Prostate-specific membrane antigen: present and future applications

D-DOPA Is a Potent, Orally Bioavailable, Allosteric Inhibitor of Glutamate Carboxypeptidase II

Glutamate carboxypeptidase-II (GCPII) is a zinc-dependent metalloenzyme implicated in numerous neurological disorders. The pharmacophoric requirements of active-site GCPII inhibitors makes them highly charged, manifesting poor pharmacokinetic (PK) properties. Herein, we describe the discovery and characterization of catechol-based inhibitors including L-DOPA, D-DOPA, and caffeic acid, with sub-micromolar potencies. Of these, D-DOPA emerged as the most promising compound, with good metabolic stability, and excellent PK properties. Orally administered D-DOPA yielded high plasma exposures (AUCplasma = 72.7 nmol·h/mL) and an absolute oral bioavailability of 47.7%. Unfortunately, D-DOPA brain exposures were low with AUCbrain = 2.42 nmol/g and AUCbrain/plasma ratio of 0.03. Given reports of isomeric inversion of D-DOPA to L-DOPA via D-amino acid oxidase (DAAO), we evaluated D-DOPA PK in combination with the DAAO inhibitor sodium benzoate and observed a >200% enhancement in both plasma and brain exposures (AUCplasma = 185 nmol·h/mL; AUCbrain = 5.48 nmol·h/g). Further, we demonstrated GCPII target engagement; orally administered D-DOPA with or without sodium benzoate caused significant inhibition of GCPII activity. Lastly, mode of inhibition studies revealed D-DOPA to be a noncompetitive, allosteric inhibitor of GCPII. To our knowledge, this is the first report of D-DOPA as a distinct scaffold for GCPII inhibition, laying the groundwork for future optimization to obtain clinically viable candidates.

18F-Prostate-Specific Membrane Antigen and 18F-Fluorodeoxyglucose PET/CT Unmasked the Characteristics of Prostate Lymphoma: A Case Report and Literature Review

Prostate lymphoma (PL) is rarely observed and may be concurrently presented with prostate adenocarcinoma. Moreover, the appearance of PL on conventional imaging is similar with prostate adenocarcinoma. Thus, most of PL is diagnosed through prostate biopsy, or accidentally found in the specimens of surgery. Prostate-specific membrane antigen (PSMA) PET/CT has improved the management of prostate adenocarcinoma. While, the question regarding whether it benefits the discovery of the characteristics of PL is unknown. A 32-year-old man presented with worsening dysuria for 1 month, and the prostate-specific antigen (PSA) concentration was normal. While the pelvic MRI showed a mass in the prostate and multiple enlarged lymph nodes in the bilateral inguinal area. Then, the diagnosis of prostate adenocarcinoma was considered, but the serum PSA was normal and he was younger than most patients. So, 18F-PSMA PET/CT was then performed to further reveal the characteristics of the lesion and guide biopsy. However, there was no abnormal PSMA uptake in the lesion of the prostate and lymph nodes of the pelvic cavity and bilateral inguinal area. These lesions presented with increased glucose metabolism on fluorodeoxyglucose (FDG) PET/CT, and the prostate biopsy was then performed. PL was confirmed based on the results of the histopathologic examination, and the patient subsequently received systemic chemotherapy plus radiotherapy. Fortunately, the symptoms and the lesions completely disappeared after radiotherapy. The clinical symptoms of PL are atypical, and PL and adenocarcinoma may be concurrently presented. Moreover, distinguishing PL from prostate adenocarcinoma based on the appearance of conventional imaging is difficult. As opposed to prostate adenocarcinoma, a high FDG-avidity and low PSMA uptake by lymphoma either in the prostate or metastases are seen. So, PSMA PET/CT combined with FDG PET/CT can non-invasively identify the characteristics and origin of PL.

