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

A PSMA-targeted doxorubicin small-molecule drug conjugate

We developed a model small-molecule drug conjugate (SMDC) that employed doxorubicin as a representative chemotherapeutic targeted to the cell membrane biomarker PSMA (prostate-specific membrane antigen) expressed on prostate cancer cells. The strategy capitalized on the clatherin-mediated internalization of PSMA to facilitate the selective uptake and release of doxorubicin in the target cells. The SMDC was prepared and assessed for binding kinetics, plasma stability, cell toxicity, and specificity towards PSMA expressing prostate cancer cell lines. We observed high affinity of the SMDC for PSMA (IC50 5 nM) with irreversible binding, as well as specific effectiveness against PSMA(+) cells. These findings validated the strategy for a small molecule-based approach in targeted cancer therapy.

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.

PSMA-targeted dendrimer as an efficient anticancer drug delivery vehicle for prostate cancer

Prostate cancer (PCa) is the second leading cause of cancer-related deaths among men in the United States. Although early-stage treatments exhibit promising 5-year survival rates, the treatment options for advanced stage disease are constrained, with short survival benefits due to the challenges associated with effective and selective drug delivery to PCa cells. Even though targeting Prostate Specific Membrane Antigen (PSMA) has been extensively explored and is clinically employed for imaging and radio-ligand therapy, the clinical success of PSMA-based approaches for targeted delivery of chemotherapies remains elusive. In this study, we combine a generation 4 hydroxy polyamidoamine dendrimer (PD) with irreversible PSMA ligand (CTT1298) to develop a PSMA-targeted nanoplatform (PD-CTT1298) for selective intracellular delivery of potent chemotherapeutics to PCa. PD-CTT1298-Cy5 exhibits a PSMA IC50 in the nanomolar range and demonstrates selective uptake in PSMA (+) PCa cells via PSMA mediated internalization. When systemically administered in a prostate tumor xenograft mouse model, PD-CTT1298-Cy5 selectively targets PSMA (+) tumors with significantly less accumulation in PSMA (-) tumors or upon blocking of the PSMA receptors. Moreover, the dendrimer clears rapidly from the off-target organs limiting systemic side-effects. Further, the conjugation of an anti-cancer agent, cabozantinib to the PSMA-targeted dendrimer translates to a significantly enhanced anti-proliferative activity in vitro compared to the free drug. These findings highlight the potential of PD-CTT1298 nanoplatform as a versatile approach for selective delivery of high payloads of potent chemotherapeutics to PCa, where dose related systemic side-effects are a major concern.

Copper-free click bioconjugation of technetium-99m complexes using strained cyclononyne derivatives

Click chemistry, in particular copper-free click reactions, has gained growing interest for radiolabelling purposes in the field of radiopharmaceutical sciences. [^99mTc][Tc(CO)₃(H₂O)₃]⁺ works as an excellent starting complex for the radiolabelling of biomolecules under mild conditions. A new chelator, investigated for the copper-free strain-promoted cycloaddition (SPAAC), was synthesised containing the 2,2'-dipicolylamine (DPA) moiety for the ^99mTc-tricarbonyl core and compared with a DPA chelator based on activated esters for conventional radiolabelling. For the copper-free click labelling procedure, a DPA containing 4,8-diazacyclononyne moiety was prepared from a sulfonyl-modified diamide (four steps, 64% yield) followed by the Nicholas reaction with butyne-1,3-diol. The ^99mTc-DPA-DACN-complex was prepared with a radiochemical conversion (RCC) of 89% after 30 min. The following SPAAC reaction with an azide-functionalised PSMA molecule was performed within 4-5 hours at 100 °C to obtain the PSMA (prostate-specific membrane antigen) targeting ^99mTc-complex with 79% RCC and without side products. For comparison, a second DPA-chelator based on a tetrafluorophenyl (TFP) ester was prepared (three steps, 64% yield) and was successfully radiolabelled with [[^99mTc]Tc(CO)₃(H₂O)₃]⁺ with 89% RCC after 20 min and >99% radiochemical purity after separation using an RP18 cartridge. The subsequent conjugation of an amine-functionalised PSMA targeting molecule was performed with 23% RCC after 150 min. Two other unknown side products were observed indicating the decomposition of the TFP ester during the labelling. All nonradioactive Re(CO)₃ complexes were synthesised from (Et₄N)₂[ReBr₃(CO)₃] (91% yield for the ^natRe-DPA-TFP ester, 76% yield for ^natRe-DPA-DACN) and characterised to confirm the identity of the ^99mTc-complexes.

