Initial Evaluation of [18F]FAPI-74 PET for Various Histopathologically Confirmed Cancers and Benign Lesions

Visual Abstract

The 18 F-labeled fibroblast activation protein inhibitor (FAPI) [ 18 F]FAPI-74 has the benefit of a higher synthetic yield and better image resolution than 68 Ga-labeled FAPI. We preliminarily evaluated the diagnostic performance of [ 18 F]FAPI-74 PET in patients with various histopathologically confirmed cancers or suspected malignancies. Methods: We enrolled 31 patients (17 men and 14 women) with lung cancer (n 5 7), breast cancer (n 5 5), gastric cancer (n 5 5), pancreatic cancer (n 5 3), other cancers (n 5 5), and benign tumors (n 5 6). Twenty-seven of the 31 patients were treatment-naïve or preoperative, whereas recurrence was suspected in the remaining 4 patients. Histopathologic confirmation was obtained for the primary lesions of 29 of the 31 patients. In the remaining 2 patients, the final diagnosis was based on the clinical course.  Cancer -associated fibroblasts are major components of the cancer stroma and play an important role in cancer invasion and metastasis in the tumor microenvironment (1). Cancer-associated fibroblasts interact with cancer cells by secreting numerous chemokines and cytokines, such as transforming growth factor b, inducing immunosuppression in the tumor microenvironment (1). Cancer-associated fibroblasts express fibroblast activation protein (FAP), and the expression levels of FAP have been reported to correlate with the prognosis in patients with cancer (1). In addition, FAP expression has been confirmed in various cancer types, with minimal expression observed in normal organs (2,3). The FAP inhibitor (FAPI) has gained attention as an excellent PET probe that can accurately detect many types of cancer compared with the conventional glucose analog [ 18 (4)(5)(6). Recently, the number of published papers reporting the excellent performance of FAPI PET has increased, and clinical trials are being conducted (NCT05262855 and NCT05641896). FAPI PET is expected to be used increasingly in cancer diagnosis and treatment planning for optimized patient management.
However, the short half-life of 68 Ga (68 min) can cause problems in production and delivery. It requires onsite production by a 68 Ga generator or cyclotron production using a solid target of 68 Zn. It also has a limitation in that one production cycle of 68 Ga allows the acquisition of only 2-3 patient scans because of the relatively low yield and short half-life. Labeling with 18 F (half-life, 110 min), such as [ 18 F]FDG, will enable more practical large-scale production. For prostate-specific membrane antigen PET, [ 68 Ga]PSMA-11 is being replaced by 18 F-labeled prostate-specific membrane antigen probes, such as [ 18 F]DCFPyL, because of the increasing cost of 68 Ga generators. Thus, 18 F-labeled FAPI ligands, such as [ 18 F]FAPI-74, may be the best option for widespread use in the global market with the added benefit of a higher synthetic yield and delivery from centralized large-scale production (5).
In this study, we preliminarily evaluated the diagnostic performance of [ 18 F]FAPI-74 PET in patients with various histopathologically confirmed cancers or suspected malignancies.

Patients
We enrolled 31 patients (age range, 38-83 y; 17 men and 14 women) in this prospective study. The inclusion criteria were as follows: patients who had been diagnosed with a malignant tumor or suspected malignancy before treatment and had undergone CT or [ 18 F]FDG PET, patients who had been diagnosed with a malignant tumor and had undergone or were to undergo chemotherapy or radiotherapy, and patients with suspected recurrence based on the clinical findings or other diagnostic imaging, such as CT or [ 18 F]FDG PET, after treatment. The patient characteristics are summarized in Table 1. The study included patients with lung cancer (n 5 7), breast cancer (n 5 5), gastric cancer (n 5 5), pancreatic cancer (n 5 3), other cancers (n 5 5), and benign tumors (n 5 6). Twenty-seven of the 31 patients were treatment-naïve or preoperative, whereas recurrence was suspected in the remaining 4 patients. Patients who were pregnant or suspected to be pregnant, pediatric patients who required sedation, and patients considered unsuitable for participation in the study were excluded. Histopathologic confirmation was obtained for the primary lesions of 29 of the 31 patients (94%) and for the metastatic lesions of 10 patients who underwent surgical resection. In 2 patients (2/31), the final diagnosis was based on the clinical course, including the results of follow-up imaging. Postoperative fibrosis was suspected in 1 patient because no remarkable changes were observed on the follow-up MRI. In another case, the patient had been followed up for more than 10 y after surgery for breast cancer. Multiple metastases in the lymph nodes (LNs), liver, and bone were detected on CT and [ 18 F]FDG PET in this patient. Among the patients with LN metastasis (n 5 12), the histopathology of the LN was confirmed via surgical resection or biopsy in 6 patients and was based on comprehensive interpretation by a nuclear medicine specialist who reported a high possibility of metastasis in the other 6 patients.
The study protocol was approved by the Institutional Review Board of the Osaka University Hospital (approval 21472-4), and the study was performed in accordance with the ethical standards laid down in the 1964 Declaration of Helsinki. Written informed consent was obtained from all patients before their inclusion in the study.

