Molecular probes for muscarinic receptors: derivatives of the M1-antagonist telenzepine

Functionalized Congeners of 2H-Chromene P2Y6 Receptor Antagonists

The P2Y6 receptor (P2Y6R), a Gq-coupled receptor, is a potential drug discovery target for various inflammatory and degenerative conditions. Antagonists have been shown to attenuate colitis, acute lung injury, etc. In the search for competitive antagonists, we have investigated the SAR of 3-nitro-2-(trifluoromethyl)-2H-chromene derivatives, although high affinity is lacking. We now reveal that long-chain amino-functionalized congeners display greatly enhanced affinity in the antagonism of UDP-induced Ca2+ mobilization in human (h) P2Y6R-transfected 1321N1 astrocytoma cells. A 6-(Boc-amino-n-heptylethynyl) analogue 30 (MRS4940) had an IC50 of 162 nM, which was a 123-fold greater affinity than the corresponding unprotected primary alkylamine, 107-fold greater than the corresponding pivaloyl derivative 30, and 132-fold selective compared to the P2Y14R. However, similar Boc-amino chains attached at the 8-position produced weak µM affinity. Thus, the P2Y6R affinity depended on the chain length, attachment point, and terminal functionality. Off-target activities, at 45 sites, were tested for acylamino derivatives 20, 24, 26, 30, 31, and 37, which showed multiple interactions, particularly at the biogenic amine receptors. The more potent analogues may be suitable for evaluation in inflammation and cancer models, which will be performed in future studies.

Differently fluorescence-labelled dibenzodiazepinone-type muscarinic acetylcholine receptor ligands with high M2R affinity

A series of fluorescent dibenzodiazepinone-type muscarinic acetylcholine M2 receptor (M2R) ligands was synthesized using various fluorescent dyes (5-TAMRA, λ ex/λ em ≈ 547/576 nm; BODIPY 630/650, λ ex/λ em ≈ 625/640 nm; pyridinium dye Py-1, λ ex/λ em ≈ 611/665 nm and pyridinium dye Py-5, λ ex/λ em ≈ 465/732 nm). All fluorescent probes exhibited high M2R affinity (pK i (radioligand competition binding): 8.75-9.62, pK d (flow cytometry): 8.36-9.19), a very low preference for the M2R over the M1 and M4 receptors and moderate to pronounced M2R selectivity compared to the M3 and M5 receptors. The presented fluorescent ligands are considered useful molecular tools for future studies using methods such as fluorescence anisotropy and BRET based MR binding assays.

Miniaturization of Fluorescence Polarization Receptor-Binding Assays Using CyDye-Labeled Ligands

Fluorescence polarization (FP) is an established technique for the study of biological interactions and is frequently used in the high-throughput screening (HTS) of potential new drug targets. This work describes the miniaturization of FP receptor assays to 1536-well formats for use in HTS. The FP assays were initially developed in 384-well microplates using CyDye-labeled nonpeptide and peptide ligands. Receptor expression levels varied from ∼1 to 10 pmols receptor per mg protein, and ligand concentrations were in the 0.5- to 1.0-nM range. The FP assays were successfully miniaturized to 1536-well formats using Cy3B-labeled ligands, significantly reducing reagent consumption, particularly the receptor source, without compromising assay reliability. Z' factor values determined for the FP receptor assays in both 384- and 1536-well formats were found to be > 0.5, indicating the assays to be robust, reliable, and suitable for HTS purposes.

A fluorescence anisotropy assay for the muscarinic M1 G-protein-coupled receptor

In the search for new chemical entities that interact with G-proteincoupled receptors (GPCRs), assays that quantify efficacy and affinity are employed. Traditional methods for measuring affinity involve radiolabeled ligands. To address the need for homogeneous biochemical fluorescent assays to characterize orthosteric ligand affinity and dissociation rates, we have developed a fluorescence anisotropy (FA) assay for the muscarinic M1 receptor that can be conducted in a 384-well plate. We used membranes from a muscarinic M1 cell line optimized for high-throughput functional assays and the previously characterized fluorescent antagonist BODIPY FL pirenzepine. The affinities of reference compounds were determined in the competitive FA assay and compared with those obtained with a competitive filter-based radioligand-binding assay using [(3)H] N-methylscopolamine. The IC(50) values produced from the FA assay were well-correlated with the radioligand-binding K(i) values (R(2) = 0.98). The dissociation of the BODIPY FL pirenzepine was readily monitored in real time using the FA assay and was sensitive to the presence of the allosteric modulator gallamine. This M1 FA assay offers advantages over traditional radioligandbinding assays as it eliminates radioactivity while allowing investigation of orthosteric or allosteric muscarinic M1 ligands in a homogeneous format.