Therapeutic potential of targeting mirnas to prostate cancer tumors: using psma as an active target

Prostate cancer (PC) is a commonly diagnosed malignancy in men and is associated with high mortality rates. Current treatments for PC include surgery, chemotherapy, and radiation therapy. However, recent advances in targeted delivery systems have yielded promising new approaches to PC treatment. As PC epithelial cells express high levels of prostate-specific membrane antigen (PSMA) on the cell surface, new drug conjugates focused on PSMA targeting have been developed. microRNAs (miRNAs) are small noncoding RNAs that regulate posttranscriptional gene expression in cells and show excellent possibilities for use in developing new therapeutics for PC. PSMA-targeted therapies based on a miRNA payload and that selectively target PC cells enhances therapeutic efficacy without eliciting damage to normal surrounding tissue. This review discusses the rationale for utilizing miRNAs to target PSMA, revealing their potential in therapeutic approaches to PC treatment. Different delivery systems for miRNAs and challenges to miRNA therapy are also explored.

Application of the Chelator-Based Clickable Radiotheranostic Platform to Moderate-Molecular-Weight Ligands

We have reported that the chelator-based clickable radiotheranostic platform, ADIBO-DOTADG-ALB (ADA), has favorable properties as a radiotheranostic platform for low-molecular-weight ligands. In this study, we evaluated the applicability of ADA to moderate-molecular-weight ligands to expand the utility of the ADA platform. As a moderate-molecular-weight ligand, we selected exendin-4, a peptide-based agonist to glucagon-like peptide-1 receptor (GLP-1R). An exendin-4-incorporated ADA derivative, exendin-4-Cys40-triazole-DOTADG-ALB (EtDA), was radiolabeled with 111In by the conjugation of exendin-4-Cys40 azide to [111In]In-ADA. The click ligation of exendin-4-Cys40 azide to [111In]In-ADA was quantitatively completed in 10 min under ambient conditions. In the in vitro cell-binding assay and albumin-binding assay, [111In]In-EtDA showed strong binding to both a GLP-1R-expressing cell and albumin. In the biodistribution assay, [111In]In-EtDA showed markedly protracted tumor uptake, which was significantly decreased by the coinjection of exendin-4-Cys40. The single photon emission computed tomography (SPECT) image of [111In]In-EtDA visualized the tumor clearly. These results indicated the utility of [111In]In-EtDA as a radiotheranostic agent, suggesting the applicability of the ADA platform to moderate-molecular-weight ligands.

A Review on the Current State and Future Perspectives of [99mTc]Tc-Housed PSMA-i in Prostate Cancer

