Patent spotlight on theranostics targeting fibroblast activation protein for personalized cancer care

ABSTRACT

Theranostics represents the state-of-the-art in precision oncology treatment, and is gaining momentum because of the potential to help improve outcomes for even late-stage cancer patients. Pairs of identical (or very similar) molecules are labeled with both diagnostic and therapeutic radionuclides, and used to both image and treat cancer. The FDA approval and commercialization of theranostics for neuroendocrine tumors and prostate cancer has spurred development of new theranostics as well as significant venture and pharma investment, such that both academic medical centers and companies are working to advance the field. One theranostic target of interest is the Fibroblast Activation Protein (FAP) because its expression in many different tumor types offers potential for a pan-cancer theranostic. In this Patent Spotlight, we present the first analysis of patents issued for FAP-targeting radiopharmaceuticals, providing perspective on current trends and challenges as well as future directions.

GRAPHICAL ABSTRACT

1. Background

In 2023 alone, over 600,000 Americans were reported (by the CDC and American Cancer Society) to have died as a result of some form of cancer, making it the second leading cause of death in the United States after heart disease. Globally, the number is approximately 10 million cancer deaths per year. The prevalence of cancer-related deaths highlights the continuing urgent need for the development of better methods for both the detection and treatment of all varieties of cancer. One such approach that is garnering much attention is the use of “theranostics,” the same (or similar) molecules bearing a targeting vector for a given tumor target tagged with either a diagnostic (β⁺, γ) or a therapeutic (α, β⁻, Auger) radionuclide [1–5]. Diagnostics are bioactive molecules that accumulate in tumors (and associated metastases) that have been labeled with either positron-emitting or gamma-emitting radionuclides. Following injection, patients receive either a positron emission tomography (PET) scan or a single photon emission computed tomography (SPECT) scan to identify any tumor and metastases retaining the imaging agent. Such scans are used initially to confirm target expression and eligibility of the patient for radiotherapy. Radiotherapeutics are tagged with cytotoxic radionuclides that accumulate in the same tumor (and metastatic) sites as the diagnostic imaging agent, and kill the cancer cells by causing, for example, single- or double-strand DNA breaks. After a positive diagnostic scan, patients often receive numerous cycles of a radiotherapeutic over several months, with additional diagnostic scans throughout and/or following treatment to confirm treatment response (Figure 1).

Figure 1.

Theranostics concept. (a) Imaging scans acquired using a tumor-targeting vector (green hexagon) labelled with a diagnostic radionuclide (yellow radioactive symbol) facilitate patient selection, individualized dosimetry, and treatment monitoring for radiopharmaceutical therapy. (b) The same vector labelled with a therapeutic radionuclide (red radioactive symbol) can then be used to administer radiopharmaceutical therapy. Reprinted with permission from [2] with permission. ^©Elsevier.

The last 10 years has seen FDA approval of theranostic pairs for neuroendocrine tumors (targeting somatostatin receptors, SSTRs [6]) and prostate cancer (targeting the prostate-specific membrane antigen, PSMA [7]). The use of molecular imaging to select patients for targeted radiotherapy is at the core of precision medicine, and early results with these neuroendocrine and prostate cancer agents has revealed not only that patients tend to tolerate these agents better than chemotherapy [8], but also that they hold the potential to improve patient prognosis and revolutionize therapy across the entire cancer spectrum.

The potential of theranostics to change the game in the fight against cancer, combined with Novartis’ successful commercialization of both the SSTR and PSMA agents, has ushered nuclear medicine into a new age of theranostics, and spurred interest from both academics and industry to identify new targets enabling development of theranostics for treatment of other cancers. Significant venture and pharma investment (billions of dollars) is backing the search, and there are now hundreds of theranostics for new targets in various stages of preclinical and clinical development. One of the most high profile targets of interest is the Fibroblast Activation Protein (FAP), with early imaging results showing uptake of a ⁶⁸Ga-FAPI tracer ([⁶⁸Ga]FAPI-04) in 28 different kinds of cancer (for a representative selection, see Figure 2) [9]. Reflecting this, there is considerable interest in developing theranostics targeting FAP for personalized cancer care, with some forecasting it will be the next billion dollar theranostics market [10]. While radiopharmaceuticals based upon FAP inhibitors offer potential for pan-cancer theranostics, they are expected to be particularly useful for imaging and treatment of tumors with a strong fibrotic response and/or high levels of cancer-associated fibroblasts, such as colorectal, breast, and pancreatic cancer. They also have the potential for diagnosis and treatment of head and neck cancers, and brain tumors such as gliomas.

Figure 2.

Maximum-intensity projections of [⁶⁸Ga]FAPI-04 PET/CT in patients reflecting 15 different histologically proven tumor entities (sorted by uptake in descending order). Ca = cancer; CCC = cholangiocellular carcinoma; CUP = carcinoma of unknown primary; MTC = medullary thyroid cancer; NET = neuroendocrine tumor. Reprinted from [9] with permission. ^©SNMMI.

