Prostate-specific membrane antigen-based therapeutics

Listeria monocytogenes: a promising vector for tumor immunotherapy

Cancer receives enduring international attention due to its extremely high morbidity and mortality. Immunotherapy, which is generally expected to overcome the limits of traditional treatments, serves as a promising direction for patients with recurrent or metastatic malignancies. Bacteria-based vectors such as Listeria monocytogenes take advantage of their unique characteristics, including preferential infection of host antigen presenting cells, intracellular growth within immune cells, and intercellular dissemination, to further improve the efficacy and minimize off-target effects of tailed immune treatments. Listeria monocytogenes can reshape the tumor microenvironment to bolster the anti-tumor effects both through the enhancement of T cells activity and a decrease in the frequency and population of immunosuppressive cells. Modified Listeria monocytogenes has been employed as a tool to elicit immune responses against different tumor cells. Currently, Listeria monocytogenes vaccine alone is insufficient to treat all patients effectively, which can be addressed if combined with other treatments, such as immune checkpoint inhibitors, reactivated adoptive cell therapy, and radiotherapy. This review summarizes the recent advances in the molecular mechanisms underlying the involvement of Listeria monocytogenes vaccine in anti-tumor immunity, and discusses the most concerned issues for future research.

PSMA Targeted Molecular Imaging and Radioligand Therapy for Prostate Cancer: Optimal Patient and Treatment Issues

Theranostics (therapy + diagnosis) targeting prostate-specific membrane antigen (PSMA) is an emerging therapeutic modality that could alter treatment strategies for prostate cancer. Although PSMA-targeted radioligand therapy (PSMA-RLT) has a highly therapeutic effect on PSMA-positive tumor tissue, the efficacy of PSMA-RLT depends on PSMA expression. Moreover, predictors of treatment response other than PSMA expression are under investigation. Therefore, the optimal patient population for PSMA-RLT remains unclear. This review provides an overview of the current status of theranostics for prostate cancer, focusing on PSMA ligands. In addition, we summarize various findings regarding the efficacy and problems of PSMA-RLT and discuss the optimal patient for PSMA-RLT.

Tumor-Targeting Ability of Novel Anti-Prostate-Specific Membrane Antigen Antibodies

Patients with prostate-specific membrane antigen (PSMA)-positive tumors can benefit from PSMA-targeted therapy; thus, we have constructed a phage-displayed synthetic antibody library for the production of novel PSMA antibodies with superior PSMA-targeting ability, favoring clinical management. The binding affinities of anti-PSMA antibodies were verified by an enzyme-linked immunosorbent assay (ELISA). Several in vitro and in vivo experiments, including cellular uptake, internalization, and cytotoxicity studies, micro single photon emission computed tomography (microSPECT)/CT, and biodistribution studies, were performed to select the most promising antibody among six different antibodies. The results showed the target affinities of our antibodies in the ELISA assays (7A, 8C, 8E, and 11A) were comparable to the existing antibodies (J591). The half-maximal effective concentrations of 7A, 8C, 8E, 11A, and J591 were 2.95, 6.64, 5.50, 2.08, and 4.79, respectively. The radiochemical yield of ¹¹¹In-labeled antibodies ranged from 30% to 50% with high radiochemical purity (>90%). In the cellular uptake studies, the accumulated radioactivity of ¹¹¹In-J591, ¹¹¹In-7A, and ¹¹¹In-11A increased over time. The internalized percentage of ¹¹¹In-11A was the highest (32.14% ± 2.06%) at 48 h after incubation, whereas that of ¹¹¹In-J591 peaked at 22.43% ± 4.38% at 24 h and dropped to 13.52% ± 3.03% at 48 h postincubation. Twenty-four hours after injection, radioactivity accumulation appeared in the LNCaP xenografts of the mice injected with ¹¹¹In-11A, ¹¹¹In-8E, ¹¹¹In-7A, and ¹¹¹In-J591 but not in the xenografts of the ¹¹¹In-8C-injected group. Marked liver uptake was noticed in all groups except the ¹¹¹In-11A-injected group. Moreover, the killing effect of ¹⁷⁷Lu-11A was superior to that of ¹⁷⁷Lu-J591 at low concentrations. In conclusion, we successfully demonstrated that 11A IgG owned the most optimal biological characteristics among several new anti-PSMA antibodies and it can be an excellent PSMA-targeting component for the clinical use.

Peptide therapeutics in the management of metastatic cancers

Cancer remains a leading health concern threatening lives of millions of patients worldwide. Peptide-based drugs provide a valuable alternative to chemotherapeutics as they are highly specific, cheap, less toxic and easier to synthesize compared to other drugs. In this review, we have discussed various modes in which peptides are being used to curb cancer. Our review highlights specially the various anti-metastatic peptide-based agents developed by targeting a plethora of cellular factors. Herein we have given a special focus on integrins as targets for peptide drugs, as these molecules play key roles in metastatic progression. The review also discusses use of peptides as anti-cancer vaccines and their efficiency as drug-delivery tools. We hope this work will give the reader a clear idea of the mechanisms of peptide-based anti-cancer therapeutics and encourage the development of superior drugs in the future.

ImmunoPET: Antibody-Based PET Imaging in Solid Tumors

Immuno-positron emission tomography (immunoPET) is a molecular imaging modality combining the high sensitivity of PET with the specific targeting ability of monoclonal antibodies. Various radioimmunotracers have been successfully developed to target a broad spectrum of molecules expressed by malignant cells or tumor microenvironments. Only a few are translated into clinical studies and barely into clinical practices. Some drawbacks include slow radioimmunotracer kinetics, high physiologic uptake in lymphoid organs, and heterogeneous activity in tumoral lesions. Measures are taken to overcome the disadvantages, and new tracers are being developed. In this review, we aim to mention the fundamental components of immunoPET imaging, explore the groundbreaking success achieved using this new technique, and review different radioimmunotracers employed in various solid tumors to elaborate on this relatively new imaging modality.

Peptides-Coated Oncolytic Vaccines for Cancer Personalized Medicine

Oncolytic Viruses (OVs) work through two main mechanisms of action: the direct lysis of the virus-infected cancer cells and the release of tumor antigens as a result of the viral burst. In this sc.enario, the OVs act as in situ cancer vaccines, since the immunogenicity of the virus is combined with tumor antigens, that direct the specificity of the anti-tumor adaptive immune response. However, this mechanism in some cases fails in eliciting a strong specific T cell response. One way to overcome this problem and enhance the priming efficiency is the production of genetically modified oncolytic viruses encoding one or more tumor antigens. To avoid the long and expensive process related to the engineering of the OVs, we have exploited an approach based on coating OVs (adenovirus and vaccinia virus) with tumor antigens. In this work, oncolytic viruses encoding tumor antigens and tumor antigen decorated adenoviral platform (PeptiCRAd) have been used as cancer vaccines and evaluated both for their prophylactic and therapeutic efficacy. We have first tested the oncolytic vaccines by exploiting the OVA model, moving then to TRP2, a more clinically relevant tumor antigen. Finally, both approaches have been investigated in tumor neo-antigens settings. Interestingly, both genetically modified oncolytic adenovirus and PeptiCRAd elicited T cells-specific anti-tumor responses. However, in vitro cross-representation experiments, showed an advantage of PeptiCRAd as regards the fast presentation of the model epitope SIINFEKL from OVA in an immunogenic rather than tolerogenic fashion. Here two approaches used as cancer oncolytic vaccines have been explored and characterized for their efficacy. Although the generation of specific anti-tumor T cells was elicited in both approaches, PeptiCRAd retains the advantage of being rapidly adaptable by coating the adenovirus with a different set of tumor antigens, which is crucial in personalized cancer vaccines clinical setting.

Global experience with PSMA-based alpha therapy in prostate cancer

PURPOSE

This review discusses the current state of prostate-specific membrane antigen (PSMA)-based alpha therapy of metastatic castration-resistant prostate cancer (mCRPC). With this in-depth discussion on the growing field of PSMA-based alpha therapy (PAT), we aimed to increase the interactions between basic scientists and physician-scientists in order to advance the field.

