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

Development of Cyclic Peptide-Based Radiotracers for uPAR

The urokinase plasminogen activator (uPA) system has garnered attention as a promising biomarker, with the uPA receptor (uPAR) playing a central role in system regulation and demonstrating strong associations with tumorigenicity, invasion, and metastasis. Radioligands targeting uPAR have emerged as powerful tools for the early diagnosis and treatment of malignant tumors. In this study, we developed and evaluated three novel cyclic peptide-based positron emission tomography (PET) radioligands, denoted as [⁶⁴Cu]CARP-1, [⁶⁴Cu]CARP-2, and [⁶⁴Cu]CARP-3, for uPAR imaging. These radioligands differ in the chiral configuration of their disulfide bond crosslinkers, which influences their binding ability and pharmacokinetic profiles. Among the three, [⁶⁴Cu]CARP-2 demonstrated optimal tumor radioactivity accumulation, specificity, and favorable pharmacokinetics in an MC38 tumor-bearing mouse model. Compared to [⁶⁴Cu]DOTA-AE105, a well-characterized radiotracer currently under clinical investigation, [⁶⁴Cu]CARP-2 exhibited reduced non-specific uptake and rapid clearance from normal tissues. These attributes highlight its potential as a diagnostic tool with improved imaging accuracy. The promising preclinical performance of [⁶⁴Cu]CARP-2 underscores its potential for further clinical investigation as a uPAR-targeting radiotracer.

Prospective phase II trial of [68Ga]Ga-NOTA-AE105 uPAR-PET/MRI in patients with primary gliomas: Prognostic value and Implications for uPAR-targeted Radionuclide Therapy

BACKGROUND

Treatment of patients with low-grade and high-grade gliomas is highly variable due to the large difference in survival expectancy. New non-invasive tools are needed for risk stratification prior to treatment. The urokinase plasminogen activator receptor (uPAR) is expressed in several cancers, associated with poor prognosis and may be non-invasively imaged using uPAR-PET. We aimed to investigate the uptake of the uPAR-PET tracer [68Ga]Ga-NOTA-AE105 in primary gliomas and establish its prognostic value regarding overall survival (OS), and progression-free survival (PFS). Additionally, we analyzed the proportion of uPAR-PET positive tumors to estimate the potential number of candidates for future uPAR-PRRT.

METHODS

In a prospective phase II clinical trial, 24 patients suspected of primary glioma underwent a dynamic 60-min PET/MRI following the administration of approximately 200 MBq (range: 83-222 MBq) [68Ga]Ga-NOTA-AE105. Lesions were considered uPAR positive if the tumor-to-background ratio, calculated as the ratio of TumorSUVmax-to-Normal-BrainSUVmean tumor-SUVmax-to-background-SUVmean, was ≥ 2.0. The patients were followed over time to assess OS and PFS and stratified into high and low uPAR expression groups based on TumorSUVmax.

RESULTS

Of the 24 patients, 16 (67%) were diagnosed with WHO grade 4 gliomas, 6 (25%) with grade 3, and 2 (8%) with grade 2. Two-thirds of all patients (67%) presented with uPAR positive lesions and 94% grade 4 gliomas. At median follow up of 18.8 (2.1-45.6) months, 19 patients had disease progression and 14 had died. uPAR expression dichotomized into high and low, revealed significant worse prognosis for the high uPAR group for OS and PFS with HR of 14.3 (95% CI, 1.8-112.3; P = 0.011), and HR of 26.5 (95% CI, 3.3-214.0; P = 0.0021), respectively. uPAR expression as a continuous variable was associated with worse prognosis for OS and PFS with HR of 2.7 (95% CI, 1.5-4.8; P = 0.0012), and HR of 2.5 (95% CI, 1.5-4.2; P = 0.00073), respectively.

CONCLUSIONS

The majority of glioma patients and almost all with grade 4 gliomas displayed uPAR positive lesions underlining the feasibility of 68Ga-NOTA-AE105 PET/MRI in gliomas. High uPAR expression is significantly correlated with worse survival outcomes for patients. Additionally, the high proportion of uPAR positive gliomas underscores the potential of uPAR-targeted radionuclide therapy in these patients.

TRAIL REGISTRATION

EudraCT No: 2016-002417-21; the Scientific Ethics Committee: H-16,035,303; the Danish Data Protection Agency: 2012-58-0004; clinical trials registry: NCT02945826, 26Oct2016, URL: https://classic.

