One-step affinity purification of recombinant urokinase-type plasminogen activator receptor using a synthetic peptide developed by combinatorial chemistry

Chloroacetamide fragment library screening identifies new scaffolds for covalent inhibition of the TEAD·YAP1 interaction

Transcriptional enhanced associate domain (TEAD) binding to co-activator yes-associated protein (YAP1) leads to a transcription factor of the Hippo pathway. TEADs are regulated by S-palmitoylation of a conserved cysteine located in a deep well-defined hydrophobic pocket outside the TEAD·YAP1 interaction interface. Previously, we reported the discovery of a small molecule based on the structure of flufenamic acid that binds to the palmitate pocket, forms a covalent bond with the conserved cysteine, and inhibits TEAD4 binding to YAP1. Here, we screen a fragment library of chloroacetamide electrophiles to identify new scaffolds that bind to the palmitate pocket of TEADs and disrupt their interaction with YAP1. Time- and concentration-dependent studies with wild-type and mutant TEAD1-4 provided insight into their reaction rates and binding constants and established the compounds as covalent inhibitors of TEAD binding to YAP1. Binding pose hypotheses were generated by covalent docking revealing that the fragments and compounds engage lower, middle, and upper sub-sites of the palmitate pocket. Our fragments and compounds provide new scaffolds and starting points for the design of derivatives with improved inhibition potency of TEAD palmitoylation and binding to YAP1.

Targeting the Urokinase-Type Plasminogen Activator Receptor (uPAR) in Human Diseases With a View to Non-invasive Imaging and Therapeutic Intervention

The interaction between the serine protease urokinase-type plasminogen activator (uPA) and its glycolipid-anchored receptor (uPAR) focalizes plasminogen activation to cell surfaces, thereby regulating extravascular fibrinolysis, cell adhesion, and migration. uPAR belongs to the Ly6/uPAR (LU) gene superfamily and the high-affinity binding site for uPA is assembled by a dynamic association of its three consecutive LU domains. In most human solid cancers, uPAR is expressed at the invasive areas of the tumor-stromal microenvironment. High levels of uPAR in resected tumors or shed to the plasma of cancer patients are robustly associated with poor prognosis and increased risk of relapse and metastasis. Over the years, a plethora of different strategies to inhibit uPA and uPAR function have been designed and investigated in vitro and in vivo in mouse models, but so far none have been implemented in the clinics. In recent years, uPAR-targeting with the intent of cytotoxic eradication of uPAR-expressing cells have nonetheless gained increasing momentum. Another avenue that is currently being explored is non-invasive imaging with specific uPAR-targeted reporter-molecules containing positron emitting radionuclides or near-infrared (NIR) florescence probes with the overarching aim of being able to: (i) localize disease dissemination using positron emission tomography (PET) and (ii) assist fluorescence guided surgery using optical imaging. In this review, we will discuss these advancements with special emphasis on applications using a small 9-mer peptide antagonist that targets uPAR with high affinity.

Peptides and pseudopeptide ligands: a powerful toolbox for the affinity purification of current and next-generation biotherapeutics

Following the consolidation of therapeutic proteins in the fight against cancer, autoimmune, and neurodegenerative diseases, recent advancements in biochemistry and biotechnology have introduced a host of next-generation biotherapeutics, such as CRISPR-Cas nucleases, stem and car-T cells, and viral vectors for gene therapy. With these drugs entering the clinical pipeline, a new challenge lies ahead: how to manufacture large quantities of high-purity biotherapeutics that meet the growing demand by clinics and biotech companies worldwide. The protein ligands employed by the industry are inadequate to confront this challenge: while featuring high binding affinity and selectivity, these ligands require laborious engineering and expensive manufacturing, are prone to biochemical degradation, and pose safety concerns related to their bacterial origin. Peptides and pseudopeptides make excellent candidates to form a new cohort of ligands for the purification of next-generation biotherapeutics. Peptide-based ligands feature excellent target biorecognition, low or no toxicity and immunogenicity, and can be manufactured affordably at large scale. This work presents a comprehensive and systematic review of the literature on peptide-based ligands and their use in the affinity purification of established and upcoming biological drugs. A comparative analysis is first presented on peptide engineering principles, the development of ligands targeting different biomolecular targets, and the promises and challenges connected to the industrial implementation of peptide ligands. The reviewed literature is organized in (i) conventional (α-)peptides targeting antibodies and other therapeutic proteins, gene therapy products, and therapeutic cells; (ii) cyclic peptides and pseudo-peptides for protein purification and capture of viral and bacterial pathogens; and (iii) the forefront of peptide mimetics, such as β-/γ-peptides, peptoids, foldamers, and stimuli-responsive peptides for advanced processing of biologics.