The topic of FAPI theranostics has been extensively reviewed in recent years [11–21]. It is not our intent to replicate these comprehensive reviews (or the primary literature they admirably survey), but rather we summarize the patent activity around FAP theranostics in this Spotlight (Supplemental Table S1). We introduce both therapeutic and diagnostic radiopharmaceuticals targeting FAP, with a focus on oncology, discuss regulatory and translational considerations, outline challenges, and limitations in the space, and reflect on the future outlook. Key features of radiopharmaceutical design and radionuclides used for labeling are emphasized and, when available, binding affinities for FAP of the compounds surveyed are included. These values are both highly dependent on experimental conditions and rely on the assay used. As such, they might differ from research group to research group and caution should thus be exercised when comparing reported values between compounds. Of further note, but beyond the scope of this article, FAP theranostics also have potential uses beyond cancer, including fibrotic conditions (e.g., lung, cardiac, liver and kidney fibrosis, Erdheim–Chester disease), inflammatory diseases (e.g., rheumatoid arthritis, Crohn´s disease), benign tumors, and scar formation [16].

2. Initial medicinal chemistry

Recruitment of normal stromal cells provides a scaffold for the growth of solid tumors. Fibroblasts are the key type of cells that provide this scaffold and during cancer cell growth they are active and express Fibroblast Activation Protein on their cell surface [22,23]. There has been interest in FAP as a theranostics target since the 1990s, and initial medicinal chemistry endeavors focused on developing a selective FAP inhibitor (FAPI) were carried out by Universiteit Antwerpen and Fox Chase Cancer Center. Their efforts, reported in the academic and patent literature, resulted in a quinoline-containing core compound that demonstrated low nM FAP inhibition (Figure 3) [22,23]. This paved the way for development of FAPI-based theranostics and, in this Spotlight, we highlight efforts to date from the patent literature (Supplemental Table S1).

Figure 3.

Common scaffold for many FAP-selective inhibitors consisting of a quinoline core functionalized with a proline-containing side chain.

3. Initial design of core connectivity (FAPI-02, FAPI-04, FAPI-46)

In several patents (WO 2019/154859 A1, WO 2019/154886 A1, US 2021/0038749 A1), from the University of Heidelberg, a series of 5-position (FAPI-01) and 6-position (FAPI-02) radiolabeled compounds were disclosed (Figure 4) [24–26]. The FAPI-01 core containing iodine-125 and a set of FAPI-02 compounds with ether linkers containing the chelator DOTA and radiometals such as gallium-68 and lutetium-177 were synthesized. An additional series, FAPI-04 with ether linkers at the 6-position containing the chelator DOTA with gallium-68, lutetium-177, and yttrium-90, were also reported. The key difference between this series and the FAPI-02 series is that the proline-derived ring is difluoro-substituted at one of the ring positions, and it is reported that the addition of the two fluoro groups increases FAP affinity and lipophilicity of the FAPI-04 compounds [27]. The bond connecting the linker to the quinoline core has also been changed from an ether linkage to an amine linkage to give new derivatives such as FAPI-46. This modification resulted in increased tumor uptake and improved pharmacokinetic properties for the resulting amino analogs [28]. Reflecting this, [⁶⁸Ga]FAPI-46 is the most commonly used PET tracer in current clinical trials (for example, an EU CTIS database search [https://euclinicaltrials.eu] reports 23 ongoing clinical trials with [⁶⁸Ga]FAPI-46). As perhaps the lead diagnostic agent in development, [⁶⁸Ga]FAPI-46 has been licensed to SOFIE Biosciences for commercialization and is in Phase 2 clinical studies in the United States (NCT05262855).

Figure 4.

Key FAP selective inhibitor scaffolds used for radiopharmaceutical development. Many consist of a quinoline core functionalized with a proline-containing side chain, either radiolabeled directly or functionalized with a linker and a chelator group for complexing radioactive metal ions.

4. FAP theranostic radiopharmaceuticals in development – current state of the art with a focus on PET and Therapy

4.1. Cyclic amino acid (FAPI-2286)

3B Pharmaceuticals disclosed a cyclic peptide compound in a series of patents which contains a linker connected to the DOTA chelator and coordinated to radiometals gallium-68, lutetium-177, and indium-111 [29–32]. In 2019, 3B Pharmaceuticals entered into a license and collaboration agreement with Clovis Oncology to carry out preclinical evaluation of ⁶⁸Ga-, ¹⁷⁷Lu- and ¹¹¹In-labeled FAPI-2286 for FAP targeted radionuclide imaging and therapy (Figure 4). This series of FAPI-2286 compounds have high affinity (K_D = 0.2 to 1.4 nM) for human FAP. Subsequently, 3B Pharmaceuticals then entered into an agreement with Clovis to carry out clinical evaluation of FAPI-2286, with Clovis subsequently being acquired by Novartis. In this study [⁶⁸Ga]FAPI-2286 is used as the diagnostic PET agent, and patients who demonstrate positive uptake of this tracer are then treated with the therapeutic [¹⁷⁷Lu]FAPI-2286. The LuMIERE Phase 1/2 trial of [¹⁷⁷Lu]FAPI-2286 being run by Novartis in patients with advanced solid tumor is ongoing (NCT04939610), and results to date have demonstrated an acceptable safety profile and promising evidence of early efficacy in patients on study with advanced or metastatic tumors [33]. [¹⁷⁷Lu]FAPI-2286 is currently the most advanced radiotherapeutic clinical trial as, to date, there have yet to be any FAP theranostic Phase 3 clinical trials conducted.

