Design,Synthesis, and Biological Evaluation of Catecholamine Vehicle for Studying Dopaminergic System

Catecholamine mimetic EDTA‐bis(tyramide) was synthesized and characterized by various spectroscopic techniques (NMR, mass spectroscopy) and λ_em 310 nm for the excitation at 270 nm. Molecular docking studies were performed with human serum albumin (PDB 1E78), showing binding pattern with amino acid residues Arg218, Arg222, and Lys444, identifies the ligand‐human serum albumin interaction for the transportation affinity of the ligand at the specific site of the target. Subsequently, binding study with human serum albumin at λ_ex = 350 nm found to be 5.847 × 10⁴ m ^?1 shows effective quenching effect. Additionally, to go more insight, acetylcholinesterase binding affinity was investigated, which shows 90% binding affinity for the 10 mm concentration. IC50 value was found 18.60 μm for MAO‐B inhibition. Finally, EDTA‐bis(tyramide) labeled with ^99mTc to investigate its in vivo radiopharmaceutical efficiency having 97% binding affinity with 98% radiochemical purity. In vivo studies were carried out for ^99mTc‐EDTA‐bis(tyramide) included blood kinetics showed a quick wash out from the circulation via renal route, and biodistribution revealed that maximum %ID/g was found in kidney at 1 h, and its scintigraphy image shows 3.96% brain uptake with respect to whole body.     

Efficient gallium‐68 radiolabeling reaction of DOTA derivatives using a resonant‐type microwave reactor

Gallium‐68 (⁶⁸Ga, t_1/2 = 68 min) can be easily obtained from a ⁶⁸Ge/⁶⁸Ga generator, and several such systems are commercially available. The use of positron emission tomography (PET) imaging using ⁶⁸Ga‐labeled radiopharmaceuticals is expected to increase in both preclinical and clinical settings. However, the chelation between a ⁶⁸Ga cation and the bifunctional macrocyclic chelates that are used for labeling bioactive substances, such as 1,4,7,10‐tetraazacyclododecane‐1,4,7,10‐tetraacetic acid (DOTA), requires a relatively long reaction time and high temperature to achieve a high radiochemical yield. Previously, we reported on a novel resonant‐type microwave reactor that can be used for radiosynthesis and the usefulness of this reactor in the PET radiosynthesis of ¹⁸F. In the present study, the usefulness of this resonant‐type microwave reactor was evaluated for the radiolabeling of model macrocyclic chelates with ⁶⁸Ga. As a result, microwave heating of resonant‐type microwave reactor notably improved the rate of the ⁶⁸Ga labeling chelate reaction in a short time period of 2 minutes, compared with the use of a conventional heating method. Additionally, it was found that the use of this reactor made it possible to decrease the amount of precursors required in the reaction and to improve the molar activity of the labeled compounds.     

Fusarinine C,a novel siderophore‐based bifunctional chelator for radiolabeling with Gallium‐68

Fusarinine C (FSC), a siderophore‐based chelator coupled with the model peptide c(RGDfK) (FSC(succ‐RGD)₃), revealed excellent targeting properties in vivo using positron emission tomography (PET). Here, we report the details of radiolabeling conditions and specific activity as well as selectivity for ⁶⁸Ga. ⁶⁸Ga labeling of FSC(succ‐RGD)₃ was optimized regarding peptide concentration, pH, temperature, reaction time, and buffer system. Specific activity (SA) of [⁶⁸Ga]FSC(succ‐RGD)₃ was compared with ⁶⁸Ga‐1,4,7‐triazacyclononane, 1‐glutaric acid‐4,7 acetic acid RGD ([⁶⁸Ga]NODAGA‐RGD). Stability was evaluated in 1000‐fold ethylenediaminetetraacetic acid (EDTA) solution (pH 7) and phosphate‐buffered saline (PBS). Metal competition tests (Fe, Cu, Zn, Al, and Ni) were carried out using [⁶⁸Ga]‐triacetylfusarinine C. High radiochemical yield was achieved within 5 min at room temperature, in particular allowing labeling with ⁶⁸Ga up to pH 8 with excellent stability in 1000‐fold EDTA solution and PBS. The 10‐fold to 20‐fold lower concentrations of FSC(succ‐RGD)₃ led to the same radiochemical yield compared with [⁶⁸Ga]NODAGA‐RGD with SA up to 1.8 TBq/µmol. Metal competition tests showed high selective binding of ⁶⁸Ga to FSC. FSC is a multivalent siderophore‐based bifunctional chelator allowing fast and highly selective labeling with ⁶⁸Ga in a wide pH range and results in stable complexes with high SA. Thus it is exceptionally well suited for the development of new ⁶⁸Ga‐tracers for in vivo molecular imaging with PET.     

