A Trifunctional Theranostic Ligand Targeting Fibroblast Activation Protein-α (FAPα)

Based on small molecules: development and application of fibroblast activation protein inhibitors radiopharmaceutical in tumor precision therapy

The discovery of biomarkers for malignant tumors is driving the development of new radiopharmaceuticals in nuclear medicine. The development and optimization of novel radiopharmaceuticals to occupy an increasingly important role in tumor diagnosis and treatment. In recent years, fibroblast activation protein (FAP) has gained attention as a promising tumor target due to its widespread expression across various tumors. FAP inhibitor (FAPI) radiopharmaceuticals are considered to be the most promising to be developed for targeting FAP due to their rapid and specific tumor targeting. This review briefly outlines the developmental history of FAP-targeted small-molecule enzyme activity inhibitors, highlighting the effective role of targeting molecules, linkers, and certain functional groups in the delivery of radioisotopes to cancerous tissues. These development strategies will serve as a reference for the further development and application of relevant radiopharmaceuticals. This review also delineates the progress on clinical FAPI as a radioisotope delivery vehicle for the targeted radioligand therapy of tumors and introduces the latest combination therapy involving FAPI radiopharmaceutical for tumor treatment. The findings provide novel therapeutic insights into the targeted radioligand therapy of tumors.

Improved FAPI-radiopharmaceutical pharmacokinetics from the perspectives of a dose escalation study

PURPOSE

This study explores the use of fibroblast activation protein inhibitors (FAPI) targeting radiopharmaceuticals as a new approach for pan-cancer treatment, focusing on key factors affecting their effectiveness. We hypothesized that adjusting the administered radiotracer dose one could enhance the tumor-to-background ratios.

METHODS

In a dose-escalation study with PC3 xenografts, all radiotracers were administered at doses between 10 and 1500 pmol, followed by biodistribution and PET/CT imaging. Their selectivity towards FAP, PREP, and DDP4, along with their stability in vivo, was assessed by biodistribution and metabolite analysis, respectively. Organ FAP expression was quantified using qPCR, and circulating FAP (sFAP) levels were measured in mouse and human blood samples via ELISA. Proof-of-principle human studies were also conducted.

RESULTS

Increasing the dose from 10 to 600 pmol significantly reduced blood uptake and enhanced tumor uptake, optimizing their in vivo performance. All radiotracers showed peak efficacy at 350-600 pmol, with altered pharmacokinetics beyond 600 pmol. Biodistribution studies validated the in vivo selectivity of all radiotracers towards FAP, even in the presence of PREP and DPP4 inhibitors, while they demonstrated remarkable stability in vivo. FAP expression was confirmed in various organs, with sFAP quantified in both healthy mice and humans. Human studies with [68Ga]Ga-DOTA.SA.FAPI revealed reduced off-target uptake (e.g., pancreas, salivary glands, heart), aligning with the preclinical findings.

CONCLUSION

The study highlights the crucial need for precise FAPI-radiotracer dosing, optimizing PET imaging, reducing radiation exposure, and enhancing treatment by accounting for FAP biology and sFAP's influence on pharmacokinetics.

Development of fibroblast activation protein-α radiopharmaceuticals: Recent advances and perspectives

Fibroblast activation protein-α (FAP) has emerged as a promising target in the field of radiopharmaceuticals due to its selective expression in cancer-associated fibroblasts (CAFs) and other pathological conditions involving fibrosis and inflammation. Recent advancements have focused on developing FAP-specific radioligands for diagnostic imaging and targeted radionuclide therapy. This perspective summarized the latest progress in FAP radiopharmaceutical development, highlighting novel radioligands, preclinical evaluations, and potential clinical applications. Additionally, we analyzed the advantages and existing problems of targeted FAP radiopharmaceuticals, and discussed the key breakthrough directions of this target, so as to improve the development and conversion of FAP-targeted radiopharmaceuticals.

[68Ga]Ga-labeled FAPI Conjugated with Gly-Pro Sequence for PET Imaging of Malignant Tumors

PURPOSE

To improve tumor uptake and prolong tumor retention, a novel fibroblast activation protein (FAP) ligand based on a quinoline-based FAP inhibitor (FAPI) conjugated with the Gly-Pro sequence and 1,4,7,10-tetraazacyclododecane-N,N',N″,N‴-tetraacetic acid (DOTA) was radiolabeled with [68Ga]GaCl3 ([68Ga]Ga-DOTA-GPFAPI-04). Due to the tumor heterogeneity, this study aimed to further validate the preclinical value of [68Ga]Ga-DOTA-GPFAPI-04 PET imaging in tumor mice models with different FAP expression levels.

