Comparative study of PSMA expression in the prostate of mouse, dog, monkey, and human

Lessons learned in application driven imaging agent design for image-guided surgery

To meet the growing demand for intraoperative molecular imaging, the development of compatible imaging agents plays a crucial role. Given the unique requirements of surgical applications compared to diagnostics and therapy, maximizing translational potential necessitates distinctive imaging agent designs. For effective surgical guidance, exogenous signatures are essential and are achievable through a diverse range of imaging labels such as (radio)isotopes, fluorescent dyes, or combinations thereof. To achieve optimal in vivo utility a balanced molecular design of the tracer as a whole is required, which ensures a harmonious effect of the imaging label with the affinity and specificity (e.g., pharmacokinetics) of a pharmacophore/targeting moiety. This review outlines common design strategies and the effects of refinements in the molecular imaging agent design on the agent's pharmacological profile. This includes the optimization of affinity, pharmacokinetics (including serum binding and target mediated background), biological clearance route, the achievable signal intensity, and the effect of dosing hereon.

Discovery of PSMA in the prostate of the common marmoset (Callithrix jacchus)

BACKGROUND

Prostate-specific membrane antigen (PSMA) is a biomarker and therapeutic target of high relevance in prostate cancer. Although upregulated PSMA expression is a well-documented feature of prostatic neoplasia in both humans and canids, to date humans are the only species known to express PSMA basally in the prostate. Thus, traditional laboratory animal species have limited utility for studying PSMA biology in the prostate or for predicting efficacy or toxicity of PSMA-targeted agents.

METHODS

PSMA expression in human, macaque, and marmoset prostates was determined by immunohistochemistry, employing an antibody with validated cross-species reactivity in a PSMA-positive control tissue; kidney.

RESULTS

We newly discover that the common marmoset endogenously expresses PSMA in non-diseased prostate, similar to humans, and thus may be a valuable preclinical model for researchers studying PSMA.

Imaging of Tumor-Associated Vascular Prostate-Specific Membrane Antigen in Woodchuck Model of Hepatocellular Carcinoma

BACKGROUND AND AIMS

Radiolabeled short peptide ligands targeting prostate-specific membrane antigen (PSMA) were developed initially for imaging and treatment of prostate cancers. While many nonprostate solid tumors including hepatocellular carcinoma (HCC) express little PSMA, their neovasculature expresses a high level of PSMA, which is avid for Gallium-68-labeled PSMA-targeting radio-ligand (⁶⁸Ga-PSMA-11) for positron emission tomography (PET). However, the lack of a spontaneous animal model of tumor-associated vascular PSMA overexpression has hindered the development and assessment of PSMA-targeting radioligands for imaging and therapy of the nonprostatic cancers. We identified detectable indigenous PSMA expression on tumor neovascular endothelia in a naturally occurring woodchuck model of HCC.

METHODS

Molecular docking was performed with 3 bait PSMA ligands and compared between human and woodchuck PSMA. Initially, PET images were acquired dynamically after intravenously injecting 37 MBq (1.0 mCi) of ⁶⁸Ga-PSMA-11 into woodchuck models of HCC. Subsequently, 10-minute static PET scans were conducted for other animals 1-hour after injection due to HCC and liver background uptake stabilization at 30-45 minutes after injection. Liver tissue samples were harvested after imaging, fresh-frozen for quantitative reverse transcription polymerase chain reaction and western blot for validation, or fixed for histology for correlation.

RESULTS

Our preclinical studies confirmed the initial clinical findings of ⁶⁸Ga-PSMA-11 uptake in HCC. The agents (ligands and antibodies) developed against human PSMA were found to be reactive against the woodchuck PSMA.

CONCLUSION

This animal model offers a unique opportunity for investigating the biogenesis of tumor-associated vascular PSMA, its functional role(s), and potentials for future treatment strategies targeting tumor vascular PSMA using already developed PSMA-targeting agents.

In silico study on radiobiological efficacy of Ac-225 and Lu-177 for PSMA-guided radiotherapy

The good efficacy of radioligand therapy (RLT) targeting prostate specific-membrane antigen (PSMA) for the treatment of metastatic castration-resistant prostate cancer (mCRPC) has been recently demonstrated in several clinical studies. However, the treatment effect of ¹⁷⁷Lu-PSMA-ligands is still suboptimal for a significant fraction of patients. In contrast to external beam radiotherapy, the radiation dose distribution itself is strongly influenced by the heterogeneous tumour microenvironment. Although microdosimetry is critical for RLT treatment outcome, it is difficult to clinically or experimentally establish the quantitative relation. We propose an in silico approach to quantitatively investigate the microdosimetry and its influence on treatment outcome for PSMA-directed RLT of two different radioisotopes ¹⁷⁷Lu and ²²⁵ Ac. The ultimate goal is optimize the combined ¹⁷⁷ Lu and ²²⁵ Ac-PSMA therapy and maximize the anti-tumour effect, while minimizing irradiation of off-target tissues.Clinical relevance- With the proposed hybrid model we show that ¹⁷⁷Lu-PSMA-ligands treatment assures a more homogeneously distributed dose and a lower dependency of the treatment outcome on the domain vascularisation. On the other hand, the ²²⁵Ac-PSMA-ligands treatment shows a much stronger efficacy in killing tumor cells with an equivalent mean dose distribution even in an hypoxic environment.

