpressed in CAFs. FAP-targeted molecular imaging agents, including FAP inhibitor (FAPI)-04 and FAPI-46, have shown promising results in tumor diagnosis. However, these molecules have relatively short tumor-retention time for peptide-targeted radionuclide therapy applications. We aimed to design a ⁶⁸Ga-labeled FAPI dimer (denoted as ⁶⁸Ga-DOTA-2P(FAPI)2) to optimize the pharmacokinetics and evaluate whether this form is more effective than its monomeric analogs. Methods: ⁶⁸Ga-DOTA-2P(FAPI)2 was synthesized based on the quinoline-based FAPI variants (FAPI-46), and its binding properties were assayed in CAFs. Preclinical pharmacokinetics was determined in FAP-positive patient-derived xenografts (PDXs) using small-animal PET and biodistribution experiments. The effective dosimetry of ⁶⁸Ga-DOTA-2P(FAPI)2 was evaluated in three healthy volunteers, and PET/ CT imaging of ⁶⁸Ga-FAPI-46 and ⁶⁸Ga-DOTA-2P(FAPI)2 was performed in three cancer patients. Results: ⁶⁸Ga-DOTA-2P(FAPI)2 was stable in phosphate-buffered saline and fetal bovine serum for 4 h. The FAPI dimer showed high affinity and specificity for FAP in-vitro and in-vivo. The tumor uptake of ⁶⁸Ga-DOTA-2P(FAPI)2 was approximately two-fold stronger than that of ⁶⁸Ga-FAPI-46 in PDXs, while the healthy organs showed low tracer uptake and fast body clearance. The effective dose of ⁶⁸Ga-DOTA-2P(FAPI)2 was 1.19E-02 mSv/MBq, calculated using OLINDA. Finally, PET/CT scans in three cancer patients revealed higher intratumoral uptake of ⁶⁸Ga-DOTA-2P(FAPI)2 than that of ⁶⁸Ga-FAPI-46 in all tumor lesions (maximum standardized uptake value: 8.1-39.0 vs. 1.7-24.0, respectively; P < 0.001). Conclusion: ⁶⁸Ga-DOTA-2P(FAPI)2 has increased tumor uptake and retention properties compared to ⁶⁸Ga-FAPI-46, and it could be a promising tracer for both diagnostic imaging and targeted therapy of malignant tumors with positive expression of FAP.

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.

Comparison of 68Ga-DOTA-FAPI and 18FDG PET/CT imaging modalities in the detection of liver metastases in patients with gastrointestinal system cancer

PURPOSE

We aimed to compare the diagnostic performance of PET/CT imaging performed with 68Ga-DOTA-FAPI and 18FDG in detection of liver metastases in patients with gastrointestinal system (GIS) cancer.

METHODS

A total of 31 patients who underwent 68Ga-DOTA-FAPI and 18F-FDG PET/CT examinations and diagnosed with GIS cancer (15 colorectal, 9 pancreas, 4 stomach and 3 other cancers) were included in the study. The presence of liver metastasis was decided based on histopathologic diagnosis, PET/CT, other radiologic examinations or tumor biomarker findings, and both PET/CT imaging findings were compared on the patient and lesion basis.

RESULTS

Of the 31 patients, 28 were found as true positive with 68Ga-DOTA-FAPI-PET/CT and 17 with 18FDG-PET/CT. Of the 98 metastatic liver lesions determined according to our diagnostic criteria, 92 were found as true positive lesions with 68Ga-DOTA-FAPI-PET/CT and 65 with 18FDG-PET/CT. There was a statistically significant difference between both imaging modalities in the patient and lesion based comparisons (p < 0.05). When semiquantitative values (SUVmax, mlr) obtained from the lesions were compared between the two imaging methods, mlr values showed statistically significant difference in all tumor subgroups (p < 0.05).

CONCLUSION

It was concluded that 68Ga-DOTA-FAPI-PET/CT was superior over 18FDG-PET/CT in detection of liver metastases of GIS cancers and it can be a complementary method especially in negative cases with 18FDG-PET/CT.

Comparison of 18F-FDG, 68Ga-FAPI, and 68Ga-DOTATATE PET/CT in a Patient With Pancreatic Neuroendocrine Tumor

ABSTRACT

We present image findings of 18F-FDG, 68Ga-FAPI, and 68Ga-DOTATATE PET/CT in a 35-year-old woman with multiple metastases of pancreatic neuroendocrine tumor. The images of PET/CTs using 3 different tracers all showed multiple foci of increased activities in the liver and pancreas body, in which 68Ga-FAPI PET/CT displayed the highest tumor-to-liver ratios. However, 68Ga-DOTATATE PET/CT detected more small metastatic lymph node and bone metastases, which were missed by both FDG and FAPI PET/CT.