A recent image of the month in the European Journal of Nuclear Medicine and Molecular Imaging reported on a 59-year-old man diagnosed with squamous cell carcinoma in the right lung [34]. FDG PET had revealed extensive metastases, and the decision was made to treat with [¹⁷⁷Lu]FAPI-2286 therapy. [⁶⁸Ga]FAPI-2286 pretreatment revealed extensive uptake in the maximum intensity projection (MIP) image (Figure 5(a)). The patient was then treated with 7.0 GBq [¹⁷⁷Lu]FAPI-2286 and follow-up [⁶⁸Ga]FAPI-2286 PET (9 weeks after treatment) showed the number of positive lesions and degree of radiopharmaceutical uptake significantly decreased on the MIP image (Figure 5(b)). This preliminary result shows clear promise for [¹⁷⁷Lu]FAPI-2286 in the management of metastatic lung cancer.

Figure 5.

a) [⁶⁸Ga]FAPI-2286 MIP PET/CT scan of a 59-year-old man diagnosed with squamous cell carcinoma in the right lung pretreatment, and b) follow up [⁶⁸Ga]FAPI-2286 MIP PET/CT scan treated following treatment with 7.0 GBq [¹⁷⁷Lu]FAPI-2286 showing a decrease in both the number of positive lesions and degree of radiopharmaceutical uptake. Reprinted from [34] with permission. ^©Springer nature.

4.2. Small molecule derivatives

Small molecule versions of both FAPI-02 and FAPI-04 labeled with carbon-11 were reported by Shanghai Jiao Tong University in 2021 (Figure 4) [35]. The ether linkage that normally forms a bond to the linker containing the chelator is instead ¹¹C-methylated. These two compounds, ¹¹C-labeled FAPI-02 (RJ1101) and ¹¹C-labeled FAPI-04 (RJ1102), are reported to have longer tumor retention times compared to the parent [⁶⁸Ga]FAPI-04.

In 2024, a fluorine-18 version of FAPI-02, FluoFAPI was disclosed by the University of Michigan (Figure 4) [36]. The fully automated radiosynthesis of this new radioligand as well as initial preclinical evaluation have been reported [37]. Biodistribution of [¹⁸F]FluoFAPI, for example, was obtained in rats and used to estimate human radiation dosimetry. Additionally, 100× expected single-dose toxicity analysis in preparation for eventual first-in-human experiments was performed. [¹⁸F]FluoFAPI was found to have favorable estimated human radiation dosimetry, and the compound demonstrated no adverse effects when injected at a dose of 100× that planned for [¹⁸F]FluoFAPI PET imaging studies.

4.3. Other diagnostic and/or theranostic compounds: FAPI-02 and FAPI-04 cores

Numerous FAP radiopharmaceuticals have been developed at the Johns Hopkins University (Supplemental Figure S1). Pomper and coworkers, for example, disclosed compounds labeled with indium-111 based upon the FAPI-02-linker (WO 2019/083990 A2) [38]. These compounds were found to exhibit high affinity and good specificity for FAP both in vitro (representative K_i = 1.26 nM) and in vivo (13–11% %ID/g in tumor up to 6 h post-injection, reduced to 0.27% when blocked with cold compound) and further development is anticipated. In 2022, Pomper et al. also disclosed the design of [⁶⁴Cu]hetero-bivalent FAPI-02-based compounds containing both a FAP selective moiety and a PSMA-targeting moiety (WO 2022/212958 A1) [39]. A representative example, FP-L, had high affinity for both FAP (K_i = 0.31 nM) and PSMA (K_i = 18.10 nM), and it was found that [⁶⁴Cu]FP-L1 was able to image both PSMA+ human prostate cancer PC3 PIP (12.06 ± 0.78 %ID/g at 2 h) and FAP+ U87 (16.96 ± 5.01 %ID/g at 2 h) tumors in a single in vivo experiment. The choice of radionuclide in this case is notable.

In 2021–2022, Shanghai Lannacheng Biotechnology disclosed the incorporation of the albumin binder, truncated Evans blue (tEB), into the general FAPI-02/04—linker – DOTA structure (CN 113,582,975 A, WO 2022/170732 A1 and CN 114,369,084 A) (Supplemental Figure S2) [40–42]. The tEB was added in hopes that it would help to optimize pharmacokinetic characteristics while increasing tumor retention and slowing down the rapid clearance rate that is an issue with several known FAPI radiotracers. Indeed, evaluation of ⁶⁸Ga-FAPI-02 showed it had cleared within a 2 h PET scan, while their tEB-modified compound was still circulating. Preclinical evaluation of the corresponding ¹⁷⁷Lu-labeled versions of these compounds in vitro and in vivo demonstrated improved binding affinity and FAP selectivity in vitro, as well as that they can be continuously taken up by tumor tissues and maintained for more than 48 h in human pancreatic cancer xenograft model mice in vivo, leading to tumor growth suppression.

