Synthesis of a F-18 labeled analog of antitumor prostaglandin Δ-pGA1 methyl ester using p-[F]Fluorobenzylamine

A fluorine-18 labeled analog of an antitumor prostaglandin Δ⁷-PGA₁ methyl ester, 15-deoxy-13,14-dihydro-Δ⁷-PGA₁ 4-[¹⁸F]fluorobenzyl amide ([¹⁸F]3), was synthesized as a tracer candidate for detecting tumors with positron emission tomography. p-[¹⁸F]Fluorobenzylamine (p-[¹⁸F]FBnA) used as a labeled precursor for the synthesis of [¹⁸F]3 was prepared by fluorination of a 4-N, N, N-trimethylammoniumbenzonitrile triflate with [¹⁸F]fluoride and subsequent reduction with borane-dimethylsulfide. Radiochemical yield and purity of p-[¹⁸F]FBnA obtained were 39–49% (decay uncorrected) and 91–96%, respectively, after C18 Sep-Pak purification. Treatment of p-[¹⁸F]FBnA with a 15-deoxy-13,14-dihydro-Δ⁷-PGA₁ N-succinimidyl ester in acetonitrile and subsequent HPLC purification gave radiochemically pure (>99%) [¹⁸F]3 with a 58% decay uncorrected yield. The total synthesis time was 70 min from the start of the radiosynthesis of p-[¹⁸F]FBnA.     

IBOX(2-(4′-dimethylaminophenyl)-6-iodobenzoxazole): a ligand for imaging amyloid plaques in the brain

It is well known that overproduction and accumulation of β-amyloid (Aβ) plaques in the brain is a key event in the pathogenesis of Alzheimer’s disease (AD). Previously it was demonstrated that [¹²⁵I]TZDM, 2-(4′-dimethylaminophenyl)-6-iodobenzothiazole, a thioflavin derivative, was an effective ligand with good in vitro and in vivo binding characteristics. To further improve the initial uptake and washout rate from the brain, important properties for in vivo imaging agents, a novel radioiodinated ligand, 2-(4′-dimethylaminophenyl)-6-iodobenzoxazole ([¹²⁵I]IBOX, 3), for detecting Aβ plaques in the brain, was synthesized and evaluated. The new iodinated ligand, IBOX, is based on an isosteric replacement of a sulfur atom of TZDM by an oxygen, by which the molecular weight is reduced while the lipophilicity of the iodinated ligand is increased. Partition coefficients (P.C.) of these two ligands were 70 and 124 for TZDM and IBOX, respectively. In vitro binding study indicated that the isosteric displacement yielded a new ligand with equal binding potency to Aβ(1–40) aggregates (K_i = 1.9 and 0.8 nM for TZDM and IBOX, respectively). Autoradiography of postmortem brain sections of a confirmed AD patient by [¹²⁵I]IBOX showed excellent labeling of plaques similar to that observed with [¹²⁵I]TZDM. More importantly, in vivo biodistribution of [¹²⁵I]IBOX in normal mice displayed superior peak brain uptake (2.08% at 30 min vs 1.57% at 60 min dose/brain for [¹²⁵I]IBOX and [¹²⁵I]TZDM, respectively). In addition, the washout from the brain was much faster for [¹²⁵I]IBOX as compared to [¹²⁵I]TZDM. Based on the data presented for [¹²⁵I]IBOX, it is predicted that the brain trapping of this new radioiodinated ligand in the Aβ containing regions will be more favorable than that of the parent compound, [¹²⁵I]TZDM. Further evaluation of [¹²⁵I]IBOX is warranted to confirm the Aβ plaque labeling properties in vivo.     

