Introduction

The rising incidence and prevalence of cancer has led to a search within the field of nuclear medicine for new ways to control and treat the diseases. Various positron emission tomography (PET)‐tracers have been employed for functional imaging [1]. Currently, 2‐¹⁸F‐fluoro‐2‐deoxy‐D‐glucose positron emission tomography/computed tomography (¹⁸F‐FDG PET/CT) is the standard molecular imaging modality for many cancers. Nevertheless, FDG is a nonspecific metabolic marker with some limitations in certain malignancies, such as well‐differentiated neuroendocrine tumors, prostate adenocarcinoma, and mucin‐producing tumors. Therefore, targeted molecular imaging is pursuing new tracers for imaging and therapy that may overcome the drawbacks of earlier agents [2].

In nuclear oncology, the theragnostic concept is an established method for specific molecular targeting, in both diagnosis and therapy. The scintigraphic detection of targets can be considered as an indirect sign that the patient may benefit from specific treatments [3].

Modern precision oncology is based on tumor biology and thus relies on accurate analysis of the relationship between tumor cells and their microenvironment to understand the underlying tumor pathophysiology [4]. In addition to cancer cells, the tumor microenvironment consists of multiple different cell types, including inflammatory cells, lymphocytes, blood vessels, fibroblasts, and the extracellular matrix [5]. Therefore, this tumor microenvironment offers numerous potential targets for novel cancer drugs.

One such target is cancer‐associated fibroblasts (CAFs), which are different from ordinary fibroblasts and abundant in the tumor microenvironment with potent effects on tumor development [6]. CAFs play a role in tumor growth and progression. A distinguishing feature of CAFs is their expression of fibroblast activation protein (FAP), a type II membrane‐bound glycoprotein [7]. FAP expression in healthy adult tissues is negligible but is highly upregulated with tissue remodeling, fibrosis, wound healing, inflammation, and cancers [8, 9]. FAP can be considered as a key co‐conspirator of cancer cells in tumor growth and metastasis [10].

Expression of FAP and its role in the majority of epithelial tumors encouraged scientists to employ it in oncologic imaging. However, early imaging with nonselective FAP antibodies was suboptimal, mainly due to their large size and long circulation time [11].

In recent years, several small‐molecule inhibitors of FAP (FAPIs) have been investigated for the diagnosis and therapy of various malignancies. A radiolabeled FAPI, recently introduced by Loktev et al. revealed acceptable characteristics for imaging, including rapid, specific, and stable uptake [12]. Also, imaging with radiolabeled FAPIs reportedly provides high contrast in tumors with rich stromal content, and they may be considered highly tumor‐specific [7].

FAP has been a target for imaging and treatment with different cold and radioactive FAP antibodies or inhibitors [13]. Here, we briefly describe the different subtypes of FAP inhibitors and their biodistribution. Moreover, we review the application of the novel promising imaging modality, radiolabeled‐FAPI PET, in nuclear oncology. Finally, we also discuss its potential role in nonmalignant conditions.

FAPI Subtypes, Biodistribution, and Dosimetry

Various radiolabeled FAPI subtypes have been used for PET imaging in preclinical and clinical settings, including, but not limited to, ⁶⁸Ga‐labeled FAPIs.

FAPI‐01 and FAPI‐02 have shown specific binding to human FAP with a rapid and almost complete internalization [7]. FAPI‐01 proved to be suboptimal due to enzymatic deiodination with efflux of the free iodine and consequently lower intracellular radioactivity after longer incubation intervals. FAPI‐02, a DOTA‐linked compound, demonstrated better pharmacokinetic and biochemical properties. FAPI‐02 eliminated more slowly than FAPI‐01 with approximately 10‐fold higher retention after 24 hours. Furthermore, a rapid internalization into FAP‐expressing cells has been reported with a high tumor uptake in patients with metastatic epithelial carcinomas [7, 14].

For optimization of tumor radiotracer uptake and retention, several FAPI‐02‐based compounds were investigated. Of synthesized FAPIs (FAPI‐02 to FAPI‐15), FAPI‐04 was the most promising radiotracer as a potential theragnostic agent [15]. Similar to FAPI‐02, FAPI‐04 showed rapid internalization into FAP‐positive tumors and fast clearance from the body, resulting in fast retention in lesions (in about 10 minutes after radiotracer injection). Furthermore, when compared to FAPI‐02, FAPI‐04 revealed 100% higher effective tumor uptake after 24 hours, which is more desirable for theragnostic application [15].

Several new compounds have been developed based on the lead compound FAPI‐04, which is characterized by rapid radiotracer uptake in FAP‐positive tumors followed by considerable background washout. Of these compounds, ⁶⁸Ga‐FAPI‐46 displayed the highest tumor‐to‐blood, tumor‐to‐muscle, and tumor‐to‐liver ratios [16].

Biodistribution has been studied on the following ¹⁷⁷lutetium‐labeled radiotracers: FAPI‐21, ‐35, ‐46, and ‐55. All of these compounds showed robust tumor accumulation with low radiotracer uptake by normal tissues. FAPI‐21 and ‐46 demonstrated higher tumor accumulation at 1 and 4 hours after injection in comparison to FAPI‐04 [16]. Depending on the tumor type, tumor accumulation can be significantly prolonged using FAPI‐46 [16].