METHODS

[68Ga]Ga-DOTA-GPFAPI-04 was synthesized and its partition coefficient was measured. The stability of [68Ga]Ga-DOTA-GPFAPI-04 was tested in phosphate-buffered saline (PBS, pH 7.4) and fetal bovine serum (FBS). Small animal PET and semi-quantitative studies were conducted in Panc-1 and A549 xenograft tumor mice models compared with [68Ga]Ga-DOTA-FAPI-04. Immunofluorescent and immunohistochemical staining and western blot assay were performed to confirm FAP expression in xenograft tumors.

RESULTS

[68Ga]Ga-DOTA-GPFAPI-04 exhibited a radiochemical purity of > 99% and high stability in PBS and FBS. [68Ga]Ga-DOTA-GPFAPI-04 had higher hydrophilic property than [68Ga]Ga-DOTA-FAPI-04 (-4.09 ± 0.05 vs -3.45 ± 0.05). Small animal PET and semi-quantitative analysis revealed Panc-1 xenograft tumor displayed higher tumor uptake of [68Ga]Ga-DOTA-GPFAPI-04 and tumor-to-background ratios compared to A549 xenograft tumor, consistent with the results of immunofluorescence, immunohistochemistry, and western blot. Moreover, [68Ga]Ga-DOTA-GPFAPI-04 demonstrated higher tumor accumulation and longer tumor retention than [68Ga]Ga-DOTA-FAPI-04 in both Panc-1 and A549 xenograft tumors. Furthermore, the FAP-binding specificity of [68Ga]Ga-DOTA-GPFAPI-04 was confirmed in vivo by co-injection of unlabeled GPFAPI-04.

CONCLUSION

[68Ga]Ga-DOTA-GPFAPI-04 showed more favorable in vivo tumor imaging and longer tumor retention compared to [68Ga]Ga-DOTA-FAPI-04, which has high potential to be a promising PET probe for detecting FAP-positive tumors.

FAP Radioligand Linker Optimization Improves Tumor Dose and Tumor-to-Healthy Organ Ratios in 4T1 Syngeneic Model

Fibroblast activation protein (FAP) has attracted considerable attention as a possible target for the radiotherapy of solid tumors. Unfortunately, initial efforts to treat solid tumors with FAP-targeted radionuclides have yielded only modest clinical responses, suggesting that further improvements in the molecular design of FAP-targeted radiopharmaceutical therapies (RPT) are warranted. In this study, we report several advances on the previously described FAP6 radioligand that increase tumor retention and accelerate healthy tissue clearance. Seven FAP6 derivatives with different linkers or albumin binders were synthesized, radiolabeled, and investigated for their effects on binding and cellular uptake. The radioligands were then characterized in 4T1 tumor-bearing Balb/c mice using both single-photon emission computed tomography (SPECT) and ex vivo biodistribution analyses to identify the conjugate with the best tumor retention and tumor-to-healthy organ ratios. The results reveal an optimized FAP6 radioligand that exhibits efficacy and safety properties that potentially justify its translation into the clinic.

Radiomolecular Theranostics With Fibroblast-Activation-Protein Inhibitors and Peptides

The advancement of theranostics, which combines therapeutic and diagnostic capabilities in oncology, has significantly impacted cancer management. This review explores fibroblast activation protein (FAP) expression in the tumor microenvironment (TME) and its association with various malignancies, highlighting its potential as a theranostic marker for PET/CT imaging using FAP-targeted tracers and for FAP-targeted radiopharmaceutical therapy. We examine the development and clinical applications of FAP inhibitors (FAPIs) and peptides, providing insights into their diagnostic accuracy, initial therapeutic efficacy, and clinical impact across diverse cancer types, as well as the synthesis of novel FAP-targeted ligands. This review aims to showcase the promising outcomes and challenges in integrating FAP-targeted approaches into cancer management.

Tailoring Fibroblast-Activation Protein Targeting for Theranostics: A Comparative Preclinical Evaluation of the 68Ga- and 177Lu-Labeled Monomeric and Dimeric Fibroblast-Activation Protein Inhibitors DOTA.SA.FAPi and DOTAGA.(SA.FAPi)2

BACKGROUND

FAP radiopharmaceuticals show promise for cancer diagnosis; however, their limited tumor residency hinders treatment. This study compared two FAPi derivatives, DOTA.SA.FAPi and DOTAGA.(SA.FAPi)2, labeled with gallium-68 and lutetium-177, aiming to determine an optimum combination for creating theranostic pairs.

METHODS

The radiotracers were studied for lipophilicity, binding to human serum proteins, and binding to human cancer-associated fibroblasts (CAFs) in vitro, including saturation and internalization/externalization studies. PET/SPECT/CT and biodistribution studies were conducted in PC3 and U87MG xenografts for [68Ga]Ga-DOTA.SA.FAPi and [68Ga]Ga-DOTAGA.(SA.FAPi)2. [177Lu]Lu-DOTA.SA.FAPi and [177Lu]Lu-DOTAGA.(SA.FAPi)2, were evaluated in PC3 xenografts. Biodistribution studies of [68Ga]Ga-DOTA.SA.FAPi were performed in healthy male and female mice.

