Stereocontrolled [11C]Alkylation of N-Terminal Glycine Schiff Bases To Obtain Dipeptides

The use of various quaternary ammonium salts as chiral phase-transfer catalysts allowed effective and stereoselective radiochemical [11C]alkylation to obtain functionalized dipeptides. We herein report a broadly applicable procedure for the asymmetric [11C]alkylation of dipeptides to give labeled N-terminal peptides by using different [11C]alkyl halides. Contended stereoselectivities of the reactions were observed by using 11C-labeled alkyl halides, [11C]methyl iodide and [11C]benzyl iodide, and diastereomeric ratios with different specialized catalysts of 95:5 and 90:10 were achieved, respectively. Accordingly, the straightforward synthesis of enantioenriched compounds should play a vital role in peptide-based radiopharmaceutical development and positron emission tomography imaging

From Carbon-11-Labeled Amino Acids to Peptides in Positron Emission Tomography: the Synthesis and Clinical Application

Radiolabeled amino acids, their derivatives and peptides have a broad scope of application and can be used as receptor ligands, as well as enzyme substrates for many different diseases as radiopharmaceutical tracers. Over the past few decades, the application of molecular imaging techniques such as positron emission tomography (PET) has gained considerable importance and significance in diagnosis in today’s advanced health care. Next to that, the availability of cyclotrons and state-of-the-art radiochemistry facilities has progressed the production of imaging agents enabling the preparation of many versatile PET radiotracers. Due to many favorable characteristics of radiolabeled amino acids and peptides, they can be used for tumor staging and monitoring the progress of therapy success, while aromatic amino acids can be employed as PET tracer to study neurological disorders. This review provides a comprehensive overview of radiosynthetic and enzymatic approaches towards carbon-11 amino acids, their analogues and peptides, with focus on stereoselective reactions, and reflects upon their clinical application

A rapid and highly enantioselective C-11C bond formation of l-[11C]phenylalanine via chiral phase-transfer catalysis

A rapid method for the synthesis of carbon-11 radiolabeled phenylalanine was developed using a chiral phase-transfer catalyst and a sub-nanomolar quantity of [11C]benzyl iodide as a radio-precursor. Based on a reported synthesis of [11C]benzyl iodide, a Schiff base precursor was evaluated for stereoselective [11C]benzylation. Extensive and interactive screening of the precursor, catalyst, base, stirring and temperature was required to achieve high stereoinduction. The result is an efficient 5-step radiolabeling method to reliably synthesize l- or d-[11C]phenylalanine with an excellent enantiomeric excess of >90% and almost quantitative radiochemical conversion of >95% (n > 5). Additionally, a phase-transfer catalyzed alkylation was utilized on the preparative scale using automated platform. The application resulted in high specific activity ranging from 85-135 GBq μmol−1 of the enantiomerically pure [11C]phenylalanine, showing that the process is robust and amenable to broad use in PET

Stereoselective 11C Labeling of a “Native” Tetrapeptide by Using Asymmetric Phase-Transfer Catalyzed Alkylation Reactions

The first 11C-labeled unmodified (“native”) peptide is described by alkylation of a tetrapeptide Schiff base, which was achieved by an automated five-step radiochemical reaction. In a proof-of-concept study, [11C]Phe-d-Trp-Lys-Thr was synthesized. This tetrapeptide is the essential pharmacophore of octreotide, an antagonist of somatostatin receptors. The asymmetric alkylation with chiral phase-transfer catalysts enabled direct labeling of a variety of isolated 11C-peptides in a highly stereoselective manner (94 % de) with acceptable radiochemical yields (9–10 %) and practical specific activities (15–35 GBq µmol–1 or 405–945 mCi µmol–1) at the end of synthesis. This novel methodology provides a powerful new radiosynthetic method to access novel, stereochemically pure carbon-11-labeled native small peptides ready for in vivo studies

Radiosynthesis of 1-iodo-2-[11C]methylpropane and 2-methyl-1-[11C]propanol and its application for alkylation reactions and C―C bond formation

The multitude of biologically active compounds requires the availability of a broad spectrum of radiolabeled synthons for the development of positron emission tomography (PET) tracers. The aim of this study was to synthesize 1-iodo-2-[11C]methylpropane and 2-methyl-1-[11C]propanol and investigate the use of these reagents in further radiosynthesis reactions. 2-Methyl-1-[11C]propanol was obtained with an average radiochemical yield of 46 ± 6% d.c. and used with fluorobenzene as starting material. High conversion rates of 85 ± 4% d.c. could be observed with HPLC, but large precursor amounts (32 mg, 333 μmol) were needed. 1-Iodo-2-[11C]methylpropane was synthesized with a radiochemical yield of 25 ± 7% d.c. and with a radiochemical purity of 78 ± 7% d.c. The labelling agent 1-iodo-2-[11C]methylpropane was coupled to thiophenol, phenol and phenylmagnesium bromide. Average radiochemical conversions of 83% d.c. for thiophenol, 40% d.c. for phenol, and 60% d.c. for phenylmagnesium bromide were obtained. In addition, [11C]2-methyl-1-propyl phenyl sulphide was isolated with a radiochemical yield of 5 ± 1% d.c. and a molar activity of 346 ± 113 GBq/μmol at the end of synthesis. Altogether, the syntheses of 1-iodo-2-[11C]methylpropane and 2-methyl-1-[11C]propanol were achieved and applied as proof of their applicability

