Anti-tumor effects of toxins targeted to the prostate specific membrane antigen

PSMA-targeted SMART molecules outfitted with SN38

Herein, we report the modular synthesis and evaluation of a prostate-specific membrane antigen (PSMA) targeted small molecule drug conjugate (SMDC) carrying the chemotherapeutic agent, SN38. Due to the fluorogenic properties of SN38, payload release kinetics from the platform was observed in buffers representing the pH conditions of systemic circulation and cellular internalization. It was found that this platform is stable with minimal payload release at physiological pH with most rapid payload release observed at pH values representing the endosome complex. We confirmed selective payload release and chemotherapeutic efficacy for PSMA(+) prostate cancer cells over PSMA(-) cells. These results demonstrate that chemotherapeutic agents with limited solubility can be conjugated to a water-soluble targeting and linker platform without attenuating efficacy.

Prostate-Specific Membrane Antigen-Targeted Turn-on Probe for Imaging Cargo Release in Prostate Cancer Cells

The tunable nature of phosphoramidate linkers enables broad applicability as pH-triggered controlled-release platforms, particularly in the context of antibody- and small-molecule-drug conjugates (ADCs and SMDCs), where there remains a need for new linker technology. Herein, we explored in-depth the release of turn-on fluorogenic payloads from a homoserinyl-based phosphoramidate acid-cleavable linker. Kinetics of payload release from the scaffold was observed in buffers representing the pH conditions of systemic circulation, early and late endosomes, and lysosomes. It was found that payload release takes place in two key consecutive steps: (1) P-N bond hydrolysis and (2) spacer immolation. These two steps were found to follow pseudo-first-order kinetics and had opposite dependencies on pH. P-N bond hydrolysis increased with decreasing pH, while spacer immolation was most rapid at physiological pH. Despite the contrasting release kinetics of these two steps, maximal payload release was observed at the mildly acidic pH (5.0-5.5), while minimal payload release occurred at physiological pH. We integrated this phosphoramidate-payload linker system into a PSMA-targeted fluorescent turn-on probe to study the intracellular trafficking and release of a fluorescent payload in PSMA-expressing prostate cancer cells. Results showed excellent turn-on and accumulation of the coumarin payload in the late endosomal and lysosomal compartments of these cells. The release properties of this linker mark it as an attractive alternative in the modular design of ADCs and SMDCs, which demand selective intracellular payload release triggered by the pH changes that accompany intracellular trafficking.

Targeted Toxins for the Treatment of Prostate Cancer

Prostate cancer is the second most common cancer and the fifth leading cause of cancer deaths worldwide. Despite improvements in diagnosis and treatment, new treatment options are urgently needed for advanced stages of the disease. Targeted toxins are chemical conjugates or fully recombinant proteins consisting of a binding domain directed against a target antigen on the surface of cancer cells and a toxin domain, which is transported into the cell for the induction of apoptosis. In the last decades, targeted toxins against prostate cancer have been developed. Several challenges, however, became apparent that prevented their direct clinical use. They comprise immunogenicity, low target antigen binding, endosomal entrapment, and lysosomal/proteasomal degradation of the targeted toxins. Moreover, their efficacy is impaired by prostate tumors, which are marked by a dense microenvironment, low target antigen expression, and apoptosis resistance. In this review, current findings in the development of targeted toxins against prostate cancer in view of effective targeting, reduction of immunogenicity, improvement of intracellular trafficking, and overcoming apoptosis resistance are discussed. There are promising approaches that should lead to the clinical use of targeted toxins as therapeutic alternatives for advanced prostate cancer in the future.

Targeted nanomedicine modalities for prostate cancer treatment

Prostate cancer (PC) is the second most common cause of death amongst men in the USA. Therapy of PC has been transformed in the past decade by introducing novel therapeutics, advanced functional imaging and diagnostic approaches, next generation sequencing, as well as improved application of existing therapies in localized PC. Treatment of PC at the different stages of the disease may include surgery, androgen deprivation therapy (ADT), chemotherapy and radiation therapy. However, although ADT has proven efficacious in PC treatment, its effectiveness may be temporary, as these tumors frequently develop molecular mechanisms of therapy resistance, which allow them to survive and proliferate even under conditions of testosterone deprivation, inhibition of androgen receptor signaling, or cytotoxic drug treatment. Importantly, ADT was found to induce key alterations which frequently result in the formation of metastatic tumors displaying a therapy refractory phenotype. Hence, to overcome these serious therapeutic impediments, novel PC cell-targeted therapeutic strategies are being developed. These include diverse platforms enabling specific enhanced antitumor drug uptake and increased intracellular accumulation. Studies have shown that these novel treatment modalities lead to enhanced antitumor activity and diminished systemic toxicity due to the use of selective targeting and decreased drug doses. The underlying mechanism of targeting and internalization is based upon the interaction between a selective ligand, conjugated to a drug-loaded nanoparticle or directly to an anti-cancer drug, and a specific plasma membrane biomarker, uniquely overexpressed on the surface of PC cells. Another targeted therapeutic approach is the delivery of unique anti-oncogenic signaling pathway-based therapeutic drugs, which are selectively cytotoxic to PC cells. The current paper reviews PC targeted modalities reported in the past 6 years, and discusses both the advantages and limitations of the various targeted treatment strategies.

Targeting prostate cancer: Prostate-specific membrane antigen based diagnosis and therapy

The high incidence rates of prostate cancer (PCa) raise demand for improved therapeutic strategies. Prostate tumors specifically express the prostate-specific membrane antigen (PSMA), a membrane-bound protease. As PSMA is highly overexpressed on malignant prostate tumor cells and as its expression rate correlates with the aggressiveness of the disease, this tumor-associated biomarker provides the possibility to develop new strategies for diagnostics and therapy of PCa. Major advances have been made in PSMA targeting, ranging from immunotherapeutic approaches to therapeutic small molecules. This review elaborates the diversity of PSMA targeting agents while focusing on the radioactively labeled tracers for diagnosis and endoradiotherapy. A variety of radionuclides have been shown to either enable precise diagnosis or efficiently treat the tumor with minimal effects to nontargeted organs. Most small molecules with affinity for PSMA are based on either a phosphonate or a urea-based binding motif. Based on these pharmacophores, major effort has been made to identify modifications to achieve ideal pharmacokinetics while retaining the specific targeting of the PSMA binding pocket. Several tracers have now shown excellent clinical usability in particular for molecular imaging and therapy as proven by the efficiency of theranostic approaches in current studies. The archetypal expression profile of PSMA may be exploited for the treatment with alpha emitters to break radioresistance and thus to bring the power of systemic therapy to higher levels.