RESULTS

All radiotracers exhibited strong binding to FAP. Their internalization rate was fast while only [177Lu]Lu-DOTAGA.(SA.FAPi)2 was retained longer in CAFs. [68Ga]Ga-DOTAGA.(SA.FAPi)2 and [177Lu]Lu-DOTAGA.(SA.FAPi)2 displayed elevated lipophilicity and affinity for human serum proteins compared to [68Ga]Ga-DOTA.SA.FAPi and [177Lu]Lu-DOTA.SA.FAPi. In vivo studies revealed slower washout of [68Ga]Ga-DOTAGA.(SA.FAPi)2 within 3 h compared to [68Ga]Ga-DOTA.SA.FAPi. The tumor-to-tissue ratios of [68Ga]Ga-DOTAGA.(SA.FAPi)2 versus [68Ga]Ga-DOTA.SA.FAPi did not exhibit any significant differences. [177Lu]Lu-DOTAGA.(SA.FAPi)2 maintained a significant tumor uptake even after 96 h p.i. compared to [177Lu]Lu-DOTA.SA.FAPi.

CONCLUSIONS

Dimeric compounds hold promise for therapy, while monomers are better suited for diagnostics. Finding the right combination is essential for effective disease management.

Synthesis and preclinical evaluation of a novel probe [18F]AlF-NOTA-IPB-GPC3P for PET imaging of GPC3 positive tumor

Glypican-3 (GPC3) is markedly overexpressed in hepatocellular carcinoma (HCC) and not expressed in normal liver tissues. In this study, a novel peptide PET imaging agent ([18F]AlF-NOTA-IPB-GPC3P) was developed to target GPC3 expressed in tumors. The overall radiochemical yield of [18F]AlF-NOTA-IPB-GPC3P was 10-15 %, and its lipophilicity, expressed as the logD value at a pH of 7.4, was -1.18 ± 0.06 (n = 3). Compared to the previously reported tracer [18F]AlF-GP2633, [18F]AlF-NOTA-IPB-GPC3P exhibited higher cellular uptake (15.13 vs 5.96) and internalized rate (80.63 % vs 35.93 %) in Huh7 cells at 120 min. Micro-PET/CT and biodistribution studies further demonstrated that [18F]AlF-NOTA-IPB-GPC3P exhibited significantly increased tumor uptake and prolonged tumor residence in Huh7 tumors compared to [18F]AlF-GP2633 (4.66 ± 0.22 % ID/g vs 0.72 ± 0.09 % ID/g at 60 min, p < 0.001; 5.05 ± 0.23 % ID/g vs 0.35 ± 0.08 % ID/g at 120 min, p < 0.001, respectively). Furthermore, the tumor-to-organ ratios of [18F]AlF-NOTA-IPB-GPC3P surpassed those of [18F]AlF-GP2633. Our results support the utilization of [18F]AlF-NOTA-IPB-GPC3P as a PET imaging agent targeting the GPC3 receptor for tumor detection.

Development, preclinical evaluation and preliminary dosimetry profiling of SB03178, a first-of-its-kind benzo[h]quinoline-based fibroblast activation protein-α-targeted radiotheranostic for cancer imaging and therapy

Fibroblast activation protein-α (FAP) is a marker of cancer-associated fibroblasts (CAFs) that constitute a significant portion of most carcinomas. Since it plays a critical role in tumor growth and metastasis, its timely detection to identify tumor lesions in early developmental stages using targeted radiopharmaceuticals has gained significant impetus. In the present work, two novel FAP-targeted precursors SB03178 and SB04033 comprising of an atypical benzo[h]quinoline construct were synthesized and either chelated to diagnostic radionuclide gallium-68 or therapeutic radionuclide lutetium-177, with ≥90% radiochemical purities and 22-76% decay-corrected radiochemical yields. natGa-labeled complexes displayed dose-dependent FAP inhibition, with binding potency of natGa-SB03178 being ∼17 times higher than natGa-SB04033. To evaluate their pharmacokinetic profiles, PET imaging and ex vivo biodistribution analyses were executed in FAP-overexpressing HEK293T:hFAP tumor-bearing mice. While both tracers displayed clear tumor visualization that was primarily FAP-arbitrated, with negligible uptake in most peripheral tissues, [68Ga]Ga-SB03178 demonstrated higher tumor uptake and superior tumor-to-background contrast ratios than [68Ga]Ga-SB04033. 177Lu-labeled SB03178 was subjected to tumor retention studies, mouse dosimetry profiling and mouse-to-human dose extrapolations also using the HEK293T:hFAP tumor model. [177Lu]Lu-SB03178 exhibited a combination of high and sustained tumor uptake, with excellent tumor-to-critical organ uptake ratios resulting in a high radiation absorbed dose to the tumor and a low estimated whole-body dose to humans. Our preliminary findings are considerably encouraging to support clinical development of [68Ga]Ga-/[177Lu]Lu-SB03178 theranostic pair for use in a vast majority of FAP-overexpressing neoplasms, particularly carcinomas.