PSMA expression on neovasculature of solid tumors

The use of prostate specific membrane antigen (PSMA) binding agents, labelled with diagnostic and therapeutic radio-isotopes is opening the potential for a new era of personalized management of prostate carcinoma. A wide variety of immunohistochemistry studies have shown PSMA also to be upregulated on the endothelial cells of the neovasculature of a wide variety of other solid tumors where it may facilitate endothelial cell sprouting and invasion through its regulation of lytic proteases that have the ability to cleave the extracellular matrix. Similar to the introduction of PSMA-targeting theranostics in prostate carcinoma, overexpression of PSMA on newly formed tumor vessels may serve as a target for imaging and subsequent treatment of cancer through the use of agents that are capable of blocking PSMA in its function or through PSMA-mediated delivery of chemotherapeutics or radiation agents. In this review, the available data on PSMA expression on tumor neovasculature in human solid tumors assessed by using immunohistochemistry are discussed.

Advances in targeted alpha therapy for prostate cancer

Amongst therapeutic radiopharmaceuticals, targeted alpha therapy (TαT) can deliver potent and local radiation selectively to cancer cells as well as the tumor microenvironment and thereby control cancer while minimizing toxicity. In this review, we discuss the history, progress, and future potential of TαT in the treatment of prostate cancer, including dosimetry-individualized treatment planning, combinations with small-molecule therapies, and conjugation to molecules directed against antigens expressed by prostate cancer cells, such as prostate-specific membrane antigen (PSMA) or components of the tumor microenvironment. A clinical proof of concept that TαT is efficacious in treating bone-metastatic castration-resistant prostate cancer has been demonstrated by radium-223 via improved overall survival and long-term safety/tolerability in the phase III ALSYMPCA trial. Dosimetry calculation and pharmacokinetic measurements of TαT provide the potential for optimization and individualized treatment planning for a precision medicine-based cancer management paradigm. The ability to combine TαTs with other agents, including chemotherapy, androgen receptor-targeting agents, DNA repair inhibitors, and immuno-oncology agents, is under investigation. Currently, TαTs that specifically target prostate cancer cells expressing PSMA represents a promising therapeutic approach. Both PSMA-targeted actinium-225 and thorium-227 conjugates are under investigation. The described clinical benefit, safety and tolerability of radium-223 and the recent progress in TαT trial development suggest that TαT occupies an important new role in prostate cancer treatment. Ongoing studies with newer dosimetry methods, PSMA targeting, and novel approaches to combination therapies should expand the utility of TαT in prostate cancer treatment.

PSMA-Targeting Positron Emission Agents for Imaging Solid Tumors Other Than Non-Prostate Carcinoma: A Systematic Review

Despite its name, prostate-specific membrane antigen (PSMA) has been shown using immunohistochemistry (IHC) to also be over-expressed in the tumor neovasculature of a wide variety of solid tumors other than prostate carcinoma. Accordingly, positron-emitting radiolabeled small molecules targeting PSMA, initially developed for positron emission tomography in prostate carcinomas, are currently being explored for their staging and restaging potential as an alternative imaging modality in other solid tumor types where 18-F-fluorodeoxyglucose (FDG)-PET imaging has low diagnostic accuracy. In this paper, the currently available literature in this field is reviewed. Preliminary, mainly retrospective studies are encouraging, with evidence of improved diagnostic sensitivity and specificity in clear cell renal carcinoma, glioma, and hepatocellular carcinoma, leading to a change in patient management in several patients. However, the results published thus far warrant confirmation by larger prospective studies additionally assessing the longitudinal impact on patient outcomes.

New Prostate Cancer Targets for Diagnosis, Imaging, and Therapy: Focus on Prostate-Specific Membrane Antigen

The rising incidence rate of the cancer in the prostate gland has increased the demand for improved diagnostic, imaging, and therapeutic approaches. Prostate-specific membrane antigen (PSMA), with folate hydrolase and carboxypeptidase and, internalization activities, is highly expressed in the epithelial cells of the prostate gland and is strongly upregulated in prostatic adenocarcinoma, with elevated expression correlating with, metastasis, progression, and androgen independence. Recently, PSMA has been an active target of investigation by several approaches, including the successful utilization of small molecule inhibitors, RNA aptamer conjugates, PSMA-based immunotherapy, and PSMA-targeted prodrug therapy. Future investigations of PSMA in prostate cancer (PCa) should focus in particular on its intracellular activities and functions. The objective of this contribution is to review the current role of PSMA as a marker for PCa diagnosis, imaging, and therapy.