METHODS

To achieve this, we discuss the potential, current status, and opportunities for alpha therapy and strategies, attempted to date, and important questions that need to be addressed. The paper reviews important concepts, including whom to treat, how to treat, what to expect regarding treatment outcome, and toxicity, and areas requiring further investigations.

RESULTS

There is much excitement about the potential of this field. Much of the potential exists because these therapies utilize unique mechanisms of action, difficult to achieve with other conventional therapies.

CONCLUSION

A better understanding of the strengths and limitations of PAT may help in creating an effective therapy for mCRPC and design a rational combinatorial approach to treatment by targeting different tumor pathways.

Bio-vehicles of cytotoxic drugs for delivery to tumor specific targets for cancer precision therapy

Abnormal structural and molecular changes in malignant tissues were thoroughly investigated and utilized to target tumor cells, hence rescuing normal healthy tissues and lowering the unwanted side effects as non-specific cytotoxicity. Various ligands for cancer cell specific markers have been uncovered and inspected for directional delivery of the anti-cancer drug to the tumor site, in addition to diagnostic applications. Over the past few decades research related to the ligand targeted therapy (LTT) increased tremendously aiming to treat various pathologies, mainly cancers with well exclusive markers. Malignant tumors are known to induce elevated levels of a variety of proteins and peptides known as cancer "markers" as certain antigens (e.g., Prostate specific membrane antigen "PSMA", carcinoembryonic antigen "CEA"), receptors (folate receptor, somatostatin receptor), integrins (Integrin αvβ3) and cluster of differentiation molecules (CD13). The choice of an appropriate marker to be targeted and the design of effective ligand-drug conjugate all has to be carefully selected to generate the required therapeutic effect. Moreover, since some tumors express aberrantly high levels of more than one marker, some approaches investigated targeting cancer cells with more than one ligand (dual or multi targeting). We aim in this review to report an update on the cancer-specific receptors and the vehicles to deliver cytotoxic drugs, including recent advancements on nano delivery systems and their implementation in targeted cancer therapy. We will discuss the advantages and limitations facing this approach and possible solutions to mitigate these obstacles. To achieve the said aim a literature search in electronic data bases (PubMed and others) using keywords "Cancer specific receptors, cancer specific antibody, tumor specific peptide carriers, cancer overexpressed proteins, gold nanotechnology and gold nanoparticles in cancer treatment" was carried out.

PSMA-Targeting Radiopharmaceuticals for Prostate Cancer Therapy: Recent Developments and Future Perspectives

Simple Summary

One of the most frequently diagnosed cancer in men is adenocarcinoma of the prostate. Once the disease is metastatic, only very limited treatment options are available, resulting in a very short median survival time of 13 months; however, this reality is gradually changing due to the discovery of prostate-specific membrane antigen (PSMA), a protein that is present in cancerous prostate tissue. Researchers have developed pharmaceuticals specific for PSMA, ranging from antibodies (mAb) to low-molecular weight molecules coupled to beta minus and alpha-emitting radionuclides for their use in targeted radionuclide therapy (TRT). TRT offers the possibility of selectively removing cancer tissue via the emission of radiation or radioactive particles within the tumour. In this article, the major milestones in PSMA ligand research and the therapeutic developments are summarised, together with a future perspective on the enhancement of current therapeutic approaches.

Abstract

Prostate cancer (PC) is the second most common cancer among men, with 1.3 million yearly cases worldwide. Among those cancer-afflicted men, 30% will develop metastases and some will progress into metastatic castration-resistant prostate cancer (mCRPC), which is associated with a poor prognosis and median survival time that ranges from nine to 13 months. Nevertheless, the discovery of prostate specific membrane antigen (PSMA), a marker overexpressed in the majority of prostatic cancerous tissue, revolutionised PC care. Ever since, PSMA-targeted radionuclide therapy has gained remarkable international visibility in translational oncology. Furthermore, on first clinical application, it has shown significant influence on therapeutic management and patient care in metastatic and hormone-refractory prostate cancer, a disease that previously had remained immedicable. In this article, we provide a general overview of the main milestones in the development of ligands for PSMA-targeted radionuclide therapy, ranging from the firstly developed monoclonal antibodies to the current state-of-the-art low molecular weight entities conjugated with various radionuclides, as well as potential future efforts related to PSMA-targeted radionuclide therapy.

Safety and preliminary immunogenicity of JNJ-64041809, a live-attenuated, double-deleted Listeria monocytogenes-based immunotherapy, in metastatic castration-resistant prostate cancer

Background

The safety and immunogenicity of JNJ-64041809 (JNJ-809), a live-attenuated, double-deleted Listeria monocytogenes (LADD Lm)-based immunotherapy targeting 4 relevant prostate cancer antigens, was evaluated in a phase 1 study in patients with metastatic castration-resistant prostate cancer (mCRPC).

Methods

Men with progressive mCRPC who had received ≥2 prior approved therapies were enrolled. Primary study objectives were to determine the recommended phase 2 dose (RP2D) and to evaluate the safety and immunogenicity of JNJ-809.

Results

A total of 26 patients received JNJ-809 (1 × 10⁸ CFU (n = 6); 1 × 10⁹ CFU (n = 20)). No dose-limiting toxicities were reported, and 1 × 10⁹ CFU was selected as the RP2D. The most common adverse events (AEs) reported were chills (92%), pyrexia (81%), and fatigue (62%). The most frequent grade ≥3 AEs were lymphopenia (27%) and hypertension (23%). Serious AEs were reported in 27% of patients including 1 patient with grade 3 intestinal obstruction. JNJ-809 transiently induced peripheral cytokines, including interferon-γ, interleukin-10, and tumor necrosis factor-α. Of the 7 patients evaluable for T cell responses at the 1 × 10⁹ CFU dose, evidence of post-treatment antigenic responses were observed in 6 to the Listeria antigen listeriolysin O and in 5 to ≥1 of the 4 encoded tumor antigens. Best overall response was stable disease in 13/25 response-evaluable patients. The study was terminated early as data collected were considered sufficient to evaluate safety and immunogenicity.

Conclusions

JNJ-809 has manageable safety consistent with other LADD Lm-based therapies. Limited antigen-specific immune responses were observed, which did not translate into objective clinical responses.

Targeted Nanobubbles Carrying Indocyanine Green for Ultrasound, Photoacoustic and Fluorescence Imaging of Prostate Cancer

Objective

To construct prostate-specific membrane antigen (PSMA)-targeting, indocyanine green (ICG)-loaded nanobubbles (NBs) for multimodal (ultrasound, photoacoustic and fluorescence) imaging of prostate cancer.

Methods

The mechanical oscillation method was used to prepare ICG-loaded photoacoustic NBs (ICG NBs). Then, PSMA-binding peptides were connected to the surface of ICG NBs using the biotin–avidin method to make targeted photoacoustic NBs, namely, PSMAP/ICG NBs. Their particle sizes, zeta potentials, and in vitro ultrasound, photoacoustic and fluorescence imaging were examined. Confocal laser scanning microscopy and flow cytometry were used to detect the binding ability of the PSMAP/ICG NBs to PSMA-positive LNCaP cells, C4-2 cells, and PSMA-negative PC-3 cells. The multimodal imaging effects of PSMAP/ICG NBs and ICG NBs were compared in nude mouse tumor xenografts.

Results

The particle size of the PSMAP/ICG NBs was approximately 457.7 nm, and the zeta potential was approximately −23.5 mV. Both confocal laser scanning microscopy and flow cytometry confirmed that the PSMAP/ICG NBs could specifically bind to both LNCaP and C4-2 cells, but they rarely bound to PC-3 cells. The ultrasound, photoacoustic and fluorescence imaging intensities of the PSMAP/ICG NBs in vitro positively correlated with their concentrations. The ultrasound and photoacoustic imaging effects of the PSMAP/ICG NBs in LNCaP and C4-2 tumor xenografts were significantly enhanced compared with those in PC-3 tumor xenografts, which were characterized by a significantly increased duration of ultrasound enhancement and heightened photoacoustic signal intensity (P < 0.05). Fluorescence imaging showed that PSMAP/ICG NBs could accumulate in LNCaP and C4-2 tumor xenografts for a relatively long period.