Recently, prostate-specific membrane antigen (PSMA) has gained momentum in tumor nuclear molecular imaging as an excellent target for both the diagnosis and therapy of prostate cancer. Since 2008, after years of preclinical research efforts, a plentitude of radiolabeled compounds mainly based on low molecular weight PSMA inhibitors (PSMA-i) have been described for imaging and theranostic applications, and some of them have been transferred to the clinic. Most of these compounds include radiometals (e.g., ⁶⁸Ga, ⁶⁴Cu, ¹⁷⁷Lu) for positron emission tomography (PET) imaging or endoradiotherapy. Nowadays, although the development of new PET tracers has caused a significant drop in single-photon emission tomography (SPECT) research programs and the development of new technetium-99m (^99mTc) tracers is rare, this radionuclide remains the best atom for SPECT imaging owing to its ideal physical decay properties, convenient availability, and rich and versatile coordination chemistry. Indeed, ^99mTc still plays a relevant role in diagnostic nuclear medicine, as the number of clinical examinations based on ^99mTc outscores that of PET agents and ^99mTc-PSMA SPECT/CT may be a cost-effective alternative for ⁶⁸Ga-PSMA PET/CT. This review aims to give an overview of the specific features of the developed [^99mTc]Tc-tagged PSMA agents with particular attention to [^99mTc]Tc-PSMA-i. The chemical and pharmacological properties of the latter will be compared and discussed, highlighting the pros and cons with respect to [⁶⁸Ga]Ga-PSMA11.

Metamorphosis of prostate specific membrane antigen (PSMA) inhibitors

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

Molecular Imaging in Primary Staging of Prostate Cancer Patients: Current Aspects and Future Trends

Accurate primary staging is the cornerstone in all malignancies. Different morphological imaging modalities are employed in the evaluation of prostate cancer (PCa). Regardless of all developments in imaging, invasive histopathologic evaluation is still the standard method for the detection and staging of the primary PCa. Magnetic resonance imaging (MRI) and computed tomography (CT) play crucial roles; however, functional imaging provides additional valuable information, and it is gaining ever-growing acceptance in the management of PCa. Targeted imaging with different radiotracers has remarkably evolved in the past two decades. [111In]In-capromab pendetide scintigraphy was a new approach in the management of PCa. Afterwards, positron emission tomography (PET) tracers such as [11C/18F]choline and [11C]acetate were developed. Nevertheless, none found a role in the primary staging. By introduction of the highly sensitive small molecule prostate-specific membrane antigen (PSMA) PET/CT, as well as recent developments in MRI and hybrid PET/MRI systems, non-invasive staging of PCa is being contemplated. Several studies investigated the role of these sophisticated modalities in the primary staging of PCa, showing promising results. Here, we recapitulate the role of targeted functional imaging. We briefly mention the most popular radiotracers, their diagnostic accuracy in the primary staging of PCa, and impact on patient management.

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

Simple Summary

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

Abstract

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

Gallium-68: methodology and novel radiotracers for positron emission tomography (2012–2017)

Commercial 68Ge/68Ga generators provide a means to produce positron emission tomography agents on site without use of a cyclotron. This development has led to a rapid growth of academic literature and patents ongallium-68 (68Ga). As 68Ga positron emission tomography agents usually involve a targeting moiety attached to a metal chelator, the development lends itself to the investigation of theragnostic applications; the 68Ga-based diagnostic is utilized to determine if the biological target is present and, if so, a therapeutic isotope (e.g., 177Lu, 225Ac) can be complexed with the same scaffold to generate a corresponding radiotherapeutic. This review considers patents issued between 2012 and 2017 that contain a 68Ga-labeled molecule indexed by Chemical Abstract Services (a division of the American Chemical Society).

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.

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.

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.

A method to determine the mode of binding for GCPII inhibitors using bio-layer interferometry

The rapid dilution of the enzyme-inhibitor complex assay to monitor the recovery of enzyme activity is a well-established assay to determine the reversibility of inhibition. Our laboratory has previously employed this method to ascertain the reversibility of known glutamate carboxypeptidase II (GCPII)-targeting agents. Due to the tedious and time-consuming nature of the assay, we sought to develop a facile method to determine the reversibility of well-characterized GCPII inhibitors using bio-layer interferometry (BLI). The results from the BLI assay are in agreement with the rapid dilution method. Herein, we report for the first time, a rapid, novel real-time BLI method to determine reversibility of inhibition.