N-[C]methylpiperidine esters as acetylcholinesterase substrates: an in vivo structure–reactivity study

A series of simple esters incorporating the N-[¹¹C]methylpiperidine structure were examined as in vivo substrates for acetylcholinesterase in mouse brain. 4-N-[¹¹C]Methylpiperidinyl esters, including the acetate, propionate and isobutyrate esters, are good in vivo substrates for mammalian cholinesterases. Introduction of a methyl group at the 4-position of the 4-piperidinol esters, to form the ester of a teritary alcohol, effectively blocks enzymatic action. Methylation of 4- N-[¹¹C]methylpiperidinyl propionate at the 3-position gives a derivative with increased in vivo reactivity toward acetylcholinesterase. Esters of piperidinecarboxylic acids (nipecotic, isonipecotic and pipecolinic acid ethyl esters) are not hydrolyzed by acetylcholinesterase in vivo, nor do they act as in vivo inhibitors of the enzyme. This study has identified simple methods to both increase and decrease the in vivo reactivity of piperidinyl esters toward acetylcholinesterase.     

Syntheses of carbon-11 labeled piperidine esters as potential in vivo substrates for acetylcholinesterase

A series of carbon-11 labeled N-methylpiperidinyl esters were prepared as potential in vivo substrates for acetylcholinesterase (AChE). Target compounds were designed based on the structure of N-[¹¹C]methylpiperidin-4-yl propionate, an ester currently used to measure AChE enzymatic activity in the human brain, to examine the structure–activity relationship for in vivo enzymatic hydrolysis. Changes in steric bulk and in the ester order (“reverse” esters) were made. Addition of methyl groups was made to both the acid side chain (synthesis of N-[¹¹C]methylmethylpiperidin-4-yl isobutyrate) and to the piperidine ring (syntheses of N-[¹¹C]methyl-4-methylpiperidin-4-yl propionate, N-[¹¹C]methyl-4-methylpiperidin-4-yl acetate, and N-[¹¹C]methyl-3-methylpiperidin-4-yl propionate). Alterations of the order of the ester heteroatoms was accomplished through syntheses of the N-[¹¹C]methyl-2,3- and 4-piperidinecarboxylic acid ethyl esters. Finally, an additional piperidine-based ester (N-[¹¹C]methylpiperidin-2-yl)methyl propionate was also prepared. All carbon-11-labeled esters were prepared by N-[¹¹C]methylation reactions, using the desmethyl precursors and no-carrier-added [¹¹C]methyltriflate, and were obtained in decay-corrected yields (not optimized) of 10–40% and high specific activities.     

Derivatives of way 100635 as potential imaging agents for 5-ht1a receptors: syntheses, radiosyntheses, and in vitro and in vivo evaluation

Analogues of the potent and selective 5-HT_1A ligand, WAY 100635, were synthesized and examined as potential candidates for imaging 5-HT_1A receptors by positron emission tomography (PET). Several of the analogues displayed nanomolar affinity for the 5-HT_1A receptor, comparable to WAY 100635. Three of these were examined in a model of human liver metabolism vis-à-vis WAY 100635. All showed a markedly lower propensity for amide hydrolysis than WAY 100635. Radiolabelling of these three potential PET radiotracers with carbon-11 was readily achieved from [¹¹C]-iodomethane, and the newly synthesized radioligands were tested in vivo in rats for binding to 5-HT_1A receptors. Whereas two of the ligands failed to bind to 5-HT_1A receptors in vivo, one was successful. The latter, [¹¹C]-7 {4-(2′-methoxyphenyl)-1-[2′-[N-(2′-pyridinyl)-2-bicyclo[2.2.2]octanecarboxamido]ethyl]-piperazine}, showed good brain penetration, hippocampal:cerebellar ratios of 10:1 at 45 min postinjection. Blocking studies with a variety of drugs demonstrated that the binding of [¹¹C]-7 in vivo was selective for 5-HT_1A receptors. [¹¹C]-7 is a promising candidate as a ligand for imaging 5-HT_1A receptors by PET.     

Rhenium-188 and technetium-99m nitridobis(N-ethoxy-N-ethyldithiocarbamate) leucocyte labelling radiopharmaceuticals: [ReN(NOET)2] and [TcN(NOET)2], NOET = Et(EtO)NCS2: Their in vitro localization and chemical behaviour

In this study, we have investigated the preparation of rhenium-188 nitridobis(N-ethoxy-N-ethyldithiocarbamate) [¹⁸⁸ReN(NOET)₂] (NOET = Et(EtO)NCS₂), analogous to the known technetium-99m radiopharmaceutical. The new ¹⁸⁸Re complex was synthesized in good yield with a satisfactory radiochemical purity, using a kit method. The subcellular localization of both radiopharmaceuticals in granulocytes was observed by microautoradiography. The uptake was independent of the radionuclide and predominantly nuclear. Furthermore, HPLC was used to characterize the ^99mTc complex before and after blood cell labelling and revealed that the intact radiopharmaceutical was involved.     