Another set of patents based on the same general idea of attaching an albumin binder to the general FAPI-04-linker-chelator were disclosed in 2022 by Boomray Co., Ltd (WO 2022/135325 A1, WO 2022/135326 A1 and WO 2022/135327 A1) (Supplemental Figure S2) [43–45]. The general idea is that the albumin-binding agent is linked with the chelator unit and the FAP inhibitor moiety to yield small molecules (the TEFAPI series) that are dual-targeted to FAP and albumin, with the goal of prolonging blood circulation and ultimately increasing the tumor uptake. The TEFAPI compounds were labeled with either gallium-68 or yttrium-86, and both imaging and blocking studies were conducted using the pancreatic cancer PDX mouse model. The labeled molecules showed significant uptake in tumors, which could be blocked indicating that the target site of TEFAPI is FAP. Albumin-binding FAP ligands (e.g., FSDD₀I, FSDD₁I, and FSDD₃I) have also been disclosed by Xiamen University (CN115974962B) [46].

Five Eleven Pharma, Inc. reported on the design of a HBED-CC chelator (non-cyclic) containing FAPI-02 (WO 2022/160338 A1) (Supplemental Figure S2) [47]. HBED-CC is an attractive choice of chelator because its thermodynamic stability constant is higher than other commonly used linkers such as NOTA and DOTA (log K_ML: HBED: 38.5; NOTA: 31.0; DOTA: 21.3), while the energy required for HBED to coordinate with ⁶⁸Ga³⁺ is lower, meaning labeling can be conducted in shorter times and at lower temperatures (e.g., room temperature, 5 min) than, for example, DOTA (e.g., 95°C, 7–10 min). The HBED-CC chelator is used to complex gallium-68 in [⁶⁸Ga]Ga-PSMA-11, an FDA-approved radiotracer for prostate cancer [7]. Similarly to the PSMA ligand, HBED-CC enabled good incorporation of gallium-68 in the case of Five Eleven Pharma’s FAP compounds (quantitative imaging at room temperature in 10–30 min) and, while there is limited information included in the patent, the inventors note that the resulting radiopharmaceuticals showed good affinity for FAP-positive tumors.

Low et al. disclosed FAPI-04 (WO 2021/155292 A1), as well as non-quinoline variations (WO 2021/207682 A2) (Supplemental Figure S2). The targeting ligand to FAP has a binding affinity to FAP in a range between about 1 nM to about 25 nM [48,49]. In the second patent (WO 2021/207682 A2), a fluorescent dye was appended. The fluorescein conjugate was demonstrated to bind FAP with high affinity (K_d ~10 nM), and allowed for the use of in vivo fluorescence imaging to evaluate the compounds in mice.

4.4. FAP-targeting diagnostic or theranostic compounds: other variations

The majority of the linker – chelator FAP-selective inhibitors in the patent literature connect the linker – chelator to the 6-position of the quinoline (FAPI-02 and FAPI-04). However, Philochem (WO 2021/160825 A1) introduced a set of linker – chelator FAP-selective inhibitors that tether the linker-chelator at the 8-position through an amine bond (OncoFAP) (Supplemental Figure S3) [50]. Fluorescent derivatives of the compounds revealed the scaffold has excellent affinity for human FAP (e.g., Conjugate 15, hFAP K_D = 0.68 nM; Conjugate 25, hFAP K_D = 1.02 nM), and preclinical experiments with SK-RC-52.hFAP renal cell carcinoma xenografts. A follow-up patent (WO 2022/171811 A1) disclosed related bivalent FAP ligands (e.g., Bi-ESV6) [51]. A further interesting part of this compound design is that instead of the more commonly used DOTA chelator, the DOTAGA chelator was used instead to coordinate gallium-68 or lutetium-177. [¹⁷⁷Lu]Bi-ESV6-DOTAGA showed very high uptake in FAP-expressing tumors (HT-1080.hFAP), but negligible uptake in non-FAP-expressing tumors (HT-1080.wt).

4.5. Boronic acid-based FAP-targeting theranostics

PNT6555 is one of the family of novel boronic acid containing FAP-targeting ligands (Supplemental Figure S4), recently developed and patented by Bachovchin and colleagues at Tufts College in conjunction with Bach Biosciences [52]. Bach subsequently licensed the compounds to Point Biopharma. This family of compounds represent more of a departure from the classical quinoline FAPI core that many of the compounds discussed in this spotlight are based upon, and thus represent a new area of FAPI chemical space. PNT6555 is the lead molecule and has been labeled with gallium-68, lutetium-177, actinium-225, and terbium-161. Preclinical work has shown minimal accumulation and retention in healthy tissues, but revealing significant tumor retention (>10 %ID/g up to 168 h post-injection) in xenograft mice implanted with tumors expressing mouse FAP derived from human embryonic kidney cells [53]. ¹⁷⁷Lu-PNT6555 exhibited significant antitumor activity and animal survival at well-tolerated doses. ²²⁵Ac-PNT6555 and ¹⁶¹Tb-PNT6555 were also efficacious, resulting in 80% and 100% survival at optimal doses, respectively. PNT6555 is being developed further by Point Biopharma, now a wholly owned subsidiary of Eli Lilly, and the Phase 1 FRONTIER trial of ¹⁷⁷Lu-PNT6555 has recently started recruiting (NCT05432193). A related series of compounds have recently been disclosed by Hangzhou Jingjiahang Biomedical Technology Co ltd (CN118955616A) [54], combining the classical quinoline core with a boronic acid moiety (Supplemental Figure S4).

