Inhibitor-Decorated Polymer Conjugates Targeting Fibroblast Activation Protein

Polymer-tethered quenched fluorescent probes for enhanced imaging of tumor associated proteases

Fluorescence-based contrast agents enable real-time detection of solid tumors and their neovasculature, making them ideal for use in image-guided surgery. Several agents have entered late-stage clinical trials or secured FDA approval, suggesting they are likely to become standard of care in cancer surgeries. One of the key parameters to optimize in contrast agent is molecular size, which dictates much of the pharmacokinetic and pharmacodynamic properties of the agent. Here, we describe the development of a class of protease-activated quenched fluorescent probes in which a N-(2-hydroxypropyl)methacrylamide copolymer is used as the primary scaffold. This copolymer core provides a high degree of probe modularity to generate structures that cannot be achieved with small molecules and peptide probes. We used a previously validated cathepsin substrate and evaluated the effects of length and type of linker as well as positioning of the fluorophore/quencher pair on the polymer core. We found that the polymeric probes could be optimized to achieve increased over-all signal and tumor-to-background ratios compared to the reference small molecule probe. Our results also revealed multiple structure-activity relationship trends that can be used to design and optimize future optical imaging probes. Furthermore, they confirm that a hydrophilic polymer is an ideal scaffold for use in optical imaging contrast probes, allowing a highly modular design that enables efficient optimization to maximize probe accumulation and overall biodistribution properties.

Fibroblast activation protein as a potential theranostic target in brain metastases of diverse solid tumours

Brain metastases are a very common and serious complication of oncological diseases. Despite the vast progress in multimodality treatment, brain metastases significantly decrease the quality of life and prognosis of patients. Therefore, identifying new targets in the microenvironment of brain metastases is desirable. Fibroblast activation protein (FAP) is a transmembrane serine protease typically expressed in tumour-associated stromal cells. Due to its characteristic presence in the tumour microenvironment, FAP represents an attractive theranostic target in oncology. However, there is little information on FAP expression in brain metastases. In this study, we quantified FAP expression in samples of brain metastases of various primary origin and characterised FAP-expressing cells. We have shown that FAP expression is significantly higher in brain metastases in comparison to non-tumorous brain tissues, both at the protein and enzymatic activity levels. FAP immunopositivity was localised in regions rich in collagen and containing blood vessels. We have further shown that FAP is predominantly confined to stromal cells expressing markers typical of cancer-associated fibroblasts (CAFs). We have also observed FAP immunopositivity on tumour cells in a portion of brain metastases, mainly originating from melanoma, lung, breast, and renal cancer, and sarcoma. There were no significant differences in the quantity of FAP protein, enzymatic activity, and FAP+ stromal cells among brain metastasis samples of various origins, suggesting that there is no association of FAP expression and/or presence of FAP+ stromal cells with the histological type of brain metastases. In summary, we are the first to establish the expression of FAP and characterise FAP-expressing cells in the microenvironment of brain metastases. The frequent upregulation of FAP and its presence on both stromal and tumour cells support the use of FAP as a promising theranostic target in brain metastases.

The Application of [68Ga]-Labeled FAPI-04 PET/CT for Targeting and Early Detection of Pancreatic Carcinoma in Patient-Derived Orthotopic Xenograft Models

[18F]FDG as a probe of PET/CT is a radiolabeled glucose analogue taken up by most cells, but its batch activity is limited. [68Ga]FAPI-04 is a promising alternative based on a fibroblast activation protein-specific inhibitor (FAPI) labeled with radiotracer FAP. Here, a series of databases suggested that FAP expression was significantly different in pancreatic cancer compared to normal tissue. The FAP-positive fibroblasts were evaluated around the tumor cells and the stroma. A patient-derived orthotopic xenograft (PDOX) model of pancreatic adenocarcinoma (PDAC) exhibits significantly higher quantitative uptake of [68Ga]FAPI-04 (P < 0.05) than [18F]FDG PET/CT in various organs. Because of relatively high (T/M) ratios, the [68Ga]FAPI-04 is excellent for B-mode ultrasound, NIRF, and PET/CT. Thus, [68Ga]FAPI-04 PET displayed a better tumor specificity and can be a potential application for the early detection of pancreatic cancer.

