Binding of the glutamate carboxypeptidase II (NAALADase) inhibitor 2-PMPA to rat brain membranes

Predicting the effect of different folate doses on [68Ga]Ga-PSMA-11 organ and tumor uptake using physiologically based pharmacokinetic modeling

BACKGROUND

Folate intake might reduce [⁶⁸Ga]Ga-PSMA-11 uptake in tissues due to a competitive binding to the PSMA receptor. For diagnostic imaging, this could impact decision making, while during radioligand therapy this could affect treatment efficacy. The relationship between folate dose, timing of dosing and tumor and organ uptake is not well established. The aim of this study was to develop a physiologically based pharmacokinetic (PBPK) model to predict the effect of folates on [⁶⁸Ga]Ga-PSMA-11 PET/CT uptake in salivary glands, kidneys and tumors.

METHODS

A PBPK model was developed for [⁶⁸Ga]Ga-PSMA-11 and folates (folic acid and its metabolite 5-MTHF), with compartments added that represent salivary glands and tumor. Reactions describing receptor binding, internalization and intracellular degradation were included. Model evaluation for [⁶⁸Ga]Ga-PSMA-11 was performed by using patient scan data from two different studies (static and dynamic), while for folates data from the literature were used for evaluation. Simulations were performed to assess the effect of different folate doses (150 µg, 400 µg, 5 mg and 10 mg) on accumulation in salivary glands, kidney and tumor, also for patients with different tumor volumes (10, 100, 500 and 1000 mL).

RESULTS

Final model evaluation showed that predictions adequately described data for both [⁶⁸Ga]Ga-PSMA-11 and folates. Predictions of a 5-MTFH dose of 150 µg and folic acid dose of 400 µg (in case of administration at the same time as [⁶⁸Ga]Ga-PSMA-11 (t = 0)) showed no clinically relevant effect on salivary glands and kidney uptake. However, the effect of a decrease in salivary glands and kidney uptake was determined to be clinically relevant for doses of 5 mg (34% decrease for salivary glands and 32% decrease for kidney) and 10 mg (36% decrease for salivary glands and 34% decrease for kidney). Predictions showed that tumor uptake was not relevantly affected by the co-administration of folate for all different folate doses (range 150 µg-10 mg). Lastly, different tumor volumes did not impact the folate effect on [⁶⁸Ga]Ga-PSMA-11 biodistribution.

CONCLUSION

Using a PBPK model approach, high doses of folate (5 and 10 mg) were predicted to show a decrease of [⁶⁸Ga]Ga-PSMA-11 salivary glands and kidney uptake, while intake by means of folate containing food or vitamin supplements showed no relevant effects. In addition, tumor uptake was not affected by folate administration in the simulated dose ranges (150 µg-10 mg). Differences in tumor volume are not expected to impact folate effects on [⁶⁸Ga]Ga-PSMA-11 organ uptake.

[99mTc]Tc-PSMA-T4-Novel SPECT Tracer for Metastatic PCa: From Bench to Clinic

Despite significant advances in nuclear medicine for diagnosing and treating prostate cancer (PCa), research into new ligands with increasingly better biological properties is still ongoing. Prostate-specific membrane antigen (PSMA) ligands show great potential as radioisotope carriers for the diagnosis and therapy of patients with metastatic PCa. PSMA is expressed in most types of prostate cancer, and its expression is increased in poorly differentiated, metastatic, and hormone-refractory cancers; therefore, it may be a valuable target for the development of radiopharmaceuticals and radioligands, such as urea PSMA inhibitors, for the precise diagnosis, staging, and treatment of prostate cancer. Four developed PSMA-HYNIC inhibitors for technetium-99m labeling and subsequent diagnosis were subjected to preclinical in vitro and in vivo studies to evaluate and compare their diagnostic properties. Among the studied compounds, the PSMA-T4 (Glu-CO-Lys-L-Trp-4-Amc-HYNIC) inhibitor showed the best biological properties for the diagnosis of PCa metastases. [99mTc]Tc-PSMA-T4 also showed effectiveness in single-photon emission computed tomography (SPECT) studies in humans, and soon, its usefulness will be extensively evaluated in phase 2/3 clinical trials.

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.