Conclusion

The targeted photoacoustic nanobubbles prepared in this study can specifically bind to PSMA-positive prostate cancer cells and have the ability to enhance ultrasound, photoacoustic and fluorescence imaging of PSMA-positive tumor xenografts. Photoacoustic imaging could visually display the intensity of the red photoacoustic signal in the tumor region, providing a more intuitive imaging modality for targeted molecular imaging. This study presents a potential multimodal contrast agent for the accurate diagnosis and assessment of prostate cancer.

Prostate-specific Membrane Antigen (PSMA) Expression in the Neovasculature of Gynecologic Malignancies: Implications for PSMA-targeted Therapy

The goal of the study was to examine expression of prostate-specific membrane antigen (PSMA) in neovasculature of gynecologic cancers, as PSMA-targeted therapy has showed a promise in treatment of advanced carcinomas. The study included cervical carcinoma (n=28), vulvar carcinoma (n=20), endometrial carcinoma (n=23), primary ovarian carcinoma (n=21), metastatic ovarian carcinoma (n=25), and normal cervix (n=12) as negative control. All cases were immunostained using anti-CD31 antibody to delineate capillary endothelial cells. In parallel, all cases were immunostained using anti-PSMA antibody. The PSMA staining was assessed in tumor capillaries and in normal tissues and scored as a percentage of CD31 staining. PSMA expression was found in the tumor neovasculature, and no significant expression was identified in vasculature of normal tissues. The extent of PSMA staining in tumor capillaries varied from high expression in ovarian and endometrial cancers, to medium expression in cervical squamous cell carcinomas, and low expression in cervical adenocarcinomas and vulvar cancers. All (100%) cases of primary ovarian carcinoma, ovarian carcinoma metastases, and primary endometrial carcinoma showed PSMA expression in tumor vasculature, which was diffuse in majority of cases. The expression of PSMA in ovarian cancer metastases was similar among different metastatic foci of the same tumor. Fifteen percent of cervical squamous cell carcinoma, 50% of cervical adenocarcinoma, and 75% of vulvar carcinomas showed no capillary expression of PSMA. In conclusion, PSMA is highly and specifically expressed in the neovasculature of ovarian, endometrial, and cervical squamous carcinoma, rendering it a potential therapeutic vascular target.

Prostate cancer: updates on current strategies for screening, diagnosis and clinical implications of treatment modalities

Prostate cancer is the most common cancer in men by way of diagnosis and a leading cause of cancer-related deaths. Early detection and intervention remains key to its optimum clinical management. This review provides the most updated information on the recent methods of prostate cancer screening, imaging and treatment modalities. Wherever possible, clinical trial data has been supplemented to provide a comprehensive overview of current prostate cancer research and development. Considering the recent success of immunotherapy in prostate cancer, we discuss cell, DNA and viruses based, as well as combinatorial immunotherapeutic strategies in detail. Furthermore, the potential of nanotechnology is increasingly being realized, especially in prostate cancer research, and we provide an overview of nanotechnology-based strategies, with special emphasis on nanotheranostics and multifunctional nanoconstructs. Understanding these recent developments is critical to the design of future therapeutic strategies to counter prostate cancer.

Prostate-Specific Membrane Antigen Targeted Therapy of Prostate Cancer Using a DUPA-Paclitaxel Conjugate

Prostate cancer (PCa) is the most prevalent cancer among men in the United States and remains the second-leading cause of cancer mortality in men. Paclitaxel (PTX) is the first line chemotherapy for PCa treatment, but its therapeutic efficacy is greatly restricted by the nonspecific distribution in vivo. Prostate-specific membrane antigen (PSMA) is overexpressed on the surface of most PCa cells, and its expression level increases with cancer aggressiveness, while being present at low levels in normal cells. The high expression level of PSMA in PCa cells offers an opportunity for target delivery of nonspecific cytotoxic drugs to PCa cells, thus improving therapeutic efficacy and reducing toxicity. PSMA has high affinity for DUPA, a glutamate urea ligand. Herein, a novel DUPA-PTX conjugate is developed using DUPA as the targeting ligand to deliver PTX specifically for treatment of PSMA expressing PCa. The targeting ligand DUPA enhances the transport capability and selectivity of PTX to tumor cells via PSMA mediated endocytosis. Besides, DUPA is conjugated with PTX via a disulfide bond, which facilitates the rapid and differential drug release in tumor cells. The DUPA-PTX conjugate exhibits potent cytotoxicity in PSMA expressing cell lines and induces a complete cessation of tumor growth with no obvious toxicity. Our findings give new insight into the PSMA-targeted delivery of chemotherapeutics and provide an opportunity for the development of novel active targeting drug delivery systems for PCa therapy.

PSMA-homing dsRNA chimeric protein vector kills prostate cancer cells and activates anti-tumor bystander responses

The treatment of metastatic androgen-resistant prostate cancer remains a challenge. We describe a protein vector that selectively delivers synthetic dsRNA, polyinosinic/polycytidylic acid (polyIC), to prostate tumors by targeting prostate specific membrane antigen (PSMA), which is overexpressed on the surface of prostate cancer cells.

The chimeric protein is built from the double stranded RNA (dsRNA) binding domain of PKR tethered to a single chain anti-PSMA antibody. When complexed with polyIC, the chimera demonstrates selective and efficient killing of prostate cancer cells. The treatment causes the targeted cancer cells to undergo apoptosis and to secrete toxic cytokines. In a bystander effect, these cytokines kill neighboring cancer cells that do not necessarily overexpress PSMA, and activate immune cells that enhance the killing effect. The strong effects of the targeted polyIC are demonstrated on both 2D cell cultures and 3D tumor spheroids.

Global comparison of targeted alpha vs targeted beta therapy for cancer: In vitro, in vivo and clinical trials

Targeted therapy for cancer is a rapidly expanding and successful approach to the management of many intractable cancers. However, many immunotherapies fail in the longer term and there continues to be a need for improved targeted cancer cell toxicity, which can be achieved by radiolabelling the targeting vector with a radioisotope. Such constructs are successful in using a gamma ray emitter for imaging. However, traditionally, a beta emitter is used for therapeutic applications. The new approach is to use the short range and highly cytotoxic alpha radiation from alpha emitters to achieve improved efficacy and therapeutic gain. This paper sets out to review all experimental and theoretical comparisons of efficacy and therapeutic gain for alpha and beta emitters labelling the same targeting vector. The overall conclusion is that targeted alpha therapy is superior to targeted beta therapy, such that the use of alpha therapy in clinical settings should be expanded.

Expression of Prostate-Specific Membrane Antigen (PSMA) in Brain Glioma and its Correlation with Tumor Grade

BACKGROUND & OBJECTIVE

Angiogenesis is an essential component of tumor growth. Expression of PSMA on the neo-vasculature of many solid tumors, including glioblastoma multi-form, has been determined. The pattern of expression suggests that PSMA may play a functional role in angiogenesis.

METHODS

expression of PSMA in different grades of brain glioma was evaluated by the immunohistochemistry method to determine the probable usefulness of anti-PSMA antibody as complementary target therapy in different grades of glioma.

RESULTS

Overall, 72 cases of low (grade I and II) and high (grade III and IV) grade gliomas were evaluated for expression of PSMA. Positive PSMA staining was observed in 12 (33.3%) of high grade and 3 (8.3%) of low grade gliomas. Although, high grade tumors more commonly had positive result for PSMA (P value=0.009), the intensity of staining was significantly stronger in low-grade tumors (P value=0.009).

CONCLUSION

Expression of PSMA in different grades of glioma might provide a basis for further investigations focusing on selective target therapy in combination with the current standard care in all glioma grades, to improve treatment efficacy.

