Fibroblast activation protein-based theranostics in cancer research: A state-of-the-art review

Abstract

In recent years, quinoline-based fibroblast activation protein (FAP) inhibitors (FAPI) have shown promising results in the diagnosis of cancer and several other diseases, making them the hotspot of much productive research. This review summarizes the literature for the state-of-the-art FAPI-PET imaging for cancer diagnosis compared with fluorodeoxyglucose (FDG)-PET. We also summarize the use of FAPI-PET for therapeutic regimen improvement and fibroblast activation protein (FAP)-targeted molecule modification strategies, as well as preliminary clinical studies regarding FAP-targeted radionuclide therapy. Our qualitative summary of the literature to date can inform future research directions, medical guidelines, and optimal clinical decision-making.

Introduction

Approximately 19.3 million new cancer diagnoses and approximately 10.0 million cancer deaths occurred worldwide in 2020, according to estimates from Global Cancer Statistics ¹. Cancers develop in complex environments composed of tumor cells and the surrounding stroma. The “seed and soil” theory emphasized the interactive importance of both components as early as 1889 ². However, diagnostic and therapeutic paradigms have predominantly targeted only tumor cells.

The widespread application of immunotherapy in clinical trials has led to increased research attention being paid to the tumor microenvironment (TME). The tumor stroma or TME comprises all the noncancer components in the tumor tissue, including cancer-associated fibroblasts (CAFs), the extracellular matrix (ECM), various types of immune cells, and intertwined blood vessels. The TME can develop an immunosuppressive niche in which tumor cells are protected from conventional therapies, resulting in treatment failure ³.

CAFs are among the most abundant components of the TME in solid tumors ⁴. However, CAFs are heterogeneous cells with both tumor-promoting and tumor-suppressive effects observed in different situations ⁵. CAFs model and remodel the ECM structure, which can become a physical barrier against the infiltration of immunocytes, which have a killer function, or a structural scaffold for intercellular interaction between tumor cells and non-tumor cells in the TME, thereby regulating tumor initiation, neovascularization, and metastasis ⁶. On the other hand, CAFs can secrete multiple chemokines and cytokines, such as transforming growth factor-β (TGFβ), CC-chemokine ligand 2 (CCL2), and interleukin-6 (IL-6), in order to recruit immunocytes with inhibitory functions in the tumor stroma, thereby facilitating immune evasion ⁷.

CAFs have several biological markers, including α-smooth muscle actin, fibroblast activation protein (FAP), and platelet-derived growth factor receptor-β (PDGFRβ). FAP is a type II integral membrane glycoprotein belonging to the serine protease family involved in ECM remodeling and fibrogenesis ⁸. In colorectal cancers, FAP expression is higher at the invasive front than in the tumor center ⁹.

Although the prognostic value of FAP in cancers has been inconsistent throughout the literature, high expression of FAP has been shown to be an independent poor prognostic marker for outcomes in lung cancer, hepatocellular carcinoma, and colon cancer in studies with large sample sizes (n = 138-449 patients) ^10-12. In a murine model, tumor growth could be potentiated by the constitutive expression of FAP, which could, in turn, be meaningfully attenuated by anti-FAP antibodies ¹³. However, sibrotuzumab (a humanized version of the murine anti-FAP antibody) failed as a treatment regimen in an early phase II trial for metastatic colorectal cancer ¹⁴. Although FAP antibodies have shown limited response in tumor therapy, small molecules targeting FAP have attracted increasing attention in the area of tumor theranostics.

Targeting FAP for tumor imaging

FAP, expressed at low levels in healthy tissues, is involved in various pathological conditions and is detected in over 90% of malignant epithelial tumors ^{8, 15}. Thus, molecular imaging (including positron emission tomography [PET] and single-photon emission computed tomography [SPECT]) targeting FAP is a promising diagnostic imaging modality.

A series of quinoline-based FAP inhibitors (FAPIs) was developed by the University Hospital Heidelberg group based on clinical and preclinical research ^16-18. The first FAPI variants (FAPI-01 and FAPI-02) were reported in 2018, demonstrating that FAPI-02 has an improved binding affinity to human FAP compared with FAPI-01 ¹⁶. Impressively, ⁶⁸Ga-FAPI-02 PET images showed favorable tracer uptake in tumor tissues and low background uptake in normal healthy organs, resulting in high-contrast images in tumor-bearing murine models and three cancer patients ¹⁶. Subsequently, FAPI-04 was identified as the best tracer from among 15 modified FAPIs for PET imaging applications, showing a higher tumor uptake in murine xenograft models than FAPI-02 ¹⁷. Notably, ⁶⁸Ga-FAPI-04 was evaluated in a cohort of 80 patients presenting with 28 types of tumors (54 primary tumors and 229 metastases). It showed overall intense tracer uptake (with a maximum standard unit value [SUVmax] > 6) and high-contrast images in various highly prevalent cancers, including sarcomas, cholangiocarcinoma, and esophageal, breast, lung, hepatocellular, colorectal, head-neck, ovarian, pancreatic, and prostate cancers ¹⁹. To improve the pharmacokinetics of this PET tracer, FAPI-46 was discovered from 15 other FAPI derivatives, and it demonstrated an enhanced tumor-to-background ratio (TBR) compared with FAPI-04 ¹⁸. Interestingly, ⁶⁸Ga-FAPI-46 PET/CT imaging acquisition at an early time point (10 min p.i.) had an equivalent lesion detection rate compared with a late time point (60 min p.i.) in a previous study ²⁰.