Preparation and preliminary bioevaluation studies of 68Ga‐NOTA‐rituximab fragments as radioimmunoscintigraphic agents for non‐Hodgkin lymphoma

Rituximab is used for the treatment of non‐Hodgkin lymphoma (NHL). This study focuses on development of ⁶⁸Ga‐labeled rituximab fragments, (⁶⁸Ga‐NOTA‐F (ab′)‐rituximab and ⁶⁸Ga‐NOTA‐F (ab′)₂‐rituximab, as PET‐imaging agents for NHL. Rituximab was digested with immobilized pepsin and papain to yield F (ab′)₂ and Fab fragments respectively that were characterized by size exclusion HPLC (SE‐HPLC) and SDS‐PAGE. They were conjugated with p‐SCN‐Bn‐NOTA, labeled with ⁶⁸Ga and characterized by SE‐HPLC. Intact rituximab was labeled with gallium‐68 for comparison. Specificity of ⁶⁸Ga‐labeled immunoconjugates was ascertained by immunoreactivity and cell binding studies in Raji cells, while biodistribution studies were performed in normal Swiss mice. Gradient SDS‐PAGE under nonreducing condition showed molecular weights of F (ab′)₂‐rituximab and F (ab′)‐rituximab as approximately 100 and 40 Kd, respectively. Radiochemical purity (RCP) of ⁶⁸Ga‐NOTA‐F (ab′)₂‐rituximab and ⁶⁸Ga‐NOTA‐F (ab′)‐rituximab were 98.2 ± 0.5% and 98.8 ± 0.2% respectively with retention times of 17.1 ± 0.1 min and 19.3 ± 0.1 min in SE‐HPLC. ⁶⁸Ga‐labeled rituximab fragments were stable in saline and serum up to 2‐hour post preparation and exhibited specificity to CD20 antigen. Immunoreactivity of ⁶⁸Ga‐labeled immunoconjugates was greater than 80%. Clearance of the fragmented radioimmunoconjugates was predominantly through renal route. Preliminary results from this study demonstrate the potential of ⁶⁸Ga‐ NOTA‐F (ab′)₂‐rituximab and ⁶⁸Ga‐NOTA‐F (ab′)‐rituximab as PET imaging agents for NHL.     

Automated production of [68Ga]Ga‐DOTANOC and [68Ga]Ga‐PSMA‐11 using a TRACERlab FXFN synthesis module

The interest in gallium‐68 labelled positron‐emission tomography probes continues to increase around the world. However, one of the barriers for routine clinical use is the cost of the automated synthesis units for relatively simple labelling procedures. Herein, we describe the adaptation of a TRACERlab FX_FN synthesis module for the automated production of gallium‐68 radiopharmaceuticals using a cation‐exchange cartridge for postprocessing of the ⁶⁸Ge/⁶⁸Ga generator eluate. The recovery of activity from the cartridge was 95.6% to 98.9% using solutions of acidified sodium chloride (5 M with pH = 1‐3). The radiosyntheses of [⁶⁸Ga]Ga‐DOTANOC and [⁶⁸Ga]Ga‐PSMA‐11 were performed using acetate sodium buffer or 4‐(2‐hydroxyethyl)piperazine‐1‐ethanesulfonic acid, with a total duration of 21 and 23 minutes, respectively, including generator elution and radiopharmaceutical dispensing. Activity yields were 77% ± 2% for [⁶⁸Ga]Ga‐PSMA‐11 and 68% ± 3% for [⁶⁸Ga]Ga‐DOTANOC (n > 100). The labelled peptides had a radiochemical purity exceeding 97%, and all quality control parameters were in conformity with the limits prescribed by the European Pharmacopoeia.     