A PSMA-targeted bispecific antibody for prostate cancer driven by a small-molecule targeting ligand

Despite the development of next-generation antiandrogens, metastatic castration-resistant prostate cancer (mCRPC) remains incurable. Here, we describe a unique semisynthetic bispecific antibody that uses site-specific unnatural amino acid conjugation to combine the potency of a T cell-recruiting anti-CD3 antibody with the specificity of an imaging ligand (DUPA) for prostate-specific membrane antigen. This format enabled optimization of structure and function to produce a candidate (CCW702) with specific, potent in vitro cytotoxicity and improved stability compared with a bispecific single-chain variable fragment format. In vivo, CCW702 eliminated C4-2 xenografts with as few as three weekly subcutaneous doses and prevented growth of PCSD1 patient-derived xenograft tumors in mice. In cynomolgus monkeys, CCW702 was well tolerated up to 34.1 mg/kg per dose, with near-complete subcutaneous bioavailability and a PK profile supporting testing of a weekly dosing regimen in patients. CCW702 is being evaluated in a first in-human clinical trial for men with mCRPC who had progressed on prior therapies (NCT04077021).

Is Hypoxia a Factor Influencing PSMA-Directed Radioligand Therapy?-An In Silico Study on the Role of Chronic Hypoxia in Prostate Cancer

Simple Summary

Tumor hypoxia is considered a critical factor associated with the resistance of conventional radiotherapy, where the X-ray-induced free radicals lead to DNA damage in a manner that is strongly dependent on the tissue oxygenation. The emerging PSMA-directed radioligand therapy (RLT) employs the α or β particles emitted by the radiopharmaceuticals to kill the tumor cells. In contrast to conventional therapy, the induced DNA damage is less dependent on the oxygenation status. Less attention has been paid to investigating whether tumor hypoxia will influence the efficacy of PSMA-directed RLT. We propose a histology-driven in silico model to quantitatively investigate the influence of tumor hypoxia on the treatment outcome for PSMA-directed RLT with 177Lu and 225Ac. Our finding suggests that hypoxia is a factor to be considered for the application of PSMA-directed RLT.

Abstract

Radioligand therapy (RLT) targeting prostate specific-membrane antigen (PSMA) is an emerging treatment for metastatic castration-resistant prostate cancer (mCRPC). It administrates 225Ac- or 177Lu-labeled ligands for the targeted killing of tumor cells. Differently from X- or γ-ray, for the emitted α or β particles the ionization of the DNA molecule is less dependent on the tissue oxygenation status. Furthermore, the diffusion range of electrons in a tumor is much larger than the volume typically spanned by hypoxic regions. Therefore, hypoxia is less investigated as an influential factor for PSMA-directed RLT, in particular with β emitters. This study proposes an in silico approach to theoretically investigate the influence of tumor hypoxia on the PSMA-directed RLT. Based on mice histology images, the distribution of the radiopharmaceuticals was simulated with an in silico PBPK-based convection–reaction–diffusion model. Three anti-CD31 immunohistochemistry slices were used to simulate the tumor microenvironment. Ten regions of interest with varying hypoxia severity were analyzed. A kernel-based method was developed for dose calculation. The cell survival probability was calculated according to the linear-quadratic model. The statistical analysis performed on all the regions of interest (ROIs) shows more heterogeneous dose distributions obtained with 225Ac compared to 177Lu. The higher homogeneity of 177Lu-PSMA-ligand treatment is due to the larger range covered by the emitted β particles. The dose-to-tissue histogram (DTH) metric shows that in poorly vascularized ROIs only 10% of radiobiological hypoxic tissue receives the target dose using 177Lu-PSMA-ligand treatment. This percentage drops down to 5% using 225Ac. In highly vascularized ROIs, the percentage of hypoxic tissue receiving the target dose increases to more than 85% and 65% for the 177Lu and 225Ac-PSMA-ligands, respectively. The in silico study demonstrated that the reduced vascularization of the tumor strongly influences the dose delivered by PSMA-directed RLT, especially in hypoxic regions and consequently the treatment outcome.

Tumor-targeted CD28 bispecific antibodies enhance the antitumor efficacy of PD-1 immunotherapy

Monoclonal antibodies that block the programmed cell death 1 (PD-1) checkpoint have revolutionized cancer immunotherapy. However, many major tumor types remain unresponsive to anti-PD-1 therapy, and even among responsive tumor types, most of the patients do not develop durable antitumor immunity. It has been shown that bispecific antibodies activate T cells by cross-linking the TCR/CD3 complex with a tumor-specific antigen (TSA). The class of TSAxCD3 bispecific antibodies have generated exciting results in early clinical trials. We have recently described another class of "costimulatory bispecifics" that cross-link a TSA to CD28 (TSAxCD28) and cooperate with TSAxCD3 bispecifics. Here, we demonstrate that these TSAxCD28 bispecifics (one specific for prostate cancer and the other for epithelial tumors) can also synergize with the broader anti-PD-1 approach and endow responsiveness-as well as long-term immune memory-against tumors that otherwise do not respond to anti-PD-1 alone. Unlike CD28 superagonists, which broadly activate T cells and induce cytokine storm, TSAxCD28 bispecifics display little or no toxicity when used alone or in combination with a PD-1 blocker in genetically humanized immunocompetent mouse models or in primates and thus may provide a well-tolerated and "off the shelf" combination approach with PD-1 immunotherapy that can markedly enhance antitumor efficacy.