The therapeutic and diagnostic potential of the prostate specific membrane antigen/glutamate carboxypeptidase II (PSMA/GCPII) in cancer and neurological disease

Prostate specific membrane antigen (PSMA) otherwise known as glutamate carboxypeptidase II (GCPII) is a membrane bound protein that is highly expressed in prostate cancer and in the neovasculature of a wide variety of tumours including glioblastomas, breast and bladder cancers. This protein is also involved in a variety of neurological diseases including schizophrenia and ALS. In recent years, there has been a surge in the development of both diagnostics and therapeutics that take advantage of the expression and activity of PSMA/GCPII. These include gene therapy, immunotherapy, chemotherapy and radiotherapy. In this review, we discuss the biological roles that PSMA/GCPII plays, both in normal and diseased tissues, and the current therapies exploiting its activity that are at the preclinical stage. We conclude by giving an expert opinion on the future direction of PSMA/GCPII based therapies and diagnostics and hurdles that need to be overcome to make them effective and viable.

Prostate specific membrane antigen knockdown impairs the tumorigenicity of LNCaP prostate cancer cells by inhibiting the phosphatidylinositol 3-kinase/Akt signaling pathway

BACKGROUND

Prostate specific membrane antigen (PSMA) can facilitate the growth, migration, and invasion of the LNCaP prostate cancer cell lines, but the underlying molecular mechanisms have not yet been clearly defined. Here, we investigated whether PSMA serves as a novel regulator of the phosphatidylinositol 3-kinase (PI3K)/Akt signaling by employing PSMA knockdown model and PI3K pharmacological inhibitor (LY294002) in LNCaP prostate cancer cells.

METHODS

PSMA knockdown had been stably established by transfecting with lentivirus-mediated siRNA in our previous study. Then, LNCaP cells were divided into interference, non-interference, and blank groups. We first testified the efficacy of PSMA knockdown in our LNCaP cell line. Then, we compared the expression of PSMA and total/activated Akt by Western blotting in the above three groups with or without LY294002 treatment. Furthermore, immunocytochemistry was performed to confirm the changes of activated Akt (p-Akt, Ser473) in groups. Besides, cell proliferation, migration, and cell cycle were measured by CCK-8 assay, Transwell analysis, and Flow cytometry respectively.

RESULTS

After PSMA knockdown, the level of p-Akt (Ser473) but not of total-Akt (Akt1/2) was significantly decreased when compared with the non-interference and blank groups. However, LY294002 administration significantly reduced the expression of p-Akt (Ser473) in all the three groups. The results of immunocytochemistry further confirmed that PSMA knockdown or LY294002 treatment was associated with p-Akt (Ser473) down-regulation. Decrease of cell proliferation, migration, and survival were also observed upon PSMA knockdown and LY294002 treatment.

CONCLUSIONS

Taken together, our results reveal that PI3K/Akt signaling pathway inhibition may serve as a novel molecular mechanism in LNCaP prostate cancer cells of PSMA knockdown and suggest that Akt (Ser473) may play a critical role as a downstream signaling target effector of PSMA in this cellular model.

Other biomarkers for detecting prostate cancer

Prostate-specific antigen (PSA) has been used for detecting prostate cancer since 1994. Although it is the best cancer biomarker available, PSA is not perfect. It lacks both the sensitivity and specificity to accurately detect the presence of prostate cancer. None of the PSA thresholds currently in use consistently identify patients with prostate cancer and exclude patients without cancer. Novel approaches to improve our ability to detect prostate cancer and predict the course of the disease are needed. Additional methods for detecting prostate cancer have been evaluated. Despite the discovery of many new biomarkers, only a few have shown some clinical value. These markers include human kallikrein 2, urokinase-type plasminogen activator receptor, prostate-specific membrane antigen, early prostate cancer antigen, PCA3, alpha-methylacyl-CoA racemase and glutathione S-transferase pi hypermethylation. We review the reports on biomarkers for prostate cancer detection, and their possible role in the clinical practice.