CLINICALTRIALS

gov/ct2/show/NCT02945826 .

Improved Positron Emission Tomography Quantification: Evaluation of a Maximum-Likelihood Scatter Scaling Algorithm

Incorrect scatter scaling of positron emission tomography (PET) images can lead to halo artifacts, quantitative bias, or reconstruction failure. Tail-fitted scatter scaling (TFSS) possesses performance limitations in multiple cases. This study aims to investigate a novel method for scatter scaling: maximum-likelihood scatter scaling (MLSS) in scenarios where TFSS tends to induce artifacts or are observed to cause reconstruction abortion. [68Ga]Ga-RGD PET scans of nine patients were included in cohort 1 in the scope of investigating the reduction of halo artifacts relative to the scatter estimation method. PET scans of 30 patients administrated with [68Ga]Ga-uPAR were included in cohort 2, used for an evaluation of the robustness of MLSS in cases where TFSS-integrated reconstructions are observed to fail. A visual inspection of MLSS-corrected images scored higher than TFSS-corrected reconstructions of cohort 1. The quantitative investigation near the bladder showed a relative difference in tracer uptake of up to 94.7%. A reconstruction of scans included in cohort 2 resulted in failure in 23 cases when TFSS was used. The lesion uptake values of cohort 2 showed no significant difference. MLSS is suggested as an alternative scatter-scaling method relative to TFSS with the aim of reducing halo artifacts and a robust reconstruction process.

uPAR (PLAUR) Marks Two Intra-Tumoral Subtypes of Glioblastoma: Insights from Single-Cell RNA Sequencing

Urokinase plasminogen activator receptor (uPAR) encoded by the PLAUR gene is known as a clinical marker for cell invasiveness in glioblastoma multiforme (GBM). It is additionally implicated in various processes, including angiogenesis and inflammation within the tumor microenvironment. However, there has not been a comprehensive study that depicts the overall functions and molecular cooperators of PLAUR with respect to intra-tumoral subtypes of GBM. Using single-cell RNA sequencing data from 37 GBM patients, we identified PLAUR as a marker gene for two distinct subtypes in GBM. One subtype is featured by inflammatory activities and the other subtype is marked by ECM remodeling processes. Using the whole-transcriptome data from single cells, we are able to uncover the molecular cooperators of PLAUR for both subtypes without presuming biological pathways. Two protein networks comprise the molecular context of PLAUR, with each of the two subtypes characterized by a different dominant network. We concluded that targeting PLAUR directly influences the mechanisms represented by these two protein networks, regardless of the subtype of the targeted cell.

Recent Progress in Fluorescent Probes for Real-Time Monitoring of Glioblastoma

Treating glioblastoma (GBM) by resecting to a large extent can prolong a patient's survival by controlling the tumor cells, but excessive resection may produce postoperative complications by perturbing the brain structures. Therefore, various imaging procedures have been employed to successfully diagnose and resect with utmost caution and to protect vital structural or functional features. Fluorescence tagging is generally used as an intraoperative imaging technique in glioma cells in collaboration with other surgical tools such as MRI and navigation methods. However, the existing fluorescent probes may have several limitations, including poor selectivity, less photostability, false signals, and intraoperative re-administration when used in clinical and preclinical studies for glioma surgery. The involvement of smart fluorogenic materials, specifically fluorescent dyes, and biomarker-amended cell-penetrable fluorescent probes have noteworthy advantages for precise glioma imaging. This review outlines the contemporary advancements of fluorescent probes for imaging glioma cells along with their challenges and visions, with the anticipation to develop next-generation smart glioblastoma detection modalities.

