Multivalent site-specific phage modification enhances the binding affinity of receptor ligands

High-throughput screening of combinatorial chemical libraries is a powerful approach for identifying targeted molecules. The display of combinatorial peptide libraries on the surface of bacteriophages offers a rapid, economical way to screen billions of peptides for specific binding properties and has impacted fields ranging from cancer to vaccine development. As a modification to this approach, we have previously created a system that enables site-specific insertion of selenocysteine (Sec) residues into peptides displayed pentavalently on M13 phage as pIII coat protein fusions. In this study, we show the utility of selectively derivatizing these Sec residues through the primary amine of small molecules that target a G protein-coupled receptor, the adenosine A1 receptor, leaving the other coat proteins, including the major coat protein pVIII, unmodified. We further demonstrate that modified Sec-phage with multivalent bound agonist binds to cells and elicits downstream signaling with orders of magnitude greater potency than that of unconjugated agonist. Our results provide proof of concept of a system that can create hybrid small molecule-containing peptide libraries and open up new possibilities for phage-drug therapies.

The second extracellular loop of the adenosine A1 receptor mediates activity of allosteric enhancers

Allosteric enhancers of the adenosine A1 receptor amplify signaling by orthosteric agonists. Allosteric enhancers are appealing drug candidates because their activity requires that the orthosteric site be occupied by an agonist, thereby conferring specificity to stressed or injured tissues that produce adenosine. To explore the mechanism of allosteric enhancer activity, we examined their action on several A1 receptor constructs, including (1) species variants, (2) species chimeras, (3) alanine scanning mutants, and (4) site-specific mutants. These findings were combined with homology modeling of the A1 receptor and in silico screening of an allosteric enhancer library. The binding modes of known docked allosteric enhancers correlated with the known structure-activity relationship, suggesting that these allosteric enhancers bind to a pocket formed by the second extracellular loop, flanked by residues S150 and M162. We propose a model in which this vestibule controls the entry and efflux of agonists from the orthosteric site and agonist binding elicits a conformational change that enables allosteric enhancer binding. This model provides a mechanism for the observations that allosteric enhancers slow the dissociation of orthosteric agonists but not antagonists.

Synthesis and evaluation of new N6-substituted adenosine-5'-N-methylcarboxamides as A3 adenosine receptor agonists

A number of N(6)-substituted adenosine-5'-N-methylcarboxamides were synthesised and their pharmacology, in terms of their receptor affinity, selectivity and cardioprotective effects, were explored. The first series of compounds, 4a-4f and 5a-5f, showed modest receptor affinity for the A(3)AR with K(i) values in the low to mid muM range. However, the incorporation of a 4-(2-aminoethyl)-2,6-di-tert-butylphenol group in the N(6)-position (in compounds 4g and 5g) significantly improved the affinity with K(i) values of 30 and 9 nM, respectively. Improvements in affinity, as well as selectivity were seen when a functionalized linker was introduced. The N(6)-phenyl series, compounds 7a-7d, demonstrated low to mid nanomolar receptor affinities (K(i)=2.3-45.0 nM), with 7b displaying 109-fold selectivity for the A(3)AR (vs A(1)). The N(6)-benzyl series 9a-9c also proved to be potent and selective A(3)AR agonists and the longer chain length linker 13 was tolerated at the A(3)AR without abrogation of affinity or selectivity. Cardioprotection was demonstrated by a simulated ischaemia cell culture assay, whereby 7b, 7c, 9a, 9b and 9c all showed cardioprotective effects at 100 nM comparable or better than the benchmark A(3)AR agonist IB-MECA, but which were indistinguishable by statistical analysis. For example, compound 9c reduced cell death by 68.0+/-3.6%.

3- and 6-Substituted 2-amino-4,5,6,7-tetrahydrothieno[2,3-c]pyridines as A1 adenosine receptor allosteric modulators and antagonists

A series of 2-amino-4,5,6,7-tetrahydrothieno[2,3-c]pyridines were prepared and evaluated as potential allosteric modulators at the A(1) adenosine receptor. The structure-activity relationships of the 3- and 6-positions of a series of 2-amino-4,5,6,7-tetrahydrothieno[2,3-c]pyridines were explored. Despite finding that 3- and 6-substituted 2-amino-4,5,6,7-tetrahydrothieno[2,3-c]pyridines possess the ability to recognize an allosteric site on the agonist-occupied A(1)AR at relatively high concentrations, the structural modifications we have performed on this scaffold favor the expression of orthosteric antagonist properties over allosteric properties. This research has identified 2-amino-4,5,6,7-tetrahydrothieno[2,3-c]pyridines as novel class of orthosteric antagonist of the A(1)AR and highlighted the close relationship between structural elements governing allosteric modulation and orthosteric antagonism of agonist function at the A(1)AR.

2-aminothienopyridazines as novel adenosine A1 receptor allosteric modulators and antagonists

A pharmacophore-based screen identified 32 compounds including ethyl 5-amino-3-(4- tert-butylphenyl)-4-oxo-3,4-dihydrothieno[3,4- d]pyridazine-1-carboxylate ( 8) as a new allosteric modulator of the adenosine A1 receptor (A1AR). On the basis of this lead, various derivatives were prepared and evaluated for activity at the human A 1AR. A number of the test compounds allosterically stabilized agonist-receptor-G protein ternary complexes in dissociation kinetic assays, but were found to be more potent as antagonists in subsequent functional assays of ERK1/2 phosphorylation. Additional experiments on the most potent antagonist, 13b, investigating A1AR-mediated [(35)S]GTPgammaS binding and [(3)H]CCPA equilibrium binding confirmed its antagonistic mode of action and also identified inverse agonism. This study has thus identified a new class of A1AR antagonists that can also recognize the receptor's allosteric site with lower potency.