Functionalized congener approach to the design of ligands for G protein-coupled receptors (GPCRs)

Functionalized congeners, in which a chemically functionalized chain is incorporated at an insensitive site on a pharmacophore, have been designed from the agonist and antagonist ligands of various G protein-coupled receptors (GPCRs). These chain extensions enable a conjugation strategy for detecting and characterizing GPCR structure and function and pharmacological modulation. The focus in many studies of functionalized congeners has been on two families of GPCRs: those responding to extracellular purines and pyrimidines-i.e., adenosine receptors (ARs) and P2Y nucleotide receptors. Functionalized congeners of small molecule as ligands for other GPCRs and non-G protein coupled receptors have also been designed. For example, among biogenic amine neurotransmitter receptors, muscarinic acetylcholine receptor antagonists and adrenergic receptor ligands have been studied with a functionalized congener approach. Adenosine A(1), A(2A), and A(3) receptor functionalized congeners have yielded macromolecular conjugates, irreversibly binding AR ligands for receptor inactivation and cross-linking, radioactive probes that use prosthetic groups, immobilized ligands for affinity chromatography, and dual-acting ligands that function as binary drugs. Poly(amidoamine) dendrimers have served as nanocarriers for covalently conjugated AR functionalized congeners. Rational methods of ligand design derived from molecular modeling and templates have been included in these studies. Thus, the design of novel ligands, both small molecules and macromolecular conjugates, for studying the chemical and biological properties of GPCRs have been developed with this approach, has provided researchers with a strategy that is more versatile than the classical medicinal chemical approaches.

Fluorescent pirenzepine derivatives as potential bitopic ligands of the human M1 muscarinic receptor

Following a recent description of fluorescence resonance energy transfer between enhanced green fluorescent protein (EGFP)-fused human muscarinic M1 receptors and Bodipy-labeled pirenzepine, we synthesized seven fluorescent derivatives of this antagonist in order to further characterize ligand-receptor interactions. These compounds carry Bodipy [558/568], Rhodamine Red-X [560/580], or Fluorolink Cy3 [550/570] fluorophores connected to pirenzepine through various linkers. All molecules reversibly bind with high affinity to M1 receptors (radioligand and energy transfer binding experiments) provided that the linker contains more than six atoms. The energy transfer efficiency exhibits modest variations among ligands, indicating that the distance separating EGFP from the fluorophores remains almost constant. This also supports the notion that the fluorophores may bind to the receptor protein. Kinetic analyses reveal that the dissociation of two Bodipy derivatives (10 or 12 atom long linkers) is sensitive to the presence of the allosteric modulator brucine, while that of all other molecules (15-24 atom long linkers) is not. The data favor the idea that these analogues might interact with both the acetylcholine and the brucine binding domains.

Functionalized congeners of tyrosine-based P2X(7) receptor antagonists: probing multiple sites for linking and dimerization

Chemically funtionalized analogues of antagonists of the P2X(7) receptor, an ATP-gated cation channel, were synthesized as tools for biophysical studies of the receptor. These functionalized congeners were intended for use in chemical conjugation with retention of biological potency. The antagonists were L-tyrosine derivatives, related to [N-benzyloxycarbonyl-O-(4-arylsulfonyl)-L-tyrosyl]benzoylpiperazine (such as MRS2409, 2). The analogues were demonstrated to be antagonists in an assay of human P2X(7) receptor function, consisting of inhibition of ATP-induced K(+) efflux in HEK293 cells expressing the recombinant receptor. The analogues were of the general structure R(1)-Tyr(OR(2))-piperazinyl-R(3), in which three positions (R(1)-R(3)) were systematically varied in structure through introduction of chemically reactive groups. Each of the three positions was designed to incorporate a 3- or 4-nitrophenyl group. The nitro groups were reduced using NaBH(4)-copper(II) acetylacetonate to amines, which were either converted to the isothiocyanate groups, as potential affinity labels for the receptor, or acylated, as models for conjugation. An alternate route to N(alpha)-3-aminobenzyloxycarbonyl functionalization was devised. The various positions of functionalization were compared for effects on biological potency, and the R(2) and R(3) positions were found to be most amenable to derivatization with retention of high potency. Four dimeric permutations of the antagonists were synthesized by coupling each of the isothiocyanate derivatives to either the precursor amine or to other amine congeners. Only dimers linked at the R(2)-position were potent antagonists. In concentration-response studies, two derivatives, a 3-nitrobenzyloxycarbonyl derivative 18 and a 4-nitrotoluenesulfonate 26b, displayed IC(50) values of roughly 100 nM as antagonists of P2X(7) receptor-mediated K(+) flux.

