GCPII imaging and cancer

Design, preclinical evaluation, and first-in-human PET study of [68Ga]Ga-PSFA-01: a PSMA/FAP heterobivalent tracer

PURPOSE

Prostate cancer (PCa), characterized by tumor heterogeneity, may exhibit low or absent prostate-specific membrane antigen (PSMA) expression in cancerous lesions, limiting the detection sensitivity of monospecific probes. Given that fibroblast activation protein (FAP) is frequently overexpressed in the tumor microenvironment (TME), we developed a PSMA/FAP dual-targeting tracer to address this limitation.

METHODS

The precursor (PSFA-01) was synthesized by coupling a quinolone-based FAP-targeting scaffold and EuK with HBED-CC via amide bonds. The dual-receptor-binding affinity and cell uptake of PSFA-01 and [natGa]Ga-PSFA-01 was evaluated in vitro. Micro-PET/CT imaging was performed on 22Rv1 and U87MG tumor-bearing mice. The feasibility of [68Ga]Ga-PSFA-01 PET/CT in a clinical setting was evaluated in a metastatic prostate cancer patient, and the results were compared with those of [68Ga]Ga-FAPI-04 and [68Ga]Ga-PSMA-11 PET/CT.

RESULTS

PSFA-01 and [natGa]Ga-PSFA-01 showed high affinity for both FAP and PSMA proteins (Ki = 0.14-1.02 nM). On micro-PET/CT imaging, the 22Rv1 tumor uptake of [68Ga]Ga-PSFA-01 (SUVmax = 3.89 ± 0.47) was higher than that of [68Ga]Ga-PSMA-11 (SUVmax = 2.96 ± 0.48). The U87MG tumor uptake of [68Ga]Ga-PSFA-01 was significantly higher (SUVmax = 7.29 ± 1.13) than [68Ga]Ga-FAPI-04 (SUVmax = 0.28 ± 0.12), showing tumor to muscle ratio as 12.68 ± 1.93 at 1 h p.i. On clinical trial, the primary tumor and metastatic lesions were distinctly identified by [68Ga]Ga-PSFA-01 (21 lesions), demonstrating superior performance compared to [68Ga]Ga-FAPI-04 (3 lesions) and [68Ga]Ga-PSMA-11 (13 lesions) in terms of lesion count and specificity.

CONCLUSIONS

[68Ga]Ga-PSFA-01 exhibited satisfactory PSMA and FAP dual-receptor-targeting properties both in vitro and in vivo. This study highlights the clinical feasibility of [68Ga]Ga-PSFA-01 PET/CT for detecting metastatic tumors of prostate cancer more sensitively compared to monomeric [68Ga]Ga-PSMA-11 and [68Ga]Ga-FAPI-04, which also suggests that a PSMA/FAP dual-targeted radionuclide therapy could potentially overcome challenges related to tumor heterogeneity and insufficient PSMA expression in PCa.

TRIAL REGISTRATION

Clinical trial registry NCT06387381, Registered 1 May 2024.

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 the standard of care in cancer surgeries. One of the key parameters to optimize in contrast agents 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 the positioning of the fluorophore/quencher pair on the polymer core. We found that the polymeric probes could be optimized to achieve increased overall 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.

Prostate-Specific Membrane Antigen Expression in Patients with Primary Prostate Cancer: Diagnostic and Prognostic Value in Positron Emission Tomography-Prostate-Specific Membrane Antigen

Prostate cancer represents a significant public health challenge, with its management requiring precise diagnostic and prognostic tools. Prostate-specific membrane antigen (PSMA), a cell surface enzyme overexpressed in prostate cancer cells, has emerged as a pivotal biomarker. PSMA's ability to increase the sensitivity of PET imaging has revolutionized its application in the clinical management of prostate cancer. The advancements in PET-PSMA imaging technologies and methodologies, including the development of PSMA-targeted radiotracers and optimized imaging protocols, led to diagnostic accuracy and clinical utility across different stages of prostate cancer. This highlights its superiority in staging and its comparative effectiveness against conventional imaging modalities. This paper analyzes the impact of PET-PSMA on prostate cancer management, discussing the existing challenges and suggesting future research directions. The integration of recent studies and reviews underscores the evolving understanding of PET-PSMA imaging, marking its significant but still expanding role in clinical practice. This comprehensive review serves as a crucial resource for clinicians and researchers involved in the multifaceted domains of prostate cancer diagnosis, treatment, and management.

Synthesis and Preclinical Evaluation of Urea-Based Prostate-Specific Membrane Antigen-Targeted Conjugates Labeled with 177Lu

177Lu-labeled small-molecule prostate-specific membrane antigen (PSMA) targeted tracers are therapeutic agents for metastatic castration-resistant prostate cancer. Optimizing molecular design holds the potential to further enhance the pharmacokinetic properties of PSMA-targeted agents while preserving their potent therapeutic effects. In this study, six novel N-[N-[(S)-1,3-dicarboxypropyl]carbamoyl]-(S)-l-lysine (DCL) urea-based PSMA ligand 2,2',2″,2‴-(1,4,7,10-tetraazacyclododecane-1,4,7,10-tetrayl)tetraacetic acid conjugates were synthesized. These conjugates feature polypeptide linkers containing the Phe-Phe peptide sequence and an aromatic fragment at the ε-NH-Lys group of the DCL fragment. The synthesis yielded products with satisfactory yields ranging from 60% to 72%, paving the way for their preclinical evaluation. The labeling of the new variants of urea-based PSMA inhibitors provided a radiochemical yield of over 95%. The 177Lu-labeled conjugates demonstrated specific and moderate affinity binding to PSMA-expressing human cancer cells PC3-pip in vitro and specific accumulation in PSMA-expressing xenografts in vivo. Based on the results, both the lipophilicity and the type of substituent in the linker significantly influence the binding properties of the PSMA inhibitor and its biodistribution profile. Specifically, the studied variants containing a bromine substituent or a hydroxyl group introduced into the aromatic fragment of the phenylalanyl residue in DCL exhibit higher affinities to PSMA compared to variants with only a chlorine-substituted aromatic fragment or variants without any substituents. The [177Lu]Lu-13C with the bromine substituent was characterized by the highest activity accumulation in blood, salivary glands, muscle, bone, and gastrointestinal tract and had inasmuch as an unfavorable pharmacokinetic profile. The negative charge of the carboxyl group in the phenyl moiety of the [177Lu]Lu-13A variant has demonstrated a positive effect on reducing the retention of activity in the liver and the kidneys (the ratio of tumor to kidneys was 1.3-fold). Low accumulation in normal tissues in vivo indicates that this novel PSMA-targeting inhibitor is a promising radioligand.

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.

Prostate-specific membrane antigen-positron emission tomography (PSMA-PET) of prostate cancer: current and emerging applications

Prostate-specific membrane antigen-positron emission tomography (PSMA-PET) is transforming the management of patients with prostate cancer. In appropriately selected patients, PSMA-PET offers superior sensitivity and specificity compared to conventional imaging (e.g., computed tomography and bone scintigraphy) as well as choline and fluciclovine PET, with the added benefit of consolidating bone and soft tissue evaluation into a single study. Despite being a newly available imaging tool, PSMA-PET has established indications, interpretation guidelines, and reporting criteria, which will be reviewed. The prostate cancer care team, from imaging specialists to those delivering treatment, should have knowledge of physiologic PSMA radiotracer uptake, patterns of disease spread, and the strengths and limitations of PSMA-PET. In this review, current and emerging applications of PSMA-PET, including appropriateness use criteria as well as image interpretation and pitfalls, will be provided with an emphasis on clinical implications.

Targeted Therapy for Prostate Cancer by Prostate-Specific Membrane Antigen-Targeted Small-Molecule Drug Conjugates

In the aging global population, prostate cancer is a worldwide health problem because the incidence rate of this disease increases at advanced ages. Although early-stage prostate cancer can be treated by total prostatectomy, the surgery causes side effects, such as incontinence and dysuria, that lower QOL. Once the disease progresses to metastatic castration-resistant prostate cancer (mCRPC), there are no effective chemotherapeutic agents without systematic side effects. Therefore, targeted therapies for mCPRC are urgently needed. Traditional antibody-drug conjugate treatments for prostate cancer have been tested in clinical trials and several side effects have been observed. Meanwhile, small-molecule drug conjugates (SMDCs) have certain advantages over antibody drug conjugates in terms of non-immunogenicity, reproducibility, and permeability. In this review, prostate-specific membrane antigen-targeted SMDCs for treating prostate cancer are summarized.