Preclinical evaluation of potential infection‐imaging probe [68Ga]Ga‐DOTA‐K‐A9 in sterile and infectious inflammation

The development of bacteria‐specific infection radiotracers is of considerable interest to improve diagnostic accuracy and enabling therapy monitoring. The aim of this study was to determine if the previously reported radiolabelled 1,4,7,10‐tetraazacyclododecane‐N,N′,N″,N?‐tetraacetic acid (DOTA) conjugated peptide [⁶⁸Ga]Ga‐DOTA‐K‐A9 could detect a staphylococcal infection in vivo and distinguish it from aseptic inflammation. An optimized [⁶⁸Ga]Ga‐DOTA‐K‐A9 synthesis omitting the use of acetone was developed, yielding 93 ± 0.9% radiochemical purity. The in vivo infection binding specificity of [⁶⁸Ga]Ga‐DOTA‐K‐A9 was evaluated by micro positron emission tomography/magnetic resonance imaging of 15 mice with either subcutaneous Staphylococcus aureus infection or turpentine‐induced inflammation and compared with 2‐deoxy‐2‐[¹⁸F]fluoro‐D‐glucose ([¹⁸F]FDG). The scans showed that [⁶⁸Ga]Ga‐DOTA‐K‐A9 accumulated in all the infected mice at injected doses ≥3.6 MBq. However, the tracer was not found to be selective towards infection, since the [⁶⁸Ga]Ga‐DOTA‐K‐A9 also accumulated in mice with inflammation. In a concurrent in vitro binding evaluation performed with a 5‐carboxytetramethylrhodamine (TAMRA) fluorescence analogue of the peptide, TAMRA‐K‐A9, the microscopy results suggested that TAMRA‐K‐A9 bound to an intracellular epitope and therefore preferentially targeted dead bacteria. Thus, the [⁶⁸Ga]Ga‐DOTA‐K‐A9 uptake observed in vivo is presumably a combination of local hyperemia, vascular leakiness and/or binding to an epitope present in dead bacteria.     

An improved assay for 68Ga‐hydroxide in 68Ga‐DOTATATE formulations intended for neuroendocrine tumour imaging

The objective of this study was to identify a more rapid assay for ⁶⁸Ga(OH)₃ impurity in ⁶⁸Ga‐DOTATATE formulations. Three methods were used to prepare ⁶⁸Ga(OH)₃ reference material (pharmacopoeial, bench titration and automated radiosynthesis), and four quality control methods for its assessment (thin layer chromatography, membrane filtration, HPLC and solid phase extraction). The optimal method of preparing ⁶⁸Ga(OH)₃ was by titrating ⁶⁸Ga³⁺ with buffered sodium hydroxide solutions to pH 5.6 ± 0.2. The precipitate was quantitatively isolated by membrane filtration (0.02 µm)/hydrochloric acid (HCl; pH 5.6) solvent, and also it remained 100% at the origin on instant thin layer chromatography with silica gel paper/HCl (pH 5.6) solvent. For ⁶⁸Ga‐DOTATATE samples, the thin layer chromatography technique was used with a single paper strip developed separately on two occasions, once in HCl (pH 5.6) and next in methanol solvent. This so‐called double‐developed (DD) method separated ⁶⁸Ga(OH)₃ impurity located at the origin, from ⁶⁸Ga‐DOTATATE plus ⁶⁸Ga³⁺ at ~R_f 0.4, and it was superior to the other methods. It assayed for the impurity similarly to the pharmacopoeial method. The advantages of the DD method were that it required inexpensive test materials and it reproducibly determined % ⁶⁸Ga(OH)₃ in ⁶⁸Ga‐DOTATATE in 12 min, 13 min earlier than the pharmacopoeial method. This time efficiency resulted in a surplus of 12% ⁶⁸Ga‐DOTATATE counts in the product vial, and this provided a contingency of radioactivity or time for the injection/imaging processes in the Nuclear Medicine Department.     

