Synthesis and Pharmacological Characterization of β2-Adrenergic Agonist Enantiomers: Zilpaterol

Adrenoceptors: Receptors, Ligands and Their Clinical Uses, Molecular Pharmacology and Assays

The nine G protein-coupled adrenoceptor subtypes are where the endogenous catecholamines adrenaline and noradrenaline interact with cells. Since they are important therapeutic targets, over a century of effort has been put into developing drugs that modify their activity. This chapter provides an outline of how we have arrived at current knowledge of the receptors, their physiological roles and the methods used to develop ligands. Initial studies in vivo and in vitro with isolated organs and tissues progressed to cell-based techniques and the use of cloned adrenoceptor subtypes together with high-throughput assays that allow close examination of receptors and their signalling pathways. The crystal structures of many of the adrenoceptor subtypes have now been determined opening up new possibilities for drug development.

DFT Study on Mechanism of Ni-Al Bimetallic-Catalyzed C-H Cyclization to Construct Tricyclic Imidazoles: Roles of NHC Ligand and AlMe3

The mechanism of the Ni-Al bimetallic-catalyzed C-H cyclization to construct tricyclic imidazoles is investigated using density functional theory calculations. The calculation result shows that the reaction mechanism involves sequential steps of substrate coordination, ligand-to-ligand hydrogen transfer (LLHT), and C-C reductive elimination to produce the final product tricyclic imidazole. The LLHT step is calculated to be the rate-determining step. The oxidative addition of the benzimidazole C-H bond to the Ni center and the insertion of the alkene into the Ni-H bond occur concertedly in the LLHT step. The effects of N-heterocyclic carbene (NHC) ligands and AlMe3 on the reactivity and regioselectivity were also analyzed. These calculation results shed light on some ambiguous suggestions from experiments.

Determination and enantioselective separation of zilpaterol in human urine after mimicking consumption of contaminated meat using high-performance liquid chromatography with tandem mass spectrometry techniques

RATIONALE

The synthetic β-adrenoreceptor agonist zilpaterol is legitimately used as an animal feed supplement in selected countries due to its known effects on lipolysis and protein biosynthesis. These pharmacological characteristics of zilpaterol have contributed to its classification as doping agent in sport by the World Anti-Doping Agency. However, the use as a feed supplement can lead to residues of the drug in edible tissues and, possibly, also in the urine of consumers.

METHODS

To provide urinary elimination profiles of microdosed zilpaterol and to determine whether the ingestion of zilpaterol below or at the acceptable daily intake level of 0.04 μg/kg bodyweight can result in an adverse analytical finding (AAF) in doping controls, healthy volunteers were administered single or multiple oral doses of 0.5 μg or 3 μg zilpaterol to mimic ingestion of contaminated cattle meat. Urine samples were collected and analyzed using a validated high-performance liquid chromatography-electrospray ionization-tandem mass spectrometry (HPLC-ESI-MS/MS) method and a newly developed chiral high-performance liquid chromatography-atmospheric pressure chemical ionization-tandem mass spectrometry (HPLC-APCI-MS/MS) method.

RESULTS

Urinary peak concentrations of zilpaterol were observed for all volunteers 1.5-12.5 h after ingestion, and maximum levels >5 ng/mL, which would constitute an AAF in doping controls, were found after the intake of 3 μg of zilpaterol on five consecutive days in one out of five study participants. Noteworthy, the enantiomeric ratio of excreted zilpaterol remained constant over time.

CONCLUSION

This study provides first insights into the urinary excretion of microdosed zilpaterol. Furthermore, a method was successfully developed and applied for the separation of the zilpaterol enantiomers with mass spectrometric detection.

Controllable construction of pharmaceutically significant scaffolds of 2,3-dihydroquinolin-4-one and benzoazepine-5-one via redox-neutral cascade hydride transfer/cyclization

The scaffolds of 2,3-dihydroquinolin-4-one and benzoazepine-5-one were controllably constructed relying on cascade condensation/redox-neutral [1,6]/[1,7]-hydride transfer/cyclization from 2-aminoacetophenone and various aldehydes as well as isatins.

Construction 7-membered ring via Ni-Al bimetal-enabled C-H cyclization for synthesis of tricyclic imidazoles

The construction of 7-membered ring via direct C7-H cyclization of benzoimidazoles with alkenes would provide a more atom- and step-economical route to tricyclic imidazoles and derivatives that widely exist in a broad range of bioactive molecules. However, transition metal-catalyzed C-H cyclization for medium-ring synthesis has been limited to reactive C-H bonds, instead, the activation of unreactive C-H bonds towards medium synthesis still remains an elusive challenge. Herein, we report a direct construction of 7-membered rings via Ni-Al co-catalyzed unreactive C7-H cyclization of benzoimidazoles with alkenes, providing a series of tricyclic imidazoles in 40-98% yield and with up to 95:5 er.