Novel Tracers and Radionuclides in PET Imaging

The use of PET imaging agents in oncology, cardiovascular disease, and neurodegenerative disease shows the power of this technique in evaluating the molecular and biological characteristics of numerous diseases. These agents provide crucial information for designing therapeutic strategies for individual patients. Novel PET tracers are in continual development and many have potential use in clinical and research settings. This article discusses the potential applications of tracers in diagnostics, the biological characteristics of diseases, the ability to provide prognostic indicators, and using this information to guide treatment strategies including monitoring treatment efficacy in real time to improve outcomes and survival.

The potential utility of [68 Ga]Ga-DOTA-FAPI-04 as a novel broad-spectrum oncological and non-oncological imaging agent-comparison with [18F]FDG

PURPOSE

This study aimed to compare the detection performance of [⁶⁸ Ga]Ga-DOTA-FAPI-04 and [¹⁸F]FDG PET/CT in the patients with various oncological and non-oncological lesions.

METHODS

A total of 123 patients underwent contemporaneous [⁶⁸ Ga]Ga-DOTA-FAPI-04 and [¹⁸F]FDG PET/CT were included in this prospective study. The maximum standard uptake value (SUVmax) was measured to compare oncological and non-oncological lesion uptake. The sensitivity, specificity, predictive values, and accuracy of [¹⁸F]FDG and [⁶⁸ Ga]Ga-DOTA-FAPI-04 PET/CT for detecting primary, metastatic, and non-oncological lesions were calculated and compared to evaluate the detection efficacy.

RESULTS

The study subjects consisted of 123 patients (69 men and 54 women; mean age 56.11 ± 11.94 years). Among the 102 patients with either newly diagnosed (82 patients) or previously treated solid tumor (20 patients), a total of 88 solid primary malignant tumors in 84/102 patients were detected. Two patients had two primary tumors each and 1 patient had three primary tumors. Among them, 58/102 and 43/102 patients had nodal (376 lesions) and distant metastases (406 lesions), respectively. Eight patients had hematological neoplasm. No malignant oncological diseases were detected in the remaining 13 patients. A total of 145 non-oncological lesions and benign tumors in 52/123 patients were detected incidentally. [⁶⁸ Ga]Ga-DOTA-FAPI-04 PET/CT demonstrated a significantly higher uptake and detection rate for the primary (SUVmax 10.98 ± 5.83 vs. 8.36 ± 6.43, p < 0.001; sensitivity 97.67 vs. 84.89%; and accuracy 96.59 vs. 82.95%, X² = 0.538, p = 0.021), nodal (SUVmax 10.50 ± 5.98 vs. 8.20 ± 6.29, p = 0.011; sensitivity 97.59 vs. 84.72%; and accuracy 97.34 vs. 84.31%, X² = 2.067, p < 0.001), and distant metastatic lesions (SUVmax 9.64 ± 6.45 vs. 6.74 ± 4.83; p < 0.001; sensitivity 98.01 vs. 65.59%; and accuracy 97.04 vs. 65.51%, X² = 4.897, p < 0.001) of solid tumor than did [¹⁸F]FDG PET/CT. [⁶⁸ Ga]Ga-DOTA-FAPI-04 PET/CT demonstrated a lower activity (SUVmax: 6.84 ± 4.67 vs. 13.09 ± 7.29, p < 0.001) and detection rate (sensitivity 50.65 vs. 96.75%, and accuracy 51.28 vs. 95.51%, X² = 5.166, p < 0.001) for multiple myeloma and lymphoma compared to [¹⁸F]FDG PET/CT. [⁶⁸ Ga]Ga-DOTA-FAPI-04 and [¹⁸F]FDG PET/CT PET/CT demonstrated a comparative activity (SUVmax 6.40 ± 3.95 vs. 5.74 ± 15.78, p = 0.729) and detection efficacy (sensitivity 86.52 vs. 72.34%, and accuracy 84.83 vs. 72.41%, X² = 9.460, p = 0.007) for non-oncological lesion and benign tumor detection.

CONCLUSIONS

Except for myeloma and lymphoma, [⁶⁸ Ga]Ga-DOTA-FAPI-04 PET/CT showed a superior detection efficacy for detecting various primary and metastatic lesions than [¹⁸F]FDG PET/CT. A comparative detection utility for non-oncological lesion was obtained with both tracers. [⁶⁸ Ga]Ga-DOTA-FAPI-04 could be used as a broad-spectrum tumor and inflammatory imaging agent in the clinical especially for various solid tumors and non-oncological lesions.

Increased 68Ga-FAPI-04 Uptake in Schmorl Node in a Patient With Gastric Cancer

ABSTRACT

A 78-year-old man with a newly diagnosed gastric adenocarcinoma underwent 18F-FDG and 68Ga-FAPI-04 PET/CT before treatment. Both 18F-FDG and 68Ga-FAPI-04 PET/CT demonstrated intense radioactivity in the gastric cancer. However, the benign Schmorl node in the inferior endplate of the T5 vertebrae showed increased uptake of 68Ga-FAPI-04, which was not FDG avid. Two months after radical gastrectomy of the gastric cancer (pT1aN0M0, IA), a follow-up CT showed that the Schmorl node in T5 vertebrae remained unchanged.