A simple method for preparation of pure 68Ga‐acetate precursor for formulation of radiopharmaceuticals: Physicochemical characteristics of the 68Ga eluate of the SnO2 based‐68Ge/68Ga column generator

Gallium‐68 radioisotope is an excellent source in clinical positron emission tomography application due to its ease of availability from germanium‐68 (⁶⁸Ge)/gallium‐68 (⁶⁸Ga) generator having a shelf life of 1 year. In this paper, a modified method for purification of the primary eluate of ⁶⁸Ge‐⁶⁸Ga generator by using a small cation exchange resin (Dowex‐50) column has been described. The breakthrough of ⁶⁸Ge before and after purification of ⁶⁸Ga eluate was 0.014% and 0.00027%, respectively. The average recovery yield of ⁶⁸Ga after purification was 84% ± 8.6% (SD, n  = 335). The results of the physiochemical studies confirmed that the ⁶⁸Ga‐acetate obtained is suitable for labeling of radiopharmaceuticals.     

Development of a novel,fibrin‐specific PET tracer

Fibrin deposition is observed in several diseases such as atherosclerosis, deep vein thrombosis, and also tumors, where it contributes to the formation of mature tumor stroma. The aim of this study was to develop a gallium‐labeled peptide tracer on the basis of the fibrin‐targeting peptide Epep for PET imaging of fibrin deposition. For this purpose, the peptide Epep was modified with a NOTA moiety for radiolabeling with ⁶⁷Ga and ⁶⁸Ga and compared with the earlier validated ¹¹¹In‐DOTA‐Epep tracer. In vitro binding assays of ⁶⁷Ga‐NOTA‐Epep displayed an enhanced retention as compared to previously published data showing binding of ¹¹¹In‐DOTA‐Epep to human (84.0 ± 0.6 vs 66.6 ± 1.4 %Dose) and mouse derived fibrin clots (83.5 ± 1.7 vs 74.2 ± 2.4% Dose). In vivo blood kinetics displayed a bi‐phasic elimination profile (t_1/2,_α = 2.6 ± 1.0 minutes and t_1/2,_β = 15.8 ± 1.3 minutes) and ex vivo biodistribution showed low blood values at 4 hours post injection and a low uptake in nontarget tissue (<0.2 %ID/g; kidneys, 1.9%ID/g). In conclusion, taking into account the ease of radiolabeling and the promising in vitro and in vivo studies, gallium‐labeled Epep displays the potential for further development towards a PET tracer for fibrin deposition.     

Investigation for the Interaction of Tyramine‐Based Anthraquinone Analogue with Human Serum Albumin by Optical Spectroscopic Technique

A newly synthesized anthraquinone derivative ‘N‐(2‐methylanthraquinone)‐4‐(2‐aminoethyl) phenol’ (Tyan) were characterized as a fluorophore from photophysical analysis by measuring the UV‐Vis absorptive (λ_ex = 325 nm) and fluorescence emitive (λ_em = 660 nm) values. Density functional theory additionally supported the spectroscopic data by modulation of highest occupied molecular orbital rather than lowest unoccupied molecular orbital due to the affect of tyramine moiety present in Tyan. The pharmacological importance of Tyan was evaluated by molecular docking with human serum albumin. The molecular docking of the Tyan was performed with the crystal structure of human serum albumin (PDB entry 1E78), which shows binding in all the three domains of human serum albumin corresponding to ?7.74 as the GScore. Moreover, the interactions of human serum albumin with Tyan were assessed employing fluorescence spectroscopy under simulative physiological conditions, and the binding constant for the interaction at 25 °C was found to be 0.6 × 10³/m .     

Synthesis and evaluation of 68Ga‐HBED‐CC‐EDBE‐folate for positron‐emission tomography imaging of overexpressed folate receptors on CT26 tumor cells