Translational assessment of a DATA-functionalized FAP inhibitor with facile 68Ga-labeling at room temperature

PURPOSE

The present study aims at evaluating the preclinical and the clinical performance of [68Ga]Ga-DATA5m.SA.FAPi, which has the advantage to be labeled with gallium-68 at room temperature.

METHODS

[68Ga]Ga-DATA5m.SA.FAPi was assessed in vitro on FAP-expressing stromal cells, followed by biodistribution and in vivo imaging on prostate and glioblastoma xenografts. Moreover, the clinical assessment of [68Ga]Ga-DATA5m.SA.FAPi was conducted on six patients with prostate cancer, aiming on investigating, biodistribution, biokinetics, and determining tumor uptake.

RESULTS

[68Ga]Ga-DATA5m.SA.FAPi is quantitatively prepared in an instant kit-type version at room temperature. It demonstrated high stability in human serum, affinity for FAP in the low nanomolar range, and high internalization rate when associated with CAFs. Biodistribution and PET studies in prostate and glioblastoma xenografts revealed high and specific tumor uptake. Elimination of the radiotracer mainly occurred through the urinary tract. The clinical data are in accordance with the preclinical data concerning the organ receiving the highest absorbed dose (urinary bladder wall, heart wall, spleen, and kidneys). Different to the small-animal data, uptake of [68Ga]Ga-DATA5m.SA.FAPi in tumor lesions is rapid and stable and tumor-to-organ and tumor-to-blood uptake ratios are high.

CONCLUSION

The radiochemical, preclinical, and clinical data obtained in this study strongly support further development of [68Ga]Ga-DATA5m.SA.FAPi as a diagnostic tool for FAP imaging.

Fibroblast Activation Protein Targeting Probe with Gly-Pro Sequence for PET of Glioblastoma

As an important cancer-associated fibroblast-specific biomarker, fibroblast activation protein (FAP) has become an attractive target for tumor diagnosis and treatment. However, most FAP-based radiotracers showed inadequate uptake and short retention in tumors. In this study, we designed and synthesized a novel FAP ligand (DOTA-GPFAPI-04) through assembling three functional moieties: a quinoline-based FAP inhibitor for specifically targeting FAP, a FAP substrate Gly-Pro as a linker for increasing the FAP protein interaction, and a 2,2',2″,2‴-(1,4,7,10-tetraazacyclododecane-1,4,7,10-tetrayl)tetraacetic acid (DOTA) chelator for radiolabeling with different radionuclides. The FAP targeting ability of DOTA-GPFAPI-04 was investigated by molecular docking studies. DOTA-GPFAPI-04 was then radiolabeled with 68Ga to give [68Ga]Ga-DOTA-GPFAPI-04 for positron emission tomography (PET) imaging of glioblastoma. [68Ga]Ga-DOTA-GPFAPI-04 exhibited a purity of >98% and high stability analyzed by radio-HPLC in saline and mouse serum. Cell uptake studies demonstrated the targeting specificity of the probe. Further in vivo pharmacokinetic studies in normal mice demonstrated the quick clearance of the probe. Moreover, compared with the widely studied [68Ga]Ga-FAPI-04, [68Ga]Ga-DOTA-GPFAPI-04 showed much higher U87MG tumor uptake values (4.467 ± 0.379 for [68Ga]Ga-DOTA-GPFAPI-04 and 1.267 ± 0.208% ID/g for [68Ga]Ga-FAPI-04 at 0.5 h post-injection, respectively). The area under the curve based on time-activity curve (TAC) analysis for tumor radioactivity in small animal models was 422.5 for [68Ga]Ga-DOTA-GPFAPI-04 and 98.14 for [68Ga]Ga-FAPI-04, respectively, demonstrating that the former had longer tumor retention time. The tumor-to-muscle (T/M) ratio for [68Ga]Ga-DOTA-GPFAPI-04 reached 9.15 in a U87MG xenograft animal model. PET imaging and blocking assays showed that [68Ga]Ga-DOTA-GPFAPI-04 had specific tumor uptake. In summary, this study demonstrates the successful synthesis and evaluation of a novel FAPI targeting probe, [68Ga]Ga-DOTA-GPFAPI-04, with a Gly-Pro sequence. It shows favorable in vivo glioblastoma imaging properties and relatively long tumor retention, highlighting DOTA-GPFAPI-04 as a promising molecular scaffold for developing FAP targeting tumor theranostic agents.