Recent Advances in Targeting the Urokinase Plasminogen Activator with Nanotherapeutics

The aberrant proteolytic landscape of the tumor microenvironment is a key contributor of cancer progression. Overexpression of urokinase plasminogen activator (uPA) and/or its associated cell-surface receptor (uPAR) in tumor versus normal tissue is significantly associated with worse clinicopathological features and poorer patient survival across multiple cancer types. This is linked to mechanisms that facilitate tumor cell invasion and migration, via direct and downstream activation of various proteolytic processes that degrade the extracellular matrix─ultimately leading to metastasis. Targeting uPA has thus long been considered an attractive anticancer strategy. However, poor bioavailability of several uPA-selective small-molecule inhibitors has limited early clinical progress. Nanodelivery systems have emerged as an exciting method to enhance the pharmacokinetic (PK) profile of existing chemotherapeutics, allowing increased circulation time, improved bioavailability, and targeted delivery to tumor tissue. Combining uPA inhibitors with nanoparticle-based delivery systems thus offers a remarkable opportunity to overcome existing PK challenges associated with conventional uPA inhibitors, while leveraging potent candidates into novel targeted nanotherapeutics for an improved anticancer response in uPA positive tumors.

Activatable Fluorescent Probes Targeting Urokinase-Type Plasminogen Activator Receptor on the Cell Membrane

Urokinase-type plasminogen activator receptor (uPAR) is a glycolipid-anchored protein located on the cell surface that is implicated in the promotion of metastasis. New fluorescent probes for the detection of uPAR expression that feature a rapid "turn-on" response are reported here. They consist of a donor-π-acceptor-based fluorophore conjugated with a uPAR-binding AE105 peptide. The resulting AE105-coupled uPAR-targeting probes are weakly emissive in aqueous buffer solutions; however, a fluorescence "turn-on" signal is instantly triggered upon specific binding to uPAR (KD =63.2 nM for P1 and 49.5 nM for P2), which restricts the rotational deactivation of the fluorophore. Applications of the probes were demonstrated in the imaging of uPAR overexpressed on the membrane of cancer cell and in a cell-based uPAR inhibitor assay.

A novel urokinase plasminogen activator receptor-targeted peptide-based probe for in-vivo molecular imaging of glioblastoma

AIM

The urokinase plasminogen activator receptor (uPAR) is a promising biomarker for cancer diagnosis and therapy. We herein fabricated a new type of uPAR-targeted imaging probe Al18F-NOTA-VC and preliminarily evaluated its potential application in PET imaging of the glioma model in vivo.

METHODS

Peptide VC was synthesized and identified by MALDI-TOF-MS. The IC50 between VC/precursor NOTA-VC and uPAR was then determined before the synthesis and purification of Al18F-NOTA-VC, followed by further studies of in-vitro properties of Al18F-NOTA-VC. Meanwhile, the AE105-based probe followed a similar procedure in-vitro test. Finally, the PET imaging properties, including uPAR-targeting ability and the metabolism of Al18F-NOTA-VC, were investigated.

RESULTS

The VC and NOTA-VC were obtained successfully and demonstrated a good affinity with uPAR. Followed by Al18F labeling successfully, excellent properties, including the serum stability, water solubility, and specificity of Al18F-NOTA-VC, were obtained in-vitro test compared with AE105 based probe. An excellent tumor uptake and renal excretion data of Al18F-NOTA-VC were acquired from in-vivo U87MG tumor model PET imaging, consistent with the subsequent biodistribution study.

CONCLUSION

In addition to the excellent specificity and high tumor/normal tissue contrast for uPAR-targeted PET imaging of U87MG tumor, Al18F-NOTA-VC possessed promising clearance ability by renal system route. These excellent properties facilitated Al18F-NOTA-VC to be a promising imaging agent for uPAR high-expressing tumors and, thus, provided a paradigm for developing peptide-based probes for uPAR-associated disease diagnosis.

Optimization and Evaluation of Al18F Labeling Using a NOTA-or RESCA1-Conjugated AE105 Peptide Antagonist of uPAR

Fluorine-18 displays almost ideal decay properties for positron emission tomography (PET) and allows for large scale production. As such, simplified methods to radiolabel peptides with fluorine-18 are highly warranted. Chelation of aluminium fluoride-18 toward specific peptides represents one method to achieve this. With the current methods, chelation of aluminium fluoride-18 can be achieved using NOTA-conjugated peptides. However, the heating to 90-100◦C that is required for this chelation approach may be deleterious to the targeting moiety of the probe. Recently, a new chelator, RESCA1, was developed allowing Al18F chelation at room temperature. Here, we optimize the labeling procedure enabling high chelation efficacy of fluoride-18 at 22◦C, even at full batch labeling. The optimized procedure was tested by Al18F-labeling of RESCA1-AE105-a uPAR targeting peptide. NOTA-AE105 was also labeled with Al18F, and the two peptides were compared head-to-head. [18F]AlF-NOTA-AE105 and [18F]AlF-RESCA1-AE105 could be produced in equal radiochemical yields (RCY), radiochemical purities (RCP) and molar activities. Additionally, the two peptides showed comparable binding affinity to uPAR and uptake in cells expressing the uPAR, when evaluated in vitro. Overall, we found that the performances of [18F]AlF-NOTA-AE105 and [18F]AlF-RESCA1-AE105 were grossly comparable, but importantly RESCA1 can be labeled with aluminium fluoride-18 at 22◦C. Consequently, this study showed that RESCA1 is superior to NOTA with respect to Al18F chelation of temperature sensitive molecules, such as thermolabile peptides and proteins as well as that full batch chelation of RESCA1 with fluoride-18 is possible.