Functionalized congeners of 1,4-dihydropyridines as antagonist molecular probes for A3 adenosine receptors

4-Phenylethynyl-6-phenyl-1,4-dihydropyridine derivatives are selective antagonists at human A3 adenosine receptors, with Ki values in a radioligand binding assay vs [125I]AB-MECA [N(6)-(4-amino-3-iodobenzyl)-5'-N-methylcarbamoyl-adenosine] in the submicromolar range. In this study, functionalized congeners of 1,4-dihydropyridines were designed as chemically reactive adenosine A3 antagonists, for the purpose of synthesizing molecular probes for this receptor subtype. Selectivity of the new analogues for cloned human A3 adenosine receptors was determined in radioligand binding in comparison to binding at rat brain A1 and A2A receptors. Benzyl ester groups at the 3- and/or 5-positions and phenyl groups at the 2- and/or 6-positions were introduced as potential sites for chain attachment. Structure-activity analysis at A3 adenosine receptors indicated that 3,5-dibenzyl esters, but not 2,6-diphenyl groups, are tolerated in binding. Ring substitution of the 5-benzyl ester with a 4-fluorosulfonyl group provided enhanced A3 receptor affinity resulting in a Ki value of 2.42 nM; however, a long-chain derivative containing terminal amine functionalization at the 4-position of the 5-benzyl ester showed only moderate affinity. This sulfonyl fluoride derivative appeared to bind irreversibly to the human A3 receptor (1 h incubation at 100 nM resulting in the loss of 56% of the specific radioligand binding sites), while the binding of other potent dihydropyridines and other antagonists was generally reversible. At the 3-position of the dihydropyridine ring, an amine-functionalized chain attached at the 4-position of a benzyl ester provided higher A3 receptor affinity than the corresponding 5-position isomer. This amine congener was also used as an intermediate in the synthesis of a biotin conjugate, which bound to A3 receptors with a Ki value of 0.60 microM.

Molecular probes for muscarinic receptors: functionalized congeners of selective muscarinic antagonists

The muscarinic agonist oxotremorine and the tricyclic muscarinic antagonists pirenzepine and telenzepine have been derivatized using a functionalized congener approach for the purpose of synthesizing high affinity ligand probes that are suitable for conjugation with prosthetic groups, for receptor cross-linking, fluorescent and radioactive detection, etc. A novel fluorescent conjugate of TAC (telenzepine amine congener), an n-decylamino derivative of the m1-selective antagonist, with the fluorescent trisulfonated pyrene dye Cascade Blue may be useful for assaying the receptor as an alternative to radiotracers. In a rat m3 receptor mutant containing a single amino acid substitution in the sixth transmembrane domain (Asn507 to Ala) the parent telenzepine lost 636-fold in affinity, while TAC lost only 27-fold. Thus, the decylamino group of TAC stabilizes the bound state and thus enhances potency by acting as a distal anchor in the receptor binding site. We have built a computer-assisted molecular model of the transmembrane regions of muscarinic receptors based on homology with the G-protein coupled receptor rhodopsin, for which a low resolution structure is known. We have coordinated the antagonist pharmacophore (tricyclic and piperazine moieties) with residues of the third and seventh helices of the rat m3 receptor. Although the decylamino chain of TAC is likely to be highly flexible and may adopt many conformations, we located one possible site for a salt bridge formation with the positively charged -NH3+ group, i.e. Asp113 in helix II.

MUSCARINIC RECEPTOR PROBES BASED ON AMINE CONGENERS OF PIRENZEPINE AND TELENZEPINE

The N-methyl groups of pirenzepine and telenzepine (M1-antagonists) were modified to produce chemically functionalized N-alkyl analogs using a "functionalized congener" approach. The derivatives were tested in binding assays vs. [3H]N-methylscopotamine in rat forebrain and cardiac membranes. The potency/selectivity were highly dependent on substitutions of the N-methyl group. The affinity in a series of n-alkyl amino derivatives progressively increased with the number of methylene groups. The amines were acylated with various reporter groups resulting in molecular probes of nanomolar affinity. The effect of chain length on aryl isothiocyanate affinity labels is explored.

High affinity acylating antagonists for muscarinic receptors

The muscarinic antagonists pirenzepine and telenzepine were derivatized as alkylamino derivatives at a site on the molecules corresponding to a region of bulk tolerance in receptor binding. The distal primary amino groups were coupled to the cross-linking reagent meta-phenylene diisothiocyanate, resulting in two isothiocyanate derivatives that were found to inhibit muscarinic receptors irreversibly and in a dose-dependent fashion. Preincubation of rat forebrain membranes with an isothiocyanate derivative followed by radioligand binding using [3H]N-methylscopolamine diminished the Bmax value, but did not affect the Kd value. The receptor binding site was not restored upon repeated washing, indicating that irreversible inhibition had occurred. IC50 values for the irreversible inhibition at rat forebrain muscarinic receptors were 0.15 nM and 0.19 nM, for derivatives of pirenzepine and telenzepine, respectively. The isothiocyanate derivative of pirenzepine was non-selective as an irreversible muscarinic inhibitor, and the corresponding derivative prepared from telenzepine was 5-fold selective for forebrain (mainly m1) vs. heart (m2) muscarinic receptors.