Development and In Vivo Evaluation of Small-Molecule Ligands for Positron Emission Tomography of Immune Checkpoint Modulation Targeting Programmed Cell Death 1 Ligand 1

A substantial portion of patients do not benefit from programmed cell death protein 1/programmed cell death 1 ligand 1 (PD-1/PD-L1) checkpoint inhibition therapies, necessitating a deeper understanding of predictive biomarkers. Immunohistochemistry (IHC) has played a pivotal role in assessing PD-L1 expression, but small-molecule positron emission tomography (PET) tracers could offer a promising avenue to address IHC-associated limitations, i.e., invasiveness and PD-L1 expression heterogeneity. PET tracers would allow for improved quantification of PD-L1 through noninvasive whole-body imaging, thereby enhancing patient stratification. Here, a large series of PD-L1 targeting small molecules were synthesized, leveraging advantageous substructures to achieve exceptionally low nanomolar affinities. Compound 5c emerged as a promising candidate (IC50 = 10.2 nM) and underwent successful carbon-11 radiolabeling. However, a lack of in vivo tracer uptake in xenografts and notable accumulation in excretory organs was observed, underscoring the challenges encountered in small-molecule PD-L1 PET tracer development. The findings, including structure-activity relationships and in vivo biodistribution data, stand to illuminate the path forward for refining small-molecule PD-L1 PET tracers.

Asymmetric Transfer Hydrogenation as a Key Step in the Synthesis of the Phosphonic Acid Analogs of Aminocarboxylic Acids

α-Aminophosphonic acids have a remarkably broad bioactivity spectrum. They can function as highly efficient transition state mimics for a variety of hydrolytic and angiotensin-converting enzymes, which makes them interesting target structures for synthetic chemists. In particular, the phosphonic acid analogs to α-aminocarboxylic acids (Pa AAs) are potent enzyme inhibitors, but many of them are only available by chiral or enzymatic resolution; sometimes only one enantiomer is accessible, and several have never been prepared in enantiopure form at all. Today, a variety of methods to access enantiopure α-aminophosphonic acids is known but none of the reported approaches can be generally applied for the synthesis of Pa AAs. Here we show that the phosphonic acid analogs of many (proteinogenic) α-amino acids become accessible by the catalytic, stereoselective asymmetric transfer hydrogenation (ATH) of α-oxo-phosphonates. The highly enantioenriched (enantiomeric excess (ee) ≥ 98 %) α-hydroxyphosphonates obtained are important pharmaceutical building blocks in themselves and could be easily converted to α-aminophosphonic acids in most studied cases. Even stereoselectively deuterated analogs became easily accessible from the same α-oxo-phosphonates using deuterated formic acid (DCO2 H).

Deciphering the role of recurrent FAD-dependent enzymes in bacterial phosphonate catabolism

Phosphonates-compounds containing a direct C-P bond-represent an important source of phosphorus in some environments. The most common natural phosphonate is 2-aminoethylphosphonate (AEP). Many bacteria can break AEP down through specialized "hydrolytic" pathways, which start with the conversion of AEP into phosphonoacetaldehyde (PAA), catalyzed by the transaminase PhnW. However, the substrate scope of these pathways is very narrow, as PhnW cannot process other common AEP-related phosphonates, notably N-methyl AEP (M1AEP). Here, we describe a heterogeneous group of FAD-dependent oxidoreductases that efficiently oxidize M1AEP to directly generate PAA, thus expanding the versatility and usefulness of the hydrolytic AEP degradation pathways. Furthermore, some of these enzymes can also efficiently oxidize plain AEP. By doing so, they surrogate the role of PhnW in organisms that do not possess the transaminase and create novel versions of the AEP degradation pathways in which PAA is generated solely by oxidative deamination.