Radiolabeled enzyme inhibitors and binding agents targeting PSMA: Effective theranostic tools for imaging and therapy of prostate cancer

Because of the broad incidence, morbidity and mortality associated with prostate-derived cancer, the development of more effective new technologies continues to be an important goal for the accurate detection and treatment of localized prostate cancer, lymphatic involvement and metastases. Prostate-specific membrane antigen (PSMA; Glycoprotein II) is expressed in high levels on prostate-derived cells and is an important target for visualization and treatment of prostate cancer. Radiolabeled peptide targeting technologies have rapidly evolved over the last decade and have focused on the successful development of radiolabeled small molecules that act as inhibitors to the binding of the N-acetyl-l-aspartyl-l-glutamate (NAAG) substrate to the PSMA molecule. A number of radiolabeled PSMA inhibitors have been described in the literature and labeled with SPECT, PET and therapeutic radionuclides. Clinical studies with these agents have demonstrated the improved potential of PSMA-targeted PET imaging agents to detect metastatic prostate cancer in comparison with conventional imaging technologies. Although many of these agents have been evaluated in humans, by far the most extensive clinical literature has described use of the ⁶⁸Ga and ¹⁷⁷Lu agents. This review describes the design and development of these agents, with a focus on the broad clinical introduction of PSMA targeting motifs labeled with ⁶⁸Ga for PET-CT imaging and ¹⁷⁷Lu for therapy. In particular, because of availability from the long-lived ⁶⁸Ge (T_1/2=270days)/⁶⁸Ga (T_1/2=68min) generator system and increasing availability of PET-CT, the ⁶⁸Ga-labeled PSMA targeted agent is receiving widespread interest and is one of the fastest growing radiopharmaceuticals for PET-CT imaging.

Preclinical Comparative Study of (68)Ga-Labeled DOTA, NOTA, and HBED-CC Chelated Radiotracers for Targeting PSMA

(68)Ga-labeled, low-molecular-weight imaging agents that target the prostate-specific membrane antigen (PSMA) are increasingly used clinically to detect prostate and other cancers with positron emission tomography (PET). The goal of this study was to compare the pharmacokinetics of three PSMA-targeted radiotracers: (68)Ga-1, using DOTA-monoamide as the chelating agent; (68)Ga-2, containing the macrocyclic chelating agent p-SCN-Bn-NOTA; and (68)Ga-DKFZ-PSMA-11, currently in clinical trials, which uses the acyclic chelating agent, HBED-CC. The PSMA-targeting scaffold for all three agents utilized a similar Glu-urea-Lys-linker construct. Each radiotracer enabled visualization of PSMA+ PC3 PIP tumor, kidney, and urinary bladder as early as 15 min post-injection using small animal PET/computed tomography (PET/CT). (68)Ga-2 demonstrated the fastest rate of clearance from all tissues in this series and displayed higher uptake in PSMA+ PC3 PIP tumor compared to (68)Ga-1 at 1 h post-injection. There was no significant difference in PSMA+ PC3 PIP tumor uptake for the three agents at 2 and 3 h post-injection. (68)Ga-DKFZ-PSMA-11 demonstrated the highest uptake and retention in normal tissues, including kidney, blood, spleen, and salivary glands and PSMA-negative PC3 flu tumors up to 3 h post-injection. In this preclinical evaluation (68)Ga-2 had the most advantageous characteristics for PSMA-targeted PET imaging.

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.