PSMA-Targeted PET Radiotracer [18F]DCFPyL as an Imaging Biomarker in Inflammatory Bowel Disease

Background

Prostate-specific membrane antigen (PSMA) is highly and specifically upregulated in active-inflamed mucosa of patients with inflammatory bowel disease (IBD). We hypothesized that this upregulation would be detectable using a PSMA-targeted positron emission tomography/computed tomography (PET/CT) imaging agent, [18F]DCFPyL, enabling non-invasive visualization of inflammation. A noninvasive means of detecting active inflammation would have high clinical value in localization and management of IBD.

Study

We performed [18F]DCFPyL imaging in three IBD patients with active disease. Abnormally increased gastrointestinal [18F]DCFPyL uptake was observed in areas with endoscopic, histologic, and immunohistochemical inflammation, demonstrating partial overlap of segments of bowel with abnormal [18F]DCFPyL uptake and active inflammation.

Conclusion

This study demonstrates that PSMA-targeted [18F]DCFPyL PET can effectively detect regions of inflamed mucosa in patients with IBD, suggesting its utility as a non-invasive imaging agent to assess location, extent, and disease activity in IBD.

The role of NFATc1 in the progression and metastasis of prostate cancer: A review on the molecular mechanisms and signaling pathways

A common type of cancer among men is the prostate cancer that kills many people every year. The multistage of this disease and the involvement of the vital organs of the body have reduced the life span and quality of life of the people involved and turned the treatment process into a complex one. NFATc1 biomarker contributes significantly in the diagnosis and treatment of this disease by increasing its expression in prostate cancer and helping the proliferation, differentiation, and invasion of cancer cells through different signaling pathways. NFATc1 is also able to target the metabolism of cancer cells by inserting specific oncogene molecules such as c-myc that it causes cell growth and proliferation. Bone is a common tissue where prostate cancer cells metastasize. In this regard, the activity of NFATc1, through the regulation of different signaling cascades, including the RANKL/RANK signaling pathway, in turn, increases the activity of osteoclasts, and as a result, bone tissue is gradually ruined. Using Silibinin as a medicinal plant extract can inhibit the activity of osteoclasts related to prostate cancer by targeting NFATc. Undoubtedly, NFATc1 is one of the effective oncogenes related to prostate cancer, which has the potential to put this cancer on the path of progression and metastasis. In this review, we will highlight the role of NFATc1 in the progression and metastasis of prostate cancer. Furthermore, we will summarize signaling pathways and molecular mechanism, through which NFATc1 regulates the process of prostate cancer.

Chest Wall Keloids Depicted by 18 F-Piflufolastat PET/CT Imaging

ABSTRACT

Keloids are pathological scars from exuberant fibroproliferative collagen response and excessive extracellular matrix production usually extending beyond the original wound margins. Although keloids are mostly of dermatological concern, they could be incidentally depicted on scintigraphic planar and PET/CT imaging and could mimic other types of skin diseases. The authors present a case of chest wall keloids documented on 18 F-piflufolastat PET/CT during the evaluation of prostate cancer recurrence.

Necessity of Pelvic Lymph Node Irradiation in Patients with Recurrent Prostate Cancer after Radical Prostatectomy in the PSMA PET/CT Era: A Narrative Review

The main prostate cancer (PCa) treatments include surgery or radiotherapy (with or without ADT). However, none of the suggested treatments eliminates the risk of lymph node metastases. Conventional imaging methods, including MRI and CT scanning, are not sensitive enough for the diagnosis of lymph node metastases; however, the novel imaging method, PSMA PET/CT scanning, has provided valuable information about the pelvic LN involvement in patients with recurrent PCa (RPCa) after radical prostatectomy. The high sensitivity and negative predictive value enable accurate N staging in PCa patients. In this narrative review, we summarize the evidence on the treatment and extent of radiation in prostate-only or whole-pelvis radiation in patients with positive and negative LN involvement on PSMA PET/CT scans.

Positron emission tomography imaging of lung cancer: An overview of alternative positron emission tomography tracers beyond F18 fluorodeoxyglucose

Lung cancer has been the leading cause of cancer-related mortality in China in recent decades. Positron emission tomography-computer tomography (PET/CT) has been established in the diagnosis of lung cancer. ¹⁸F-FDG is the most widely used PET tracer in foci diagnosis, tumor staging, treatment planning, and prognosis assessment by monitoring abnormally exuberant glucose metabolism in tumors. However, with the increasing knowledge on tumor heterogeneity and biological characteristics in lung cancer, a variety of novel radiotracers beyond ¹⁸F-FDG for PET imaging have been developed. For example, PET tracers that target cellular proliferation, amino acid metabolism and transportation, tumor hypoxia, angiogenesis, pulmonary NETs and other targets, such as tyrosine kinases and cancer-associated fibroblasts, have been reported, evaluated in animal models or under clinical investigations in recent years and play increasing roles in lung cancer diagnosis. Thus, we perform a comprehensive literature review of the radiopharmaceuticals and recent progress in PET tracers for the study of lung cancer biological characteristics beyond glucose metabolism.

[18F]-Labeled PARP-1 PET imaging of PSMA targeted alpha particle radiotherapy response

The growing interest and clinical translation of alpha particle (α) therapies brings with it new challenges to assess target cell engagement and to monitor therapeutic effect. Noninvasive imaging has great potential to guide α-treatment and to harness the potential of these agents in the complex environment of disseminated disease. Poly(ADP) ribose polymerase 1 (PARP-1) is among the most abundantly expressed DNA repair enzymes with key roles in multiple repair pathways-such as those induced by irradiation. Here, we used a third-generation PARP1-specific radiotracer, [18F]-PARPZ, to delineate castrate resistant prostate cancer xenografts. Following treatment with the clinically applied [225Ac]-PSMA-617, positron emission tomography was performed and correlative autoradiography and histology acquired. [18F]-PARPZ was able to distinguish treated from control (saline) xenografts by increased uptake. Kinetic analysis of tracer accumulation also suggests that the localization of the agent to sites of increased PARP-1 expression is a consequence of DNA damage response. Together, these data support expanded investigation of [18F]-PARPZ to facilitate clinical translation in the ⍺-therapy space.

Metamorphosis of prostate specific membrane antigen (PSMA) inhibitors

Prostate-specific membrane antigen (PSMA), also called glutamate carboxypeptidase II (GCP(II)), is a Zn-dependent metalloprotease that is known as a well prostate cancer indication and a potential targeting towards anti-cancer medicines and drug delivery. Because of its centrality in the diagnostics and treatment of prostate cancer, several types of inhibitors are designed with particular scaffolds. In this study, important groups of related inhibitors as well as reported experimental and computational studies are being reviewed, in which we examined three functional groups on each group of structures. The importance of computational biochemistry and the necessity of extensive research in this area on PSMA and its effective ligands are recommended.

The Importance of Prostate-Specific Membrane Antigen Expression in Carotid Body Paragangliomas

Objective:

Prostate-specific membrane antigen (PSMA) is a transmembrane protein expressed in prostate cancer. It is, however, also expressed in the neovasculature of some non-prostatic solid tumors. Carotid body paragangliomas (CBPs) are highly vascular neoplasms. In this study, we aimed to investigate the possible role of PSMA expression in CBPs. There are no studies in the literature that report to have investigated the relationship between PSMA and CBPs.

Methods:

This study is a retrospective analysis of cases diagnosed with CBP based on their demographic, clinical, radiological, surgical and immunohistochemical findings. Immunohistochemical examination results of Ki-67, S100, synaptophysin, chromogranin were retrieved from patient files. Then, the paraffin blocks of CBPs specimens, stained by PSMA-antibody by immunohistochemical methods were examined histopathologically.

Results:

The number of patients operated on for CBP was 12 (four men and eight women). Ten out of 12 specimens were suitable for staining and histopathological examination. Capsular and/or vascular invasions of tumors were seen in complicated cases. Intratumoral vascular PSMA expression was seen in all specimens except one. Extratumoral vascular PSMA expression was not detected in any of the cases. Tumoral cell PSMA staining was seen in six of ten cases.

Conclusion:

We found higher intratumoral vascular expressions of PSMA nearly in all CBPs, but we could not assess the statistical significance because of the small number of specimens. These data might be a guide for future studies that are planned for either diagnostic or therapeutic approaches to CBPs.