Synthesis of 4-Deoxy-4-Fluoro-d-Sedoheptulose: A Promising New Sugar to Apply the Principle of Metabolic Trapping

Fluorinated carbohydrates are important tools for understanding the deregulation of metabolic fluxes and pathways. Fluorinating specific positions within the sugar scaffold can lead to enhanced metabolic stability and subsequent metabolic trapping in cells. This principle has, however, never been applied to study the metabolism of the rare sugars of the pentose phosphate pathway (PPP). In this study, two fluorinated derivatives of d-sedoheptulose were designed and synthesized: 4-deoxy-4-fluoro-d-sedoheptulose (4DFS) and 3-deoxy-3-fluoro-d-sedoheptulose (3DFS). Both sugars are taken up by human fibroblasts but only 4DFS is phosphorylated. Fluorination of d-sedoheptulose at C-4 effectively halts the enzymatic degradation by transaldolase and transketolase. 4DFS thus has a high potential as a new PPP imaging probe based on the principle of metabolic trapping. Therefore, the synthesis of potential radiolabeling precursors for 4DFS for future radiofluorinations with fluorine-18 is presented.

The Microbial Degradation of Natural and Anthropogenic Phosphonates

Phosphonates are compounds containing a direct carbon-phosphorus (C-P) bond, which is particularly resistant to chemical and enzymatic degradation. They are environmentally ubiquitous: some of them are produced by microorganisms and invertebrates, whereas others derive from anthropogenic activities. Because of their chemical stability and potential toxicity, man-made phosphonates pose pollution problems, and many studies have tried to identify biocompatible systems for their elimination. On the other hand, phosphonates are a resource for microorganisms living in environments where the availability of phosphate is limited; thus, bacteria in particular have evolved systems to uptake and catabolize phosphonates. Such systems can be either selective for a narrow subset of compounds or show a broader specificity. The role, distribution, and evolution of microbial genes and enzymes dedicated to phosphonate degradation, as well as their regulation, have been the subjects of substantial studies. At least three enzyme systems have been identified so far, schematically distinguished based on the mechanism by which the C-P bond is ultimately cleaved-i.e., through either a hydrolytic, radical, or oxidative reaction. This review summarizes our current understanding of the molecular systems and pathways that serve to catabolize phosphonates, as well as the regulatory mechanisms that govern their activity.

Fluorinated Analogues to the Pentuloses of the Pentose Phosphate Pathway

Fluorinated carbohydrates are valuable tools for enzymological studies due to their increased metabolic stability compared to their non-fluorinated analogues. Replacing different hydroxyl groups within the same monosaccharide by fluorine allows to influence a wide range of sugar-receptor interactions and enzymatic transformations. In the past, this principle was frequently used to study the metabolism of highly abundant carbohydrates, while the metabolic fate of rare sugars is still poorly studied. Rare sugars, however, are key intermediates of many metabolic routes, such as the pentose phosphate pathway (PPP). Here we present the design and purely chemical synthesis of a set of three deoxyfluorinated analogues of the rare sugars d-xylulose and d-ribulose: 1-deoxy-1-fluoro-d-ribulose (1DFRu), 3-deoxy-3-fluoro-d-ribulose (3DFRu) and 3-deoxy-3-fluoro-d-xylulose (3DFXu). Together with a designed set of potential late-stage radio-fluorination precursors, they have the potential to become useful tools for studies on the complex equilibria of the non-oxidative PPP.

On the Road towards Small-Molecule Programmed Cell Death 1 Ligand 1 Positron Emission Tomography Tracers: A Ligand-Based Drug Design Approach

PD-1/PD-L1 immune checkpoint blockade for cancer therapy showed promising results in clinical studies. Further endeavors are required to enhance patient stratification, as, at present, only a small portion of patients with PD-L1-positive tumors (as determined by PD-L1 targeted immunohistochemistry; IHC) benefit from anti-PD-1/PD-L1 immunotherapy. This can be explained by the heterogeneity of tumor lesions and the intrinsic limitation of multiple biopsies. Consequently, non-invasive in vivo quantification of PD-L1 on tumors and metastases throughout the entire body using positron emission tomography (PET) imaging holds the potential to augment patient stratification. Within the scope of this work, six new small molecules were synthesized by following a ligand-based drug design approach supported by computational docking utilizing lead structures based on the (2-methyl-[1,1'-biphenyl]-3-yl)methanol scaffold and evaluated in vitro for potential future use as PD-L1 PET tracers. The results demonstrated binding affinities in the nanomolar to micromolar range for lead structures and newly prepared molecules, respectively. Carbon-11 labeling was successfully and selectively established and optimized with very good radiochemical conversions of up to 57%. The obtained insights into the significance of polar intermolecular interactions, along with the successful radiosyntheses, could contribute substantially to the future development of small-molecule PD-L1 PET tracers.