Since the dodecane tetraacetic acid (DOTA) ligand was used as a chelator, FAPI-04/46 could also be radiolabeled with therapeutic nuclides such as ⁹⁰Y, ¹⁷⁷Lu, and ²²⁵Ac. In addition, the NOTA ligand was used in FAPI-74 for labeling with both ¹⁸F and ⁶⁸Ga, showing favorable image contrasts in PET/CT imaging in various cancers ²¹. Similarly, the chelator bis((1-(2-(tert-butoxy)-2-oxoethyl)1H-imidazol-2-yl) methyl) glycine was applied to [^99mTc]Tc-Labeled FAPI tracers, and [^99mTc]Tc-FAPI-34 SPECT was performed on two patients with ovarian and pancreatic cancers ²².

It should be noted that increased FAPI uptake has been reported in many non-oncological conditions, including inflammatory lesions, fibrotic disease, trauma, arthritis, degenerative bone disease, immunoglobulin 4 [IgG-4] related diseases, connective tissue disease, and atherosclerosis ^{23, 24}. Thus, imaging interpretation with ⁶⁸Ga/¹⁸F-FAPI PET/CT should be interpreted with caution to avoid misdiagnosis. Interestingly, increased FAPI uptake in non-oncological conditions could open possibilities for broader use of FAPI for corresponding diseases. For example, FAP-specific PET/CT could be used in the discrimination between inflammatory and fibrotic activity in IgG-4 related diseases ²⁵, evaluation of the progression in atherosclerotic plaques ²⁶, and assessment of the disease activity in fibrotic interstitial lung diseases ^{27, 28}.

Comparing ⁶⁸Ga-FAPI and ¹⁸F-FDG uptake in various types of cancer

As FAPI is a novel PET tracer in cancer imaging, it is critical to evaluate lesion detection rates and diagnostic efficacy for FAPI compared with ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG), the dominant tracer in oncology. To the best of our knowledge, Chen et al. conducted the first head-to-head study comparing ⁶⁸Ga-FAPI-04 and ¹⁸F-FDG PET/CT in a cohort of 75 patients (54 patients identified at initial assessment and 21 patients with recurrence detection) with 12 different tumor entities. This prospective study demonstrated that ⁶⁸Ga-FAPI-04 had a higher sensitivity as compared with ¹⁸F-FDG in identifying primary tumors (98.2% vs. 82.1%, P = 0.021), lymph node metastases (86.4% vs. 45.5%, P = 0.004), and bone and visceral metastases (83.8% vs. 59.5%, P = 0.004) ²⁹. However, the limited number of patients harboring each cancer type enrolled in this study did not allow for a subgroup comparison in terms of diagnostic efficacy for the same tumor type. Representative MIP images of both ¹⁸-FDG PET/CT and ⁶⁸Ga-FAPI PET/CT in 8 patients with different types of cancer are shown in Figure 1. Some studies have recently compared the diagnostic efficacy of ⁶⁸Ga-FAPI and 18F-FDG in various types of tumors.

Figure 1.

Representative comparison of 8 patients with different tumor entities undergoing both ¹⁸F-FDG PET and ⁶⁸Ga-FAPI-04 PET imaging within less than 1 week. Solid arrows indicate primary tumors, while the dotted arrows indicate metastasis lesions. NPC: nasopharyngeal carcinoma.

Head and neck cancer

⁶⁸Ga-FAPI-04 PET/CT demonstrated improved sensitivity when compared with ¹⁸F-FDG PET/CT for cancers of unknown primary origin (CUP), while the sensitivity for detecting primary tumors was comparable in nasopharyngeal carcinoma (NPC), oral squamous cell carcinoma, and Waldeyer's tonsillar ring cancer ^30-34. Impressively, ⁶⁸Ga-FAPI-04 PET/CT pinpointed 39% (7/18) of primary head and neck CUP tumors among primary patients with negative ¹⁸F-FDG findings ³⁰. In another cohort of 45 patients with NPC, ⁶⁸Ga-FAPI-04 PET/CT showed higher radiotracer uptake than ¹⁸F-FDG for primary tumors, regional lymph nodes, and distant metastases, resulting in higher sensitivity for the detection of lymph nodes and distant metastases ³¹. Interestingly, Qin et al. reported that ⁶⁸Ga-FAPI-04 PET/CT detected a smaller number of positive lymph nodes compared with ¹⁸F-FDG PET/CT (the detected number of positive lymph nodes was 48 vs. 100) ³². However, no suspicious FDG-avid lymph nodes were confirmed histologically. This study indicated that ⁶⁸Ga-FAPI PET/CT might be more specific than ¹⁸F-FDG for differentiating reactive lymph nodes from tumor metastatic lymph nodes ³⁵, and it is very likely that the FDG-positive/FAPI-negative lymph nodes are reactive lymph nodes. However, this finding requires validation in future research.

Both studies mentioned above demonstrated that ⁶⁸Ga-FAPI outperformed ¹⁸F-FDG PET/CT in evaluating skull base and intracranial invasion in NPC ^{31, 32}. In a study of 10 patients with oral squamous cell carcinoma (OSCC), both ⁶⁸Ga-FAPI-04 PET/CT and ¹⁸F-FDG PET/CT had comparable sensitivity and specificity for detecting primary tumors (100% vs. 100%) and cervical lymph node metastases (81.3% vs. 87.5%, P = 0.32; 93.3% vs. 81.3%, P = 0.16) ³³. However, both radiotracers have relatively high physiological uptakes in the oral mucosa, potentially leading to a compromised target-to-blood pool ratio (TBR) ³³. For cancers of Waldeyer's tonsillar ring, ⁶⁸Ga-FAPI-04 PET/CT showed a higher detection rate in primary tumors but an inferior detection rate in lymph node metastases compared with ¹⁸F-FDG PET/CT. However, ⁶⁸Ga-FAPI-04 PET/CT demonstrated a higher TBR than ¹⁸F-FDG PET/CT for detecting primary tumors (10.90 vs. 4.11) ³⁴.