FAPI PET/CT Imaging-An Updated Review

Despite revolutionizing the field of oncological imaging, Positron Emission Tomography (PET) with [18F]Fluorodeoxyglucose (FDG) as its workhorse is limited by a lack of specificity and low sensitivity in certain tumor subtypes. Fibroblast activation protein (FAP), a type II transmembrane glycoprotein, is expressed by cancer-associated fibroblasts (CAFs) that form a major component of the tumor stroma. FAP holds the promise to be a pan-cancer target, owing to its selective over-expression in a vast majority of neoplasms, particularly epithelial cancers. Several radiolabeled FAP inhibitors (FAPI) have been developed for molecular imaging and potential theranostic applications. Preliminary data on FAPI PET/CT remains encouraging, with extensive multi-disciplinary clinical research currently underway. This review summarizes the existing literature on FAPI PET/CT imaging with an emphasis on diagnostic applications, comparison with FDG, pitfalls, and future directions.

Synthesis and Preclinical Evaluation of Novel 68Ga-Labeled (R)-Pyrrolidin-2-yl-boronic Acid-Based PET Tracers for Fibroblast Activation Protein-Targeted Cancer Imaging

Fibroblast activation protein (FAP) is a membrane-tethered serine protease overexpressed in the reactive stromal fibroblasts of >90% human carcinomas, which makes it a promising target for developing radiopharmaceuticals for the imaging and therapy of carcinomas. Here, we synthesized two novel (R)-pyrrolidin-2-yl-boronic acid-based FAP-targeted ligands: SB02055 (DOTA-conjugated (R)-(1-((6-(3-(piperazin-1-yl)propoxy)quinoline-4-carbonyl)glycyl)pyrrolidin-2-yl)boronic acid) and SB04028 (DOTA-conjugated ((R)-1-((6-(3-(piperazin-1-yl)propoxy)quinoline-4-carbonyl)-D-alanyl)pyrrolidin-2-yl)boronic acid). ^natGa- and ⁶⁸Ga-complexes of both ligands were evaluated in preclinical studies and compared to previously reported ^natGa/⁶⁸Ga-complexed PNT6555. Enzymatic assays showed that FAP binding affinities (IC₅₀) of ^natGa-SB02055, ^natGa-SB04028 and ^natGa-PNT6555 were 0.41 ± 0.06, 13.9 ± 1.29 and 78.1 ± 4.59 nM, respectively. PET imaging and biodistribution studies in HEK293T:hFAP tumor-bearing mice showed that while [⁶⁸Ga]Ga-SB02055 presented with a nominal tumor uptake (1.08 ± 0.37 %ID/g), [⁶⁸Ga]Ga-SB04028 demonstrated clear tumor visualization with ~1.5-fold higher tumor uptake (10.1 ± 0.42 %ID/g) compared to [⁶⁸Ga]Ga-PNT6555 (6.38 ± 0.45 %ID/g). High accumulation in the bladder indicated renal excretion of all three tracers. [⁶⁸Ga]Ga-SB04028 displayed a low background level uptake in most normal organs, and comparable to [⁶⁸Ga]Ga-PNT6555. However, since its tumor uptake was considerably higher than [⁶⁸Ga]Ga-PNT6555, the corresponding tumor-to-organ uptake ratios for [⁶⁸Ga]Ga-SB04028 were also significantly greater than [⁶⁸Ga]Ga-PNT6555. Our data demonstrate that (R)-(((quinoline-4-carbonyl)-d-alanyl)pyrrolidin-2-yl)boronic acid is a promising pharmacophore for the design of FAP-targeted radiopharmaceuticals for cancer imaging and radioligand therapy.

Advanced Fibroblast Activation Protein-Ligand Developments: FAP Imaging Agents: A Review of the Structural Requirements

Fibroblast activation protein-α (FAP) has attracted increasing attention as a selective marker of cancer-associated fibroblasts (CAFs) and more broadly, of activated fibroblasts in tissues undergoing remodeling of their ECM due to chronic inflammation, fibrosis, or wound healing. Since FAP is critical to the initiation of metastatic growth, its expression will serve as a molecular marker to detect tumors at an earlier stage of development compared to currently available methods. The design of high affinity small molecule FAP inhibitor will allow for noninvasive imaging of activated fibroblast in cancer patients. Small molecule inhibitors of FAP are being developed for targeted radiotherapy of tumors.