68Ga-FAPI Uptake of Thyroiditis in a Patient With Breast Cancer

ABSTRACT

68Ga-fibroblast activation protein-specific inhibitor (FAPI)-04 PET/CT was performed in a 49-year-old woman diagnosed with breast cancer. In PET/CT imaging, intense 68Ga-FAPI uptake was observed in the primary tumor, axillary lymph nodes, and also in the thyroid gland, whereas pathological 18F-FDG uptake was not observed in the thyroid gland. On thyroid ultrasonography, parenchyma was heterogeneous, and an area of focal thyroiditis was observed in the superior part of the right lobe. Biochemical parameters were found to be consistent with thyroiditis. This case shows that FAPI uptake in the thyroid gland may be associated with thyroiditis and should be evaluated clinically.

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

OBJECTIVES

This study is set out to estimate the radiation-absorbed doses to normal organs and tumor tissue using low-dose 177Lu-FAPI04 dosimetry to determine the safety and theranostic potential of fibroblast activation protein-targeted radionuclide therapy.

PATIENTS AND METHODS

Four patients with metastatic advanced-stage cancer were administered low-dose 177Lu-FAPI04 for dosimetry measurements. Data acquisition for dosimetry of normal organs and tumors was performed by whole-body and 3D SPECT/CT imaging at 4, 24, 48, and 96 hours after administering 177Lu-FAPI04. Blood samples were drawn at 5, 15, 30, 60, 60, 120, and 180 minutes, and at 24, 48, and 96 hours for bone marrow dosimetry calculations.

RESULTS

Mean absorbed doses per megabecquerel were 0.25 ± 0.16 mGy (range, 0.11-0.47 mGy), 0.11 ± 0.08 mGy (range, 0.06-0.22 mGy), and 0.04 ± 0.002 mGy (range, 0.04-0.046 mGy) for kidneys, liver, and bone marrow, respectively. The respective maximum estimated amount of radioactivity to reach radiation-absorbed dose limits were 120.9 ± 68.6 GBq, 47.5 ± 2.8 GBq, 397.8 ± 217.1 GBq, and 52.4 ± 15.3 GBq for kidneys, bone marrow, liver, and total body. The mean absorbed dose per megabecquerel was 0.62 ± 0.55 mGy for bone metastases, 0.38 ± 0.22 mGy for metastatic lymph nodes, 0.33 ± 0.21 mGy for liver metastases, and 0.37 ± 0.29 for metastatic soft tissue. The maximum absorbed dose in a tumor lesion was 1.67 mGy/MBq for bone, 0.6 mGy/MBq for lymph node, 0.62 mGy/MBq for liver, and 1 mGy/MBq for soft tissue.

CONCLUSIONS

The mean absorbed dose to organs at risk with 177Lu-FAPI04 is reasonably low, allowing for low tumor-absorbed dose rates by administering a higher dose. Further research on optimizing therapeutic efficacy and using alternative radioisotopes is necessary, along with an individualized dosimetric approach.

False-Positive 68Ga-Fibroblast Activation Protein-Specific Inhibitor Uptake of Benign Lymphoid Tissue in a Patient With Breast Cancer

ABSTRACT

68Ga-fibroblast activation protein-specific inhibitor (FAPI)-04 PET/CT was performed in a patient with left lower outer quadrant breast cancer who had 18F-FDG PET/CT imaging. 68Ga-FAPI-04 PET/CT showed higher accumulation of radiotracer in primary tumor and axillary lymph nodes than 18F-FDG PET/CT. In addition, focal increased FAPI uptake was observed in another nodular lesion in the lower inner quadrant in the same breast, which was considered malignant. However, Tru-Cut biopsy of this lesion was reported as benign lymphoid tissue. This case showed that all FAPI accumulation in breast tissue should not be interpreted in favor of malignancy; histopathological confirmation is required.

Head-to-head comparison of [68Ga]Ga-FAPI-04 and [18F]-FDG PET/CT in evaluating the extent of disease in gastric adenocarcinoma

BACKGROUND

[¹⁸F]-Fluorodeoxyglucose (FDG) positron emission tomography-computed tomography (PET/CT) may sometimes be suboptimal for imaging gastric adenocarcinoma. The recently introduced [⁶⁸Ga]Ga-FAPI-04 (FAPI) PET/CT targets tumor stroma and has shown considerable potential in evaluating the extent of disease in a variety of tumors.

METHODS

We performed a head-to-head prospective comparison of FAPI and FDG PET/CT in the same group of 13 patients with gastric adenocarcinoma who presented for either initial staging (n = 10) or restaging (n = 3) of disease. Lesion detection and maximum standardized uptake value (SUV_max) were compared between the two types of radiotracers.