Synthesis and Biological Evaluation of Substrate-Based Imaging Agents for the Prostate-Specific Membrane Antigen

Prostate-specific membrane antigen (PSMA) is an attractive target for the imaging of prostate cancer (PCa) due to the elevated expression on the surface of prostate tumor cells. Most PSMA-targeted low molecular weight imaging agents are inhibitors of PSMA. We have synthesized a series of substrate-based PSMA-targeted imaging agents by mimicking poly-γ-glutamyl folic acid, an endogenous substrate of PSMA. In vitro the γ-linked polyglutamate conjugates proved to be better substrates than the corresponding α-linked glutamates. However, in vivo imaging studies of γ-ray-emitting and γ-linked glutamates did not demonstrate selective uptake in PSMA-pos-itive over PSMA-negative tumors. Subsequent chromatographic studies and in silico molecular dynamics simulations indicated that hydrolysis of the substrates is slow and access to the enzymatic active site is limited. These results inform the design of future substrate-based imaging agents for PSMA.

Structural Insight into the Pharmacophore Pocket of Human Glutamate Carboxypeptidase II

Inhibition of glutamate carboxypeptidase II (GCPII) has been shown to be neuroprotective in multiple preclinical models in which dysregulated glutamatergic transmission is implicated. Herein, we report crystal structures of the human GCPII complexed with three glutamate mimetics/derivatives, 2-(phosphonomethyl)pentanedioic acid (2-PMPA), quisqualic acid (QA), and L-serine O-sulfate (L-SOS), at 1.72, 1.62, and 2.10 A resolution, respectively. Despite the structural differences between the distal parts of the inhibitors, all three compounds share similar binding modes in the pharmacophore (i.e., S1') pocket of GCPII, where they are stabilized by a combination of polar and van der Waals interactions. The structural diversity of the distal parts of the inhibitors leads to rearrangements of the S1' site that are necessary for efficient interactions between the enzyme and an inhibitor. The set of structures presented here, in conjunction with the available biochemical data, illustrates a flexibility of the GCPII pharmacophore pocket and highlights the structural features required for potent GCPII inhibition. These findings could facilitate the rational structure-based drug design of new GCPII inhibitors in the future.

Endogenous N-acetylaspartylglutamate reduced NMDA receptor-dependent current neurotransmission in the CA1 area of the hippocampus

N-Acetylaspartylglutamate (NAAG) is a neuropeptide found in high concentrations in the brain. Using whole-cell recordings of CA1 pyramidal neurons in acute hippocampal slices, we found that either (i) the application of exogenous NAAG or (ii) an increase of endogenous extracellular NAAG, caused by the inhibition of its catabolic enzyme glutamate carboxypeptidase II (GCP II), resulted in a significant reduction in the amplitude of the isolated NMDA receptor (NMDAR) component of the evoked excitatory postsynaptic current (EPSC). Conversely, reduction of endogenous extracellular NAAG caused by either (i) perfusion with a soluble form of pure human GCP II or (ii) affinity purified antibodies against NAAG, enhanced the amplitude of the isolated NMDAR current. Bath application of GCP II inhibitor induced a progressive loss of spontaneous NMDAR miniatures. Furthermore, NAAG blocked the induction of long-term potentiation at Schaffer collateral axons-CA1 pyramidal neuron synapses. All together, these results suggest that NAAG acts as an endogenous modulator of NMDARs in the CA1 area of the hippocampus.

Glutamate carboxypeptidase II (NAALADase) inhibition as a novel therapeutic strategy

GCP II inhibition decreases extracellular excitotoxic glutamate and increases extracellular NAAG, both of which provide neuroprotection. We have demonstrated with our potent and selective GCP II inhibitors efficacy in models of stroke, ALS and neuropathic pain. GCP II inhibition may have significant potential benefits over existing glutamate-based neuroprotection strategies. The upstream mechanism seems selective for excitotoxic induced glutamate release, as GCP II inhibitors in normal animals induced no change in basal glutamate. This suggestion has recently been corroborated by Lieberman and coworkers24 who found that both NAAG release and increase in GCP II activity appear to be induced by electrical stimulation in crayfish nerve fibers and that subsequent NAAG hydrolysis to glutamate contributes, at least in part, to subsequent NMDA receptor activation. Interestingly, even at relatively high doses of compounds, GCP II inhibition did not appear to be associated with learning/memory deficits in animals. Additionally, quantitative neurophysiological testing data and visual analog scales for 'psychedelic effects' in Phase I single dose and repeat dose studies showed GCP II inhibition to be safe and well tolerated by both healthy volunteers and diabetic patients. GCP II inhibition may represent a novel glutamate regulating strategy devoid of the side effects that have hampered the development of postsynaptic glutamate receptor antagonists.