As Similar As Possible, As Different As Necessary - On-Site Laboratory Teaching during the COVID-19 Pandemic

Most of the available information on studying under the challenging conditions brought about by the COVID-19 pandemic emphasizes a variety of aspects on how to digitalize the whole teaching process. Thus, several useful and potentially game-changing strategies have been reported recently. In contrast to the digitalization of teaching, in this article, we focus on the reverse process: transitioning back to offline teaching, which is unavoidable especially for the acquisition of practical skills during chemistry studies. In this work, we describe our own experience acquired during the Organic Chemistry practical course at the University of Vienna, which was held in June 2020 and onwards. The article contains descriptions of precautions and measures that were taken, additional materials, and necessary changes made in order to safely continue on-site course teaching. We anticipate that this set of precautions can be used in an adapted fashion for any type of laboratory course. Further, we offer a critical analysis of students' and instructors' opinions concerning the changes and well-being during the course. Those opinions were collected via a detailed survey. From our experience, with careful planning and responsible behavior, a return to on-site education is possible and warmly welcomed by all involved participants. The detailed description of our course may also be useful for those who need to start a new organic laboratory course or want to improve an existing one.

Towards understanding the magnetism of Os(IV) complexes: an ab initio insight

The magnetism of a recently synthesized trans-[Os^IVCl₄(κN₁-Hind)₂] complex (5d⁴-system), where Hind = 2H-indazole, was studied experimentally and theoretically. Relativistic CASSCF/CASPT2 calculations for this and model [Os^IVCl₆]^2- complexes were employed to understand the nature of the low-lying multiplets. It is found that despite strong metal-ligand covalency they are basically characterized by the total angular pseudo-momentum J̃ originating from the spin-orbit coupling of the ground-state spin S = 1 with the orbital pseudo-momentum L̃ = 1 of the Os^IV ion. The strong spin-orbit interaction also preserves the dominant J̃ = 0 character of the non-magnetic ground state in the trans-[Os^IVCl₄(κN₁-Hind)₂] complex despite significant deviation of the ligand environment of Os^IV from octahedral symmetry. At the same time the spin-orbit admixture of all multiplets arising from the t_2g⁴ strong-field electronic configuration is indispensable for the correct description of magnetic properties of Os^IV complexes. Moreover, based on ab initio calculations, we argue that the charge-transfer states play an important role in the magnetism of the present and probably other 5d complexes, a situation never encountered for 3d and 4f compounds.

Biosynthesis of the Fungal Organophosphonate Fosfonochlorin Involves an Iron(II) and 2-(Oxo)glutarate Dependent Oxacyclase

The fungal metabolite Fosfonochlorin features a chloroacetyl moiety that is unusual within known phosphonate natural product biochemistry. Putative biosynthetic genes encoding Fosfonochlorin in Fusarium and Talaromyces spp. were investigated through reactions of encoded enzymes with synthetic substrates and isotope labelling studies. We show that early biosynthetic steps for Fosfonochlorin involves the reduction of phosphonoacetaldehyde to form 2-hydroxyethylphosphonic acid, followed by oxidative intramolecular cyclization of the resulting alcohol to form ( S )-epoxyethylphosphonic acid. The latter reaction is catalyzed by FfnD, a rare example of a non-heme iron / 2-(oxo)glutarate dependent oxacyclase. In contrast, FfnD behaves as a more typical oxygenase with ethylphosphonic acid, producing ( S )-1-hydroxyethylphosphonic acid. FfnD thus represents a new example of a ferryl generating enzyme that can suppress the typical oxygen rebound reaction that follows abstraction of a substrate hydrogen by a ferryl oxygen, thereby directing the substrate radical towards a fate other than hydroxylation.