Synthesis and Evaluation of 68Ga-Labeled (2S,4S)-4-Fluoropyrrolidine-2-Carbonitrile and (4R)-Thiazolidine-4-Carbonitrile Derivatives as Novel Fibroblast Activation Protein-Targeted PET Tracers for Cancer Imaging

Fibroblast activation protein α (FAP-α) is a cell-surface protein overexpressed on cancer-associated fibroblasts that constitute a substantial component of tumor stroma and drive tumorigenesis. FAP is minimally expressed by most healthy tissues, including normal fibroblasts. This makes it a promising pan-cancer diagnostic and therapeutic target. In the present study, we synthesized two novel tracers, [⁶⁸Ga]Ga-SB03045 and [⁶⁸Ga]Ga-SB03058, bearing a (2S,4S)-4-fluoropyrrolidine-2-carbonitrile or a (4R)-thiazolidine-4-carbonitrile pharmacophore, respectively. [⁶⁸Ga]Ga-SB03045 and [⁶⁸Ga]Ga-SB03058 were evaluated for their FAP-targeting capabilities using substrate-based in vitro binding assays, and in PET/CT imaging and ex vivo biodistribution studies in an HEK293T:hFAP tumor xenograft mouse model. The IC₅₀ values of ^natGa-SB03045 (1.59 ± 0.45 nM) and ^natGa-SB03058 (0.68 ± 0.09 nM) were found to be lower than those of the clinically validated ^natGa-FAPI-04 (4.11 ± 1.42 nM). Contrary to the results obtained in the FAP-binding assay, [⁶⁸Ga]Ga-SB03058 demonstrated a ~1.5 fold lower tumor uptake than that of [⁶⁸Ga]Ga-FAPI-04 (7.93 ± 1.33 vs. 11.90 ± 2.17 %ID/g), whereas [⁶⁸Ga]Ga-SB03045 (11.8 ± 2.35 %ID/g) exhibited a tumor uptake comparable to that of [⁶⁸Ga]Ga-FAPI-04. Thus, our data suggest that the (2S,4S)-4-fluoropyrrolidine-2-carbonitrile scaffold holds potential as a promising pharmacophore for the design of FAP-targeted radioligands for cancer diagnosis and therapy.

Albumin-Mediated Size Exclusion Chromatography: The Apparent Molecular Weight of PSMA Radioligands as Novel Parameter to Estimate Their Blood Clearance Kinetics

A meticulously adjusted pharmacokinetic profile and especially fine-tuned blood clearance kinetics are key characteristics of therapeutic radiopharmaceuticals. We, therefore, aimed to develop a method that allowed the estimation of blood clearance kinetics in vitro. For this purpose, ¹⁷⁷Lu-labeled PSMA radioligands were subjected to a SEC column with human serum albumin (HSA) dissolved in a mobile phase. The HSA-mediated retention time of each PSMA ligand generated by this novel 'albumin-mediated size exclusion chromatography' (AMSEC) was converted to a ligand-specific apparent molecular weight (MW_app), and a normalization accounting for unspecific interactions between individual radioligands and the SEC column matrix was applied. The resulting normalized MW_app,norm. could serve to estimate the blood clearance of renally excreted radioligands by means of their influence on the highly size-selective process of glomerular filtration (GF). Based on the correlation between MW and the glomerular sieving coefficients (GSCs) of a set of plasma proteins, GSC_calc values were calculated to assess the relative differences in the expected GF/blood clearance kinetics in vivo and to select lead candidates among the evaluated radioligands. Significant differences in the MW_app,norm. and GSC_calc values, even for stereoisomers, were found, indicating that AMSEC might be a valuable and high-resolution tool for the preclinical selection of therapeutic lead compounds for clinical translation.

Preclinical evaluation of FAP-2286 for fibroblast activation protein targeted radionuclide imaging and therapy

PURPOSE

Fibroblast activation protein (FAP) is a membrane-bound protease that has limited expression in normal adult tissues but is highly expressed in the tumor microenvironment of many solid cancers. FAP-2286 is a FAP-binding peptide coupled to a radionuclide chelator that is currently being investigated in patients as an imaging and therapeutic agent. The potency, selectivity, and efficacy of FAP-2286 were evaluated in preclinical studies.

METHODS

FAP expression analysis was performed by immunohistochemistry and autoradiography on primary human cancer specimens. FAP-2286 was assessed in biochemical and cellular assays and in in vivo imaging and efficacy studies, and was further evaluated against FAPI-46, a small molecule-based FAP-targeting agent.

RESULTS

Immunohistochemistry confirmed elevated levels of FAP expression in multiple tumor types including pancreatic, breast, and sarcoma, which correlated with FAP binding by FAP-2286 autoradiography. FAP-2286 and its metal complexes demonstrated high affinity to FAP recombinant protein and cell surface FAP expressed on fibroblasts. Biodistribution studies in mice showed rapid and persistent uptake of 68Ga-FAP-2286, 111In-FAP-2286, and 177Lu-FAP-2286 in FAP-positive tumors, with renal clearance and minimal uptake in normal tissues. 177Lu-FAP-2286 exhibited antitumor activity in FAP-expressing HEK293 tumors and sarcoma patient-derived xenografts, with no significant weight loss. In addition, FAP-2286 maintained longer tumor retention and suppression in comparison to FAPI-46.