Synthesis and evaluation of a novel urea-based 68Ga-complex for imaging PSMA binding in tumor

INTRODUCTION

Prostate specific membrane antigen (PSMA) is a well-established target for diagnostic and therapeutic applications for prostate cancer. It is know that [⁶⁸Ga]PSMA 11 ([⁶⁸Ga]Glu-NH-CO-NH-Lys(Ahx)-HBED-CC) is the most well studied PET imaging agent for detecting over expressed PSMA binding sites of tumors in humans. In an effort to provide new agents with improved characteristics for PET imaging, we report a novel [⁶⁸Ga]-Glu-NH-CO-NH-Lys(Ahx)-linker-HBED-CC conjugate with a novel O-(carboxymethyl)-L-tyrosine, as the linker group.

METHODS

Radiosynthesis was performed by a direct method. In vitro binding and cell internalization of [⁶⁸Ga]10 was investigated in PSMA positive LNCaP cell lines. Biodistribution and MicroPET imaging studies were performed in LNCaP tumor bearing mice.

RESULTS

In vitro binding to LNCaP cells showed that ^natGa labeled O-(carboxymethyl)-L-tyrosine conjugate, [^natGa]10, displayed excellent affinity and specificity (IC₅₀ = 16.5 nM) a value comparable to that of PSMA 11. In vitro cell binding and internalization showed excellent uptake and retention; [⁶⁸Ga]10 displayed significantly higher cellular internalization than [⁶⁸Ga]PSMA 11 (12.5 vs 7.4% ID/10⁶ cells at 1 h). Biodistribution studies in LNCaP tumor-bearing mice exhibited a high specific uptake in PSMA expressing tumors and fast clearance in normal organs (19.7 tumor/blood; 20.7 tumor/muscle at 1 h after iv injection). MicroPET imaging studies in mice confirmed that [⁶⁸Ga]10 displayed excellent uptake and distinctive tumor localization, which was blocked by iv injection of a competing drug, 2-PMPA.

CONCLUSIONS

The preliminary results strongly suggest that [⁶⁸Ga]10 may be promising candidates as a PET imaging radiotracer for detecting PSMA expression in prostate cancer.

Novel PSCA targeting scFv-fusion proteins for diagnosis and immunotherapy of prostate cancer

PURPOSE

Despite great progress in the diagnosis and treatment of localized prostate cancer (PCa), there remains a need for new diagnostic markers that can accurately distinguish indolent and aggressive variants. One promising approach is the antibody-based targeting of prostate stem cell antigen (PSCA), which is frequently overexpressed in PCa. Here, we show the construction of a molecular imaging probe comprising a humanized scFv fragment recognizing PSCA genetically fused to an engineered version of the human DNA repair enzyme O6-alkylguanine-DNA alkyltransferase (AGT), the SNAP-tag, enabling specific covalent coupling to various fluorophores for diagnosis of PCa. Furthermore, the recombinant immunotoxin (IT) PSCA(scFv)-ETA' comprising the PSCA(scFv) and a truncated version of Pseudomonas exotoxin A (PE, ETA') was generated.

METHODS

We analyzed the specific binding and internalization behavior of the molecular imaging probe PSCA(scFv)-SNAP in vitro by flow cytometry and live cell imaging, compared to the corresponding IT PSCA(scFv)-ETA'. The cytotoxic activity of PSCA(scFv)-ETA' was tested using cell viability assays. Specific binding was confirmed on formalin-fixed paraffin-embedded tissue specimen of early and advanced PCa.

RESULTS

Alexa Fluor® 647 labeling of PSCA(scFv)-SNAP confirmed selective binding to PSCA, leading to rapid internalization into the target cells. The recombinant IT PSCA(scFv)-ETA' showed selective binding leading to internalization and efficient elimination of target cells.

CONCLUSIONS

Our data demonstrate, for the first time, the specific binding, internalization, and cytotoxicity of a scFv-based fusion protein targeting PSCA. Immunohistochemical staining confirmed the specific ex vivo binding to primary PCa material.

Enterolactone glucuronide and β-glucuronidase in antibody directed enzyme prodrug therapy for targeted prostate cancer cell treatment

Evidence from preclinical and animal studies demonstrated an anticancer effect of flaxseed lignans, particularly enterolactone (ENL), against prostate cancer. However, extensive first-pass metabolism following oral lignan consumption results in their systemic availability primarily as glucuronic acid conjugates (ENL-Gluc) and their modest in vivo effects. To overcome the unfavorable pharmacokinetics and improve their effectiveness in prostate cancer, antibody-directed enzyme prodrug therapy (ADEPT) might offer a novel strategy to allow for restricted activation of ENL from circulating ENL-Gluc within the tumor environment. The anti-prostate-specific membrane antigen (PSMA) antibody D7 was fused with human β-glucuronidase (hβG) via a flexible linker. The binding property of the fusion construct, D7-hβG, against purified or cell surface PSMA was determined by flow cytometry and Octet Red 384 system, respectively, with a binding rate constant, K _d, of 2.5 nM. The enzymatic activity of D7-hβG was first tested using the probe, 4-methylumbelliferone glucuronide. A 3.8-fold greater fluorescence intensity was observed at pH 4.5 at 2 h compared with pH 7.4. The ability of D7-hβG to activate ENL from ENL-Gluc was tested and detected using LC-MS/MS. Enhanced generation of ENL was observed with increasing ENL-Gluc concentrations and reached 3613.2 ng/mL following incubation with 100 μM ENL-Gluc at pH 4.5 for 0.5 h. D7-hβG also decreased docetaxel IC₅₀ value from 23 nM to 14.9 nM in C4-2 cells. These results confirmed the binding and activity of D7-hβG and additional in vitro investigation is needed to support the future possibility of introducing this ADEPT system to animal models.

Cell growth inhibition and apoptotic effects of a specific anti-RTFscFv antibody on prostate cancer, but not glioblastoma, cells

Background: Single chain antibody (scFv) has shown interesting results in cancer immunotargeting approaches, due to its advantages over monoclonal antibodies. Regeneration and tolerance factor (RTF) is one of the most important regulators of extracellular and intracellular pH in eukaryotic cells. In this study, the inhibitory effects of a specific anti-RTF scFv were investigated and compared between three types of prostate cancer and two types of glioblastoma cells.  Methods: A phage antibody display library of scFv was used to select specific scFvs against RTF using panning process. The reactivity of a selected scFv was assessed by phage ELISA. The anti-proliferative and apoptotic effects of the antibody on prostate cancer (PC-3, Du-145 and LNCaP) and glioblastoma (U-87 MG and A-172) cell lines were investigated by MTT and Annexin V/PI assays.  Results: A specific scFv with frequency 35% was selected against RTF epitope. This significantly inhibited the proliferation of the prostate cells after 24 h. The percentages of cell viability (using 1000 scFv/cell) were 52, 61 and 73% for PC-3, Du-145 and LNCaP cells, respectively, compared to untreated cells. The antibody (1000 scFv/cell) induced apoptosis at 50, 40 and 25% in PC-3, Du-145 and LNCaP cells, respectively. No growth inhibition and apoptotic induction was detected for U-87 and A172 glioblastoma cells.  Conclusions: Anti-RTFscFv significantly reduced the proliferation of the prostate cancer cells. The inhibition of cell growth and apoptotic induction effects in PC-3 cells were greater than Du-145 and LNCaP cells. This might be due to higher expression of RTF antigen in PC-3 cells and/or better accessibility of RTF to scFv antibody. The resistance of glioblastoma cells to anti-RTF scFv offers the existence of mechanism(s) that abrogate the inhibitory effect(s) of the antibody to RTF. The results suggest that the selected anti-RTF scFv antibody could be an effective new alternative for prostate cancer immunotherapy.

In Vivo 3T Magnetic Resonance Imaging Using a Biologically Specific Contrast Agent for Prostate Cancer: A Nude Mouse Model

We characterized in vivo a functional superparamagnetic iron-oxide magnetic resonance contrast agent that shortens the T2 relaxation time in magnetic resonance imaging (MRI) of prostate cancer xenografts. The agent was developed by conjugating Molday ION™ carboxyl-6 (MIC6), with a deimmunized mouse monoclonal antibody (muJ591) targeting prostate-specific membrane antigen (PSMA). This functional contrast agent could be used as a noninvasive method to detect prostate cancer cells that are PSMA positive and more readily differentiate them from surrounding tissues for treatment. The functional contrast agent was injected intravenously into mice and its effect was compared to both MIC6 (without conjugated antibody) and phosphate-buffered saline (PBS) injection controls. MR imaging was performed on a clinical 3T MRI scanner using a multiecho spin echo (MESE) sequence to obtain T2 relaxation time values. Inductively coupled plasma atomic emission spectroscopy was used to confirm an increase in elemental iron in injected mice tumours relative to controls. Histological examination of H&E stained tissues showed normal morphology of the tissues collected.