Overall Retention of Methyl Stereochemistry during B12-Dependent Radical SAM Methyl Transfer in Fosfomycin Biosynthesis

Methylcobalamin-dependent radical S-adenosylmethionine (SAM) enzymes methylate non-nucleophilic atoms in a range of substrates. The mechanism of the methyl transfer from cobalt to the receiving atom is still mostly unresolved. Here we determine the stereochemical course of this process at the methyl group during the biosynthesis of the clinically used antibiotic fosfomycin. In vitro reaction of the methyltransferase Fom3 using SAM labeled with 1H, 2H, and 3H in a stereochemically defined manner, followed by chemoenzymatic conversion of the Fom3 product to acetate and subsequent stereochemical analysis, shows that the overall reaction occurs with retention of configuration. This outcome is consistent with a double-inversion process, first in the SN2 reaction of cob(I)alamin with SAM to form methylcobalamin and again in a radical transfer of the methyl group from methylcobalamin to the substrate. The methods developed during this study allow high-yield in situ generation of labeled SAM and recombinant expression and purification of the malate synthase needed for chiral methyl analysis. These methods facilitate the broader use of in vitro chiral methyl analysis techniques to investigate the mechanisms of other novel enzymes.

Discovery of a New, Recurrent Enzyme in Bacterial Phosphonate Degradation: (R)-1-Hydroxy-2-aminoethylphosphonate Ammonia-lyase

Phosphonates represent an important source of bioavailable phosphorus in certain environments. Accordingly, many microorganisms (particularly marine bacteria) possess catabolic pathways to degrade these molecules. One example is the widespread hydrolytic route for the breakdown of 2-aminoethylphosphonate (AEP, the most common biogenic phosphonate). In this pathway, the aminotransferase PhnW initially converts AEP into phosphonoacetaldehyde (PAA), which is then cleaved by the hydrolase PhnX to yield acetaldehyde and phosphate. This work focuses on a pyridoxal 5'-phosphate-dependent enzyme that is encoded in >13% of the bacterial gene clusters containing the phnW-phnX combination. This enzyme (which we termed PbfA) is annotated as a transaminase, but there is no obvious need for an additional transamination reaction in the established AEP degradation pathway. We report here that PbfA from the marine bacterium Vibrio splendidus catalyzes an elimination reaction on the naturally occurring compound (R)-1-hydroxy-2-aminoethylphosphonate (R-HAEP). The reaction releases ammonia and generates PAA, which can be then hydrolyzed by PhnX. In contrast, PbfA is not active toward the S enantiomer of HAEP or other HAEP-related compounds such as ethanolamine and d,l-isoserine, indicating a very high substrate specificity. We also show that R-HAEP (despite being structurally similar to AEP) is not processed efficiently by the PhnW-PhnX couple in the absence of PbfA. In summary, the reaction catalyzed by PbfA serves to funnel R-HAEP into the hydrolytic pathway for AEP degradation, expanding the scope and the usefulness of the pathway itself.

Discovery of melanin-concentrating hormone receptor 1 in brown adipose tissue

Although extensive research on brown adipose tissue (BAT) has stimulated optimism in the battle against obesity and diabetes, BAT physiology and organ crosstalk are not fully understood. Besides BAT, melanin‐concentrating hormone (MCH) and its receptor (MCHR1) play an important role in energy homeostasis. Because of the link between hypothalamic MCH neurons and sympathetic BAT activation via β‐adrenoceptors, we investigated the expression and physiological role of the MCHR1 in BAT. MCHR1 was detected in rodent and human BAT with RT‐qPCR and western blot analyses. In vivo imaging in rats used the glucose analog [¹⁸F]FDG and the MCHR1‐tracer [¹¹C]SNAP‐7941. We found that the β3‐adrenoceptor (ADRB3) agonist CL316,243 increased [¹¹C]SNAP‐7941 uptake in BAT. Additionally, a pharmacological concentration of SNAP‐7941—a low‐affinity ADRB3 ligand—stimulated [¹⁸F]FDG uptake, reflecting BAT activation. In cultured human adipocytes, CL316,243 induced MCHR1 expression, further supporting a direct interaction between MCHR1 and ADRB3. These findings characterized MCHR1 expression in rodent and human BAT for the first time, including in vitro and in vivo data demonstrating a link between MCHR1 and the β3‐adrenergic system. The presence of MCHR1 in BAT emphasizes the role of BAT in energy homeostasis and may help uncover treatment approaches for obesity.

