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.


Introduction
Positron emission tomography (PET) is a functional imaging technique extensively used in oncology to diagnose tumors early, even in the absence of morphological abnormalities. Hybrid imaging modalities, including PET/computed tomography (PET/CT) and PET/magnetic resonance imaging (PET/MRI), are currently available and may allow to combine functional and morphological information on cancer patients. Different PET radiotracers evaluating different metabolic pathways or receptor statuses may be used in this setting [1][2][3][4]. Although many PET radiotracers are currently available, fluorine-18 fluorodeoxyglucose ([ 18 F]F-FDG) is still the most widely used PET radiotracer in oncology [2][3][4]. [ 18 F]F-FDG uptake is related to glucose metabolism, and increased glucose metabolism is one of the hallmarks of many cancer types. However, [ 18 F]F-FDG has known limitations, such as its high physiological uptake in many normal tissues (hampering the detection of tumor lesions in these sites), its low uptake in certain tumor types (as several well-differentiated tumors), and a lack of specificity (as several diseases may be characterized by increasing glucose metabolism); these limitations represent the basis for the continuous development of new PET radiotracers in oncology [2][3][4].
Recently, fibroblast activation protein (FAP) expression in cancer-associated fibroblasts (CAFs) was evaluated as a possible target for PET imaging in oncology [5,6]. CAFs are the main component of tumor microenvironment, which has a pivotal role in cancer development, including tumor growth, tumor invasion and metastatic spread [7]. FAP is a transmembrane glycoprotein enzyme, which is overexpressed on the cell surface of activated CAFs of multiple tumor types and, in particular, in many epithelial carcinomas (especially in those characterized by a strong desmoplastic reaction, as they can comprise up to 90% of the tumor mass). Conversely, there is a low expression of FAP in ubiquitous resting fibroblasts of healthy tissues [7]. However, FAP expression is not cancer specific but activated fibroblasts in nonmalignant diseases may overexpress FAP [7,8].
Several radiolabeled FAP inhibitors (FAPI) targeting FAP expression in CAFs and characterized by rapid renal clearance and high tumor-to-background uptake ratio (TBR) have been developed to allow early cancer detection through PET imaging [9]. Several recent studies comparing radiolabeled FAPI and [ 18 F]F-FDG as 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.

Literature Search
The review question was the diagnostic comparison of radiolabeled FAPI and [ 18 F]F-FDG as PET radiotracers in oncology. The literature search results using a systematic approach are reported in Figure 1. The comprehensive computer literature search from PubMed/MEDLINE and Cochrane library database revealed 162 records. Reviewing titles and abstracts, 136 records were excluded: 55 because they were not in the field of interest of this review; 12 reviews, editorials, letters or comments; and 69 case reports or small case series (< 8 patients). Twenty-six articles were selected and retrieved in full-text version. No additional studies were found screening the references of the selected articles. Finally, 26 articles (925 patients) including data on the comparison between radiolabeled FAPI and [ 18 F]F-FDG as PET radiotracers in oncology were included in the systematic review . The characteristics of the studies selected for the systematic review are presented in Table 1,  Table 2, Table 3. The overall quality assessment of the studies is reported in Figure 2.

Qualitative Synthesis (Systematic Review) 2.2.1. Basic Study and Patient Characteristics
Through the comprehensive computer literature search, 26 full-text articles including data on the head-to-head comparison of radiolabeled FAPI and [ 18 F]F-FDG in cancer patients were selected (Table 1)  . All the selected articles were published in the last two years. Countries from Asia, Europe, North America and Africa were represented; the most frequent country was China followed by Germany and Turkey. About the type of study, 88% of the studies were monocentric, 12% were multicentric, 54% were retrospective and 46% were prospective. Different types of tumors were evaluated in the selected studies. The number of patients performing PET with radiolabeled FAPI and [ 18 F]F-FDG ranged from 8 to 123. The median age of the patients included ranged from 44 to 70 years; the male percentage was highly variable from 0% to 96%.

Technical Aspects
Heterogeneous technical aspects among the included studies were found ( Table 2). The most frequent FAPI radiotracer used was [ 68 Ga]Ga-DOTA-FAPI-04. The hybrid imaging modality was PET/CT in most of the studies; PET/MRI was also performed in 23% of included studies. The time between [ 18 F]F-FDG PET and radiolabeled FAPI PET ranged from one day to 89 days, even if the most frequent time range was within one week. The radiopharmaceutical injected activity largely varied among the included studies. Notably, fasting was requested only before [ 18 F]F-FDG injection, but not before radiolabeled FAPI injection. The most frequent time from the radiopharmaceutical injection to PET image acquisition was one hour for both [ 18 F]F-FDG and FAPI radiotracers. The PET image analysis was performed by using qualitative (visual) analysis and additional semi-quantitative analysis through the calculation of the maximal standardized uptake values (SUVmax) in all the studies. For qualitative analysis an area of increased radiopharmaceutical uptake was considered abnormal at [ 18 F]F-FDG PET and radiolabeled FAPI PET if this uptake was higher than the background region, excluding sites of physiological uptake.