Proteolyzed Variant of IgG with Free C-Terminal Lysine as a Biomarker of Prostate Cancer

Simple Summary

We have discovered that immunoglobulins digested with plasmin, one of the enzymes of blood clotting cascade acquire a capability to bind to one of the chains of plasminogen. We investigate here the mechanisms and localization of such binding. We also show that levels of this digested immunoglobulin molecule are higher in patients with prostate cancer. Therefore, this digested immunoglobulin could serve as a biomarker for the detection of patients with prostate cancer from patients with benign prostate hyperplasia. We observed that the diagnostic accuracy of blood levels of digested immunoglobulins is dramatically higher than that of PSA.

Abstract

The differential diagnosis of prostate cancer is problematic due to the lack of markers with high diagnostic accuracy. We previously demonstrated the increased binding of IgG to human plasminogen (PLG) in plasma of patients with prostate cancer (PC) compared to healthy controls. Heavy and light chains of PLG (PLG-H and PLG-L) were immobilized on 96-well plates and the binding of IgG to PLG-H and PLG-L was analyzed in serum from 30 prostate cancer (PC) patients, 30 patients with benign prostatic hyperplasia (BPH) and 30 healthy controls using enzyme-linked immunosorbent assay (ELISA). Our results demonstrate that IgG from PC sera bind to PLG-H but not to PLG-L. This interaction occurred through the free IgG C-terminal lysine (Lys) that becomes exposed as a result of IgG conformational changes associated with proteolysis. Circulating levels of modified IgG with exposed C-terminal Lys (IgG-Lys) were significantly higher in PC patients than in healthy controls and in BPH. We used Receiver Operating Characteristic (ROC) analysis to calculate the sensitivity (SN) and specificity (SP) of circulating IgG-Lys for differentiating PC from BPH as 77% and 90%, respectively. The area under the curve (AUC) was 0.87. We demonstrated that the diagnostic accuracy of circulating levels of IgG-Lys is much higher than diagnostic accuracy of total PSA (tPSA).

PET/CT variants and pitfalls in prostate cancer: What you might see on PET and should never forget

2-deoxy-2-[¹⁸F]fluorodeoxyglucose (FDG) positron emission tomography (PET) gained an impressive role in the diagnostic management of many oncological diseases, even though its use in imaging prostate cancer (PC) is limited to selected cases, mostly advanced stage of PC and selection for prostate specific antigen membrane (PSMA) radioligand therapy (RLT). In the past years, several PET tracers have been developed for both staging and restaging PC. The three most employed PET molecules in daily practice are [¹¹C] or [¹⁸F]F-Choline, [¹⁸F]F-Fluciclovine (Anti-1- amino-3-[18F]Fluorocyclobutane-1-Carboxylic Acid, also known as (Anti-[¹⁸F]FACBC), [⁶⁸Ga]Ga-PSMA and recently FDA approved the first Fluorinated PSMA-based named [¹⁸F]F-DCFPyl. Each one has its own physiological and peculiarity which are worth exploring. Moreover, an increasing number of case reports and studies have reported tracers' variants, pitfalls, or even non-prostatic diseases (benign and malignant) incidentally detected. In prostate oncology, PET can be performed with several indications in different stages of disease, as highlighted in the EAU Guidelines on PC. A correct scan interpretation depends on the knowledge of both the physiological distribution of the tracers and the uptake of possible variants and pitfalls. The aim of this critical review is to provide a comprehensive knowledge of physiological distribution of these three tracers, as well as an updated overview of variants and pitfalls.

A prostate-specific membrane antigen (PSMA)-targeted prodrug with a favorable in vivo toxicity profile

Prostate-specific membrane antigen (PSMA) is a promising target for the treatment of advanced prostate cancer (PC) and various solid tumors. Although PSMA-targeted radiopharmaceutical therapy (RPT) has enabled significant imaging and prostate-specific antigen (PSA) responses, accumulating clinical data are beginning to reveal certain limitations, including a subgroup of non-responders, relapse, radiation-induced toxicity, and the need for specialized facilities for its administration. To date non-radioactive attempts to leverage PSMA to treat PC with antibodies, nanomedicines or cell-based therapies have met with modest success. We developed a non-radioactive prodrug, SBPD-1, composed of a small-molecule PSMA-targeting moiety, a cancer-selective cleavable linker, and the microtubule inhibitor monomethyl auristatin E (MMAE). SBPD-1 demonstrated high binding affinity to PSMA (K_i = 8.84 nM) and selective cytotoxicity to PSMA-expressing PC cell lines (IC₅₀ = 3.90 nM). SBPD-1 demonstrated a significant survival benefit in two murine models of human PC relative to controls. The highest dose tested did not induce toxicity in immunocompetent mice. The high specific targeting ability of SBPD-1 to PSMA-expressing tumors and its favorable toxicity profile warrant its further development.

[68Ga]Ga-PSMA-11 in prostate cancer: a comprehensive review

Imaging of the prostate-specific membrane antigen (PSMA) has become an important tool for managing patients with recurrent prostate cancer, and one of the most frequently employed radiopharmaceuticals is [68Ga]Ga-PSMA-11. Herein, we summarize the preclinical development and the clinical applications of [68Ga]Ga-PSMA-11 and present side-by-side comparisons with other radiopharmaceuticals or imaging modalities, in order to assist imagers and clinicians in recommending, performing, and interpreting the results of [68Ga]Ga-PSMA-11 PET scans in patients with prostate cancer.

Toward the Discovery and Development of PSMA Targeted Inhibitors for Nuclear Medicine Applications

BACKGROUND

The rising incidence rate of prostate cancer (PCa) has promoted the development of new diagnostic and therapeutic radiopharmaceuticals during the last decades. Promising improvements have been achieved in clinical practice using prostate specific membrane antigen (PSMA) labeled agents, including specific antibodies and small molecular weight inhibitors. Focusing on molecular docking studies, this review aims to highlight the progress in the design of PSMA targeted agents for a potential use in nuclear medicine.

RESULTS

Although the first development of radiopharmaceuticals able to specifically recognize PSMA was exclusively oriented to macromolecule protein structure such as radiolabeled monoclonal antibodies and derivatives, the isolation of the crystal structure of PSMA served as the trigger for the synthesis and the further evaluation of a variety of low molecular weight inhibitors. Among the nuclear imaging probes and radiotherapeutics that have been developed and tested till today, labeled Glutamate-ureido inhibitors are the most prevalent PSMA-targeting agents for nuclear medicine applications.

CONCLUSION

PSMA represents for researchers the most attractive target for the detection and treatment of patients affected by PCa using nuclear medicine modalities. [99mTc]MIP-1404 is considered the tracer of choice for SPECT imaging and [68Ga]PSMA-11 is the leading diagnostic for PET imaging by general consensus. [18F]DCFPyL and [18F]PSMA-1007 are clearly the emerging PET PSMA candidates for their great potential for a widespread commercial distribution. After paving the way with new imaging tools, academic and industrial R&Ds are now focusing on the development of PSMA inhibitors labeled with alpha or beta minus emitters for a theragnostic application.

Gallium-68 prostate-specific membrane antigen ([68Ga]Ga-PSMA-11) PET for imaging of thyroid cancer: a feasibility study

Background

Prostate-specific membrane antigen (PSMA) is expressed in the microvasculature of thyroid cancer. This suggests the potential use of PSMA as a diagnostic agent in patients with aggressive forms of thyroid cancer. The purpose of the current study was to determine the feasibility and utility of [⁶⁸Ga]Ga-PSMA-11 PET/MRI in thyroid cancer patients.

Methods

Eligible patients for this prospective pilot study were adults with a history of pathology-proven thyroid cancer who had abnormal radiotracer uptake on an 2-[¹⁸F]FDG PET and/or ¹³¹I scintigraphy performed in the 12 months prior to study enrollment. Patients underwent a [⁶⁸Ga]Ga-PSMA-11 PET/MRI, and comparison was made to the prior qualifying 2-[¹⁸F]FDG PET CT/MRI for lesion location and relative intensity.

