Kenji Arimitsu

Synthesis and Biological Evaluation of Novel 18F-Labeled Probes Targeting Prostate-Specific Membrane Antigen for Positron Emission Tomography of Prostate Cancer.

Prostate-specific membrane antigen (PSMA) is a membrane protein highly expressed on prostate cancer cells and a potential imaging target for diagnosis. 18F-DCFPyL has been recently developed as an effective probe with high diagnostic accuracy for prostate cancer imaging. However, its radiochemical yield is low. We developed new PSMA probes using succinimidyl 4-18F-fluorobenzoate (18F-SFB), a rapid and effective 18F-labeling agent, taking advantage of the high radiochemical yield of this compound. We evaluated the probes as PET probes for PSMA imaging. Methods: Four 18F-labeled probes, 18F-8a, 18F-8b, 18F-10a, and 18F-10b, were synthesized using 18F-SFB, and their affinity for PSMA and partition coefficients (log D) were evaluated in vitro. Biodistribution studies were performed in human prostate cancer xenograft–bearing mice. PET images were obtained using 2 compounds, 18F-8a and 18F-10a, and a toxicologic study of 18F-10a was performed. Results: Four 18F-labeled asymmetric urea compounds, conjugated with 18F-SFB, were synthesized at a radiochemical yield of 30%–50% (decay-corrected), with a radiochemical purity greater than 95%. The radiochemical yield was 10–15 times higher than that of 18F-DCFPyL, the probe currently used in clinical studies. All 4 compounds showed high affinity for PSMA. 18F-8a and 18F-10a had a particularly high binding affinity (Ki values of 3.35 and 2.23 nM, respectively). In the biodistribution study, the accumulation of 18F-8a (13.3 ± 2.2 percentage injected dose per gram [%ID/g]) and 18F-10a (14.0 ± 3.1 %ID/g) in PSMA-positive human prostate (LNCaP) tumors was higher than that of the other 2 compounds and similar to that of 18F-DCFPyL (16.0 ± 2.9 %ID/g). 18F-10a showed the lowest hepatic and intestinal accumulation among the 4 compounds and slightly slower blood clearance than others. In the PET imaging studies, 18F-8a and 18F-10a were clearly visualized in LNCaP in xenograft-bearing mice. 18F-10a showed higher LNCaP-to-liver ratios than 18F-8a. We confirmed the safety profiles of 18F-10a; the no-observed-adverse-effects level was larger than 13.2 μg/kg. Conclusion: A novel 18F-labeled asymmetric urea compound, 18F-10a, had a high radiochemical yield, high binding affinity for PSMA, and pharmacokinetic profiles suitable for a PSMA imaging probe. We believe that 18F-10a can be effectively and safely used in this type of imaging.

Synthesis and evaluation of 18F-labeled mitiglinide derivatives as positron emission tomography tracers for β-cell imaging.

Measuring changes in β-cell mass in vivo during progression of diabetes mellitus is important for understanding the pathogenesis, facilitating early diagnosis, and developing novel therapeutics for this disease. However, a non-invasive method has not been developed. A novel series of mitiglinide derivatives (o-FMIT, m-FMIT and p-FMIT; FMITs) were synthesized and their binding affinity for the sulfonylurea receptor 1 (SUR1) of pancreatic islets were evaluated by inhibition studies. (+)-(S)-o-FMIT had the highest affinity of our synthesized FMITs (IC50=1.8μM). (+)-(S)-o-[(18)F]FMIT was obtained with radiochemical yield of 18% by radiofluorination of racemic precursor 7, hydrolysis, and optical resolution with chiral HPLC; its radiochemical purity was >99%. In biodistribution experiments using normal mice, (+)-(S)-o-[(18)F]FMIT showed 1.94±0.42% ID/g of pancreatic uptake at 5min p.i., and decreases in radioactivity in the liver (located close to the pancreas) was relatively rapid. Ex vivo autoradiography experiments using pancreatic sections confirmed accumulation of (+)-(S)-o-[(18)F]FMIT in pancreatic β-cells. These results suggest that (+)-(S)-o-[(18)F]FMIT meets the basic requirements for an radiotracer, and could be a candidate positron emission tomography tracer for in vivo imaging of pancreatic β-cells.