Dependency of the [18F]fluoromisonidazole uptake on oxygen delivery and tissue oxygenation in the porcine liver

We have previously shown that the accumulation of fluorine-18-labeled fluoromisonidazole ([¹⁸F]FMISO) is inversely correlated to tissue oxygenation, allowing the quantification of porcine liver tissue hypoxia in vivo. We determined the activity from administered [¹⁸F]FMISO in relation to the hepatic oxygen availability and the partial pressure of oxygen in tissue (tPO₂) to define a critical oxygen delivery on a regional basis. [¹⁸F]FMISO was injected 2 h after onset of regional liver hypoxia due to arterial occlusion of branches of the hepatic artery in 10 domestic pigs. During the experimental procedure the fractional concentration of inspired oxygen (FiO₂) was set to 0.67 in group A ( N=5) and to 0.21 in group B ( N=5) animals. Immediately before sacrifice, the tPO₂ was determined in normal flow and flow-impaired liver segments. The standardized uptake values (SUV) for [¹⁸F]FMISO was calculated from 659 single tissue samples obtained 3 h after injection of approximately 10 MBq/kg body weight [¹⁸F]FMISO and was compared with the regional total hepatic oxygen delivery (DO₂) calculated from the regional arterial and portal venous flow (based on ¹⁴¹Ce- and ^99mTc-microspheres measurements) and the oxygen content of the arterial and portal venous blood. In 121 tPO₂-measured liver tissue samples, the mean DO₂ was significantly decreased in occluded liver tissue samples [group A: 0.063 (0.044–0.089); group B: 0.046 (0.032–0.066)] compared to normal flow segments [group A: 0.177 (0.124–0.252); group B: 0.179 (0.128–0.25) mL·min⁻¹·g⁻¹; geometric mean (95% confidence limits); p < 0.01 in group A and p < 0.001 in group B]. The tPO₂ of occluded segments [group A: 5.1 (3.2–8.1); group B: 3.9 (2.4–6.2) mm Hg] was significantly decreased compared to normal flow segments [group A: 20.2 (12.6–32.5); group B: 22.4 (14.3–35.2) mm Hg; p < 0.01 in group A and p < 0.001 in group B]. Three hours after [¹⁸F]FMISO administration, the mean [¹⁸F]FMISO SUV determined in tPO₂-measured occluded segments was significantly higher [group A: 4.08 (3.12–5.34), group B: 5.43 (4.14–7.13)] compared to normal liver tissue [group A: 1.57 (1.2–2.06), group B: 1.5 (1.16–1.93); p < 0.001 for both groups]. The [¹⁸F]FMISO SUV allowed prediction of the tPO₂ with satisfying accuracy in hypoxic regions using the exponential regression curve { [¹⁸F]FMISO=1.05+6.7^{(−0.117 tPO₂}); r²=0.75;p < 0.001}. In addition, regardless of ventilation conditions, a significant exponential relationship between the DO₂ and the [¹⁸F]FMISO SUV was found ( r²=0.39,p < 0.001). Our results suggest that the reduction of the oxygen delivery below the critical range of 0.1–0.11 mL·min⁻¹·g⁻¹ regularly causes liver tissue hypoxia. The severity of hypoxia is reflected by the [¹⁸F]FMISO accumulation and allows the in vivo estimation of the tPO₂ in hypoxic regions.     