CONCLUSION

In preclinical models, radiolabeled FAP-2286 demonstrated high tumor uptake and retention, as well as potent efficacy in FAP-positive tumors. These results support clinical development of 68Ga-FAP-2286 for imaging and 177Lu-FAP-2286 for therapeutic use in a broad spectrum of FAP-positive tumors.

Rational Design and Pharmacomodulation of Protein-Binding Theranostic Radioligands for Targeting the Fibroblast Activation Protein

The fibroblast activation protein (FAP), overexpressed on cancer-associated fibroblasts (CAFs), has become a valuable target for tumor diagnosis and therapy. However, most FAP-based radioligands show insufficient tumor uptake and retention. In this study, three novel albumin-binding FAP ligands (denoted as FSDD₀I, FSDD₁I, and FSDD₃I) were labeled with ⁶⁸Ga and ¹⁷⁷Lu to overcome these limitations. Cell-based studies and molecular docking assays were performed to identify the specificity and protein-binding properties for FAP. Positron emission tomography (PET) scans in human hepatocellular carcinoma patient-derived xenografts (HCC-PDXs) animal models revealed longer blood retention of ⁶⁸Ga-FSDD₀I than ⁶⁸Ga-FAPI-04, ⁶⁸Ga-FSDD₁I, and ⁶⁸Ga-FSDD₃I. Remarkably, ⁶⁸Ga-FSDD₃I had prominent tumor-to-nontarget (T/NT) ratios. The prominent tumor retention properties of ¹⁷⁷Lu-FSDD₀I in single photon emission computed tomography (SPECT) imaging and biodistribution studies were demonstrated. In summary, this study reports a proof-of-concept study of albumin-binding radioligands for FAP-targeted imaging and targeted radionuclide therapy (TRT).

Albumin Binder-Conjugated Fibroblast Activation Protein Inhibitor Radiopharmaceuticals for Cancer Therapy

Fibroblast activation protein (FAP) has become an attractive target for diagnosis and therapy, and a series of FAP inhibitor (FAPI)-based radiotracers has been developed and had excellent performance for diagnosis outcomes in clinical applications. Yet, their fast clearance and insufficient tumor retention have hampered their further clinical application in cancer treatment. In this study, we developed 2 albumin binder-conjugated FAPI radiotracers, TEFAPI-06 and TEFAPI-07. They were derived from FAPI-04 and were optimized by conjugating 2 types of well-studied albumin binders, 4-(p-iodophenyl) butyric acid moiety (TEFAPI-06) and truncated Evans blue moiety (TEFAPI-07), to try to overcome the above limitations at the expense of prolonging the blood circulation. Methods: TEFAPI-06 and TEFAPI-07 were synthesized and labeled with 68Ga, 86Y, and 177Lu successfully. A series of cell assays was performed to identify the binding affinity and FAP specificity in vitro. PET imaging, SPECT imaging, and biodistribution studies were performed to evaluate the pharmacokinetics in pancreatic cancer patient-derived xenograft (PDX) animal models. The cancer treatment efficacy of 177Lu-TEFAPI-06 and 177Lu-TEFAPI-07 were evaluated in pancreatic cancer PDX-bearing mice. Results: The binding affinities (dissociation constants) to FAP of 68Ga-TEFAPI-06 and 68Ga-TEFAPI-07 were 10.16 ± 2.56 nM and 7.81 ± 2.28 nM, respectively, which were comparable with that of 68Ga-FAPI-04. Comparative PET imaging of HT-1080-FAP and HT-1080 tumor-bearing mice and a blocking study showed the FAP-targeting ability in vivo of these 2 tracers. Compared with 177Lu-FAPI-04, PET imaging, SPECT imaging, and biodistribution studies of TEFAPI-06 and TEFAPI-07 demonstrated their remarkably enhanced tumor accumulation and retention, respectively. Notable tumor growth inhibition by 177Lu-TEFAPI-06 and 177Lu-TEFAPI-07 were observed, whereas the control group and the group treated by 177Lu-FAPI-04 showed a slight therapeutic effect. Conclusion: Two albumin binder-conjugated FAPI radiopharmaceuticals have been developed and evaluated in vitro and in vivo. Significantly improved tumor uptake and retention were observed, compared with the original FAPI tracer. Both 177Lu-TEFAPI-06 and 177Lu-TEFAPI-07 showed remarkable growth inhibition of PDX tumors, whereas the side effects were almost negligible, demonstrating that these radiopharmaceuticals are promising for further clinical translational studies.