The development of a high-affinity conformation-sensitive antibody mimetic using a biocompatible copolymer carrier (iBody)

Peptide display methods are a powerful tool for discovering new ligands of pharmacologically relevant targets. However, the selected ligands often suffer from low affinity. Using phage display, we identified a new bicyclic peptide binder of prostate-specific membrane antigen (PSMA), a metalloprotease frequently overexpressed in prostate cancer. We show that linking multiple copies of a selected low-affinity peptide to a biocompatible water-soluble N-(2-hydroxypropyl)methacrylamide copolymer carrier (iBody) improved binding of the conjugate by several orders of magnitude. Furthermore, using ELISA, enzyme kinetics, confocal microscopy, and other approaches, we demonstrate that the resulting iBody can distinguish between different conformations of the target protein. The possibility to develop stable, fully synthetic, conformation-selective antibody mimetics has potential applications for molecular recognition, diagnosis and treatment of many pathologies. This strategy could significantly contribute to more effective drug discovery and design.

Structure-activity relationship and biochemical evaluation of novel fibroblast activation protein and prolyl endopeptidase inhibitors with α-ketoamide warheads

Peptidomimetic inhibitors of fibroblast activation protein (FAP) are regarded as promising tools for tumor targeting in vivo. Even though several peptidomimetic compounds with nanomolar potency have been described, broad chemical space for further modification remained unexplored. Therefore, we set to analyze the structure-activity relationship (SAR) of pseudopeptide compound series with α-ketoamide warheads in order to explore the contributions of the P1' and P2' moieties to the inhibitory potency. A series of novel inhibitors bearing varied P1' and/or P2' moieties was synthesized by combining a Passerini reaction-Amine Deprotection-Acyl Migration (PADAM) approach with peptide coupling and subsequent oxidation. The resulting compounds inhibited FAP and the related prolyl endopeptidase (PREP) with potencies in the nanomolar to sub-nanomolar range. The most potent FAP inhibitor IOCB22-AP446 (6d, IC₅₀ = 89 pM) had about 36-fold higher inhibition potency than the most potent inhibitor published to date. The compounds were selective over FAP's closest homolog DPP-IV, were stable in human and mouse plasma and in mouse microsomes, and displayed minimal cytotoxicity in tissue cultures.

In Vitro and In Situ Activity-Based Labeling of Fibroblast Activation Protein with UAMC1110-Derived Probes

Fibroblast activation protein (FAP) is a proline-selective protease that belongs to the S9 family of serine proteases. It is typically highly expressed in the tumor microenvironment (TME) and especially in cancer-associated fibroblasts, the main cell components of the tumor stroma. The exact role of its enzymatic activity in the TME remains largely unknown. Hence, tools that enable selective, activity-based visualization of FAP within the TME can help to unravel FAP’s function. We describe the synthesis, biochemical characterization, and application of three different activity-based probes (biotin-, Cy3-, and Cy5-labeled) based on the FAP-inhibitor UAMC1110, an in-house developed molecule considered to be the most potent and selective FAP inhibitor available. We demonstrate that the three probes have subnanomolar FAP affinity and pronounced selectivity with respect to the related S9 family members. Furthermore, we report that the fluorescent Cy3- and Cy5-labeled probes are capable of selectively detecting FAP in a cellular context, making these chemical probes highly suitable for further biological studies. Moreover, proof of concept is provided for in situ FAP activity staining in patient-derived cryosections of urothelial tumors.

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

PURPOSE

Fibroblast activation protein-α (FAPα) is uniquely expressed in activated fibroblasts, including cancer-associated fibroblasts that populate tumor stroma and contribute to proliferation and immunosuppression. Radiolabeled FAPα inhibitors enable imaging of multiple human cancers, but time-dependent clearance from tumors currently limits their utility as FAPα-targeted radiotherapeutics. We sought to increase the area under the curve (AUC) by constructing a trifunctional ligand that binds FAPα with high affinity and also binds albumin and theranostic radiometals.