Mimicking Intermolecular Interactions of Tight Protein-Protein Complexes for Small-Molecule Antagonists

Tight protein-protein interactions (K_d <100 nm) that occur over a large binding interface (>1000 Ų ) are highly challenging to disrupt with small molecules. Historically, the design of small molecules to inhibit protein-protein interactions has focused on mimicking the position of interface protein ligand side chains. Here, we explore mimicry of the pairwise intermolecular interactions of the native protein ligand with residues of the protein receptor to enrich commercial libraries for small-molecule inhibitors of tight protein-protein interactions. We use the high-affinity interaction (K_d =1 nm) between the urokinase receptor (uPAR) and its ligand urokinase (uPA) to test our methods. We introduce three methods for rank-ordering small molecules docked to uPAR: 1) a new fingerprint approach that represents uPA's pairwise interaction energies with uPAR residues; 2) a pharmacophore approach to identify small molecules that mimic the position of uPA interface residues; and 3) a combined fingerprint and pharmacophore approach. Our work led to small molecules with novel chemotypes that inhibited a tight uPAR⋅uPA protein-protein interaction with single-digit micromolar IC₅₀ values. We also report the extensive work that identified several of the hits as either lacking stability, thiol reactive, or redox active. This work suggests that mimicking the binding profile of the native ligand and the position of interface residues can be an effective strategy to enrich commercial libraries for small-molecule inhibitors of tight protein-protein interactions.

A Computational Investigation of Small-Molecule Engagement of Hot Spots at Protein-Protein Interaction Interfaces

The binding affinity of a protein-protein interaction is concentrated at amino acids known as hot spots. It has been suggested that small molecules disrupt protein-protein interactions by either (i) engaging receptor protein hot spots or (ii) mimicking hot spots of the protein ligand. Yet, no systematic studies have been done to explore how effectively existing small-molecule protein-protein interaction inhibitors mimic or engage hot spots at protein interfaces. Here, we employ explicit-solvent molecular dynamics simulations and end-point MM-GBSA free energy calculations to explore this question. We select 36 compounds for which high-quality binding affinity and cocrystal structures are available. Five complexes that belong to three classes of protein-protein interactions (primary, secondary, and tertiary) were considered, namely, BRD4•H4, XIAP•Smac, MDM2•p53, Bcl-xL•Bak, and IL-2•IL-2Rα. Computational alanine scanning using MM-GBSA identified hot-spot residues at the interface of these protein interactions. Decomposition energies compared the interaction of small molecules with individual receptor hot spots to those of the native protein ligand. Pharmacophore analysis was used to investigate how effectively small molecules mimic the position of hot spots of the protein ligand. Finally, we study whether small molecules mimic the effects of the native protein ligand on the receptor dynamics. Our results show that, in general, existing small-molecule inhibitors of protein-protein interactions do not optimally mimic protein-ligand hot spots, nor do they effectively engage protein receptor hot spots. The more effective use of hot spots in future drug design efforts may result in smaller compounds with higher ligand efficiencies that may lead to greater success in clinical trials.

Small Molecules Engage Hot Spots through Cooperative Binding To Inhibit a Tight Protein-Protein Interaction

Protein-protein interactions drive every aspect of cell signaling, yet only a few small-molecule inhibitors of these interactions exist. Despite our ability to identify critical residues known as hot spots, little is known about how to effectively engage them to disrupt protein-protein interactions. Here, we take advantage of the ease of preparation and stability of pyrrolinone 1, a small-molecule inhibitor of the tight interaction between the urokinase receptor (uPAR) and its binding partner, the urokinase-type plasminogen activator uPA, to synthesize more than 40 derivatives and explore their effect on the protein-protein interaction. We report the crystal structure of uPAR bound to previously discovered pyrazole 3 and to pyrrolinone 12. While both 3 and 12 bind to uPAR and compete with a fluorescently labeled peptide probe, only 12 and its derivatives inhibit the full uPAR·uPA interaction. Compounds 3 and 12 mimic and engage different hot-spot residues on uPA and uPAR, respectively. Interestingly, 12 is involved in a π-cation interaction with Arg-53, which is not considered a hot spot. Explicit-solvent molecular dynamics simulations reveal that 3 and 12 exhibit dramatically different correlations of motion with residues on uPAR. Free energy calculations for the wild-type and mutant uPAR bound to uPA or 12 show that Arg-53 interacts with uPA or with 12 in a highly cooperative manner, thereby altering the contributions of hot spots to uPAR binding. The direct engagement of peripheral residues not considered hot spots through π-cation or salt-bridge interactions could provide new opportunities for enhanced small-molecule engagement of hot spots to disrupt challenging protein-protein interactions.