Digestive system cancer

⁶⁸Ga-FAPI-04 PET/CT demonstrated improved sensitivity in liver, gastric, and pancreatic cancers when compared with ¹⁸F-FDG PET/CT ^36-45, while the sensitivities of both tracers were comparable in colorectal and esophageal cancers ^{39, 46}. For liver cancer, including hepatocellular carcinoma and intrahepatic cholangiocarcinoma, ⁶⁸Ga-FAPI-04 has been demonstrated to have a higher sensitivity in detecting primary liver tumors (partly attributed to higher tumor uptake and lower hepatic background uptake as compared with ¹⁸F-FDG) as well as extrahepatic metastases ^36-38. In gastric cancer, the sensitivity of ⁶⁸Ga-FAPI-04 PET/CT was higher than that of ¹⁸F-FDG PET/CT in detecting primary tumors, lymph nodes, and distant metastases ^{39-42, 45}, especially for signet-ring cell carcinoma and mucinous adenocarcinoma.

In gastric cancer, Pang et al. and Qin et al. reported that the FAPI-derived SUVmax in primary and metastatic lesions was significantly higher than the FDG-derived SUVmax ^{39, 40}, while Jiang et al. and Kuten et al. reported that FAPI-derived TBR was higher than FDG-derived TBR without accompanying differences in the SUVmax ^{41, 42}. In colorectal cancer, ⁶⁸Ga-FAPI-04 PET/CT has been demonstrated to have equal sensitivity in primary tumor detection compared to ¹⁸F-FDG PET/CT (6/6, 100%), although the FAPI-derived SUVmax was statistically significantly higher than the FDG-derived SUVmax in a previous study ³⁹.

Although ⁶⁸Ga-FAPI-04 has significantly higher uptake compared with ¹⁸F-FDG in esophageal cancer, their sensitivity in detecting primary tumors was shown to be comparable ⁴⁶. Pancreatic tumors are characterized by intense stromal desmoplastic reactions surrounding cancer cells, and CAFs are the main effector cells involved in this desmoplastic reaction. As expected, ⁶⁸Ga-FAPI-04 PET/CT shows higher sensitivity in detecting primary tumors, including lymph nodes, and metastases than ¹⁸F-FDG PET/CT in pancreatic cancer, mainly due to the intense ⁶⁸Ga-FAPI uptake in pancreatic tumor lesions ⁴³. However, non-specific ⁶⁸Ga-FAPI uptake in tumor-induced pancreatitis should be noted, as intense ⁶⁸Ga-FAPI uptake is normally observed throughout the whole pancreas in this disease. This phenomenon is frequently observed in tumors located in the head of the pancreas. As a solution, dual-time point ⁶⁸Ga-FAPI PET/CT (1 h early-point and 3 h late-point scans) may help differentiate pancreatitis from malignancy ^{43, 44}.

Breast cancer

⁶⁸Ga-FAPI-04 PET/CT was demonstrated to detect a greater number of cancerous lesions with a higher SUVmax for primary tumors, including lymph nodes, and distant metastases compared with ¹⁸F-FDG PET/CT in a cohort of 48 breast cancer patients ⁴⁷. Kömek et al. reported a similar conclusion in a cohort of 20 patients ⁴⁸.

Lung Adenocarcinoma

Both ¹⁸F-FAPI-42 and ¹⁸F-FDG had comparable detection rates (100%) for primary tumors in a cohort of 34 patients ⁴⁹. Moreover, ¹⁸F-FAPI-42 showed higher SUVmax compared to ¹⁸F-FDG in the lymph nodes, pleura, bones, and other tissue lesions (P < 0.05), as well as better TBR compared to ¹⁸F-FDG in brain lesions (9.53 ± 12.07 vs. 1.01 ± 0.49, P < 0.0001). Therefore, ¹⁸F-FAPI-42 may have advantages over ¹⁸F-FDG for the primary staging of lung adenocarcinoma. However, ¹⁸F-FAPI-42 is inferior in detecting brain lesions compared to contrast-enhanced magnetic resonance imaging (CE-MRI) (56 vs. 34, P = 0.002). Interestingly, one recent study demonstrated intense ⁶⁸Ga-FAPI activity but minimal ¹⁸F-FDG uptake in a malignant pulmonary ground-glass opacity (GGO) nodule ⁵⁰. Further studies are needed to determine the effectiveness of ⁶⁸Ga-FAPI and ¹⁸F-FDG PET/CT for characterizing GGO nodules.

Sarcomas

The detection rates for primary lesions observed with ⁶⁸Ga-FAPI-46 PET/CT and ¹⁸F-FDG PET/CT were equally high (33/43 vs. 35/43) for bone or soft tissue sarcomas. Moreover, ⁶⁸Ga-FAPI-46 PET/CT allowed for detecting additional distant metastatic lesions that were not detected via ¹⁸F-FDG PET/CT in 6/43 patients ⁵¹.

Hematological neoplasms

Head-to-head comparison studies of hematological neoplasms are relatively rare compared to studies evaluating solid tumors. Lan et al. reported that ⁶⁸Ga-FAPI-04 PET/CT demonstrated inferior sensitivity (50.65 vs. 96.75%) and accuracy (51.28 vs. 95.51%, P < 0.001) than ¹⁸F-FDG in a subgroup of eight patients with hematological neoplasms, including multiple myeloma and lymphoma ⁵².