PROCEDURES

RPS-309 comprised a FAPα-targeting moiety, an albumin-binding group, and 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA). Inhibition of recombinant human FAPα (rhFAPα) was determined by colorimetric assay. Affinity for human serum albumin (HSA) was determined by high-performance affinity chromatography. The tissue distribution of [⁶⁸Ga]Ga-RPS-309 in SW872 tumor xenograft-bearing mice was imaged by microPET/CT and quantified by biodistribution studies performed from 30 min to 3 h post injection (p.i.). The biodistribution of [¹⁷⁷Lu]Lu-RPS-309 was determined at 4, 24, and 96 h p.i.

RESULTS

RPS-309 inhibits rhFAPα with IC₅₀ = 7.3 ± 1.4 nM. [⁶⁸Ga]Ga-RPS-309 is taken up specifically by FAPα-expressing cells and binds HSA with K_d = 4.6 ± 0.1 μM. Uptake of the radiolabeled ligand in tumors was evident from 30 min p.i. (> 5 %ID/g) and was significantly reduced by co-injection of RPS-309. Specific skeletal uptake was also observed. Activity in tumors was constant through 4 h p.i., but cleared significantly by 24 h. The AUC in this period was 127 (%ID/g) × h.

CONCLUSIONS

RPS-309 is a high-affinity FAPα inhibitor with prolonged plasma residence. Introduction of the albumin-binding group did not compromise FAPα binding. Although initial tumor uptake was high and FAPα-specific, RPS-309 also progressively cleared from tumors. Nevertheless, RPS-309 incorporates multiple sites in which structural diversity can be introduced, and therefore serves as a platform for future structure-activity relationship studies.

Regulation of Fibroblast Activation Protein by Transforming Growth Factor Beta-1 in Glioblastoma Microenvironment

The proline-specific serine protease fibroblast activation protein (FAP) can participate in the progression of malignant tumors and represents a potential diagnostic and therapeutic target. Recently, we demonstrated an increased expression of FAP in glioblastomas, particularly those of the mesenchymal subtype. Factors controlling FAP expression in glioblastomas are unknown, but evidence suggests that transforming growth factor beta (TGFbeta) can trigger mesenchymal changes in these tumors. Here, we investigated whether TGFbeta promotes FAP expression in transformed and stromal cells constituting the glioblastoma microenvironment. We found that both FAP and TGFbeta-1 are upregulated in glioblastomas and display a significant positive correlation. We detected TGFbeta-1 immunopositivity broadly in glioblastoma tissues, including tumor parenchyma regions in the immediate vicinity of FAP-immunopositive perivascular stromal cells. Wedemonstrate for the first time that TGFbeta-1 induces expression of FAP in non-stem glioma cells, pericytes, and glioblastoma-derived endothelial and FAP⁺ mesenchymal cells, but not in glioma stem-like cells. In glioma cells, this effect is mediated by the TGFbeta type I receptor and canonical Smad signaling and involves activation of FAP gene transcription. We further present evidence of FAP regulation by TGFbeta-1 secreted by glioma cells. Our results provide insight into the previously unrecognized regulation of FAP expression by autocrine and paracrine TGFbeta-1 signaling in a broad spectrum of cell types present in the glioblastoma microenvironment.

The role of the biotin linker in polymer antibody mimetics, iBodies, in biochemical assays

We investigated the effect of a linker between the polymer backbone and biotin, and the influence of the number of biotins per polymer chain as well as the biotin position on the polymer chain on the efficacy of the ELISA or pull-down assays.