4.6. Alternative labeling strategies – [¹⁸F]AlF

Given the ready availability of fluorine-18 in multi-Curie amounts from small medical cyclotrons, ¹⁸F-labeling of FAPI radiopharmaceuticals is appealing to facilities that operate their own particle accelerators. Direct ¹⁸F-labeling of small molecule FAPI inhibitors was described above (Figure 4) [36,37], but there is also interest in chelating [¹⁸F]AlF as an alternative to gallium-68. Indeed, one of the early patents from Heidelberg mentioned above (WO 2019/154886 A1) includes FAPI-74 (Supplemental Figure S5), a tracer labeled via chelation of [¹⁸F]AlF by NOTA [25]. The patent disclosed uptake of the imaging agent in both HT-1080-FAP tumor bearing mice and a patient with non-small cell lung cancer, paving the way for further evaluation. A recent report from Osaka University compared [¹⁸F]FDG PET with [¹⁸F]FAPI-74 PET in patients with breast, pancreatic, and gastric/esophageal cancer (Supplemental Figure S6), as well as lung and other cancers and benign tumors [55]. [¹⁸F]FAPI-74 imaging revealed higher uptake in primary cancer lesions than nonmalignant lesions, but there was appreciable uptake in some of the nonmalignant lesions. [¹⁸F]FAPI-74 showed significantly increased uptake compared to [¹⁸F]FDG in primary lesions, lymph node metastases, and other metastases, and also detected more metastatic lesions than [¹⁸F]FDG in several patients. [¹⁸F]FAPI-74 is the lead fluorine-18 PET radiopharmaceutical for FAP in development at this point. It has been licensed to GE Healthcare, who is partnering with SOFIE Biosciences for further development, and the imaging agent is in Phase 2 clinical trials in the United States (NCT05641896).

In 2021, the Affiliated Tumor Hospital of Shandong First Medical, University of Shandong, also disclosed a FAPI-04 compound with a linker bonded to the chelator NOTA, and labeled it with [¹⁸F]AlF (CN 114,853,732 A) (Supplemental Figure S5) [56]. The analog is essentially the same as [¹⁸F]FAPI-74, but with the difluoro motif on the proline ring. Several other variations around this same concept have also been disclosed by different groups in a number of other patents (CN 112,933,253 A and WO 2022/032844 A1) [57,58]. For example, (S)-[¹⁸F]-AlF-NOTA-Bn-FAPI-4, disclosed by Shanghai Institute of Health Sciences (WO 2022/032844 A1) which showed good uptake in tumor bearing mice (1.12 %ID/g in tumor), which could be blocked (0.22 %ID/g) by administration of AlF-FAPI-75 (100 µg) 30 min prior to administration of the radiotracer. (S)-[¹⁸F]-AlF-NOTA-Bn-FAPI-4 has many of the classical features of FAPI radiopharmaceuticals including the quinoline core and the difluoroproline sidechain, but incorporation of a thiourea into the linker to the chelator group is uncommon.

5. FAPI radiopharmaceuticals for SPECT imaging

The majority of theranostic agents targeting FAP highlighted above utilize PET as the diagnostic component. However, since 80% of nuclear medicine studies (~40 million per year) still utilize ^99mTc SPECT/CT, ^99mTc-labeled FAP-targeting radiopharmaceuticals are also of interest, particularly in markets that do not yet have access to PET/CT scanners and/or radiochemistry infrastructure to prepare short-lived PET drugs. Moreover, technetium-99 m can be used in conjunction with rhenium-188, a beta emitter used for radiotherapy, given the similarity of their chemistry to give a theranostic pair [59]. Early on, the most prolific applicant in this theranostic space was Molecular Insight Pharmaceuticals, Inc., who filed a patent in 2010 disclosing technetium-99 m and rhenium-188 theranostics targeting FAP (WO 2010/036814) [60]. Molecular Insight was subsequently acquired by Progenics Pharmaceuticals, Inc. in 2013, who then merged with Lantheus in 2020. It is unclear if Lantheus is continuing development of these agents, but they are active in the FAPI area having recently acquired the rights for NTI-1309 from Noria Therapeutics, Inc [61].

More recently, two patents from Peking University (CN 111,991,570 A and WO 2022/017375 A1) introduced the synergistic use of tricine (N-tris(hydroxymethyl)methyl glycine) and TPPTS (triphenylphosphine trisulfonate) as ligands to coordinate technetium-99 m to a FAPI-04 scaffold resulting in SPECT imaging agents for FAP specific tumors (Supplemental Figure S7) [62,63]. The invention discloses ^99mTc-HFAPi and ^99mTc-HpFAPi. In binding experiments with recombinant human FAP-α protein (rhFAP-α), the %ADs were 4.4% and 4.3% for ^99mTc-HFAPi and ^99mTc-HpFAPi, respectively, and these were reduced to 0.95% and 1.1% in blocking experiments. Both biodistribution and SPECT imaging in the U87MG tumor-bearing mouse model revealed good tumor uptake of both compounds (on the order of 15–20 %ID/g at 1 h post-injection) that was significantly reduced (to ~ 1–2 %ID/g) upon blocking, and pharm-tox studies for both compounds revealed no obvious drug acute toxicity. The latter patent (WO 2022/017375 A1) [63] and associated publication [64] also includes details of a clinical study comparing ^99mTc-HFAPi SPECT to FDG PET in a 47-year-old female patient with a right breast tumor showed that both imaging techniques revealed breast cancer primary tumor and lymph node metastases, but that ^99m Tc-HFAPi showed lower brain and kidney uptake. Although oncology trials with ^99mTc-HFAPi do not appear to have started yet, in China the agent is being used in a prospective study to investigate its potential usefulness in the diagnosis, treatment response assessment, and follow-up of pulmonary fibrosis (NCT05859763).