Optimized indirect (76)Br-bromination of antibodies using N-succinimidyl para-[76Br]bromobenzoate for radioimmuno PET

Monoclonal antibody 38S1 was radiobrominated with the positron emitter ⁷⁶Br (T_1/2= 16.2 h). Indirect labeling was performed using N-succinimidyl para-(tri-methylstannyl)benzoate (SPMB) as the precursor molecule. SPMB was labeled using Chloramine-T yielding N-succinimidyl para-[⁷⁶Br]bromobenzoate, which was then conjugated to the antibody. Optimization of the labeling conditions and further conjugation gave a total yield ( mean±max error) of 49±2%. The immunoreactivity of the antibodies was retained after labeling. Thus, antibodies intended for positron emission tomography can be labeled with ⁷⁶Br, which gives high yields and preserved immunoreactivity when using the SPMB technique described.     

Fluorine-18 and carbon-11 labeled amphetamine analogs—Synthesis, distribution, binding characteristics in mice and rats and a PET study in monkey

No-carrier-added (NCA) (±)-p-[¹⁸F]fluoroamphetamine (2a) and (±)-6-[¹⁸F]fluoro-3,4-methylene-dioxyamphetamine (2b) were synthesized through a multistep synthesis by nucleophilic substitution of the appropriate precursors (p-nitrobenzaldehyde, 1a and 6-nitropiperonal 1b, respectively) with [¹⁸F]fluoride followed by condensation with nitroethane and reduction with LAH in 20–30% yield (EOB) in a synthesis time of 90–109 min from EOB. NCA (−)-[¹¹C]methamphetamine (4a) and (±)-3,4-methylenedioxy-N-[¹¹C]methamphetamine (4b) were synthesized by methylation of the appropriate desmethyl precursors 3a and 3b with [¹¹C]H₃I in 40–60% yield (EOB) in a synthesis time of 30 min from EOB. Animal studies in mouse and rat revealed that the relative tissue uptake of these radiotracers was kidneys > lungs > liver > spleen > brain > heart > blood. The uptakes of these radiotracers in mouse brain were high and similar at 5 min post-injection (approx. 5%/g) but radioactivity then declined rapidly (approx. 1%/g at 60 min post-injection). For compounds 2a and 2b, the activity in the femur did not increase with time indicating in vivo defluorination may not be the major route of metabolism. Monoamine uptake inhibitors (nomifensine, fluoxetine and nisoxetine) did not inhibit but enhance the uptake of (−)-[¹¹C]methamphetamine (4a) in the rat brain by greater than 50%. A PET study in a Rhesus monkey revealed that the uptakes of (−)-[¹¹C]methamphetamine in different brain regions were similar and the retention of the radioactivity in these regions remained constant throughout the study. Analysis of arterial plasma by HPLC showed that 50% of radioactivity remained as 4a at 60 min post-injection.     

Methodological aspects for in vitro characterization of receptor binding using C-labeled receptor ligands: A detailed study with the benzodiazepine receptor antagonist [C]Ro 15-1788

As a complement to in vivo studies with positron emission tomography (PET), it is desirable to perform in vitro characterization of newly developed ¹¹C tracers. In this report we describe the technique for determination of receptor-ligand kinetics utilizing ligands labeled with the short-lived radionuclide ¹¹C. The limitations and advantages are discussed. The benzodiazepine antagonist [¹¹C]Ro 15-1788 was used as a model substance, and the use of storage phosphor plates for quantification of radioactivity was validated. Storage phosphor plates showed an excellent linear range (˜10³) and acceptable resolution (˜ 0.5 mm). Receptor-ligand kinetics, including depletion, association and dissociation, saturation and displacement were evaluated with good results through the use of short-lived radiotracers and storage phosphor plates.     

Cytotoxic and genotoxic effect of the [166Dy]Dy/166Ho-EDTMP in vivo generator system in mice

Multiple myeloma and other hematological malignancies have been treated by myeloablative radiotherapy/chemotherapy and subsequent stem cell transplantation. [¹⁶⁶Dy]Dy/¹⁶⁶Ho-ethylenediaminetetramethylene phosphonate (EDTMP) forms a stable in vivo generator system with selective skeletal uptake in mice; therefore, it could work as a potential and improved agent for marrow ablation. Induced bone marrow cytotoxicity and genotoxicity are determined by the reduction of reticulocytes (RET) and elevation of micronucleated reticulocyte (MN-RET) in peripheral blood and ablation by bone marrow histological studies. The aim of this study was to determine the bone marrow cytotoxic and genotoxic effect of the [¹⁶⁶Dy]Dy/¹⁶⁶Ho-EDTMP in vivo generator system in mice and to evaluate by histopathology its myeloablative potential.