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.

The use of a uPAR-targeted probe for photothermal cancer therapy prolongs survival in a xenograft mouse model of glioblastoma

The development of tumor-targeted probes that can efficiently reach cancerous tissue is an important focus of preclinical research. Photothermal cancer therapy (PTT) relies on light-absorbing molecules, which are directed towards tumor tissue and irradiated with an external source of light. This light is transformed into heat, causing localized hyperthermia and tumor death. The fluorescent probe indocyanine green (ICG) is already used as an imaging agent both preclinically and in clinical settings, but its use for PTT is yet to be fully exploited due to its short retention time and non-specific tumor targeting. Therefore, increasing ICG tumor uptake is necessary to improve treatment outcome. The urokinase-type plasminogen activator receptor, uPAR, is overexpressed in multiple tumor types. ICG-Glu-Glu-AE105, consisting of the uPAR-targeting peptide AE105 conjugated to ICG, has shown great potential for fluorescence-guided surgery. In this study, ICG-Glu-Glu-AE105 was evaluated as photothermal agent in a subcutaneous mouse model of human glioblastoma. We observed that the photothermal abilities of ICG-Glu-Glu-AE105 triggered high temperatures in the tumor during PTT, leading to tumor death and prolonged survival. This confirms the potential of ICG-Glu-Glu-AE105 as photothermal agent and indicates that it could be used as an add-on to the application of the probe for fluorescence-guided surgery.

IRDye800CW labeled uPAR-targeting peptide for fluorescence-guided glioblastoma surgery: Preclinical studies in orthotopic xenografts

Glioblastoma (GBM) is a devastating cancer with basically no curative treatment. Even with aggressive treatment, the median survival is disappointing 14 months. Surgery remains the key treatment and the postoperative survival is determined by the extent of resection. Unfortunately, the invasive growth with irregular infiltrating margins complicates an optimal surgical resection. Precise intraoperative tumor visualization is therefore highly needed and molecular targeted near-infrared (NIR) fluorescence imaging potentially constitutes such a tool. The urokinase-type Plasminogen Activator Receptor (uPAR) is expressed in most solid cancers primarily at the invading front and the adjacent activated peritumoral stroma making it an attractive target for targeted fluorescence imaging. The purpose of this study was to develop and evaluate a new uPAR-targeted optical probe, IRDye800CW-AE344, for fluorescence guided surgery (FGS).

Methods: In the present study we characterized the fluorescent probe with regard to binding affinity, optical properties, and plasma stability. Further, in vivo imaging characterization was performed in nude mice with orthotopic human patient derived glioblastoma xenografts, and we performed head-to-head comparison within FGS between our probe and the traditional procedure using 5-ALA. Finally, the blood-brain barrier (BBB) penetration was characterized in a 3D BBB spheroid model.

Results: The probe effectively visualized GBM in vivo with a tumor-to-background ratio (TBR) above 4.5 between 1 to 12 h post injection and could be used for FGS of orthotopic human glioblastoma xenografts in mice where it was superior to 5-ALA. The probe showed a favorable safety profile with no evidence of any acute toxicity. Finally, the 3D BBB model showed uptake of the probe into the spheroids indicating that the probe crosses the BBB.

Conclusion: IRDye800CW-AE344 is a promising uPAR-targeted optical probe for FGS and a candidate for translation into human use.