The primary lesion detection rates for ⁶⁸Ga-FAPI and ¹⁸F-FDG PET/CT (based on published data) in various types of cancer are summarized in Figure 2. ⁶⁸Ga-FAPI PET/CT outperformed ¹⁸F-FDG PET/CT in CUP, breast cancer, hepatocellular carcinoma, intrahepatic cholangiocarcinoma, gastric cancer, and pancreatic cancer. In these diseases, ⁶⁸Ga-FAPI PET/CT may have the potential to replace ¹⁸F-FDG in the future. Conversely, ⁶⁸Ga-FAPI PET/CT was inferior to ¹⁸F-FDG PET/CT for diagnosing hematological neoplasms. However, it should be noted that Figure 2 is not based on a systematic literature search of PubMed/MEDLINE and Cochrane Library database analysis, and the number of patients enrolled in previous studies was very small. It should also be noted that the results of Figure 2 only aimed at comparing the detection rate of primary tumors between ¹⁸F-FDG and FAPI PET/CT based on existing investigations, including retrospective or smaller prospective studies. For more information, a systematic review was conducted by Treglia et al. to compare radiolabeled FAPI and ¹⁸F-FDG PET/CT in oncologic imaging ⁵³. Clear comparability, especially superiority or inferiority, between FAPI vs. FDG should be demonstrated in future studies.

Figure 2.

Published studies comparing fibroblast activation protein inhibitor-positron emission tomography (FAPI) vs. fluorodeoxyglucose positron emission tomography (FDG) in the diagnosis of various types of cancer (detection rate of primary tumors). The corresponding references are presented in the figure.

Overall, we conclude that ⁶⁸Ga-FAPI PET/CT has comparable or improved diagnostic performance in imaging various cancers compared to 18F-FDG PET/CT, particularly in cancer types that normally show a low-to-moderate ¹⁸F-FDG uptake ^{54, 55}. Lower background uptake in normal organs leads to higher TBRs with ⁶⁸Ga-FAPI when compared with ¹⁸F-FDG ⁵⁶. As stroma components can comprise up to 50-90% of the tumor environment, stroma-targeted PET/CT imaging could be more sensitive than glucose metabolism-based PET imaging in detecting small lesions or lesions with low glucometabolic activity ⁵⁷. Moreover, ⁶⁸Ga-FAPI PET/CT provides advantages over ¹⁸F-FDG PET/CT because it does not require fasting and imaging acquisition at early time points following injection (i.e., 10-60 min after tracer administration). However, it is too arbitrary to say FAPI PET/CT will replace FDG PET/CT based on the current clinical studies. The Role of FAPI PET in cancer imaging and management should be further explored in larger prospective trials before any conclusion should be reached.

Changes in cancer management according to ⁶⁸Ga/¹⁸F-FAPI PET/CT

In addition to studies comparing ⁶⁸Ga-FAPI and ¹⁸F-FDG PET/CT for the diagnosis of various types of cancer, several studies have explored the added value of ⁶⁸Ga-FAPI PET/CT in cancer management compared with standard care of imaging (SCI). In oral squamous cell carcinoma, ⁶⁸Ga-FAPI-04 PET/CT showed superior sensitivity (81.3% vs. 50.0%) and specificity (93.3% vs. 61.5%) in detecting lymph node metastases compared with CE-MRI ³³. Specifically, compared with CE-MRI, ⁶⁸Ga-FAPI PET/CT upgraded and underestimated the T stage in 4/39 and 2/39 patients with NPC, respectively (Figure 3A) ³¹. Zhao et al. reported that ⁶⁸Ga-FAPI-04 PET/CT improved tumor staging in patients with esophageal cancer, compared to contrast-enhanced CT and ¹⁸F-FDG PET/CT (Figure 3B) ⁵⁸. Pang et al. and Röhrich et al. reported that the use of ⁶⁸Ga-FAPI-04/46 PET/CT led to changes in oncologic management in 1/23 patients and 7/19 patients, respectively, due to the upstaging of the TNM stage as compared with contrast-enhanced computed tomography (CE-CT) ^{43, 44}. Regarding anal canal carcinoma, ⁶⁸Ga-FAPI-46 PET/CT led to a change in the TNM classification in 3/6 patients as compared with MRI and CT in a previous study (i.e., based on FAPI-positive nodes in two patients with ill-defined results on MRI and FAPI-negative lesions in one patient with suspected pulmonary lesions on CT) ⁵⁹.

Figure 3.

A. Imaging findings in a 51-year-old treatment-naive male patient with nasopharyngeal carcinoma. ¹⁸F-fluorodeoxyglucose (FDG) (upper row) and ⁶⁸Ga-labeled fibroblast activation protein inhibitor (FAPI) positron emission tomography/computed tomography (PET/CT) (lower row) reveal abnormal activity in the nasopharynx. However, intense ⁶⁸Ga-FAPI uptake is observed in the skull base (white arrow) along with normal FDG uptake, confirmed by magnetic resonance imaging. The TNM stage was upgraded from T2N2 (FDG-based) to T3N2 (FAPI-based). B. Imaging findings in a 56-year-old treatment-naive male patient with esophageal squamous cell carcinoma. ¹⁸F-FDG PET/CT for tumor staging to decide the most proper treatment strategy. Maximum intensity projection (MIP) image ¹⁸F-FDG PET/CT reveals an intense FDG-avid mass in the mid-esophagus, while the MIP image of ⁶⁸Ga-FAPI-04 shows intense uptake of FAPI in the primary tumor and paraesophageal lymph node. This FAPI-positive lymph node, suggestive of nodal metastasis, was later confirmed by histopathology. Tumor staging was upgraded to stage IIIB based on FAPI. Adapted with permission from ³¹, copyright 2021 Springer, and ⁵⁸, copyright 2020 Springer.