6. Regulatory considerations for translational and approval

Translation (and ultimately approval) of radiopharmaceuticals is a complex and costly business, and the same is expected for the FAP-targeting theranostics discussed in the present Spotlight. The choice of which radiopharmaceutical to translate for a given target, and what mechanism to translate it under, often involves balancing local regulatory nuances with intellectual property (IP) and licensing considerations, availability of appropriate labeling precursor(s), supply of radionuclide(s) from cyclotrons, generators or local suppliers, and financial/reimbursement considerations, while the cost to bring a new theranostic pair to market is hundreds of millions of dollars (or more).

From a regulatory perspective, a common paradigm for translation of theranostics that have shown promise in preclinical studies, for example, involves initial clinical evaluation in countries with straightforward regulations governing human studies. Thus, initial human use is frequently undertaken in Germany, where the most widely used tool to introduce new radiopharmaceuticals into the clinic is article 13, section 2b, of the German Drug Law (Arzneimittelgesetz §13 2b). This rule allows a physician to administer pharmaceuticals to an individual patient as long as the drug is prepared under the immediate supervision of the physician. This was the mechanism used originally to evaluate [⁶⁸Ga]FAPI-04 in numerous different cancer types at Heidelberg [9], as well as many of the theranostic agents targeting somatostatin receptors and PSMA. In the case of radiotherapeutics, where there is potential direct benefit to patients, such initial human use can also happen under compassionate use rules (i.e., Arzneimittelgesetz §13 2b), such as has happened for FAPI radiotherapeutics such as [¹⁷⁷Lu]FAPI-46 and other in Germany (and elsewhere) [19,65].

Once these initial studies have shown promise, then the next step is larger clinical studies, including multisite Phase 2 and Phase 3 trials. The total revenue of the global pharmaceutical market was $1.6t in 2023, of which 53.3% was from North America and 22.7% was Europe [66]. Given this, Phase 2 and 3 clinical trials frequently take place in the United States and/or Europe (possibly with additional countries elsewhere) because companies typically seek US (FDA) and European (EMA) marketing authorization first as the market size and reimbursement enables them to recoup development costs and, eventually, turn a profit. Notably, FDA usually requires demonstration of safety and efficacy in US patients (i.e., clinical trial data from other countries usually are not sufficient), and this should also be accounted for during development. This type of approach was taken with the large NETTER-1 [67] and VISION [68] trials that led to Novartis gaining approval of Lutathera (EMA: 2017, FDA: 2018) and Pluvicto (EMA and FDA: 2022), respectively. Following initial US marketing authorization, further approvals by other regulatory agencies round the world tend to follow. Lutathera, for example, was approved in the UK in 2018, Japan in 2021 and Australia in 2024, while Pluvicto was approved in the UK in 2022 and Australia in 2024, but is not yet approved in Japan.

This showcases how the global R and D infrastructure, regulatory differences between countries, and worldwide partnerships between academics, biotech and pharma can be leveraged for the development, translation, and approval of theranostics. The same general pathway is already enabling development of FAP radiopharmaceuticals including, for example, ¹⁷⁷Lu-FAPI-2286 (Figures 4 and 5) together with its respective imaging agent, the first pair to enter clinical development for commercialization. The 2286 theranostic pair was developed by 3B Pharmaceuticals in Berlin [29–32] and, following promising preclinical results [53,69], first-in-human work was completed with Baum and colleagues at Zentralklinik Bad Berka, under the German Medical Products Act (Arzneimittelgesetz §13 2b) [70]. Promising initial human results led to a licensing deal between 3B and Clovis Oncology, a US biopharmaceutical company, for development and commercialization in the United States. Clovis began LuMIERE, a Phase 1/2, Multicenter, Open-label, Non-randomized Study to Investigate Safety and Tolerability, Pharmacokinetics, Dosimetry, and Preliminary Activity of ¹⁷⁷Lu-FAP-2286 in Patients with an Advanced Solid Tumor (NCT04939610) but, unfortunately, filed for Chapter 11 bankruptcy during the study. Clovis was ultimately acquired by Novartis, who is continuing the LuMIERE trial and, as noted above, early results are encouraging [33,34].

7. Economic considerations

There are unique issues surrounding economics and reimbursement when it comes to radiopharmaceuticals. In the United States for example, historically diagnostic radiopharmaceuticals have enjoyed a period of pass through where their cost is covered, before being bundled with the imaging procedure so that manufacturing facilities take a loss or stop offering the radiopharmaceutical. Gratifyingly, extensive lobbying efforts led to a change in the regulations, and the Centers for Medicare & Medicaid Services (CMS) began unbundling diagnostic radiopharmaceuticals on 1 January 2025 [71]. This is beneficial for FAP diagnostics, as it makes their development financially viable going forward, and there is a general argument that molecular imaging reduces the cost of care for cancer patients [72].