177Lu-Labeled Phosphoramidate-Based PSMA Inhibitors: The Effect of an Albumin Binder on Biodistribution and Therapeutic Efficacy in Prostate Tumor-Bearing Mice

Prostate-specific membrane antigen (PSMA) continues to be an active biomarker for small-molecule PSMA-targeted imaging and therapeutic agents for prostate cancer and various non-prostatic tumors that are characterized by PSMA expression on their neovasculature. One of the challenges for small-molecule PSMA inhibitors with respect to delivering therapeutic payloads is their rapid renal clearance. In order to overcome this pharmacokinetic challenge, we outfitted a ¹⁷⁷Lu-labeled phosphoramidate-based PSMA inhibitor (CTT1298) with an albumin-binding motif (CTT1403) and compared its in vivo performance with that of an analogous compound lacking the albumin-binding motif (CTT1401). The radiolabeling of CTT1401 and CTT1403 was achieved using click chemistry to connect ¹⁷⁷Lu-DOTA-N₃ to the dibenzocyclooctyne (DBCO)-bearing CTT1298 inhibitor cores. A direct comparison in vitro and in vivo performance was made for CTT1401 and CTT1403; the specificity and efficacy by means of cellular uptake and internalization, biodistribution, and therapeutic efficacy were determined for both compounds. While both compounds displayed excellent uptake and rapid internalization in PSMA+ PC3-PIP cells, the albumin binding moiety in CTT1403 conferred clear advantages to the PSMA-inhibitor scaffold including increased circulating half-life and prostate tumor uptake that continued to increase up to 168 h post-injection. This increased tumor uptake translated into superior therapeutic efficacy of CTT1403 in PSMA+ PC3-PIP human xenograft tumors.

Anti-tumor effects of a recombinant anti-prostate specific membrane antigen immunotoxin against prostate cancer cells

Background

To evaluate anti-prostate cancer effects of a chimeric tumor-targeted killer protein.

Methods

We established a novel fusion gene, immunocasp-3, composed of NH2-terminal leader sequence fused with an anti-prostate-specific membrane antigen (PSMA) antibody (J591), the furin cleavage sequences of diphtheria toxin (Fdt), and the reverse coding sequences of the large and small subunits of caspase-3 (revcaspase-3). The expressing level of the immunocasp-3 gene was evaluated by using the reverse transcription-PCR (RT-PCR) and western blot analysis. Cell viability assay and cytotoxicity assay were used to evaluate its anti-tumor effects in vitro. Apoptosis was confirmed by electron microscopy and Annexin V-FITC staining. The antitumor effects of immunocasp-3 were assessed in nude mice xenograft models containing PSMA-overexpressing LNCaP cells.

Results

This study shows that the immunocasp-3 proteins selectively recognized and induced apoptotic death in PSMA-overexpressing LNCaP cells in vitro, where apoptotic cells were present in 15.3% of the cells transfected with the immunocasp-3 expression vector at 48 h after the transfection, in contrast to 5.5% in the control cells. Moreover, LNCaP cells were significantly killed under the condition of the co-culture of the immunocasp-3-secreting Jurkat cells and more than 50% of the LNCaP cells died when the two cell lines were co-cultured within 5 days. In addition, The expression of immunocasp-3 also significantly suppressed tumor growth and greatly prolonged the animal survival rate in vivo.

Conclusion

A novel fusion gene, immunocasp-3, may represent a viable approach to treating PSMA-positive prostate cancer.

Structure-Activity Relationship of (18)F-Labeled Phosphoramidate Peptidomimetic Prostate-Specific Membrane Antigen (PSMA)-Targeted Inhibitor Analogues for PET Imaging of Prostate Cancer

A series of phosphoramidate-based prostate specific membrane antigen (PSMA) inhibitors of increasing lipophilicity were synthesized (4, 5, and 6), and their fluorine-18 analogs were evaluated for use as positron emission tomography (PET) imaging agents for prostate cancer. To gain insight into their modes of binding, they were also cocrystallized with the extracellular domain of PSMA. All analogs exhibited irreversible binding to PSMA with IC50 values ranging from 0.4 to 1.3 nM. In vitro assays showed binding and rapid internalization (80-95%, 2 h) of the radiolabeled ligands in PSMA(+) cells. In vivo distribution demonstrated significant uptake in CWR22Rv1 (PSMA(+)) tumor, with tumor to blood ratios of 25.6:1, 63.6:1, and 69.6:1 for [(18)F]4, [(18)F]5, and [(18)F]6, respectively, at 2 h postinjection. Installation of aminohexanoic acid (AH) linkers in the phosphoramidate scaffold improved their PSMA binding and inhibition and was critical for achieving suitable in vivo imaging properties, positioning [(18)F]5 and [(18)F]6 as favorable candidates for future prostate cancer imaging clinical trials.

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.

A high-affinity [(18)F]-labeled phosphoramidate peptidomimetic PSMA-targeted inhibitor for PET imaging of prostate cancer

INTRODUCTION

In this study, a structurally modified phosphoramidate scaffold, with improved prostate-specific membrane antigen (PSMA) avidity, stability and in vivo characteristics, as a PET imaging agent for prostate cancer (PCa), was prepared and evaluated.

METHODS

p-Fluorobenzoyl-aminohexanoate and 2-(3-hydroxypropyl)glycine were introduced into the PSMA-targeting scaffold yielding phosphoramidate 5. X-ray crystallography was performed on the PSMA/5 complex. [(18)F]5 was synthesized, and cell uptake and internalization studies were conducted in PSMA(+) LNCaP and CWR22Rv1 cells and PSMA(-) PC-3 cells. In vivo PET imaging and biodistribution studies were performed at 1 and 4 h post injection in mice bearing CWR22Rv1 tumor, with or without blocking agent.

RESULTS

The crystallographic data showed interaction of the p-fluorobenzoyl group with an arene-binding cleft on the PSMA surface. In vitro studies revealed elevated uptake of [(18)F]5 in PSMA(+) cells (2.2% in CWR22Rv1 and 12.1% in LNCaP) compared to PSMA(-) cells (0.08%) at 4 h. In vivo tumor uptake of 2.33% ID/g and tumor-to-blood ratio of 265:1 was observed at 4 h.

CONCLUSIONS

We have successfully synthesized, radiolabeled and evaluated a new PSMA-targeted PET agent. The crystal structure of the PSMA/5 complex highlighted the interactions within the arene-binding cleft contributing to the overall complex stability. The high target uptake and rapid non-target clearance exhibited by [(18)F]5 in PSMA(+) xenografts substantiates its potential use for PET imaging of PCa.

ADVANCES IN KNOWLEDGE

The only FDA-approved imaging agent for PCa, Prostascint®, targets PSMA but suffers from inherent shortcomings. The data acquired in this manuscript confirmed that our new generation of [(18)F]-labeled PSMA inhibitor exhibited promising in vivo performance as a PET imaging agent for PCa and is well-positioned for subsequent clinical trials. Implications for Patient Care Our preliminary data demonstrate that this tracer possesses the required imaging characteristics to be sensitive and specific for PCa imaging in patients at all stages of the disease.

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.

PSMA-targeted stably linked "dendrimer-glutamate urea-methotrexate" as a prostate cancer therapeutic

One of the important criteria for achieving efficient nanoparticle-based targeted drug delivery is that the drug is not prematurely released at off-target sites. Here we report the preclinical evaluation of a serum-stable dendrimer-based drug conjugate capable of actively targeting into prostate cancer (PC) cells, delivered through the prostate-specific membrane antigen (PSMA). Multiple molecules of PSMA-binding small molecule glutamate urea (GLA; targeting agent) and the drug methotrexate (MTX) were conjugated to generation 5 PAMAM dendrimer (G5) through Cu-free "click" chemistry. The GLA was conjugated through a stable amide bond, and the MTX was conjugated either through ester (Es)- or amide (Am)-coupling, to generate G5-GLA(m)-(Es)MTX(n) and G5-GLA(m)-(Am)MTX(n), respectively. In serum-containing medium, free MTX was slowly released from "G5-GLA(m)-(Es)MTX(n)", with ~8% MTX released from the dendrimer in 72 h, whereas the MTX on G5-GLA(m)-(Am)MTX(n) was completely stable. The G5-GLA(m)-(Am)MTX(n) bound and internalized into PSMA-expressing LNCaP cells, but not into PSMA-negative PC3 cells. The conjugate-inhibited recombinant dihydrofolate reductase and induced potent cytotoxicity in the LNCaP cells, but not in the PC3 cells. Similar to the action of free GLA, stable amide-linked dendrimer-GLA was capable of inhibiting the enzyme N-acetylated α-linked acidic dipeptidase (NAALADase) activity of PSMA. The G5-GLA(m)-MTX(n) may serve as a serum-stable nanoparticle conjugate to specifically and effectively target and treat PSMA-overexpressing prostate tumors.

