Transglutaminase 2 (TGase 2)-catalyzed transamidation represents an important post-translational mechanism for protein modification with implications in physiological and pathophysiological conditions, including fibrotic and neoplastic processes. Consequently, this enzyme is considered a promising target for the diagnosis of and therapy for these diseases. In this study, we report on the synthesis and kinetic characterization of N-acryloyllysine piperazides as irreversible inhibitors of TGase 2. Systematic structural modifications on 54 new compounds were performed with a major focus on fluorine-bearing substituents due to the potential of such compounds to serve as radiotracer candidates for positron emission tomography. The determined inhibitory activities ranged from 100 to 10 000 M s, which resulted in comprehensive structure-activity relationships. Structure-activity correlations using various substituent parameters accompanied by covalent docking studies provide an advanced understanding of the molecular recognition for this inhibitor class within the active site of TGase 2. Selectivity profiling of selected compounds for other transglutaminases demonstrated an excellent selectivity toward transglutaminase 2. Furthermore, an initial pharmacokinetic profiling of selected inhibitors was performed, including the assessment of potential membrane permeability and liver microsomal stability.

Both enantiomers of [ 18 F]flubatine are promising radioligands for neuroimaging of α4β2 nicotinic acetylcholine receptors (nAChRs) by positron emission tomography (PET). To support clinical studies in patients with early Alzheimer's disease, a detailed examination of the metabolism in vitro and in vivo has been performed. (+)-and (−)-flubatine, respectively, were incubated with liver microsomes from mouse and human in the presence of NADPH (β-nicotinamide adenine dinucleotide 2 -phosphate reduced tetrasodium salt). Phase I in vitro metabolites were detected and their structures elucidated by LC-MS/MS (liquid chromatography-tandem mass spectrometry). Selected metabolite candidates were synthesized and investigated for structural confirmation. Besides a high level of in vitro stability, the microsomal incubations revealed some species differences as well as enantiomer discrimination with regard to the formation of monohydroxylated products, which was identified as the main metabolic pathway in this assay. Furthermore, after injection of 250 MBq (+)-[ 18 F]flubatine (specific activity > 350 GBq/µmol) into mouse, samples were prepared from brain, liver, plasma, and urine after 30 min and investigated by radio-HPLC (high performance liquid chromatography with radioactivity detection). For structure elucidation of the radiometabolites of (+)-[ 18 F]flubatine formed in vivo, identical chromatographic conditions were applied to LC-MS/MS and radio-HPLC to compare samples obtained in vitro and in vivo. By this correlation approach, we assigned three of four main in vivo radiometabolites to products that are exclusively C-or N-hydroxylated at the azabicyclic ring system of the parent molecule.

Monocarboxylate transporters 1 and 4 (MCT1 and MCT4) are involved in tumor development and progression. Their expression levels are related to clinical disease prognosis. Accordingly, both MCTs are promising drug targets for treatment of a variety of human cancers. The noninvasive imaging of these MCTs in cancers is regarded to be advantageous for assessing MCT‐mediated effects on chemotherapy and radiosensitization using specific MCT inhibitors. Herein, we describe a method for the radiosynthesis of [18F]FACH ((E)‐2‐cyano‐3‐{4‐[(3‐[18F]fluoropropyl)(propyl)amino]‐2‐methoxyphenyl}acrylic acid), as a novel radiolabeled MCT1/4 inhibitor for imaging with PET. A fluorinated analog of α‐cyano‐4‐hydroxycinnamic acid (FACH) was synthesized, and the inhibition of MCT1 and MCT4 was measured via an L‐[14C]lactate uptake assay. Radiolabeling was performed by a two‐step protocol comprising the radiosynthesis of the intermediate (E)/(Z)‐[18F]tert‐Bu‐FACH (tert‐butyl (E)/(Z)‐2‐cyano‐3‐{4‐[(3‐[18F]fluoropropyl)(propyl)amino]‐2‐methoxyphenyl}acrylate) followed by deprotection of the tert‐butyl group. The radiofluorination was successfully implemented using either K[18F]F‐K2.2.2‐carbonate or [18F]TBAF. The final deprotected product [18F]FACH was only obtained when [18F]tert‐Bu‐FACH was formed by the latter procedure. After optimization of the deprotection reaction, [18F]FACH was obtained in high radiochemical yields (39.6 ± 8.3%, end of bombardment (EOB) and radiochemical purity (greater than 98%).