Interleukin-6: Molecule in the Intersection of Cancer, Ageing and COVID-19

Interleukin-6 (IL-6) is a cytokine with multifaceted effects playing a remarkable role in the initiation of the immune response. The increased level of this cytokine in the elderly seems to be associated with the chronic inflammatory setting of the microenvironment in aged individuals. IL-6 also represents one of the main signals in communication between cancer cells and their non-malignant neighbours within the tumour niche. IL-6 also participates in the development of a premetastatic niche and in the adjustment of the metabolism in terminal-stage patients suffering from a malignant disease. IL-6 is a fundamental factor of the cytokine storm in patients with severe COVID-19, where it is responsible for the fatal outcome of the disease. A better understanding of the role of IL-6 under physiological as well as pathological conditions and the preparation of new strategies for the therapeutic control of the IL-6 axis may help to manage the problems associated with the elderly, cancer, and serious viral infections.

Targeting fibroblast activation protein (FAP): next generation PET radiotracers using squaramide coupled bifunctional DOTA and DATA5m chelators

Background

Fibroblast activation protein (FAP) is a proline selective serine protease that is overexpressed in tumor stroma and in lesions of many other diseases that are characterized by tissue remodeling. In 2014, a most potent FAP-inhibitor (referred to as UAMC1110) with low nanomolar FAP-affinity and high selectivity toward related enzymes such as prolyl oligopeptidase (PREP) and the dipeptidyl-peptidases (DPPs): DPP4, DPP8/9 and DPP2 were developed. This inhibitor has been adopted recently by other groups to create radiopharmaceuticals by coupling bifunctional chelator-linker systems. Here, we report squaric acid (SA) containing bifunctional DATA^5m and DOTA chelators based on UAMC1110 as pharmacophor. The novel radiopharmaceuticals DOTA.SA.FAPi and DATA^5m.SA.FAPi with their non-radioactive derivatives were characterized for in vitro inhibitory efficiency to FAP and PREP, respectively and radiochemical investigated with gallium-68. Further, first proof-of-concept in vivo animal study followed by ex vivo biodistribution were determined with [⁶⁸Ga]Ga-DOTA.SA.FAPi.

Results

[⁶⁸Ga]Ga-DOTA.SA.FAPi and [⁶⁸Ga]Ga-DATA^5m.SA.FAPi showed high complexation > 97% radiochemical yields after already 10 min and high stability over a period of 2 h. Affinity to FAP of DOTA.SA.FAPi and DATA^5m.SA.FAPi and its ^natGa and ^natLu-labeled derivatives were excellent resulting in low nanomolar IC₅₀ values of 0.7–1.4 nM. Additionally, all five compounds showed low affinity for the related protease PREP (high IC₅₀ with 1.7–8.7 μM). First proof-of-principle in vivo PET-imaging animal studies of the [⁶⁸Ga]Ga-DOTA.SA.FAPi precursor in a HT-29 human colorectal cancer xenograft mouse model indicated promising results with high accumulation in tumor (SUV_mean of 0.75) and low background signal. Ex vivo biodistribution showed highest uptake in tumor (5.2%ID/g) at 60 min post injection with overall low uptake in healthy tissues.

Conclusion

In this work, novel PET radiotracers targeting fibroblast activation protein were synthesized and biochemically investigated. Critical substructures of the novel compounds are a squaramide linker unit derived from the basic motif of squaric acid, DOTA and DATA^5m bifunctional chelators and a FAP-targeting moiety. In conclusion, these new FAP-ligands appear promising, both for further research and development as well as for first human application.

Molecular recognition of fibroblast activation protein for diagnostic and therapeutic applications

Fibroblast activation protein (FAP) is a non-classical serine protease expressed predominantly in conditions accompanied by tissue remodeling, particularly cancer. Due to its plasma membrane localization, FAP represents a promising molecular target for tumor imaging and treatment. The unique enzymatic activity of FAP facilitates development of diagnostic and therapeutic tools based on molecular recognition of FAP by substrates and small-molecule inhibitors, in addition to conventional antibody-based strategies. In this review, we provide background on the pathophysiological role of FAP and discuss its potential for diagnostic and therapeutic applications. Furthermore, we present a detailed analysis of the structural patterns crucial for substrate and inhibitor recognition by the FAP active site and determinants of selectivity over the related proteases dipeptidyl peptidase IV and prolyl endopeptidase. We also review published data on targeting of the tumor microenvironment with FAP antibodies, FAP-targeted prodrugs, activity-based probes and small-molecule inhibitors. We describe use of a recently developed, selective FAP inhibitor with low-nanomolar potency in inhibitor-based targeting strategies including synthetic antibody mimetics based on hydrophilic polymers and inhibitor conjugates for PET imaging. In conclusion, recent advances in understanding of the molecular structure and function of FAP have significantly contributed to the development of several tools with potential for translation into clinical practice.