Discriminatory Role of Detergent-Resistant Membranes in the Dimerization and Endocytosis of Prostate-Specific Membrane Antigen

Prostate-specific membrane antigen (PSMA) is a type-II membrane glycoprotein that was initially identified in LNCaP cells. It is expressed at elevated levels in prostate cancer. In view of the correlation between the expression levels of PSMA and disease grade and stage, PSMA is considered to be one of the most promising biomarkers in the diagnosis and treatment of prostate cancer. In LNCaP cells PSMA undergoes internalization via clathrin-coated pits followed by accumulation in the endosomes. PSMA associates with different types of detergent-resistant membranes (DRMs) along the secretory pathway. Its mature form is mainly insoluble in Lubrol WX, but does not associate with Triton X-100-DRMs. To understand the mechanism of PSMA internalization we investigated its association during internalization with DRMs. For this purpose, internalization was induced by antibody cross-linking. We demonstrate at the biochemical and cell biological levels that: [i] exclusively homodimers of PSMA are associated with Lubrol WX-DRMs, [ii] antibody-induced cross-linking of PSMA molecules results in a time-dependent partitioning into another DRMs type, namely Triton X-100-DRMs, and [iii] concomitant with its association with Triton-X-100-DRMs internalization of PSMA occurs along tubulin filaments. In a previous work (Colombatti et al. (2009) PLoS One 4: e4608) we demonstrated that the small GTPases RAS and RAC1 and the MAPKs p38 and ERK1/2 are activated during antibody cross-linking. As downstream effects of this activation we observed a strong induction of NF-kB associated with an increased expression of IL-6 and CCL5 genes and that IL-6 and CCL5 enhanced the proliferative potential of LNCaP cells synergistically. These observations together with findings reported here hypothesize a fundamental role of DRMs during activation of PSMA as platforms for trafficking, endocytosis and signalling. Understanding these mechanisms constitutes an essential prerequisite for utilization of PSMA as a therapeutically suitable target in prostate cancer.

An anti-PSMA bivalent immunotoxin exhibits specificity and efficacy for prostate cancer imaging and therapy

Prostate specific membrane antigen (PSMA) is overexpressed on prostate tumor cells and the neovascular endothelia various solid tumors. A bivalent immunotoxin generated by fusing a fold-back single-chain diabody derived from the Fv fragments of an anti-PSMA monoclonal antibody with a truncated diphtheria toxin (DT) containing the activity and translocation domains [A-dmDT390-scfbDb(PSMA)] might be suitable for targeted therapy of tumors that overexpress PSMA. In this study, a PSMA-positive and a PSMA-negative prostate cancer cell lines were treated with immunotoxin A-dmDT390-scfbDb(PSMA) in order to study the tumor targeting specificity and therapeutic potential of the immunotoxin. The cellular uptake and selective toxicity of the immunotoxin were evident in monolayer cultures of PSMA-positive LNCaP prostate cancer cells but not in cultures of PSMA-negative PC-3 prostate cancer cells. Cellular accumulation of A-dmDT390-scfbDb(PSMA) increased with increasing incubation times and concentrations in LNCaP cells. The proportion of apoptotic LNCaP cells increased upon incubation with increasing doses of the fold-back immunotoxin. Optical imaging and MRI with the Alexa Fluor 680-labeled A-dmDT390-scfbDb(PSMA) confirmed the specific targeting and therapeutic efficacy of this immunotoxin towards PSMA-positive LNCaP solid tumor xenografts in athymic nude mice.

Cloning and characterization of canine prostate-specific membrane antigen

BACKGROUND

Prostate-specific membrane antigen (PSMA) is a promising biomarker in the diagnosis of prostate cancer and a potential target for antibody-based therapeutic strategies. We isolated the canine PSMA cDNA and investigated the cellular and biochemical characteristics of the recombinant protein as a potential target for animal preclinical studies of antibody based-therapies.

METHODS

Canine PSMA cDNA was isolated by PCR, cloned into expression vectors and transfected into COS-1 and MDCK cells. The biosynthesis and glycosylation of the recombinant protein were investigated in pulse-chase experiments, the cellular localization by confocal laser microscopy, the mode of association of PSMA with the membrane with solubilization in different detergents and its quaternary structure in sucrose-density gradients.

RESULTS

Canine PSMA shows 91% amino acid homology to human PSMA, whereby the major difference is a longer cytoplasmic tail of canine PSMA compared to its human counterpart. Canine PSMA is trafficked efficiently along the secretory pathway, undergoes homodimerization when it acquires complex glycosylated mature form. It associates with detergent-resistant membranes, which act as platforms along its intracellular trafficking. Confocal analysis revealed canine PSMA at the cell surface, Golgi, and the endoplasmic reticulum. A similar distribution is revealed for human PSMA, yet with reduced cell surface levels.

CONCLUSIONS

The cloning, expression, biosynthesis, processing and localization of canine PSMA in mammalian cells is described. We demonstrate that canine PSMA reveals similar characteristics to human PSMA rendering this protein useful as a translational model for investigations of prostate cancer as well as a suitable antigen for targeted therapy studies in dogs.

PSMA-targeted SPECT agents: mode of binding effect on in vitro performance

BACKGROUND

The enzyme-biomarker prostate-specific membrane antigen (PSMA) is an active target for imaging and therapeutic applications for prostate cancer. The internalization of PSMA has been shown to vary with inhibitors' mode of binding: irreversible, slowly reversible, and reversible.

METHODS

In the present study, PSMA-targeted clickable derivatives of an irreversible phosphoramidate inhibitor DBCO-PEG(4) -CTT-54 (IC(50) = 1.0 nM) and a slowly reversible phosphate inhibitor, DBCO-PEG(4) -CTT-54.2 (IC(50) = 6.6 nM) were clicked to (99m) Tc(CO)(3) -DPA-azide to assemble a PSMA-targeted SPECT agent. The selectivity, percent uptake, and internalization of these PSMA-targeted SPECT agents were evaluated in PSMA-positive and PSMA-negative cells.

RESULTS

In vitro studies demonstrated that PSMA-targeted SPECT agents exhibited selective cellular uptake in the PSMA-positive LNCaP cells compared to PSMA-negative PC3 cells. More importantly, it was found that (99m) Tc(CO)(3) -DPA-DBCO-PEG(4) -CTT-54 based on an irreversible PSMA inhibitor core, exhibited greater uptake and internalization than (99m) Tc(CO)(3) -DPA-DBCO-PEG(4) -CTT-54.2 constructed from a slowly reversible PSMA inhibitor core.