Synthesis of [ 18 F]FAPI-74 and [ 18 F]FDG
[ 18 F]FAPI-74 solution was synthesized using CFN-MPS200 (Sumitomo Heavy Industries), according to a previous study (6). [ 18 F]fluoride eluted with 0.5 M sodium acetate buffer and precursor solution (a mixture of dimethyl sulfoxide, 10 mM aluminum chloride, 20% w/v ascorbic acid, and 4 mM FAPI-74 precursor) was mixed and fluorinated for 5 min at room temperature, followed by 15  PET/CT Scanning PET/CT scanning was performed 60 min after the intravenous injection of [ 18 F]FAPI-74 (240 6 31 MBq). The patients were monitored for adverse events from the time of administration to the end of the PET scan. PET/CT images were acquired using a Biograph Vision 600 (Siemens Healthineers) in continuous bed motion (matrix, 440 3 440; pixel size, 1.65 mm) at 3.5 mm/s for 4 frames. The PET images were reconstructed to a slice thickness of 3 mm with an increment of 3 mm using ordered-subset expectation maximization with 3 iterations per 5 subsets and was gauss-filtered to a transaxial resolution of 3 mm in full width at half maximum. Attenuation was corrected using unenhanced low-dose CT data (effective dose, 50 mAs). The CT scans were reconstructed to a slice thickness of 3 mm in increments of 3 mm.   Image Analysis SUV measurements were performed via volume-of-interest analysis of the [ 18 F]FAPI-74 PET images using Syngovia software (Siemens Healthineers). SUV max and the tumor-to-normal background (T/N) ratio were compared between the primary or local recurrent lesions of malignant tumors (n 5 21) and nonmalignant lesions (n 5 8). Volumes of interest of normal background were placed on the normal areas surrounding each tumor lesion where physiologic accumulation did not overlap. We defined nonmalignant lesions as postoperative/therapeutic changes and benign/low-malignancy tumors, such as thymoma (type B1), granuloma, and solitary fibrous tumor (intermediate lesion with low malignancy potential). The SUV max of the primary lesions, LN metastases, and other metastases, as well as the number of detected metastatic lesions on [ 18

Statistical Analyses
Comparisons between the 2 groups were performed using the Wilcoxon signed-rank test for paired data or Mann-Whitney U test for unpaired data by SPSS (version 25.0; IBM Corp.). The differences were considered statistically significant at a P value of less than 0.05.

RESULTS
All patients were monitored for adverse or serious adverse events due to the [ 18 F]FAPI-74 injection. The symptoms were monitored after injection until the end of the PET scan, and no adverse events occurred due to its administration. No clinically significant changes were observed in heart rate, oxygen saturation, or body temperature before or after administration of the  1A and B). However, some of the benign tumors and treatment-related changes showed high uptake on [ 18 F]FAPI-74 PET, such as granuloma, postoperative changes, and radiation pneumonitis (Supplemental Table 2 (Fig. 5).