Optimization of TNM staging via FAPI PET/CT results in improving oncologic management. For example, the course of clinical management was changed in 13 (30%) patients with sarcomas following ⁶⁸Ga-FAPI-PET in a previous investigation, including major changes (e.g., changes in therapeutic strategy) in seven (16%) patients ⁵¹. However, ⁶⁸Ga-FAPI PET/CT imaging is not always superior to conventional SCI. For example, ⁶⁸Ga-FAPI-04 PET/CT detected fewer lesions as compared with MRI of the liver (85% [41/ 48] vs. 100% [48/48], P = 0.34) in a cohort of 32 patients with liver cancer ³⁶. Therefore, it is recommended that ⁶⁸Ga-FAPI PET/CT be used as a tool complementary to ¹⁸F-FDG PET/CT and SCI. Additional investigation is required, as current studies investigating the advantages and disadvantages of ⁶⁸Ga-FAPI PET/CT are highly limited.

In addition to changes in TNM classification, a clear contour with a favorable TBR improves target volume delineation for radiotherapy. Syed et al. first introduced ⁶⁸Ga-FAPI PET/CT images in tumor radiotherapy for evaluating gross tumor volume (GTV) contouring ⁶⁰. Four thresholds (three-, five-, seven- and ten times that of the ⁶⁸Ga-FAPI uptake in normal tissue surrounding the tumor) were used to generate FAPI-GTV values (FAPI × 3, 5, 7 and 10) in 14 patients with head and neck cancers, which were subsequently compared with GTV values measured conventionally via CE-CT and MRI (CT-GTV). The area covered by FAPI-GTV (FAPI × 3 and 5) was significantly different compared to the area covered by CT-GTV, and the FAPI × 3 threshold was recommended as the best among the imaging modalities ⁶⁰. Similarly, ¹⁸F-FAPI-74 × 3 and ⁶⁸Ga-FAPI-04 × 2 thresholds were considered optimal with respect to radiotherapy planning for lung cancer and locally recurrent pancreatic cancer ^{21, 61}. Windisch et al. compared FAPI-GTV and MRI-GTV values in glioblastoma and found that the FAPI-GTV values for all thresholds were greater than the MRI-GTV values ⁶². In addition to using background FAPI uptake, Zhao et al. used demarcations at 20%, 30%, and 40% of the SUVmax as thresholds for FAPI-GTV; their results demonstrated that ⁶⁸Ga-FAPI-04 × 40% of the SUVmax was ideal for reflecting the actual tumor volumes in esophageal cancer (Figure 4) ⁴⁶. Interestingly, PET/CT with different FAPI variants (⁶⁸Ga-FAPI-02, ⁶⁸Ga-FAPI-46, and ⁶⁸Ga-FAPI-74) acquired at three time points (10 min, 1 h, and 3 h) with a threshold of 25-35% of the SUVmax was used to delineate three FAPI-GTVs in adenoid cystic carcinomas, all of which were more accurate than CT-GTVs (based on CE-CT and CE-MRI); the ⁶⁸Ga-FAPI PET/CT images acquired 1 h post-injection were presumed to reflect the ideal time point for contouring ⁶³. Without implementing the FAPI threshold mentioned above, Koerber et al. and Ristau et al. reported that ⁶⁸Ga-FAPI-04/46 PET/CT improved target volume delineation in 6/6 anal canal carcinoma patients and 6/7 esophageal cancer patients ^{59, 64}.

Figure 4.

Radiation treatment plan for a 57-year-old male patient with lower esophageal cancer based on (A) contrast-enhanced CT (tumor length, 4 cm; GTV volume, 39.32 cm³); and (B) CT + FAPI ×20% (tumor length, 7.5 cm; GTV volume, 41.73 cm³). Adapted with permission from ⁴⁶, copyright 2021 Elsevier.

The published data on the impact of FAPI PET/CT on radiotherapy are shown in Table 1. However, differing FAPI variants and thresholds for various types of tumors may be obstacles to the widespread use of FAPI PET/CT in GTV contouring. A well-designed prospective study with a large patient population is warranted to evaluate the overall survival benefit from FAPI-derived GTVs compared with GTVs derived from SCI in a heterogeneous grouping of cancers and imaging modalities.

Table 1.

Studies summarizing the impact of fibroblast activation protein inhibitor positron emission tomography/computed tomography (FAPI PET/CT) on the efficacy of radiotherapy.

