Manufacturing 6-[18F]Fluoro-L-DOPA via Flow Chemistry-Enhanced Photoredox Radiofluorination.

In this study, we introduce a practical methodology for the synthesis of PET probes by seamlessly combining flow chemistry with photoredox radiofluorination. The clinical PET tracer 6-[18F]FDOPA was smoothly prepared in a 24.3% non-decay-corrected yield with over 99.0% radiochemical purity (RCP) and enantiomeric excess (ee), notably by a simple cartridge-based purification. The flow chemistry-enhanced photolabeling method supplies an efficient and versatile solution for the synthesis of 6-[18F]FDOPA and for more PET tracer development.

Surface modification by poly(ethylene glycol) with different end-grafted groups: Experimental and theoretical study.

Dihydroxyphenylalanine (DOPA) is extensively reported to be a surface-independent anchor molecule in bioadhesive surface modification and antifouling biomaterial fabrication. However, the mechanisms of DOPA adsorption on versatile substrates and the comparison between experimental results and theoretical results are less addressed. We report the adsorption of DOPA anchored monomethoxy poly(ethylene glycol) (DOPA-mPEG) on substrates and surface wettability as well as antifouling property in comparison with thiol and hydroxyl anchored mPEG (mPEG-SH and mPEG-OH). Gold and hydroxylated silicon were used as model substrates to study the adsorptions of mPEGs. The experimental results showed that the DOPA-mPEG showed higher affinity to both gold and silicon wafers, and the DOPA-mPEG modified surfaces had higher resistance to protein adsorption than those of mPEG-SH and mPEG-OH. It is revealed that the surface wettability is primary for surface fouling, while polymer flexibility is the secondary parameter. We present ab initio calculations of the adsorption of mEGs with different end-functionalities on Au and hydroxylated silicon wafer (Si-OH), where the binding energies are obtained. It is established that monomethoxy ethylene glycol (mEG) with DOPA terminal DOPA-mEG is clearly favored for the adsorption with both gold and Si-OH surfaces due to the bidentate Au-O interactions and the bidentate O-H bond interactions, in agreement with experimental evidence.

Nucleophilic Synthesis of 6-l-[18F]FDOPA. Is Copper-Mediated Radiofluorination the Answer?

Positron emission tomography employing 6-l-[18F]fluoro-3,4-dihydroxyphenylalanine (6-l-[18F]FDOPA) is currently a highly relevant clinical tool for detection of gliomas, neuroendocrine tumors and evaluation of Parkinson's disease progression. Yet, the deficiencies of electrophilic synthesis of 6-l-[18F]FDOPA hold back its wider use. To fulfill growing clinical demands for this radiotracer, novel synthetic strategies via direct nucleophilic 18F-radiloabeling starting from multi-Curie amounts of [18F]fluoride, have been recently introduced. In particular, Cu-mediated radiofluorination of arylpinacol boronates and arylstannanes show significant promise for introduction into clinical practice. In this short review these current developments will be discussed with a focus on their applicability to automation.

Synthesis of high-molar-activity [18F]6-fluoro-L-DOPA suitable for human use via Cu-mediated fluorination of a BPin precursor.

[18F]6-fluoro-L-DOPA ([18F]FDOPA) is a diagnostic radiopharmaceutical for positron emission tomography (PET) imaging that is used to image Parkinson's disease, brain tumors, and focal hyperinsulinism of infancy. Despite these important applications, [18F]FDOPA PET remains underutilized because of synthetic challenges associated with accessing the radiotracer for clinical use; these stem from the need to radiofluorinate a highly electron-rich catechol ring in the presence of an amino acid. To address this longstanding challenge in the PET radiochemistry community, we have developed a one-pot, two-step synthesis of high-molar-activity [18F]FDOPA by Cu-mediated fluorination of a pinacol boronate (BPin) precursor. The method is fully automated, has been validated to work well at two separate sites (an academic facility with a cyclotron on site and an industry lab purchasing [18F]fluoride from an outside vendor), and provides [18F]FDOPA in reasonable radiochemical yield (2.44 ± 0.70 GBq, 66 ± 19 mCi, 5 ± 1%), excellent radiochemical purity (>98%) and high molar activity (76 ± 30 TBq/mmol, 2,050 ± 804 Ci/mmol), n = 26. Herein we report a detailed protocol for the synthesis of [18F]FDOPA that has been successfully implemented at two sites and validated for production of the radiotracer for human use.