Preclinical Dosimetry, Imaging, and Targeted Radionuclide Therapy Studies of Lu-177-Labeled Albumin-Binding, PSMA-Targeted CTT1403

PURPOSE

Prostate-specific membrane antigen (PSMA) continues to be the hallmark biomarker for prostate cancer as it is expressed on nearly all prostatic tumors. In addition, increased PSMA expression correlates with castration resistance and progression to the metastatic stage of the disease. Recently, we combined both an albumin-binding motif and an irreversible PSMA inhibitor to develop the novel PSMA-targeted radiotherapeutic agent, CTT1403. This molecule was novel in the field of PSMA-targeted agents as its key motifs resulted in extended blood circulation time and tumor uptake, rapid and extensive internalization into PSMA+ cells, and promising therapeutic efficacy. The objective of this study was to perform IND-enabling translational studies on CTT1403 in rodent models.

PROCEDURES

A dose optimization study was performed in PC3-PIP (PSMA+) tumor-bearing mice. Treatment groups were randomly selected to receive one to three 14-MBq injections of CTT1403. Control groups included (1) saline, (2) non-radioactive [¹⁷⁵Lu]CTT1403, or (3) two injections of 14 MBq CTT1751, a Lu-177-labeled non-targeted albumin-binding moiety. Tumor growth was monitored up to 120 days. Small-animal single photon emission tomography/X-ray computed tomography imaging was performed with CTT1403 and CTT1751 in PC3-PIP tumor-bearing mice to visualize infiltration of the Lu-177-labeled agent into the tumor. In preparation for a first-in-human study, human absorbed doses were estimated based on rat biodistribution out to 5 weeks to determine a safe CTT1403 therapy dose in humans.

RESULTS

Two to 3 injections of 14 MBq CTT1403 yielded significant tumor growth inhibition and increased survival compared with all control groups and mice receiving 1 injection of 14 MBq CTT1403. Five of 12 mice receiving 2 or 3 injections of CTT1403 survived to the 120-day post-treatment study endpoint. Dosimetry identified the kidneys as the dose-limiting organ, with an equivalent dose of 5.18 mSv/MBq, resulting in a planned maximum dose of 4.4 GBq for phase 1 clinical trials.

CONCLUSIONS

The preclinical efficacy and dosimetry of CTT1403 suggest that this agent has significant potential to be safe and effective in humans.

Alzheimer's Disease, and Breast and Prostate Cancer Research: Translational Failures and the Importance to Monitor Outputs and Impact of Funded Research

Dementia and cancer are becoming increasingly prevalent in Western countries. In the last two decades, research focused on Alzheimer's disease (AD) and cancer, in particular, breast cancer (BC) and prostate cancer (PC), has been substantially funded both in Europe and worldwide. While scientific research outcomes have contributed to increase our understanding of the disease etiopathology, still the prevalence of these chronic degenerative conditions remains very high across the globe. By definition, no model is perfect. In particular, animal models of AD, BC, and PC have been and still are traditionally used in basic/fundamental, translational, and preclinical research to study human disease mechanisms, identify new therapeutic targets, and develop new drugs. However, animals do not adequately model some essential features of human disease; therefore, they are often unable to pave the way to the development of drugs effective in human patients. The rise of new technological tools and models in life science, and the increasing need for multidisciplinary approaches have encouraged many interdisciplinary research initiatives. With considerable funds being invested in biomedical research, it is becoming pivotal to define and apply indicators to monitor the contribution to innovation and impact of funded research. Here, we discuss some of the issues underlying translational failure in AD, BC, and PC research, and describe how indicators could be applied to retrospectively measure outputs and impact of funded biomedical research.

A comparative study of peptide-based imaging agents [68Ga]Ga-PSMA-11, [68Ga]Ga-AMBA, [68Ga]Ga-NODAGA-RGD and [68Ga]Ga-DOTA-NT-20.3 in preclinical prostate tumour models

INTRODUCTION

Peptide-based imaging agents targeting prostate-specific membrane antigen (PSMA) have revolutionized the evaluation of biochemical recurrence of prostate cancer (PCa) but lacks sensitivity at very low serum prostate specific antigen (PSA) levels. Once recurrence is suspected, other positron emission tomography (PET) radiotracers could be of interest to discriminate between local and distant relapse. We studied [¹⁸F]fluorodeoxyglucose ([¹⁸F]FDG) targeting glucose metabolism, [¹⁸F]fluorocholine ([¹⁸F]FCH) targeting membrane metabolism and peptide-based imaging agents [⁶⁸Ga]Ga-PSMA-11, [⁶⁸Ga]Ga-AMBA, [⁶⁸Ga]Ga-NODAGA-RGD and [⁶⁸Ga]Ga-DOTA-NT-20.3 targeting PSMA, gastrin releasing peptide receptor (GRPr), αvβ3 integrin and neurotensin type 1 receptor (NTSR1) respectively, in different PCa tumour models.

METHODS

Mice were xenografted with 22Rv1, an androgen-receptor (AR)-positive, PCa cell line that expresses PSMA and PC3, an AR-negative one that does not express PSMA. PET imaging using the different radiotracers was performed sequentially and the uptake characteristics compared to one other. NTSR1 and PSMA expression levels were analysed in tumours by immunohistochemistry.