The ⁶⁸Ga is a positron‐emitting radionuclide that can be combined with bifunctional chelating agents and bioactive substances for use as positron‐emission tomography (PET) diagnostic agents. The HBED‐CC is an acyclic chelating agent that is rapidly labeled with ⁶⁸Ga under mild conditions. To target cancer cells, bioactive substances can be conjugated to the carboxyl terminus of HBED‐CC. Because folic acid strongly binds to folate receptors that are overexpressed on the surfaces of many types of cancer cells, it was coupled with HBED‐CC through a small polyethylene glycol‐based linker (EDBE) to generate an active, receptor‐selective targeting system. The HBED‐CC‐EDBE‐folate (HCEF) precursor was readily labeled with ⁶⁸Ga in 5 minutes at room temperature (98% radiochemical yield; 99% radiochemical purity after isolation). In cellular uptake tests, higher uptakes of ⁶⁸Ga‐HCEF were observed for the CT26 and KB cell lines (which express folate receptors) than for the A549 cell line (which does not). Finally, in vivo micro‐PET measurements over 2 hours of binding in BALB/c mice into which CT26 tumors had been transplanted showed the selective accumulation of ⁶⁸Ga‐HCEF in the folate receptor‐expressing CT26 tumors. These results confirmed the potential of ⁶⁸Ga‐HCEF as a PET diagnostic agent for tumors that express folate receptors.     

Synthesis,radiofluorination and first evaluation of (±)‐[18F]MDL 100907 as serotonin 5‐HT2A receptor antagonist for PET

In some psychiatric disorders 5‐HT_2A receptors play an important role. In order to investigate those in vivo there is an increasing interest in obtaining a metabolically stable, subtype selective and high affinity radioligand for receptor binding studies using positron emission tomography (PET). Combining the excellent in vivo properties of [¹¹C]MDL 100907 for PET imaging of 5‐HT_2A receptors and the more suitable half‐life of fluorine‐18, MDL 100907 was radiofluorinated in four steps using 1‐(2‐bromoethyl)‐4‐[¹⁸F]fluorobenzene as a secondary labelling precursor. The complex reaction required an overall reaction time of 140 min and (±)‐[¹⁸F]MDL 100907 was obtained with a specific activity of at least 30 GBq/µmol (EOS) and an overall radiochemical yield of 1–2%. In order to verify its binding to 5‐HT_2A receptors, in vitro rat brain autoradiography was conducted showing the typical distribution of 5‐HT_2A receptors and a very low non‐specific binding of about 6% in frontal cortex, using ketanserin or spiperone for blocking. Thus, [¹⁸F]MDL 100907 appears to be a promising new 5‐HT_2A PET ligand. Copyright © 2008 John Wiley & Sons, Ltd.     

Synthesis of an 18F‐labelled high affinity β1‐adrenoceptor PET radioligand based on ICI 89,406

To date, some non‐selective β‐adrenoceptor (β‐AR) positron emission tomography (PET) radioligands are in clinical use, but no PET radioligand for the selective imaging of cardiac β₁‐ARs is clinically available. Therefore, the aim of this study was to develop a potential high‐affinity PET radioligand for the β₁‐subtype of ARs. Here, the synthesis and in vitro evaluation of (S)‐ and (R)‐N‐[2‐[3‐(2‐cyano‐phenoxy)‐2‐hydroxy‐propylamino]‐ethyl]‐N′‐[4‐(2‐fluoro‐ethoxy)‐phenyl]‐urea ( 8a–b ), derivatives of the well‐characterized β₁‐AR selective antagonist, ICI 89,406, are described. The (S)‐isomer 8a shows both higher β₁‐AR selectivity and β₁‐AR affinity than the (R)‐enantiomer 8b (selectivity: 40 800 vs 1580; affinity: K_I1=0.049 nM vs K_I1=0.297 nM). Therefore, the ¹⁸F‐labelled analogue 8e of compound 8a was synthesized. While the direct nucleophilic ¹⁸F‐fluorination of the tosylate precursor 8d produced 8e in low radiochemical yields (?2.9% decay‐corrected) and specific activities (?3.5 GBq/µmol at the end of synthesis (EOS), n=9) the alternative two‐step synthesis of 8e from ethylene glycol di‐p‐tosylate 9 , [¹⁸F]fluoride ion and phenol precursor 8f gave satisfying results (16.4±3.2% radiochemical yield (decay‐corrected), 99.7±0.5% radiochemical purity, 40±8 GBq/µmol specific activity at the EOS within 174±3 min from the end of bombardment (EOB) (n=5)). Copyright © 2006 John Wiley & Sons, Ltd.     