Properties of Electropolymerized Dopamine and Its Analogues.

This work reports a study of electropolymerization kinetics, film thickness, stability, and antifouling properties of polydopamine (PDA) and its three analogues: poly(3-(3,4-dihydroxyphenyl)-l-alanine) (PL-DOPA), poly(5-hydroxytryptophan) (PL-5-HTP), and poly(Adrenalin) (PAdrenalin). It was observed that the number of the hydroxyl groups on the benzene ring and the type (primary vs secondary) of amine group significantly affect the electropolymerization kinetics and thus the thickness of the obtained polymer films. Monomers with two hydroxyl groups (except Adrenalin) resulted in films that were thicker (∼10-15 nm) than the one with only one hydroxyl group (PL-5-HTP) (∼5-8 nm) under similar conditions. Adrenalin containing a secondary amino group could not be deposited onto the ITO substrate, while the other three compounds containing a primary amino group completely covered the ITO. The PDA films had better electrochemical stability than the other films. No film showed stable antifouling surfaces against protein.

Understanding the Role of Aggregation in the Broad Absorption Bands of Eumelanin.

In this work, we investigate the relationship between the complex hierarchical assembly structure of eumelanin, its characteristic broad absorption band, and the highly unusual nonlinear dynamics revealed by pump-probe or transient absorption microscopy. Melanin-like nanoparticles (MelNPs), generated by spontaneous oxidation of dopamine, were created with uniform but adjustable size distributions, and kinetically controlled oxidation was probed with a wide range of characterization methods. This lets us explore the broad absorption bands of eumelanin models at different assembly levels, such as small subunit fractions (single monomeric and oligomeric units and small oligomer stacks), stacked oligomer fractions (protomolecules), and large-scale aggregates of protomolecules (parental particles). Both the absorption and pump-probe dynamics are very sensitive to these structural differences or to the size of intact particles (a surprising result for an organic polymer). We show that the geometric packing order of protomolecules in long-range aggregation is key secondary interactions to extend the absorption band of eumelanin to the low energy spectrum and produce drastic changes in the transient absorption spectrum.

Melanin Precursor Influence on Structural Colors from Artificial Melanin Particles: PolyDOPA, Polydopamine, and Polynorepinephrine.

Polydopamine (PDA) is of interest as a mimetic material of melanin to produce structural color materials. Herein, to investigate the influence of the material composition of the artificial melanin particles on structural color, we demonstrated the preparation of core-shell particles by polymerization of norepinephrine or 3,4-dihydroxyphenylalanine, which are melanin precursors similar to dopamine, in the presence of polystyrene particles. It was revealed that the arrays of the prepared particles produce high-visibility structural color because of absorption of scattering light. Although poly(3,4-dihydroxyphenylalanine) showed the same tendency as PDA which was previous studied, polynorepinephrine can easily produce a smooth and thick shell layer compared with that of PDA, and pellets consisting of the particles showed angle-dependent structural color. Therefore, the artificial melanin particles that produce angle-dependent structural color became stable than ever before. These results indicated that material compositions of artificial melanin particles have influence on structural color, and an important finding for application as a coloring material was obtained.

Evaluation of two nucleophilic syntheses routes for the automated synthesis of 6-[18F]fluoro-l-DOPA.

Two different strategies for the nucleophilic radiosynthesis of [18F]F-DOPA were evaluated regarding their applicability for an automated routine production on an Ecker&Ziegler Modular-Lab Standard module. Initially, we evaluated a promising 5-step synthesis based on a chiral, cinchonidine-derived phase-transfer catalyst (cPTC) being described to give the product in high radiochemical yields (RCY), high specific activities (AS) and high enantiomeric excesses (ee). However, the radiosynthesis of [18F]F-DOPA based on this strategy showed to be highly complex, giving the intermediate products as well as the final product in insufficient yields for automatization. Furthermore, the automatization proved to be problematic due to incomplete radiochemical conversions and the formation of precipitates during the enantioselective reaction step. Furthermore, the required use of HI at 180°C during the last reaction step led to partial decomposition of lines and seals of the module which further counteracts an automatization. Further on, we evaluated a 3-step synthesis using the commercially available, enantiomerically pure precursor AB1336 for automatization. This synthesis approach gave much better results and [18F]F-DOPA could be produced fully automated within 114min in RCYs of 20±1%, ee of >96%, a radiochemical purity (RCP) of >98% and specific activities of up to 2.2GBq/µmol.