Results

Twelve patients underwent [⁶⁸Ga]Ga-PSMA-11 PET/MRI, one of which was excluded from analysis due to debulking surgery prior to the PSMA PET. Of the remaining patients, 7/11 had differentiated disease (3 papillary, 2 follicular, 2 Hurthle cell) and 4/11 had dedifferentiated disease (2 poorly differentiated papillary, 2 anaplastic). Out of 43 lesions, 41 were visually 2-[¹⁸F]FDG positive (uptake greater than background, detection rate 95.3%) and 28 were PSMA positive (uptake greater than background, detection rate 65.1%). Uptake was heterogeneous between patients, and in some cases within patients. 3/11 patients (1 poorly differentiated papillary, 2 follicular) had PSMA uptake which was greater than FDG uptake. For the remaining 8 patients, 2-[¹⁸F]FDG uptake was greater than PSMA. Using one eligibility guideline in the prostate cancer literature for PSMA radioligand therapy (RLT), 8/11 could be considered eligible for possible future PSMA RLT. This was not predictable based on thyroid cancer subtype.

Conclusions

[⁶⁸Ga]Ga-PSMA-11 PET demonstrated lower detection rate when compared to 2-[¹⁸F]FDG PET for thyroid cancer lesion visualization. Thyroid cancer subtype alone may not be sufficient to predict PSMA uptake, and radiotracer uptake may vary between patients and even within patients.

Engineered Fragments of the PSMA-Specific 5D3 Antibody and Their Functional Characterization

Prostate-Specific Membrane Antigen (PSMA) is an established biomarker for the imaging and experimental therapy of prostate cancer (PCa), as it is strongly upregulated in high-grade primary, androgen-independent, and metastatic lesions. Here, we report on the development and functional characterization of recombinant single-chain Fv (scFv) and Fab fragments derived from the 5D3 PSMA-specific monoclonal antibody (mAb). These fragments were engineered, heterologously expressed in insect S2 cells, and purified to homogeneity with yields up to 20 mg/L. In vitro assays including ELISA, immunofluorescence and flow cytometry, revealed that the fragments retain the nanomolar affinity and single target specificity of the parent 5D3 antibody. Importantly, using a murine xenograft model of PCa, we verified the suitability of fluorescently labeled fragments for in vivo imaging of PSMA-positive tumors and compared their pharmacokinetics and tissue distribution to the parent mAb. Collectively, our data provide an experimental basis for the further development of 5D3 recombinant fragments for future clinical use.

The Role of PET-CT Imaging in Prostate Cancer

Prostate cancer is the commonest malignancy to affect men in the United Kingdom. Extraprostatic disease detection at staging and in the setting of biochemical recurrence is essential in determining treatment strategy. Conventional imaging including computed tomography and bone scintigraphy are limited in their ability to detect sites of loco-regional nodal and metastatic bone disease, particularly at clinically relevant low prostate-specific antigen levels. The use of positron emission tomography-computed tomography has helped overcome these deficiencies and is leading a paradigm shift in the management of prostate cancer using a wide range of radiopharmaceuticals. Their mechanisms of action, utility in both staging and biochemical recurrence, and comparative strengths and weaknesses will be covered in this article.

Auger radiopharmaceutical therapy targeting prostate-specific membrane antigen in a micrometastatic model of prostate cancer

Auger radiopharmaceutical therapy is a promising strategy for micrometastatic disease given high linear energy transfer and short range in tissues, potentially limiting normal tissue toxicities. We previously demonstrated anti-tumor efficacy of a small-molecule Auger electron emitter targeting the prostate-specific membrane antigen (PSMA), 2-[3-[1-carboxy-5-(4-[¹²⁵I]iodo-benzoylamino)-pentyl]-ureido]-pentanedioic acid), or ¹²⁵I-DCIBzL, in a mouse xenograft model. Here, we investigated the therapeutic efficacy, long-term toxicity, and biodistribution of ¹²⁵I-DCIBzL in a micrometastatic model of prostate cancer (PC).

Methods: To test the therapeutic efficacy of ¹²⁵I-DCIBzL in micrometastatic PC, we used a murine model of human metastatic PC in which PSMA+ PC3-ML cells expressing firefly luciferase were injected intravenously in NSG mice to form micrometastatic deposits. One week later, 0, 0.37, 1.85, 3.7, 18.5, 37, or 111 MBq of ¹²⁵I-DCIBzL was administered (intravenously). Metastatic tumor burden was assessed using bioluminescence imaging (BLI). Long-term toxicity was evaluated via serial weights and urinalysis of non-tumor-bearing mice over a 12-month period, as well as final necropsy.

Results: In the micrometastatic PC model, activities of 18.5 MBq ¹²⁵I-DCIBzL and above significantly delayed development of detectable metastatic disease by BLI and prolonged survival in mice. Gross metastases were detectable in control mice and those treated with 0.37-3.7 MBq ¹²⁵I-DCIBzL at a median of 2 weeks post-treatment, versus 4 weeks for those treated with 18.5-111 MBq ¹²⁵I-DCIBzL (P<0.0001 by log-rank test). Similarly, treatment with ≥18.5 MBq ¹²⁵I-DCIBzL yielded a median survival of 11 weeks, compared with 6 weeks for control mice (P<0.0001). At 12 months, there was no appreciable toxicity via weight, urinalysis, or necropsy evaluation in mice treated with any activity of ¹²⁵I-DCIBzL, which represents markedly less toxicity than the analogous PSMA-targeted α-particle emitter. Macro-to-microscale dosimetry modeling demonstrated lower absorbed dose in renal cell nuclei versus tumor cell nuclei due to lower levels of drug uptake and cellular internalization in combination with the short range of Auger emissions.

Conclusion: PSMA-targeted radiopharmaceutical therapy with the Auger emitter ¹²⁵I-DCIBzL significantly delayed development of detectable metastatic disease and improved survival in a micrometastatic model of PC, with no long-term toxicities noted at 12 months, suggesting a favorable therapeutic ratio for treatment of micrometastatic PC.

PSMA ADC monotherapy in patients with progressive metastatic castration-resistant prostate cancer following abiraterone and/or enzalutamide: Efficacy and safety in open-label single-arm phase 2 study

BACKGROUND

Prostate-specific membrane antigen (PSMA) is a well-established therapeutic and diagnostic target overexpressed in both primary and metastatic prostate cancers. PSMA antibody-drug conjugate (PSMA ADC) is a fully human immunoglobulin G1 anti-PSMA monoclonal antibody conjugated to monomethylauristatin E, which binds to PSMA-positive cells and induces cytotoxicity. In a phase 1 study, PSMA ADC was well tolerated and demonstrated activity as measured by reductions in serum prostate-specific antigen (PSA) and circulating tumor cells (CTCs). To further assess PSMA ADC, we conducted a phase 2 trial in metastatic castration-resistant prostate cancer (mCRPC) subjects who progressed following abiraterone/enzalutamide (abi/enz) therapy.

METHODS

A total of 119 (84 chemotherapy-experienced and 35 chemotherapy-naïve) subjects were administered PSMA ADC 2.5 or 2.3 mg/kg IV q3w for up to eight cycles. Antitumor activity (best percentage declines in PSA and CTCs from baseline and tumor responses through radiological imaging), exploratory biomarkers, and safety (monitoring of adverse events [AEs], clinical laboratory tests, and Eastern Cooperative Oncology Group performance status) were assessed.

RESULTS

PSA declines ≥50% occurred in 14% of all treated (n = 113) and 21% of chemotherapy-naïve subjects (n = 34). CTC declines ≥50% were seen in 78% of all treated (n = 77; number of subjects with ≥5 CTCs at baseline and a posttreatment result) and 89% of chemotherapy-naïve subjects (n = 19); 47% of all treated and 53% of chemotherapy-naïve subjects had a transition from ≥5 to less than 5 CTCs/7.5 mL blood at some point during the study. PSA and CTC reductions were associated with high PSMA expression (CTCs or tumor tissue) and low neuroendocrine serum markers. In the chemotherapy-experienced group, the best overall radiologic response to PSMA ADC treatment was stable disease in 51 (60.7%) subjects; 5.7% of subjects in the chemotherapy-naïve group had partial responses. The most common treatment-related AEs ≥Common Terminology Criteria for AE (CTCAE) grade 3 were neutropenia, fatigue, electrolyte imbalance, anemia, and neuropathy. The most common serious AEs were dehydration, hyponatremia, febrile neutropenia, and constipation. Two subjects who received 2.5 mg/kg died of sepsis.

CONCLUSIONS

PSMA ADC demonstrated some activity with respect to PSA declines, CTC conversions/reductions, and radiologic assessments in abi/enz treated mCRPC subjects. Clinically significant treatment-related AEs included neutropenia and neuropathy.