Tris-(Nitrilotriacetic Acid)-Decorated Polymer Conjugates as Tools for Immobilization and Visualization of His-Tagged Proteins

Recombinant proteins are commonly expressed with artificial affinity tags for purification, immobilization and characterization. The most frequently used tag, His-tag, is a sequence of consecutive histidine residues fused to the protein of interest. Specialized small molecules that bind His-tag are primarily used for purification, while antibodies are used for protein analysis. However, various issues may be encountered with the use of antibodies. Low inherent stability, the difficulty of introducing chemical modifications, and often-unreliable batch-to-batch consistency are among the limiting factors that call for better alternatives. Recently described polymer conjugates of N-(2-hydroxypropyl) methacrylamide and low-molecular-weight functional ligands, so-called iBodies, are antibody mimetics capable of replacing antibodies in biochemical applications. We tailored this system for methods utilizing His-tag by accessorizing the polymer carrier with tris-nitrilotriacetic acid targeting ligands. These anti-polyHis iBodies are additionally accessorized with fluorophores, enabling detection, and biotin ligands, enabling immobilization. Here, we characterized anti-polyHis iBodies and explored their use as antibody mimetics. We tested their stability, as well as the influence of different metal mediators and His-tag lengths on binding. With high affinity and stability, iBodies represent a new alternative for immobilization and visualization of His-tagged proteins.

Novel Small Molecule-Derived, Highly Selective Substrates for Fibroblast Activation Protein (FAP)

Fibroblast activation protein (FAP) is a proline-selective serine protease. It is hardly expressed in healthy adult tissue but upregulated in tissue remodeling sites associated with several diseases including epithelial cancer types, atherosclerosis, arthritis and fibrosis. Ongoing research aims at clinical implementation of FAP as a biomarker for these diseases. Several immunochemical methods that quantify FAP expression have been reported. An alternative/complementary approach focuses on quantification of FAP's enzymatic activity. Developing an activity-based assay for FAP has nonetheless proven challenging because of selectivity issues with respect to prolyl oligopeptidase (PREP). Here, we present substrate-type FAP probes that are structurally derived from a FAP-inhibitor (UAMC1110) that we published earlier. Both cleavage efficiency and FAP-selectivity of the best compounds in the series equal or surpass the most advanced peptide-based FAP substrates reported to date. Finally, proof-of-concept is provided that 4-aminonaphthol containing probes can spatially localize FAP activity in biological samples.

Inhibitor-conjugated harmonic nanoparticles targeting fibroblast activation protein

The recent progress in the engineering of nanosized inorganic materials presenting tailored physical properties and reactive surface for post-functionalization has opened promising avenues for the use of nanoparticles (NPs) in diagnosis and therapeutic intervention. Surface decoration of metal oxide NPs with ligands modulating circulation time, cellular uptake, affinity and extravasation through active targeting led to efficient cancer specific bioimaging probes. The most relevant cancer biomarkers studied so far include surface and transmembrane cancer cell receptors. More recently, tumor microenvironments and more specifically the fibroblastic element of the tumor stroma have emerged as a valuable target for diagnosis and treatment of several types of cancers. In this study, a low molecular weight ligand targeting fibroblast activation protein α (FAP), which is specifically expressed by activated fibroblasts of the tumor stroma, was synthesized. This ligand demonstrated nanomolar inhibition of FAP with high selectivity with respect to prolyl oligopeptidase (PREP) and dipeptidyl peptidase (DPP) IV, as well as good biocompatibility toward a human lung tissue model. Bismuth ferrite (BFO) harmonic nanoparticles (HNPs) conjugated to this ligand showed target-specific association to FAP as demonstrated by reverse ELISA-type assay using Human Fibroblast Activation Protein alpha/FAP Alexa Fluor® 594-conjugated Antibody and multiphoton multispectral microscopy experiments. These functionalized HNPs may provide new nanocarriers to explore the role of FAP in tumorigenesis and to target the fibroblastic component of the tumor microenvironment.