The economics of therapeutics is perhaps more complex. Radiotherapeutics like Lutathera, Pluvicto and, presumably, FAPI therapeutics that may be approved in the future, at first look are expensive (ca. $40-50k per cycle in the US), but cost can vary considerably between markets and may come down as additional agents and/or generics come to market. However, radiopharmaceuticals are always going to have some inherent costs because the nature of radioactive decay means they cannot be produced in bulk and stockpiled like regular pharmaceuticals. Nevertheless, there is a balance to be struck between routine operating costs, companies recovering R and D costs and making some revenue, and maintaining affordability and access, especially in developing countries. The cost of theranostics can also not be considered in a vacuum – with the rising number of cancer cases putting strain on health systems in both developed and developing countries, theranostics are a part of the total cost of care for each individual patient and thus individual theranostics will need to be considered for a given patient population in each country. Since FAPI therapeutics are only in Phase 2 trials at this point, there is insufficient data to accurately determine cost–benefit ratios. Once larger studies have been completed, the data analyzed and benefits understood, the cost-benefit will need to be determined to FAPI theranostics and specific economic models built.

8. Challenges and limitations

Development of FAP theranostics has enormous potential as we outline in this Spotlight. However, in order to give a balanced perspective, it is also important to consider some of the challenges associated with the development of these agents. Like any pharmaceutical development, there are the overarching challenges associated with the cost and time drug development takes. This was an issue with Clovis going bankrupt during the LuMIERE trial, not because of any issue with ¹⁷⁷Lu-FAPI-2286, but because sales of Rubraca, their approved ovarian cancer drug, were hit by competition from other treatments and declining diagnoses during the COVID-19 pandemic [73]. There is also the risk that clinical trials could run into issues for any number of reasons (e.g., toxicity, lack of efficacy, failure to meet endpoints). Although not related to FAP, such was the case for Johnson and Johnson in the case of their Phase 1 trial of JNJ-69086420, an ²²⁵Ac-labeled antibody targeting kallikrein-related peptidase 2 in metastatic castrate-resistant prostate cancer. Four patient deaths have caused J&J to pivot to a new dosing strategy, capping exposure at either 500 µCi (split between two 250 µCi doses), or a single dose of 400 µCi [74]. These are standard issues in (radio)pharmaceutical development that drug hunters need to navigate.

There are also challenges associated specifically with radiopharmaceutical development. Chief among these is ensuring an adequate supply of radionuclide, particularly therapeutic radionuclides such as lutetium-177 and actinium-225, both for clinical trials and routine provision of approved theranostics. This has been a complicating factor for BMS and RayzeBio who paused their ACTION-1 phase 3 study of RYZ101 (²²⁵Ac-DOTATATE) owing to a shortage of actinium-225 [75], and for Novartis who struggled with early supply of Pluvicto following regulatory approval due to difficulties producing lutetium-177 [76] that have since been resolved [77]. In response to these issues, considerable work is going into building the radionuclide supply chain but, as more and more theranostics (including FAP) enter clinical trials and ultimately receive regulatory approval, all while geopolitical complications abound, it will be important to grow and secure the critical isotope supply chain to ensure continued growth of theranostics is sustainable. There are also challenges with limited infrastructure and workforce shortages that are confounding the unprecedented growth that is occurring in theranostics [3]. Efforts are underway to address both of these issues, with increased corporate investment in new manufacturing sites and introduction of new training programs by academic medical centers and professional societies, but these take time to materialize. This growth is particularly important in order to enable widespread utilization of theranostics, especially in remote areas. In the meantime it is recommended that corporate sponsors, CROs, and academic medical centers all work diligently to simplify the process of conducting clinical trials so that studies for the many new theranostics under development can be accommodated without overwhelming the theranostic infrastructure.

There are issues inherent to theranostics targeting FAP. As noted above, there is variability in FAP expression levels across different types of tumors and/or patient populations as well as non-cancer specific FAP expression in normal tissues that can lead to false positives in imaging studies and side effects from FAP radiotherapy. This needs to be accounted for in trial design, and strategies for mitigation of the issues implemented. The other issue alluded to throughout this paper is rapid clearance of FAP-targeting molecules from the body, and the need to design radiopharmaceuticals that achieve adequate tumor retention for effective radiotherapy.

There are also questions about the right choice of radionuclide for FAP therapeutics. Given that FAP is highly expressed in cancer-associated fibroblasts of the tumor stroma, it has been suggested that these cells are present at the invasive front of the tumor stroma but have lower expression in the tumor center, and questions remain about whether short-range alpha-emitters are appropriate for FAP theranostics [78]. However, preclinical studies with ²²⁵Ac-Labeled FAPI-04 and ²²⁵Ac-FAPI-46 in pancreatic cancer xenograft mouse models showed significant tumor growth suppression compared to controls [79,80] which, taken in conjunction with the encouraging preclinical results for ²²⁵Ac-PNT6555 described above, demonstrate proof-of-concept and suggest initial human studies are worth undertaking.