Study	Patients No.	Tumor type	FAPI variants	Compared imaging modalities	Background	FAPI thresholds	Results / optimal threshold
Windisch et al. 62	12	Glioblastoma	68Ga-FAPI-02 and 68Ga-FAPI-04	CE-MRI	Healthy appearing contralateral brain parenchyma	FAPI × 5, 7, and 10	FAP × 5, × 7 and × 10 increase the MRI-GTV (statistical significance
Syed et al. 60	14	Head and neck cancers	68Ga-FAPI	CE-CT and MRI	Healthy appearing surrounding tissue	FAPI × 3, 5, 7, and 10	FAPI × 3 (about 20-25% SUVmax)
Röhrich et al. 63	12	Adenoid cystic carcinomas	68Ga-FAPI-02, 68Ga-FAPI-46, and 68Ga-FAPI-74	CE-CT and CE-MRI	NA	25-35% of SUVmax at three time points (10 min, 1 h, and 3 h)	The FAPI images acquired 1 h p.i. were considered ideal for contouring
Ristau et al. 64	7	Esophageal cancer	68Ga-FAPI-04 and 68Ga-FAPI-46	Standard CT	Not mentioned	Not mentioned	FAPI PET/CT imaging improved target volume delineation in 6/7 patients
Zhao et al. 46	21	Esophageal cancer	68Ga-FAPI-04	CE-CT	NA	FAPI × 20%, 30%, and 40% SUVmax	FAPI × 20% SUVmax
Giesel et al. 21	10	Lung cancer	18F-FAPI-74 and 68Ga-FAPI-74	CE-CT	Blood-pool	FAPI × 1.5, 2, 2.5, and 3	FAPI × 3 (about 40-50% SUVmax)
Koerber et al. 59	6	Treatment-naïve carcinoma of the anal canal	68Ga-FAPI-04 and 68Ga-FAPI-46	MRI	Not mentioned	Not mentioned	Modified dose concepts in two patients, improved target volume delineation in six patients
Liermann et al. 61	7	Locally recurrent pancreatic cancer	68Ga-FAPI-04	CE-CT	Healthy appearing surrounding tissue	FAPI × 1.5, 2, and 2.5	FAPI × 2

CE: contrast-enhanced; CT: computed tomography; MRI: magnetic resonance imaging; NA: not applicable

Improvement in the FAPI probe and FAP-targeted radionuclide therapy

As a pan-cancer target with an excellent TBR, FAP is considered an attractive target for radionuclide therapy. FAPI variants labeled with therapeutic radionuclides (such as ¹³¹I, ⁹⁰Y, ¹⁷⁷Lu, and ²²⁵Ac) have been assessed in both preclinical and clinical studies. For example, Ma et al. synthesized ¹³¹I-FAPI-04 and used it to suppress tumor growth in U87MG glioma xenografts ⁶⁵. FAPI-04 labeled with ²²⁵Ac demonstrated statistically significant tumor-suppressive effects compared with the control group in a study of pancreatic cancer xenografts ⁶⁶. Similarly, ¹⁷⁷Lu-FAPI-46 and ²²⁵Ac-FAPI-46 showed tumor growth suppression in pancreatic cancer mouse models without an obvious decrease in body weight; no radionuclide therapy-related side effects were observed in these tumor xenografts ⁶⁷. However, tumor uptake was only 0.3% ID/g at 3 h p.i. and 0.1 % ID/g at 24 h p.i. for ¹⁷⁷Lu-FAPI-46 and ²²⁵Ac-FAPI-46 ⁶⁷, respectively, and these results require additional clarification.

Strategies for prolonging the blood circulation of drug molecules by adding albumin-binder moieties and harnessing the polyvalency effects of multimeric peptides are widely used to enhance the tumor uptake and retention of radiopharmaceuticals ^{68, 69}. For example, a series of albumin binder (truncated Evans blue) modified FAPI-02 related radiopharmaceuticals has been synthesized and radiolabeled with ¹⁷⁷LuCl₃ (named ¹⁷⁷Lu-EB-FAPI-B1, B2, B3, B4, Figure 5A). Improved tumor accumulation and retention of these compounds were observed until 96 h post-injection, especially for ¹⁷⁷Lu-EB-FAPI-B1. ¹⁷⁷Lu-EB-FAPI-B1 demonstrated notable tumor growth inhibitions in the U87MG tumor model with negligible side effects, indicating that ¹⁷⁷Lu-EB-FAPI-B1 is a promising theranostic agent for future clinical transformation ⁷⁰. Similarly, FAPI-04 conjugated with albumin binder (4-[p-iodophenyl] butyric acid moiety, truncated Evans blue moiety, lauric acid [C12], and palmitic acid [C16]) was developed (TEFAPI-06, TEFAPI-07, FAPI-C12, and FAPI-C16) to improve tumor retention (Figure 5B), and novel FAPI-variants showed notable tumor growth inhibition after radiolabeling with ¹⁷⁷Lu in pancreatic cancer patient-derived xenografts (PDXs) and HT-1080-FAP xenografts ^{71, 72}. Another albumin binder (Lys [4-p-chlorophenyl] butyric acid)-conjugated FAP-targeting peptide (Alb-FAPtp-01) showed higher tumor uptake as compared with FAPI-04 after radiolabelling with ⁶⁸Ga ⁷³. Multimerization has been used as another strategy to improve tumor uptake and retention. Recently, the FAPI dimer DOTA-2P(FAPI)₂ was synthesized based on the structure of FAPI-46 (Figure 5C), and it demonstrated increased tumor uptake and retention properties compared to FAPI-46 in PDXs of hepatocellular carcinoma ⁷⁴. Moreover, PET/CT scans in three cancer patients revealed higher intratumoral uptake of ⁶⁸Ga-DOTA-2P(FAPI)2 compared to ⁶⁸Ga-FAPI-46 in 21 tumor lesions (SUVmax: 8.1-39.0 vs. 1.7-24.0; P < 0.001) ⁷⁴.

Figure 5.

A. Chemical structure and each part of the functional groups of ¹⁷⁷Lu-EB-FAPI-B1 (without PEG), ¹⁷⁷Lu-EB-FAPI-B2 (with PEG: n = 1), ¹⁷⁷Lu-EB-FAPI-B3 (with PEG: n = 2) and ¹⁷⁷Lu-EB-FAPI-B4 (with PEG: n = 3) (FAP targeting motif: FAPI-02). Adapted with permission from ⁷⁰, copyright 2021 Ivyspring. B-D. (B) Chemical structure of TEFAPI-06/07 (FAP targeting motif: FAPI-04). Adapted with permission from ⁷¹, copyright 2021 Journal of Nuclear Medicine. (C) Chemical structure of DOTA-2P(FAPI)₂ (FAP targeting motif: FAPI-46). Adapted with permission from ⁷⁴, copyright 2021 Journal of Nuclear Medicine. (D) Chemical structure of DOTAGA.(SA.FAPi)₂ (FAP targeting motif: SA.FAPi) Adapted with permission from ⁸³, copyright 2021 Mary Ann Liebert.