RESULTS

[¹⁸F]FDG displayed low but sufficient uptake to visualize PC3 and 22Rv1 derived tumours. We also observed a low efficacy of [¹⁸F]FCH PET imaging and a low [⁶⁸Ga]Ga-NODAGA-RGD tumour uptake in those tumours. As expected, an elevated tumour uptake was obtained for [⁶⁸Ga]Ga-PSMA-11 in 22Rv1 derived tumour although no uptake was measured in the androgen independent cell line PC3, derived from a bone metastasis of a high-grade PCa. Moreover, in PC3 cell line, we obtained good tumour uptake, high tumour-to-background contrast using [⁶⁸Ga]Ga-AMBA and [⁶⁸Ga]Ga-DOTA-NT-20.3. Immunohistochemistry analysis confirmed high NTSR1 expression in PC3 derived tumours and conversely high PSMA expression in 22Rv1 derived tumours.

CONCLUSION

PET imaging using [⁶⁸Ga]Ga-AMBA and [⁶⁸Ga]Ga-DOTA-NT-20.3 demonstrates that GRPr and NTSR1 could represent viable alternative targets for diagnostic or therapeutic applications in PCa with limited PSMA expression levels. More preclinical and clinical studies will follow to explore this potential.

ADVANCES IN KNOWLEDGE AND IMPLICATIONS FOR PATIENT

Peptide-based imaging agents targeting PSMA represent a major progress in the evaluation of biochemical recurrence of PCa but sometimes yield false negative results in some lesions. Continuing efforts have thus been made to evaluate other radiotracers. Our preclinical results suggest that [⁶⁸Ga]labelled bombesin and neurotensin analogues could serve as alternative PET radiopharmaceuticals for diagnostic or therapy in cases of PSMA-negative PCa.

A PSMA-Targeting CD3 Bispecific Antibody Induces Antitumor Responses that Are Enhanced by 4-1BB Costimulation

Patients with hematologic cancers have improved outcomes after treatment with bispecific antibodies that bind to CD3 on T cells and that redirect T cells toward cancer cells. However, clinical benefit against solid tumors remains to be shown. We made a bispecific antibody that targets both the common prostate tumor-specific antigen PSMA and CD3 (PMSAxCD3) and provide evidence for tumor inhibition in several preclinical solid tumor models. Mice expressing the human extracellular regions of CD3 and PSMA were generated to examine antitumor efficacy in the presence of an intact immune system and PSMA expression in normal tissues. PSMAxCD3 accumulated in PSMA-expressing tissues and tumors as detected by immuno-PET imaging. Although PSMAxCD3 induced T-cell activation and showed antitumor efficacy in mice with low tumor burden, PSMAxCD3 lost efficacy against larger solid tumors, mirroring the difficulty of treating solid tumors in the clinic. Costimulatory receptors can enhance T-cell responses. We show here that costimulation can enhance the antitumor efficacy of PSMAxCD3. In particular, 4-1BB stimulation in combination with PSMAxCD3 enhanced T-cell activation and proliferation, boosted efficacy against larger tumors, and induced T-cell memory, leading to durable antitumor responses. The combination of CD3 bispecific antibodies and anti-4-1BB costimulation represents a therapeutic approach for the treatment of solid tumors.

Peripheral ganglia in healthy rats as target structures for the evaluation of PSMA imaging agents

Background

The recent implementation of PET with prostate specific membrane antigen (PSMA)-specific radiotracers into the clinical practice has resulted in the significant improvement of accuracy in the detection of prostate carcinoma (PCa). PSMA-expression in ganglia has been regarded as an important pitfall in prostate carcinoma-PET diagnostics but has not found any practical use for diagnosis or therapy.

Methods

We explored this phenomenon and demonstrated the applicability of peripheral ganglia in healthy rats as surrogates for small PSMA positive lesions for the preclinical evaluation of diagnostic PCa PET probes. Healthy rats were measured with PET/CT using the tracers [¹⁸F]DCFPyL, [Al¹⁸F]PSMA-11 and [⁶⁸Ga]PSMA-11. Sections of ganglia were stained with an anti-PSMA antibody. [¹⁸F]DCFPyL uptake in ganglia was compared to that in LNCaP tumor xenografts in mice.

Results

Whereas [¹⁸F]DCFPyL and [⁶⁸Ga]PSMA-11 were stable in vivo and accumulated in peripheral ganglia, [Al¹⁸F]PSMA-11 suffered from fast in vivo deflourination resulting in high bone uptake. Ganglionic PSMA expression was confirmed by immunohistochemistry. [¹⁸F]DCFPyL uptake and signal-to-noise ratio in the superior cervical ganglion was not significantly different from LNCaP xenografts.

Conclusions

Our results demonstrated the non-inferiority of the novel model compared to conventionally used tumor xenografts in immune compromised rodents with regard to reproducibility and stability of the PSMA signal. Furthermore, the model involves less expense and efforts while it is permanently available and avoids tumor-growth associated animal morbidity and distress. To the best of our knowledge, this is the first tumor-free model suitable for the in vivo evaluation of tumor imaging agents.