Glutamate carboxypeptidase II levels in rodent brain using [125I]DCIT quantitative autoradiography

The ability to visualize quantitatively glutamate carboxypeptidase II (GCPII) levels in vivo could advance our understanding of its function in health and disease. In the current study, we synthesized and evaluated a radiolabeled (iodine-125) analog of N-[N-[(S)-1,3-dicarboxypropyl]carbamoyl]-S-3-iodo-L-tyrosine (DCIT), a potent antagonist of GCPII activity. We examined the regional distribution of [125I]DCIT binding in the rodent brain using quantitative autoradiography in order to confirm the validity of this radioligand as a marker of GCPII in the brain. The ultimate goal is to develop an imaging agent for assessing GCPII levels in the living brain. The specific binding of [125I]DCIT to rat brain followed a regional distribution consistent with previous studies describing regional brain GCPII gene expression and activity. We found a modest rostrocaudal gradient in which specific binding of [125I]DCIT to GCPII was lowest in cortical regions, with increasing levels of binding in midbrain structures and high levels of binding in hindbrain and brainstem. Autoradiography of [125I]DCIT in GCPII knockout and wild type mouse brain showed a gene-dose dependency confirming the selectivity of this radioligand for GCPII. We propose that [125I]DCIT is a selective radioligand that can be used to quantify brain GCPII levels in vitro using quantitative autoradiography.

Effect of N-acetylaspartylglutamate (NAAG) on non-quantal and spontaneous quantal release of acetylcholine at the neuromuscular synapse of rat

N-Acetylaspartylglutamate (NAAG), known to be present in rat motor neurons, may participate in neuronal modulation of non-quantal secretion of acetylcholine (ACh) from motor nerve terminals. Non-quantal release of ACh was estimated by the amplitude of the endplate membrane hyperpolarization (H-effect) caused by inhibition of nicotinic receptors by (+)-tubocurarine and acetylcholinesterase by armin (diethoxy-p-nitrophenyl phosphate). Application of exogenous NAAG decreased the H-effect in a dose-dependent manner. The reduction of the H-effect by NAAG was completely removed when N-acetyl-beta-aspartylglutamate (betaNAAG) or 2-(phosphonomethyl)-pentanedioic acid (2-PMPA) was used to inhibit glutamate carboxypeptidase II (GCP II), a presynaptic Schwann cell membrane-associated ectoenzyme that hydrolyzes NAAG to glutamate and N-acetylaspartate. Bath application of glutamate decreased the H-effect similarly to the action of NAAG but N-acetylaspartate was without effect. Inhibition of NMDA receptors by dl-2-amino-5-phosphopentanoic acid, (+)-5-methyl-10,11-dihydro-5H-dibenzocyclohepten-5,10-imine (MK801), and 7-chlorokynurenic acid or inhibition of muscle nitric oxide synthase (NO synthase) by N(G)-nitro-l-arginine methyl ester and 3-bromo-7-nitroindazole completely prevented the decrease of the H-effect by NAAG. These results suggest that glutamate, produced by enzymatic hydrolysis of bath-applied NAAG, can modulate non-quantal secretion of ACh from the presynaptic terminal of the neuromuscular synapse via activation of postsynaptic NMDA receptors and synthesis of nitric oxide (NO) in muscle fibers. NAAG also increased the frequency of miniature endplate potentials (mEPPs) generated by spontaneous quantal secretion of ACh, whereas the mean amplitude and time constants for rise time and for decay of mEPPs did not change.

Is prostate-specific membrane antigen a multifunctional protein?

Prostate-specific membrane antigen (PSMA) is a metallopeptidase expressed predominantly in prostate cancer (PCa) cells. PSMA is considered a biomarker for PCa and is under intense investigation for use as an imaging and therapeutic target. Although the clinical utility of PSMA in the detection and treatment of PCa is evident and is being pursued, very little is known about its basic biological function in PCa cells. The purpose of this review is to highlight the possibility that PSMA might be a multifunctional protein. We suggest that PSMA may function as a receptor internalizing a putative ligand, an enzyme playing a role in nutrient uptake, and a peptidase involved in signal transduction in prostate epithelial cells. Insights into the possible functions of PSMA should improve the diagnostic and therapeutic values of this clinically important molecule.