Radioligands Targeting Fibroblast Activation Protein (FAP)

Simple Summary

FAP-targeted radiotracers, recently introduced in cancer treatment, accumulate in Cancer-Associated Fibroblasts (CAFs). CAFs are present in tumor lesions but do not correspond to genuine cancer cells, although they behave in an abnormal and disease-promoting manner. One of their characteristic features, the expression of the surface protein FAP, can be utilized to discriminate between cancerous and healthy tissues. By the choice of an appropriate radionuclide, FAP-targeted tracers can be used for imaging or therapy in many cancer types. Therefore, the first successful application of FAP-targeted imaging has led to an enormous and growing interest in nuclear medicine and radiopharmacy.

Abstract

Targeting fibroblast activation protein (FAP) in cancer-associated fibroblasts (CAFs) has attracted significant attention in nuclear medicine. Since these cells are present in most cancerous tissues and FAP is rarely expressed in healthy tissues, anti-FAP tracers have a potential as pan-tumor agents. Compared to the standard tumor tracer [¹⁸F]FDG, these tracers show better tumor-to-background ratios (TBR) in many indications. Unlike [¹⁸F]FDG, FAP-targeted tracers do not require exhausting preparations, such as dietary restrictions on the part of the patient, and offer the possibility of radioligand therapy (RLT) in a theragnostic approach. Although a radiolabeled antibody was clinically investigated as early as the 1990s, the breakthrough event for FAP-targeting in nuclear medicine was the introduction and clinical application of the so-called FAPI-tracers in 2018. From then, the development and application of FAP-targeted tracers became hot topics for the radiopharmaceutical and nuclear medicine community, and attracted the interest of pharmaceutical companies. The aim of this review is to provide a comprehensive overview of the development of FAP-targeted radiopharmaceuticals and their application in nuclear medicine.

New Frontiers in Cancer Imaging and Therapy Based on Radiolabeled Fibroblast Activation Protein Inhibitors: A Rational Review and Current Progress

Over the past decade, the tumor microenvironment (TME) has become a new paradigm of cancer diagnosis and therapy due to its unique biological features, mainly the interconnection between cancer and stromal cells. Within the TME, cancer-associated fibroblasts (CAFs) demonstrate as one of the most critical stromal cells that regulate tumor cell growth, progression, immunosuppression, and metastasis. CAFs are identified by various biomarkers that are expressed on their surfaces, such as fibroblast activation protein (FAP), which could be utilized as a useful target for diagnostic imaging and treatment. One of the advantages of targeting FAP-expressing CAFs is the absence of FAP expression in quiescent fibroblasts, leading to a controlled targetability of diagnostic and therapeutic compounds to the malignant tumor stromal area using radiolabeled FAP-based ligands. FAP-based radiopharmaceuticals have been investigated strenuously for the visualization of malignancies and delivery of theranostic radiopharmaceuticals to the TME. This review provides an overview of the state of the art in TME compositions, particularly CAFs and FAP, and their roles in cancer biology. Moreover, relevant reports on radiolabeled FAP inhibitors until the year 2021 are highlighted-as well as the current limitations, challenges, and requirements for those radiolabeled FAP inhibitors in clinical translation.

Rational Design, Pharmacomodulation, and Synthesis of [68Ga]Ga-Alb-FAPtp-01, a Selective Tumor-Associated Fibroblast Activation Protein Tracer for PET Imaging of Glioma

Dynamic changes in the tumor-associated fibroblast activation protein (FAP) expression in tumors of different stages may be helpful for prognostic evaluation and treatment response monitoring, making this protein a promising surveillance biomarker for timely diagnosis of malignant tumors and effective planning of patient care. To prospectively verify the diagnostic efficacy value of the developed FAP tracers, [⁶⁸Ga]Ga-FAPtp and [⁶⁸Ga]Ga-Alb-FAPtp-01, dynamic/static positron emission tomography (PET)/computed tomography scans were acquired for tumor-targeting studies in vivo and in comparison with the well-established clinically used tracer [⁶⁸Ga]Ga-FAPI-04. The optimized rationally designed FAP-targeting PET tracer, [⁶⁸Ga]Ga-Alb-FAPtp-01, with albumin-binding capability demonstrated prominent tumor uptake over time. The mean standard uptake value (SUV) and the tumor/muscle (T/M) ratio were as high as 1.775 ± 0.179 SUV and T/M = 5.9, 1.533 ± 0.222 SUV and T/M = 6.7, and 1.425 ± 0.204 SUV and T/M = 9.5, respectively, at 1, 2, and 3 h. Its improved tumor uptake and pharmacokinetics suggest that the [⁶⁸Ga]Ga-Alb-FAPtp-01 tracer can noninvasively detect FAP activation in vivo, permitting a precise definition of its roles in tumors of different stages and yielding insights regarding FAP-targeted radiotherapeutic strategies at the molecular level.