Radiotracer Biodistribution and Main Outcome Measures
Regarding the normal tissue biodistribution of radiolabeled FAPI in comparison to [ 18 F]F-FDG, all the included studies showed a lower radiolabeled FAPI uptake in the normal brain, liver, and oral mucosa, compared to [ 18 F]F-FDG .
The main outcome measures about the head-to-head comparison among [ 18 F]F-FDG and FAPI radiotracers are listed in Table 3 and include comparison of radiopharmaceutical uptake and tumor-to-background uptake ratio (TBR) in tumor lesions, and comparison in the detection of primary tumor lesions and/or metastases.
About the comparison of the uptake of [ 18 F]F-FDG and FAPI radiotracers in tumor lesions, there are discrepant findings among the included articles. A significantly higher uptake of radiolabeled FAPI, compared to [ 18 F]F-FDG, was reported only in some articles and only for some types of tumors, most frequently in gastrointestinal tumors, liver tumors and breast cancer. Conversely, when investigated, most of the included articles clearly demonstrated a significant higher TBR for FAPI radiotracers, compared to [ 18 F]F-FDG.
Overall, taking into account the radiotracer uptake and TBR values, compared to [ 18 F]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 [ 18 F]F-FDG PET were gastrointestinal tumors, liver tumors, breast cancer and nasopharyngeal carcinoma.

Discussion
Compared to the previous systematic reviews on FAPI imaging [8,36,37], our systematic review was focused on the head-to-head diagnostic comparison on [ 18 F]F-FDG PET and radiolabeled FAPI PET in oncology, and therefore, only studies or subsets of studies performing both these imaging methods in cancer patients were selected. We believe that the head-to-head comparison should be preferred, compared to indirect comparison, to obtain more solid evidence.
Overall, we found several advantages of radiolabeled FAPI PET, compared to [ 18 F]F-FDG in oncology. First of all, about the patient preparation, compared to [ 18 F]F-FDG, radiolabeled FAPI PET, does not require fasting or any dietary preparation, as glucose metabolic pathways are not involved; thus, a higher patient compliance is expected, compared to [ 18 F]F-FDG, as radiolabeled FAPI PET is feasible even in patients with high serum glucose levels (e.g., diabetic patients).
Most of the FAPI radiotracers included in this systematic review were labeled with 68 Ga obtained from a 68 Ge/ 68 Ga generator; thus, the radiotracer can be produced on site also in small PET centers without an on-site cyclotron. On the other hand, the 68 Ga activity obtained from a generator may be limited, taking into account batch size and short radionuclide half-life. Furthermore, the price of 68 Ge/ 68 Ga generators should be considered. To overcome these drawbacks, FAPI radiolabeling with the longer-lived radionuclide 18 F was recently investigated [38]. Moreover, aside from the reduced availability of 68 Ge/ 68 Ga generators, we would like to underline that FAPI radiotracers labeled with 68 Ga, which are the most used FAPI radiopharmaceuticals, are affected by a lower resolution for PET imaging with respect to FAPI radiotracers labeled with 18 F, due to the high positron energy of 68 Ga, compared to 18 F [38].
About the normal tissue biodistribution of radiolabeled FAPI in comparison to [ 18 F]F-FDG, all the included studies showed a lower radiolabeled FAPI uptake in the normal brain, liver, and oral mucosa, compared to [ 18 F]F-FDG. Therefore, this is the rationale for the better detection of primary or metastatic lesions in these organs . As radiolabeled FAPI seems to present lower background activity, compared to [ 18 F]F-FDG, considering the equal or higher uptake in tumoral lesions, this may finally result in a sharper contrast . Overall, taking into account radiotracer uptake and TBR values, compared to [ 18 F]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 [ 18 F]F-FDG PET, were gastrointestinal tumors, liver tumors, breast cancer and nasopharyngeal carcinoma.
Notably, compared to [ 18 F]F-FDG, the limitation of the reduced specificity still remains with radiolabeled FAPI. As a matter of fact, [ 18 F]F-FDG is known to accumulate in acute inflammation, whereas recent studies have demonstrated the increased radiolabeled FAPI uptake, due to FAP activation in chronic inflammation, causing a fibrotic reaction [8,39].
Even if the results reported by the studies included in this systematic review seem promising regarding the role of radiolabeled FAPI PET in oncology, more research studies focused on specific tumor types are still needed to clearly define the role of radiolabeled FAPI PET/CT of PET/MRI in oncology and to define whether radiolabeled FAPI may substitute [ 18 F]F-FDG (e.g., in some tumor types with low glucose metabolism) or have a complementary role (e.g., possible use in patients with inconclusive findings at [ 18 F]F-FDG PET).
However, the real-world scenario is still characterized by the reduced availability of radiolabeled FAPI worldwide, compared to [ 18 F]F-FDG, and a small number of available research data comparing these radiotracers in specific oncological settings is currently available [39,40].
Some limitations of our systematic review should be underlined. First of all, the wellrecognized clinical and methodological heterogeneity of the included studies hampered a pooled analysis (meta-analysis) and the achievement of definitive conclusions about the review question. To this regard, a meta-analysis on radiolabeled FAPI compared to [ 18 F]F-FDG should be performed about specific tumor types, but unfortunately the number of articles on specific tumor types is still limited. Furthermore, some biases of the included studies should be recognized, such as a lack of adequate reference standard in some studies and the possible publication bias, particularly in studies including a low number of patients. We have tried to limit the publication bias excluding case reports and small case series from this systematic review.
Based on current literature data, we cannot still suggest the alternative or complementary use of radiolabeled FAPI PET compared to [ 18 F]F-FDG PET in oncology. Further head-to-head comparison studies among radiolabeled FAPI and [ 18 F]F-FDG for specific tumor types are warranted, and in particular, cost-effectiveness analyses are strongly suggested to better define the future role of radiolabeled FAPI PET in oncology, compared to [ 18 F]F-FDG PET.