CONCLUSIONS

We have demonstrated that a PSMA-targeted SPECT agent can be assembled efficiently using copper-less click chemistry. In addition, we demonstrated that mode of binding has an effect on internalization and percent uptake of PSMA-targeted SPECT agents; with the irreversible targeting agent demonstrating superior uptake and internalization in PSMA+ cells. The approach demonstrated in this work now supports a modular approach for the assembly of PSMA-targeted imaging and therapeutic agents.

A single-chain fragment against prostate specific membrane antigen as a tool to build theranostic reagents for prostate cancer

Prostate carcinoma is the most common non-cutaneous cancer in developed countries and represents the second leading cause of death. Early stage androgen dependent prostate carcinoma responds well to conventional therapies, but relatively few treatment options exist for patients with hormone-refractory prostate cancer. One of the most suitable targets for antibody-mediated approaches is prostate specific membrane antigen (PSMA) which is a well known tumour associated antigen. PSMA is a type II integral cell-surface membrane protein that is not secreted, and its expression density and enzymatic activity are increased progressively in prostate cancer compared to normal prostate epithelium, thereby making PSMA an ideal target for monoclonal antibody imaging and therapy. To obtain a small protein that can better penetrate tissue, we have engineered a single-chain variable fragment (scFv) starting from the variable heavy and light domains of the murine anti-PSMA monoclonal antibody D2B. scFvD2B was analysed in vitro for activity, stability, internalisation ability and in vivo for targeting specificity. Maintenance of function and immunoreactivity as well as extremely high radiolabelling efficiency and radiochemical purity were demonstrated by in vitro assays and under different experimental conditions. Despite its monovalent binding, scFvD2B retained a good strength of binding and was able to internalise around 40% of bound antigen. In vivo we showed its ability to specifically target only PSMA expressing prostate cancer xenografts. Due to these advantageous properties, scFvD2B has the potential to become a good theranostic reagent for early detection and therapy of prostate cancers.

A phosphoramidate-based prostate-specific membrane antigen-targeted SPECT agent

BACKGROUND

Prostate-specific membrane antigen (PSMA) remains an active target for imaging and therapeutic applications for prostate cancer.

METHODS

In the present study, an irreversible phosphoramidate inhibitor, CTT-54 (IC50 = 14 nM), has been modified to deliver 99mTc-(CO)3-DTPA as a SPECT imaging payload to PSMA+ cells in vivo and in vitro. Percent uptake, competitive binding, and internalization will evaluate the imaging agent in vitro. Preliminary biodistribution and imaging will be utilized for in vivo evaluation.

RESULTS

In vitro studies demonstrate that the radiotracer 99mTc-(CO)3-DTPA-CTT-54 exhibits increasing cellular uptake in the PSMA+ LNCaP cells over time. More importantly, it was found that 99mTc-(CO)3-DTPA-CTT-54 is rapidly internalized into LNCaP cells, presumably through the PSMA enzyme-inhibitor complex. In a pilot biodistribution study, increasing accumulation of the radiotracer in LNCaP xenografts was observed from 2 to 4 hr and significant clearance from non-target tissues.

CONCLUSIONS

While DTPA may not represent the ideal chelate structure for 99mTc(CO)3, the data provides proof-of-concept support for the development of a next-generation phosphoramidate-based PSMA inhibitor-conjugates for use as SPECT imaging agents.

Tumor-associated antigens for specific immunotherapy of prostate cancer

Prostate cancer (PCa) is the most common noncutaneous cancer diagnosis and the second leading cause of cancer-related deaths among men in the United States. Effective treatment modalities for advanced metastatic PCa are limited. Immunotherapeutic strategies based on T cells and antibodies represent interesting approaches to prevent progression from localized to advanced PCa and to improve survival outcomes for patients with advanced disease. CD8⁺ cytotoxic T lymphocytes (CTLs) efficiently recognize and destroy tumor cells. CD4⁺ T cells augment the antigen-presenting capacity of dendritic cells and promote the expansion of tumor-reactive CTLs. Antibodies mediate their antitumor effects via antibody-dependent cellular cytotoxicity, activation of the complement system, improving the uptake of coated tumor cells by phagocytes, and the functional interference of biological pathways essential for tumor growth. Consequently, several tumor-associated antigens (TAAs) have been identified that represent promising targets for T cell- or antibody-based immunotherapy. These TAAs comprise proteins preferentially expressed in normal and malignant prostate tissues and molecules which are not predominantly restricted to the prostate, but are overexpressed in various tumor entities including PCa. Clinical trials provide evidence that specific immunotherapeutic strategies using such TAAs represent safe and feasible concepts for the induction of immunological and clinical responses in PCa patients. However, further improvement of the current approaches is required which may be achieved by combining T cell- and/or antibody-based strategies with radio-, hormone-, chemo- or antiangiogenic therapy.

Induction of in vitro and in vivo NK cell cytotoxicity using high-avidity immunoligands targeting prostate-specific membrane antigen in prostate carcinoma

Cancer that might develop as host natural killer (NK) cells fail to detect ligands for their activating NK receptors. Immunoligands represent promising immunotherapeutic tools to overcome this deficit. These are fusion proteins containing a single-chain antibody fragment (scFv) to target an available tumor antigen and ULBP2 to activate host NK cells by targeting the activatory receptor NKG2D. Prostate-specific membrane antigen (PSMA) is an integral non-shed type 2 membrane protein that is highly and specifically expressed on prostate epithelial cells and strongly upregulated in prostate cancer. Here, we compare the impact of various anti-PSMA immunoligand formats on the therapeutic efficacy against prostate carcinoma cells by activating NK cells via NKG2D. Shortening of the linker separating the heavy and light chain antibody domain leads to the formation of dimers, trimers, and higher molecular mass oligomers. NK cells are most efficiently activated by multimeric immunoligands, thus showing an altered cytokine release pattern. The high avidity format is also superior in in vitro NK-mediated tumor cell targeting as shown in cytotoxicity assays. Finally, the efficacy of a multimeric immunoligand is shown in a prostate carcinoma mouse xenograft model showing a strong activity against advanced established tumors.

Targeting the prostate-specific membrane antigen for prostate cancer therapy

Prostate cancer remains a leading cause of death for men in Western civilization. Despite the effectiveness of surgical prostatectomy, radiotherapy and hormonal therapy, a significant proportion of patients progress to advanced metastatic disease for which there are currently no curative treatment options. Therefore, new therapeutic approaches need to be considered. The prostate-specific membrane antigen is a cell-surface glycoprotein that is highly and specifically expressed on prostate epithelial cells and strongly upregulated in prostate cancer at all stages. These characteristics make it an attractive target for antibody-based imaging and therapies and the first anti-prostate-specific membrane antigen agents have already entered clinical trials. The proposed strategies include targeted toxins and radiotherapeutics as well as immunotherapeutic agents and vaccines.

Flow cytometric detection of prostate tumor cells using chemoaffinity labels

BACKGROUND

The enzyme-biomarker prostate-specific membrane antigen (PSMA) is an emerging target for imaging and therapeutic applications for prostate cancer. However, the use of PSMA for detecting circulating prostate tumor cells remains under-explored. The present study focuses on the specific labeling of PSMA+ prostate cancer cells with a fluorescent PSMA inhibitor and the quantitation of PSMA+ cells in blood by flow cytometry (FC) using a gating strategy to separate labeled PSMA+ cells from peripheral blood mononuclear cells.