Whereas earlier DOTA‐based tracers (FAPI‐02, ‐04, and ‐46) were developed with a focus on potential therapeutic applications, the NOTA‐derivate FAPI‐74 was developed as an exclusively diagnostic ligand, with slightly shorter tumor retention [17]. Recent analysis of biodistribution and dosimetry of ¹⁸F‐FAPI‐74 PET/CT and ⁶⁸Ga‐FAPI‐74 PET/CT (by NOTA chelator ligand for ¹⁸F and ⁶⁸Ga) in 10 lung cancer patients revealed high image contrast and low radiation burden.

All currently available ⁶⁸Ga‐labeled FAPI subtypes have renal excretion with activity in the urinary bladder and otherwise minimal background activity in normal tissues. Usually, the radiotracer is seen in the renal collecting system and urinary bladder without accumulation in the renal parenchyma [14].

Some researchers have reported dosimetry results of FAPI PET tracers. Giesel et al. performed the preliminary dosimetry for ⁶⁸Ga‐FAPI‐02 and ⁶⁸Ga‐FAPI‐04 in two metastasized breast cancer patients. PET/CT acquisition was performed at 10 minutes, 1 hour, and 3 hours after radiotracer injection. The effective total body dose was 1.80E‐22 mSv/MBq for ⁶⁸Ga‐FAPI‐02 and 1.64E‐22 mSv/MBq for ⁶⁸Ga‐FAPI‐04. The authors predicted that by injection of a dose of about 200 MBq, the effective dose from ⁶⁸Ga‐FAPI PET/CT would be 3–4 mSv. The urinary bladder received the highest effective dose for both ⁶⁸Ga‐FAPI‐02 and ‐04 [14].

The dosimetry of ⁶⁸Ga‐FAPI‐46 was investigated by Meyer et al. in six different cancer patients. Serial imaging was performed at 10 minutes, 1 hour, and 3 hours after injection of 214–246 MBq of ⁶⁸Ga‐FAPI‐46. The average effective total body dose was 7.80E‐03 mSv/MBq. Organs with the highest effective doses were the bladder wall (2.41E‐03 mSv/MBq), ovaries (1.15E‐03 mSv/MBq), and red marrow (8.49E‐04 mSv/MBq). The effective total body dose from 200 MBq of ⁶⁸Ga‐FAPI‐46 was 1.56 ± 0.26 mSv. This, along with 3.7 mSv from low‐dose CT, leads to approximately 5.3 mSv total effective dose. They concluded that ⁶⁸Ga‐FAPI‐46 PET/CT has a favorable dosimetry profile for imaging [18].

In summary, of the various FAPI tracers that have been developed, many of them have shown optimal imaging characteristics with favorable dosimetry. In particular, FAPI‐21 and FAPI‐46 seem to be the most appropriate agents for theragnostics.

Imaging Protocol

At present there are no special patient preparation recommendations prior to performing ⁶⁸Ga‐FAPI PET/CT, such as fasting or discontinuing medications. However, adequate hydration is necessary for all agents with urinary excretion. Due to high and rapid renal excretion of most FAPI‐based PET radiotracers, intravenous hydration with 500 mL of normal saline and injection of 20 mg of furosemide from 15 minutes before to 30 minutes after radiotracer administration have been implemented [19]. ⁶⁸Ga‐FAPI is injected intravenously but a definite injection activity has not been established. Authors have used the range of 100–370 MBq [14, 18, 19]. The appropriate injection activity is yet to be established for different scanners.

In several clinical studies, especially for dosimetry purposes, different acquisition times for whole‐body ⁶⁸Ga‐FAPI PET/CT scans were evaluated, mainly at three time points after radiotracer injection: 10 minutes, 1 hour, and 3 hours [14, 15, 18]. Early time‐point imaging at 10 minutes with ⁶⁸Ga‐FAPI is possible, but it should be noted that the tumor‐to‐background contrast ratio improves with time [18].

Based on our experience, we orally hydrate patients and begin acquisition 30 minutes after injection of 200 MBq ⁶⁸Ga‐FAPI.

Oncologic Application

The overexpression of FAP in the microenvironment of many epithelial tumors is associated with a worse prognosis. Therefore, targeting this enzyme for molecular imaging and endoradiotherapy may be a novel strategy in cancer management [7]. ⁶⁸Ga‐FAPI PET/CT with various FAPI ligands (which will all be referred to as FAPI below) has been used for targeting FAP in tumor stroma [20]. The first human studies were reported in 2018 [12, 15]. To date, multiple clinical studies have been performed to evaluate the potential efficiency of FAPI PET in clinical oncology.

Advantages

FAP is expressed minimally or not at all in normal cells [12]. Therefore, imaging with FAPIs provides absent or low background activity. When compared with ¹⁸F‐FDG, this feature is particularly useful for detection of malignant lesions in the brain and liver due to its higher contrast [2, 12, 44].

FAP is also an encouraging tracer in the evaluation of small lesions such as peritoneal carcinomatosis [21, 30, 45] and leptomeninges [46]. Since even tiny tumors require supporting stromal tissue, it is hypothesized that tumors as small as 3–5 mm could be detected with FAPI PET [12].

Moreover, the uptake of ⁶⁸Ga‐FAPI is independent from the blood glucose level, which has important implications for patient preparation and particular utility in diabetic patients (Figure 30.1) [47].