Small-molecule PSMA ligands. Current state,SAR and perspectives

Prostate cancer (PC) is the prevalent malignancy widespread among men in the Western World. Prostate specific membrane antigen (PSMA) is an established PC marker and has been considered as a promising biological target for anti-PC drug delivery and diagnostics. The protein was found to be overexpressed in PC cells, including metastatic, and the neovasculature of solid tumors. These properties make PSMA-based approach quite appropriate for effective PC imaging and specific drug therapy. Through the past decade, a variety of PSMA-targeted agents has been systematically evaluated. Small-molecule compounds have several advantages over other classes, such as improved pharmacokinetics and rapid blood clearance. These low-weight ligands have similar structure and can be divided into three basic categories in accordance with the type of their zinc-binding core-head. Several PSMA binders are currently undergoing clinical trials generally for PC imaging. The main goal of the present review is to describe the recent progress achieved within the title field and structure activity relationships (SAR) disclosed for different PSMA ligands. Recent in vitro and in vivo studies for each type of the compounds described have also been briefly summarized.     

Brain imaging techniques for diagnosis and drugs against schizophrenia

Positron emission tomography studies in schizophrenia have used two main approaches; metabolic imaging with deoxyglucose or glucose and dopamine receptor imaging with ligands for the dopamine receptor. The metabolic studies have generally shown relatively reduced metabolic rates in the frontal cortex, consistent with most studies of cerebral blood flow. Relatively low metabolic rates in the basal ganglia have also been observed in some studies. With labeled neuroleptics, brain images with the greatest uptake in the basal gangila have been obtained, and clinical studies comparing normals and schizophrenics are still in progress. For both approaches, the problem of biological heterogeneity in schizophrenia must be fully considered so that PET can reach its full potential as a tool to develop new drugs, to select a particular drug for the individual patients, and ultimately to understand the pathophysiology of schizophrenia.     

PSMA specific antibodies and their diagnostic and therapeutic use

Prostate-specific membrane antigen (PSMA) is a membrane-bound glycoprotein highly restricted to prostatic epithelial cells. PSMA expression is increased in association with prostatic cancer, particularly in hormone refractory disease. Given its membrane-bound character, PSMA is an ideal sentinel molecule for use in targeting prostatic cancer cells. Monoclonal antibodies specific for PSMA are available, beginning with the antibody 7E11.C5 which originally defined PSMA and which has been developed for use in cancer detection via immunoscintiscanning in the ProstaScint? test. Newer second generation antibodies specific for both linear amino acid sequence epitopes and protein conformational epitopes on the extracellular domain of PSMA have been reported. Although most of these are murine antibodies, both humanised and fully human examples have been developed. These antibodies are beginning to work their way into clinical applications for potential improved diagnostic and therapeutic uses. Results to date suggest that antibodies specific for extracellular epitopes are significantly better for clinical uses in vivo than the 7E11.C5 antibody that is specific for an intracellular epitope. Current knowledge relating to PSMA-specific antibodies and their clinical uses and potential is described and evaluated.     

PSMA specific antibodies and their diagnostic and therapeutic use

Prostate-specific membrane antigen (PSMA) is a membrane-bound glycoprotein highly restricted to prostatic epithelial cells. PSMA expression is increased in association with prostatic cancer, particularly in hormone refractory disease. Given its membrane-bound character, PSMA is an ideal sentinel molecule for use in targeting prostatic cancer cells. Monoclonal antibodies specific for PSMA are available, beginning with the antibody 7E11.C5 which originally defined PSMA and which has been developed for use in cancer detection via immunoscintiscanning in the ProstaScint test. Newer second generation antibodies specific for both linear amino acid sequence epitopes and protein conformational epitopes on the extracellular domain of PSMA have been reported. Although most of these are murine antibodies, both humanised and fully human examples have been developed. These antibodies are beginning to work their way into clinical applications for potential improved diagnostic and therapeutic uses. Results to date suggest that antibodies specific for extracellular epitopes are significantly better for clinical uses in vivo than the 7E11.C5 antibody that is specific for an intracellular epitope. Current knowledge relating to PSMA-specific antibodies and their clinical uses and potential is described and evaluated.     