METHODS

Suspensions of PSMA+ (LNCaP) and PSMA- (DU145) cells were incubated with the fluorescent PSMA inhibitor FAMX-CTT-54. Incubation parameters (time, temperature, and label concentration) were varied to optimize cell labeling. A gating protocol based on double fluorescent labeling of CD45 and PSMA was developed for the quantitiation of LNCaP cells in the presence of white blood cells from bovine blood. Nonfluorescent beads were added to the labeled cell mixture and served as internal standard for precise cellular quantification of LNCaP cells by flow cytometry.

RESULTS

The fluorescent PSMA inhibitor FAMX-CTT-54 was specific for PSMA+ cells. The minimum time and concentration of FAMX-CTT-54 for effective labeling of PSMA+ cell suspensions at 37°C was 7.5 min and 35 nM, respectively; no labeling was observed on PSMA- cells. Co-incubation or pre-incubation of PSMA+ cells with the unlabeled PSMA inhibitor CTT-54 resulted in a concentration-dependent reduction in fluorescent labeling with FAMX-CTT-54 thereby confirming that the labeling was specific for PSMA. In blood samples in which LNCaP cells were added, an average of five cells were detected in a 115 µl sample of the most dilute sample examined (29 cells/ml); three cells were expected theoretically. The greater loss of labeling of PSMA+ cells with FAMX-CTT-54 when pre-incubated with CTT-54 is consistent with the irreversible mode of binding of CTT-54 to PSMA and subsequent internalization of the PSMA-inhibitor complex.

CONCLUSIONS

The results suggest that fluorescent PSMA inhibitors can be utilized to effectively detect and quantify PSMA+ cells by FC. These results support the use of such compounds in the application of FC to detect, quantify, and characterize circulating prostate tumor cells.

Saporin toxin-conjugated monoclonal antibody targeting prostate-specific membrane antigen has potent anticancer activity

BACKGROUND

Prostate-specific membrane antigen (PSMA) provides an attractive target for monoclonal antibody targeted therapies in the treatment of prostate cancer (PC). In this study, we generated an immunotoxin by linking a humanized anti-PSMA monoclonal antibody (hJ591) to the ribosome-inactivating protein toxin saporin. The hJ591-saporin immunoconjugate was evaluated for antitumor activity against PC cells.

METHODS

PSMA-positive cell lines, LNCaP and CWR22Rv1 and a PSMA-negative cell line, PC-3, were used in these experiments. The hJ591 was biotinylated and mixed with streptavidin-saporin (SAZAP). The binding ability of hJ591-SAZAP and the extent of internalization into the cells were tested. The viability of cells treated with hJ591-SAZAP was also examined and the apoptotic cells were measured. Lastly, the anticancer effect of hJ591-SAZAP was investigated in vivo.

RESULTS

The binding ability of hJ591-SAZAP to PSMA was equivalent to that of unconjugated J591. Internalization of hJ591-SAZAP was clearly detected in PSMA-positive, but not in PSMA-negative cell lines. IC(50) of hJ591-SAZAP was 0.14 nM, 1.99 nM, and more than 100 nM in LNCaP, CWR22Rv1, and PC-3 cells, respectively. After 72 hr of hJ591-SAZAP treatment, the percentage of apoptotic cells was 60.29% and 40.73% in LNCaP and CWR22Rv1 cells, respectively, compared to 4.70% in PC-3 cells. The hJ591-SAZAP also had anticancer activity in a LNCaP xenograft model.

CONCLUSIONS

Our findings show that hJ591-SAZAP conjugate has potent and selective antitumor effects on PSMA-positive PC cells in vitro and in vivo. This study supports development of PSMA antibody-toxin conjugates for therapy of PC.

Promising tumor-associated antigens for future prostate cancer therapy

Prostate cancer (CaP) is one of the most prevalent malignant diseases among men in Western countries. There is currently no cure for metastatic castrate-resistant CaP, and median survival for these patients is about 18 months; the high mortality rate seen is associated with widespread metastases. Progression of CaP from primary to metastatic disease is associated with several molecular and genetic changes that can affect the expression of specific tumor-associated antigens (TAAs) or receptors on the cell surface. Targeting TAAs is emerging as an area of promise for controlling late-stage and recurrent CaP. Several reviews have summarized the progress made in targeting signaling pathways for CaP but will not be discussed here. We describe some important CaP TAAs. These include prostate stem-cell antigen, prostate-specific membrane antigen, MUC1, epidermal growth factor receptor, platelet-derived growth factor and its receptor, urokinase plasminogen activator and its receptor, and extracellular matrix metalloproteinase inducer. We summarize recent advancements in our understanding of their role in CaP metastasis, as well as potential therapeutic options for targeting CaP TAAs. We also discuss the origin, identification, and characterization of prostate cancer stem cells (CSCs) and the potential benefits of targeting prostate CSCs to overcome chemoresistance and CaP recurrence.

Three conformational antibodies specific for different PSMA epitopes are promising diagnostic and therapeutic tools for prostate cancer

BACKGROUND

The prostate specific membrane antigen (PSMA) represents an attractive antigen for antibody-based diagnostic and therapeutic intervention in prostate cancer, since it is highly restricted to the prostate and overexpressed in all tumor stages. The present work describes the in vitro characterization of the three anti-PSMA monoclonal antibodies (mAbs) 3/A12, 3/E7, and 3/F11 in comparison to the mAb J591.

METHODS

The mAbs were tested for saturation and competitive binding on C4-2 prostate cancer cells by flow cytometry. Immunohistochemical staining was conducted on frozen prostate normal and cancer tissues as well as on lymph node metastases. Similarly, potential crossreactivities were tested on a broad panel of human normal tissues.

RESULTS

The anti-PSMA mAbs showed a strong binding to C4-2 cells with mean half-maximal saturation concentrations of about 14 nM for 3/A12, 17 nM for 3/E7, 9 nM for 3/F11, and 16 nM for J591. Competitive binding studies revealed that our three mAbs bind to different extracellular PSMA epitopes. The mAbs showed comparable staining of epithelial cells for all tested normal and tumorous prostate tissues. Extraprostatic staining was observed on secretory cells of the salivary glands and on the brush border of the duodenal columnar epithelium. J591 additionally showed positive staining of the normal breast epithelium.

CONCLUSIONS

Due to their specific binding characteristics, the anti-PSMA mAbs 3/A12, 3/E7, and 3/F11 show great promise for diagnostic and therapeutic applications in prostate cancer.

In vitro targeted photodynamic therapy with a pyropheophorbide--a conjugated inhibitor of prostate-specific membrane antigen

BACKGROUND

The lack of specific delivery of photosensitizers (PSs), represents a significant limitation of photodynamic therapy (PDT) of cancer. The biomarker prostate-specific membrane antigen (PSMA) has attracted considerable attention as a target for imaging and therapeutic applications for prostate cancer. Although recent efforts have been made to conjugate inhibitors of PSMA with imaging agents, there have been no reports on PS-conjugated PSMA inhibitors for targeted PDT of prostate cancer. The present study focuses on the use of a PSMA inhibitor-conjugate of pyropheophorbide-a (Ppa-conjugate 2) for targeted PDT to achieve apoptosis in PSMA+ LNCaP cells.

METHODS

Confocal laser scanning microscopy with a combination of nuclear staining and immunofluorescence methods were employed to monitor the specific imaging and PDT-mediated apoptotic effects on PSMA-positive LNCaP and PSMA-negative (PC-3) cells.