Purpose The adenosine A2A receptor has emerged as a therapeutic target for multiple diseases, and thus the non-invasive imaging of the expression or occupancy of the A2A receptor has potential to contribute to diagnosis and drug development. We aimed at the development of a metabolically stable A2A receptor radiotracer and report herein the preclinical evaluation of [18F]FLUDA, a deuterated isotopologue of [18F]FESCH. Methods [18F]FLUDA was synthesized by a two-step one-pot approach and evaluated in vitro by autoradiographic studies as well as in vivo by metabolism and dynamic PET/MRI studies in mice and piglets under baseline and blocking conditions. A single-dose toxicity study was performed in rats. Results [18F]FLUDA was obtained with a radiochemical yield of 19% and molar activities of 72–180 GBq/μmol. Autoradiography proved A2A receptor–specific accumulation of [18F]FLUDA in the striatum of a mouse and pig brain. In vivo evaluation in mice revealed improved stability of [18F]FLUDA compared to that of [18F]FESCH, resulting in the absence of brain-penetrant radiometabolites. Furthermore, the radiometabolites detected in piglets are expected to have a low tendency for brain penetration. PET/MRI studies confirmed high specific binding of [18F]FLUDA towards striatal A2A receptor with a maximum specific-to-non-specific binding ratio in mice of 8.3. The toxicity study revealed no adverse effects of FLUDA up to 30 μg/kg, ~ 4000-fold the dose applied in human PET studies using [18F]FLUDA. Conclusions The new radiotracer [18F]FLUDA is suitable to detect the availability of the A2A receptor in the brain with high target specificity. It is regarded ready for human application.

BackgroundThe level of expression of cannabinoid receptor type 2 (CB2R) in healthy and diseased brain has not been fully elucidated. Therefore, there is a growing interest to assess the regional expression of CB2R in the brain. Positron emission tomography (PET) is an imaging technique, which allows quantitative monitoring of very low amounts of radiolabelled compounds in living organisms at high temporal and spatial resolution and, thus, has been widely used as a diagnostic tool in nuclear medicine. Here, we report on the radiofluorination of N-aryl-oxadiazolyl-propionamides at two different positions in the lead structure and on the biological evaluation of the potential of the two tracers [18F]1 and [18F]2 as CB2 receptor PET imaging agents.ResultsHigh binding affinity and specificity towards CB2 receptors of the lead structure remained unaffected by the structural changes such as the insertion of the aliphatic and aromatic fluorine in the selected labelling sites of 1 and 2. Aliphatic and aromatic radiofluorinations were optimized, and [18F]1 and [18F]2 were achieved in radiochemical yields of ≥30% with radiochemical purities of ≥98% and specific activities of 250 to 450 GBq/μmol. Organ distribution studies in female CD1 mice revealed that both radiotracers cross the blood–brain barrier (BBB) but undergo strong peripheral metabolism. At 30 min after injection, unmetabolized [18F]1 and [18F]2 accounted for 60% and 2% as well as 68% and 88% of the total activity in the plasma and brain, respectively. The main radiometabolite of [18F]2 could be identified as the free acid [18F]10, which has no affinity towards the CB1 and CB2 receptors but can cross the BBB.ConclusionsN-aryl-oxadiazolyl-propionamides can successfully be radiolabelled with 18F at different positions. Fluorine substitution at these positions did not affect affinity and specificity towards CB2R. Despite a promising in vitro behavior, a rather rapid peripheral metabolism of [18F]1 and [18F]2 in mice and the generation of brain permeable radiometabolites hamper the application of these radiotracers in vivo. However, it is expected that future synthetic modification aiming at a replacement of metabolically susceptible structural elements of [18F]1 and [18F]2 will help to elucidate the potential of this class of compounds for CB2R PET studies.

