Conjugable A3 adenosine receptor antagonists for the development of functionalized ligands and their use in fluorescent probes

Illuminating the monoamine transporters: Fluorescently labelled ligands to study dopamine, serotonin and norepinephrine transporters

Fluorescence microscopy has revolutionized the visualization of physiological processes in live-cell systems. With the recent innovations in super resolution microscopy, these events can be examined with high precision and accuracy. The development of fluorescently labelled small molecules has provided a significant advance in understanding the physiological relevance of targeted proteins that can now be visualized at the cellular level. One set of physiologically important target proteins are the monoamine transporters (MATs) that play an instrumental role in maintaining monoamine signalling homeostasis. Understanding the mechanisms underlying their regulation and dysregulation is fundamental to treating several neuropsychiatric conditions such as attention deficit hyperactivity disorder (ADHD), anxiety, depression and substance use disorders. Herein, we describe the rationale behind the small molecule design of fluorescently labelled ligands (FLL) either as MAT substrates or inhibitors as well as their applications to advance our understanding of this class of transporters in health and disease.

Fluorescent A2A and A3 adenosine receptor antagonists as flow cytometry probes

Adenosine receptor (AR) ligands are being developed for metabolic, cardiovascular, neurological, and inflammatory diseases and cancer. The ease of drug discovery is contingent on the availability of pharmacological tools. Fluorescent antagonist ligands for the human A2A and A3ARs were synthesized using two validated pharmacophores, 1,3-dipropyl-8-phenylxanthine and triazolo[1,5-c]quinazolin-5-yl)amine, which were coupled to eight reporter fluorophores: AlexaFluor, JaneliaFluor (JF), cyanine, and near infrared (NIR) dyes. The conjugates were first screened using radioligand binding in HEK293 cells expressing one of the three AR subtypes. The highest affinities at A2AAR were Ki 144-316 nM for 10, 12, and 19, and at A3AR affinity of Ki 21.6 nM for 19. Specific binding of JF646 conjugate MRS7774 12 to the HEK293 cell surface A2AAR was imaged using confocal microscopy. Compound 19 MRS7535, a triazolo[1,5-c]quinazolin-5-yl)amine containing a Sulfo-Cy7 NIR dye, was suitable for A3AR characterization in whole cells by flow cytometry (Kd 11.8 nM), and its bitopic interaction mode with an A3AR homology model was predicted. Given its affinity and selectivity (11-fold vs. A2AAR, ~ 50-fold vs. A1AR and A2BAR) and a good specific-to-nonspecific binding ratio, 19 could be useful for live cell or potentially a diagnostic in vivo NIR imaging tool and/or therapy targeting the A3AR.

Lighting Up the Plasma Membrane: Development and Applications of Fluorescent Ligands for Transmembrane Proteins

Despite the fact that transmembrane proteins represent the main therapeutic targets for decades, complete and in-depth knowledge about their biochemical and pharmacological profiling is not fully available. In this regard, target-tailored small-molecule fluorescent ligands are a viable approach to fill in the missing pieces of the puzzle. Such tools, coupled with the ability of high-precision optical techniques to image with an unprecedented resolution at a single-molecule level, helped unraveling many of the conundrums related to plasma proteins' life-cycle and druggability. Herein, we review the recent progress made during the last two decades in fluorescent ligand design and potential applications in fluorescence microscopy of voltage-gated ion channels, ligand-gated ion channels and G-coupled protein receptors.

Illuminating the norepinephrine transporter: fluorescent probes based on nisoxetine and talopram

The utilization of fluorescent ligands to study the monoamine transporters (MATs) has increased our knowledge of their function and distribution in live cell systems. In this study, we extend SAR for nisoxetine and talopram as parent compounds, to identify high affinity rhodamine-labeled fluorescent probes for the norepinephrine transporter (NET). Nisoxetine-based fluorescent probe 6 demonstrated high binding affinity (K _i = 43 nM) for NET and an overall selectivity compared to the other transporters for dopamine (DAT; K _i = 1540 nM) and serotonin (SERT; K _i = 785 nM) in competitive radioligand binding assays. Using confocal microscopy, compound 6 was shown to stain both NET and SERT, but not DAT, at low nanomolar concentrations, in transporter-expressing cells.