RESULTS

Our results demonstrated that PDT-mediated effects by Ppa-conjugate 2 were specific to LNCaP cells, but not PC-3 cells. Cell permeability was detected as early as 2 hr by HOE33342/PI double staining, becoming more intense by 4 hr. Evidence for the apoptotic caspase cascade being activated was based on the appearance of poly-ADP-ribose polymerase (PARP) p85 fragment. Terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) assay detected DNA fragmentation 16 hr post-PDT, confirming apoptotic events.

CONCLUSIONS

Cell permeability by HOE33342/PI double staining as well as PARP p85 fragment and TUNEL assays confirm cellular apoptosis in PSMA+ cells when treated with PS-inhibitor conjugate 2 and subsequently irradiated. It is expected that the PSMA targeting small-molecule of this conjugate can serve as a delivery vehicle for PDT and other therapeutic applications for prostate cancer.

Cell-Surface labeling and internalization by a fluorescent inhibitor of prostate-specific membrane antigen

BACKGROUND

[corrected] Prostate-specific membrane antigen (PSMA) remains an attractive target for imaging and therapeutic applications for prostate cancer. Recent efforts have been made to conjugate inhibitors of PSMA with imaging agents. Compared to antibodies, small-molecule inhibitors of PSMA possess apparent advantages for in vivo applications. To date, there are no reports on the cellular fate of such constructs once bound the extracellular domain of PSMA. The present study was focused on precisely defining the binding specificity, time-dependent internalization, cellular localization, and retention of inhibitor conjugates targeted to PSMA on LNCaP cells. A novel fluorescent inhibitor was prepared as a model to examine these processes.

METHODS

Fluorescence microscopy of LNCaP and PC-3 cell lines was used to monitor the specificity, time-dependent internalization, cellular localization, and retention of a fluorescent PSMA inhibitor.

RESULTS

Fluorescent inhibitor 2 was found to be a potent inhibitor (IC50 = 0.35 nM) of purified PSMA. Its high affinity for PSMA on living cells was confirmed by antibody blocking and competitive binding experiments. Specificity for LNCaP cells was demonstrated as no labeling by 2 was observed for negative control PC-3 cells. Internalization of 2 by viable LNCaP cells was detected after 30 min incubation at 37 degrees C, followed by accumulation in the perinuclear endosomes. It was noted that internalized fluorescent inhibitor can be retained within endosomes for up to 150 min without loss of signal.

CONCLUSIONS

Our results suggest that potent, small-molecule inhibitors of PSMA can be utilized as carriers for targeted delivery for prostate cancer for future imaging and therapeutic applications.

Phase I Trial of the Prostate-Specific Membrane Antigen–Directed Immunoconjugate MLN2704 in Patients With Progressive Metastatic Castration-Resistant Prostate Cancer

Purpose

MLN2704 is an immunoconjugate designed to deliver the maytansinoid antimicrotubule agent drug maytansinoid-1 directly to prostate-specific membrane antigen (PSMA)–expressing cells via the PSMA-targeted monoclonal antibody MLN591. This novel immunoconjugate has shown cytotoxic anti–prostate cancer activity. This study investigated the safety profile, pharmacokinetics, immunogenicity, and preliminary antitumor activity of MLN2704.

Patients and Methods

Patients with progressive, metastatic, castration-resistant prostate cancer received MLN2704 intravenously over 2.5 hours. Dose-limiting toxicity (DLT), maximum-tolerated dose (MTD), pharmacokinetics, immunogenicity, and antitumor activity were assessed.

Results

Twenty-three patients received MLN2704 at doses of 18 to 343 mg/m2. Eighteen of these patients received ≥ three doses at 4-week intervals. Pharmacokinetics of conjugate levels were dose proportional. There was no correlation between clearance and body-surface area. MLN2704 was nonimmunogenic. Study drug–related grade 3 toxicities occurred in three (13%) of 23 patients, including uncomplicated febrile neutropenia (the only DLT) in one patient, reversible elevations in hepatic transaminases, leukopenia, and lymphopenia. No grade 4 toxicities were observed. The most frequent grade 1 or 2 toxicities included fatigue, nausea, and diarrhea. Neuropathy occurred in eight (35%) of 23 patients, including five of six patients treated at 343 mg/m2. Two (22%) of the nine patients treated at 264 or 343 mg/m2 had sustained a more than 50% decrease in prostate-specific antigen versus baseline, accompanied by measurable tumor regression in the patient treated at 264 mg/m2.

Conclusion

Therapeutic doses of MLN2704 can be administered safely on a repetitive basis. An MTD was not defined. MLN2704 is being administered at more frequent intervals in ongoing trials to determine an optimal dosing schedule.

Tissue expression and enzymologic characterization of human prostate specific membrane antigen and its rat and pig orthologs

BACKGROUND

Prostate specific membrane antigen (PSMA), also called glutamate carboxypeptidase II (GCPII), is a target enzyme for diagnosis and treatment of prostate cancer. Moreover, it is upregulated in the vasculature of most solid tumors and is therefore a potential target for the generation of novel antineoplastics. In this context, we analyze the possibility of using rat and pig as animal models for enzymologic and in vivo studies.

METHODS

We prepared the recombinant extracellular part of human, rat, and pig GCPII in S2 cell media and characterized the activity and inhibition profiles of the three orthologs by radioenzymatic assay. We performed Western blot analysis of GCPII expression in human, rat, and pig tissues using the monoclonal antibody GCP-04 and confirmed these findings by activity measurements and immunohistochemistry.

RESULTS

The three recombinant proteins show similar specific enzymatic activities and inhibition profiles. Tissue expression analysis revealed that most of the pig and human tissues show at least some GCPII-positivity, while the expression pattern in rat is more restricted. Moreover, tissues such as prostate and testes exhibit different GCPII expression levels among the species studied.

CONCLUSIONS

The rat and pig orthologs of GCPII seem to be suitable to approximate human GCPII in enzymologic studies. However, the diffuse expression pattern of GCPII in animal and human tissues could be a caveat for the potential utilization of GCPII-targeted anticancer drugs. Furthermore, variations in GCPII tissue distribution among the species studied should be considered when using rat or pig as models for antineoplastic drug discovery.

High expression of PSM-E correlated with tumor grade in prostate cancer: a new alternatively spliced variant of prostate-specific membrane antigen

BACKGROUND

Prostate-specific membrane antigen (PSMA) overexpressed in prostate cancer (PCa) has been targeted for therapy and diagnosis of PCa. In the current study, PSMA cDNA was cloned from PCa tissue by RT-PCR. After sequencing, a new spliced variant of PSMA (PSM-E) was discovered and its specificity in PCa was evaluated.

METHODS

PSM-E and PSMA mRNA were measured in LNCaP, PC-3 and prostate or nonprostatic malignancies. Following transfection of PC-3 with PSM-E cDNA in the pcDNA3.0 vector, PSM-E expression was measured by immunofluorescence and Western-blot. PSM-E and PSMA mRNA levels were quantified by a real-time PCR assay in normal prostate (n = 7), benign prostatic hyperplasia (BPH) (n = 22) and PCa (n = 41). The correlation between their levels and tumor grade was analyzed.