Bispecific radioligands targeting prostate‐specific membrane antigen and gastrin‐releasing peptide receptors on the surface of prostate cancer cells

Metastases of prostate cancer usually show highly heterogeneous or partly lost prostate‐specific membrane antigen (PSMA) expression. In order to image and treat both PSMA positive and negative tissues, the extension of PSMA tracer specificity to other receptors, also highly expressed on the surface of prostate cancer cells, has been suggested. Prostate cancer cells usually express both PSMA and gastrin‐releasing peptide (GRP) receptors; thus, bispecific heterodimeric molecules, addressing both targets at the same time, may significantly improve prostate cancer imaging and therapy. This article summarizes preclinical data regarding low‐molecular weight molecules targeting PSMA and GRP receptors.     

Preclinical assessment of 68Ga‐PSMA‐617 entrapped in a microemulsion delivery system for applications in prostate cancer PET/CT imaging

It has in recent years been reported that microemulsion (ME) delivery systems provide an opportunity to improve the efficacy of a therapeutic agent whilst minimising side effects and also offer the advantage of favourable treatment regimens. The prostate‐specific membrane antigen (PSMA) targeting agents PSMA‐11 and PSMA‐617, which accumulate in prostate tumours, allow for [⁶⁸Ga]Ga³⁺‐radiolabelling and positron emission tomography/computed tomography (PET) imaging of PSMA expression in vivo. We herein report the formulation of [⁶⁸Ga]Ga‐PSMA‐617 into a ME ≤40 nm including its evaluation for improved cellular toxicity and in vivo biodistribution. The [⁶⁸Ga]Ga‐PSMA‐617‐ME was tested in vitro for its cytotoxicity to HEK293 and PC3 cells. [⁶⁸Ga]Ga‐PSMA‐617‐ME was administered intravenously in BALB/c mice followed by microPET/computed tomography (CT) imaging and ex vivo biodistribution determination. [⁶⁸Ga]Ga‐PSMA‐617‐ME indicated negligible cellular toxicity at different concentrations. A statistically higher tolerance towards the [⁶⁸Ga]Ga‐PSMA‐617‐ME occurred at 0.125 mg/mL by HEK293 cells compared with PC3 cells. The biodistribution in wild‐type BALB/C mice showed the highest amounts of radioactivity (%ID/g) presented in the kidneys (31%) followed by the small intestine (10%) and stomach (9%); the lowest uptake was seen in the brain (0.5%). The incorporation of [⁶⁸Ga]Ga‐PSMA‐617 into ME was successfully demonstrated and resulted in a stable nontoxic formulation as evaluated by in vitro and in vivo means.     

Synthesis and automated fluorine-18 radiolabeling of new PSMA-617 derivatives with a CuAAC radiosynthetic approach

In the last decade, the development of new radiopharmaceuticals for the imaging and therapy of prostate cancer has been a highly active and important area of research, especially focusing on the prostate-specific membrane antigen (PSMA), an antigen which is upregulated in prostate, as well as in other tumor cells. A large variety of PSMA ligands have been radiolabeled, to date. Among the various derivatives, PSMA-617 resulted to be one of the most interesting in terms of interaction with the antigen and clinical properties, and its lutetium-177 labeled version has recently been approved by regulatory agencies for therapeutic purposes. For this reasons, the radiolabeling with fluorine-18 of a PSMA-617 derivative might be of interest. Beside other methodologies to radiolabel macromolecules with fluorine-18, the “click-chemistry” approach resulted to be very useful, and the copper-catalyzed azide-alkyne cycloaddition (CuAAC) is considered one of most efficient and reliable. This paper proposes the synthesis of a suitable precursor for the radiolabeling with fluorine-18 of a new PSMA-617 derivative. The whole radiosynthetic procedure has been fully automated, and the final product, which proved to be stable in plasma, has been obtained with radiochemical yield and purity suitable for subsequent preclinical studies.     