Alternative and New Radiopharmaceutical Agents for Lung Cancer

BACKGROUND

FDG PET/CT imaging has an established role in lung cancer (LC) management. Whilst it is a sensitive technique, FDG PET/CT has a limited specificity in the differentiation between LC and benign conditions and is not capable of defining LC heterogeneity since FDG uptake varies between histotypes.

OBJECTIVE

To get an overview of new radiopharmaceuticals for the study of cancer biology features beyond glucose metabolism in LC.

METHODS

A comprehensive literature review of PubMed/Medline was performed using a combination of the following keywords: "positron emission tomography", "lung neoplasms", "non-FDG", "radiopharmaceuticals", "tracers".

RESULTS

Evidences suggest that proliferation markers, such as 18F-Fluorothymidine and 11CMethionine, improve LC staging and are useful in evaluating treatment response and progression free survival. 68Ga-DOTA-peptides are already routinely used in pulmonary neuroendocrine neoplasms (NENs) management and should be firstly performed in suspected NENs. 18F-Fluoromisonidazole and other radiopharmaceuticals show a promising impact on staging, prognosis assessment and therapy response in LC patients, by visualizing hypoxia and perfusion. Radiolabeled RGD-peptides, targeting angiogenesis, may have a role in LC staging, treatment outcome and therapy. PET radiopharmaceuticals tracing a specific oncogene/signal pathway, such as EGFR or ALK, are gaining interest especially for therapeutic implications. Other PET tracers, like 68Ga-PSMA-peptides or radiolabeled FAPIs, need more development in LC, though, they are promising for therapy purposes.

CONCLUSION

To date, the employment of most of the described tracers is limited to the experimental field, however, research development may offer innovative opportunities to improve LC staging, characterization, stratification and response assessment in an era of increased personalized therapy.

Expression of PSMA in tumor neovasculature of high grade sarcomas including synovial sarcoma, rhabdomyosarcoma, undifferentiated sarcoma and MPNST

AIMS

PSMA (prostate specific membrane antigen) is physiologically expressed in normal prostate tissue. It is overexpressed in prostate cancer cells and has been suggested as a target for antibody-based radioligand therapy. As PSMA expression so far has not been systematically analyzed in soft tissue tumors, the current study aims at investigating a large cohort of different subtypes.

METHODS AND RESULTS

Immunohistochemistry was used to detect PSMA expression in 779 samples of soft tissue tumors and Ewing sarcoma as a primary bone malignancy. CD34 coexpression was employed to study PSMA expression in the neovasculature. PSMA expression was found in the tumor-associated neovasculature of 151/779 soft tissue/bone tumors (19.38%) and was more frequent in malignant tumors compared to tumors with intermediate or benign biological potential (p=0.078). Strong neovascular PSMA expression was predominantly observed in subsets of different sarcomas including 3/20 rhabdomyosarcomas (15%), 4/21 malignant peripheral nerve sheath tumors (19.05%), 6/16 synovial sarcomas (35.29%) and 6/33 undifferentiated pleomorphic sarcomas (18.18%).

CONCLUSION

We conclude that PSMA is expressed in the neovasculature of a subset of soft tissue tumors to a variable extent. Our observation of strong PSMA expression predominantly occurring in sarcomas might provide a rationale to evaluate PSMA-targeted radioligand therapy in these entities.

Neovascular PSMA expression is a common feature in malignant neoplasms of the thyroid

Aim

PSMA (prostate-specific membrane antigen) is physiologically expressed in normal prostate tissue and over expressed in prostate cancer cells, therefore constituting a potential target for antibody-based radioligand therapy. Very recent imaging findings reported PSMA-PET/CT uptake in various thyroid lesions. We were therefore encouraged to systematically analyse PSMA expression in different benign and malignant thyroid lesions.

Methods

Immunohistochemistry was used to detect PSMA expression in 101 thyroid lesions, while neovasculature was identified by CD34 immunostaining.

Results

PSMA expression in the neovasculature was significantly more frequent in malignant tumors (36/63; 57.1%) compared to benign diseases (5/38; 13.2%; p = 0.0001). In addition, PSMA expression levels in the neovasculature of poorly and undifferentiated thyroid cancers were significantly higher compared to differentiated thyroid tumors (p = 0.021). However, one case with a strong expression in follicular adenoma was identified.

Conclusions

We conclude that neovascular PSMA expression is common in thyroid cancer but may also rarely be found in benign thyroid diseases, such as follicular adenoma. High expression in the tumor-associated neovasculature is predominantly found in poorly differentiated and undifferentiated (anaplastic) thyroid cancer. This knowledge is highly relevant when interpreting PSMA/PET-CT scans from patients with prostate cancer. In addition, our findings might provide a rationale for further evaluation of PSMA-targeted anti-neovascular or radioligand therapy in metastatic dedifferentiated thyroid cancer.

Repeatability of [68Ga]DKFZ11-PSMA PET Scans for Detecting Prostate-specific Membrane Antigen-positive Prostate Cancer

PURPOSE

We studied the effect of varying specific activity of [68Ga]DKFZ-PSMA11 ([68Ga]DP11) on repeated imaging of prostate-specific membrane antigen-positive (PSMA+) xenograft tumors.

PROCEDURES

Athymic nude mice bearing PC3-PIP (PSMA+) and PC3 (PSMA-) bilateral flank tumors were assessed to study intra- and inter-day repeatability of [68Ga]DP11 imaging in mice administered [68Ga]DP11 or [67Ga]DP11 (as a dilution tracer) using imaging and biodistribution studies.

RESULTS

Region of interest (ROI) analysis of the [68Ga]DP11 imaging study indicated that the uptake was constant on the same day or consecutive days. Prior imaging with [68Ga]DP11 did not significantly influence the subsequent uptake of [68Ga]DP11. Uptake of [68Ga]DP11 (60 min) and [67Ga]DP11 (24 h) in PC3-PIP tumors was 12.37 ± 4.19 %ID/g and 12.49 ± 6.88 %ID/g, respectively; [68Ga]DP11 was 13.83 ± 3.77 and 17.76 ± 1.84 on same-day and 15.98 ± 5.82 %ID/g on second-day imaging.

CONCLUSIONS

This study demonstrates that [68Ga]DP11, in a given PSMA+ lesion, is constant under several same-day or serial-day imaging conditions.

Prostate specific membrane antigen (PSMA) expression in non-small cell lung cancer

OBJECTIVES

PSMA (prostate-specific membrane antigen) is overexpressed in prostate cancer cells and is reported to be a promising target for antibody-based radioligand therapy in patients with metastasized prostate cancer. Since PSMA expression is not restricted to prostate cancer, the underlying study investigates PSMA expression in non-small cell lung cancer (NSCLC).

MATERIAL AND METHODS

Immunohistochemistry was used to identify PSMA expression in n = 275 samples of NSCLC tissue specimens. By means of CD34 co-expression, the level of PSMA expression in tumor associated neovasculature was investigated. The impact of PSMA expression on clinicopathologic parameters and prognosis was evaluated.

RESULTS

PSMA tumor cell expression in NSCLC is as low as 6% and was predominantly found in squamous cell carcinoma (p = 0.002). Neovascular PSMA expression was found in 49% of NSCLC. High neovascular PSMA expression was associated with higher tumor grading (G3/G4) (p < 0.001). Neither for PSMA tumor cell expression, nor for PSMA neovascular cell expression prognostic effects were found for the investigated NSCLC cases.

CONCLUSION

Here, we report on the expression of PSMA in NSCLC tissue samples. Against the background of a potential treatment with radiolabeled PSMA ligands, our data might serve for the future identification of patients who could benefit from this therapeutic option.

Neovascular Prostate-Specific Membrane Antigen Expression Is Associated with Improved Overall Survival under Palliative Chemotherapy in Patients with Pancreatic Ductal Adenocarcinoma

Aims

Expression of PSMA (prostate-specific membrane antigen) has been demonstrated in various cancers, including pancreatic ductal adenocarcinoma (PDAC). However, PSMA expression in PDAC-associated neovasculature has so far not been systematically analyzed.