Enriched ¹⁶⁶Dy₂O₃ was irradiated and [¹⁶⁶Dy]DyCl₃ was added to EDTMP in phosphate buffer (pH 8.0) in a molar ratio of 1:1.75. QC was determined by TLC. Dy-EDTMP complex was prepared the same way with nonirradiated dysprosium oxide. A group of BALB/c mice were intraperitoneally injected with the radiopharmaceutical and two groups of control animals were injected with the cold complex and with 0.9% sodium chloride, respectively. A blood sample was taken at the beginning of the experiments and every 48 h for 12 days postinjection. The animals were sacrificed, organs of interest taken out and the radioactivity determined. The femur was used for histological studies. Flow cytometry analysis was used to quantify the frequency of RET and MN-RET in the blood samples. The MCNP4B Monte Carlo computer code was used for dosimetry calculations.

Radiochemical purity was 99% and the mean specific activity was 1.3 MBq/mg. The RET and MN-RET frequency were statistically different in the treatment at the end of the 12-day period demonstrating cytotoxicity and genotoxicity induced by the in vivo generator system. The histology studies show that there was complete, or almost complete, acellularity, which means significant suppression of the bone marrow activity. Bone marrow absorbed dose was 18–23 Gy. [¹⁶⁶Dy]Dy/¹⁶⁶Ho-EDTMP induces cytotoxicity, genotoxicity and severe myelosuppression in mice. Potentially, it is a good agent for use in humans.     

Synthesis of 6-Methyl[C]-2′-deoxyuridine and evaluation of Its In Vivo distribution in Wistar rats

In this study, 6-methyl-2′-deoxyuridine, an artificial nucleoside, was labeled with ¹¹C in the methyl position. Tissue distribution of 6-methyl[¹¹C]-2′-deoxyuridine was investigated in normal Wistar rats and compared to the behavior of the natural nucleoside [methyl-¹¹C]thymidine. High renal clearance, up to at least 20% of the injected activity, was noticed during the 20 min period following injection. Tissue distribution as determined by dynamic PET studies of both ¹¹C-labeled nucleosides was significantly different for most of the organs.     

1,25(OH)2D3对关节滑膜蛋白聚糖-4基因表达及分泌的影响

 

目的 探讨维生素D体内活性形式1,25(OH)₂D₃对关节滑膜蛋白聚糖-4 (proteoglycan 4,PRG4)基因表达及分泌的影响。方法 新西兰大白兔,分离膝关节滑膜体外培养。首先用10 nM浓度1,25(OH)₂D₃进行试验。Real time-PCR检测PRG4基因表达,ELISA 法检测培养液中PRG4分泌量的变化,确定1,25(OH)₂D₃作用后PRG4基因表达和PRG4分泌的最佳时间点。之后用不同浓度1,25(OH)₂D₃进行刺激,在最佳时间点测定1,25(OH)₂D₃对关节滑膜PRG4基因表达及分泌的影响。结果 Real time-PCR分析显示,1,25(OH)₂D₃作用6 h后PRG4 mRNA水平开始上调,24 h时PRG4 mRNA表达水平达峰值。ELISA分析结果显示,1,25(OH)₂D₃作用12 h时滑膜培养液中PRG4蛋白含量开始明显升高,48 h时滑膜培养液中PRG4蛋白含量达最高。Real time-PCR分析显示,在0.1~100 nM 浓度范围内1,25(OH)₂D₃均促进关节滑膜PRG4基因表达,随着1,25(OH)₂D₃浓度的提高,PRG4基因表达上调逐渐提高。ELISA分析显示,1,25(OH)₂D₃在0.1~100 nM浓度范围内均促进关节滑膜分泌PRG4,随着1,25(OH)₂D₃浓度的提高,滑膜分泌PRG4量逐渐增加。结论 1,25(OH)₂D₃对滑膜PRG4基因表达及分泌有明显的促进作用。

     