C-H Bond Cleavage Is Rate-Limiting for Oxidative C-P Bond Cleavage by the Mixed Valence Diiron-Dependent Oxygenase PhnZ

PhnZ utilizes a mixed valence diiron(II/III) cofactor and O₂ to oxidatively cleave the carbon-phosphorus bond of (R)-2-amino-1-hydroxyethylphosphonic acid to form glycine and orthophosphate. The active site residues Y24 and E27 are proposed to mediate induced-fit recognition of the substrate and access of O₂ to one of the active site Fe ions. H62 is proposed to deprotonate the C1-hydroxyl of the substrate during catalysis. Kinetic isotope effects (KIEs), pH-rate dependence, and site-directed mutagenesis were used to probe the rate-determining transition state and the roles of these three active site residues. Primary deuterium KIE values of 5.5 ± 0.3 for ^D(V) and 2.2 ± 0.4 for ^D(V/K) were measured with (R)-2-amino[1-²H₁]-1-hydroxyethylphosphonic acid, indicating that cleavage of the C1-H bond of the substrate is rate-limiting. This step is also rate-limiting for PhnZ Y24F, as shown by a significant deuterium KIE value of 2.3 ± 0.1 for ^D(V). In contrast, a different reaction step appears to be rate-limiting for the PhnZ E27A and H62A variants, which exhibited ^D(V) values near unity. A solvent KIE of 2.2 ± 0.3 for ^D₂O(V) is observed for PhnZ. Significant solvent KIE values are also observed for the PhnZ Y24F and E27A variants. In contrast, the PhnZ H62A variant does not show a significant solvent KIE, suggesting that H62 is mediating proton transfer in the transition state. A proton inventory study with PhnZ indicates that 1.5 ± 0.6 protons are in flight in the rate-determining step. Overall, the rate-determining transition state for oxidative C-P bond cleavage by PhnZ is proposed to involve C-H bond cleavage that is coupled to deprotonation of the substrate C1-hydroxyl by H62.

SNAPshots of the MCHR1: a Comparison Between the PET-Tracers [18F]FE@SNAP and [11C]SNAP-7941

PURPOSE

The melanin-concentrating hormone receptor 1 (MCHR1) has become an important pharmacological target, since it may be involved in various diseases, such as diabetes, insulin resistance, and obesity. Hence, a suitable positron emission tomography radiotracer for the in vivo assessment of the MCHR1 pharmacology is imperative. The current paper contrasts the extensive in vitro, in vivo, and ex vivo assessments of the radiotracers [18F]FE@SNAP and [11C]SNAP-7941 and provides comprehensive information about their biological and physicochemical properties. Furthermore, it examines their suitability for first-in-man imaging studies.

PROCEDURES

Kinetic real-time cell-binding studies with [18F]FE@SNAP and [11C]SNAP-7941 were conducted on adherent Chines hamster ovary (CHO-K1) cells stably expressing the human MCHR1 and MCHR2. Small animal imaging studies on mice and rats were performed under displacement and baseline conditions, as well as after pretreatment with the P-glycoprotein/breast cancer resistant protein inhibitor tariquidar. After the imaging studies, detailed analyses of the ex vivo biodistribution were performed. Ex vivo metabolism was determined in rat blood and brain and analyzed at various time points using a quantitative radio-HPLC assay.

RESULTS

[11C]SNAP-7941 demonstrates high uptake on CHO-K1-hMCHR1 cells, whereas no uptake was detected for the CHO-K1-hMCHR2 cells. In contrast, [18F]FE@SNAP evinced binding to CHO-K1-hMCHR1 and CHO-K1-hMCHR2 cells. Imaging studies with [18F]FE@SNAP and [11C]SNAP-7941 showed an increased brain uptake after tariquidar pretreatment in mice, as well as in rats, and exhibited a significant difference between the time-activity curves of the baseline and blocking groups. Biodistribution of both tracers demonstrated a decreased uptake after displacement. [11C]SNAP-7941 revealed a high metabolic stability in rats, whereas [18F]FE@SNAP was rapidly metabolized.