Methods and Results

We analyzed PSMA expression in 81 PDAC tissue samples from 61 patients. Microvessel density (MVD) was assessed by software-based image analysis and showed a mean MVD of 63.7 microvessels/0.785 mm². PSMA was practically absent in tumor tissue (5.3%) and PDAC cell lines (0/7) but could be detected in tumor-associated neovasculature in 53.2% of cases. There was no association between neovascular PSMA expression and clinicopathological tumor characteristics. Samples with PSMA+ neovasculature showed increased MVD; however, this result was not statistically significant (p > 0.05). Presence of PSMA+ neovessels correlated with overall survival under palliative chemotherapy (894 versus 400 days; HR 0.42; 95% CI: 0.12 to 0.87; p < 0.05).

Conclusion

PSMA expression in tumor-associated neovasculature is a common feature and associated with improved overall survival under palliative chemotherapy in PDAC. Our results point towards a possible association between PSMA expression and response to therapy which might be based on enhanced intratumoral bioavailability of systemic chemotherapy.

PSMA-Based [(18)F]DCFPyL PET/CT Is Superior to Conventional Imaging for Lesion Detection in Patients with Metastatic Prostate Cancer

PURPOSE

Current standard of care conventional imaging modalities (CIM) such as X-ray computed tomography (CT) and bone scan can be limited for detection of metastatic prostate cancer and therefore improved imaging methods are an unmet clinical need. We evaluated the utility of a novel second-generation low molecular weight radiofluorinated prostate-specific membrane antigen (PSMA)-targeted positron emission tomography (PET) radiotracer, [(18)F]DCFPyL, in patients with metastatic prostate cancer.

PROCEDURES

Nine patients with suspected prostate cancer recurrence, eight with CIM evidence of metastatic prostate cancer and one with biochemical recurrence, were imaged with [(18)F]DCFPyL PET/CT. Eight of the patients had contemporaneous CIM for comparison. A lesion-by-lesion comparison of the detection of suspected sites of metastatic prostate cancer was carried out between PET and CIM. Statistical analysis for estimated proportions of inter-modality agreement for detection of metastatic disease was calculated accounting for intra-patient correlation using general estimating equation (GEE) intercept-only regression models.

RESULTS

One hundred thirty-nine sites of PET positive [(18)F]DCFPyL uptake (138 definite, 1 equivocal) for metastatic disease were detected in the eight patients with available comparison CIM. By contrast, only 45 lesions were identified on CIM (30 definite, 15 equivocal). When lesions were negative or equivocal on CIM, it was estimated that a large portion of these lesions or 0.72 (95 % confidence interval (CI) 0.55-0.84) would be positive on [(18)F]DCFPyL PET. Conversely, of those lesions negative or equivocal on [(18)F]DCFPyL PET, it was estimated that only a very small proportion or 0.03 (95 % CI 0.01-0.07) would be positive on CIM. Delayed 2-h-post-injection time point PET yielded higher tumor radiotracer uptake and higher tumor-to-background ratios than an earlier 1-h-post-injection time point.

CONCLUSIONS

A novel PSMA-targeted PET radiotracer, [(18)F]DCFPyL, was able to a large number of suspected sites of prostate cancer, many of which were occult or equivocal by CIM. This study provides strong preliminary evidence for the use of this second-generation PSMA-targeted PET radiotracer for detection of metastatic prostate cancer and lends further support for the importance of PSMA-targeted PET imaging in prostate cancer.

Pearls and pitfalls in clinical interpretation of prostate-specific membrane antigen (PSMA)-targeted PET imaging

BACKGROUND

The rapidly expanding clinical adaptation of prostate-specific membrane antigen (PSMA)-targeted PET imaging in the evaluation of patients with prostate cancer has placed an increasing onus on understanding both the potential pearls of interpretation as well as limitations of this new technique. As with any new molecular imaging modality, accurate characterization of abnormalities on PSMA-targeted PET imaging can be accomplished only if one is aware of the normal distribution pattern, physiological variants of radiotracer uptake, and potential sources of false-positive and false-negative imaging findings. In recent years, a growing number of reports have come to light describing incidental non-prostatic benign or malignant pathologies with high uptake on PSMA-targeted PET imaging. In this review, we have summarized the published literature regarding the potential pearls and technical and interpretive pitfalls of this imaging modality. Knowledge of these limitations can increase the confidence of interpreting physicians and thus improve patient care.

CONCLUSIONS

As PSMA-targeted PET is expected to be evaluated in larger prospective trials, the dissemination of potential diagnostic pitfalls and the biologic underpinning of those findings will be of increased importance.

Molecular imaging for prostate cancer: Performance analysis of 68Ga-PSMA PET/CT versus choline PET/CT

INTRODUCTION

There is a need for a precise and reliable imaging to improve the management of prostate cancer. In recent years the PET/CT with choline has changed the handling of prostate cancer in Europe, and it is commonly used for initial stratification or for the diagnosis of a biochemical recurrence, although it does not lack limitations. Other markers are being tested, including the ligand of prostate-specific membrane antigen (PSMA), that seems to offer encouraging prospects. The goal of this piece of work was to critically review the role of choline and PSMA PET/CT in prostate cancer.

EVIDENCE ACQUISITION

A systematic literature review of databases PUBMED/MEDLINE and EMBASE was conducted searching for articles fully published in English on the PET marker in prostate cancer and its clinical application.

EVIDENCE SYNTHESIS AND DISCUSSION

It seems as 68Ga-PSMA PET/CT is better than PET/CT in prostate cancer to detect primary prostate lesions, initial metastases in the lymph nodes and recurrence. However, further research is required to obtain high-level tests. Also, other PET markers are studied. Moreover, the emergence of a new PET/MR camera could change the performance of PET imaging.

Novel Monoclonal Antibodies Recognizing Human Prostate-Specific Membrane Antigen (PSMA) as Research and Theranostic Tools

BACKGROUND

Prostate-specific membrane antigen (PSMA) is a validated target for the imaging and therapy of prostate cancer. Here, we report the detailed characterization of four novel murine monoclonal antibodies (mAbs) recognizing human PSMA as well as PSMA orthologs from different species.

METHODS

Performance of purified mAbs was assayed using a comprehensive panel of in vitro experimental setups including Western blotting, immunofluorescence, immunohistochemistry, ELISA, flow cytometry, and surface-plasmon resonance. Furthermore, a mouse xenograft model of prostate cancer was used to compare the suitability of the mAbs for in vivo applications.

RESULTS

All mAbs demonstrate high specificity for PSMA as documented by the lack of cross-reactivity to unrelated human proteins. The 3F11 and 1A11 mAbs bind linear epitopes spanning residues 226-243 and 271-288 of human PSMA, respectively. 3F11 is also suitable for the detection of PSMA orthologs from mouse, pig, dog, and rat in experimental setups where the denatured form of PSMA is used. 5D3 and 5B1 mAbs recognize distinct surface-exposed conformational epitopes and are useful for targeting PSMA in its native conformation. Most importantly, using a mouse xenograft model of prostate cancer we show that both the intact 5D3 and its Fab fragment are suitable for in vivo imaging.

CONCLUSIONS

With apparent affinities of 0.14 and 1.2 nM as determined by ELISA and flow cytometry, respectively, 5D3 has approximately 10-fold higher affinity for PSMA than the clinically validated mAb J591 and, therefore, is a prime candidate for the development of next-generation theranostics to target PSMA. Prostate 77:749-764, 2017. © 2017 Wiley Periodicals, Inc.

Emerging Molecular Biomarkers in Advanced Prostate Cancer: Translation to the Clinic

Recent clinical and preclinical studies focused on understanding the molecular landscape of castration-resistant prostate cancer (CRPC) have provided insights into mechanisms of treatment resistance, disease heterogeneity, and potential therapeutic targets. This work has served as a framework for several ongoing clinical studies focused on bringing novel observations into the clinic in the form of tissue, liquid, and imaging biomarkers. Resistance in CRPC typically is driven through reactivation of androgen receptor (AR) signaling, which can occur through AR-activating point mutations, amplification, splice variants (such as AR-V7), or other bypass mechanisms. Detection of AR aberrations in the circulation negatively impacts response to subsequent AR-directed therapies such as abiraterone and enzalutamide. Other potentially clinically relevant alterations in CRPC include defects in DNA damage repair (at either the somatic or germline level) in up to 20% of patients (with implications for PARP1 inhibitor therapy), PI3K/PTEN/Akt pathway activation, WNT signaling pathway alterations, cell cycle gene alterations, and less common but potentially targetable alterations involving RAF and FGFR2. Imaging biomarkers that include those focused on incorporating overexpressed androgen-regulated genes/proteins, such as prostate-specific membrane antigen (PSMA) and dihydrotestosterone (DHT) in combination with CT, can noninvasively identify patterns of AR-driven distribution of CRPC tumor cells, monitor early metastatic lesions, and potentially capture heterogeneity of response to AR-directed therapies and other therapeutics. This article focuses on the current state of clinical biomarker development and future directions for how they might be implemented into the clinic in the near term to improve risk stratification and treatment selection for patients.