Molecular imaging of the urokinase plasminogen activator receptor: opportunities beyond cancer

The urokinase plasminogen activator receptor (uPAR) plays a multifaceted role in almost any process where migration of cells and tissue-remodeling is involved such as inflammation, but also in diseases as arthritis and cancer. Normally, uPAR is absent in healthy tissues. By its carefully orchestrated interaction with the protease urokinase plasminogen activator and its inhibitor (plasminogen activator inhibitor-1), uPAR localizes a cascade of proteolytic activities, enabling (patho)physiologic cell migration. Moreover, via the interaction with a broad range of cell membrane proteins, like vitronectin and various integrins, uPAR plays a significant, but not yet completely understood, role in differentiation and proliferation of cells, affecting also disease progression. The implications of these processes, either for diagnostics or therapeutics, have received much attention in oncology, but only limited beyond. Nonetheless, the role of uPAR in different diseases provides ample opportunity to exploit new applications for targeting. Especially in the fields of oncology, cardiology, rheumatology, neurology, and infectious diseases, uPAR-targeted molecular imaging could offer insights for new directions in diagnosis, surveillance, or treatment options.

Molecular Targeted NIR-II Probe for Image-Guided Brain Tumor Surgery

Optical imaging strategies for improving delineation of glioblastoma (GBM) is highly desired for guiding surgeons to distinguish cancerous tissue from healthy and precious brain tissue. Fluorescence imaging (FLI) in the second near-infrared window (NIR-II) outperforms traditional NIR-I imaging with better tissue penetration, higher spatial and temporal resolution, and less auto fluorescence and scattering. Because of high expression in GBM and many other tumors, urokinase Plasminogen Activator Receptor (uPAR) is an attractive and well proven target for FLI. Herein we aim to combine the benefit of a NIR-II fluorophore with a high affinity uPAR targeting small peptide. A targeted NIR-II fluorescent probe was developed by conjugating an in-house synthesized NIR-II fluorophore, CH1055, and a uPAR targeting peptide, AE105. To characterize the in vivo distribution and targeting properties, a dynamic imaging was performed in orthotopic GBM bearing nude mice (n = 8). Additionally, fluorescence guided surgery of orthotopic GBM was performed in living animals. CH1055-4Glu-AE105 was easily synthesized with >75% yield and >98% HPLC evaluated purity. The retention time of the probe on analytical HPLC was 15.9 min and the product was verified by mass spectrometry. Dynamic imaging demonstrated that the uPAR targeting probe visualized orthotopic GBM through the intact skull with a tumor-to-background ratio (TBR) of 2.7 peaking at 96 h. Further, the orthotopic GBM was successfully resected in small animals guided by the NIR-II FLI. By using a small uPAR targeting NIR-II probe, FLI allows us to specifically image and detect GBM. A real-time imaging setup further renders FLI guided tumor resection, and the probe developed in this work is a promising candidate for clinical translation.

Noninvasive Detection of HER2 Expression in Gastric Cancer by 64Cu-NOTA-Trastuzumab in PDX Mouse Model and in Patients