Regarding the clinical investigation of FAPI-targeted radionuclide therapy, Lindner et al. first reported that a patient with advanced breast cancer was treated with 2.9 GBq of ⁹⁰Y-FAPI-04, resulting in a statistically significant reduction in pain medication ¹⁷. Other FAPI variants (FAPI-46 and DOTA.SA.FAPi) radiolabeled with therapeutic nuclides (¹⁵³Sm, ⁹⁰Y, and ¹⁷⁷Lu) were evaluated in several scattered case reports (Table 2) ^{75, 22, 76-78}. Subsequently, a few preliminary studies on FAP-targeted radionuclide treatment were reported.

Table 2.

Radionuclide therapy targeting fibroblast activation protein (FAP).

Study	Patients No.	Tumor type	FAPI agent	Total cycles	Treatment cycle/ patient	Median injected activity	Response (RECIST)	Treatment-related adverse events in all treatment cycles
Assadi et al. 81	18	Ovarian cancer, sarcoma, colon cancer, breast cancer, pancreatic cancer, prostate cancer, cervical cancer, round-cell tumor, lung cancer, anaplastic thyroid cancer, cholangiocarcinoma	177Lu-FAPI-46	36	1-4	3.7 GBq (1.85-13.7 GBq)	12 SD, 6 PD	1 patient suffered thrombocytopenia (G1), leukopenia (G1), and anaemia (G3) (CTCAE v4.03)
Ballal et al. 83	15	Thyroid Cancer	177Lu-DOTAGA.(SA.FAPi)2	45	2-4	8.2 GBq (5.5-14 GBq)	NA	Diarrhoea (G1 in 1 pt) (CTCAE v5.0)
Baum et al. 79	11	Pancreatic cancer, breast cancer, ovarian cancer, and rectum cancer	177Lu-FAP-2286	22	1-3	5.8 GBq (2.4-9.9 GBq)a	2 SD, 9 PD	Hemoglobin (G1 in 2 pts, G2 in 4 pts, and G3 in 1 pt), leukopenia (G2 in 1 pt, and G3 in 2 pts, non-G3 in 2 pts), thrombocytopenia (G3 in 1 pts),b pain flare-up (G3 in 1 pts) (CTCAE v5.0)
Ferdinandus et al. 80	9	Sarcoma, pancreatic cancer	90Y-FAPI-46	13	1-3	3.8 (3.25-5.40) GBq for the first cycle and 7.4 (7.3-7.5) GBq for any subsequent cycle	4 SD, 4 PD	Hemoglobin (G1 in 2 pts, G2 in 2 pts, and G3 in 4 pts), kidney adverse events (G1 in 1 pt, and G2 in 2 pts), liver adverse events (G1 in 1 pt, G2 in 2 pts, G3 in 1 pt, and G4 in 1 pt), pancreatobiliary adverse events (G1 in 1 pt, G3 in 1 pt, and G4 in 1 pt) (CTCAE v5.0)
Kuyumcu et al. 82	4	Breast cancer, thymic carcinoma, thyroid cancer, ovarian carcinosarcoma	177Lu-FAPI-04	4	1	0.27 GBq (0.26-0.28 GBq)	NA	NA
Lindner et al. 22	2	Ovarian cancer and pancreatic cancer	90Y‐FAPI‐46	2	1	6 GBq	NA	NA
Jokar et al. 78	1	Breast cancer	177Lu-FAPI-46	2	2	3.7 GBq	NA	NA
Rathke et al. 77	1	Metachronous metastasized breast cancer and colorectal cancer	90Y-FAPI-46	4	4	35.5 GBq	SD for breast cancer and PR for colorectal cancer after 1 cycle, but PD after 4 cycles	NA
Ballal S et al. 76	1	Breast cancer	177Lu-DOTA.SA.FAPi	1	1	3.2 GBq	Decrease in the intensity of headaches	NA
Lindner et al. 17	1	Breast cancer	90Y-FAPI-04	1	1	2.9 GBq	Statistically significant reduction in pain medication	NA
Kratochwil et al. 75	1	Sarcoma	153Sm-FAPI-46 90Y-FAPI-46	3	3	20 GBq for 153Sm and 8 GBq for 90Y	SD	NA

FAPI: fibroblast activation protein inhibitor; RECIST: Response Evaluation Criteria in Solid Tumors; SD: stable disease; PD: progressive disease; PR: partial response; NA: not applicable; CTCAE: Common Terminology Criteria for Adverse Events.

aThis footnote indicates the presentation of means rather than medians.

^bOne patient with hemoglobin (G3), leukopenia (G3), and thrombocytopenia (G3).