Equilibrium modeling of 5-HT(2A) receptors with [18F]deuteroaltanserin and PET: feasibility of a constant infusion paradigm

ABSTRACT. [¹⁸F]Altanserin has emerged as a promising positron emission tomography (PET) ligand for serotonin-2A (5-HT_2A) receptors. The deuterium substitution of both of the 2′-hydrogens of altanserin ([¹⁸F]deuteroaltanserin) yields a metabolically more stable radiotracer with higher ratios of parent tracer to radiometabolites and increased specific brain uptake than [¹⁸F]altanserin. The slower metabolism of the deuterated analog might preclude the possibility of achieving stable plasma and brain activities with a bolus plus constant infusion within a reasonable time frame for an ¹⁸F-labeled tracer (T_1/2 110 min). Thus, the purpose of this study was to test the feasibility in human subjects of a constant infusion paradigm for equilibrium modeling of [¹⁸F]deuteroaltanserin with PET. Seven healthy male subjects were injected with [¹⁸F]deuteroaltanserin as a bolus plus constant infusion lasting 10 h postinjection. PET acquisitions and venous blood sampling were performed throughout the infusion period. Linear regression analysis revealed that time-activity curves for both specific brain uptake and plasma [¹⁸F]deuteroaltanserin concentration stabilized after about 5 h. This permitted equilibrium modeling and estimation of V^′₃ (ratio of specific uptake to total plasma parent concentration) and the binding potential V₃ (ratio of specific uptake to free plasma parent concentration). Cortical/cerebellar ratios were increased by 26% relative to those we previously observed with [¹⁸F]altanserin using similar methodology in a somewhat older subject sample. These results demonstrate feasibility of equilibrium imaging with [¹⁸F]deuteroaltanserin and suggest that it may be superior to [¹⁸F]altanserin as a PET radioligand.     

Radiosynthesis of [F] N-3-fluoropropyl-2-β-carbomethoxy-3-β-(4-iodophenyl) nortropane and the first human study with positron emission tomography

A procedure for the routine preparation of [¹⁸F]FP-CIT has been developed. Purification of the final product was achieved by preparative HPLC using phenethyl column without decomposition or epimerization. [¹⁸F] labeled-N-fluoropropyl-2β-carbomethoxy-3β-(4-iodophenyl)nortropane was prepared and PET imaging was performed on human subjects. A high uptake into striatal regions was observed. HPLC plasma analysis using [¹⁸F]FP-CIT indicated the presence of only one metabolite. By directly comparing the behavior of these three radiotracers ([¹⁸F]DOPA, [¹²³I]FP-CIT, and [¹⁸F]FP-CIT) in the same subjects, we can enhance our understanding of the dopaminergic system as well as the relative potential of these techniques in a clinical research setting.     

Remote processing, delivery and injection of H2[O] produced from a N2/H2 gas target using a simple and compact apparatus

We report here a simple apparatus for remote trapping and processing of H₂[¹⁵O] produced from the N₂/H₂ target. The system performs a three step operation for H₂[¹⁵O] delivery at the PET imaging facility which includes the following: (i) collecting the radiotracer in sterile water; (ii) adjusting preparation pH through removal of radiolytically produced ammonia, while at the same time adjusting solution isotonicity; and (iii) delivery of the radiotracer preparation to the injection syringe in a sterile and pyrogen-free form suitable for human studies. The processing apparatus is simple, can be remotely operated and fits inside a Capintec Dose Monitoring Chamber for direct measurement of accumulated radioactivity. Using this system, 300 mCi of H₂[¹⁵O] (15 μA of 8 MeV D⁺ on target) is transferred from target through 120 m × 3.18 mm o.d. Impolene tubing to yield 100 mCi of H₂[¹⁵O] which is isotonic, neutral and suitable for human studies.

A remote hydraulically driven system for i.v. injection of the H₂[¹⁵O] is also described. The device allows for direct measurement of syringe dose while filling, and for easy, as well as safe transport of the injection syringe assembly to the patient's bedside via a shielded delivery cart. This cart houses a hydraulic piston that allows the physician to “manually” inject the radiotracer without directly handling the syringe.     