The purpose of this study was to establish the quality control and quantify the novel ⁶⁴Cu-NOTA-Trastuzumab in gastric cancer patient-derived xenografts (PDX) mice models and patients by applying the molecular imaging technique. Trastuzumab was labeled with ⁶⁴Cu using NCS-Bz-NOTA as bifunctional chelator, and hIgG1 was labeled with the same procedures as a negative control agent. HER2-positive (case 176, n = 12) and HER2-negative (case 168, n = 3) PDX models were established and validated by Western blot, DNA amplification, and immunohistochemistry (IHC). Both models were conducted for micro-PET imaging by tail injection of 18.5 MBq of ⁶⁴Cu-NOTA-Trastuzumab or ⁶⁴Cu-NOTA-hIgG1. Radioprobe uptake in tumor and main organs was quantified by region of interested (ROI) analysis of the micro-PET images and autoradiography. Finally, gastric cancer patients were enrolled in preliminary ⁶⁴Cu-NOTA-Trastuzumab PET/CT scans. NOTA-Trastuzumab was efficiently radiolabeled with ⁶⁴Cu over a 99% radiochemical purity and 17.5 GBq/μmol specific activity. The immune activity was preserved as the nonmodified antibody, and the radiopharmaceutical proved to be stable for up to 5 half-decay lives of ⁶⁴Cu both in vitro and in vivo. Two serials of PDX gastric cancer models were successfully established: case 176 for HER2 positive and case 168 for HER2 negative. In micro-PET imaging studies, ⁶⁴Cu-NOTA-Trastuzumab exhibits a significant higher tumor uptake (11.45 ± 0.42 ID%/g) compared with ⁶⁴Cu-NOTA-IgG1 (3.25 ± 0.28 ID%/g, n = 5, p = 0.0004) at 36 h after intravenous injection. Lower level uptake of ⁶⁴Cu-NOTA-Trastuzumab (6.35 ± 0.48 ID%/g) in HER2-negative PDX tumor models further confirmed specific binding of the radioprobe. Interestingly, the coinjection of 2.0 mg of Trastuzumab (15.52 ± 1.97 ID%/g) or 2.0 mg of hIgG1 (15.64 ± 3.54 ID%/g) increased the ⁶⁴Cu-NOTA-Trastuzumab tumor uptake in PDX tumor (HER2⁺) models compared with ⁶⁴Cu-NOTA-Trastuzumab alone ( p < 0.05) at 36 h postinjection. There were good correlations between micro-PET images and IHC ( n = 4) and autoradiography in PDX (HER2⁺) tumor tissues. Therefore, ⁶⁴Cu-NOTA-Trastuzumab successfully translated to clinical PET imaging, and ⁶⁴Cu-NOTA-Trastuzumab PET/CT scan in gastric cancer patients showed good detection ability. In conclusion, we reported quality control and application of novel ⁶⁴Cu-NOTA-Trastuzumab for HER2 expression in PDX gastric cancer mice models and gastric cancer patients. Moreover, ⁶⁴Cu-NOTA-Trastuzumab holds great potential for noninvasive PET detection, staging, and follow-up of HER2 expression in gastric cancer.

Antibody-based PET of uPA/uPAR signaling with broad applicability for cancer imaging

Mounting evidence suggests that the urokinase plasminogen activator (uPA) and its receptor (uPAR) play a central role in tumor progression. The goal of this study was to develop an 89Zr-labeled, antibody-based positron emission tomography (PET) tracer for quantitative imaging of the uPA/uPAR system. An anti-uPA monoclonal antibody (ATN-291) was conjugated with a deferoxamine (Df) derivative and subsequently labeled with 89Zr. Flow cytometry, microscopy studies, and competitive binding assays were conducted to validate the binding specificity of Df-ATN-291 against uPA. PET imaging with 89Zr-Df-ATN-291 was carried out in different tumors with distinct expression levels of uPA. Biodistribution, histology examination, and Western blotting were performed to correlate tumor uptake with uPA or uPAR expression. ATN-291 retained uPA binding affinity and specificity after Df conjugation. 89Zr-labeling of ATN-291 was achieved in good radiochemical yield and high specific activity. Serial PET imaging demonstrated that, in most tumors studied (except uPA- LNCaP), the uptake of 89Zr-Df-ATN-291 was higher compared to major organs at 120 h post-injection, providing excellent tumor contrast. The tumor-to-muscle ratio of 89Zr-Df-ATN-291 in U87MG was as high as 45.2 ± 9.0 at 120 h p.i. In vivo uPA specificity of 89Zr-Df-ATN-291 was confirmed by successful pharmacological blocking of tumor uptake with ATN-291 in U87MG tumors. Although the detailed mechanisms behind in vivo 89Zr-Df-ATN-291 tumor uptake remained to be further elucidated, quantitative PET imaging with 89Zr-Df-ATN-291 in tumors can facilitate oncologists to adopt more relevant cancer treatment planning.

uPAR-targeted optical near-infrared (NIR) fluorescence imaging and PET for image-guided surgery in head and neck cancer: proof-of-concept in orthotopic xenograft model

Purpose

Urokinase-like Plasminogen Activator Receptor (uPAR) is overexpressed in a variety of carcinoma types, and therefore represents an attractive imaging target. The aim of this study was to assess the feasibility of two uPAR-targeted probes for PET and fluorescence tumor imaging in a human xenograft tongue cancer model.