Materials and Methods
The reporting of this systematic review conforms to the updated "Preferred Reporting Items for a Systematic Review and Meta-Analysis" (PRISMA) statement, a reporting guidance to identify, select, appraise, and synthesize studies in systematic reviews [41].

Search Strategy
Two authors (G.T. and B.M.) independently performed a comprehensive computer literature search of PubMed/MEDLINE and Cochrane library databases to find relevant articles comparing radiolabeled FAPI and [ 18 F]F-FDG as PET radiotracers in oncology.
A search algorithm based on a combination of these terms was used: ((FDG) OR (fluorodeoxyglucose)) AND ((FAPI) OR (FAP) OR (fibroblast activation protein)). No beginning date limit was used. The search was updated until 28 August 2021. No language restriction was used. To expand the search, references of the retrieved articles were also screened for additional studies.

Study Selection
Studies or subsets of studies comparing radiolabeled FAPI and [ 18 F]F-FDG as PET radiotracers in oncology were eligible for inclusion in the systematic review. The exclusion criteria were (a) articles not within the field of interest of this review, including studies not comparing these radiopharmaceuticals or those comparing them, but in other field than in oncology; (b) review articles, editorials, letters, comments, conference proceedings related to the review question; and (c) case reports or small case series related to the review question (<8 patients).
Two researchers (G.T. and B.M.) independently reviewed the titles and abstracts of the retrieved articles, applying the inclusion and exclusion criteria mentioned above. Articles were rejected if they were clearly ineligible. The same two researchers then independently reviewed the full-text version of the remaining articles to assess their eligibility for inclusion. Disagreements were resolved in an online consensus meeting involving all the co-authors.

Data Extraction
For each included study, information was collected by two authors independently (G.T. and B.M.) concerning basic study (authors, year of publication, country of origin, study design), patient characteristics (type or cancer evaluated, number of patients who underwent PET with both radiotracers, mean/median age, sex ratio), technical aspects (type of radiotracers, PET hybrid imaging modality and tomographs, time between PET with radiolabeled FAPI and [ 18 F]F-FDG, radiotracer injected activity, time interval between radiotracer injection and image acquisition, image analysis and reference standard). Furthermore, main findings of the included studies about the comparison among [ 18 F]F-FDG and FAPI radiotracers were extracted. In particular, the results on the comparison of radiopharmaceutical uptake, tumor-to-background uptake ratio (TBR) in tumor lesions, and detection of primary tumor lesions and/or metastases were extracted from the original studies.

Quality Assessment
The overall quality of the studies included in the systematic review was critically appraised by two authors (G.T. and B.M.) based on the revised "Quality Assessment of Diagnostic Accuracy Studies" tool (QUADAS-2) [42].

Statistical Analysis
Due to the significant methodological and clinical heterogeneity (considering the different types of tumors evaluated) a statistical analysis was not performed to avoid additional statistical heterogeneity [40,43,44].

Conclusions
Literature data about the comparison of [ 18 F]F-FDG and radiolabeled FAPI as PET radiotracers in oncology are rapidly increasing. Overall, taking into account radiotracer uptake and TBR values, compared to [ 18 F]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 [ 18 F]F-FDG PET were gastrointestinal tumors, liver tumors, breast cancer and nasopharyngeal carcinoma. Further comparison studies are inevitably needed to better evaluate the best field of application of each PET radiotracer.
Author Contributions: Conceptualization, G.T. and R.S.; methodology, G.T. and R.S.; formal analysis, G.T.; data curation, all the co-authors: H.R., Z.K., K.A. and R.S.; writing-original draft preparation, G.T.; writing-review and editing, R.S. All authors have read and agreed to the published version of the manuscript.

Conflicts of Interest:
The authors declare no conflict of interest.