Synthesis of ß-Hydroxytyrosine, A Component of Burkholdines.

The preparation of four stereoisomers of ß-hydroxytyrosine containing burkholdines is described. Enantio-pure syn ß-hydroxytyrosine was synthesized using Sharpless aminohydroxylation. To obtain anti ß-hydroxytyrosine, the cinnamate derivative was oxidized to give the optical active diol derivative by the AD-mix and subsequently, the a-hydroxy group was converted to amine. Deprotection of the acid-sensitive ß-hydroxytyrosine derivatives was successively accomplished by brief immersion in 4N HCl/dioxane. All prepared stereoisomers of ß-hydroxytyrosine were available for solid and solution phase peptide synthesis and amino acid analysis.

Construction of Compact Polyelectrolyte Multilayers Inspired by Marine Mussel: Effects of Salt Concentration and pH As Observed by QCM-D and AFM.

Biomimetic multilayers based on layer-by-layer (LbL) assembly were prepared as functional films with compact structure by incorporating the mussel-inspired catechol cross-linking. Dopamine-modified poly(acrylic acid) (PAADopa) was synthesized as a polyanion to offer electrostatic interaction with the prelayer polyethylenimine (PEI) and consecutively cross-linked by zinc to generate compact multilayers with tunable physicochemical properties. In situ layer-by-layer growth and cross-linking were monitored by a quartz crystal microbalance with dissipation (QCM-D) to reveal the kinetics of the process and the influence of Dopa chemistry. Addition of Dopa enhanced the mass adsorption and led to the formation of a more compact structure. An increase of ionic strength induced an increase in mass adsorption in the Dopa-cross-linked multilayers. This is a universal approach for coating of various surfaces such as Au, SiO2, Ti, and Al2O3. Roughness observed by AFM in both wet and dry conditions was compared to confirm the compact morphology of Dopa-cross-linked multilayers. Because of the pH sensitivity of Dopa moiety, metal-chelated Dopa groups can be turned into softer structure at higher pH as revealed by reduction of Young's modulus determined by MFP-3D AFM. A deeper insight into the growth and mechanical properties of Dopa-cross-linked polyelectrolyte multilayers was addressed in the present study. This allows a better control of these systems for bioapplications.

Automated production at the curie level of no-carrier-added 6-[(18)F]fluoro-L-dopa and 2-[(18)F]fluoro-L-tyrosine on a FASTlab synthesizer.

An efficient, fully automated, enantioselective multi-step synthesis of no-carrier-added (nca) 6-[(18)F]fluoro-L-dopa ([(18)F]FDOPA) and 2-[(18)F]fluoro-L-tyrosine ([(18)F]FTYR) on a GE FASTlab synthesizer in conjunction with an additional high- performance liquid chromatography (HPLC) purification has been developed. A PTC (phase-transfer catalyst) strategy was used to synthesize these two important radiopharmaceuticals. According to recent chemistry improvements, automation of the whole process was implemented in a commercially available GE FASTlab module, with slight hardware modification using single use cassettes and stand-alone HPLC. [(18)F]FDOPA and [(18)F]FTYR were produced in 36.3 ± 3.0% (n = 8) and 50.5 ± 2.7% (n = 10) FASTlab radiochemical yield (decay corrected). The automated radiosynthesis on the FASTlab module requires about 52 min. Total synthesis time including HPLC purification and formulation was about 62 min. Enantiomeric excesses for these two aromatic amino acids were always >95%, and the specific activity of was >740 GBq/µmol. This automated synthesis provides high amount of [(18)F]FDOPA and [(18)F]FTYR (>37 GBq end of synthesis (EOS)). The process, fully adaptable for reliable production across multiple PET sites, could be readily implemented into a clinical good manufacturing process (GMP) environment.