CONCLUSIONS

Both radiotracers demonstrate appropriate imaging properties for the MCHR1. However, the pronounced metabolic stability as well as superior selectivity and affinity of [11C]SNAP-7941 underlines the decisive superiority over [18F]FE@SNAP.

An Oxidative Pathway for Microbial Utilization of Methylphosphonic Acid as a Phosphate Source

Methylphosphonic acid is synthesized by marine bacteria and is a prominent component of dissolved organic phosphorus. Consequently, methylphosphonic acid also serves as a source of inorganic phosphate (Pi) for marine bacteria that are starved of this nutrient. Conversion of methylphosphonic acid into Pi is currently only known to occur through the carbon-phosphorus lyase pathway, yielding methane as a byproduct. In this work, we describe an oxidative pathway for the catabolism of methylphosphonic acid in Gimesia maris DSM8797. G. maris can use methylphosphonic acid as Pi sources despite lacking a phn operon encoding a carbon-phosphorus lyase pathway. Instead, the genome contains a locus encoding homologues of the non-heme Fe(II) dependent oxygenases HF130PhnY* and HF130PhnZ, which were previously shown to convert 2-aminoethylphosphonic acid into glycine and Pi. GmPhnY* and GmPhnZ1 were produced in E. coli and purified for characterization in vitro. The substrate specificities of the enzymes were evaluated with a panel of synthetic phosphonates. Via ³¹P NMR spectroscopy, it is demonstrated that the GmPhnY* converts methylphosphonic acid to hydroxymethylphosphonic acid, which in turn is oxidized by GmPhnZ1 to produce formic acid and Pi. In contrast, 2-aminoethylphosphonic acid is not a substrate for GmPhnY* and is therefore not a substrate for this pathway. These results thus reveal a new metabolic fate for methylphosphonic acid.

Synthetic Phosphonic Acids as Potent Tools to Study Phosphonate Enzymology

Phosphonic acids are highly stable phosphorus-containing compounds, which have been proposed as important intermediates in the global phosphorus cycle. Biogenic phosphonates as well as their synthetic analogues play an important role as potential enzyme inhibitor drugs and as alternative phosphorus source for microbes. Despite these properties, their metabolism is still poorly understood. New degradative pathways and unknown compounds are identified at fast pace. However, most of these pathways include a variety of unique enzymatic transformations, which are difficult to characterize – especially without sufficient amounts of the potential substrates and intermediates of the postulated transformations in hands. Thus, there is a great need for the development of synthetic methodologies to access phosphonic acids in high yields and in enantiomerically pure form for the use in enzymatic studies and in studies on the biological activity of newly isolated natural products, which are often only obtained in low yields. In this Synpacts article we aim at highlighting our recent contributions to this field.

1 Introduction2 Phosphonates as Alternative Phosphorus Source

3 The Application of Phosphonates in Enzymatic Studies

4 Conclusion

Optimization of the Automated Synthesis of [11C]mHED-Administered and Apparent Molar Activities

The tracer [¹¹C]meta-Hydroxyephedrine ([¹¹C]mHED) is one of the most applied PET tracers for cardiac imaging, whose radiosynthesis was already reported in 1990. While not stated in the literature, separation difficulties and an adequate formulation of the product are well known challenges in its production. Furthermore, the precursor (metaraminol) is also a substrate for the norepinephrine transporter, and can therefore affect the image quality. This study aims at optimizing the synthetic process of [¹¹C]mHED and investigating the effect of the apparent molar activity (sum of mHED and metaraminol) in patients and animals. The main optimization was the improved separation through reverse phase-HPLC by a step gradient and subsequent retention of the product on a weakly-cationic ion exchange cartridge. The µPET/µCT was conducted in ten rats (ischemic model) and the apparent molar activity was correlated to the VOI- and SUV-ratio of the myocardium/intra-ventricular blood pool. Moreover, nine long-term heart transplanted and five Morbus Fabry patients underwent PET and MRI imaging for detection of changes in the sympathetic innervation. In summary, the fully-automated synthesis and optimized purification method of [¹¹C]mHED is easily applicable and reproducible. Moreover, it was shown that the administered apparent molar activities had a negligible effect on the imaging quality.