Synthesis and Preclinical Evaluation of the First Carbon-11 Labeled PET Tracers Targeting Substance P1-7

Two potent SP1-7 peptidomimetics have been successfully radiolabeled via [11C]CO2-fixation with excellent yields, purity, and molar activity. l-[11C]SP1-7-peptidomimetic exhibited promising ex vivo biodistribution profile. Metabolite analysis showed that l-[11C]SP1-7-peptidomimetic is stable in brain and spinal cord, whereas rapid metabolic degradation occurs in rat plasma. Metabolic stability can be significantly improved by substituting l-Phe for d-Phe, preserving 70% more of intact tracer and resulting in better brain and spinal cord tracer retention. Positron emission tomography (PET) scanning confirmed moderate brain (1.5 SUV; peak at 3 min) and spinal cord (1.0 SUV; peak at 10 min) uptake for l- and d-[11C]SP1-7-peptidomimetic. A slight decrease in SUV value was observed after pretreatment with natural peptide SP1-7 in spinal cord for l-[11C]SP1-7-peptidomimetic. On the contrary, blocking using cold analogues of l- and d-[11C]tracers did not reduce the tracers' brain and spinal cord exposure. In summary, PET scanning of l- and d-[11C]SP1-7-peptidomimetics confirms rapid blood-brain barrier and blood-spinal-cord barrier penetration. Therefore, further validation of these two tracers targeting SP1-7 is needed in order to define a new PET imaging target and select its most appropriate radiopharmaceutical

Novel Thienopyrimidine-Based PET Tracers for P2Y12Receptor Imaging in the Brain

The P2Y12 receptor (P2Y12R) is uniquely expressed on microglia in the brain, and its expression level directly depends on the microglial activation state. Therefore, P2Y12R provides a promising imaging marker for distinguishing the pro- and anti-inflammatory microglial phenotypes, both of which play crucial roles in neuroinflammatory diseases. In this study, three P2Y12R antagonists were selected from the literature, radiolabeled with carbon-11 or fluorine-18, and evaluated in healthy Wistar rats. Brain imaging was performed with and without blocking of efflux transporters P-glycoprotein and breast cancer resistance protein using tariquidar. Low brain uptake in healthy rats was observed for all tracers at baseline conditions, whereas blocking of efflux transporters resulted in a strong (6-7 fold) increase in brain uptake for both of them. Binding of the most promising tracer, [18F]3, was further evaluated by in vitro autoradiography on rat brain sections, ex vivo metabolite studies, and in vivo P2Y12R blocking studies. In vitro binding of [18F]3 on rat brain sections indicated high P2Y12R targeting with approximately 70% selective and specific binding. At 60 min post-injection, over 95% of radioactivity in the brain accounted for an intact tracer. In blood plasma, still 40% intact tracer was found, and formed metabolites did not enter the brain. A moderate P2Y12R blocking effect was observed in vivo by positron emission tomography (PET) imaging with [18F]3 (p = 0.04). To conclude, three potential P2Y12R PET tracers were obtained and analyzed for P2Y12R targeting in the brain. Unfortunately, the brain uptake appeared low. Future work will focus on the design of P2Y12R inhibitors with improved physicochemical characteristics to reduce efflux transport and increase brain penetration

Preclinical PET Neuroimaging of [ 11

Activation of retinoid X receptors (RXRs) has been proposed as a therapeutic mechanism for the treatment of neurodegeneration, including Alzheimer's and Parkinson's diseases. We previously reported radiolabeling of a Food and Drug Administration-approved RXR agonist, bexarotene, by copper-mediated [ 11 C]CO 2 fixation and preliminary positron emission tomography (PET) neuroimaging that demonstrated brain permeability in nonhuman primate with regional binding distribution consistent with RXRs. In this study, the brain uptake and saturability of [ 11 C]bexarotene were studied in rats and nonhuman primates by PET imaging under baseline and greater target occupancy conditions. [ 11 C]Bexarotene displays a high proportion of nonsaturable uptake in the brain and is unsuitable for RXR occupancy measurements in the central nervous system

Preclinical PET Neuroimaging of [C]Bexarotene

Activation of retinoid X receptors (RXRs) has been proposed as a therapeutic mechanism for the treatment of neurodegeneration, including Alzheimer's and Parkinson's diseases. We previously reported radiolabeling of a Food and Drug Administration-approved RXR agonist, bexarotene, by copper-mediated [ 11 C]CO 2 fixation and preliminary positron emission tomography (PET) neuroimaging that demonstrated brain permeability in nonhuman primate with regional binding distribution consistent with RXRs. In this study, the brain uptake and saturability of [ 11 C]bexarotene were studied in rats and nonhuman primates by PET imaging under baseline and greater target occupancy conditions. [ 11 C]Bexarotene displays a high proportion of nonsaturable uptake in the brain and is unsuitable for RXR occupancy measurements in the central nervous system