Harmonic nanoparticles, functionalized with a selective inhibitor of FAP, provide imaging probes targeting the fibroblastic element of the tumor stroma.

Identification of Protein Targets of Bioactive Small Molecules Using Randomly Photomodified Probes

Identifying protein targets of bioactive small molecules often requires complex, lengthy development of affinity probes. We present a method for stochastic modification of small molecules of interest with a photoactivatable phenyldiazirine linker. The resulting isomeric mixture is conjugated to a hydrophilic copolymer decorated with biotin and a fluorophore. We validated this approach using known inhibitors of several medicinally relevant enzymes. At least a portion of the stochastic derivatives retained their binding to the target, enabling target visualization, isolation, and identification. Moreover, the mix of stochastic probes could be separated into fractions and tested for binding affinity. The structure of the active probe could be determined and the probe resynthesized to improve binding efficiency. Our approach can thus enable rapid target isolation, identification, and visualization, while providing information required for subsequent synthesis of an optimized probe.

Afterglow Resonance Energy Transfer Inhibition for Fibroblast Activation Protein-α Assay

Traditional photoluminescence resonance energy transfer (PRET)-based sensors are widely applied, but still suffer from the severe background interference from in situ excitation. The afterglow nature of the persistent luminescence nanoparticles (PLNPs) allows optosensing after the stoppage of in situ illumination, and thus subtly overcomes that interference. We proposed a simple strategy for functionalizing PLNPs for bioanalytical applications and the new afterglow resonance energy transfer (ARET)-based assay for quantitative determination and imaging of fibroblast activation protein-alpha (FAPα) in live cells using Au-decorated Cr³⁺_0.004:ZnGa₂O₄ as donor and Cy5.5-KGPNQC-SH as acceptor. The ARET between the donor and acceptor quenches the afterglow of the donor, and the cleavage of peptide KGPNQC by FAPα inhibits the ARET and restores the afterglow of the donor. The ARET-based assay of FAPα, with the linear range of 0.1-2.0 mg·L^-1 (1.2-22.9 nM), LOD of 11 μg·L^-1 (115 pM), and RSD of 3.9% (for 0.5 mg·L^-1 FAPα, n = 5), displays higher sensitivity, lower limit of detection (LOD), and better anti-interference capability than the corresponding PRET-based assay. Besides, the ARET-based sensors are lighted up by the FAPα-positive U87MG and MDA-MB-435 cells, but kept in the dark when incubated in the FAPα-negative AD293 cells. The proposed ARET-based sensor can detect FAPα of U87MG and MDA-MB-435 living cells in human serum with the spiked recoveries of 95.6-103%. Our data demonstrated a simple and effective strategy for bridging PLNPs to bioanalytical applications, and an attractive ARET assay of FAPα.

Pro-tumorigenic roles of fibroblast activation protein in cancer: back to the basics

Fibroblast activation protein (FAP) is a cell-surface serine protease that acts on various hormones and extracellular matrix components. FAP is highly upregulated in a wide variety of cancers, and is often used as a marker for pro-tumorigenic stroma. It has also been proposed as a molecular target of cancer therapies, and, especially in recent years, a great deal of research has gone into design and testing of diverse FAP-targeted treatments. Yet despite this growing field of research, our knowledge of FAP's basic biology and functional roles in various cancers has lagged behind its use as a tumor-stromal marker. In this review, we summarize and analyze recent advances in understanding the functions of FAP in cancer, most notably its prognostic value in various tumor types, cellular effects on various cell types, and potential as a therapeutic target. We highlight outstanding questions in the field, the answers to which could shape preclinical and clinical studies of FAP.