68Ga-DOTA-FAPI-04 PET/CT in Erdheim-Chester Disease

Recent Clinical Implications of FAPI: Imaging and Therapy

ABSTRACT

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

Potential utility of [68Ga]DOTA-FAPI-04 as a broad-spectrum benign disease imaging agent-comparison with [18F]FDG and [99mTc]MDP

OBJECTIVES

To compare the detection performance of [68Ga]DOTA-FAPI-04 positron emission tomography/computed tomography (PET/CT) and [18F]FDG PET/CT for patients with various benign diseases and that of [68Ga]DOTA-FAPI-04 PET/CT and [99mTc]MDP single-photon emission computed tomography/computed tomography (SPECT/CT) for patients with benign bone lesions.

METHODS

This prospective study included 89 patients who underwent [18F]FDG and [68Ga]DOTA-FAPI-04 PET/CT and 22 patients who underwent [99mTc]MDP SPECT/CT and [68Ga]DOTA-FAPI-04 PET/CT. Detection performance of [68Ga]DOTA-FAPI-04, [18F]FDG PET/CT, and [99mTc]MDP SPECT/CT for benign lesions was compared using the T/B value, T value, and true positive rate. Paired sample t-tests were conducted for comparisons.

RESULTS

This study comprised 53 men and 58 women (mean age, 53 ± 15.75 years). [68Ga]DOTA-FAPI-04 demonstrated a higher uptake and detection rate for fibrotic disease (SUVmax [FAPI vs FDG: 6.26 ± 1.61 vs 2.38 ± 1.26], p1 < 0.001; T/B value [FAPI vs FDG: 9.38 ± 2.78 vs 1.95 ± 1.33], p2 < 0.001; and true positive rate 100% vs 26.3%), infectious disease (T/B value [FAPI vs FDG: 14.15 ± 12.42 vs 6.70 ± 5.87], p2 < 0.05; and true positive rate 94% vs 82%), and benign tumor (T/B value [FAPI vs FDG: 6.49 ± 5.85 vs 1.96 ± 2.03], p2 < 0.05; and true positive rate 70% vs 52%) than [18F]FDG. [68Ga]DOTA-FAPI-04 demonstrated a lower uptake and a comparable detection rate for benign bone disease (T/B value [FAPI vs FDG: 5.46 ± 2.91 vs 23.58 ± 15.37], p < 0.001; and true positive rate 90% vs 99%) than [99mTc]MDP.

CONCLUSION

[68Ga]DOTA-FAPI-04 could be used as a benign disease imaging agent to complement traditional nuclide imaging agents.

CLINICAL RELEVANCE STATEMENT

Our study showed that [68Ga]DOTA-FAPI-04 PET/CT could provide reliable imaging evidence for clinical detection and diagnosis of various benign lesions, such as inflammatory, infectious, fibrotic diseases, and benign tumors.

KEY POINTS

• [68Ga]DOTA-FAPI-04 demonstrated a higher uptake and detection rate for fibrotic disease, infectious disease, and benign tumor than [18F]FDG. • [68Ga]DOTA-FAPI-04 demonstrated an equivalent detection efficacy to [18F]FDG for inflammatory diseases. • [68Ga]DOTA-FAPI-04 demonstrated a lower uptake and a comparable detection rate for benign bone disease than [99mTc]MDP.

68 Ga-FAPI PET/CT Imaging : Unveiling the Hidden Pitfalls

ABSTRACT

68 Ga-FAPI has been introduced recently in many nuclear medicine centers. Many pitfalls have been discussed previously. However, not all pitfalls are currently addressed in the literature. To our knowledge, this is the first reported case to address nonspecific 68 Ga-FAPI expression at anorectal sphincter. A 69-year-old woman was diagnosed with intrahepatic cholangiocarcinoma. A 68 Ga-FAPI scan was performed after diagnostic workup and chemoradiotherapy. In addition to 68 Ga-FAPI-avid lesion at primary site, several nononcological ancillary findings have been observed. This includes unaddressed 68 Ga-FAPI-avid focus within anorectal sphincter. Moreover, the same noteworthy findings were observed in the musculoskeletal and gynecological systems.