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

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

Prostate-specific membrane antigen as a target for cancer imaging and therapy

The prostate-specific membrane antigen (PSMA) is a molecular target whose use has resulted in some of the most productive work toward imaging and treating prostate cancer over the past two decades. A wide variety of imaging agents extending from intact antibodies to low-molecular-weight compounds permeate the literature. In parallel there is a rapidly expanding pool of antibody-drug conjugates, radiopharmaceutical therapeutics, small-molecule drug conjugates, theranostics and nanomedicines targeting PSMA. Such productivity is motivated by the abundant expression of PSMA on the surface of prostate cancer cells and within the neovasculature of other solid tumors, with limited expression in most normal tissues. Animating the field is a variety of small-molecule scaffolds upon which the radionuclides, drugs, MR-detectable species and nanoparticles can be placed with relative ease. Among those, the urea-based agents have been most extensively leveraged, with expanding clinical use for detection and more recently for radiopharmaceutical therapy of prostate cancer, with surprisingly little toxicity. PSMA imaging of other cancers is also appearing in the clinical literature, and may overtake FDG for certain indications. Targeting PSMA may provide a viable alternative or first-line approach to managing prostate and other cancers.

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.

Synthesis and evaluation of constrained phosphoramidate inhibitors of prostate-specific membrane antigen

Prostate-specific membrane antigen (PSMA) is a cell-surface enzyme-biomarker that is actively pursued for targeted delivery of imaging and therapeutic agents for prostate cancer. Our lab has developed PSMA inhibitors based on a phosphoramidate scaffold, which has shown both high selectivity for PSMA-positive tumors and rapid clearance in vivo when radiolabeled with (18)F. However, this scaffold exhibits hydrolytic instability under low pH and high temperature conditions, barring the use of other imaging or therapeutic radionuclides such as (68)Ga or (177)Lu. Previous studies in our lab have shown a trend in increasing acid stability as the distance between the phosphoramidate core and the α-carboxylate of the P1 residue is increased. Therefore, a new generation of phosphoramidate inhibitors was developed based on trans-4-hydroxyproline as the P1 residue to restrict the interaction of the α-carboxylate to the phosphoramidate core. These hydroxyproline inhibitors demonstrated comparable IC50 values to earlier generations as well as enhanced thermal and acid stability.

Multiplex detection of functional G protein-coupled receptors harboring site-specifically modified unnatural amino acids

We developed a strategy for identifying positions in G protein-coupled receptors that are amenable to bioorthogonal modification with a peptide epitope tag under cell culturing conditions. We introduced the unnatural amino acid p-azido-l-phenylalanine (azF) into human CC chemokine receptor 5 (CCR5) at site-specific amber codon mutations. We then used strain-promoted azide–alkyne [3+2] cycloaddition to label the azF-CCR5 variants with a FLAG peptide epitope-conjugated aza-dibenzocyclooctyne (DBCO) reagent. A microtiter plate-based sandwich fluorophore-linked immunosorbent assay was used to probe simultaneously the FLAG epitope and the receptor using infrared dye-conjugated antibodies so that the extent of DBCO incorporation, corresponding nominally to labeling efficiency, could be quantified ratiometrically. The extent of incorporation of DBCO at the various sites was evaluated in the context of a recent crystal structure of maraviroc-bound CCR5. We observed that labeling efficiency varied dramatically depending on the topological location of the azF in CCR5. Interestingly, position 109 in transmembrane helix 3, located in a hydrophobic cavity on the extracellular side of the receptor, was labeled most efficiently. Because the bioorthogonal labeling and detection strategy described might be used to introduce a variety of different peptide epitopes or fluorophores into engineered expressed receptors, it might prove to be useful for a wide range of applications, including single-molecule detection studies of receptor trafficking and signaling mechanism.

⁶⁴Cu-labeled inhibitors of prostate-specific membrane antigen for PET imaging of prostate cancer