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.

Influence of Androgen Deprivation Therapy on the Uptake of PSMA-Targeted Agents: Emerging Opportunities and Challenges

Prostate-specific membrane antigen (PSMA) is an attractive target for both diagnosis and therapy because of its high expression in the vast majority of prostate cancers. Development of small molecules for targeting PSMA is important for molecular imaging and radionuclide therapy of prostate cancer. Recent evidence implies that androgen-deprivation therapy increase PSMA-ligand uptake in some cases. The reported upregulations in PSMA-ligand uptake after exposure to second-generation antiandrogens such as enzalutamide and abiraterone might disturb PSMA-targeted imaging for staging and response monitoring of patients undergoing treatment with antiandrogen-based drugs. On the other hand, second-generation antiandrogens are emerging as potential endoradio-/chemosensitizers. Therefore, the enhancement of the therapeutic efficiency of PSMA-targeted theranostic methods can be listed as a new capability of antiandrogens. In this manuscript, we will present what is currently known about the mechanism of increasing PSMA uptake following exposure to antiandrogens. In addition, we will discuss whether these above-mentioned antiandrogens could play the role of endoradio-/chemosensitizers in combination with the well-established PSMA-targeted methods for pre-targeting of prostate cancer.

Induction of PSMA and Internalization of an Anti-PSMA mAb in the Vascular Compartment

Angiogenesis is critical for tumor growth and survival and involves interactions between cancer and endothelial cells. Prostate-specific membrane antigen (PSMA/FOLH1) is expressed in the neovasculature of several types of cancer. However, the study of neovascular PSMA expression has been impeded as human umbilical vein endothelial cell (HUVEC) cultures are PSMA-negative and both tumor xenografts and patient-derived xenograft (PDX) models are not known to express PSMA in their vasculature. Therefore, PSMA expression was examined in HUVECs, in vitro and in vivo, and we tested the hypothesis that cancer cell-HUVEC crosstalk could induce the expression of PSMA in HUVECs. Interestingly, conditioned media from several cancer cell lines induced PSMA expression in HUVECs, in vitro, and these lines induced PSMA, in vivo, in a HUVEC coimplantation mouse model. Furthermore, HUVECs in which PSMA expression was induced were able to internalize J591, a mAb that recognizes an extracellular epitope of PSMA as well as nanoparticles bearing a PSMA-binding ligand/inhibitor. These findings offer new avenues to study the molecular mechanism responsible for tumor cell induction of PSMA in neovasculature as well as the biological role of PSMA in neovasculature. Finally, these data suggest that PSMA-targeted therapies could synergize with antiangiogenic and/or other antitumor agents and provide a promising model system to test therapeutic modalities that target PSMA in these settings.

IMPLICATIONS

Cancer cells are able to induce PSMA expression in HUVECs, in vitro and in vivo, allowing internalization of PSMA-specific mAbs and nanoparticles bearing a PSMA-binding ligand/inhibitor. Mol Cancer Res; 14(11); 1045-53. ©2016 AACR.

In vivo imaging of prostate cancer using an anti-PSMA scFv fragment as a probe

We aimed to evaluate a fluorescent-labeled single chain variable fragment (scFv) of the anti-PSMA antibody as a specific probe for the detection of prostate cancer by in vivo fluorescence imaging. An orthotopic model of prostate cancer was generated by injecting LNCaP cells into the prostate lobe. ScFvD2B, a high affinity anti-PSMA antibody fragment, was labeled using a near-infrared fluorophore to generate a specific imaging probe (X770-scFvD2B). PSMA-unrelated scFv-X770 was used as a control. Probes were injected intravenously into mice with prostate tumors and fluorescence was monitored in vivo by fluorescence molecular tomography (FMT). In vitro assays showed that X770-scFvD2B specifically bound to PSMA and was internalized in PSMA-expressing LNCaP cells. After intravenous injection, X770-scFvD2B was detected in vivo by FMT in the prostate region. On excised prostates the scFv probe co-localized with the cancer cells and was found in PSMA-expressing cells. The PSMA-unrelated scFv used as a control did not label the prostate cancer cells. Our data demonstrate that scFvD2B is a high affinity contrast agent for in vivo detection of PSMA-expressing cells in the prostate. NIR-labeled scFvD2B could thus be further developed as a clinical probe for imaging-guided targeted biopsies.

Clinical translation of (177)Lu-labeled PSMA-617: Initial experience in prostate cancer patients

OBJECTIVE

PSMA-617 is reported to exhibit very high binding affinity towards PSMA receptors, over-expressed on prostate cancer cells and therefore, (177)Lu-labeled PSMA-617 is expected to play a pivotal role in the clinical management of patients suffering from ca prostate. The objective of the present study is to formulate the patient dose of (177)Lu-labeled PSMA-617, pre-clinical studies in animal model and clinical investigation in limited number of prostate cancer patients as well evaluating its potential for theranostic application.

EXPERIMENTAL

Patient dose of 7.4 GBq (200 mCi) of (177)Lu-labeled PSMA-617 was prepared by incubating 100 μg of PSMA-617 with (177)LuCl3 at 95 °C for 15 minutes. Radiochemical purity as well as in-vitro stability of the preparation was determined by PC and HPLC methods. The pharmacokinetic behavior and in-vivo distribution of the agent were studied by carrying out biodistribution studies in normal male Wistar rats. Preliminary clinical investigation was performed in 7 patients suffering from prostate cancer.

RESULTS

The complex was prepared with >98% radiochemical purity under the optimized reaction protocols and the preparation exhibited adequate in-vitro stability. Biodistribution studies revealed no significant uptake in any of the major organ/tissue along with major clearance through renal pathway. Clinical studies showed similar distribution in lesions and physiologic areas of uptake as seen in diagnostic (68)Ga-PSMA-11 PET scans performed earlier.

CONCLUSION

Preliminary clinical studies indicated the promising potential of the agent for theranostic applications. However, further investigations in large pool of patients are warranted to establish the theranostic potential of the agent.

The clinical and therapeutic uses of MDM2 and PSMA and their potential interaction in aggressive cancers

Prostate-specific membrane antigen (PSMA) overexpression is observed in the neovasculature of solid tumors, but not in the vasculature of normal tissues. Increased PSMA expression is positively associated with tumor stage and grade, although its function in cancer remains unclear. Mouse double minute 2 (MDM2) is a negative regulator of the p53 tumor suppressor and is reported to regulate VEGF expression and angiogenesis. Both proteins have been considered as biomarkers and therapeutic targets for advanced solid tumors. Our work and a recent microarray-based gene profiling study suggest there could be signaling interplay between MDM2 and PSMA. We herein review the mechanisms underlining the outgrowth of tumors associated with PSMA and MDM2, their potential interaction and how this may be applied to anticancer therapeutics.

Nano-Targeted Delivery of Toremifene, an Estrogen Receptor-α Blocker in Prostate Cancer

PURPOSE

Estrogen Receptor-α (ERα) expression is increased in prostate cancer and acts as an oncogene. We propose that blocking of estrogen hormone binding to ERα using the ERα blocker toremifene will reduce the tumorigenicity of prostate cancer, and nano-targeted delivery of toremifene will improve anticancer efficacy. We report the synthesis and use in an orthotopic mouse model of PLGA-PEG nanoparticles encapsulating toremifene and nanoparticles encapsulating toremifene that are also conjugated to anti-PSMA for targeted prostate tumor delivery.

METHODS

Human prostate cancer cell line PC3M and a nude mouse model were used to test efficacy of nano-targeted and nano-encapsulated toremifene versus free toremifene on the growth and differentiation of tumor cells.