正电子显像技术在肿瘤免疫治疗个体化用药中的研究进展

肿瘤免疫治疗是近年来肿瘤治疗领域的重大突破,在多种恶性肿瘤中展示出显著的抗肿瘤效果.然而,临床使用提示不同患者对肿瘤免疫治疗的反应存在明显的个体差异,如何进行个体化治疗成为这类治疗的研究热点.正电子显像技术(PET)以其空间分辨率高、灵敏度高、不受内源性物质影响、定位定量准确等特点,有望在肿瘤免疫治疗个体化用药中发挥重...     

Development of radiotracers for oncology--the interface with pharmacology

There is an increasing role for positron emission tomography (PET) in oncology, particularly as a component of early phase clinical trials. As a non-invasive functional imaging modality, PET can be used to assess both pharmacokinetics and pharmacodynamics of novel therapeutics by utilizing radiolabelled compounds. These studies can provide crucial information early in the drug development process that may influence the further development of novel therapeutics. PET imaging probes can also be used as early biomarkers of clinical response and to predict clinical outcome prior to the administration of therapeutic agents. We discuss the role of PET imaging particularly as applied to phase 0 studies and discuss the regulations involved in the development and synthesis of novel radioligands. The review also discusses currently available tracers and their role in the assessment of pharmacokinetics and pharmacodynamics as applied to oncology.     

前列腺特异膜抗原及其变异体DNA检测在临床应用初探

目的初步探讨前列腺特异性膜抗原（PSMA）及其变异体（PSMA5）DNA检测在前列腺癌诊断中的临床意义。方法用自建的荧光定量逆转录聚合酶链反应（FQ-RT-PCR）检测方法检测前列腺增生及前列腺癌患者及正常对照组外周血单个核细胞和血浆中PSMA和PSMA5DNA的含量。结果在前列腺癌及前列腺增生患者外周血单个核细胞中PSMADNA含量及水平明显高于正常对照组。而PSMA5DNA含量在前列腺癌患者的外周血单个核细胞中明显高于PSMADNA。结论 PSMA及PSMA5DNA在外周血单个核细胞中的差异提示了作为前列腺癌的肿瘤标志物,PSMA5更具有肿瘤特异性,对其DNA的检测为前列腺肿瘤标志物的研究指明了一条新的道路。     

Synthesis and evaluation of technetium-99m- and rhenium-labeled inhibitors of the prostate-specific membrane antigen (PSMA)

The prostate-specific membrane antigen (PSMA) is increasingly recognized as a viable target for imaging and therapy of cancer. We prepared seven (99m)Tc/Re-labeled compounds by attaching known Tc/Re chelating agents to an amino-functionalized PSMA inhibitor (lys-NHCONH-glu) with or without a variable length linker moiety. K i values ranged from 0.17 to 199 nM. Ex vivo biodistribution and in vivo imaging demonstrated the degree of specific binding to engineered PSMA+ PC3 PIP tumors. PC3-PIP cells are derived from PC3 that have been transduced with the gene for PSMA. Despite demonstrating nearly the lowest PSMA inhibitory potency of this series, [(99m)Tc(CO)3( L1)] (+) ( L1 = (2-pyridylmethyl)2N(CH2) 4CH(CO2H)NHCO-(CH2) 6CO-NH-lys-NHCONH-glu) showed the highest, most selective PIP tumor uptake, at 7.9 +/- 4.0% injected dose per gram of tissue at 30 min postinjection. Radioactivity cleared from nontarget tissues to produce a PIP to flu (PSMA-PC3) ratio of 44:1 at 120 min postinjection. PSMA can accommodate the steric requirements of (99m)Tc/Re complexes within PSMA inhibitors, the best results achieved with a linker moiety between the epsilon amine of the urea lysine and the chelator.     