Experimental design and results

Tumor growth of tongue cancer was monitored by bioluminescence imaging (BLI) and MRI. Either ICG-Glu-Glu-AE105 (fluorescent agent) or ⁶⁴Cu-DOTA-AE105 (PET agent) was injected systemically, and fluorescence imaging or PET/CT imaging was performed. Tissue was collected for micro-fluorescence imaging and histology. A clear fluorescent signal was detected in the primary tumor with a mean in vivo tumor-to-background ratio of 2.5. Real-time fluorescence-guided tumor resection was possible, and sub-millimeter tumor deposits could be localized. Histological analysis showed co-localization of the fluorescent signal, uPAR expression and tumor deposits. In addition, the feasibility of uPAR-guided robotic cancer surgery was demonstrated. Also, uPAR-PET imaging showed a clear and localized signal in the tongue tumors.

Conclusions

This study demonstrated the feasibility of combining two uPAR-targeted probes in a preclinical head and neck cancer model. The PET modality provided preoperative non-invasive tumor imaging and the optical modality allowed for real-time fluorescence-guided tumor detection and resection. Clinical translation of this platform seems promising.

UPAR targeted molecular imaging of cancers with small molecule-based probes

Molecular imaging can allow the non-invasive characterization and measurement of biological and biochemical processes at the molecular and cellular levels in living subjects. The imaging of specific molecular targets that are associated with cancers could allow for the earlier diagnosis and better treatment of diseases. Small molecule-based probes play prominent roles in biomedical research and have high clinical translation ability. Here, with an emphasis on small molecule-based probes, we review some recent developments in biomarkers, imaging techniques and multimodal imaging in molecular imaging and highlight the successful applications for molecular imaging of cancers.

Improved positron emission tomography imaging of glioblastoma cancer using novel 68Ga-labeled peptides targeting the urokinase-type plasminogen activator receptor (uPAR)

The urokinase-type plasminogen activator receptor (uPAR) is overexpressed in several cancers including glioblastoma (GBM) and is an established biomarker for metastatic potential. The uPAR-targeting peptide AE105-NH₂ (Ac-Asp-Cha-Phe-(D)Ser-(D)Arg-Tyr-Leu-Trp-Ser-CONH₂) is a promising candidate for non-invasive positron emission tomography (PET) imaging of uPAR. Despite the optimal physical properties of ⁶⁸Ga for peptide-based PET imaging, low tumor uptakes have previously been reported using ⁶⁸Ga-labeled AE105-NH₂-based tracers. In an attempt to optimize the tumor uptake, we developed three novel tracers with alkane (AOC) and polyethylene glycol (PEG) spacers inserted between AE105-NH₂ and the radio metal chelator 2-(4,7-bis(carboxymethyl)-1,4,7-triazonan-1-yl)pentanedioic acid (NODAGA). The resulting tracers NODAGA-AOC-AE105-NH₂, NODAGA-PEG₃-AE105-NH₂ and NODAGA-PEG₈-AE105-NH₂ were compared to the non-spacer version, NODAGA-AE105-NH₂. Following radiolabeling with ⁶⁸Ga, we evaluated the in vitro and in vivo performance in mice bearing subcutaneous tumors derived from the uPAR-expressing human GBM cell line U87MG. In vivo PET/CT imaging showed that introduction of PEG spacers more than doubled the in vivo tumor uptake after 1 h compared with the non-spacer version: ⁶⁸Ga-NODAGA-PEG₃-AE105-NH₂ (2.08 ± 0.37%ID/g) and ⁶⁸Ga-NODAGA-PEG₈-AE105-NH₂ (2.01 ± 0.22%ID/g) vs. ⁶⁸Ga-NODAGA-AE105-NH₂ (0.70 ± 0.40%ID/g), p < 0.05. In addition, ⁶⁸Ga-NODAGA-PEG₈-AE105-NH₂ showed significantly higher (p < 0.05) tumor-to-background contrast (3.68 ± 0.23) than the other tracers. The specific tumor-targeting property of ⁶⁸Ga-NODAGA-PEG₈-AE105-NH₂ was established by effectively blocking the tumor uptake with co-injection of unlabeled AE105-NH₂ (1 h: unblocked 2.01 ± 0.22%ID/g vs. blocked 1.24 ± 0.09%ID/g, p < 0.05). Ex vivo biodistribution confirmed the improved tumor uptakes of the PEG-modified tracers. ⁶⁸Ga-NODAGA-PEG₈-AE105-NH₂ is thus a promising candidate for human translation for PET imaging of GBM.