Performance of 68Ga-Labeled Fibroblast Activation Protein Inhibitor PET/CT in Evaluation of Erdheim-Chester Disease: A Comparison with 18F-FDG PET/CT

Erdheim-Chester disease (ECD) involves multiple organs and tissues and has diverse manifestations, which makes it difficult to distinguish lesions caused by ECD from those caused by other diseases. Variable degrees of fibrosis are present in ECD. Therefore, we conducted a prospective cohort study to explore the ability of 68Ga fibroblast activation protein inhibitor (68Ga-FAPI) PET/CT to detect lesions in ECD patients. Methods: Fourteen patients diagnosed with ECD, as confirmed by histology, were included in this study. For every patient, 68Ga-FAPI PET/CT and 18F-FDG PET/CT were conducted within 1 wk. The positive rate and SUVmax of the lesions in the involved organs were compared between the examinations. Results: The most commonly involved organs were bone (100%), heart (57.1%), lung (57.1%), kidney (42.9%), and peritoneum or omentum (35.7%); other common manifestations were intracranial infiltration (50%) and cutaneous infiltration (35.7%). 68Ga-FAPI PET/CT detected 64 of 67 lesions in 14 patients, whereas 18F-FDG PET/CT detected 51 of 67 lesions (P = 0.004). The SUVmax for 68Ga-FAPI PET/CT was significantly higher than the SUVmax for 18F-FDG PET/CT of the heart (4.9 ± 2.4 vs. 2.8 ± 1.2, respectively; P = 0.050), lung or pleura (6.8 ± 4.9 vs. 3.1 ± 1.3, respectively; P = 0.025), peritoneum or omentum (5.7 ± 3.6 vs. 2.8 ± 1.7, respectively; P = 0.032), and kidney or perinephric infiltration (4.9 ± 1.2 vs. 2.9 ± 1.1, respectively; P = 0.009). Conclusion: The detectivity of 68Ga-FAPI PET/CT is superior to that of 18F-FDG PET/CT. Moreover, 68Ga-FAPI PET/CT has a better image contrast and higher SUVmax for lesions in multiple organs including the heart, lungs, peritoneum, and kidneys. 68Ga-FAPI PET/CT is a promising tool to assess pathologic features and disease extent in ECD patients.

FAP expression in alpha cells of Langherhans insulae-implications for FAPI radiopharmaceuticals' use

PURPOSE

Radiopharmaceuticals targeting fibroblast activation protein (FAP) alpha are increasingly studied for diagnostic and therapeutic applications. We discovered FAP expression at immunohistochemistry (IHC) in the alpha cells of the Langerhans insulae of few patients. Therefore, we planned an investigation aimed at describing FAP expression in the pancreas and discussing the implications for radioligand applications.

METHODS

We retrospectively included 40 patients from 2 institutions (20 pts each) according to the following inclusion/exclusion criteria: (i) pathology proven pancreatic ductal adenocarcinoma and neuroendocrine tumors (NET), 10 pts per each group at each center; (ii) and availability of paraffin-embedded tissue; and (iii) clinical-pathological records. We performed IHC analysis and applied a semiquantitative visual scoring system (0, negative staining; 1, present in less than 30%; 2, present in more than 30% of the area). FAP expression was assessed according to histology-NET (n = 20) vs ductal adenocarcinoma (n = 20)-and to previous treatments within the adenocarcinoma group. The local ethics committee approved the study (No. INT 21/16, 28 January 2016).