Development of 99mTc-labeled asymmetric urea derivatives that target prostate-specific membrane antigen for single-photon emission computed tomography imaging

Prostate-specific membrane antigen (PSMA) is expressed strongly in prostate cancers and is, therefore, an attractive diagnostic and radioimmunotherapeutic target. In contrast to previous reports of PMSA-targeting (99m)Tc-tricarbonyl complexes that are cationic or lack a charge, no anionic (99m)Tc-tricarbonyl complexes have been reported. Notably, the hydrophilicity conferred by both cationic and anionic charges leads to rapid hepatobiliary clearance, whereas an anionic charge might better enhance renal clearance relative to a cationic charge. Therefore, an improvement in rapid clearance would be expected with either cationic or anionic charges, particularly anionic charges. In this study, we designed and synthesized a novel anionic (99m)Tc-tricarbonyl complex ([(99m)Tc]TMCE) and evaluated its use as a single-photon emission computed tomography (SPECT) imaging probe for PSMA detection. Direct synthesis of [(99m)Tc]TMCE from dimethyl iminodiacetate, which contains both the asymmetric urea and succinimidyl moiety important for PSMA binding, was performed using our microwave-assisted one-pot procedure. The chelate formation was successfully achieved even though the precursor included a complicated bioactive moiety. The radiochemical yield of [(99m)Tc]TMCE was 12-17%, with a radiochemical purity greater than 98% after HPLC purification. [(99m)Tc]TMCE showed high affinity in vitro, with high accumulation in LNCaP tumors and low hepatic retention in biodistribution and SPECT/CT studies. These findings warrant further evaluation of [(99m)Tc]TMCE as an imaging agent and support the benefit of this strategy for the design of other PSMA imaging probes.

Synthesis and evaluation of (−)- and (+)-[11C]galanthamine as PET tracers for cerebral acetylcholinesterase imaging

Improved radiopharmaceuticals for imaging cerebral acetylcholinesterase (AChE) are needed for the diagnosis of Alzheimers disease (AD). Thus, (11)C-labeled (-)-galanthamine and its enantiomers were synthesized as novel agents for imaging the localization and activity of AChE by positron emission tomography (PET). C-11 was incorporated into (-)- and (+)-[(11)C]galanthamine by N-methylation of norgalanthamines with [(11)C]methyl triflate. Simple accumulation of (11)C in the brain was measured in an in vivo biodistribution study using mice, whilst donepezil was used as a blocking agent in analogous in vivo blocking studies. In vitro autoradiography of rat brain tissue was performed to investigate the distribution of (-)-[(11)C]galanthamine, and confirmed the results of PET studies in mice. The radiochemical yields of N-methylation of (-)- and (+)-norgalanthamines were 13.7% and 14.4%, respectively. The highest level of accumulation of (11)C in the brains of mice was observed at 10 min after administration (2.1% ID/g). Intravenous pretreatment with donepezil resulted in a 30% decrease in accumulation of (-)-[(11)C]galanthamine in the striatum; however, levels in the cerebellum were unchanged. In contrast, use of (+)-[(11)C]galanthamine led to accumulation of radioactivity in the striatum equal to that in the cerebellum, and these levels were unaffected by pretreatment with donepezil. In in vitro autoradiography of regional radioactive signals of brain sections showed that pretreatment with either (-)-galanthamine or donepezil blocked the binding of (-)-[(11)C]galanthamine to the striatum, while sagittal PET imaging revealed accumulation of (-)-[(11)C]galanthamine in the brain. These results indicate that (-)-[(11)C]galanthamine showed specific binding to AChE, whereas (+)-[(11)C]-galanthamine accumulated in brain tissue by non-specific binding. Thus, optically pure (-)-[(11)C]galanthamine could be a useful PET tracer for imaging cerebral AChE.

Synthesis and evaluation of a radioiodinated trisaccharide derivative as a synthetic substrate for a sensitive N-acetylglucosaminyltransferase V radioassay.