[(18)F]6-fluoro-3,4-dihydroxy-L-phenylalanine--recent modern syntheses for an elusive radiotracer.

[(18)F]6-fluoro-3,4-dihydroxy-L-phenylalanine ([(18)F]F-DOPA) has been known to be a useful radiotracer for over 30 years. Its widespread clinical use has been hampered by the lack of a robust, high yielding synthesis. This review summarises new developments in radiochemistry that are providing solutions to long standing problems involved in the synthesis of this important but elusive radiotracer. Considerable advances in nucleophilic synthesis have been achieved by optimising multistep strategies and using both hypervalent iodine chemistry and transition metal-mediated fluorinations allowing for the production of high specific activity [(18)F]F-DOPA.

6-[18F]fluoro-L-DOPA: a well-established neurotracer with expanding application spectrum and strongly improved radiosyntheses.

For many years, the main application of [(18)F]F-DOPA has been the PET imaging of neuropsychiatric diseases, movement disorders, and brain malignancies. Recent findings however point to very favorable results of this tracer for the imaging of other malignant diseases such as neuroendocrine tumors, pheochromocytoma, and pancreatic adenocarcinoma expanding its application spectrum. With the application of this tracer in neuroendocrine tumor imaging, improved radiosyntheses have been developed. Among these, the no-carrier-added nucleophilic introduction of fluorine-18, especially, has gained increasing attention as it gives [(18)F]F-DOPA in higher specific activities and shorter reaction times by less intricate synthesis protocols. The nucleophilic syntheses which were developed recently are able to provide [(18)F]F-DOPA by automated syntheses in very high specific activities, radiochemical yields, and enantiomeric purities. This review summarizes the developments in the field of [(18)F]F-DOPA syntheses using electrophilic synthesis pathways as well as recent developments of nucleophilic syntheses of [(18)F]F-DOPA and compares the different synthesis strategies regarding the accessibility and applicability of the products for human in vivo PET tumor imaging.

Endogenous compounds labeled with radionuclides of short half-life-some perspectives.

In the article, the strategy and synthesis of some endogenous compounds labeled mainly with (11) C are presented. There are some examples illustrating how endogenous labeled compounds in connection with positron emission tomography have unique properties to describe various biological processes, and a few examples of the use of tracers labeled with (13) N and (15) O are also discussed. Labeled endogenous compounds may be an important asset to describe the conditions and the status of biological systems and might therefore be a key for the future search of individualized medicine.

Production at the Curie level of no-carrier-added 6-18F-fluoro-L-dopa.

UNLABELLED: 6-(18)F-fluoro-L-dopa ((18)F-FDOPA) has proven to be a useful radiopharmaceutical for the evaluation of presynaptic dopaminergic function using PET. In comparison to electrophilic synthesis, the no-carrier-added (NCA) nucleophilic method has several advantages. These include much higher available activity and specific activity. Recently, we have described an NCA enantioselective synthesis using a chiral phase-transfer catalyst. However, some chemicals were difficult to implement into a commercially available synthesizer, restricting access to this radiopharmaceutical to only a few PET centers. METHODS: In this paper, 2 important chemical improvements are proposed to simplify production of (18)F-FDOPA, resulting in straightforward automation of the synthesis in a commercially available module. RESULTS: First, a fast, simple, and reliable synthesis of 2-(18)F-fluoro-4,5-dimethoxybenzyl iodide on a solid-phase support was developed. Second, a phase-transfer catalyst alkylation of a glycine derivative at room temperature was used to enable enantioselective carbon-carbon bond formation. After hydrolysis and high-performance liquid chromatography purification, a high enantiomeric excess of (18)F-FDOPA (≈ 97%) was obtained using a chiral catalyst available from a biphenyl 3 substrate. The total synthesis time was 63 min, and the decay-corrected radiochemical yield was 36% ± 3% (n = 8). CONCLUSION: By exploiting the advantages of this NCA approach, using a starting activity of 185 GBq of NCA (18)F-fluoride, high activities of (18)F-FDOPA (>45 GBq) with high specific activity (≥ 753 GBq/µmol) are now available at the end of synthesis for use in clinical investigations.