PSMA expression in the Hi-Myc model; extended utility of a representative model of prostate adenocarcinoma for biological insight and as a drug discovery tool

Prostate-specific membrane antigen (PSMA), also known as glutamate carboxypeptidase II (GCPII), is highly overexpressed in primary and metastatic prostate cancer (PCa). This has led to the development of radiopharmaceuticals for targeted imaging and therapy under current clinical evaluation. Despite this progress, the exact biological role of the protein in prostate cancer development and progression has not been fully elucidated. This is in part because the human PSMA and mouse PSMA (mPSMA) have different patterns of anatomical expression which confound study in the most widely utilized model organisms. Most notably, mPSMA is not expressed in the healthy murine prostate. Here, we reveal that mPSMA is highly upregulated in the prostate adenocarcinoma of the spontaneous Hi-Myc mouse model, a highly accurate and well characterized mouse model of prostate cancer development. Antibody detection and molecular imaging tools are used to confirm that mPSMA is expressed from early prostatic intraepithelial neoplasia (PIN) through adenocarcinoma.

Preparation of 68Ga-PSMA-11 with a Synthesis Module for Micro PET-CT Imaging of PSMA Expression during Prostate Cancer Progression

Objective

To synthesize ⁶⁸Ga-Glu-urea-Lys(Ahx)-HBED-CC (⁶⁸Ga-PSMA-11) with a synthesis module and investigate PET-CT imaging to monitor PSMA expression during prostate cancer (PCa) progression and tumor growth in mice bearing subcutaneous PCa xenografts.

Method

The radiochemical purity and stability of  ⁶⁸Ga-PSMA-11 were determined via radio-HPLC. The PCa cell lines of different PSMA expression levels (PC3, VCAP±, CWR22RV1+, and LNCaP++) were selected to mimic the PCa progression. ⁶⁸Ga-PSMA-11 biodistribution was studied by dissection method and in vivo imaging with micro PET-CT. The expression levels of PSMA in tumor cells and tissues were analyzed by immunofluorescence, flow cytometry, and western blot. The correlation between PSMA expression and radio-uptake was also evaluated. 2-PMPA preadministration served as a block group.

Results

The radiochemical purity of  ⁶⁸Ga-PSMA-11 was 99.6 ± 0.1% and stable in vitro for 2 h. The equilibrium binding constant (Kd) of  ⁶⁸Ga-PSMA-11 to LNCaP, CWR22Rv1, PC-3, and VCAP cells was 4.3 ± 0.8 nM, 16.4 ± 1.3 nM, 225.3 ± 20.8 nM, and 125.6 ± 13.1 nM, respectively. Results of tumor uptake (% ID and % ID/g or % ID/cm³) of  ⁶⁸Ga-PSMA-11 in biodistribution and micro PET imaging were LNCaP > CWR22RV1 > PC-3 and VCAP due to different PSMA expression levels. It was confirmed by flow cytometry, western blot, and immunofluorescence. Tumor uptake (% ID/cm³) of  ⁶⁸Ga-PSMA-11 increased with the tumor anatomical volume in quadratic polynomial fashion and reached the peak (when tumor volume was 0.5 cm³) earlier than tumor uptake (% ID). Tumor uptake (% ID/cm³) of  ⁶⁸Ga-PSMA-11 based on functional volume correlated well with the PSMA expression in a linear manner (y = 9.35x + 2.59, R² = 0.8924, and p < 0.0001); however, low dose 2-PMPA causes rapid renal clearance of increased tumor/kidney uptake of  ⁶⁸Ga-PSMA-11.

Conclusions

The ⁶⁸Ga-PSMA-11 PET-CT imaging could invasively evaluate PSMA expression during PCa progression and tumor growth with % ID/cm³ (based on functional volume) as an important index. Low dose 2-PMPA preadministration might be a choice to decrease kidney uptake of  ⁶⁸Ga-PSMA-11.

A Perspective on the Evolving Story of PSMA Biology, PSMA-Based Imaging, and Endoradiotherapeutic Strategies

In this review, we cover the evolution of knowledge on the biology of prostate-specific membrane antigen (PSMA) and its translation to therapy. The usual key to discovery is a realistic model for experimentation and for testing a hypothesis. A realistic model is especially needed in the case of the human prostate, which differs significantly from the prostate of species often used as research models. We will emphasize the genetic characterization of PSMA, the nature of the PSMA protein, and its role as a carboxypeptidase, with differing important substrates and products in different tissues. We give special prominence to the importance of PSMA as a target for imaging and therapy in prostate cancer and its underdeveloped role for imaging and targeting the neovasculature of tumors other than prostate cancer. Lastly, we bring attention to its importance in other nonprostatic tissues.

Targeting PSMA with a Cu-64 Labeled Phosphoramidate Inhibitor for PET/CT Imaging of Variant PSMA-Expressing Xenografts in Mouse Models of Prostate Cancer

PURPOSE

Prostate-specific membrane antigen (PSMA) is highly up-regulated in prostate tumor cells, providing an ideal target for imaging applications of prostate cancer. CTT-1297 (IC50 = 27 nM) is an irreversible phosphoramidate inhibitor of PSMA that has been conjugated to the CB-TE1K1P chelator for incorporation of Cu-64. The resulting positron emission tomography (PET) agent, [(64)Cu]ABN-1, was evaluated for selective uptake both in vitro and in vivo in PSMA-positive cells of varying expression levels. The focus of this study was to assess the ability of [(64)Cu]ABN-1 to detect and distinguish varying levels of PSMA in a panel of prostate tumor-bearing mouse models.