Urokinase-Type Plasminogen Activator Receptor as a Potential PET Biomarker in Glioblastoma

Glioblastoma is one of the most malignant types of human cancer, and the prognosis is poor. The development and validation of novel molecular imaging biomarkers has the potential to improve tumor detection, grading, risk stratification, and treatment monitoring of gliomas. The aim of this study was to explore the potential of PET imaging of the urokinase-type plasminogen activator receptor (uPAR) in glioblastoma.

METHODS

The uPAR messenger RNA expression of tumors from 19 glioblastoma patients was analyzed, and a cell culture derived from one of these patients was used to establish an orthotopic xenograft model of glioblastoma. Tumor growth was monitored using bioluminescence imaging. Five to six weeks after inoculation, all mice were scanned with small-animal PET/CT using two new uPAR PET ligands ((64)Cu-NOTA-AE105 and (68)Ga-NOTA-AE105) and, for comparison, O-(2-(18)F-fluoroethyl)-l-tyrosine ((18)F-FET). One MRI scan was obtained for each mouse to confirm tumor location. The uPAR specificity of (64)Cu-NOTA-AE105 was confirmed by alignment of hematoxylin- and eosin-stained and uPAR immunohistochemistry-stained slides of the brain with the activity distribution as determined using autoradiography.

RESULTS

uPAR expression was found in all 19 glioblastoma patient tumors, and high expression of uPAR correlated with decreased overall survival (P = 0.04). Radiolabeling of NOTA-AE105 with (64)Cu and (68)Ga was straightforward, resulting in a specific activity of approximately 20 GBq/μmol and a radiochemical purity of more than 98% for (64)Cu-NOTA-AE105 and more than 97% for (68)Ga-NOTA-AE105. High image contrast resulting in clear tumor delineation was found for both (68)Ga-NOTA-AE105 and (64)Cu-NOTA-AE105. Absolute uptake in tumor was higher for (18)F-FET (3.5 ± 0.8 percentage injected dose [%ID]/g) than for (64)Cu-NOTA-AE105 (1.2 ± 0.4 %ID/g) or (68)Ga-NOTA-AE105 (0.4 ± 0.1 %ID/g). A similar pattern was observed in background brain tissue, where uptake was 1.9 ± 0.1 %ID/g for (18)F-fluorothymidine, compared with 0.05 ± 0.01 %ID/g for (68)Ga-NOTA-AE105 and 0.11 ± 0.02 %ID/g for (64)Cu-NOTA-AE105. The result was a significantly higher tumor-to-background ratio for both (68)Ga-NOTA-AE105 (7.6 ± 2.1, P < 0.05) and (64)Cu-NOTA-AE105 (10.6 ± 2.3, P < 0.01) than for (18)F-FET PET (1.8 ± 0.3). Autoradiography of brain slides confirmed that the accumulation of (64)Cu-NOTA-AE105 corresponded well with uPAR-positive cancer cells.

CONCLUSION

On the basis of our translational study, uPAR PET may be a highly promising imaging biomarker for glioblastoma. Further clinical exploration of uPAR PET in glioblastoma is therefore justified.

Mapping the topographic epitope landscape on the urokinase plasminogen activator receptor (uPAR) by surface plasmon resonance and X-ray crystallography

The urokinase-type plasminogen activator receptor (uPAR or CD87) is a glycolipid-anchored membrane protein often expressed in the microenvironment of invasive solid cancers and high levels are generally associated with poor patient prognosis (Kriegbaum et al., 2011 [1]). uPAR is organized as a dynamic modular protein structure composed of three homologous Ly6/uPAR domains (LU).This internally flexible protein structure of uPAR enables an allosteric regulation of the interactions with its two principal ligands: the serine protease urokinase-type plasminogen activator (uPA) and the provisional matrix protein vitronectin (Vn) (Mertens et al., 2012; Gårdsvoll et al., 2011; Madsen et al., 2007 [2–4]). The data presented here relates to the non-covalent trapping of one of these biologically relevant uPAR-conformations by a novel class of monoclonal antibodies (Zhao et al., 2015 [5]) and to the general mapping of the topographic epitope landscape on uPAR. The methods required to achieve these data include: (1) recombinant expression and purification of a uPAR-hybrid protein trapped in the desired conformation [patent; WO 2013/020898 A12013]; (2) developing monoclonal antibodies with unique specificities using this protein as antigen; (3) mapping the functional epitope on uPAR for these mAbs by surface plasmon resonance with a complete library of purified single-site uPAR mutants (Zhao et al., 2015; Gårdsvoll et al., 2006 [5,6]); and finally (4) solving the three-dimensional structures for one of these mAbs by X-ray crystallography alone and in complex with uPAR [deposited in the PDB database as 4QTH and 4QTI, respectively].