S(N)1-type substitution reactions of N-protected ß-hydroxytyrosine esters: stereoselective synthesis of ß-aryl and ß-alkyltyrosines.

The title compounds were prepared by aldol reaction of anisaldehyde and the respective N,N-dibenzyl glycinates. Deprotection of the nitrogen atom with Pearlman's catalyst delivered the unprotected ß-hydroxytyrosine esters, which were further N-protected as N,N-phthaloyl (Phth) and N-fluorenylmethylcarbonyloxy (Fmoc) derivatives. The Friedel-Crafts reaction with various arenes was studied employing these alcohols as electrophiles. It turned out that the facial diastereoselectivitiy depends on the nitrogen protecting group and on the ester group. The unprotected substrates (NH(2)) gave preferentially syn-products but the anti-selectivity increased when going from NHFmoc over NPhth to NBn(2). If the ester substituent was varied the syn-preference increased in the order Me < Et < iPr. The reactions were shown to be fully stereoconvergent and proceeded under kinetic product control. A model is suggested to explain the facial diastereoselectivity based on a conformationally locked benzylic cation intermediate. The reactions are preparatively useful for the N-unprotected isopropyl ester, which gave Friedel-Crafts alkylation products with good syn-selectivity (anti/syn=21:79 to 7:93), and for the N,N-dibenzyl-protected methyl ester, which led preferentially to anti-products (anti/syn=80:20 to >95:5). Upon acetylation of the latter compound to the respective acetate, Bi(OTf)(3) -catalyzed alkylation reactions became possible, in which silyl enol ethers served as nucleophiles. The respective alkylation products were obtained in high yield and with excellent anti-selectivitiy (anti/syn≥95:5).

Adhesive performance of biomimetic adhesive-coated biologic scaffolds.

Surgical repair of a discontinuity in traumatized or degenerated soft tissues is traditionally accomplished using sutures. A current trend is to reinforce this primary repair with surgical grafts, meshes, or patches secured with perforating mechanical devices (i.e., sutures, staples, or tacks). These fixation methods frequently lead to chronic pain and mesh detachment. We developed a series of biodegradable adhesive polymers that are synthetic mimics of mussel adhesive proteins (MAPs), composed of 3,4-dihydroxyphenylalanine (DOPA)-derivatives, polyethylene glycol (PEG), and polycaprolactone (PCL). These polymers can be cast into films, and their mechanical properties, extent of swelling, and degradation rate can be tailored through the composition of the polymers as well as blending with additives. When coated onto a biologic mesh used for hernia repair, these adhesive constructs demonstrated adhesive strengths significantly higher than fibrin glue. With further development, a precoated bioadhesive mesh may represent a new surgical option for soft tissue repair.

Clinical value of 18-fluorine-fluorodihydroxyphenylalanine positron emission tomography/computed tomography in the follow-up of medullary thyroid carcinoma.