Molecular probes for the human adenosine receptors

Adenosine receptors, G protein-coupled receptors (GPCRs) that are activated by the endogenous ligand adenosine, have been considered potential therapeutic targets in several disorders. To date however, only very few adenosine receptor modulators have made it to the market. Increased understanding of these receptors is required to improve the success rate of adenosine receptor drug discovery. To improve our understanding of receptor structure and function, over the past decades, a diverse array of molecular probes has been developed and applied. These probes, including radioactive or fluorescent moieties, have proven invaluable in GPCR research in general. Specifically for adenosine receptors, the development and application of covalent or reversible probes, whether radiolabeled or fluorescent, have been instrumental in the discovery of new chemical entities, the characterization and interrogation of adenosine receptor subtypes, and the study of adenosine receptor behavior in physiological and pathophysiological conditions. This review summarizes these applications, and also serves as an invitation to walk another mile to further improve probe characteristics and develop additional tags that allow the investigation of adenosine receptors and other GPCRs in even finer detail.

Potent and selective A3 adenosine receptor antagonists bearing aminoesters as heterobifunctional moieties

A₃ adenosine receptors were found to have a role in different pathological states, such as glaucoma, renal fibrosis, neuropathic pain and cancer. Consequently, it is important to utilize any molecular tool which could help to study these conditions. In the present study we continue our search for potent A₃ adenosine receptor ligands which could be successively conjugated to other molecules with the aim of obtaining more potent (e.g. allosteric ligand conjugation) or detectable ligands (e.g. fluorescent molecule or biotin conjugation). Specifically, different aminoester moieties were introduced at the 5 position of the pyrazolo[4,3-e]-1,2,4-triazolo[1,5-c]pyrimidine core. The ester functionalization represents the candidate for the subsequent conjugation. All the reported compounds are potent hA₃ adenosine receptor antagonists and some of them exhibited high selectivity against the other adenosine receptors. The main structural terms of ligand recognition and selectivity were disclosed by molecular modelling studies. Molecular docking results led to the characterization of an alternative binding mode for antagonists at the orthosteric binding site of the hA₃ adenosine receptor, evaluated and assessed by classical molecular dynamics simulations.

Allosteric Antagonism of the A2A Adenosine Receptor by a Series of Bitopic Ligands

Allosteric antagonism by bitopic ligands, as reported for many receptors, is a distinct modulatory mechanism. Although several bitopic A_2A adenosine receptor (A_2AAR) ligand classes were reported as pharmacological tools, their receptor binding and functional antagonism patterns, i.e., allosteric or competitive, were not well characterized. Therefore, here we systematically characterized A_2AAR binding and functional antagonism of two distinct antagonist chemical classes. i.e., fluorescent conjugates of xanthine amine congener (XAC) and SCH442416. Bitopic ligands were potent, weak, competitive or allosteric, based on the combination of pharmacophore, linker and fluorophore. Among antagonists tested, XAC, XAC245, XAC488, SCH442416, MRS7352 showed K_i binding values consistent with K_B values from functional antagonism. Interestingly, MRS7396, XAC-X-BY630 (XAC630) and 5-(N,N-hexamethylene)amiloride (HMA) were 9–100 times weaker in displacing fluorescent MRS7416 binding than radioligand binding. XAC245, XAC630, MRS7396, MRS7416 and MRS7322 behaved as allosteric A_2AAR antagonists, whereas XAC488 and MRS7395 antagonized competitively. Schild analysis showed antagonism slopes of 0.42 and 0.47 for MRS7396 and XAC630, respectively. Allosteric antagonists HMA and MRS7396 were more potent in displacing [³H]ZM241385 binding than MRS7416 binding. Sodium site D52N mutation increased and decreased affinity of HMA and MRS7396, respectively, suggesting possible preference for different A_2AAR conformations. The allosteric binding properties of some bitopic ligands were rationalized and analyzed using the Hall two-state allosteric model. Thus, fluorophore tethering to an orthosteric ligand is not neutral pharmacologically and may confer unexpected properties to the conjugate.