N-acetylglucosaminyltransferase V (GnT-V) is one of the most relevant glycosyltransferases to tumor invasion and metastasis. Based on previous findings of molecular recognition between GnT-V and synthetic substrates, we designed and synthesized a p-iodophenyl-derivatized trisaccharide, 2-(4-iodophenyl)ethyl 6-O-[2-O-(2-acetamido-2-deoxy-β-D-glucopyranosyl)-α-d-mannopyranosyl]-β-D-glucopyranoside (IPGMG, 1) and its radiolabeled form, [(125)I]IPGMG ([(125)I]1), for use in assays of GnT-V activity in vitro. The tributyltin derivative, 2-[4-(n-tributylstannyl)phenyl]ethyl 6-O-[2-O-(3,4,6-tri-O-acetyl-2-acetamido-2-deoxy-β-D-glucopyranosyl)-3,4,6-tri-O-acetyl-α-D-mannopyranosyl]-2,3,4-tri-O-acetyl-β-D-glucopyranoside (21), was synthesized as a precursor for the preparation of [(125)I]1. The iododestannylation of 21 using hydrogen peroxide as an oxidant followed by deacetylation yielded [(125)I]1. When [(125)I]1 was incubated in GnT-V-expressing cells with a UDP-GlcNAc donor, the production of β1-6GlcNAc-bearing IPGMG (IPGGMG, 2) was confirmed by radio-HPLC. In kinetic analysis, 1 was found to be a good substrate with a K(m) of 23.7 μM and a V(max) of 159 pmol/h. μg protein. [(125)I]1 would therefore be a useful synthetic substrate for the quantitative determination of GnT-V activity.

Development of 111In-labeled exendin(9-39) derivatives for single-photon emission computed tomography imaging of insulinoma

Insulinoma is a tumor derived from pancreatic β-cells, and the resulting hyperinsulinemia leads to characteristic hypoglycemia. Recent studies have reported the frequent overexpression of glucagon-like peptide-1 receptor (GLP-1R) in human insulinomas, suggesting that the binding of a radiolabeled compound to GLP-1R is useful for the imaging of such tumors. Exendin(9-39), a fragment peptide of exendin-3 and -4, binds GLP-1R with high affinity and acts as an antagonist. Accordingly, radiolabeled exendin(9-39) derivatives have also been investigated as insulinoma imaging probes that might be less likely to induce hypoglycemia. In this study, we synthesized a novel indium-111 (111In)-benzyl-diethylenetriaminepentaacetic acid (111In-BnDTPA)-conjugated exendin(9-39), 111In-BnDTPA-exendin(9-39), and evaluated its utility as a probe for the SPECT imaging of insulinoma. natIn-BnDTPA-exendin(9-39) exhibited a high affinity for GLP-1R (IC50=2.5nM), stability in plasma, and a specific activity that improved following reactions with a solvent and solubilizer. Regarding the in vivo biodistribution of 111In-BnDTPA-exendin(9-39) in INS-1 tumor-bearing mice, high uptake levels were observed in tumors (14.6%ID/g at 15min), with corresponding high tumor-to-blood (T/B), tumor-to-muscle (T/M), and tumor-to-pancreas (T/P) ratios (T/B=2.55, T/M=22.7, T/P=2.7 at 1h). The pre-administration of excess nonradioactive exendin(9-39) significantly reduced accumulation in both the tumor and pancreas (76% and 68% inhibition, respectively) at 1h after 111In-BnDTPA-exendin(9-39) injection, indicating that the GLP-1R mediated a majority of 111In-BnDTPA-exendin(9-39) uptake in the tumor and pancreas. Finally, 111In-BnDTPA-exendin(9-39) SPECT/CT studies in mice yielded clear images of tumors at 30min post-injection. These results suggest that 111In-BnDTPA-exendin(9-39) could be a useful SPECT molecular imaging probe for the detection and exact localization of insulinomas.

Preparation of Chiral Ligands Connected with Quaternary Ammonium Group for Recyclable Catalytic Asymmetric Transfer Hydrogenation in Ionic Liquid

Reuse of chiral ruthenium catalyst in catalytic asymmetric transfer hydrogenation (CATH) has attracted attention from economic and environmental viewpoints, and reactions using ionic liquids (ILs) as solvent are recognized as one of the most useful methods for reuse of the catalyst. We synthesized (1S,2S)-N-(p-toluenesulfonyl)-1,2-diphenylethylenediamine (TsDPEN) derivatives with various ionic moieties, and investigated the effect of their structure with respect to catalytic ability and recyclability in CATH with ILs. Ligand 3a having an imidazolium group showed the best results, and significant differences were observed depending on the structure of the ionic moiety or the length of the alkyl chain connecting the ligand site and the ionic moiety. Among various prochiral ketones used as substrates at various cycles, 3a showed a relatively good result.