Regulation of glutamate carboxypeptidase II hydrolysis of N ‐acetylaspartylglutamate (NAAG) in crayfish nervous tissue is mediated by glial glutamate and acetylcholine receptors

Glutamate carboxypeptidase II (GCPII), a glial ectoenzyme, is responsible for N-acetylaspartylglutamate (NAAG) hydrolysis. Its regulation in crayfish nervous tissue was investigated by examining uptake of [3H]glutamate derived from N-acetylaspartyl-[3H]glutamate ([3H]NAAG) to measure GCPII activity. Electrical stimulation (100 Hz, 10 min) during 30 min incubation with [3H]NAAG increased tissue [3H]glutamate tenfold. This was prevented by 2-(phosphonomethyl)-pentanedioic acid (2-PMPA), a GCPII inhibitor, suggesting that stimulation increased the hydrolysis of [3H]NAAG and metabolic recycling of [3H]glutamate. Antagonists of glial group II metabotropic glutamate receptors (mGLURII), NMDA receptors and acetylcholine (ACh) receptors that mediate axon-glia signaling in crayfish nerve fibers decreased the effect of stimulation by 58-83%, suggesting that glial receptor activation leads to stimulation of GCPII activity. In combination, they reduced [3H]NAAG hydrolysis during stimulation to unstimulated control levels. Agonist stimulation of mGLURII mimicked the effect of electrical stimulation, and was prevented by antagonists of GCPII or mGLURII. Raising extracellular K+ to three times the normal level stimulated [3H]NAAG release and GCPII activity. These effects were also blocked by antagonists of GCPII and mGLUR(II). No receptor antagonist or agonist tested or 2-PMPA affected uptake of [3H]glutamate. We conclude that NAAG released from stimulated nerve fibers activates its own hydrolysis via stimulation of GCPII activity mediated through glial mGLURII, NMDA and ACh receptors.

Kinetics and inhibition of glutamate carboxypeptidase II using a microplate assay

Glutamate carboxypeptidase II (GCPII or prostate-specific membrane antigen or NAALADase) is an enzyme that catalyzes the hydrolysis of the neuropeptide N-acetylaspartylglutamate (NAAG) to N-acetylaspartate (NAA) and glutamate (G). Inhibitors of GCPII provide neuroprotection in a variety of animal models of central nervous system disorders. Neuroprotection is probably the result of increased NAAG concentrations and decreased levels of excess toxic glutamate. Consequently, GCPII inhibitors could be useful therapeutic agents where increased glutamate levels are the result of increased GCPII activity. Current GCPII in vitro activity assays are cumbersome or have limited sensitivity. In this report we describe a microplate assay to study GCPII inhibition that is most sensitive, efficient, and generates little waste. GCPII turnover number (k(cat)) was 4s(-1) and the binding constant (K(m)) for NAAG and GCPII was 130nM. The apparent association rate constant for GCPII and NAAG (k(cat)/K(m)) was 3 x 10(7)M(-1)s(-1). Inhibition studies with the GCPII inhibitor 2-(phosphonomethyl)pentanedioic acid (2-PMPA) demonstrated competitive inhibition with a K(i)=0.2nM.

11C-MCG: Synthesis, Uptake Selectivity, and Primate PET of a Probe for Glutamate Carboxypeptidase II (NAALADase)

Imaging of glutamate carboxypeptidase II (GCP II), also known as N-acetylated α-linked l-amino dipeptidase (NAALADase), may enable study of glutamatergic transmission, prostate cancer, and tumor neovasculature in vivo. Our goal was to develop a probe for GCP II for use with positron emission tomography (PET). Radiosynthesis of 11C–MeCys–C(O)–Glu or 11C-( S)-2-[3-(( R)-1-carboxy-2-methylsulfanyl-ethyl)-ureido]-pentanedioic acid (11C-MCG), an asymmetric urea and potent ( Ki = 1.9 nM) inhibitor of GCP II, was performed by C-11 methylation of the free thiol. Biodistribution of 11C-MCG was assayed in mice, and quantitative PET was performed in a baboon. 11C-MCG was obtained in 16% radiochemical yield at the end of synthesis with specific radioactivities over 167 GBq/mmol (4000 Ci/mmol) within 30 min after the end of bombardment. At 30 min postinjection, 11C-MCG showed 33.0 ± 5.1%, 0.4 ± 0.1%, and 1.1 ± 0.2% ID/g in mouse kidney (target tissue), muscle, and blood, respectively. Little radioactivity gained access to the brain. Blockade with unlabeled MCG or 2-(phosphonomethyl)pentanedioic acid (PMPA), another potent inhibitor of GCP II, provided sevenfold and threefold reductions, respectively, in binding to target tissue. For PET, distribution volumes (DVs) were 1.38 then 0.87 pre- and postblocker (PMPA). Little metabolism of 11C-MCG occurred in the mouse or baboon. These results suggest that 11C-MCG may be useful for imaging GCP II in the periphery.