A new class of orthosteric uPAR·uPA small-molecule antagonists are allosteric inhibitors of the uPAR·vitronectin interaction

The urokinase receptor (uPAR) is a GPI-anchored cell surface receptor that is at the center of an intricate network of protein-protein interactions. Its immediate binding partners are the serine proteinase urokinase (uPA), and vitronectin (VTN), a component of the extracellular matrix. uPA and VTN bind at distinct sites on uPAR to promote extracellular matrix degradation and integrin signaling, respectively. Here, we report the discovery of a new class of pyrrolone small-molecule inhibitors of the tight ∼1 nM uPAR·uPA protein-protein interaction. These compounds were designed to bind to the uPA pocket on uPAR. The highest affinity compound, namely 7, displaced a fluorescently labeled α-helical peptide (AE147-FAM) with an inhibition constant Ki of 0.7 μM and inhibited the tight uPAR·uPAATF interaction with an IC50 of 18 μM. Biophysical studies with surface plasmon resonance showed that VTN binding is highly dependent on uPA. This cooperative binding was confirmed as 7, which binds at the uPAR·uPA interface, also inhibited the distal VTN·uPAR interaction. In cell culture, 7 blocked the uPAR·uPA interaction in uPAR-expressing human embryonic kidney (HEK-293) cells and impaired cell adhesion to VTN, a process that is mediated by integrins. As a result, 7 inhibited integrin signaling in MDA-MB-231 cancer cells as evidenced by a decrease in focal adhesion kinase (FAK) phosphorylation and Rac1 GTPase activation. Consistent with these results, 7 blocked breast MDA-MB-231 cancer cell invasion with IC50 values similar to those observed in ELISA and surface plasmon resonance competition studies. Explicit-solvent molecular dynamics simulations show that the cooperativity between uPA and VTN is attributed to stabilization of uPAR motion by uPA. In addition, free energy calculations revealed that uPA stabilizes the VTNSMB·uPAR interaction through more favorable electrostatics and entropy. Disruption of the uPAR·VTNSMB interaction by 7 is consistent with the cooperative binding to uPAR by uPA and VTN. Interestingly, the VTNSMB·uPAR interaction was less favorable in the VTNSMB·uPAR·7 complex suggesting potential cooperativity between 7 and VTN. Compound 7 provides an excellent starting point for the development of more potent derivatives to explore uPAR biology.

uPAR targeted radionuclide therapy with (177)Lu-DOTA-AE105 inhibits dissemination of metastatic prostate cancer

The urokinase-type plasminogen activator receptor (uPAR) is implicated in cancer invasion and metastatic development in prostate cancer and provides therefore an attractive molecular target for both imaging and therapy. In this study, we provide the first in vivo data on an antimetastatic effect of uPAR radionuclide targeted therapy in such lesions and show the potential of uPAR positron emission tomography (PET) imaging for identifying small foci of metastatic cells in a mouse model of disseminating human prostate cancer. Two radiolabeled ligands were generated in high purity and specific activity: a uPAR-targeting probe ((177)Lu-DOTA-AE105) and a nonbinding control ((177)Lu-DOTA-AE105mut). Both uPAR flow cytometry and ELISA confirmed high expression levels of the target uPAR in PC-3M-LUC2.luc cells, and cell binding studies using (177)Lu-DOTA-AE105 resulted in a specific binding with an IC50 value of 100 nM in a competitive binding experiment. In vivo, uPAR targeted radionuclide therapy significantly reduced the number of metastatic lesions in the disseminated metastatic prostate cancer model, when compared to vehicle and nontargeted (177)Lu groups (p < 0.05) using bioluminescence imaging. Moreover, we found a significantly longer metastatic-free survival, with 65% of all mice without any disseminated metastatic lesions present at 65 days after first treatment dose (p = 0.047). In contrast, only 30% of all mice in the combined control groups treated with (177)Lu-DOTA-AE105mut or vehicle were without metastatic lesions. No treatment-induced toxicity was observed during the study as evaluated by observing animal weight and H&E staining of kidney tissue (dose-limiting organ). Finally, uPAR PET imaging using (64)Cu-DOTA-AE105 detected all small, disseminated metastatic foci when compared with bioluminescence imaging in a cohort of animals during the treatment study. In conclusion, uPAR targeted radiotherapy resulted in a significant reduction in the number of metastatic lesions in a human metastatic prostate cancer model. Furthermore, we have provided the first evidence of the potential for identification of small metastatic lesions using uPAR PET imaging in disseminated prostate cancer, illustrating the promising strategy of uPAR theranostics in prostate cancer.