BACKGROUND: 18-Fluorine-fluorodihydroxyphenylalanine positron emission tomography ((18)F-DOPA PET) is a sensitive method for detecting medullary thyroid carcinoma (MTC). The advent of PET/computed tomography (CT) has enabled more sensitive and specific lesion identification using various tracers in many other tumors. The aim of this study was therefore to test the hypothesis that combined (18)F-DOPA PET/CT more accurately detects MTC lesions than each modality does alone. METHODS: Twenty-eight consecutive (18)F-DOPA PET, CT, and (18)F-DOPA PET/CT scans of patients followed up for sporadic MTC or multiple endocrine neoplasia 2 syndrome-associated MTC were reviewed retrospectively in randomized sequence by two blinded readers, one a nuclear medicine physician and the other a radiologist, with extensive experience interpreting such images. RESULTS: Of 18 lesions detected concurrently by the three modalities, PET identified all as positive for MTC, but was unable to definitively localize 4 (22%) lesions. CT could definitively localize all 18 lesions, but could not definitively diagnose or exclude MTC in 6 (33%) lesions. Further, CT falsely identified as MTC-negative one lesion that was judged to be MTC-positive by both PET and PET/CT. Only PET/CT scans accurately characterized and localized all 18 lesions. On a per patient basis, the sensitivity of (18)F-DOPA PET/CT for MTC was 74% and the specificity, 100%. In the present series, no truly MTC-positive (18)F-DOPA PET/CT case was found in patients with basal human calcitonin (hCt) levels under 60 pg/mL, and conversely, no truly MTC-negative PET/CT case was found in patients with basal hCt over 120 pg/mL. Between hCt concentrations of 60 and 150 pg/mL true-negative, false-negative, and true-positive scans all were obtained. (18)F-DOPA PET/CT had 100% sensitivity and specificity when hCt at the time of scanning was over 150 pg/mL, the threshold at which the 2009 American Thyroid Association guidelines recommend performing additional imaging including (18)F-DOPA PET/CT. CONCLUSIONS: (18)F-DOPA PET/CT allows for a more accurate diagnosis and localization of MTC lesions than do (18)F-DOPA PET or CT alone. Supporting the recent American Thyroid Association recommendations on additional imaging in MTC, (18)F-DOPA PET/CT appears to have 100% sensitivity in patients with hCt over 150 pg/mL.

Three-step, "one-pot" radiosynthesis of 6-fluoro-3,4-dihydroxy-L-phenylalanine by isotopic exchange.

UNLABELLED: The (18)F-labeled aromatic amino acid 6-fluoro-3,4-dihydroxy-L-phenylalanine (6-(18)F-fluoro-L-DOPA) is widely used as a radiopharmaceutical in neurologic and oncologic PET. In this study, a novel approach to the preparation of carrier-added (CA) 6-(18)F-fluoro-L-DOPA in 3 radiosynthesis steps was developed and evaluated; in this approach, direct nucleophilic (18)F fluorination of a protected amino acid derivative was used. The method currently used for the routine preparation of 6-(18)F-fluoro-L-DOPA by electrophilic labeling is limited to the production of small amounts of activity at high costs. Alternative syntheses based on the advantage of large-scale production of nucleophilic (18)F-fluoride, however, either have resulted in insufficient enantiomeric purity or are difficult to automate because of the complexity of the necessary multiple steps. METHODS: An isotopic exchange reaction on the precursor (2S,5S)-tert-butyl-5-(4-benzyloxy-2-fluoro-5-formylbenzyl)-2-tert-butyl-3-methyl-4-oxoimidazolidine-1-carboxylate was used. The formyl group served as the activating group in the (18)F-for-(19)F exchange with tetrabutylammonium bicarbonate for anion activation in N,N-dimethylformamide. The intermediate was converted to a hydroxy group by Baeyer-Villiger oxidation with meta-chloroperbenzoic acid. After final deprotection with hydrobromic acid, CA 6-(18)F-fluoro-L-DOPA was isolated by high-performance liquid chromatography. RESULTS: The precursor was obtained by an 11-step organic synthesis. The optimized isotopic (18)F exchange proceeded with a radiochemical yield of about 50%. The complete preparation and isolation of CA 6-(18)F-fluoro-L-DOPA thus far are possible with a radiochemical yield of about 22%, within a synthesis time of 105 min, and at a much higher specific activity than with the electrophilic method. The enantiomeric excess of the desired L-isomer was greater than 96%. CONCLUSION: The pathway to 6-(18)F-fluoro-L-DOPA by isotopic exchange not only is more efficient but also is suited to automation as a "one-pot" procedure.

Automated synthesis of n.c.a. [18F]FDOPA via nucleophilic aromatic substitution with [18F]fluoride.

An improved, automated synthesis of [(18)F]FDOPA including four synthetic steps (fluorination, reductive iodination, alkylation and hydrolysis) is reported with each step optimized individually. In a home-made automatic synthesizer, 9064+/-3076 MBq of [(18)F]FDOPA were produced within 120 min from EOB (n=5). Radiochemical purity and enantiomeric excess were both >or= 95%. Specific activity was ca. 50 GBq/micromol at EOS. This automatically operable synthesis is well suited for the multi-patient-dose routine production of n.c.a. [(18)F]FDOPA.