RESULTS

Treatment with free toremifene resulted in a significant reduction in growth of prostate tumor and proliferation, and its nano-targeting resulted in greater reduction of prostate tumor growth, greater toremifene tumor uptake, and enhanced tumor necrosis. Tumors from animals treated with nano-encapsulated toremifene conjugated with anti-PSMA showed about a 15-fold increase of toremifene compared to free toremifene.

CONCLUSIONS

Our data provide evidence that blocking ERα by toremifene and targeting prostate cancer tissues with anti-PSMA antibody on the nanoparticles' surface repressed the tumorigenicity of prostate cancer cells in this mouse model.

Self-adjuvanted mRNA vaccination in advanced prostate cancer patients: a first-in-man phase I/IIa study

Background

CV9103 is a prostate-cancer vaccine containing self-adjuvanted mRNA (RNActive®) encoding the antigens PSA, PSCA, PSMA, and STEAP1. This phase I/IIa study evaluated safety and immunogenicity of CV9103 in patients with advanced castration-resistant prostate-cancer.

Methods

44 Patients received up to 5 intra-dermal vaccinations. Three dose levels of total mRNA were tested in Phase I in cohorts of 3–6 patients to determine a recommended dose. In phase II, 32 additional patients were treated at the recommended dose. The primary endpoint was safety and tolerability, the secondary endpoint was induction of antigen specific immune responses monitored at baseline and at weeks 5, 9 and 17.

Results

The most frequent adverse events were grade 1/2 injection site erythema, injection site reactions, fatigue, pyrexia, chills and influenza-like illness. Possibly treatment related urinary retention occurred in 3 patients. The recommended dose was 1280 μg. A total of 26/33 evaluable patients treated at 1280 μg developed an immune response, directed against multiple antigens in 15 out of 33 patients. One patient showed a confirmed PSA response. In the subgroup of 36 metastatic patients, the Kaplan-Meier estimate of median overall survival was 31.4 months [95 % CI: 21.2; n.a].

Conclusions

The self-adjuvanted RNActive® vaccine CV9103 was well tolerated and immunogenic.

The technology is a versatile, fast and cost-effective platform allowing for creation of vaccines. The follow-up vaccine CV9104 including the additional antigens prostatic acid phosphatase (PAP) and Muc1 is currently being tested in a randomized phase IIb trial to assess the clinical benefit induced by this new vaccination approach.

Trial registration

EU Clinical Trials Register: EudraCT number 2008-003967-37, registered 27 Jan 2009.

Electronic supplementary material

The online version of this article (doi:10.1186/s40425-015-0068-y) contains supplementary material, which is available to authorized users.

Vectors for the delivery of radiopharmaceuticals in cancer therapeutics

Internal radiation using radiopharmaceuticals promises efficient cancer therapeutics. The specificity and selectivity required for screening and pinpointing tumor cells for cell-kill has been made possible by targeted ligands based on 'magic bullet' and tracer principle- theories nearing a century. Overexpression of certain receptors has been exploited using biomolecules for targeting. The pragmatic analysis, however, is not as promising compared with the theoretical knowledge of available gamut of vectors and targets. The complex interplay of in vitro and in vivo parameters, and the effect of radionuclides involve a systematic assessment of radiopharmaceuticals as diagnostic and therapeutic agent. This review presents different vectors with their pros and cons, present status and recent design variations followed by a future perspective based on novel approaches.

Immunotherapy for prostate cancer: recent developments and future challenges

Since the approval of sipuleucel-T for men with metastatic castrate resistant prostate cancer in 2010, great strides in the development of anti-cancer immunotherapies have been made. Current drug development in this area has focused primarily on antigen-specific (i.e. cancer vaccines and antibody based therapies) or checkpoint inhibitor therapies, with the checkpoint inhibitors perhaps gaining the most attention as of late. Indeed, drugs blocking the inhibitory signal generated by the engagement of cytotoxic T-lymphocyte antigen-4 (CTLA-4) and programmed cell death-1 (PD-1) found on T-cells has emerged as potent means to combat the immunosuppressive milieu. The anti-CTLA-4 monoclonal antibody ipilimumab has already been approved in advanced melanoma and two phase III trials evaluating ipilimumab in men with metastatic castrate-resistant prostate cancer are underway. A phase III trial evaluating ProstVac-VF, a poxvirus-based therapeutic prostate cancer vaccine, is also underway. While there has been reason for encouragement over the past few years, many questions regarding the use of immunotherapies remain. Namely, it is unclear what stage of disease is most likely to benefit from these approaches, how best to incorporate said treatments with each other and into our current treatment regimens and which therapy is most appropriate for which disease. Herein we review some of the recent advances in immunotherapy as related to the treatment of prostate cancer and outline some of the challenges that lie ahead.

Synthesis and preclinical evaluation of DOTAGA-conjugated PSMA ligands for functional imaging and endoradiotherapy of prostate cancer

Background

Due to its high expression in prostate cancer, PSMA (prostate-specific membrane antigen) represents an ideal target for both diagnostic imaging and endoradiotherapeutic approaches. Based on a previously published highly specific PSMA ligand ([⁶⁸Ga]DOTA-FFK(Sub-KuE)), we developed a corresponding metabolically stable 1,4,7,10-tetraazacyclododececane,1-(glutaric acid)-4,7,10-triacetic acid (DOTAGA) construct for theranostic treatment of prostate cancer.

Methods

All ligands were synthesized by a combined solid phase and solution phase synthesis strategy. The affinity of the ^natgallium and lutetium complexes to PSMA and the internalization efficiency of the radiotracers were determined on PSMA-expressing LNCaP cells. The ⁶⁸Ga- and ¹⁷⁷Lu-labelled ligands were further investigated for lipophilicity, binding specificity, metabolic stability, as well as biodistribution and μPET in LNCaP-tumour-bearing mice.

Results

Radiochemical yields for ⁶⁸Ga (3 nmol, 5.0 M NaCl/2.7 M HEPES (approximately 5/1), pH 3.5 to 4.5, 5 min, 95°C) and ¹⁷⁷Lu labelling (0.7 nmol, 0.1 M NH₄OAc, pH 5.5, 30 min, 95°C) were almost quantitative, resulting in specific activities of 250 to 300 GBq/μmol for the ⁶⁸Ga analogues and 38 GBq/μmol for ¹⁷⁷Lu complexes. Due to metabolic instability of l-amino acid spacers, d-amino acids were implemented resulting in a metabolically stable DOTAGA ligand. Compared to the DOTA ligand, the DOTAGA derivatives showed higher hydrophilicity (logP = −3.6 ± 0.1 and −3.9 ± 0.1 for ⁶⁸Ga and ¹⁷⁷Lu, respectively) and improved affinity to PSMA resulting in an about twofold increased specific internalization of the ⁶⁸Ga- and ¹⁷⁷Lu-labelled DOTAGA analogue. Especially, [⁶⁸Ga]DOTAGA-ffk(Sub-KuE) exhibits favourable pharmacokinetics, low unspecific uptake and high tumour accumulation in LNCaP-tumour-bearing mice.

Conclusions

The pair of diagnostic/therapeutic PSMA-ligands [⁶⁸Ga/¹⁷⁷Lu]DOTAGA-ffk(Sub-KuE) possess remarkable potential for the management of prostate cancer.

Next-generation therapy for residual prostate cancer

Prostate cancer claimed an estimated 136,500 lives globally in 2011. Ironically, the best existing treatment strategies provide survival benefits and missed opportunities to further improve outcomes. Prostatectomy provides the greatest survival benefit, albeit the risk of systemic recurrence increases dramatically with extracapsular involvement. To date, further systemic treatment is not generally prescribed for these 'high-risk' patients until such time as advanced disease is diagnosed based on persistent high PSA levels and/or when larger tumors are confirmed by imaging. This recurrent form of the disease is most often terminal. Androgen deprivation therapy (ADT) provides outstanding early control for these patients, which is rather tragic as the early benefits of ADT are lost within 2 years for most men, as the cancer again progresses to an incurable 'late-stage' castration-resistant form of the disease with a median survival of approximately 18 months. We review the potential of targeted α-therapy as an adjuvant with minimal side effects for early-stage high-risk patients to be administered immediately following prostatectomy and/or during ADT.