GCPII imaging and cancer

Glutamate carboxypeptidase II (GCPII) in the central nervous system is referred to as the prostate-specific membrane antigen (PSMA) in the periphery. PSMA serves as a target for imaging and treatment of prostate cancer and because of its expression in solid tumor neovasculature has the potential to be used in this regard for other malignancies as well. An overview of GCPII/PSMA in cancer, as well as a discussion of imaging and therapy of prostate cancer using a wide variety of PSMA-targeting agents is provided.     

以前列腺特异性膜抗原为靶点的主动靶向载体研究进展

前列腺特异性膜抗原(prostate-specific membrane antigen,PSMA)是一种特异性表达于前列腺恶性实体肿瘤新生毛细血管内皮细胞上的糖蛋白。PSMA是一种较前列腺特异性抗原、前列腺干细胞抗原等更加特异、敏感的肿瘤标志物,在前列腺癌的诊断和靶向治疗中显示出良好的应用前景。但限于相关临床研究的缺乏,要在临床上推广应用PSMA靶向疗法,还需要进行大量的研究以验证其临床安全性、稳定性和有效性等。本文综述了以核苷酸适配体和单克隆抗体作为靶向头基的药物载体治疗前列腺癌的研究进展,并讨论了该领域未来可能的发展方向。     

Fine‐tuning of the automated [18F]PSMA‐1007 radiosynthesis

Radiolabeled prostate‐specific membrane antigen (PSMA) targeting PET‐tracers have become desirable radiopharmaceuticals for the imaging of prostate cancer (PC). Recently, the PET radiotracer [¹⁸F]PSMA‐1007 was introduced as an alternative to [⁶⁸Ga]Ga‐PSMA‐11, for staging and diagnosing biochemically recurrent PC. We incorporated a one‐step procedure for [¹⁸F]PSMA‐1007 radiosynthesis, using both Synthra RNplus and GE TRACERlab FxFN automated modules, in accordance with the recently described radiolabeling procedure. Although the adapted [¹⁸F]PSMA‐1007 synthesis resulted in repeatable radiochemical yields (55 ± 5%, NDC), suboptimal radiochemical purities of 87 ± 8% were obtained using both modules. As described here, modifications made to the radiolabeling and the solid‐phase extraction purification steps reduced synthesis time to 32 minutes and improved radiochemical purity to 96.10%, using both modules, without shearing the radiochemical yield.     

Folate-mediated targeting: from diagnostics to drug and gene delivery

The covalent attachment of the vitamin folic acid to almost any molecule yields a conjugate that can be endocytosed into folate receptor-bearing cells. Because folate receptors are significantly overexpressed in the majority of human cancers, this methodology is currently being investigated for the selective delivery of imaging and therapeutic agents to tumor tissue. Phase I and II clinical studies for the first folate-containing imaging agent were initiated in 1999, and clinical trials of folate-targeted therapeutic agents should soon follow. This review will summarize folate-mediated drug delivery and highlight those techniques undergoing active preclinical or clinical investigation.     

Development of polymeric microbubbles targeted to prostate-specific membrane antigen as prototype of novel ultrasound contrast agents

Ultrasound-targeted microbubbles (MBs) offer new opportunities to enhance the capabilities of diagnostic ultrasound (US) imaging to specific pathological tissue. Herein, we report on the design and development of a novel prototype of US contrast agent based on polymeric MBs targeted to prostate-specific membrane antigen (PSMA) for use in the diagnosis of prostate cancer (PCa). First, a set of air-filled MBs by a variety of biocompatible polymers were prepared and characterized in terms of morphology and echogenic properties after exposure to US. MBs derived from poly(D,L-lactic-co-glycolic acid) (PLGA)-poly(ethylene glycol) (PEG) copolymer resulted as the most effective in terms of reflectivity. Such polymer was therefore preconjugated with a urea-based PSMA inhibitor molecular probe (DCL), and the obtained MBs were investigated in vitro for their targeting efficacy toward PSMA positive PCa (LNCaP) cells. Fluorescence microscopy proved a specific and efficient adhesion of targeted MBs to LNCaP cells. To our knowledge, this work reports the first model of polymeric MBs appropriately engineered to target PSMA, which might be further optimized and used for PCa diagnosis and potential carriers for selective drug delivery.     