RESULTS

All ten primary gastric tumors were FAPI-positive (100% detection rate), whereas only five were also FDG-positive (50%). SUV_max was not significantly different, but the tumor-to-background ratio was higher for FAPI (mean, median, and range of 4.5, 3.2, and 0.8-9.7 for FDG and 12.9, 11.9, and 2.2-23.9 for FAPI, P = 0.007). The level of detection of regional lymph node involvement was comparable. FAPI showed a superior detection rate for peritoneal carcinomatosis (100% vs. none). Two patients with widespread peritoneal carcinomatosis underwent a follow-up FAPI scan after chemotherapy: one showed partial remission and the other showed progressive disease.

CONCLUSIONS

The findings of this pilot study suggest that FAPI PET/CT outperforms FDG PET/CT in detecting both primary gastric adenocarcinoma and peritoneal carcinomatosis from gastric cancer. FAPI PET/CT also shows promise for monitoring response to treatment in patients with peritoneal carcinomatosis from gastric cancer; however, larger trials are needed to validate these preliminary findings.

Comparison of [68 Ga]Ga-FAPI-04 and [18F]-FDG for the detection of primary and metastatic lesions in patients with gastric cancer: a bicentric retrospective study

INTRODUCTION

The low sensitivity of [¹⁸F]-fluorodeoxyglucose ([¹⁸F]-FDG) for the diagnosis of gastric cancer limits its application. In this study, we aimed to investigate the potential advantage of [⁶⁸ Ga]Ga-FAPI-04 over [¹⁸F]-FDG in the evaluation of gastric cancer.

METHODS

This was a bicentric retrospective analysis of a prospective parent study (clinical trial: HS-KY-2020-826 (Huashan Hospital) and DF-2020-102 (Shanghai East Hospital)). Thirty-eight patients with gastric cancer (31 with adenocarcinoma and 7 with signet ring cell carcinoma) were included in this study. All of the participants underwent [⁶⁸ Ga]Ga-FAPI-04 and [¹⁸F]-FDG imaging by positron emission tomography (PET)/computed tomography (CT) or PET/magnetic resonance (MR). The scans were interpreted by two experienced nuclear medicine physicians, and the maximum standardized uptake value (SUV_max) was calculated. Histopathological findings obtained from biopsy or resected surgical specimens were used as a reference for the final diagnosis.

RESULTS

For the detection of primary gastric cancer, the sensitivities of [⁶⁸ Ga]Ga-FAPI-04 PET and [¹⁸F]-FDG PET were 100% (38/38) and 82% (31/38), respectively (P = 0.016). Four cases of adenocarcinoma and three cases of signet ring cell carcinoma were missed by [¹⁸F]-FDG PET. The mean SUV_max of [⁶⁸ Ga]Ga-FAPI-04 in tumours greater than 4 cm (11.0 ± 4.5) was higher than that in tumours less than 4 cm (4.5 ± 3.2) (P = 0.0015). The mean SUV_max of [⁶⁸ Ga]Ga-FAPI-04 was higher in T2-4 tumours (9.7 ± 4.4) than in T1 tumours (3.1 ± 1.5) (P = 0.0002). For the detection of metastatic lesions, the sensitivities of [⁶⁸ Ga]Ga-FAPI-04 PET and [¹⁸F]-FDG PET in 10 patients with regional lymph node metastasis and distant metastasis were 6/10 and 5/10, respectively.

CONCLUSION

In this selected cohort, [⁶⁸ Ga]Ga-FAPI-04 PET had a superior detection rate than [¹⁸F]-FDG PET for primary gastric cancer. [⁶⁸ Ga]Ga-FAPI-04 PET could provide better performance with regard to gastric cancer diagnosis and staging. Prospective clinical trials are warranted.

Increased uptake of 68Ga-DOTA-FAPI-04 in bones and joints: metastases and beyond

PURPOSE

To describe the uptake of ⁶⁸Gallium-labelled fibroblast activation protein inhibitor (⁶⁸Ga-FAPI) in the bones and joints for better understanding of the role of ⁶⁸Ga-FAPI PET in benign and malignant bone lesions and joint diseases.

METHODS

All 129 ⁶⁸Ga-FAPI PET/MR or PET/CT scans from June 1, 2020, to February 20, 2021, performed at our PET center were retrospectively reviewed. Foci of elevated ⁶⁸Ga-FAPI uptake in the bones and joints were identified. All lesions were divided into malignant and benign diseases. Benign lesions included osteofibrous dysplasia, periodontitis, degenerative bone diseases, arthritis, and other inflammatory or trauma-related abnormalities. The number, locations, and SUVmax of all lesions were recorded and analyzed. The detectability of ⁶⁸Ga-FAPI PET and ¹⁸F-FDG PET in patients who had two scans was also compared.