Improved PET imaging of uPAR expression using new (64)Cu-labeled cross-bridged peptide ligands: comparative in vitro and in vivo studies

The correlation between uPAR expression, cancer cell invasion and metastases is now well-established and has prompted the development of a number of uPAR PET imaging agents, which could potentially identify cancer patients with invasive and metastatic lesions. In the present study, we synthesized and characterized two new cross-bridged ⁶⁴Cu-labeled peptide conjugates for PET imaging of uPAR and performed a head-to-head comparison with the corresponding and more conventionally used DOTA conjugate. Based on in-source laser-induced reduction of chelated Cu(II) to Cu(I), we now demonstrate the following ranking with respect to the chemical inertness of their complexed Cu ions: DOTA-AE105 << CB-TE2A-AE105 < CB-TE2A-PA-AE105, which is correlated to their corresponding demetallation rate. No penalty in the uPAR receptor binding affinity of the targeting peptide was encountered by conjugation to either of the macrobicyclic chelators (IC₅₀ ~ 5-10 nM) and high yields and radiochemical purities (>95%) were achieved in all cases by incubation at 95ºC. In vivo, they display identical tumor uptake after 1h, but differ significantly after 22 hrs, where the DOTA-AE105 uptake remains surprisingly high. Importantly, the more stable of the new uPAR PET tracers, ⁶⁴Cu-CB-TE2A-PA-AE105, exhibits a significantly reduced liver uptake compared to ⁶⁴Cu-DOTA-AE105 as well as ⁶⁴Cu-CB-TE2A-AE105, (p<0.0001), emphasizing that our new in vitro stability measurements by mass spectrometry predicts in vivo stability in mice. Specificity of the best performing ligand, ⁶⁴Cu-CB-TE2A-PA-AE105 was finally confirmed in vivo using a non-binding ⁶⁴Cu-labeled peptide as control (⁶⁴Cu-CB-TE2A-PA-AE105^mut). This control PET-tracer revealed significantly reduced tumor uptake (p<0.0001), but identical hepatic uptake compared to its active counterpart (⁶⁴Cu-CB-TE2A-PA-AE105) after 1h. In conclusion, our new approach using in-source laser-induced reduction of Cu(II)-chelated PET-ligands provides useful information, which are predictive for the tracer stability in vivo in mice. Furthermore, the increased stability of our new macrobicyclic ⁶⁴Cu-CB-TE2A-PA-AE105 PET ligand is paralleled by an excellent imaging contrast during non-invasive PET scanning of uPAR expression in preclinical mouse cancer models. The translational promises displayed by this PET-tracer for future clinical cancer patient management remains, however, to be investigated.

Structure-driven design of radionuclide tracers for non-invasive imaging of uPAR and targeted radiotherapy. The tale of a synthetic peptide antagonist

Research performed during the last two decades has provided a wealth of information to highlight the role of the urokinase-type plasminogen activator receptor (uPAR) in the progression and dissemination of invasive and metastatic cancer. In parallel, our perception of the structure-function relationships in uPAR has been refined to such a level that a rational design of uPAR function as well as compounds specifically targeting defined functions of uPAR are now realistic options. This knowledge opens new avenues for developing therapeutic intervention regimens targeting uPAR as well as for monitoring the effects of such treatments by non-invasive imaging using e.g. positron emission tomography. This mini-review will focus on recent advancements in translational research devoted to non-invasive targeting of uPAR, with a view to molecular imaging of its expression in live individuals as well as specific eradication of these cells by targeted radiotherapy.