Dual‐time‐point 64Cu‐PSMA‐617‐PET/CT in patients suffering from prostate cancer

Regardless of its high positron energy, ⁶⁸Ga‐labeled PSMA ligands have become standard of care in metabolic prostate cancer imaging. ⁶⁴Cu, a radionuclide with a much longer half‐life (12.7 h), is available for PSMA labeling allowing imaging much later than ⁶⁸Ga. In this study, the diagnostic performance of ⁶⁴Cu‐labeled PSMA was compared between early and late scans. Sixteen men (median age: 70 y) with prostate cancer in different stages underwent ⁶⁴Cu‐PSMA‐617‐PET/CT 2 and 22 hours post tracer injection. Pathologic and physiologic uptakes were analyzed for both points of time. Pathologic tracer accumulations occurred in 12 patients. Five patients presented with pathologic uptake in 17 different lymph nodes, two patients showed pathologic bone uptake in nine lesions, and seven patients had pathologic PSMA uptake in eight prostatic lesions. Physiologic uptake of the renal parenchyma, urine bladder, and salivary glands decreased over time, while the physiologic uptake of liver and bowel increased. In the present study, ⁶⁴Cu‐PSMA‐617‐PET demonstrated to be feasible for imaging prostate cancer for both the primary tumor site and metastases. Later imaging showed no additional, clinically relevant benefit compared with the early scans. At least the investigated time points we chose did not vindicate the additional expenditure.     

Synthesis and preliminary evaluation of 2‐arylhydroxyquinoline derivatives for tau imaging

Alzheimer's disease (AD) is the most common cause of dementia. Senile plaques, consisting of β‐amyloid, and neurofibrillary tangles (NFTs), composed of tau protein, are representative pathological hallmarks of AD. It is believed that the accumulation of NFTs precedes the onset of clinical symptoms of AD and correlates with the progression of memory dysfunction. Thus, the use of noninvasive detection techniques including radiolabeled probes and positron emission tomography (PET) will facilitate early diagnosis or staging of AD. In this study, we synthesized and evaluated novel hydroxylated 2‐arylquinoline derivatives as tau imaging PET probes. The binding affinities of compounds for tau were evaluated by fluorescent staining of the AD hippocampal section and a competitive binding assay using [¹⁸F]THK‐523. THK‐951 showed high binding affinity for tau pathology in an AD brain section and K18Δ280K fibrils (K_i = 20.7 nM); thus, we radiosynthesized a ¹¹C‐labeled THK‐951 and further studied its potential as a tau PET probe. The [¹¹C]THK‐951 demonstrated excellent kinetics in a normal mouse brain (3.23% ID/g at 2 min postinjection and 0.15% ID/g at 30 min postinjection) and showed the labeling of NFTs in an AD brain section by autoradiography assay. These findings indicate the availability of [¹¹C]THK‐951 for in vivo PET imaging of tau pathology in AD. Copyright © 2013 John Wiley & Sons, Ltd.     

Targeting novel antigens for prostate cancer treatment: focus on prostate-specific membrane antigen

Prostate-specific membrane antigen (PSMA) is a relatively omnipresent, but unique Type II dimeric transmembrane protein with a multiplicity of functions and has been shown to be a reasonable target for immunological approaches such as vaccines or more directed therapy with radioactively labelled monoclonal antibodies against PSMA. Given the abundance of various glycoprotein and carbohydrate antigens expressed on the surface of prostate cancer cells and cell lines, PSMA stands out as another 'self' antigen which is not only expressed on cancer cells, but on neovasculature. Although vaccines are varied in their design and target goal, recent technology has afforded researchers the opportunity to induce recruitment of multiple effector cell populations, cytokines and factors which can elicit both cellular and humoral responses. This review serves to present unique approaches in vaccine development which can induce immunological responsiveness with potential impact on disease progression and to introduce PSMA as a potential target for multimodality therapies.