Increased therapeutic efficacy of the prostate-specific oncolytic adenovirus Ad[I/PPT-E1A] by reduction of the insulator size and introduction of the full-length E3 region

Conditionally replicating adenoviruses are developing as a complement to traditional cancer therapies. Ad[I/PPT-E1A] is an E1B/E3-deleted virus that replicates exclusively in prostate cells, since the expression of E1A is controlled by the recombinant 1.4 kb prostate-specific PPT promoter. The transcriptional integrity of PPT is maintained by the 3.0 kb mouse H19 insulator that was introduced directly upstream of the PPT sequence. In order to increase the cloning capacity to be able to reintroduce E3 sequences in the 35.7 kb Ad[I/PPT-E1A] genome, various shorter insulators were examined in a luciferase reporter gene assay. It was found that the 1.6 kb core H19 insulator (i) improves the activity of PPT, compared to the 3.0 kb full-length insulator, while still maintaining prostate cell specificity and releasing 1.4 kb of space for insertion of additional sequences. To improve the ability of the virus to efficiently lyse infected cells and persist in vivo, we inserted the adenovirus death protein (ADP) or the full-length adenovirus E3 region. The oncolytic activity of PPT-E1A-based viruses was studied using MTS, crystal violet and replication assays. The virus with the reintroduced full-length E3-region (Ad[i/PPT-E1A, E3]) showed the highest cytopathic effects in vitro. Furthermore, this virus suppressed the growth of aggressively growing prostate tumors in vivo. Therefore, we conclude that Ad[i/PPT-E1A, E3] is a prostate-specific oncolytic adenovirus with a high potential for treating localized prostate cancer.

Circulating biomarkers for prostate cancer

Due to its significant applicability for early detection, risk prediction and follow-up evaluation, prostate specific antigen (PSA) has revolutionized our ability to treat prostate cancer patients. With the prevalent use of PSA for early detection during the last two decades, disease characteristics have been altered towards early detected, localized tumors with a high chance of cure following local therapy. This advantage faces the risk of overdetection and overtreatment. In addition, PSA lacks both, sensitivity and specificity to accurately detect patients at risk of prostate cancer. Therefore, novel biomarkers are urgently needed to improve identification of men at risk of having the disease and to predict the natural behaviour of the tumor. Recent advances in the evaluation of high-throughput technologies have led to the discovery of novel candidate markers for prostate cancer. This article will briefly discuss current PSA-based strategies and review several novel biomarkers for prostate cancer, detectable in blood.

Update on fused capromab pendetide imaging of prostate cancer

The primary objective of this overview is to apprise clinical urologists and oncologists of the current state of fused multimodality imaging of prostate cancer, which can be applied to optimize treatment by ensuring that a patient's disease is characterized as well as current imaging technology permits. The focus of this study is the monoclonal antibody capromab pendetide, which targets prostate specific membrane antigen (PSMA), a type II membrane glycoprotein strongly associated with prostate cancer. Identifying where capromab pendetide uptake occurs can be done accurately if this functional imaging modality is combined with a modality that provides anatomic detail, such as computed tomography (CT) or magnetic resonance imaging (MRI). Image fusion, or coregistration, which is overlaying the functional images of capromab pendetide uptake on the anatomic CT or MRI images, provides a detailed map of cancer localization inside and outside the prostate gland. This same principle of fusing functional images on anatomic images is the basis for enormous growth of positron emission tomography with CT during the past 2 years. Positron emission tomography imaging has a different functionality base than does capromab pendetide, and thus the 2 modalities should be complementary. However, the key to both functional imaging modalities is accurate fusion with anatomic images, which is illustrated in our case reports. The cases cited demonstrate the need to optimize every phase of imaging from patient preparation to reading and reporting increased PSMA concentration seen on the fused images. Reference is also made to applying capromab pendetide/CT fused imaging to radiation therapy planning.