PROCEDURES

CTT-1297 was conjugated to the CB-TE1K1P chelator using click chemistry and radiolabeled with Cu-64. Internalization and binding affinity of [(64)Cu]ABN-1 was evaluated in the following cell lines having varying levels of PSMA expression: LNCaP late-passage > LNCaP early passage ≈ C4-2B > CWR22rv1 and PSMA-negative PC-3 cells. PET/X-ray computed tomography imaging was performed in NCr nude mice with subcutaneous tumors of the variant PSMA-expressing cell lines.

RESULTS

[(64)Cu]ABN-1 demonstrated excellent uptake in PSMA-positive cells in vitro, with ∼80 % internalization at 4 h for each PSMA-positive cell line with uptake (fmol/mg) correlating to PSMA expression levels. The imaging data indicated significant tumor uptake in all models. The biodistribution for late-passage LNCaP (highest PSMA expression) demonstrated the highest specific uptake of [(64)Cu]ABN-1 with tumor-to-muscle and tumor-to-blood ratios of 30 ± 11 and 21 ± 7, respectively, at 24 h post-injection. [(64)Cu]ABN-1 cleared through all tissues except for PSMA-positive kidneys.

CONCLUSION

[(64)Cu]ABN-1 demonstrated selective uptake in PSMA-positive cells and tumors, which correlated to the level of PSMA expression. The data reported herein suggest that [(64)Cu]ABN-1 will selectively target and image variant PSMA expression and in the future will serve as a non-invasive method to follow the progression of prostate cancer in men.

An FDA oncology analysis of CD3 bispecific constructs and first-in-human dose selection

We retrospectively examined the nonclinical studies conducted with 17 CD3 bispecific constructs in support of first-in-human (FIH) trials in oncology. We also collected information on the design of dose-finding clinical trials. Sponsors have used different MABEL approaches for FIH dose selection. To better assess acceptable approaches, FIH doses were computed from nonclinical studies and compared to the maximum tolerated doses (MTDs) in patients, to the highest human doses (HHDs) when an MTD was not identified, or to the recommended human dose (RHD) for blinatumomab. We concluded that approaches based on receptor occupancy, highest non-severely toxic dose, or no-observed adverse effect level are not acceptable for selecting the FIH dose as they resulted in doses close to or above the MTDs, HHDs, or the RHD. A FIH dose corresponding to 10%-30% pharmacologic activity (PA) was an acceptable approach. A FIH dose corresponding to 50% PA was acceptable for all except one construct, potentially due to its biological or structural properties. The most common toxicities in animals and patients were those related to cytokine release. Doses were better tolerated when intra-animal or intra-patient dose escalation was used. Exposing naïve patients to an MTD achieved with intra-patient dose escalation design may be unsafe.

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.

DNA vaccination for prostate cancer: key concepts and considerations

While locally confined prostate cancer is associated with a low five year mortality rate, advanced or metastatic disease remains a major challenge for healthcare professionals to treat and is usually terminal. As such, there is a need for the development of new, efficacious therapies for prostate cancer. Immunotherapy represents a promising approach where the host's immune system is harnessed to mount an anti-tumour effect, and the licensing of the first prostate cancer specific immunotherapy in 2010 has opened the door for other immunotherapies to gain regulatory approval. Among these strategies DNA vaccines are an attractive option in terms of their ability to elicit a highly specific, potent and wide-sweeping immune response. Several DNA vaccines have been tested for prostate cancer and while they have demonstrated a good safety profile they have faced problems with low efficacy and immunogenicity compared to other immunotherapeutic approaches. This review focuses on the positive aspects of DNA vaccines for prostate cancer that have been assessed in preclinical and clinical trials thus far and examines the key considerations that must be employed to improve the efficacy and immunogenicity of these vaccines.

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.

Drug discovery in prostate cancer mouse models

INTRODUCTION

The mouse is an important, though imperfect, organism with which to model human disease and to discover and test novel drugs in a preclinical setting. Many experimental strategies have been used to discover new biological and molecular targets in the mouse, with the hopes of translating these discoveries into novel drugs to treat prostate cancer in humans. Modeling prostate cancer in the mouse, however, has been challenging, and often drugs that work in mice have failed in human trials.

AREAS COVERED

The authors discuss the similarities and differences between mice and men; the types of mouse models that exist to model prostate cancer; practical questions one must ask when using a mouse as a model; and potential reasons that drugs do not often translate to humans. They also discuss the current value in using mouse models for drug discovery to treat prostate cancer and what needs are still unmet in field.

EXPERT OPINION

With proper planning and following practical guidelines by the researcher, the mouse is a powerful experimental tool. The field lacks genetically engineered metastatic models, and xenograft models do not allow for the study of the immune system during the metastatic process. There remain several important limitations to discovering and testing novel drugs in mice for eventual human use, but these can often be overcome. Overall, mouse modeling is an essential part of prostate cancer research and drug discovery. Emerging technologies and better and ever-increasing forms of communication are moving the field in a hopeful direction.