Imaging of Prostate Cancer Using Urokinase-Type Plasminogen Activator Receptor PET

Urokinase-type plasminogen activator receptor (uPAR) overexpression is an important biomarker for aggressiveness in cancer including prostate cancer (PC) and provides independent clinical information in addition to prostate-specific antigen and Gleason score. This article focuses on uPAR PET as a new diagnostic and prognostic imaging biomarker in PC. Many preclinical uPAR-targeted PET imaging studies using AE105 in cancer models have been undertaken with promising results. A major breakthrough was obtained with the recent human translation of uPAR PET in using ⁶⁴Cu- and ⁶⁸Ga-labelled versions of AE105, respectively. Clinical results from patients with PC included in these studies are encouraging and support continuation with large-scale clinical trials.

Safety, Dosimetry, and Tumor Detection Ability of 68Ga-NOTA-AE105: First-in-Human Study of a Novel Radioligand for uPAR PET Imaging

The overexpression of urokinase-type plasminogen activator receptors (uPARs) represents an established biomarker for aggressiveness in most common malignant diseases, including breast cancer (BC), prostate cancer (PC), and urinary bladder cancer (UBC), and is therefore an important target for new cancer therapeutic and diagnostic strategies. In this study, uPAR PET imaging using a ⁶⁸Ga-labeled version of the uPAR-targeting peptide (AE105) was investigated in a group of patients with BC, PC, and UBC. The aim of this first-in-human, phase I clinical trial was to investigate the safety and biodistribution in normal tissues and uptake in tumor lesions. Methods: Ten patients (6 PC, 2 BC, and 2 UBC) received a single intravenous dose of ⁶⁸Ga-NOTA-AE105 (154 ± 59 MBq; range, 48-208 MBq). The biodistribution and radiation dosimetry were assessed by serial whole-body PET/CT scans (10 min, 1 h, and 2 h after injection). Safety assessment included measurements of vital signs with regular intervals during the imaging sessions and laboratory blood screening tests performed before and after injection. In a subgroup of patients, the in vivo stability of ⁶⁸Ga-NOTA-AE105 was determined in collected blood and urine. PET images were visually analyzed for visible tumor uptake of ⁶⁸Ga-NOTA-AE105, and SUVs were obtained from tumor lesions by manually drawing volumes of interest in the malignant tissue. Results: No adverse events or clinically detectable pharmacologic effects were found. The radioligand exhibited good in vivo stability and fast clearance from tissue compartments primarily by renal excretion. The effective dose was 0.015 mSv/MBq, leading to a radiation burden of 3 mSv when the clinical target dose of 200 MBq was used. In addition, radioligand accumulation was seen in primary tumor lesions as well as in metastases. Conclusion: This first-in-human, phase I clinical trial demonstrates the safe use and clinical potential of ⁶⁸Ga-NOTA-AE105 as a new radioligand for uPAR PET imaging in cancer patients.

PET imaging of urokinase-type plasminogen activator receptor (uPAR) in prostate cancer: current status and future perspectives

Overexpression of urokinase-type plasminogen activator receptors (uPAR) represents an important biomarker for aggressiveness in most common malignant diseases, including prostate cancer (PC). Accordingly, uPAR expression either assessed directly in malignant PC tissue or assessed directly in plasma (intact/cleaved forms)—provides independent additional clinical information to that contributed by PSA, Gleason score, and other relevant pathological and clinical parameters. In this respect, non-invasive molecular imaging by positron emission tomography (PET) offers a very attractive technology platform, which can provide the required quantitative information on the uPAR expression profile, without the need for invasive procedures and the risk of missing the target due to tumor heterogeneity. These observations support non-invasive PET imaging of uPAR in PC as a clinically relevant diagnostic and prognostic imaging method. In this review, we will focus on the recent development of uPAR PET and the relevance within prostate cancer imaging. Novel antibody and small-molecule radiotracers-targeting uPAR, including a series of uPAR-targeting PET ligands, based on the high affinity peptide ligand AE105, have been synthesized and tested in vitro and in vivo in preclinical murine xenograft models and, recently, in a first-ever clinical uPAR PET study in cancer patients, including patients with PC. In this phase I study, a high and specific uptake of the tracer ⁶⁴Cu-DOTA-AE105 was found in both primary tumors and lymph node metastases. The results are encouraging and support large-scale clinical trials to determine the utility of uPAR PET in the management of patients with PC with the goal of improving outcome.