A novel TARP-promoter-based adenovirus against hormone-dependent and hormone-refractory prostate cancer

TARP (T cell receptor gamma-chain alternate reading frame protein) is a protein that in males is uniquely expressed in prostate epithelial cells and prostate cancer cells. We have previously shown that the transcriptional activity of a chimeric sequence comprising the TARP promoter (TARPp) and the PSA enhancer (PSAe) is strictly controlled by testosterone and highly restricted to cells of prostate origin. Here we report that a chimeric sequence comprising TARPp and the PSMA enhancer (PSMAe) is highly active in testosterone-deprived prostate cancer cells, while a regulatory sequence comprising PSAe, PSMAe, and TARPp (PPT) has high prostate-specific activity both in the presence and in the absence of testosterone. Therefore, the PPT sequence may, in a gene therapy setting, be beneficial to prostate cancer patients that have been treated with androgen withdrawal. A recombinant adenovirus vector with the PPT sequence, shielded from interfering adenoviral sequences by the mouse H19 insulator, yields high and prostate-specific transgene expression both in cell cultures and when prostate cancer, PC-346C, tumors were grown orthotopically in nude mice. Intravenous virus administration reveals both higher activity and higher selectivity for the insulator-shielded PPT sequence than for the immediate-early CMV promoter. Therefore, we believe that an adenovirus with therapeutic gene expression controlled by an insulator-shielded PPT sequence is a promising candidate for gene therapy of prostate cancer.

Human dendritic cells genetically engineered to express cytosolically retained fragment of prostate-specific membrane antigen prime cytotoxic T-cell responses to multiple epitopes

The ability of two plasmid DNA vaccines to stimulate lymphocytes from normal human donors and to generate antigen-specific responses is demonstrated. The first vaccine (truncated; tPSMA) encodes for only the extracellular domain of prostate-specific membrane antigen (PSMA). The product, expressed following transfection with this vector, is retained in the cytosol and degraded by the proteasomes. For the "secreted" (sPMSA) vaccine, a signal peptide sequence is added to the expression cassette and the expressed protein is glycosylated and directed to the secretory pathway. Monocyte-derived dendritic cells (DCs) are transiently transfected with either sPSMA or tPSMA plasmids. The DCs are then used to activate autologous lymphocytes in an in vitro model of DNA vaccination. Lymphocytes are boosted following priming with transfected DCs or with peptide-pulsed monocytes. Their reactivity is tested against tumor cells or peptide-pulsed T2 target cells. Both tPSMA DCs and sPSMA DCs generate antigen-specific cytotoxic T-cell responses. The immune response is restricted toward one of the four PSMA-derived epitopes when priming and boosting is performed with sPSMA. In contrast, tPSMA-transfected DCs prime T cells toward several PSMA-derived epitopes. Subsequent repeated boosting with transfected DCs, however, restricts the immune response to a single epitope due to immunodominance.

Dipeptidyl peptidase IV activity and/or structure homologues (DASH) and their substrates in cancer

Post-translational modification of proteins is an important regulatory event. Numerous biologically active peptides that play an essential role in cancerogenesis contain an evolutionary conserved proline residue as a proteolytic-processing regulatory element. Proline-specific proteases could therefore be viewed as important "check-points". Limited proteolysis of such peptides may lead to quantitative but, importantly, due to the change of receptor preference, also qualitative changes of their signaling potential. Dipeptidyl peptidase-IV (DPP-IV, EC 3.4.14.5, identical with CD26) was for many years believed to be a unique cell membrane protease cleaving X-Pro dipeptides from the N-terminal end of peptides and proteins. Subsequently, a number of other molecules were discovered, exhibiting various degree of structural homology and DPP-IV-like enzyme activity, capable of cleaving similar set of substrates. These comprise for example, seprase, fibroblast activation protein alpha, DPP6, DPP8, DPP9, attractin, N-acetylated-alpha-linked-acidic dipeptidases I, II and L, quiescent cell proline dipeptidase, thymus-specific serine protease and DPP IV-beta. It is tempting to speculate their potential participation on DPP-IV biological function(s). Disrupted expression and enzymatic activity of "DPP-IV activity and/or structure homologues" (DASH) might corrupt the message carried by their substrates, promoting abnormal cell behavior. Consequently, modulation of particular enzyme activity using e.g. DASH inhibitors, specific antibodies or DASH expression modification may be an attractive therapeutic concept in cancer treatment. This review summarizes recent information on the interactions between DASH members and their substrates with respect to their possible role in cancer biology.