Small-molecule inhibition of the uPAR·uPA interaction: synthesis, biochemical, cellular, in vivo pharmacokinetics and efficacy studies in breast cancer metastasis

The uPAR·uPA protein-protein interaction (PPI) is involved in signaling and proteolytic events that promote tumor invasion and metastasis. A previous study had identified 4 (IPR-803) from computational screening of a commercial chemical library and shown that the compound inhibited uPAR·uPA PPI in competition biochemical assays and invasion cellular studies. Here, we synthesize 4 to evaluate in vivo pharmacokinetic (PK) and efficacy studies in a murine breast cancer metastasis model. First, we show, using fluorescence polarization and saturation transfer difference (STD) NMR, that 4 binds directly to uPAR with sub-micromolar affinity of 0.2 μM. We show that 4 blocks invasion of breast MDA-MB-231, and inhibits matrix metalloproteinase (MMP) breakdown of the extracellular matrix (ECM). Derivatives of 4 also inhibited MMP activity and blocked invasion in a concentration-dependent manner. Compound 4 also impaired MDA-MB-231 cell adhesion and migration. Extensive in vivo PK studies in NOD-SCID mice revealed a half-life of nearly 5h and peak concentration of 5 μM. Similar levels of the inhibitor were detected in tumor tissue up to 10h. Female NSG mice inoculated with highly malignant TMD-MDA-MB-231 in their mammary fat pads showed that 4 impaired metastasis to the lungs with only four of the treated mice showing severe or marked metastasis compared to ten for the untreated mice. Compound 4 is a promising template for the development of compounds with enhanced PK parameters and greater efficacy.

New peptide receptor radionuclide therapy of invasive cancer cells: in vivo studies using 177Lu-DOTA-AE105 targeting uPAR in human colorectal cancer xenografts

The proposition of uPAR as a potential target in cancer therapy is advanced by its predominant expression at the invasive front of colorectal cancer (CRC) and its value as prognostic biomarker for poor survival in this disease. In this study, we provide the first in vivo proof-of-concept for a theranostic approach as treatment modality in a human xenograft colorectal cancer model.

METHODS

A DOTA-conjugated 9-mer high affinity uPAR binding peptide (DOTA-AE105) was radiolabeled with (64)Cu and (177)Lu, for PET imaging and targeted radionuclide therapy study, respectively. Human uPAR-positive CRC HT-29 cells were inoculated in Nude mice and treated with (177)Lu-DOTA-AE105 once a visible tumor had formed. To evaluate the true effect of the targeted radiotherapy, two controls groups were included in this study, one receiving a (177)Lu-labeled non-binding control peptide and one receiving vehicle. All animals were treated day 0 and 7. A parallel (18)F-FLT PET/CT study was performed on day 0, 1, 3 and 6. Dosimetry calculations were based on a biodistribution study, where organs and tissue of interest were collected 0.5, 1.0, 2.0, 4.0 and 24h post injection of (177)Lu-DOTA-AE105. Toxicity was assessed by recording mouse weight and by H&E staining of kidneys in each treatment group.

RESULTS

uPAR-positive HT-29 xenograft was clearly visualized by PET/CT imaging using (64)Cu-DOTA-AE105. Subsequently, these xenograft transplants were locally irradiated using (177)Lu-DOTA-AE105, where a significant effect on tumor size and the number of uPAR-positive cells in the tumor was found (p<0.05). Evaluations of biodistribution and dosimetry revealed highest accumulation of radioactivity in kidneys and tumor tissue. (18)F-FLT PET/CT imaging study revealed a significant correlation between (18)F-FLT tumor uptake and efficacy of the radionuclide therapy. A histological examination of the kidneys from one animal in each treatment group did not reveal any gross abnormalities and the general performance of all treated animals also showed no indications of radioactivity-induced toxicity.

CONCLUSION

These findings document for the first time the in vivo efficacy of an uPAR-targeted radionuclide therapeutic intervention on both tumor size and its content of uPAR expressing cells thus setting the stage for future translation into clinical use.

Quantitative PET of human urokinase-type plasminogen activator receptor with 64Cu-DOTA-AE105: implications for visualizing cancer invasion

Expression levels of the urokinase-type plasminogen activator receptor (uPAR) represent an established biomarker for poor prognosis in a variety of human cancers. The objective of the present study was to explore whether noninvasive PET can be used to perform a quantitative assessment of expression levels of uPAR across different human cancer xenograft models in mice and to illustrate the clinical potential of uPAR PET in future settings for individualized therapy.