Generation and characterisation of a canine EGFP-HMGA2 prostate cancer in vitro model

The architectural transcription factor HMGA2 is abundantly expressed during embryonic development. In several malignant neoplasias including prostate cancer, high re-expression of HMGA2 is correlated with malignancy and poor prognosis. The let-7 miRNA family is described to regulate HMGA2 negatively. The balance of let-7 and HMGA2 is discussed to play a major role in tumour aetiology. To further analyse the role of HMGA2 in prostate cancer a stable and highly reproducible in vitro model system is precondition. Herein we established a canine CT1258-EGFP-HMGA2 prostate cancer cell line stably overexpressing HMGA2 linked to EGFP and in addition the reference cell line CT1258-EGFP expressing solely EGFP to exclude EGFP-induced effects. Both recombinant cell lines were characterised by fluorescence microscopy, flow cytometry and immunocytochemistry. The proliferative effect of ectopically overexpressed HMGA2 was determined via BrdU assays. Comparative karyotyping of the derived and the initial CT1258 cell lines was performed to analyse chromosome consistency. The impact of the ectopic HMGA2 expression on its regulator let-7a was analysed by quantitative real-time PCR. Fluorescence microscopy and immunocytochemistry detected successful expression of the EGFP-HMGA2 fusion protein exclusively accumulating in the nucleus. Gene expression analyses confirmed HMGA2 overexpression in CT1258-EGFP-HMGA2 in comparison to CT1258-EGFP and native cells. Significantly higher let-7a expression levels were found in CT1258-EGFP-HMGA2 and CT1258-EGFP. The BrdU assays detected an increased proliferation of CT1258-HMGA2-EGFP cells compared to CT1258-EGFP and native CT1258. The cytogenetic analyses of CT1258-EGFP and CT1258-EGFP-HMGA2 resulted in a comparable hyperdiploid karyotype as described for native CT1258 cells. To further investigate the impact of recombinant overexpressed HMGA2 on CT1258 cells, other selected targets described to underlie HMGA2 regulation were screened in addition. The new fluorescent CT1258-EGFP-HMGA2 cell line is a stable tool enabling in vitro and in vivo analyses of the HMGA2-mediated effects on cells and the development and pathogenesis of prostate cancer.

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.

Glutamate carboxypeptidase II in diagnosis and treatment of neurologic disorders and prostate cancer

Glutamate carboxypeptidase II (GCPII) is a membrane-bound binuclear zinc metallopeptidase with the highest expression levels found in the nervous and prostatic tissue. Throughout the nervous system, glia-bound GCPII is intimately involved in the neuron-neuron and neuron-glia signaling via the hydrolysis of N-acetylaspartylglutamate (NAAG), the most abundant mammalian peptidic neurotransmitter. The inhibition of the GCPII-controlled NAAG catabolism has been shown to attenuate neurotoxicity associated with enhanced glutamate transmission and GCPII-specific inhibitors demonstrate efficacy in multiple preclinical models including traumatic brain injury, stroke, neuropathic and inflammatory pain, amyotrophic lateral sclerosis, and schizophrenia. The second major area of pharmacological interventions targeting GCPII focuses on prostate carcinoma; GCPII expression levels are highly increased in androgen-independent and metastatic disease. Consequently, the enzyme serves as a potential target for imaging and therapy. This review offers a summary of GCPII structure, physiological functions in healthy tissues, and its association with various pathologies. The review also outlines the development of GCPII-specific small-molecule compounds and their use in preclinical and clinical settings.

Assessment of an 18F-labeled phosphoramidate peptidomimetic as a new prostate-specific membrane antigen-targeted imaging agent for prostate cancer

Prostate-specific membrane antigen (PSMA) is a transmembrane protein commonly found on the surface of late-stage and metastatic prostate cancer and a well-known imaging biomarker for staging and monitoring therapy. Although (111)In-labeled capropmab pendetide is the only approved agent available for PSMA imaging, its clinical use is limited because of its slow distribution and clearance that leads to challenging image interpretation. A small-molecule approach using radiolabeled urea-based PSMA inhibitors as imaging agents has shown promise for prostate cancer imaging. The motivation of this work is to explore phosphoramidates as a new class of potent PSMA inhibitors to develop more effective prostate cancer imaging agents with improved specificity and clearance properties.

METHODS

N-succinimidyl-4-(18)F-fluorobenzoate ((18)F-SFB) was conjugated to S-2-((2-(S-4-amino-4-carboxybutanamido)-S-2-carboxyethoxy)hydroxyphosphorylamino)-pentanedioic acid (Phosphoramidate (1)), yielding S-2-((2-(S-4-(4-(18)F-fluorobenzamido)-4-carboxybutanamido)-S-2-carboxyethoxy)hydroxyphosphorylamino)-pentanedioic acid (3). In vivo studies were conducted in mice bearing either LNCaP (PSMA-positive) or PC-3 (PSMA-negative) tumors. PET images were acquired at 1 and 2 h with or without a preinjection of a nonradioactive version of the fluorophosphoramidate. Tissue distribution studies were performed at the end of the 2 h imaging sessions.

RESULTS

Phosphoramidate (1) and its fluorobenzamido conjugate (2) were potent inhibitors of PSMA (inhibitory concentration of 50% [IC(50)], 14 and 0.68 nM, respectively). PSMA-mediated tumor accumulation was noted in the LNCaP versus the PC-3 tumor xenografts. The LNCaP tumor uptake was also blocked by the administration of nonradioactive (2) prior to imaging studies. With the exception of the kidneys, tumor-to-tissue and tumor-to-blood ratios were greater than 5:1 at 2 h. The strong kidney uptake may be due to the known PSMA expression in the mouse kidney, because significant reduction (>6-fold) in kidney activity was seen in mice injected with (2).