RESULTS

Elevated uptake of ⁶⁸Ga-FAPI in/around the bones and joints was found in 82 cases (63.57%). A total of 295 lesions were identified, including 94 (31.9%) malignant lesions (all were metastases) and 201 (68.1%) benign lesions. The benign lesions consisted of 13 osteofibrous dysplasia, 48 degenerative bone disease, 33 periodontitis, 56 arthritis, and 51 other inflammatory or trauma-related abnormalities. The spine, shoulder joint, alveolar ridge, and pelvis were the most commonly involved locations. Bone metastases were mainly distributed in the spine, pelvis, and ribs. Among benign diseases, periodontitis and arthritis are site-specific. The mean SUVmax of bone metastases was significantly higher than that of benign diseases (7.14 ± 4.33 vs. 3.57 ± 1.60, p < 0.001), but overlap existed. The differences in SUVmax among subgroups of benign diseases were statistically significant (p < 0.001), with much higher uptake in periodontitis (4.45 ± 1.17). ⁶⁸Ga-FAPI PET identified much more lesions than ¹⁸F-FDG PET (104 vs. 48) with higher uptake value.

CONCLUSION

⁶⁸Ga-FAPI accumulated in both bone metastases and some benign diseases of the bones and joints. Although the uptake of ⁶⁸Ga-FAPI was often higher in bone metastases, this finding cannot be used to distinguish between benign and malignant lesions. ⁶⁸Ga-FAPI PET also has the potential to locate and evaluate the extent of both malignant tumor and benign diseases in bones and joints.

TRIAL REGISTRATION

NCT04554719, NCT04605939. Registered September 8, 2020 and October 25, 2020-retrospectively registered, http://clinicaltrails.gov/show/NCT04554719 ; http://clinicaltrails.gov/show/NCT04605939.

68Ga-FAPI-PET/CT improves diagnostic staging and radiotherapy planning of adenoid cystic carcinomas - Imaging analysis and histological validation

BACKGROUND

Adenoid cystic carcinomas (ACCs) are rare epithelial tumors mostly situated in the head and neck region and characterized by infiltrative growth. The tumor stroma of ACCs includes cancer-associated fibroblasts (CAFs) expressing Fibroblast Activation Protein (FAP), a new target for positron emission tomography (PET) imaging. Here we describe the value of PET/ computed tomography (PET/CT) imaging using ⁶⁸Ga-labelled FAP-Inhibitors (⁶⁸Ga-FAPI-PET/CT) and their clinical potential for staging and radiotherapy planning in 12 ACC patients (7 primary, 5 recurrent).

PATIENTS AND METHODS

Patients underwent contrast enhanced staging CT (ceCT) and magnetic resonance imaging (ceMRI) before ⁶⁸Ga-FAPI - PET/CT. PET-scans were acquired 10, 60 and 180 minutes after administration of 150-250 MBq of ⁶⁸Ga-labelled FAPI tracers. SUV_max and SUV_mean values of ACCs and healthy organs were obtained using a 60% of maximum iso-contour. FAP and alpha smooth muscle actin (α-SMA) immunohistochemistry was performed in 13 cases (3 with and 10 without ⁶⁸Ga FAPI-PET/CT). Staging and radiotherapy planning based on ⁶⁸Ga-FAPI-PET/CT versus ceCT/MRI alone were compared.

RESULTS

We observed elevated tracer uptake in all ACCs. Immunohistochemistry showed FAP-expressing CAFs in the tumor. Compared to conventional staging, ⁶⁸Ga-FAPI-PET/CT led to upstaging in 2/12 patients and to detection of additional metastases in 3 patients, thus in total 42% of patients had their staging altered. Moreover, ⁶⁸Ga-FAPI PET improved the accuracy of target volume delineation for radiotherapy, as compared to CT and MRI.

CONCLUSION

⁶⁸Ga-FAPI-PET/CT is a promising imaging modality for ACC, increasing the accuracy of staging exams and radiotherapy planning volumes, as compared conventional to CT and MRI.

The Role of Fibroblast Activation Protein Ligands in Oncologic PET Imaging

Fibroblast activation protein inhibitor emerges as a novel and highly promising agent for diagnostic and possibly theranostic application in various malignant and non-malignant diseases. FAPI impresses with its selective expression in several pathologies, ligand induced internalization, and presence in a large variety of malignancies. Current studies indicate that FAPI is equal or even superior to the current standard oncological tracer fluorodeoxyglucose in several oncological diseases. It seems to present lower background activity, stronger uptake in tumorous lesions and thus sharper contrasts. For improved comprehension of fibroblast activation, protein expression and clinicopathologic conditions, further studies are of essence.

State-of-the-art of FAPI-PET imaging: a systematic review and meta-analysis

INTRODUCTION

Fibroblast activation protein-α (FAPα) is overexpressed on cancer-associated fibroblasts in approximately 90% of epithelial neoplasms, representing an appealing target for therapeutic and molecular imaging applications. [⁶⁸ Ga]Ga-labelled radiopharmaceuticals-FAP-inhibitors (FAPI)-have been developed for PET. We systematically reviewed and meta-analysed published literature to provide an overview of its clinical role.