Hapten synthesis and antibody production for the development of a paper immunosensor for lean meat powder zilpaterol

An anti-zilpaterol mAb with an IC₅₀ of 0.31 ng mL⁻¹ and a limit of detection (LOD) of 0.02 ng mL⁻¹ has been developed. For semi-quantitative detection in pork samples, the visual LOD is 0.5 ng mL⁻¹ and the cut-off value is 5 ng mL⁻¹.

Stereocontrolled addition of Grignard reagents to oxa-bridged benzazepines: highly efficient synthesis of functionalized benzazepine scaffolds

An efficient and highly diastereoselective synthesis of 2-substituted benzo[b]azepin-5-ol via stereocontrolled addition of Grignard reagents to oxa-bridged benzazepines has been developed. The reaction proceeds efficiently starting from versatile skeletons with mild reaction conditions as well as simple operation. Furthermore, 2-substituted benzazepinones could been obtained by simple Dess–Martin oxidation in excellent yields.

An efficient and highly diastereoselective synthesis of 2-substituted benzo[b]azepin-5-ol via stereocontrolled addition of Grignard reagents to oxa-bridged benzazepines has been developed.

Asymmetric Synthesis of 1-Benzazepine Derivatives via Copper-Catalyzed Intramolecular Reductive Cyclization

An asymmetric construction of enantioenriched 2,3-substituted-1-benzazepine derivatives containing a cyclic tertiary amine moiety was developed by copper-catalyzed reductive intramolecular cyclization of (E)-dienyl arenes with a tethered ketimine. This protocol involves tandem chemo-, regio-, and enantioselective hydrocupration and asymmetric cyclization in the presence of a chiral bisphosphine-copper catalyst. Under mild conditions, a broad range of 1-benzazepine derivatives was obtained in good to high yields with high degrees of diastereoselectivity and enantioselectivity.

Identification of Buctopamine and Mebuctopamine, a β2 Receptor Agonist and Its Metabolite, in Swine Hair and Feed Additives

4-[2-(t-Butylamino)-1-hydroxyethyl]phenol (buctopamine, 4), a new β₂ receptor agonist (β₂-agonist), was found to be an adulterant in feed additives for swine in Taiwan, where using β₂-agonists in food-production animals is prohibited. Buctopamine and its metabolite, 4-[2-(t-butylamino)-1-hydroxyethyl]-2-methoxyphenol (mebuctopamine, 2), were detected in swine hair specimens. Authentic compounds 2 and 4 were synthesized with 98.6% and 97.7% purity, respectively, as reference standards for analysis, and both compounds were more hydrophilic than ractopamine and clenbuterol. In a preliminary pharmacological evaluation, compounds 2 and 4 exhibited moderate human β₂ receptor binding affinity and did not show significant affinities for the human α₁, α₂, β₁, and β₃ receptors. After addition of compounds 2-4 into the β₂-agonist library, a multiresidue analysis of 26 β₂-agonists by using triple quadrupole LC/MS/MS for routine screening conducted by regulatory authorities was established, in which the common limits of quantification for the 26 β₂-agonists in swine feed and hair are 10 and 25 ng/g, respectively. In addition, the illegal use of buctopamine (4) has been effectively prevented. The results of this study are also useful for controlling the illegal use of new β₂-agonists in food-production animals.

Recent structural advances of β1 and β2 adrenoceptors yield keys for ligand recognition and drug design

Because they represent attractive drug targets, adrenoceptors have been widely studied. Recent progress in structural data of β-adrenoceptors allows us to understand and predict key interactions in ligand recognition and receptor activation. Nevertheless, an important aspect of this process has only begun to be explored: the stabilization of a conformational state of these receptors upon contact with a ligand and the capacity of a ligand to influence receptor conformation through allosteric modulation, biased signaling, and selectivity. The aim of the present Perspective is to identify the well-defined orthosteric binding site and possible allosteric sites and to analyze the importance of the ligand-receptor interaction in the stabilization of certain receptor conformations. For this purpose, we have reviewed recent advances made through the use of X-ray data from ligand-β-adrenoceptor (including ADRB1 and ADRB2) crystal structures. Most importantly, implications in the medicinal chemistry field are explored in relation to drug design.

Impact of substrate elasticity on human hematopoietic stem and progenitor cell adhesion and motility

In the bone marrow, hematopoietic stem cells (HSCs) reside in endosteal and vascular niches. The interactions with the niches are essential for the maintenance of HSC number and properties. Although the molecular nature of these interactions is quite well understood, little is known about the role of physical parameters such as matrix elasticity. Osteoblasts, the major cellular component of the endosteal HSC niche, flatten during HSC mobilization. We show that this process is accompanied by osteoblast stiffening, demonstrating that not only biochemical signals but also mechanical properties of the niche are modulated. HSCs react to stiffer substrates with increased cell adhesion and migration, which could facilitate the exit of HSCs from the niche. These results indicate that matrix elasticity is an important factor in regulating the retention of HSCs in the endosteal niche and should be considered in attempts to propagate HSCs in vitro for clinical applications.