RESULTS

The population consisted of 24 males and 16 females, with a median age of 68 and a range of 14-84 years; 8/20 adenocarcinoma patients received chemotherapy. In all the Langerhans insulae (40/40), pancreatic alpha cells were found to express FAP, with a score of 2. No difference was found among NET (20/20) and adenocarcinoma (20/20), nor according to neoadjuvant chemotherapy in the adenocarcinoma cohort (received or not received).

CONCLUSION

Pancreatic Langerhans islet alpha cells normally express FAP. This is not expected to influence the diagnostic accuracy of FAP-targeting tracers. In the therapeutic setting, our results suggest the need to better elucidate FAPI radioligands' effects on the Langerhans insulae function.

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.

Positron emission tomography and computed tomography with [68Ga]Ga-fibroblast activation protein inhibitors improves tumor detection and staging in patients with pancreatic cancer

PURPOSE

This study aimed to investigate the diagnostic performance of [68Ga]Ga-FAPI PET/CT for primary and metastatic pancreatic carcinoma lesions and compare the results with those of [18F]-fluorodeoxyglucose ([18F]FDG) PET/CT.

METHODS

Patients with suspected or diagnosed pancreatic malignancy, who underwent contemporaneous [18F]FDG and [68Ga]Ga-FAPI PET/CT between June 2020 and January 2021, were retrospectively analyzed. Routine contrast-enhanced CT (CE-CT) is performed in all patients as standardized care. Findings were confirmed by histopathology or radiographic follow-up. We compared radiotracer uptake, diagnostic performance, and TNM (tumor-node-metastasis) classifications.

RESULTS

We evaluated 36 participants (25/36 men; median age, 60 years), including 26 patients with pancreatic malignancies and ten patients with pancreatic benign lesions. [68Ga]Ga-FAPI PET/CT showed higher radiotracer uptake and higher sensitivity than [18F]FDG PET/CT in evaluating primary tumors (SUVmax, 21.4 vs. 4.8; sensitivity, 100% vs. 73.1%), involved lymph nodes (SUVmax, 8.6 vs. 2.7; sensitivity, 81.8% vs. 59.1%), and metastases (SUVmax, 7.9 vs. 3.5; sensitivity, 91.5% vs. 44.0%); Compared with [18F]FDG, [68Ga]Ga-FAPI PET/CT upstaged six patients' TNM staging (6/23, 26.1%) and changed two patients' clinical management (2/23, 8.7%). Compared with CE-CT, [68Ga]Ga-FAPI PET/CT upgraded TNM staging in five patients (5/23, 21.7%) and changed the therapeutic regimen in only one patient (1/23, 4.3%). Intense [68Ga]Ga-FAPI uptake was observed throughout the pancreas in 12/26 pancreatic malignancies; dual-time point [68Ga]Ga-FAPI PET/CT may differentiate pancreatitis from malignancy.

CONCLUSIONS

Compared with [18F]FDG PET/CT, [68Ga]Ga-FAPI PET/CT shows higher sensitivity in detecting primary pancreatic tumors, involved lymph nodes, and metastases and is superior in terms of TNM staging. Prospective trials with larger patient population are needed to evaluate whether [68Ga]Ga-FAPI PET/CT could elicit treatment modification in pancreatic cancer when compared with standard of care imaging.

Portal Biliopathy and Cavernous Transformation of the Portal Vein Revealed by 68Ga-FAPI PET/CT

ABSTRACT

A 47-year-old man presented with right upper abdominal pain for 1 month. Contrast-enhanced CT revealed hilar bile duct stenosis with dilatation of the intrahepatic bile ducts, and his serum CA19-9 and CA242 levels were significantly elevated. 18F-FDG and 68Ga-FAPI PET/CT were performed for differential diagnosis. 18F-FDG PET/CT showed only mild FDG uptake in the hepatic hilum. Astonishingly, in 68Ga-FAPI PET/CT, intense radioactivity was presented on the same region, which indicated massive fibroblasts aggregation in hepatic hilum. The patient was finally diagnosed as portal biliopathy caused by cavernous transformation of the portal vein.

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.

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.