In vitro Radiopharmaceutical Evidence for MCHR1 Binding Sites in Murine Brown Adipocytes

[¹¹C]SNAP-7941 and its radiofluorinated, fluoro-ethyl derivative [¹⁸F]FE@SNAP have been developed as the first positron emission tomography tracers for melanin-concentrating hormone receptor 1 (MCHR1) imaging. Accumulation of these MCHR1 PET-tracers in rat brown adipose tissue (BAT) in vivo provided first indication of MCHR1 expression in rodent BAT. To rule out off-target binding, affinity of both MCHR1 ligands toward adrenergic beta-3 receptors (ADRB3) was examined. Further, specific binding of [¹¹C]SNAP-7941 to brown adipocytes and effects of MCHR1 ligands on brown adipocyte activation were investigated. SNAP-7941 and FE@SNAP evinced to be highly selective toward MCHR1. [¹¹C]SNAP-7941 binding to brown adipocytes was shown to be mainly MCHR1-specific. This data strongly indicates MCHR1 expression in rodent BAT and moreover, a peripheral, anti-obesity effect of MCHR1 antagonists directly exerted in BAT is proposed. Moreover, MCHR1 expression in murine brown adipocytes was confirmed by protein and mRNA analysis. We conclude that MCHR1 PET imaging contributes to basic research in endocrinology by elucidating the involvement of the MCH system in peripheral tissues, such as BAT.

PcxL and HpxL are flavin-dependent, oxime-forming N-oxidases in phosphonocystoximic acid biosynthesis in Streptomyces

Several oxime-containing small molecules have useful properties, including antimicrobial, insecticidal, anticancer, and immunosuppressive activities. Phosphonocystoximate and its hydroxylated congener, hydroxyphosphonocystoximate, are recently discovered oxime-containing natural products produced by Streptomyces sp. NRRL S-481 and Streptomyces regensis NRRL WC-3744, respectively. The biosynthetic pathways for these two compounds are proposed to diverge at an early step in which 2-aminoethylphosphonate (2AEPn) is converted to (S)-1-hydroxy-2-aminoethylphosphonate ((S)-1H2AEPn) in S. regensis but not in Streptomyces sp. NRRL S-481). Subsequent installation of the oxime moiety into either 2AEPn or (S)-1H2AEPn is predicted to be catalyzed by PcxL or HpxL from Streptomyces sp. NRRL S-481 and S. regensis NRRL WC-3744, respectively, whose sequence and predicted structural characteristics suggest they are unusual N-oxidases. Here, we show that recombinant PcxL and HpxL catalyze the FAD- and NADPH-dependent oxidation of 2AEPn and 1H2AEPn, producing a mixture of the respective aldoximes and nitrosylated phosphonic acid products. Measurements of catalytic efficiency indicated that PcxL has almost an equal preference for 2AEPn and (R)-1H2AEPn. 2AEPn was turned over at a 10-fold higher rate than (R)-1H2AEPn under saturating conditions, resulting in a similar but slightly lower kcat/Km We observed that (S)-1H2AEPn is a relatively poor substrate for PcxL but is clearly the preferred substrate for HpxL, consistent with the proposed biosynthetic pathway in S. regensis. HpxL also used both 2AEPn and (R)-1H2AEPn, with the latter inhibiting HpxL at high concentrations. Bioinformatic analysis indicated that PcxL and HpxL are members of a new class of oxime-forming N-oxidases that are broadly dispersed among bacteria.

Determination of the Absolute Configuration of (-)-Hydroxynitrilaphos and Related Biosynthetic Questions

The ongoing search for bioactive natural products has led to the development of new genome-based screening approaches to identify possible phosphonate producing microorganisms. From the identified phosphonate producers, several until now unknown phosphonic acid natural products were isolated, including (hydroxy)nitrilaphos (4 and 5) and (hydroxy)phosphonocystoximate (7 and 6). We present the synthesis of phosphonocystoximate via an aldoxime intermediate. Chlorination and coupling with methyl N-acetylcysteinate furnished 6 after global deprotection. The obtained experimental data confirm the previously assigned structure of the natural product. We were also able to determine the absolute configuration of (-)-hydroxynitrilaphos. Chiral resolution of diethyl cyanohydroxymethylphosphonate (24) with Noe's lactol furnished both enantiomers of 4. Conversion of (+)-24 to (R)-2-amino-1-hydroxyethylphosphonic acid by reduction of the cyano-group showed (-)-hydroxynitrilaphos ultimately to be S-configured. Further, we present a ¹³ C-isotope labeling strategy for 4 and 5 that will possibly solve the question of whether hydroxynitrilaphos is a biosynthetic intermediate or a downstream product of hydroxyphosphonocystoximate biosynthesis.