Maternal inflammation leads to impaired glutamate homeostasis and up-regulation of glutamate carboxypeptidase II in activated microglia in the fetal/newborn rabbit brain

Astrocyte dysfunction and excessive activation of glutamatergic systems have been implicated in a number of neurologic disorders, including periventricular leukomalacia (PVL) and cerebral palsy (CP). However, the role of chorioamnionitis on glutamate homeostasis in the fetal and neonatal brains is not clearly understood. We have previously shown that intrauterine endotoxin administration results in intense microglial 'activation' and increased pro-inflammatory cytokines in the periventricular region (PVR) of the neonatal rabbit brain. In this study, we assessed the effect of maternal inflammation on key components of the glutamate pathway and its relationship to astrocyte and microglial activation in the fetal and neonatal New Zealand white rabbit brain. We found that intrauterine endotoxin exposure at gestational day 28 (G28) induced acute and prolonged glutamate elevation in the PVR of fetal (G29, 1day post-injury) and postnatal day 1 (PND1, 3days post-injury) brains along with prominent morphological changes in the astrocytes (soma hypertrophy and retracted processes) in the white matter tracts. There was a significant increase in glutaminase and N-Methyl-d-Aspartate receptor (NMDAR) NR2 subunit expression along with decreased glial L-glutamate transporter 1 (GLT-1) in the PVR at G29, that would promote acute dysregulation of glutamate homeostasis. This was accompanied with significantly decreased TGF-β1 at PND1 in CP kits indicating ongoing neuroinflammation. We also show for the first time that glutamate carboxypeptidase II (GCPII) was significantly increased in the activated microglia at the periventricular white matter area in both G29 and PND1 CP kits. This was confirmed by in vitro studies demonstrating that LPS activated primary microglia markedly upregulate GCPII enzymatic activity. These results suggest that maternal intrauterine endotoxin exposure results in early onset and long-lasting dysregulation of glutamate homeostasis, which may be mediated by impaired astrocyte function and GCPII upregulation in activated microglia.

Prostate-Specific Membrane Antigen PET/CT: False-Positive Results due to Sarcoidosis?

We report on a 72-year-old male patient who developed sarcoidosis of the mediastinal lymph nodes, the liver, and the prostate 11 years ago. Seven years later, he underwent transurethral resection of the prostate by laser due to hematuria. Pathology of the resected chips showed a ‘granulomatous prostatitis with epitheloid cells’. Malignancy was histologically excluded at that time. Four years later, he was diagnosed with an undifferentiated prostate carcinoma, with a Gleason score of 5 + 4 = 9. After initiation of antihormonal therapy, he underwent radical prostatectomy and pelvic lymphadenectomy, which revealed a pT3b pN1 carcinoma with infiltrated resection margins. Three months later, the prostate-specific antigen level was 1.4 ng/ml, and a local recurrence was suspected by ultrasound; consequently, a ⁶⁸Ga-prostate-specific membrane antigen (PSMA) PET/CT was performed. This examination seemed to confirm the local recurrence, a right pelvic lymph node metastasis, and a hepatic metastasis. However, ultrasound with contrast medium could not confirm the metastatic spread to the liver. In palliative intention, radiotherapy of the pelvis was done. After 50 Gy, the supposed recurrence had markedly shrunk, and an additional boost dose with 16.2 Gy was applied. Two years later, the patient is still free of disease. Due to this clinical development, we doubt the diagnosis of a fulminant progression of the prostate cancer as suspected by PSMA-PET/CT. Instead, we suspect a recurrence of the previously proven sarcoidosis leading to false-positive results. Our focus in this report is on the interaction between PSMA-PET/CT and sarcoidosis. Another report on a case of sarcoidosis of the spleen seems to confirm this possibility [Kobe et al: Clin Nucl Med 2015;40: 897–898].

Prostate-specific membrane antigen (PSMA)-mediated laminin proteolysis generates a pro-angiogenic peptide

Prostate-specific membrane antigen (PSMA) is a membrane-bound glutamate carboxypeptidase expressed in a number of tissues. PSMA participates in various biological functions depending on the substrate available in the particular tissue; in the brain, PSMA cleaves the abundant neuropeptide N-acetyl-aspartyl-glutamate to regulate release of key neurotransmitters, while intestinal PSMA cleaves polyglutamated peptides to supply dietary folate. PSMA expression is also progressively upregulated in prostate cancer where it correlates with tumor progression as well as in tumor vasculature, where it regulates angiogenesis. The previous research determined that PSMA cleavage of small peptides generated via matrix metalloprotease-mediated proteolysis of the extracellular matrix protein laminin potently activated endothelial cells, integrin signaling and angiogenesis, although the specific peptide substrates were not identified. Herein, using enzymatic analyses and LC/MS, we unequivocally demonstrate that several laminin-derived peptides containing carboxy-terminal glutamate moieties (LQE, IEE, LNE) are bona fide substrates for PSMA. Subsequently, the peptide products were tested for their effects on angiogenesis in various models. We report that LQ, the dipeptide product of PSMA cleavage of LQE, efficiently activates endothelial cells in vitro and enhances angiogenesis in vivo. Importantly, LQE is not cleaved by an inactive PSMA enzyme containing an active site mutation (E424S). Endothelial cell activation by LQ was dependent on integrin beta-1-induced activation of focal adhesion kinase. These results characterize a novel PSMA substrate, provide a functional rationale for the upregulation of PSMA in cancer cells and tumor vasculature and suggest that inhibition of PSMA could lead to the development of new angiogenic therapies.

(2S)-2-(3-(1-Carboxy-5-(4-211At-Astatobenzamido)Pentyl)Ureido)-Pentanedioic Acid for PSMA-Targeted α-Particle Radiopharmaceutical Therapy

Alpha-particle emitters have a high linear energy transfer and short range, offering the potential for treating micrometastases while sparing normal tissues. We developed a urea-based, ²¹¹At-labeled small molecule targeting prostate-specific membrane antigen (PSMA) for the treatment of micrometastases due to prostate cancer (PC).

METHODS

PSMA-targeted (2S)-2-(3-(1-carboxy-5-(4-²¹¹At-astatobenzamido)pentyl)ureido)-pentanedioic acid (²¹¹At- 6: ) was synthesized. Cellular uptake and clonogenic survival were tested in PSMA-positive (PSMA+) PC3 PIP and PSMA-negative (PSMA-) PC3 flu human PC cells after ²¹¹At- 6: treatment. The antitumor efficacy of ²¹¹At- 6: was evaluated in mice bearing PSMA+ PC3 PIP and PSMA- PC3 flu flank xenografts at a 740-kBq dose and in mice bearing PSMA+, luciferase-expressing PC3-ML micrometastases. Biodistribution was determined in mice bearing PSMA+ PC3 PIP and PSMA- PC3 flu flank xenografts. Suborgan distribution was evaluated using α-camera images, and microscale dosimetry was modeled. Long-term toxicity was assessed in mice for 12 mo.

RESULTS

²¹¹At- 6: treatment resulted in PSMA-specific cellular uptake and decreased clonogenic survival in PSMA+ PC3 PIP cells and caused significant tumor growth delay in PSMA+ PC3 PIP flank tumors. Significantly improved survival was achieved in the newly developed PSMA+ micrometastatic PC model. Biodistribution showed uptake of ²¹¹At- 6: in PSMA+ PC3 PIP tumors and in kidneys. Microscale kidney dosimetry based on α-camera images and a nephron model revealed hot spots in the proximal renal tubules. Long-term toxicity studies confirmed that the dose-limiting toxicity was late radiation nephropathy.

CONCLUSION

PSMA-targeted ²¹¹At- 6: α-particle radiotherapy yielded significantly improved survival in mice bearing PC micrometastases after systemic administration. ²¹¹At- 6: also showed uptake in renal proximal tubules resulting in late nephrotoxicity, highlighting the importance of long-term toxicity studies and microscale dosimetry.