MATERIALS AND METHODS

The search, limited to January 1st, 2018-March 31st, 2021, was performed on MedLine and Embase databases using all the possible combinations of terms "FAP", "FAPI", "PET/CT", "positron emission tomography", "fibroblast", "cancer-associated fibroblasts", "CAF", "molecular imaging", and "fibroblast imaging". Study quality was assessed using the QUADAS-2 criteria. Patient-based and lesion-based pooled sensitivities/specificities of FAPI PET were computed using a random-effects model directly from the STATA "metaprop" command. Between-study statistical heterogeneity was tested (I²-statistics).

RESULTS

Twenty-three studies were selected for systematic review. Investigations on staging or restaging head and neck cancer (n = 2, 29 patients), abdominal malignancies (n = 6, 171 patients), various cancers (n = 2, 143 patients), and radiation treatment planning (n = 4, 56 patients) were included in the meta-analysis. On patient-based analysis, pooled sensitivity was 0.99 (95% CI 0.97-1.00) with negligible heterogeneity; pooled specificity was 0.87 (95% CI 0.62-1.00), with negligible heterogeneity. On lesion-based analysis, sensitivity and specificity had high heterogeneity (I² = 88.56% and I² = 97.20%, respectively). Pooled sensitivity for the primary tumour was 1.00 (95% CI 0.98-1.00) with negligible heterogeneity. Pooled sensitivity/specificity of nodal metastases had high heterogeneity (I² = 89.18% and I² = 95.74%, respectively). Pooled sensitivity in distant metastases was good (0.93 with 95% CI 0.88-0.97) with negligible heterogeneity.

CONCLUSIONS

FAPI-PET appears promising, especially in imaging cancers unsuitable for [¹⁸F]FDG imaging, particularly primary lesions and distant metastases. However, high-level evidence is needed to define its role, specifically to identify cancer types, non-oncological diseases, and clinical settings for its applications.

68Ga-FAPI-04 Versus 18F-FDG PET/CT in the Detection of Hepatocellular Carcinoma

Background

Fibroblast activation protein (FAP) is commonly expressed in activated stromal fibroblasts in various epithelial tumours. Recently, ⁶⁸Ga-FAPI-04 has been used for tumour imaging in positron emission tomography/computed tomography (PET/CT). This study aimed to compare the diagnostic performances of ⁶⁸Ga-FAPI-04 PET/CT and ¹⁸F-FDG PET/CT in hepatocellular carcinoma (HCC), and to assess factors associated with ⁶⁸Ga-FAPI-04 uptake in HCC.

Materials and Methods

Twenty-nine patients with suspiciously HCC who received both ¹⁸F-FDG and ⁶⁸Ga-FAPI-04 PET/CT were included in this retrospective study. The results were interpreted by two experienced nuclear medicine physicians independently. The maximum and mean standardized uptake values (SUV_max and SUV_mean) were measured in the lesions and liver background, respectively. The tumour-to-background ratio (TBR) was then calculated as lesion's SUV_max divided by background SUV_mean.

Results

A total of 35 intrahepatic lesions in 25 patients with HCC were finally involved in the statistical analysis. ⁶⁸Ga-FAPI-04 PET/CT showed a higher sensitivity than ¹⁸F-FDG PET/CT in detecting intrahepatic HCC lesions (85.7% vs. 57.1%, P = 0.002), including in small (≤ 2 cm in diameter; 68.8% vs. 18.8%, P = 0.008) and well- or moderately-differentiated (83.3% vs. 33.3%, P = 0.031) tumors. SUV_max was comparable between ⁶⁸Ga-FAPI-04 and ¹⁸F-FDG (6.96 ± 5.01 vs. 5.89 ± 3.38, P > 0.05), but the TBR was significantly higher in the ⁶⁸Ga-FAPI-04 group compared with the ¹⁸F-FDG group (11.90 ± 8.35 vs. 3.14 ± 1.59, P < 0.001). SUV_max and the TBR in ⁶⁸Ga-FAPI-04 positive lesions were associated with tumour size (both P < 0.05), but not the remaining clinical and pathological features (all P > 0.05).

Conclusions

⁶⁸Ga-FAPI-04 PET/CT is more sensitive than ¹⁸F-FDG PET/CT in detecting HCC lesions, and ⁶⁸Ga-FAPI-04 uptake is correlated mainly with tumour size.

Clinical translational evaluation of Al18F-NOTA-FAPI for fibroblast activation protein-targeted tumour imaging

PURPOSE

In this study, a novel aluminium-[¹⁸F]fluoride (Al¹⁸F)-labelled 1,4,7‑triazacyclononane-N,N',N″-triacetic acid (NOTA)-conjugated fibroblast activation protein inhibitor (FAPI) probe, named Al¹⁸F-NOTA-FAPI, was developed for fibroblast activation protein (FAP)-targeted tumour imaging; it could deliver hundreds of millicuries of radioactivity using automated synthesis. The tumour detection efficacy of Al¹⁸F-NOTA-FAPI was further validated in both preclinical and clinical translational studies.