RESULTS

PSM-E cDNA is identical to PSMA except for a 97-nucleotide region and a 93-nucleotide region. PSM-E and PSMA mRNA were detected in PCa and LNCaP, not in PC-3; PSMA could be detected in some nonprostatic tumors whereas PSM-E not. The expression of PSM-E protein was detected in transfected cells. Significant difference of PSM-E mRNA levels was observed among normal prostate, BPH and PCa (P < 0.001), and PSM-E levels increased with increasing Gleason score (r = 0.514, P < 0.001). PSMA mRNA levels were higher in BPH and PCa than in normal prostate (P < 0.001), but no difference between BPH and PCa, no significant correlation was observed between PSMA levels and Gleason score (r = 0.229, P = 0.057).

CONCLUSIONS

PSM-E may be a potential prognostic indicator for PCa progression and may be a new target antigen for therapy of PCa.

Advances in specific immunotherapy for prostate cancer

OBJECTIVES

The absence of effective therapies for advanced prostate cancer has entailed an intensive search for novel treatments. This review presents an overview of specific immunotherapeutic strategies for prostate cancer.

METHODS

Current literature was reviewed regarding the identification of tumor antigens and the design of T-cell- and antibody-based immunotherapy for prostate cancer. The PubMed database was searched using the key words antibodies, clinical trials, dendritic cells, immunotherapy, prostate cancer, and T cells.

RESULTS

T cells and antibodies are powerful components of the specific antitumor immune response. CD8+ cytotoxic T lymphocytes (CTLs) efficiently destroy tumor cells. CD4+ T cells improve the antigen-presenting capacity of dendritic cells (DCs) and support the stimulation of tumor-reactive CTLs. Monoclonal antibodies exhibit their antitumor effects via antibody-dependent cellular cytotoxicity and complement activation. Consequently, much attention has been given to the identification of tumor antigens that represent attractive targets for specific immunotherapy. Several prostate cancer-related antigens were described and used in clinical trials. Such studies were based on the administration of peptides, proteins, or DNA. Furthermore, men with prostate cancer were vaccinated with peptide-, protein-, or RNA-loaded DCs, which display an extraordinary capacity to induce tumor-reactive T cells. Monoclonal antibodies directed against surface antigens were also used. Clinical trials revealed that immunotherapeutic strategies represent safe and feasible concepts for the induction of immunologic and clinical responses in men with prostate cancer.

CONCLUSIONS

Specific immunotherapy represents a promising treatment modality for prostate cancer. Further improvement of the current approaches is required and may be achieved by combining T-cell- and antibody-based vaccination strategies with radio-, hormone-, chemo-, or antiangiogenic therapy.

Phosphoramidate derivatives of hydroxysteroids as inhibitors of prostate-specific membrane antigen

Prostate-specific membrane antigen (PSMA) is a membrane-bound cell surface peptidase which is over-expressed in prostate cancer cells. The enzymatic activities of PSMA are understood but the role of the enzyme in prostate cancer remains conjectural. We previously confirmed the existence of a hydrophobic binding site remote from the enzyme's catalytic center. To explore the specificity and accommodation of this binding site, we prepared a series of six glutamate-containing phosphoramidate derivatives of various hydroxysteroids (1a-1f). The inhibitory potencies of the individual compounds of the series were comparable to a simple phenylalkyl analog (8), and in all cases IC50 values were sub-micromolar. Molecular docking was used to develop a binding model for these inhibitors and to understand their relative inhibitory potencies against PSMA.

The molecular pruning of a phosphoramidate peptidomimetic inhibitor of prostate-specific membrane antigen

To identify the pharmacophore of a phosphoramidate peptidomimetic inhibitor of prostate-specific membrane antigen (PSMA), a small analog library was designed and screened for inhibitory potency against PSMA. The design of the lead inhibitor was based upon N-acyl derivatives of endogenous substrate folyl-gamma-Glu and incorporates a phosphoramidate group to interact with the PSMA catalytic zinc atoms. The scope of the analog library was designed to test the importance of various functional groups to the inhibitory potency of the lead phosphoramidate. The IC(50) for the lead phosphoramidate inhibitor was 35 nM while the IC(50) values for the analog library presented a range from 0.86 nM to 4.1 microM. Computational docking, utilizing a recently solved X-ray crystal structure of the recombinant protein, along with enzyme inhibition data, was used to propose a pharmacophore model for the PSMA active site.

Substrate specificity of prostate-specific membrane antigen

A series of putative dipeptide substrates of prostate-specific membrane antigen (PSMA) was prepared that explored alpha- and beta/gamma-linked acidic residues at the P1 position and various chromophores at the P2 position, while keeping the P1' residue constant as L-Glu. Four chromophores were examined, including 4-phenylazobenzoyl, 1-pyrenebutyryl, 9-anthracenylcarboxyl-gamma-aminobutyryl, and 4-nitrophenylbutyryl. When evaluating these chromophores, it was found that a substrate containing 4-phenylazobenzoyl at the P2 position was consumed most efficiently. Substitution at the P1 position with acidic residues showed that only gamma-linked L-Glu and D-Glu were recognized by the enzyme, with the former being more readily proteolyzed. Lastly, binding modes of endogenous substrates and our best synthetic substrate (4-phenylazobenzoyl-Glu-gamma-Glu) were proposed by computational docking studies into an X-ray crystal structure of the PSMA extracellular domain.

Amphipathic Peptide-Based Fusion Peptides and Immunoconjugates for the Targeted Ablation of Prostate Cancer Cells

We describe the design, generation, and in vitro evaluation of targeted amphipathic fusion peptides and immunoconjugates for the ablation of prostate cancer cells. The overexpression of the prostate-specific membrane antigen (PSMA) was exploited as means to specifically deliver cytotoxic peptides to prostate cancer cells. Cationic amphipathic lytic peptides were chosen as cytotoxic agents due to their ability to depolarize mitochondrial membranes and induce apoptosis. Specific delivery of the lytic peptide was facilitated by PSMA-targeting peptides and antibodies. Our results indicate that although the use of PSMA-targeted peptides only modestly enhanced the cytotoxic activity of the lytic peptide, peptide-antibody conjugates were two orders of magnitude more potent than untargeted peptide. In addition to quantifying the cytotoxic activities of the individual constructs, we also investigated the mechanisms of cell death induced by the fusion peptides and immunoconjugates. Although fusion peptides induced oncotic/necrotic death in cells, treatment with immunoconjugates resulted in apoptotic death. In summary, immunoconjugates based on lytic peptides are a promising class of therapeutics for prostate cancer therapy and warrant further investigation.

Immunotherapy for prostate cancer

Whereas androgen deprivation and chemotherapy have become the cornerstone of therapy for advanced prostate cancer, novel therapies are being developed that may expand upon currently available treatments. The identification of antigens expressed by prostate tissue and/or prostate cancer that are recognized by T cells or antibodies creates opportunities to develop novel immunotherapeutic approaches including tumor vaccines. Proteins expressed in prostate cancer-including prostate-specific antigen, prostatic acid phosphatase, and prostate membrane antigen-have been used as immunologic targets for immunotherapy. Moreover, innovations in cancer genomics and proteomics also will aid in the identification of immunologic targets. Emerging trials have demonstrated that immunotherapy may generate not only immune responses in patients but also clinical responses. Future studies will be directed at capitalizing on these findings.