9. Conclusion and future perspectives

In the past several years, there has been unprecedented growth in nuclear medicine and theranostics, with some reports estimating the field will be worth $30 billion by 2030. The field has seen enormous investment in development of theranostics for new targets such as Fibroblast Activation Protein, which has shown to be expressed in many different tumor types and offers potential to develop theranostics for numerous different types of cancer. Reflecting this, there has been a corresponding jump in the patent literature with filings disclosing novel FAP selective inhibitors labeled with a variety of diagnostic and therapeutic radionuclides and, while questions remain around the idea of FAP theranostics [81,82], some predict it will be a billion dollar theranostic target [10] and the molecule of the century [83]!

This Patent Spotlight highlights recent trends in the design of novel FAP inhibitors for theranostic applications. One reason that FAP-selective inhibitors have attracted much attention is for their versatility and potential for the same compound to be used as a diagnostic/theranostic pair depending on the radioisotope used. The two main approaches that many research groups have reported focus on utilizing either small molecule derivatives labeled directly labeled with radionuclides such as ¹¹C, ¹⁸F or ¹²⁵I, or inhibitors bearing a suitable chelating group in order to complex radioactive metal ions such as ⁶⁸Ga, ¹⁷⁷Lu, or ²²⁵Ac. Whichever strategy is taken, a key aspect of developing successful FAPI theranostics is making sure there is slow enough blood circulation so as to achieve sufficient tumor residence time for effective diagnosis and treatment. The strategies being pursued appending albumin binders to prolong blood circulation and increase tumor uptake will likely be pivotal to addressing this issue, and brand-new patents continue to emerge utilizing this approach. Ratio Therapeutics, Inc., for example, disclosed a new series of FAPI inhibitors while this Spotlight was undergoing revisions (WO2025029608A1) [84]. The compounds in question (Supplemental Figure S8) have albumin-binding features that lead to increased circulatory residence times, but interestingly, it is the extremely high affinity to FAP (K_d <1 pM) that is believed to result in significant tumor residence times.

There remain questions about whether FAP is a true biomarker (e.g., FAP expression correlates with tumor size and can be used to monitor response to therapy), or if it is simply overexpressed in different tumors. Either is useful and it will become clearer as more work is done. In particular, the currently ongoing LuMIERE clinical trial with [¹⁷⁷Lu]FAPI-2286 (and [⁶⁸Ga]FAPI-2286) being conducted by Novartis, as well as Phase 2 diagnostic trials of [¹⁸F]FAPI-74 and [⁶⁸Ga]FAPI-46 that SOFIE Biosciences is leading, will provide extremely valuable data, and are expected to inspire further R&D efforts related to the design of novel FAP-targeting theranostics. There are also other key questions around development of FAPI theranostics that need answering. The issue of whether there is sufficient FAPI throughout tumors to warrant the use of short-range alpha emitters needs to be determined. Preclinical data look promising, but clinical studies are urgently needed to answer this question in a clinical context.

Finally, since there are currently no FAP theranostics in Phase 3 clinical trials, at the current rate of progress we do not anticipate approval of a FAP theranostic by FDA or other regulatory agencies for at least 5–10 years.

Funding Statement

The work was funded by U.S. National Institutes of Health, National Institute of Biomedical Imaging and Bioengineering [R01EB021155] and the Neuroendocrine Tumor Research Foundation [Grant no. 1159892].

Article highlights

• “Theranostics” is an emerging area in which the same (or very similar) molecules bearing a targeting vector for a given tumor target are tagged with either a diagnostic (β⁺, γ) or a therapeutic (α, β⁻, Auger) radionuclide.

• The diagnostic agent is used initially to confirm target expression and eligibility of the patient for radiotherapy. Patients then receive several cycles of the corresponding radiotherapeutic over several months, with additional diagnostic scans throughout and/or following treatment to confirm response.

• The last 10 years has seen FDA approval of theranostic pairs for neuroendocrine tumors and prostate cancer.

• The commercialization and remarkable treatment results in both instances has ushered nuclear medicine into a new age of theranostics, and spurred interest from both academics and industry in development of new theranostics for treatment of other cancers.

• Billions of dollars of venture and pharma investment is driving a search for new theranostics, and there are now hundreds of emerging theranostic radiopharmaceuticals for new targets in various stages of preclinical and clinical development.

• Once such target of interest is the Fibroblast Activation Protein (FAP) because early PET imaging results showed uptake of a ⁶⁸Ga-FAPI tracer in 28 different kinds of cancer, suggesting broad applicability to personalized cancer care. Reflecting this, there is considerable interest in developing theranostics for FAP.

• The two main approaches that many research groups have reported focus on utilizing either direct radiolabeling of small molecule FAP inhibitors, or inhibitors bearing a suitable chelating group in order to complex radioactive metal ions.

• The majority of FAP theranostics disclosed build off a quinoline-containing core compound with low nM FAP inhibition.

• LuMIERE is a Phase 1/2 trial of [¹⁷⁷Lu]FAPI-2286 currently being run by Novartis for patients with advanced solid tumors.

• To date there are no FAP theranostics in Phase 3 clinical trials.

Declaration of interest

The authors have no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.

Disclosures

The authors are co-inventors of WO2024197111A2, discussed in this article. Financial support from NIH (R01EB021155 to PJHS) and the Neuroendocrine Tumor Research Foundation (2023 NETRF Investigator Award ID: 1159892 to BLV) is gratefully acknowledged.

Supplementary material

Supplemental data for this article can be accessed online at https://doi.org/10.1080/20468954.2025.2500811.