METHODS

The radiolabelling procedure of Al¹⁸F-NOTA-FAPI was optimized. Cell uptake and competitive binding assays were completed with the U87MG and A549 cell lines to evaluate the affinity and specificity of the Al¹⁸F-NOTA-FAPI probe. The biodistribution, pharmacokinetics, radiation dosimetry and tumour imaging efficacy of the Al¹⁸F-NOTA-FAPI probe were researched in healthy Kunming (KM) and/or U87MG model mice. After the approval of the ethical committee, the Al¹⁸F-NOTA-FAPI probe was translated into the clinic for PET/CT imaging of the first 10 cancer patients.

RESULTS

The radiolabelling yield of Al¹⁸F-NOTA-FAPI was 33.8 ± 3.2% using manual synthesis (n = 10), with a radiochemical purity over 99% and the specific activity of 9.3-55.5 MBq/nmol. The whole body effective dose of Al¹⁸F-NOTA-FAPI was estimated to be 1.24E - 02 mSv/MBq, which was lower than several other FAPI probes (⁶⁸Ga-FAPI-04, ⁶⁸Ga-FAPI-46 and ⁶⁸Ga-FAPI-74). In U87MG tumour-bearing mice, Al¹⁸F-NOTA-FAPI showed good tumour detection efficacy based on the results of micro PET/CT imaging and biodistribution studies. In an organ biodistribution study of patients, Al¹⁸F-NOTA-FAPI showed a lower SUVmean than 2-[¹⁸F]-fluoro-2-deoxy-D-glucose (2-[¹⁸F]FDG) in most organs, especially in the liver (1.1 ± 0.2 vs. 2.0 ± 0.9), brain (0.1 ± 0.0 vs. 5.9 ± 1.3), and bone marrow (0.9 ± 0.1 vs. 1.7 ± 0.4). Meanwhile, Al¹⁸F-NOTA-FAPI did not show extensive bone uptake, and was able to detect more lesions than 2-[¹⁸F]FDG in the PET/CT imaging of several patients.

CONCLUSION

The Al¹⁸F-NOTA-FAPI probe was successfully fabricated and applied in fibroblast activation protein-targeted tumour PET/CT imaging, which showed excellent imaging quality and tumour detection efficacy in U87MG tumour-bearing mice as well as in cancer patients.

TRIAL REGISTRATION

Chinese Clinical Trial Registry ChiCTR2000038080. Registered 09 September 2020. http://www.chictr.org.cn/showproj.aspx?proj=61192.

Head-to-head intra-individual comparison of biodistribution and tumor uptake of 68Ga-FAPI and 18F-FDG PET/CT in cancer patients

PURPOSE

FAPI ligands (fibroblast activation protein inhibitor), a novel class of radiotracers for PET/CT imaging, demonstrated in previous studies rapid and high tumor uptake. The purpose of this study is the head-to-head intra-individual comparison of ⁶⁸Ga-FAPI versus standard-of-care ¹⁸F-FDG in PET/CT in organ biodistribution and tumor uptake in patients with various cancers.

MATERIAL AND METHODS

This international retrospective multicenter analysis included PET/CT data from 71 patients from 6 centers who underwent both ⁶⁸Ga-FAPI and ¹⁸F-FDG PET/CT within a median time interval of 10 days (range 1-89 days). Volumes of interest (VOIs) were manually drawn in normal organs and tumor lesions to quantify tracer uptake by SUVmax and SUVmean. Furthermore, tumor-to-background ratios (TBR) were generated (SUVmax tumor/ SUVmax organ).

RESULTS

A total of 71 patients were studied of, which 28 were female and 43 male (median age 60). In 41 of 71 patients, the primary tumor was present. Forty-three of 71 patients exhibited 162 metastatic lesions. ⁶⁸Ga-FAPI uptake in primary tumors and metastases was comparable to ¹⁸F-FDG in most cases. The SUVmax was significantly lower for ⁶⁸Ga-FAPI than ¹⁸F-FDG in background tissues such as the brain, oral mucosa, myocardium, blood pool, liver, pancreas, and colon. Thus, ⁶⁸Ga-FAPI TBRs were significantly higher than ¹⁸F-FDG TBRs in some sites, including liver and bone metastases.

CONCLUSION

Quantitative tumor uptake is comparable between ⁶⁸Ga-FAPI and ¹⁸F-FDG, but lower background uptake in most normal organs results in equal or higher TBRs for ⁶⁸Ga-FAPI. Thus, ⁶⁸Ga-FAPI PET/CT may yield improved diagnostic information in various cancers and especially in tumor locations with high physiological ¹⁸F-FDG uptake.

Head-to-head intra-individual comparison of biodistribution and tumor uptake of 68Ga-FAPI and 18F-FDG PET/CT in cancer patients

Purpose

FAPI ligands (fibroblast activation protein inhibitor), a novel class of radiotracers for PET/CT imaging, demonstrated in previous studies rapid and high tumor uptake. The purpose of this study is the head-to-head intra-individual comparison of ⁶⁸Ga-FAPI versus standard-of-care ¹⁸F-FDG in PET/CT in organ biodistribution and tumor uptake in patients with various cancers.