Detection and quantitation of glutamate carboxypeptidase II in human blood

BACKGROUND

Glutamate carboxypeptidase II (GCPII) is a transmembrane enzyme that cleaves N-acetyl-L-aspartyl-L-glutamate (NAAG) in the brain. GCPII is highly expressed in the prostate and prostate cancer and might be associated with prostate cancer progression. Another exopeptidase, plasma glutamate carboxypeptidase (PGCP), was reported to be similar to GCPII and to share its NAAG-hydrolyzing activity.

METHODS

We performed a radioenzymatic assay with [(3) H]NAAG as a substrate to detect and quantify the enzymatic activity of GCPII in plasma. Using a specific antibody raised against native GCPII (2G7), we immunoprecipitated GCPII from human plasma. We also cloned two PGCP constructs, expressed them in insect cells, and tested them for their NAAG-hydrolyzing activity.

RESULTS

We detected GCPII protein in human plasma and found that its concentration ranges between 1.3 and 17.2 ng/ml in volunteers not diagnosed with prostate cancer. Recombinant PGCP was enzymatically active but exhibited no NAAG-hydrolyzing activity.

CONCLUSION

GCPII is present in human blood, and its concentration within a healthy population varies. Recombinant PGCP does not hydrolyze NAAG, suggesting that GCPII alone is responsible for the NAAG-hydrolyzing activity observed in human blood. The potential correlation between GCPII serum levels and the disease status of prostate cancer patients will be further investigated.

Prostate-specific membrane antigen as a marker of pancreatic cancer cells

The aim of this study was to identify the expression of prostate-specific membrane antigen (PSMA) and analyze the correlation between PSMA with clinical characteristics in patients with pancreatic cancer. The expression of PSMA protein and mRNA was detected by immunohistochemistry and real-time quantitative polymerase chain reaction in pancreatic cancer tissues, pancreatic intraepithelial neoplasia or normal pancreatic tissues, respectively. And clinical characteristics and prognosis of patients were investigated. PSMA was expressed in pancreatic cancer cells, both in protein and mRNA levels. Moreover, the PSMA levels were associated with the prognosis of patients with pancreatic ductal adenocarcinoma. The overall survival time of pancreatic cancer patients with high expression of PSMA was significantly shorter than that of the low ones. Moreover, the PSMA levels were correlated with clinicopathological features including the histological grade and pathological tumor-node-metastasis stage. PSMA is involved in the carcinogenesis of pancreatic cancer, and it might serve as a potential therapeutic target for pancreatic cancer.

Elevated expression of prostate cancer-associated genes is linked to down-regulation of microRNAs

Background

Recent evidence suggests that the prostate cancer (PCa)-specific up-regulation of certain genes such as AMACR, EZH2, PSGR, PSMA and TRPM8 could be associated with an aberrant expression of non-coding microRNAs (miRNA).

Methods

In silico analyses were used to search for miRNAs being putative regulators of PCa-associated genes. The expression of nine selected miRNAs (hsa-miR-101, -138, -186, -224, -26a, -26b, -374a, -410, -660) as well as of the aforementioned PCa-associated genes was analyzed by quantitative PCR using 50 malignant (Tu) and matched non-malignant (Tf) tissue samples from prostatectomy specimens as well as 30 samples from patients with benign prostatic hyperplasia (BPH). Then, correlations between paired miRNA and target gene expression levels were analyzed. Furthermore, the effect of exogenously administered miR-26a on selected target genes was determined by quantitative PCR and Western Blot in various PCa cell lines. A luciferase reporter assay was used for target validation.

Results

The expression of all selected miRNAs was decreased in PCa tissue samples compared to either control group (Tu vs Tf: -1.35 to -5.61-fold; Tu vs BPH: -1.17 to -5.49-fold). The down-regulation of most miRNAs inversely correlated with an up-regulation of their putative target genes with Spearman correlation coefficients ranging from -0.107 to -0.551. MiR-186 showed a significantly diminished expression in patients with non-organ confined PCa and initial metastases. Furthermore, over-expression of miR-26a reduced the mRNA and protein expression of its potential target gene AMACR in vitro. Using the luciferase reporter assay AMACR was validated as new target for miR-26a.

Conclusions

The findings of this study indicate that the expression of specific miRNAs is decreased in PCa and inversely correlates with the up-regulation of their putative target genes. Consequently, miRNAs could contribute to oncogenesis and progression of PCa via an altered miRNA-target gene-interaction.

Circulating tumor cells from prostate cancer patients interact with E-selectin under physiologic blood flow

Hematogenous metastasis accounts for the majority of cancer-related deaths, yet the mechanism remains unclear. Circulating tumor cells (CTCs) in blood may employ different pathways to cross blood endothelial barrier and establish a metastatic niche. Several studies provide evidence that prostate cancer (PCa) cell tethering and rolling on microvascular endothelium via E-selectin/E-selectin ligand interactions under shear flow theoretically promote extravasation and contribute to the development of metastases. However, it is unknown if CTCs from PCa patients interact with E-selectin expressed on endothelium, initiating a route for tumor metastases. Here we report that CTCs derived from PCa patients showed interactions with E-selectin and E-selectin expressing endothelial cells. To examine E-selectin-mediated interactions of PCa cell lines and CTCs derived from metastatic PCa patients, we used fluorescently-labeled anti-prostate specific membrane antigen (PSMA) monoclonal antibody J591-488 which is internalized following cell-surface binding. We employed a microscale flow device consisting of E-selectin-coated microtubes and human umbilical vein endothelial cells (HUVECs) on parallel-plate flow chamber simulating vascular endothelium. We observed that J591-488 did not significantly alter the rolling behavior in PCa cells at shear stresses below 3 dyn/cm(2). CTCs obtained from 31 PCa patient samples showed that CTCs tether and stably interact with E-selectin and E-selectin expressing HUVECs at physiological shear stress. Interestingly, samples collected during disease progression demonstrated significantly more CTC/E-selectin interactions than samples during times of therapeutic response (p=0.016). Analysis of the expression of sialyl Lewis X (sLe(x)) in patient samples showed that a small subset comprising 1.9-18.8% of CTCs possess high sLe(x) expression. Furthermore, E-selectin-mediated interactions between prostate CTCs and HUVECs were diminished in the presence of anti-E-selectin neutralizing antibody. CTC-Endothelial interactions provide a novel insight into potential adhesive mechanisms of prostate CTCs as a means to initiate metastasis.

The prostate-specific membrane antigen: lessons and current clinical implications from 20 years of research

OBJECTIVE

Despite a multitude of detection and treatment advances in the past 2 decades, prostate cancer remains the second leading cause of deaths due to cancer among men in the United States. Technological evolution and expanding knowledge of tumor biomarkers have invigorated exploration in prostate cancer therapeutics. Prostate-specific membrane antigen (PSMA) was one of the first prostate cancer biomarkers successfully cloned. Since then, it has been characterized as the prototypical cell-surface marker for prostate cancer and has been the subject of intense clinical inquiry. In this article, we review the relevant research in PSMA on the 20th anniversary of its cloning.

METHODS AND MATERIALS

A PubMed search using the keywords "prostate-specific membrane antigen" or "glutamate carboxypeptidase II" provided 1019 results. An additional 3 abstracts were included from scientific meetings. Articles were vetted by title and abstract with emphasis placed on those with clinically relevant findings.

RESULTS

Sixty articles were selected for inclusion. PSMA was discovered and cloned in 1993. Its structure and function were further delineated in the ensuing decade. Consensus sites of expression in normal physiology are prostate, kidney, nervous system, and small intestine. PSMA has been implicated in the neovasculature of several tumors including urothelial and renal cell carcinomas. In prostate cancer, expression of PSMA is directly related to the Gleason grade. PSMA has been tested both in imaging and therapeutics in a number of prostate cancer clinical trials. Several recent approaches to target PSMA include the use of small molecule inhibitors, PSMA-based immunotherapy, RNA aptamer conjugates, and PSMA-targeted prodrug therapy. Future study of PSMA in prostate cancer might focus on its intracellular functions and possible role in tumor neurogenesis.