Prostate-specific membrane antigen (PSMA) is a well-recognized target for identification and therapy of a variety of cancers. Here we report five (64)Cu-labeled inhibitors of PSMA, [(64)Cu]3-7, which are based on the lysine-glutamate urea scaffold and utilize a variety of macrocyclic chelators, namely NOTA(3), PCTA(4), Oxo-DO3A(5), CB-TE2A(6), and DOTA(7), in an effort to determine which provides the most suitable pharmacokinetics for in vivo PET imaging. [(64)Cu]3-7 were prepared in high radiochemical yield (60-90%) and purity (>95%). Positron emission tomography (PET) imaging studies of [(64)Cu]3-7 revealed specific accumulation in PSMA-expressing xenografts (PSMA+ PC3 PIP) relative to isogenic control tumor (PSMA- PC3 flu) and background tissue. The favorable kinetics and high image contrast provided by CB-TE2A chelated [(64)Cu]6 suggest it as the most promising among the candidates tested. That could be due to the higher stability of [(64)Cu]CB-TE2A as compared with [(64)Cu]NOTA, [(64)Cu]PCTA, [(64)Cu]Oxo-DO3A, and [(64)Cu]DOTA chelates in vivo.

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.

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

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

The growing impact of bioorthogonal click chemistry on the development of radiopharmaceuticals

Click chemistry has become a ubiquitous chemical tool with applications in nearly all areas of modern chemistry, including drug discovery, bioconjugation, and nanoscience. Radiochemistry is no exception, as the canonical Cu(I)-catalyzed azide-alkyne cycloaddition, strain-promoted azide-alkyne cycloaddition, inverse electron demand Diels-Alder reaction, and other types of bioorthogonal click ligations have had a significant impact on the synthesis and development of radiopharmaceuticals. This review will focus on recent applications of click chemistry ligations in the preparation of imaging agents for SPECT and PET, including small molecules, peptides, and proteins labeled with radionuclides such as (18)F, (64)Cu, (111)In, and (99m)Tc.

PET imaging in prostate cancer: focus on prostate-specific membrane antigen

Prostate cancer (PCa) is the second leading cause of cancer-related death in American men. Positron emission tomography/computed tomography (PET/CT) with emerging radiopharmaceuticals promises accurate staging of primary disease, restaging of recurrent disease, detection of metastatic lesions and, ultimately, for predicting the aggressiveness of disease. Prostate-specific membrane antigen (PSMA) is a well-characterized imaging biomarker of PCa. Because PSMA levels are directly related to androgen independence, metastasis and progression, PSMA could prove an important target for the development of new radiopharmaceuticals for PET. Preclinical data for new PSMA-based radiotracers are discussed and include new (89)Zr- and (64)Cu-labeled anti-PSMA antibodies and antibody fragments, (64)Cu-labeled aptamers, and (11)C-, (18)F-, (68)Ga-, (64)Cu-, and (86)Y-labeled low molecular weight inhibitors of PSMA. Several of these agents, namely (68)Ga- HBED-CC conjugate 15, (18)F-DCFBC 8, and BAY1075553 are particularly promising, each having detected sites of PCa in initial clinical studies. These early clinical results suggest that PET/CT using PSMA-targeted agents, especially with compounds of low molecular weight, will make valuable contributions to the management of PCa.

Targeting prostate cancer cells with PSMA inhibitor-guided gold nanoparticles

Prostate-specific membrane antigen (PSMA) is a notable biomarker for diagnostic and therapeutic applications in prostate cancer. Gold nanoparticles (AuNPs) provide an attractive nanomaterial platform for combining a variety of targeting, imaging, and cytotoxic agents into a unified device for biomedical research. In this study, we present the generation and evaluation of the first AuNP system functionalized with a small molecule phosphoramidate peptidomimetic inhibitor for the targeted delivery to PSMA-expressing prostate cancer cells. The general approach involved the conjugation of streptavidin-coated AuNPs with a biotin-linked PSMA inhibitor (CTT54) to generate PSMA-targeted AuNPs. In vitro evaluations of these targeted AuNPs were conducted to determine PSMA-mediated and time-dependent binding to PSMA-positive LNCaP cells. The PSMA-targeted AuNPs exhibited significantly higher and selective binding to LNCaP cells compared to control non-targeted AuNPs, thus demonstrating the feasibility of this approach.