METHODS

To accomplish our objective, a linear, high-affinity uPAR peptide antagonist, AE105, was conjugated with DOTA and labeled with (64)Cu ((64)Cu-DOTA-AE105). Small-animal PET was performed in 3 human cancer xenograft mice models, expressing different levels of human uPAR, and the tumor uptake was correlated with the uPAR expression level determined by uPAR enzyme-linked immunosorbent assay. The tumor uptake pattern of this tracer was furthermore compared with (18)F-FDG uptake, and finally the correlation between sensitivity toward 5-fluorouracil therapy and uPAR expression level was investigated.

RESULTS

The uPAR-targeting PET tracer was produced in high purity and with high specific radioactivity. A significant correlation between tumor uptake of (64)Cu-DOTA-AE105 and uPAR expression was found (R(2) = 0.73; P < 0.0001) across 3 cancer xenografts, thus providing a strong argument for specificity. A significantly different uptake pattern of (64)Cu-DOTA-AE105, compared with that of (18)F-FDG, was observed, thus emphasizing the additional information that can be obtained on tumor biology using (64)Cu-DOTA-AE105 PET. Furthermore, a significant correlation between baseline uPAR expression and sensitivity toward 5-fluorouracil was revealed, thus illustrating the possible potentials of uPAR PET in a clinical setting.

CONCLUSION

Our results clearly demonstrate that the peptide-based PET tracer (64)Cu-DOTA-AE105 enables the noninvasive quantification of uPAR expression in tumors in vivo, thus emphasizing its potential use in a clinical setting to detect invasive cancer foci and for individualized cancer therapy.

Mimicry of the regulatory role of urokinase in lamellipodia formation by introduction of a non-native interdomain disulfide bond in its receptor

The high-affinity interaction between the urokinase-type plasminogen activator (uPA) and its glycolipid-anchored receptor (uPAR) plays a regulatory role for both extravascular fibrinolysis and uPAR-mediated adhesion and migration on vitronectin-coated surfaces. We have recently proposed that the adhesive function of uPAR is allosterically regulated via a "tightening" of its three-domain structure elicited by uPA binding. To challenge this proposition, we redesigned the uPAR structure to limit its inherent conformational flexibility by covalently tethering domains DI and DIII via a non-natural interdomain disulfide bond (uPAR(H47C-N259C)). The corresponding soluble receptor has 1) a smaller hydrodynamic volume, 2) a higher content of secondary structure, and 3) unaltered binding kinetics towards uPA. Most importantly, the purified uPAR(H47C-N259C) also displays a gain in affinity for the somatomedin B domain of vitronectin compared with uPAR(wt), thus recapitulating the improved affinity that accompanies uPA-uPAR(wt) complex formation. This functional mimicry is, intriguingly, operational also in a cellular setting, where it controls lamellipodia formation in uPAR-transfected HEK293 cells adhering to vitronectin. In this respect, the engineered constraint in uPAR(H47C-N259C) thus bypasses the regulatory role of uPA binding, resulting in a constitutively active uPAR. In conclusion, our data argue for a biological relevance of the interdomain dynamics of the glycolipid-anchored uPAR on the cell surface.

Structure-based engineering of species selectivity in the interaction between urokinase and its receptor: implication for preclinical cancer therapy

The high affinity interaction between the urokinase-type plasminogen activator (uPA) and its glycolipid-anchored receptor (uPAR) is decisive for cell surface-associated plasminogen activation. Because plasmin activity controls fibrinolysis in a variety of pathological conditions, including cancer and wound healing, several intervention studies have focused on targeting the uPA.uPAR interaction in vivo. Evaluations of such studies in xenotransplanted tumor models are, however, complicated by the pronounced species selectivity in this interaction. We now report the molecular basis underlying this difference by solving the crystal structure for the murine uPA.uPAR complex and demonstrate by extensive surface plasmon resonance studies that the kinetic rate constants for this interaction can be swapped completely between these orthologs by exchanging only two residues. This study not only discloses the structural basis required for a successful rational design of the species selectivity in the uPA.uPAR interaction, which is highly relevant for functional studies in mouse models, but it also suggests the possible development of general inhibitors that will target the uPA.uPAR interaction across species barriers.