Synthesis and biological evaluation of an 111In-labeled exendin-4 derivative as a single-photon emission computed tomography probe for imaging pancreatic β-cells

A non-invasive method of pancreatic β-cell mass measurement is needed to enhance our understanding of the pathogenesis of diabetes, facilitate the early diagnosis of this disease, and promote the development of novel therapeutics. Here, we described the synthesis of a novel indium-111 (111In) exendin-4 derivative, [Lys12(In-BnDTPA-Ahx)]exendin-4, through a process involving isothiocyanate-benzyl-DTPA (BnDTPA) and 6-aminohexanoic acid (Ahx) attached to an ɛ-amino group at the lysine-12 residue. We further evaluated the potential use of this derivative as a SPECT probe for pancreatic β-cell imaging. An in vitro binding assay revealed that [Lys12(natIn-BnDTPA-Ahx)]exendin-4 has a high affinity for GLP-1 receptors (IC50=0.43nM). In biodistribution experiments involving normal mice, high [Lys12(111In-BnDTPA-Ahx)]exendin-4 uptake was observed in the pancreas (21.8 ± 4.0%ID/g) and was maintained for 2h after injection. Pre-injection of excess exendin(9-39) markedly reduced the pancreatic uptake of [Lys12(111In-BnDTPA-Ahx)]exendin-4 (95.2%), indicating that the uptake of this tracer is specific and mediated by GLP-1 receptors. Ex vivo autoradiography experiments involving pancreatic sections from MIP-GFP mice confirmed the accumulation of [Lys12(111In-BnDTPA-Ahx)]exendin-4 in pancreatic β-cells. Finally, in mice, [Lys12(111In-BnDTPA-Ahx)]exendin-4 SPECT/CT yielded clear images of the pancreas at 30min post-injection. In conclusion, SPECT with [Lys12(111In-BnDTPA-Ahx)]exendin-4 enables to visualize β-cells in vivo non-invasively.

Continuous-Flow Synthesis of N-Succinimidyl 4-[18F]fluorobenzoate Using a Single Microfluidic Chip

In the field of positron emission tomography (PET) radiochemistry, compact microreactors provide reliable and reproducible synthesis methods that reduce the use of expensive precursors for radiolabeling and make effective use of the limited space in a hot cell. To develop more compact microreactors for radiosynthesis of 18F-labeled compounds required for the multistep procedure, we attempted radiosynthesis of N-succinimidyl 4-[18F]fluorobenzoate ([18F]SFB) via a three-step procedure using a microreactor. We examined individual steps for [18F]SFB using a batch reactor and microreactor and developed a new continuous-flow synthetic method with a single microfluidic chip to achieve rapid and efficient radiosynthesis of [18F]SFB. In the synthesis of [18F]SFB using this continuous-flow method, the three-step reaction was successfully completed within 6.5 min and the radiochemical yield was 64 ± 2% (n = 5). In addition, it was shown that the quality of [18F]SFB synthesized on this method was equal to that synthesized by conventional methods using a batch reactor in the radiolabeling of bovine serum albumin with [18F]SFB.

Gibberellin Biosynthetic Inhibitors Make Human Malaria Parasite Plasmodium falciparum Cells Swell and Rupture to Death

Malaria remains as one of the most devastating infectious disease, and continues to exact an enormous toll in medical cost and days of labor lost especially in the tropics. Effective malaria control and eventual eradication remain a huge challenge, with efficacious antimalarials as important intervention/management tool. Clearly new alternative drugs that are more affordable and with fewer side effects are desirable. After preliminary in vitro assays with plant growth regulators and inhibitors, here, we focus on biosynthetic inhibitors of gibberellin, a plant hormone with many important roles in plant growth, and show their inhibitory effect on the growth of both apicomplexa, Plasmodium falciparum and Toxoplasma gondii. Treatment of P. falciparum cultures with the gibberellin biosynthetic inhibitors resulted in marked morphological changes that can be reversed to a certain degree under hyperosmotic environment. These unique observations suggest that changes in the parasite membrane permeability may explain the pleiotropic effects observed within the intracellular parasites.