Ester-modified TcO[SN(R)S/S] mixed ligand complexes: synthesis and preliminary evaluation

Two novel ^99mTc-(SNS/S) complexes: a mono-ester compound carrying an ethyl ester group on the tridentate ligand, ^99mTcO[C₂H₅OOCCH₂N(CH₂CH₂S)₂][SC₆H₄CH₃], 3, and a diester compound, carrying a second ethyl ester group on the monodentate ligand, ^99mTcO[C₂H₅OOCCH₂N(CH₂CH₂S)₂][SC₆H₄COOC₂H₅], 4, were synthesized. The corresponding oxorhenium(V) complexes, 1 and 2 were also synthesized. Enzymatic hydrolysis demonstrated that 3 remains intact after 10 min incubation while 4 is totally converted to an unidentified hydrophilic complex. Tissue distribution data in mice revealed that both complexes, 3 and 4, exhibit significant initial brain uptake (1.42 and 1.01% of injected dose at 5 minutes post injection respectively) and fast blood clearance.     

Synthesis and biological evaluation of 1-(4-[18F]fluorobenzyl)-4-[(5,6-dimethoxy-1-oxoindan-2-yl)methyl]piperidine for in vivo studies of acetylcholinesterase

We synthesized and evaluated 1-(4-fluorobenzyl)-4-[(5,6-dimethoxy-1-oxoindan-2-yl)methyl]piperidine (4-FDP), which is an analog of donepezil. The 4-[¹⁸F]FDP was prepared by reductive alkylation of debenzylated donepezil with 4-[¹⁸F]fluorobenzaldehyde in high radiochemical yield (decay-corrected, 40–52%) and with high effective specific activity (30–38 GBq/μmol). Tissue distribution studies in mice demonstrated nonspecific distribution of the 4-[¹⁸F]FDP in brain regions, suggesting that this radioligand may not be a suitable agent for in vivo studies of acetylcholinesterase (AChE), despite its potent in vitro biological activity.     

Synthesis of [1-C], [2-C], [1-C](H3) and [2-C](H3)acetate for in vivo studies of myocardium using PET

Four isotopically-labelled acetates ([1-¹¹C], [2-¹¹C], [1-¹¹C](²H₃) and [2-¹¹C](²H₃)acetate) were synthesized and used in positron emission tomography (PET) studies of pig myocardium. The [1-¹¹C]acetates were synthesized by carboxylation of the appropriate ¹H or ²H methyl Grignard reagents immobilized on a C₂ solid phase extraction column (SPE). Purification by reverse-phase HPLC, resulted in 35–45% decay-corrected radiochemical yield with a total synthesis time of 25 min, and a radiochemical purity higher than 99%. The [2-¹¹C]acetates were synthesized by carboxylation of ¹¹C-labelled ¹H or ²H methyl lithium. Purification as above resulted in 35–55% decay-corrected radiochemical yield with a total synthesis time of 30 min, and a radiochemical purity higher than 99%. Position-specific labelling was assessed by ¹³C-labelling and NMR.

Multiple isotopic labelling by the combination of position-specific ¹¹C-labelling and ²H substitution, has the potential to highlight different aspects of a complex biochemical system using a selected set of tracers in comparative PET studies. An illustration of this principle is given using acetate, where citric acid cycle metabolism results in a position-specific kinetic for the ¹¹C-label, and deuteration opens up the possibility for the proton-abstracting processes within the citric acid cycle to be assessed.     

Synthesis of [F]fluoroetanidazole: A potential new tracer for imaging hypoxia

A method has been developed for the synthesis of [¹⁸F]fluoroetanidazole, a potential tracer for imaging hypoxia with positron emission tomography. The compound is prepared by an active ester coupling reaction between the 2,3,5,6-tetrafluorophenyl ester of 2-nitroimidazole acetic acid and [¹⁸F]fluoroethylamine. [¹⁸F]Fluoroethylamine is prepared from N-[2-(toluene-4-sulfonyloxy)-ethyl]-phthalimide and [¹⁸F]fluoride and purified by distillation. The overall reaction takes about 90 min and gives a yield, uncorrected, of about 25%. Purification on a reversed-phase column is straightforward.