Detailed evaluation of different 68Ge/68Ga generators: an attempt toward achieving efficient 68Ga radiopharmacy

The present study is aimed at carrying out a comparative performance evaluation of different types of ⁶⁸Ge/⁶⁸Ga generators to identify the best choice for use in ⁶⁸Ga‐radiopharmacy. Over the 1 year period of evaluation, the elution yields from the CeO₂‐based and SiO₂‐based ⁶⁸Ge/⁶⁸Ga generators remained almost consistent, in contrast to the sharp decrease observed in the elution yields from TiO₂ and SnO₂‐based generators. The level of ⁶⁸Ge impurity in ⁶⁸Ga eluates from the CeO₂ and SiO₂‐based ⁶⁸Ge/⁶⁸Ga generator was always <10^?3%, while this level increased from 10^?3% to 10^?1% in case of TiO₂ and SnO₂‐based generators. The level of chemical impurities in ⁶⁸Ga eluates from CeO₂ and SiO₂‐based ⁶⁸Ge/⁶⁸Ga generators was negligibly low (<0.1 ppm) in contrast to the significantly higher level (1–20 ppm) of such impurities in eluates from other two generators. As demonstrated by radiolabeling studies carried out using DOTA‐coupled dimeric cyclic RGD peptide derivative (DOTA‐RGD₂), CeO₂‐PAN and SiO₂‐based generators are directly amenable for radiopharmaceutical preparation, whereas the other generators can be only used after post‐elution purification of ⁶⁸Ga eluates. Clinically relevant dose of ⁶⁸Ga‐DOTA‐RGD₂ was prepared in a hospital radiopharmacy for non‐invasive visualization of tumors in breast cancer patients using positron emission tomography.     

Synthesis and biological evaluation of 68Ga‐labeled Pteroyl‐Lys conjugates for folate receptor‐targeted tumor imaging

In order to develop novel ⁶⁸Ga‐labeled PET tracers for folate receptor imaging, two DOTA‐conjugated Pteroyl‐Lys derivatives, Pteroyl‐Lys‐DOTA and Pteroyl‐Lys‐DAV‐DOTA, were designed, synthesized and radiolabeled with ⁶⁸Ga. Biological evaluations of the two radiotracers were performed with FR‐positive KB cell line and athymic nude mice bearing KB tumors. Both ⁶⁸Ga‐DOTA‐Lys‐Pteroyl and ⁶⁸Ga‐DOTA‐DAV‐Lys‐Pteroyl exhibited receptor specific binding in KB cells in vitro. The tumor uptake values of ⁶⁸Ga‐DOTA‐Lys‐Pteroyl and ⁶⁸Ga‐DOTA‐DAV‐Lys‐Pteroy were 10.06 ± 0.59%ID/g and 11.05 ± 0.60%ID/g at 2 h post‐injection, respectively. Flank KB tumor was clearly visualized with ⁶⁸Ga‐DOTA‐DAV‐Lys‐Pteroyl by Micro‐PET imaging at 2 h post‐injection, suggesting the feasibility of using ⁶⁸Ga‐labeled Pteroyl‐Lys conjugates as a novel class of FR targeted probes.     

Synthesis,radiolabeling, and evaluation of gastrin releasing peptide receptor antagonist 68Ga‐HBED‐CC‐RM26

The acyclic chelator HBED‐CC has attained huge clinical significance owing to high thermodynamic and kinetic stability of ⁶⁸Ga‐HBED‐CC chelate. It provides an excellent platform for quick preparation of ⁶⁸Ga‐based radiotracers in high yield. Thus, the present study aimed at conjugation of gastrin releasing peptide receptor (GRPr) antagonist, RM26, with HBED‐CC chelator for ⁶⁸Ga‐labeling. In vitro and vivo behavior of the peptide tracer, ⁶⁸Ga‐HBED‐CC‐PEG₂‐RM26, was assessed and compared with ⁶⁸Ga‐NODAGA‐PEG₂‐RM26. The peptide tracers, ⁶⁸Ga‐HBED‐CC‐PEG₂‐RM26 and ⁶⁸Ga‐NODAGA‐PEG₂‐RM26, prepared either by wet chemistry or formulated using freeze‐dried kits exhibited excellent radiochemical yield and in vitro stability. The two peptide tracers cleared rapidly from the blood. Biodistribution studies in normal mice demonstrated slightly higher or comparable uptake of ⁶⁸Ga‐HBED‐CC‐PEG₂‐RM26 in GRPr‐expressing organs pancreas, stomach, and intestine. The preliminary studies suggest high potential of ⁶⁸Ga‐HBED‐CC‐PEG₂‐RM26 for further investigation as a GRPr imaging agent and the wide scope of HBED‐CC chelator in development of ⁶⁸Ga‐based peptide tracers.     