For example, in a study of 22 cycles of ¹⁷⁷Lu-FAP-2286 administered to 11 patients with diverse adenocarcinomas (mean injected activity, 5.8 GBq), grade 3 adverse events occurred in three patients, and no grade 4/5 adverse events occurred ⁷⁹. Ferdinandus et al. reported 13 cycles of ⁹⁰Y-FAPI-46 administered to nine patients (with a mean injected activity of 3.8 GBq for the first cycle and a mean injected activity of 7.4 GBq for any subsequent cycle), with new grade 3/4 adverse events occurring in four patients ⁸⁰. In contrast, only one patient had a new grade 3 adverse event in a study of 36 cycles of ¹⁷⁷Lu-FAPI-46 administered to 18 patients (median injected activity, 3.7 GBq) ⁸¹. The measured mean absorbed dose of ¹⁷⁷Lu-FAPI-04 was 0.37 Gy/GBq in tumor lesions ⁸², much lower than that of ⁹⁰Y-FAPI-46 (median, 1.28 Gy/GBq) and ¹⁷⁷Lu-FAP-2286 (3.00 Gy/GBq in bone metastases) ^{80, 79}. Recently, another FAPI dimer, DOTA(SA.FAPi)₂, was developed and synthesized by Ballal et al. (Figure 5D) ⁸³. Radionuclide therapy with ¹⁷⁷Lu-DOTA(SA.FAPi)₂ was administered to 15 patients with radioiodine-refractory differentiated thyroid cancer (DTC; RR-DTC). The results of that study revealed that the numbers of patients with complete response, partial response, and stable disease were 0, 4, and 3, respectively. None of the patients experienced grade 3/4 hematological, renal, or hepato-toxicity.

However, most of the studies mentioned above showed mixed responses to FAP-targeted radionuclide therapy because of the different tumors and patient conditions evaluated (Table 2). It should be noted that the latest studies were mainly aimed at evaluating the feasibility and safety of FAP-targeted radionuclide therapy; the number of patients enrolled in these studies was very limited, and the patient cohorts were heterogeneous. In addition, most patients received FAP-targeted radionuclide therapy as the last line of treatment with poor performance status. Although the tumor half-life of FAP-2286 (average of 44 h for bone and 32 h for single liver metastases) is prolonged compared to FAPI-02/04, it is still shorter than the tumor half-life of PSMA ^{79, 84}.

PSMA-targeted radionuclide therapy is reportedly very effective with beta-radionuclides such as ¹⁷⁷Lu. Moreover, ¹⁷⁷Lu-PSMA directly targets cancer cells in radiotherapy, while the ionizing radiation of radiolabeled FAPIs mainly kills CAFs and indirectly kills cancer cells adjacent to CAFs via crossfire effects. These reasons may partially explain the difference in treatment response to targeted radionuclide therapy between PSMA and FAPI. Therefore, further research to enhance the therapeutic efficacy of FAP-targeted radionuclide is of great importance, including optimizing the chemical structure of the FAPI vector (e.g., multimerization and chemical conjugation with albumin binder), shortening the time interval between treatments, increasing the administered dose of therapeutic radionuclide, and combination treatments with other types of treatment (e.g., immunotherapy, external-beam radiotherapy, and molecular targeted therapy).

It has been reported that a tumor size of 1-2 mm requires the formation of stroma to support the tumor ⁵⁷. Thus, radionuclide therapy targeting FAP may be highly effective for treating advanced cancer patients with widespread metastases. Moreover, in an autochthonous model of pancreatic ductal adenocarcinoma, depleting FAP-positive CAFs induced T-cell accumulation in cancer cells and synergistically enhanced anti-tumor effects within PD-L1 immunotherapy ⁸⁵. Therefore, exploring optimal combination therapies with radionuclide therapy targeting FAP, especially with respect to immunotherapy, is warranted in future research. It must be noted that FAP is overexpressed in various epithelial cancers and is also expressed in many non-oncological diseases ²³. In a cohort of 91 patients, 81.3% of the presenting patients had non-tumor FAPI uptake, including degenerative lesions and physiological uptake in normal salivary glands, mammary glands, and the uterus ⁸⁶. In order to select patients who are most likely to benefit from this therapeutic regimen, careful pre-therapeutic evaluation with ⁶⁸Ga-FAPI PET/CT is recommended prior to FAP-targeted radionuclide therapy.

Conclusion

FAPI variants labeled with ⁶⁸Ga or ¹⁸F have shown impressive results in a broad spectrum of cancers. Well-designed clinical trials with large patient populations are needed to define the role of this diagnostic agent, as ¹⁸F-FDG is the dominant tracer in clinical oncology at present. In addition to CAFs, intense FAP expression is also related to fibrosis, arthritis, atherosclerosis, and autoimmune diseases. Thus, FAPI uptake in non-malignant diseases must be carefully identified. Regarding FAP-targeted radionuclide therapy, one direction for future research is improving the pharmacokinetic properties of tracers via chemical modification. The other potential direction is to explore optimal combination therapies (e.g., combining with external-beam radiotherapy, chemotherapy, and immunotherapy) to synergistically enhance anti-tumor efficacy. Overall, FAPI-based imaging and therapy of cancer have been a highly vibrant research field over the past few years. We look forward to future studies and rapid translation of the most promising FAPI ligands into the clinical arena to benefit patients with various types of cancer.

Abbreviations

¹⁸F-FDG

¹⁸F-fluorodeoxyglucose

CAFs

cancer-associated fibroblasts

CCL2

CC-chemokine ligand 2

CE-CT

contrast-enhanced computed tomography

CE-MRI

contrast-enhanced magnetic resonance imaging

CUP

cancers of unknown primary origin

DOTA

dodecane tetraacetic acid

ECM

extracellular matrix

FAP

fibroblast activation protein

FAPIs

FAP inhibitors

GGO

ground-glass opacity

GTV

gross tumor volume

IL-6

interleukin-6

OSCC

oral squamous cell carcinoma

PDGFRβ

platelet-derived growth factor receptor-β

PDXs

patient-derived xenografts

PET

positron emission tomography

p.i.

post injection

SCI

standard care of imaging

TBR

target-to-blood pool ratio

TGFβ

transforming growth factor-β

TME

tumor microenvironment