Phosphonate and phosphinate analogues of N-acylated gamma-glutamylglutamate. potent inhibitors of glutamate carboxypeptidase II

Phosphonate and phosphinate analogues of N-acylated gamma-glutamylglutamate were tested for the ability to inhibit glutamate carboxypeptidase II (GCP II). All of the compounds inhibit GCP II with IC(50) values in the low nanomolar range. The comparison of the results to previously reported inhibitory studies of the same compounds toward folylpoly-gamma-glutamyl synthetase (FPGS) and gamma-glutamyl hydrolase (gamma-GH) provides insight into structural and mechanistic features of each enzyme. Potential utility of these compounds as diagnostic agents and probes to understand folate or antifolate poly-gamma-glutamates metabolism is also described.

Separation methods applicable to prostate cancer diagnosis and monitoring therapy

During the last decade, significant research has been conducted using prostate-specific antigen (PSA) in the basic and clinical sciences and many advances have occurred in the clinical use of PSA for detecting and monitoring prostate cancer (PCa). Separation methods including gel-permeation chromatography, isoelectric focusing, lectin-affinity chromatography, polyacrylamide gel electrophoresis and high-performance liquid chromatography have made significant contributions to the discovery and identification of different molecular forms of PSA. Furthermore, the measurement of free and total PSA has improved the ability of PSA to detect early PCa. However, unnecessary biopsies are still needed for men with slightly elevated PSA values. On the other hand, PSA is not adequate for staging newly diagnosed PCa and prognosticating the course in individual cases. The possible application of separation methods in the basic science of prostate cancer may be associated with identification of more cancer-specific forms of PSA and discoveries of other serum proteins useful not only for detecting, but also for staging and prognosticating PCa. Such novel markers might lead to a better understanding of PCa aggressiveness and to developments in the clinical field of treatment.

Multicenter ProstaScint imaging findings in 2154 patients with prostate cancer. The ProstaScint Imaging Centers

OBJECTIVES

To report the results of a retrospective study of 2290 ProstaScint scans of 2154 patients with prostate carcinoma done at 15 institutions.

METHODS

The results were analyzed by logistic regression after stratification of the patients into four groups: group 1, newly diagnosed; group 2, after radical prostatectomy with a rising prostate-specific antigen (PSA) level; group 3, after radiation therapy with a rising PSA level; and group 4, after hormonal therapy.

RESULTS

The PSA level and ProstaScint scans positive in the prostate bed (P <0.001) and for pelvic metastases (P <0.001), but not for extrapelvic metastases, correlated significantly in group 1 patients. In group 2, the association for detecting fossa recurrence was weaker (P = 0.033) and was insignificant for pelvic and extrapelvic metastases. Patients in group 3 also exhibited a weak PSA-ProstaScint association for detecting fossa recurrence (P = 0.038), and was insignificant for pelvic and extrapelvic metastases. No significant PSA-ProstaScint correlation was found in patients in group 4 for fossa recurrence, pelvic or extrapelvic metastases. The distribution of positive ProstaScint results among the prostate/prostate bed, pelvic nodes, and extrapelvic nodes was nearly equal for all groups, except that a significantly greater percentage of extrapelvic metastases was found in the hormonal group (group 4). The ProstaScint results were independent of the Gleason score for 260 patients before and 285 patients after therapy.

CONCLUSIONS

The results of this study underscore the complementary diagnostic value of ProstaScint to PSA level and Gleason score as an independent indicator of prostate cancer recurrence and metastases and in identifying extrapelvic metastases in both newly diagnosed and recurrent prostate cancer.