Radiopharmaceutical chemistry for positron emission tomography

Molecular imaging is an emerging technology that allows the visualization of interactions between molecular probes and biological targets. Molecules that either direct or are subject to homeostatic controls in biological systems could be labeled with the appropriate radioisotopes for the quantitative measurement of selected molecular interactions during normal tissue homeostasis and again after perturbations of the normal state. In particular, positron emission tomography (PET) offers picomolar sensitivity and is a fully translational technique that requires specific probes radiolabeled with a usually short-lived positron-emitting radionuclide. PET has provided the capability of measuring biological processes at the molecular and metabolic levels in vivo by the detection of the gamma rays formed as a result of the annihilation of the positrons emitted. Despite the great wealth of information that such probes can provide, the potential of PET strongly depends on the availability of suitable PET radiotracers. However, the development of new imaging probes for PET is far from trivial and radiochemistry is a major limiting factor for the field of PET. In this review, we provided an overview of the most common chemical approaches for the synthesis of PET-labeled molecules and highlighted the most recent developments and trends. The discussed PET radionuclides include ¹¹C (t_1/2 = 20.4 min), ¹³N (t_1/2 = 9.9 min), ¹⁵O (t_1/2 = 2 min), ⁶⁸Ga (t_1/2 = 68 min), ¹⁸F (t_1/2 = 109.8 min), ⁶⁴Cu (t_1/2 = 12.7 h), and ¹²⁴I (t_1/2 = 4.12 d).     

Synthesis,labelling and first evaluation of [18F]R91150 as a serotonin 5‐HT2A receptor antagonist for PET

In psychiatric disorders such as anxiety, depression and schizophrenia, 5‐HT_2A receptors play an important role. In order to investigate them in vivo there is an increasing interest in selective and high‐affinity radioligands for receptor binding studies using positron emission tomography (PET). Since available radioligands have disadvantages, R91150, which is a selective and high‐affinity ligand for 5‐HT_2A receptors, was labelled with fluorine‐18. This was accomplished in six steps via 4‐[¹⁸F]fluorophenol and 1‐(3‐bromopropoxy)‐4‐[¹⁸F]fluorobenzene within 190 min starting from no‐carrier‐added [¹⁸F]fluoride. The overall radiochemical yield was 3.8±2% and the specific activity was at least 335 GBq/µmol at the end of the synthesis. First ex vivo studies in mice proved the uptake of [¹⁸F]R91150 in the brain. Radiometabolite studies revealed no radiometabolites in the brain, whereas in the plasma at least two could be detected 30 min p.i. Further preclinical studies are encouraged to evaluate the potential of this new 5‐HT_2A ligand as a radiotracer for PET. Copyright © 2008 John Wiley & Sons, Ltd.     

Functional reversal of (−)‐Stepholidine analogues by replacement of benzazepine substructure using the ring‐expansion strategy

(?)‐Stepholidine is an active ingredient of the Chinese herb Stephania and naturally occurring tetrahydroprotoberberine alkaloid with mixed dopamine receptor D₁ agonistic and dopamine receptor D₂ antagonistic activities. In this work, a series of novel hexahydrobenzo[4,5]azepino [2,1‐a]isoquinolines were designed and synthesized as ring‐expanded analogues of (?)‐Stepholidine. Initial pharmacological assays demonstrated that a benzazepine replacement was associated with significant increase in selectivity and functional reversal at dopamine receptor D₁. Compound‐(?)‐ 15e (K_i = 5.32 ± 0.01 nm ) is more potent than (?)‐Stepholidine (K_i = 13 nm ) and was identified as a selective dopamine receptor D₁ antagonist (IC₅₀ = 0.14 μm ). Moreover, molecular modeling suggested that (?)‐ 15e might exert its dopamine receptor D₁ antagonistic activities through interacting with the transmembrane helix 7 of dopamine receptor D₁.