Immunotherapy for prostate cancer

Whereas androgen deprivation and chemotherapy have become the cornerstone of therapy for advanced prostate cancer, novel therapies are being developed that may expand upon currently available treatments. The identification of antigens expressed by prostate tissue and/or prostate cancer that are recognized by T cells or antibodies creates opportunities to develop novel immunotherapeutic approaches including tumor vaccines. Proteins expressed in prostate cancer including prostate-specific antigen, prostatic acid phosphatase, and prostate membrane antigen have been used as immunologic targets for immunotherapy. Moreover, innovations in cancer genomics and proteomics also will aid in the identification of immunologic targets. Emerging trials have demonstrated that immunotherapy may not only generate immune responses in patients, but also clinical responses. Future studies will be directed at capitalizing on these findings.

A recombinant PSMA-specific single-chain immunotoxin has potent and selective toxicity against prostate cancer cells

Prostate cancer is the most commonly diagnosed form of cancer and the second leading cancer-related death among men in the Western civilization. Since no effective therapy exists for this tumor after progression beyond resectable boundaries, there is an urgent need for new treatment strategies. Prostate specific membrane antigen (PSMA) represents an excellent target on prostate cancer cells, and therefore specific immunotherapy may be a novel therapeutic option for the management of this tumor. We constructed a fully recombinant immunotoxin (A5-PE40) from a single-chain antibody fragment (scFv) against cell-adherent PSMA and a truncated form of Pseudomonas exotoxin A (PE40) lacking its natural binding domain Ia. The scFv A5 was obtained from a mAb elicited with native PSMA by phage display technology and direct selection on cells carrying the antigen. The bacterially expressed and purified immunotoxin A5-PE40 specifically binds to PSMA-positive prostate cancer cells and induces a 50% reduction of viability (IC50) at a concentration of 20 pM, while PSMA-negative cells remain unaffected. Due to its high and specific toxicity this recombinant immunotoxin is a promising candidate for therapeutic applications in patients with prostate cancer.

Differing effects of microtubule depolymerizing and stabilizing chemotherapeutic agents on t-SNARE–mediated apical targeting of prostate-specific membrane antigen

Prostate-specific membrane antigen (PSMA) is a protein up-regulated in the vast majority of prostate cancers. Antibodies to PSMA have proved highly specific for prostate cancer cells, and the therapeutic potential of such antibodies is currently being assessed in clinical trials. We have previously shown that PSMA at the cell surface of polarized epithelial cells is predominantly expressed at the apical plasma membrane and that microtubule depolymerization abolishes apical PSMA targeting. In the current report, we implicate a functional role for a target membrane soluble N-ethylmaleimide-sensitive factor adaptor protein receptor, syntaxin 3, in the microtubule-dependent apical targeting of PSMA. PSMA and syntaxin 3 are similarly localized to the apical plasma membrane of the prostatic epithelium and Madin-Darby canine kidney cells. Introduction of a point mutation into syntaxin 3 abolishes its polarized distribution and causes PSMA to be targeted in a nonpolarized fashion. Additionally, treatment of polarized Madin-Darby canine kidney cells with vinblastine, a microtubule depolymerizing chemotherapeutic agent, causes both syntaxin 3 and PSMA to redistribute in a nonpolarized fashion. However, following treatment with the microtubule stabilizing chemotherapeutic agent Taxotere, both syntaxin 3 and PSMA continue to localize in a polarized manner at the apical plasma membrane. Thus, microtubule depolymerizing and stabilizing chemotherapeutic drugs might exact similar cytotoxic effects but have disparate effects on protein targeting. This phenomenon might have important clinical implication, especially related to antibody-mediated immunotherapy, and could potentially be exploited for therapeutic benefit.

Expression of prostate-specific membrane antigen in normal and malignant human tissues

BACKGROUND

Prostate-specific membrane antigen (PSMA) is upregulated in androgen-dependent prostate carcinoma and it has been targeted for immunotherapy and diagnosis of this cancer. However, this protein is also expressed in other tissues. The objective of this study is to investigate its expression in normal and malignant human tissues.

METHODS

Using monoclonal antibodies 24.4E6 (specific for residues 638-657) and 7E11.C5 (specific for the transmembrane domain of PSMA), immunohistochemical detection of PSMA was performed in surgical specimens.

RESULTS

Prostate-specific membrane antigen was detected in the epithelium of prostate, urinary bladder, proximal tubules of kidney, liver, esophagus, stomach, small intestine, colon, breast, fallopian tubes and testicular seminiferous tubules, hippocampal neurons and astrocytes, ependyma, cortex and medulla of the adrenal gland, and ovary stroma. It was also detected in neoplasms of the prostate, kidney, urinary bladder, stomach, small intestine, colon, lung, adrenal gland, and testis. It was not detected in normal seminal vesicles or the lung.

CONCLUSIONS

These findings demonstrate that PSMA is widely distributed in normal tissues, and, depending on the tumors, its expression is up- or down-regulated, or unchanged. The broad distribution of PSMA may make it suitable for the diagnosis and therapy of a wide variety of tumors.

A new generation of monoclonal and recombinant antibodies against cell-adherent prostate specific membrane antigen for diagnostic and therapeutic targeting of prostate cancer

BACKGROUND

Prostate-specific membrane antigen (PSMA) is an excellent candidate for targeting prostate cancer by virtue of its restricted expression on prostatic epithelial cells and its upregulation on prostatic carcinoma cells. PSMA is expressed on the cell surface displaying a specific three-dimensional structure. Therefore, only antibodies with a high cell binding activity will have an important impact on antibody-based imaging and therapy.

METHODS

Monoclonal antibodies (mAbs) and single chain antibody fragments (scFvs) were prepared from spleen cells of mice that had been immunized either with purified PSMA or a cell lysate of prostate cancer LNCaP cells containing native PSMA. mAbs and scFvs were screened for reactivity with purified PSMA and binding to PSMA-expressing LNCaP cells.

RESULTS

From mice immunized with purified PSMA, we obtained three mAbs (K7, K12, D20) and four scFvs (G0, G1, G2, G4), which were highly reactive with the isolated antigen, but showed weak or no reaction with viable LNCaP cells. From mice immunized with unpurified LNCaP lysate, we obtained three mAbs (3/E7, 3/F11, 3/A12), and one scFv (A5), which were reactive with purified PSMA, also showing a strong and specific binding to viable LNCaP cells and PSMA-transfected cells.

CONCLUSIONS

Our results suggest that only the mAbs and scFvs, that were elicited with unpurified LNCaP lysate and not with purified PSMA will be useful agents for diagnostic imaging and therapeutic applications of prostate cancer.

Immunotoxins for targeted cancer therapy

Immunotoxins are proteins that contain a toxin along with an antibody or growth factor that binds specifically to target cells. Nearly all protein toxins work by enzymatically inhibiting protein synthesis. For the immunotoxin to work, it must bind to and be internalized by the target cells, and the enzymatic fragment of the toxin must translocate to the cytosol. Once in the cytosol, 1 molecule is capable of killing a cell, making immunotoxins some of the most potent killing agents. Various plant and bacterial toxins have been genetically fused or chemically conjugated to ligands that bind to cancer cells. Among the most active clinically are those that bind to hematologic tumors. At present, only 1 agent, which contains human interleukin-2 and truncated diphtheria toxin, is approved for use in cutaneous T-cell lymphoma. Another, containing an anti-CD22 Fv and truncated Pseudomonas exotoxin, has induced complete remissions in a high proportion of cases of hairy-cell leukemia. Refinement of existing immunotoxins and development of new immunotoxins are underway to improve the treatment of cancer.