Material and Methods

This international retrospective multicenter analysis included PET/CT data from 71 patients from 6 centers who underwent both ⁶⁸Ga-FAPI and ¹⁸F-FDG PET/CT within a median time interval of 10 days (range 1–89 days). Volumes of interest (VOIs) were manually drawn in normal organs and tumor lesions to quantify tracer uptake by SUVmax and SUVmean. Furthermore, tumor-to-background ratios (TBR) were generated (SUVmax tumor/ SUVmax organ).

Results

A total of 71 patients were studied of, which 28 were female and 43 male (median age 60). In 41 of 71 patients, the primary tumor was present. Forty-three of 71 patients exhibited 162 metastatic lesions. ⁶⁸Ga-FAPI uptake in primary tumors and metastases was comparable to ¹⁸F-FDG in most cases. The SUVmax was significantly lower for ⁶⁸Ga-FAPI than ¹⁸F-FDG in background tissues such as the brain, oral mucosa, myocardium, blood pool, liver, pancreas, and colon. Thus, ⁶⁸Ga-FAPI TBRs were significantly higher than ¹⁸F-FDG TBRs in some sites, including liver and bone metastases.

Conclusion

Quantitative tumor uptake is comparable between ⁶⁸Ga-FAPI and ¹⁸F-FDG, but lower background uptake in most normal organs results in equal or higher TBRs for ⁶⁸Ga-FAPI. Thus, ⁶⁸Ga-FAPI PET/CT may yield improved diagnostic information in various cancers and especially in tumor locations with high physiological ¹⁸F-FDG uptake.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00259-021-05307-1.

In Vitro Evaluation of the Squaramide-Conjugated Fibroblast Activation Protein Inhibitor-Based Agents AAZTA5.SA.FAPi and DOTA.SA.FAPi

Recently, the first squaramide-(SA) containing FAP inhibitor-derived radiotracers were introduced. DATA^5m.SA.FAPi and DOTA.SA.FAPi with their non-radioactive complexes showed high affinity and selectivity for FAP. After a successful preclinical study with [⁶⁸Ga]Ga-DOTA.SA.FAPi, the first patient studies were realized for both compounds. Here, we present a new squaramide-containing compound targeting FAP, based on the AAZTA⁵ chelator 1,4-bis-(carboxylmethyl)-6-[bis-(carboxymethyl)-amino-6-pentanoic-acid]-perhydro-1,4-diazepine. For this molecule (AAZTA⁵.SA.FAPi), complexation with radionuclides such as gallium-68, scandium-44, and lutetium-177 was investigated, and the in vitro properties of the complexes were characterized and compared with those of DOTA.SA.FAPi. AAZTA⁵.SA.FAPi and its derivatives labelled with non-radioactive isotopes demonstrated similar excellent inhibitory potencies compared to the previously published SA.FAPi ligands, i.e., sub-nanomolar IC₅₀ values for FAP and high selectivity indices over the serine proteases PREP and DPPs. Labeling with all three radiometals was easier and faster with AAZTA⁵.SA.FAPi compared to the corresponding DOTA analogue at ambient temperature. Especially, scandium-44 labeling with the AAZTA derivative resulted in higher specific activities. Both DOTA.SA.FAPi and AAZTA⁵.SA.FAPi showed sufficiently high stability in different media. Therefore, these FAP inhibitor agents could be promising for theranostic approaches targeting FAP.

FAPI Uptake in a Vertebral Body Fracture in a Patient With Lung Cancer: A FAPI Imaging Pitfall

ABSTRACT

A left pulmonary nodule was identified by CT scan in a 53-year-old woman who had a car accident 10 days earlier. 18F-FDG PET/CT showed multiple FDG-avid lesions located at the left lung nodule, mediastinal lymph nodules, and L4 vertebral body. 68Ga-FAPI PET/CT was performed for further evaluation. However, 68Ga-FAPI demonstrated intense FAPI uptake in the accident-related fracture of the L4 vertebral body. This case documents that the fracture of the vertebral body may cause FAPI uptake, and nuclear clinicians evaluating 68Ga-FAPI imaging should be aware of this potential pitfall.

Use of 68Ga-FAPI PET/CT for Evaluation of Peritoneal Carcinomatosis Before and After Cytoreductive Surgery

ABSTRACT

A 55-year-old woman underwent 18F-FDG and 68Ga-FAPI PET/CT for tumor detection and staging under the prospective study NCT04416165. Both scans showed a mass lesion with increased tracer uptake in the pancreas. Moreover, 68Ga-FAPI PET/CT showed a greater number of abnormal foci in the mesentery and omentum than that shown with 18F-FDG. These abnormal foci were suspected to be metastases from pancreatic cancer. The patient subsequently underwent a cytoreductive surgery, and postoperative histopathology confirmed the diagnosis of acinar cell carcinoma of pancreas. Follow-up 68Ga-FAPI PET/CT (3 months after surgery) showed an excellent response with decreasing 68Ga-FAPI activity in the whole abdomen.