Approaches to high-performance preparative chromatography of proteins

Preparative liquid chromatography is widely used for the purification of chemical and biological substances. Different from high-performance liquid chromatography for the analysis of many different components at minimized sample loading, high-performance preparative chromatography is of much larger scale and should be of high resolution and high capacity at high operation speed and low to moderate pressure drop. There are various approaches to this end. For biochemical engineers, the traditional way is to model and optimize a purification process to make it exert its maximum capability. For high-performance separations, however, we need to improve chromatographic technology itself. We herein discuss four approaches in this review, mainly based on the recent studies in our group. The first is the development of high-performance matrices, because packing material is the central component of chromatography. Progress in the fabrication of superporous materials in both beaded and monolithic forms are reviewed. The second topic is the discovery and design of affinity ligands for proteins. In most chromatographic methods, proteins are separated based on their interactions with the ligands attached to the surface of porous media. A target-specific ligand can offer selective purification of desired proteins. Third, electrochromatography is discussed. An electric field applied to a chromatographic column can induce additional separation mechanisms besides chromatography, and result in electrokinetic transport of protein molecules and/or the fluid inside pores, thus leading to high-performance separations. Finally, expanded-bed adsorption is described for process integration to reduce separation steps and process time.

Imaging of urokinase-type plasminogen activator receptor expression using a 64Cu-labeled linear peptide antagonist by microPET

PURPOSE

Malignant tumors are capable of degrading the surrounding extracellular matrix, resulting in local invasion or metastasis. Urokinase-type plasminogen activator (uPA) and its cell surface receptor (uPAR) are central molecules in one of the major protease systems involved in extracellular matrix degradation. Noninvasive imaging of this receptor in vivo with radiolabeled peptides that specifically target uPAR may therefore be useful to decipher the potential invasiveness of malignant lesions.

EXPERIMENTAL DESIGN

In this study, we developed a (64)Cu-labeled uPAR-binding peptide for positron emission tomography (PET) imaging. A linear, high-affinity uPAR-binding peptide antagonist AE105 was conjugated with 1,4,7,10-tetraazadodecane-N,N',N'',N'''-tetraacetic acid (DOTA) and labeled with (64)Cu for microPET imaging of mice bearing U87MG human glioblastoma (uPAR positive) and MDA-MB-435 human breast cancer (uPAR negative).

RESULTS

Surface plasmon resonance measurements show that AE105 with DOTA conjugated at the alpha-amino group (DOTA-AE105) has high affinity toward uPAR. microPET imaging reveals a rapid and high accumulation of (64)Cu-DOTA-AE105 in uPAR-positive U87MG tumors (10.8 +/- 1.5%ID/g at 4.5 hours, n = 3) but not in uPAR-negative MDA-MB-435 tumors (1.2 +/- 0.6%ID/g at 4.5 hours, n = 3). Specificity of this peptide-based imaging of uPAR was validated by further control experiments. First, a nonbinding variant of AE105 carrying a single amino acid replacement (Trp-->Glu) does not target U87MG tumors in vivo. Second, targeting of U87MG tumors by (64)Cu-DOTA-AE105 is specifically inhibited by a nonlabeled antagonist.

CONCLUSION

The successful demonstration of the ability of a (64)Cu labeled uPAR-specific probe to visualize uPAR expression in vivo may allow clinical translation of this class of radiopharmaceuticals for uPAR-positive cancer detection and patient stratification for uPA/uPAR system-based cancer therapy.

Mapping of the Vitronectin-binding Site on the Urokinase Receptor

The urokinase-type plasminogen activator receptor (uPAR) has been implicated as a modulator of several biochemical processes that are active during tumor invasion and metastasis, e.g. extracellular proteolysis, cell adhesion, and cell motility. The structural basis for the high affinity interaction between the urokinase-type plasminogen activator (uPA) and uPAR, which focuses cell surface-associated plasminogen activation in vivo, is now thoroughly characterized by site-directed mutagenesis studies and x-ray crystallography. In contrast, the structural basis for the interaction between uPAR and the extracellular matrix protein vitronectin, which is involved in the regulation of cell adhesion and motility, remains to be clarified. In this study, we have identified the functional epitope on uPAR that is responsible for its interaction with the full-length, extended form of vitronectin by using a comprehensive alanine-scanning library of purified single-site uPAR mutants (244 positions tested). Interestingly, the five residues identified as "hot spots" for vitronectin binding form a contiguous epitope consisting of two exposed loops connecting the central fourstranded beta-sheet in uPAR domain I (Trp(32), Arg(58), and Ile(63)) as well as a proximal region of the flexible linker peptide connecting uPAR domains I and II (Arg(91) and Tyr(92)). This binding topology provides the molecular basis for the observation that uPAR can form a ternary complex with uPA and vitronectin. Furthermore, it raises the intriguing possibility that the canonical receptor and inhibitor for uPA (uPAR and PAI-1) may have reached a convergent solution for binding to the somatomedin B domain of vitronectin.