DISCUSSION
In this study, we evaluated patients with various histopathologically confirmed cancers and benign lesions using [ 18  Previous studies reported higher uptake of FAPI PET than of [ 18 F]FDG in many types of cancers, including gastric, pancreatic, ovarian, and head and neck (7)(8)(9)(10). It has been reported that   FAPI PET detected a higher number of metastatic lesions than [ 18 F]FDG PET did in patients with various types of cancers presenting with inconclusive [ 18 F]FDG PET findings (7). The number of positive lesions was significantly higher in patients with gastric, lung, liver, and nasopharyngeal cancers. FAPI PET showed excellent sensitivity for peritoneal carcinomatosis of gastric, pancreatic, and ovarian cancers (8)(9)(10). We also observed peritoneal carcinomatosis in patients with pancreatic cancer, which could not be detected on [ 18 F]FDG PET (Fig. 3). FAPI PET, including [ 18 F]FAPI-74 PET, will contribute to accurate clinical staging and lead to proper patient management.
Two studies compared the uptake between FAPI PET and [ 18 F]FDG PET in patients with histopathologically confirmed non-small cell lung cancer (11,12). Both studies reported that there was no statistically significant difference in the primary lesions in terms of SUV max , T/N ratio, or lesion detection. However, FAPI PET was significantly superior to [ 18 F]FDG PET in the detection of LN, pleural, and bone metastases. In our cohort, we enrolled mainly preoperative patients without metastatic lesions. Although primary lesion detectability was similar in larger tumors, consistent with previous reports, [ 18 F]FAPI-74 PET showed higher uptake in the ground-glass opacity lesions than did [ 18 F]FDG PET, as shown in Figure 5.
FAPI PET is reported to be superior to [ 18 F]FDG PET in detecting primary tumors, with high sensitivity in patients with breast cancer, as well as in detecting LN, hepatic, bone, and cerebral metastases because of its lower background activity (13)(14)(15)(16) FAPI PET images must be interpreted carefully in terms of specificity. There are several pitfalls of noncancer uptake in benign lesions. A previous study reported that benign uptake occurs in bone degeneration, wound healing, the endometrium, and inflammation, including pancreatitis and pneumonia (17,18). Benign tumors sometimes show high FAPI uptake in renal angiomyolipomas, thyroid adenomas, necrotizing granulomas, and splenic hemangioma (18). Therefore, it is necessary to carefully interpret the possibility of benign or inflammatory uptake on [ 18 F]FAPI-74 PET.
In this study, we observed a high uptake in postoperative changes or scars, granulomas, and radiation pneumonitis. One case study reported that FAPI PET showed high uptake in tuberculous granuloma (19). Although there have been no systematic reports of FAP expression in granulomas, granulomatous lesions may show high FAP expression. Regarding radiation pneumonitis, Qin et al. reported increased FAP expression and FAPI uptake in a rat model of radiation-induced lung damage (20). Further, R€ ohrich et al. reported increased uptake in fibrotic lung disease (21). In our study, there were no residual tumors among the high-uptake lesions on FAPI PET in a patient with lung cancer after chemoradiation therapy. It has been suggested that FAP expression is associated with radiation-induced fibrotic changes. Therefore, caution is required when using [ 18 F]FAPI-74 PET to assess the therapeutic efficacy in lung cancer after radiation therapy.
Compared with [ 68 Ga]-labeled FAPI PET probes, [ 18 F]FAPI-74 might have the advantage of the shorter positron range of 18 F (mean range in water, 0.6 mm) than of 68 Ga (3.5 mm), thereby yielding a better image resolution, especially in small lesions (22). However, hepatobiliary excretion might affect the detectability of biliary tract cancer on [ 18 F]FAPI-74 PET.
This study had some limitations. First, the number of patients was small, which should be finalized in the final report with a certain number of patients for each cancer type. Second, we performed [ 18 F]FAPI-74 PET after [ 18 F]FDG PET with a median interval of 18 d, and some [ 18 F]FDG PET images were acquired using different PET scanners; thus, there may have been some resulting bias in the comparison. Third, we did not perform a comparison between immunohistochemical staining and FAP. Such a comparison should be included in future studies to confirm the specific uptake of [ 18 F]FAPI-74 on PET and elucidate the FAP pathology in cancerassociated fibroblasts in greater detail.

CONCLUSION
[ 18 F]FAPI-74 PET showed higher uptake and detection rates in primary and metastatic lesions than did [ 18 F]FDG PET. [ 18 F]FAPI-74 PET is a promising novel diagnostic modality for various tumors, especially for precise staging before treatment, including characterization of tumor lesions before surgery. Moreover, 18 F-labeled FAPI ligand might serve a higher demand in clinical care in the future.

DISCLOSURE
This study was funded by the QiSS program of OPERA (grant JPMJOP1721) from the Japan Science and Technology Agency. Frederik Giesel is an advisor at ABX, Telix, SOFIE Biosciences, and a-Fusion and has shares in a consultancy group for iTheranostics. No other potential conflict of interest relevant to this article was reported.