CONCLUSIONS

Twenty years from its discovery, PSMA represents a viable biomarker and treatment target in prostate cancer. Research to delineate its precise role in prostate carcinogenesis and within the therapeutic armamentarium for patients with prostate cancer remains encouraging.

Tribody: robust self-assembled trimeric targeting ligands with high stability and significantly improved target-binding strength

The C-terminal coiled-coil region of mouse and human cartilage matrix protein (CMP) self-assembles into a parallel trimeric complex. Here, we report a general strategy for the development of highly stable trimeric targeting ligands (tribodies), against epidermal growth factor receptor (EGFR) and prostate-specific membrane antigen (PSMA) as examples, by fusing a specific target-binding moiety with a trimerization domain derived from CMP. The resulting fusion proteins can efficiently self-assemble into a well-defined parallel homotrimer with high stability. Surface plasmon resonance (SPR) analysis of the trimeric targeting ligands demonstrated significantly enhanced target-binding strength compared with the corresponding monomers. Cellular-binding studies confirmed that the trimeric targeting ligands have superior binding strength toward their respective receptors. Significantly, the EGFR-binding tribody was considerably accumulated in the tumor of mice bearing xenografted EGFR-positive tumors, indicating its effective cancer-targeting feature under in vivo conditions. Our results demonstrate that CMP-based self-assembly of tribodies can be a general strategy for the facile and robust generation of trivalent targeting ligands for a wide variety of in vitro and in vivo applications.

Combination of the PI3K inhibitor ZSTK474 with a PSMA-targeted immunotoxin accelerates apoptosis and regression of prostate cancer

The phosphoinositide 3-kinase (PI3K) pathway is activated in most advanced prostate cancers, yet so far treatments with PI3K inhibitors have been at best tumorostatic in preclinical cancer models and do not show significant antitumor efficacy in clinical trials. Results from tissue culture experiments in prostate cancer cells suggest that PI3K inhibitors should be combined with other cytotoxic agents; however, the general toxicity of such combinations prevents translating these experimental data into preclinical and clinical models. We investigated the emerging concept of tumor-targeted synthetic lethality in prostate cancer cells by using the pan-PI3K inhibitor ZSTK474 and the immunotoxin J591PE, a protein chimera between the single-chain variable fragment of the monoclonal antibody J591 against the prostate-specific membrane antigen (PSMA) and the truncated form of the Pseudomonas aeruginosa exotoxin A (PE38QQR). The combination of ZSTK474 and J591PE increased apoptosis within 6 hours and cell death (monitored at 24-48 hours) in the PSMA-expressing cells LNCaP, C4-2, and C4-2Luc but not in control cells that do not express PSMA (PC3 and BT549 cells). Mechanistic analysis suggested that induction of apoptosis requires Bcl-2-associated death promoter (BAD) dephosphorylation and decreased expression of myeloid leukemia cell differentiation protein 1 (MCL-1). A single injection of ZSTK474 and J591PE into engrafted prostate cancer C4-2Luc cells led to consistent and stable reduction of luminescence within 6 days. These results suggest that the combination of a PI3K inhibitor and a PSMA-targeted protein synthesis inhibitor toxin represents a promising novel strategy for advanced prostate cancer therapy that should be further investigated.

Effect of chelators on the pharmacokinetics of (99m)Tc-labeled imaging agents for the prostate-specific membrane antigen (PSMA)

Technetium-99m, the most commonly used radionuclide in nuclear medicine, can be attached to biologically important molecules through a variety of chelating agents, the choice of which depends upon the imaging application. The prostate-specific membrane antigen (PSMA) is increasingly recognized as an important target for imaging and therapy of prostate cancer (PCa). Three different (99m)Tc-labeling methods were employed to investigate the effect of the chelator on the biodistribution and PCa tumor uptake profiles of 12 new urea-based PSMA-targeted radiotracers. This series includes hydrophilic ligands for radiolabeling with the [(99m)Tc(CO)3](+) core (L8-L10), traditional NxSy-based chelating agents with varying charge and polarity for the (99m)Tc-oxo core (L11-L18), and a (99m)Tc-organohydrazine-labeled radioligand (L19). (99m)Tc(I)-Tricarbonyl-labeled [(99m)Tc]L8 produced the highest PSMA+ PC3 PIP to PSMA- PC3 flu tumor ratios and demonstrated the lowest retention in normal tissues including kidney after 2 h. These results suggest that choice of chelator is an important pharmacokinetic consideration in the development of (99m)Tc-labeled radiopharmaceuticals targeting PSMA.

A multivalent approach of imaging probe design to overcome an endogenous anion binding competition for noninvasive assessment of prostate specific membrane antigen

2[(3-Amino-3-carboxypropyl)(hydroxy)(phosphinyl)methyl]pentane-1,5-dioic acid) (GPI) is a highly potent inhibitor of prostate specific membrane antigen (PSMA) with a rapid in vivo clearance profile from nontarget organs including kidneys, but its use for imaging of PSMA is impeded by an endogenous anion (serum phosphate) competition, which compromises its specific binding to the antigen. Multipresentation of a targeting molecule on a single entity has been recognized as a practical way for imaging sensitivity enhancement. Herein, we demonstrate a multivalent approach based on a (64)Cu-specific bifunctional chelator scaffold to overcome the endogenous phosphate competition thus enabling the utility of GPI conjugates for in vivo detection of PSMA and imaging quantification. Both monomeric (H2CBT1G) and dimeric (H2CBT2G) conjugates were synthesized and labeled with (64)Cu for in vitro and in vivo evaluations. A 4-fold enhancement of PSMA binding affinity was observed for H2CBT2G as compared to H2CBT1G from the PSMA competitive binding assays performed on LNCaP cells. In vivo PET imaging studies were conducted on mouse xenograft models established with a PSMA(+) cell line, LNCaP, and PSMA(-) PC3 and H2009 cell lines. (64)Cu-CBT2G showed significantly higher LNCaP tumor uptake than (64)Cu-CBT1G at 1, 4, and 24 h postinjection (p.i.) (p < 0.05). In addition, tumor uptake of (64)Cu-CBT2G remained steady out to 24 h p.i. (1.46 ± 0.54, 1.12 ± 0.56, and 1.00 ± 0.50% ID/g at 1, 4, and 24 h p.i., respectively), while (64)Cu-CBT1G showed a great decrease from 1 to 4 h p.i. The PSMA imaging specificity of both H2CBT1G and H2CBT2G was demonstrated by their low uptake in PSMA(-) tumors (PC3 and H2009) and further confirmed by a significant signal reduction in PSMA(+) LNCaP tumors in the blockade study. In addition, the LNCaP tumor uptake (% ID/g) of (64)Cu-CBT2G was found to be in a positive linear correlation with the tumor size (R(2) = 0.92, 0.94, and 0.93 for 1 h, 4 h, and 24 h p.i.). This may render the probe with potential application in the management of patients with prostate cancer.

PSMA-targeted dendrimers: a patent evaluation (WO2012078534)

Prostate-specific membrane antigen (PSMA) is a cell surface protein expressed primarily in prostate cancer and solid tumor neovasculature. PSMA has been a target for both imaging and therapeutic investigations due to its increased expression in advanced disease and in prostate cancer patients receiving hormonal therapy. The recent approval of new androgen receptor pathway targeted drugs for prostate cancer has sharply increased this interest in creating novel probes to measure this disease-specific biomarker. Monoclonal antibodies and small molecules targeting PSMA, however, have been investigated for diagnostic and therapeutic purposes, with only limited success. In fact, only one agent (monoclonal antibody agent - capromab pendetide) has been FDA-approved for human use. PSMA-targeting dendrimers may provide a targeting method that might maintain the specificity of monoclonal antibodies, achieve high levels of tumor concentration (similar to small molecule antagonist ligands) and avoid accumulation in non-target sites of PSMA expression (such as renal tubules). The current patent application provides theoretical evidence for the advantage of PSMA-targeting dendrimers with in vitro data for PSMA binding, but lacks sufficient preclinical animal binding, dosimetry and toxicology data.