Characterization of a novel metastatic prostate cancer cell line of LNCaP origin

BACKGROUND

The LNCaP cell line was originally isolated from the lymph node of a patient with metastatic prostate cancer. Many cell lines have been derived from LNCaP by selective pressures to study different aspects of prostate cancer progression. When injected subcutaneously into male athymic nude mice, LNCaP and its derivatives rarely metastasize.

METHODS

Here, we describe the characteristics of a new LNCaP derivative, JHU-LNCaP-SM, which was generated by long term passage in normal cell culture conditions.

RESULTS

Short tandem repeat (STR) analysis and genomic sequencing verified JHU-LNCaP-SM derivation from parental LNCaP cells. JHU-LNCaP-SM cells express the same mutated androgen receptor (AR) but unlike LNCaP, are no longer androgen dependent for growth. The cells demonstrate an attenuated androgen responsiveness in transcriptional assays and retain androgen sensitive expression of PSA, AR, and PSMA. Unlike parental LNCaP, JHU-LNCaP-SM cells quickly form subcutaneous tumors in male athymic nude mice, reliably metastasize to the lymph nodes and display a striking intra-tumoral and spreading hemorrhagic phenotype as tumor xenografts.

CONCLUSIONS

The JHU-LNCaP-SM cell line is a new isolate of LNCaP, which facilitates practical, preclinical studies of spontaneous metastasis of prostate cancer through lymphatic tissues.

Selection and characterization of Anticalins targeting human prostate-specific membrane antigen (PSMA)

Although prostate carcinoma (PCa) is by far the most commonly diagnosed neoplasia in men, corresponding diagnostic and therapeutic modalities have limited efficacy at present. Anticalins comprise a novel class of binding proteins based on a non-immunoglobulin scaffold that can be engineered to specifically address molecular targets of interest. Here we report the selection and characterization of Anticalins that recognize human prostate-specific membrane antigen (PSMA), a membrane-tethered metallopeptidase constituting a disease-related target for imaging and therapy of PCa as well as solid malignancies in general. We used a randomized lipocalin library based on the human lipocalin 2 (Lcn2) scaffold together with phage display and ELISA screening to select PSMA-specific variants. Five Anticalin candidates from the original panning were expressed in Escherichia coli as soluble monomeric proteins, revealing affinities toward PSMA down to the low nanomolar range. Binding characteristics of the most promising candidate were further improved via affinity maturation by applying error-prone PCR followed by selection via phage display as well as bacterial surface display under more stringent conditions. In BIAcore measurements, the dissociation constant of the best Anticalin was determined as ∼500 pM, with a substantially improved dissociation rate compared with the first-generation candidate. Finally, immunofluorescence microscopy revealed specific staining of PSMA-positive tumor cell lines while flow cytometric analysis confirmed the ability of the selected Anticalins to detect PSMA on live cells. Taken together, Anticalins resulting from this study offer a viable alternative to antibody-based PSMA binders for biomedical applications, including in vivo imaging of PCa or neovasculature of solid tumors.

Comparison of Prostate-Specific Membrane Antigen-Based 18F-DCFBC PET/CT to Conventional Imaging Modalities for Detection of Hormone-Naïve and Castration-Resistant Metastatic Prostate Cancer

Conventional imaging modalities (CIMs) have limited sensitivity and specificity for detection of metastatic prostate cancer. We examined the potential of a first-in-class radiofluorinated small-molecule inhibitor of prostate-specific membrane antigen (PSMA), N-[N-[(S)-1,3-dicarboxypropyl]carbamoyl]-4-(18)F-fluorobenzyl-l-cysteine ((18)F-DCFBC), to detect metastatic hormone-naïve (HNPC) and castration-resistant prostate cancer (CRPC).

METHODS

Seventeen patients were prospectively enrolled (9 HNPC and 8 CRPC); 16 had CIM evidence of new or progressive metastatic prostate cancer and 1 had high clinical suspicion of metastatic disease. (18)F-DCFBC PET/CT imaging was obtained with 2 successive PET scans starting at 2 h after injection. Patients were imaged with CIM at approximately the time of PET. A lesion-by-lesion analysis of PET to CIM was performed in the context of either HNPC or CRPC. The patients were followed with available clinical imaging as a reference standard to determine the true nature of identified lesions on PET and CIM.

RESULTS

On the lesion-by-lesion analysis, (18)F-DCFBC PET was able to detect a larger number of lesions (592 positive with 63 equivocal) than CIM (520 positive with 61 equivocal) overall, in both HNPC and CRPC patients. (18)F-DCFBC PET detection of lymph nodes, bone lesions, and visceral lesions was superior to CIM. When intrapatient clustering effects were considered, (18)F-DCFBC PET was estimated to be positive in a large proportion of lesions that would be negative or equivocal on CIM (0.45). On follow-up, the sensitivity of (18)F-DCFBC PET (0.92) was superior to CIM (0.71). (18)F-DCFBC tumor uptake was increased at the later PET time point (~2.5 h after injection), with background uptake showing a decreasing trend on later PET.

CONCLUSION

PET imaging with (18)F-DCFBC, a small-molecule PSMA-targeted radiotracer, detected more lesions than CIM and promises to diagnose and stage patients with metastatic prostate cancer more accurately than current imaging methods.

[(18)F]Fluorobenzoyllysinepentanedioic Acid Carbamates: New Scaffolds for Positron Emission Tomography (PET) Imaging of Prostate-Specific Membrane Antigen (PSMA)

Radiolabeled urea-based low-molecular weight inhibitors of the prostate-specific membrane antigen (PSMA) are under intense investigation as imaging and therapeutic agents for prostate and other cancers. In an effort to provide agents with less nontarget organ uptake than the ureas, we synthesized four (18)F-labeled inhibitors of PSMA based on carbamate scaffolds. 4-Bromo-2-[(18)F]fluorobenzoyllysineoxypentanedioic acid (OPA) carbamate [(18)F]23 and 4-iodo-2-[(18)F]fluorobenzoyllysine OPA carbamate [(18)F]24 in particular exhibited high target-selective uptake in PSMA+ PC3 PIP tumor xenografts, with tumor-to-kidney ratios of >1 by 4 h postinjection, an important benchmark. Because of its high tumor uptake (90% injected dose per gram of tissue at 2 h postinjection) and high tumor-to-organ ratios, [(18)F]23 is promising for clinical translation. Prolonged tumor-specific uptake demonstrated by [(18)F]24, which did not reach equilibrium during the 4 h study period, suggests carbamates as alternative scaffolds for mitigating dose to nontarget tissues.

Design of composite inhibitors targeting glutamate carboxypeptidase II: the importance of effector functionalities

Inhibitors targeting human glutamate carboxypeptidase II (GCPII) typically consist of a P1' glutamate-derived binding module, which warrants the high affinity and specificity, linked to an effector function that is positioned within the entrance funnel of the enzyme. Here we present a comprehensive structural and computational study aimed at dissecting the importance of the effector function for GCPII binding and affinity. To this end we determined crystal structures of human GCPII in complex with a series of phosphoramidate-based inhibitors harboring effector functions of diverse physicochemical characteristics. Our data show that higher binding affinities of phosphoramidates, compared to matching phosphonates, are linked to the presence of additional hydrogen bonds between Glu424 and Gly518 of the enzyme and the amide group of the phosphoramidate. While the positioning of the P1' glutamate-derived module within the S1' pocket of GCPII is invariant, interaction interfaces between effector functions and residues lining the entrance funnel are highly varied, with the positively charged arginine patch defined by Arg463, Arg534 and Arg536 being the only 'hot-spot' common to several studied complexes. This variability stems in part from the fact that the effector/GCPII interfaces generally encompass isolated areas of nonpolar residues within the entrance funnel and resulting van der Waals contacts lack the directionality typical for hydrogen bonding interactions. The presented data unravel a complexity of binding modes of inhibitors within non-prime site(s) of GCPII and can be exploited for the design of novel GCPII-specific compounds.

PDB ID CODES

Atomic coordinates of the present structures together with the experimental structure factor amplitudes were deposited at the RCSB Protein Data Bank under accession codes 4P44 (complex with JRB-4-81), 4P45 (complex with JRB-4-73), 4P4B (complex with CTT54), 4P4D (complex with MP1C), 4P4E (complex with MP1D), 4P4F (complex with NC-2-40), 4P4I (complex with T33) and 4P4J (complex with T33D).