Phosphonodifluoropyruvate is a mechanism-based inhibitor of phosphonopyruvate decarboxylase from Bacteroides fragilis

Bacteroides fragilis, a human pathogen, helps in the formation of intra-abdominal abscesses and is involved in 90% of anaerobic peritoneal infections. Phosphonopyruvate decarboxylase (PnPDC), a thiamin diphosphate (ThDP)-dependent enzyme, plays a key role in the formation of 2-aminoethylphosphonate, a component of the cell wall of B. fragilis. As such PnPDC is a possible target for therapeutic intervention in this, and other phosphonate producing organisms. However, the enzyme is of more general interest as it appears to be an evolutionary forerunner to the decarboxylase family of ThDP-dependent enzymes. To date, PnPDC has proved difficult to crystallize and no X-ray structures are available. In the past we have shown that ThDP-dependent enzymes will often crystallize if the cofactor has been irreversibly inactivated. To explore this possibility, and the utility of inhibitors of phosphonate biosynthesis as potential antibiotics, we synthesized phosphonodifluoropyruvate (PnDFP) as a prospective mechanism-based inhibitor of PnPDC. Here we provide evidence that PnDFP indeed inactivates the enzyme, that the inactivation is irreversible, and is accompanied by release of fluoride ion, i.e., PnDFP bears all the hallmarks of a mechanism-based inhibitor. Unfortunately, the enzyme remains refractive to crystallization.

Towards the biodegradation pathway of fosfomycin

Three functionalised propylphosphonic acids were synthesised to study C-P bond cleavage in R. huakuii PMY1. (R)-1-Hydroxy-2-oxopropylphosphonic acid [(R)-5] was prepared by chiral resolution of (±)-dimethyl 1-hydroxy-2-methylallyllphosphonate [(±)-12], followed by ozonolysis and deprotection. The N-(l-alanyl)-substituted (1R,2R)-2-amino-1-hydroxypropylphosphonic acid 10, a potential precursor for 2-oxopropylphosphonic acid (5) in cells, was obtained by coupling the aminophosphonic acid with benzotriazole-activated Z-l-alanine and hydrogenolytic deprotection. (1R*,2R*)-1,2-Dihydroxy-3,3,3-trifluoropropylphosphonic acid, a potential inhibitor of C-P bond cleavage after conversion into its 2-oxo derivative in the cell, was accessed from trifluoroacetaldehyde hydrate via hydroxypropanenitrile 21, which was silylated and reduced to the aldehyde (±)-23. Diastereoselective addition of diethyl trimethylsilyl phosphite furnished diastereomeric α-siloxyphosphonates. The less polar one was converted to the desired racemic phosphonic acid (±)-(1R*,2R*)-9 as its ammonium salt.

The Stereochemical Course of the α-Hydroxyphosphonate-Phosphate Rearrangement

The phosphonate–phosphate rearrangement is an isomerisation of α-hydroxyphosphonates bearing electron-withdrawing substituents at the α-carbon atom. We studied the stereochemical course of this rearrangement with respect to phosphorus. A set of four diastereomeric α-hydroxyphosphonates was prepared by a Pudovik reaction from two diastereomeric cyclic phosphites. The hydroxyphosphonates were separated and rearranged with Et₃N as base. In analogy to trichlorphon, which was the first reported compound undergoing this rearrangement. All four hydroxyphosphonates could be rearranged to 2,2-dichlorovinyl phosphates. Single-crystal X-ray structure analyses of the α-hydroxyphosphonates and the corresponding phosphates allowed us to show that the rearrangement proceeds with retention of configuration on the phosphorus atom.