S y n t h e s i s o f N a t u r a l P r o d u c t P r e c u r s o r s b y E n z y m a t i c B a e y e r -V i l l i g e r O x i d a t i o nAbstract: The Baeyer-Villiger oxidation of the 2-substituted ketones 1 and 3 with the coupled system cyclohexanone monooxygenase from Acinetobacter NCIMB 9871 / formate dehydrogenase from Pseudomonas sp. 101 provides the lactones (R)-2 and (R)-4 with high enantiomeric excess which are precursors in the synthesis of lipoic acid. The symmetrically trisubstituted ketone 5 was oxidised to the lactones 6a and 6b in a ratio of approx. 3:1. The absolute configuration of 6a and 6b was determined by hydrolysis of the racemic lactone with PLE yielding the hydroxycarboxylic acid (-)-7 with known absolute configuration.In a recent review, Hoberg pointed out the biological and chemical relevance of seven-membered oxacycles. 2 One of the most powerful and elegant methods for expanding six-membered rings is the well-known Baeyer-Villiger oxidation. 3,4 Unfortunately, there are only very few examples for achieving at least some enantioselectivity when oxidising prochiral or racemic ketones using chiral metal complexes, 5 chiral auxiliaries, 6 or the Sharpless epoxidation system. 7 However, to our knowledge there is no method that could rival with the enzymatic Baeyer-Villiger oxidation concerning the stereoselectivities as well as the spectrum of substrates. 8-10 Among several BaeyerVilliger monooxygenases, cyclohexanone monooxygenase from Acinetobacter NCIMB 9871 (CHMO; EC 1.14.13.22) has been studied most intensively. 11-13 Some interesting applications for the synthesis of precursors of natural products like azadirachtin 14 and ionomycin 15 were published.In principle, there are two possibilities to carry out CHMO-catalysed Baeyer-Villiger oxidations: with whole cells or with isolated and enriched enzymes, respectively. The first method often suffers from over-metabolism and side reactions. This is also true if recombinant strains like a cyclohexanone monooxygenase expressing baker's yeast 16 or Escherichia coli 17,18 are used. The second approach has some advantages. The reaction process can be executed in a simpler manner using well-defined concentrations of enzyme and substrate. However, this method has a considerable disadvantage; it requires an effective NADPH regeneration. The latter problem has probably been the reason for the fact that, barring one example, 19 no Baeyer-Villiger oxidations using CHMO on a preparative scale have been reported. In contrast to NADH, regeneration of NADPH by enzymatic methods is much more difficult. 20 Tishkov et al. developed a protein-engineered formate dehydrogenase (FDH) from Pseudomonas sp. 101 by multiple site-directed mutagenesis which accepts NADP + to an extent making it suitable for NADPH regeneration. 21,22 Until now, three examples of application of this new promising enzyme are known. 23-25 FDH was first used for cofactor regeneration in a dehydrogenase-catalysed reduction of acetophenone. 23 The coupling of CHMO with the NADP + -dependent ...

Phosphodiesterases (PDEs) are enzymes that play a major role in cell signalling by hydrolysing the secondary messengers cyclic adenosine monophosphate (cAMP) and/or cyclic guanosine monophosphate (cGMP) throughout the body and brain. Altered cyclic nucleotide-mediated signalling has been associated with a wide array of disorders, including neurodegenerative disorders. Recently, PDE5 has been shown to be involved in neurodegenerative disorders such as Alzheimer’s disease, but its precise role has not been elucidated yet. To visualize and quantify the expression of this enzyme in brain, we developed a radiotracer for specific PET imaging of PDE5. A quinoline-based lead compound has been structurally modified resulting in the fluoroethoxymethyl derivative ICF24027 with high inhibitory activity towards PDE5 (IC50 = 1.86 nM). Radiolabelling with fluorine-18 was performed by a one-step nucleophilic substitution reaction using a tosylate precursor (RCY(EOB) = 12.9% ± 1.8%; RCP > 99%; SA(EOS) = 70–126 GBq/μmol). In vitro autoradiographic studies of [18F]ICF24027 on different mouse tissue as well as on porcine brain slices demonstrated a moderate specific binding to PDE5. In vivo studies in mice revealed that [18F]ICF24027 was metabolized under formation of brain penetrable radiometabolites making the radiotracer unsuitable for PET imaging of PDE5 in brain.

TRPC6 (transient receptor potential cation channels; canonical subfamily C, member 6) is widespread localized in mammalian tissues like kidney and lung and associated with progressive proteinuria and pathophysiological pulmonary alterations, e.g., reperfusion edema or lung fibrosis. However, the understanding of TRPC6 channelopathies is still at the beginning stages. Recently, by chemical diversification of (+)-larixol originating from Larix decidua resin traditionally used for inhalation, its methylcarbamate congener, named SH045, was obtained and identified in functional assays as a highly potent, subtype-selective inhibitor of TRPC6. To pave the way for use of SH045 in animal disease models, this study aimed at developing a capable bioanalytical method and to provide exploratory pharmacokinetic data for this promising derivative. According to international guidelines, a robust and selective LC-MS/MS method based on MRM detection in positive ion mode was established and validated for quantification of SH045 in mice plasma, whereby linearity and accuracy were demonstrated for the range of 2–1600 ng/mL. Applying this method, the plasma concentration time course of SH045 following single intraperitoneal administration (20 mg/kg body weight) revealed a short half-life of 1.3 h. However, the pharmacological profile of SH045 is promising, as five hours after administration, plasma levels still remained sufficiently higher than published low nanomolar IC50 values. Summarizing, the LC-MS/MS method and exploratory pharmacokinetic data provide essential prerequisites for experimental pharmacological TRPC6 modulation and translational treatment of TRPC6 channelopathies.