CONCLUSION

(18)F-labeled phosphoramidate (3) is a representative of a new class of PSMA targeting peptidomimetic molecules that shows great promise as imaging agents for detecting PSMA+ prostate tumors.

Preclinical evaluation of novel glutamate-urea-lysine analogues that target prostate-specific membrane antigen as molecular imaging pharmaceuticals for prostate cancer

Prostate-specific membrane antigen (PSMA) is expressed in normal human prostate epithelium and is highly up-regulated in prostate cancer. We previously reported a series of novel small molecule inhibitors targeting PSMA. Two compounds, MIP-1072, (S)-2-(3-((S)-1-carboxy-5-(4-iodobenzylamino)pentyl)ureido)pentanedioic acid, and MIP-1095, (S)-2-(3-((S)-1carboxy-5-(3-(4-iodophenyl)ureido)pentyl)ureido)pentanedioic acid, were selected for further evaluation. MIP-1072 and MIP-1095 potently inhibited the glutamate carboxypeptidase activity of PSMA (K(i) = 4.6 +/- 1.6 nmol/L and 0.24 +/- 0.14 nmol/L, respectively) and, when radiolabeled with (123)I, exhibited high affinity for PSMA on human prostate cancer LNCaP cells (K(d) = 3.8 +/- 1.3 nmol/L and 0.81 +/- 0.39 nmol/L, respectively). The association of [(123)I]MIP-1072 and [(123)I]MIP-1095 with PSMA was specific; there was no binding to human prostate cancer PC3 cells, which lack PSMA, and binding was abolished by coincubation with a structurally unrelated NAALADase inhibitor, 2-(phosphonomethyl)pentanedioic acid (PMPA). [(123)I]MIP-1072 and [(123)I]MIP-1095 internalized into LNCaP cells at 37 degrees C. Tissue distribution studies in mice showed 17.3 +/- 6.3% (at 1 hour) and 34.3 +/- 12.7% (at 4 hours) injected dose per gram of LNCaP xenograft tissue, for [(123)I]MIP-1072 and [(123)I]MIP-1095, respectively. [(123)I]MIP-1095 exhibited greater tumor uptake but slower washout from blood and nontarget tissues compared with [(123)I]MIP-1072. Specific binding to PSMA in vivo was shown by competition with PMPA in LNCaP xenografts, and the absence of uptake in PC3 xenografts. The uptake of [(123)I]MIP-1072 and [(123)I]MIP-1095 in tumor-bearing mice was corroborated by single-photon emission computed tomography/computed tomography (SPECT/CT) imaging. PSMA-specific radiopharmaceuticals should provide a novel molecular targeting option for the detection and staging of prostate cancer.

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.

Histopathological and immunohistochemical characterization of canine prostate cancer

BACKGROUND

In this study we try to identify the origin of canine prostate cancer (cPC) by classifying the tumors histological subtypes and relate these subtypes to their combined expressional characteristics of several tissue specific and differentiation markers.

METHODS

cPCs were examined histomorphologically and by immunohistochemical detection of the cytokeratin markers CK14, HMWCK, CK5, CK18, and CK7, and of the markers UPIII, PSA and PSMA.

RESULTS

Histopathologically, six growth patterns could be differentiated. The most frequent patterns were solid, cribriform and micropapillary growth patterns, while sarcomatoid, small acinar/ductal, and tubulo-papillary growth patterns were less frequent present. Solid growth patterns were significantly (P = 0.027) more often seen in castrated dogs. Immunohistochemically, about half of the cPC cases showed expression of PSA (8/20) and PSMA (10/20); 85% and 60% of the cPC expressed UPIII (17/20) and CK7 (12/20), while 13 and 12 cPC expressed CK5 and CK14, respectively; all cPC expressed CK18. CK14 was significantly more often and UPIII less frequent expressed in the solid growth patterns than in the micropapillary and cribriform patterns, respectively.

CONCLUSIONS

Canine prostate cancer appear to be more aggressive and of a less differentiated type than most common human prostate cancers. Comparing the expression patterns of the markers in cPC to those in normal canine prostate tissue, cPC most likely originates from the collecting ducts rather than from the peripheral acini. Given also the fact that canine prostate cancer is unresponsive to androgen withdrawal therapy, canine prostate cancer mostly resembles human, androgen refractory, poorly differentiated prostate cancer.

Comparative characterization of the canine normal prostate in intact and castrated animals

BACKGROUND

Prostate diseases in the dog are generally regarded as representative for their human counterparts. We characterized the normal canine prostate in comparison to the normal human prostate.

METHODS

Prostates of dogs were examined histomorphologically and by immunohistochemical detection of the markers CK14, HMWCK, CK5, CK18, CK7, UPIII, PSA, and PSMA.

RESULTS

Histomorphologically, the canine prostate lacks the human zonal differentiation, has much more prominent acini, while comprising less stromal tissue. In general, the canine prostate epithelium displayed a highly differentiated character, with no cells expressing CK14, minimal amounts of cells expressing HMWCK/CK5 and the vast majority of cells expressing CK18 and PSA. After castration, the prostate epithelium regressed, and the remaining tubules were largely populated by cells showing a ductal phenotype (HMWCK+/CK5+/CK18+/CK7+).

CONCLUSIONS

The human and canine prostate are histologically differently organized. The general scheme of cellular differentiation of the prostate epithelium may however be applicable to both species.

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.