Dual acting antioxidant A1 adenosine receptor agonists

Herein we report the synthesis and biological evaluation of some potent and selective A(1) adenosine receptor agonists, which incorporate a functionalised linker attached to an antioxidant moiety. N(6)-(2,2,5,5-Tetramethylpyrrolidin-1-yloxyl-3-ylmethyl)adenosine (VCP28, 2e) proved to be an agonist with high affinity (K(i)=50nM) and good selectivity (A(3)/A(1) > or = 400) for the A(1) adenosine receptor. N(6)-[4-[2-[1,1,3,3-Tetramethylisoindolin-2-yloxyl-5-amido]ethyl]phenyl]adenosine (VCP102, 5a) has higher binding affinity (K(i)=7 nM), but lower selectivity (A(3)/A(1)= approximately 3). All compounds bind weakly (K(i)>1 microM) to A(2A) and A(2B) receptors. The combination of A(1) agonist activity and antioxidant activity has the potential to produce cardioprotective effects.

2-Aminothiophene-3-carboxylates and carboxamides as adenosine A1 receptor allosteric enhancers

Three series of 2-amino-4,5,6,7-tetrahydrobenzo[b]thiophene and 2-amino-5,6,7,8-tetrahydrocyclohepta[b]thiophenes with 3-carboxylates and carboxamides have been prepared using the Gewald synthesis and evaluated as A(1)AR allosteric enhancers. The structure-activity relationships of these classes of compound are described. A number of compounds, notably 7b, are more potent and efficacious than PD81,723 (1).

6-aryl-8H-indeno[1,2-d]thiazol-2-ylamines: A1 adenosine receptor agonist allosteric enhancers having improved potency

Allosteric enhancers (AEs) of the A(1) adenosine receptor (A(1)AR) have potential as drugs for treating neurological, cardiovascular, and renal diseases. This report describes the synthesis and evaluation of a series of 6-aryl-8H-indeno[1,2-d]thiazol-2-ylamines that exhibited AE activity at the A(1)AR. Palladium-mediated condensation of arylboronic acids with 5-bromoindan-1-one generated arylindanones 2a-aj for iodine-catalyzed condensation with thiourea, generating 2-aminothiazolium salts 3a-aj. Binding studies using membranes from cells stably expressing human A(1)ARs, A(2A)ARs, or A(3)ARs evaluated AE activity and receptor subtype selectivity. The EC(50) of the AE activities of compounds 3m-o, 3x, and 3ae were 2.2, 1.5, 0.9, 1.0, and 3.0 muM, respectively, substantially lower than that of the well characterized 2-amino-3-aroylthiophene (PD 81,723), >10 microM. The new compounds also have substantially higher maximal AE activity. These compounds had no AE activity at the A(2A)AR and only minimal activity at the A(3)AR.

Allosteric enhancers of A1 adenosine receptors increase receptor-G protein coupling and counteract Guanine nucleotide effects on agonist binding

Endogenous ligands of G protein-coupled receptors bind to orthosteric sites that are topologically distinct from allosteric sites. Certain aminothiophenes such as (2-amino-4,5-dimethyl-3-thienyl)-[3-(trifluromethyl)-phenyl]-methanone (PD81,723) and 2-amino-4,5,6,7-tetrahydro-benzo[b]thiophen-3-yl)-biphenyl-4-yl-methanone (ATL525) are positive allosteric regulators, or enhancers, of the human A1 adenosine receptor (A1AR). In equilibrium binding assays, 125I-N6-aminobenzyladenosine (125I-ABA) binds to two affinity states of A1AR with KD-high (0.33 microM) and KD-low ( approximately 10 nM). Enhancers have little effect on KD-high but convert all A1AR binding sites to the high-affinity state. Enhancers decrease the potency of guanosine 5'-O-(3-thio)triphosphate (GTPgammaS) as an inhibitor of agonist binding by 100-fold and increase agonist-stimulated guanine nucleotide exchange. The association of 125I-ABA to high-affinity receptors on Chinese hamster ovary (CHO)-hA1 membranes does not follow theoretical single-site association kinetics but is approximated by a bi-exponential equation with t1/2 values of 1.85 and 12.8 min. Allosteric enhancers selectively increase the number of slow binding sites, possibly by stabilizing newly formed receptor-G protein complexes. A new rapid assay method scores enhancer activity on a scale from 0 to 100 based on their ability to prevent the rapid dissociation of 125I-ABA from A1AR in response to GTPgammaS. Compared with PD81,723, ATL525 (100 microM) scores higher (27 versus 79) and has less antagonist activity. ATL525 functionally enhances A1 signaling to inhibit cAMP accumulation in CHO-hA1 cells. These data suggest that simultaneously binding orthosteric and allosteric enhancer ligands convert the A1AR from partly to fully coupled to G proteins and prevents rapid uncoupling upon binding of GTPgammaS.

2-Amino-3-benzoylthiophene allosteric enhancers of A1 adenosine agonist binding: new 3, 4-, and 5-modifications

2-Amino-3-aroylthiophenes are agonist allosteric enhancers (AE) at the A(1) adenosine receptor (A(1)AR). Here we report the syntheses of three kinds of novel 2-aminothiophenes and assays of their AE activity at the human A(1)AR (hA(1)AR), namely, (1) 2-amino-4,5-diphenylthiophene-3-carboxylates, 3a-h, (2) 2-amino-3-benzoyl-4,5-diphenylthiophenes, 7a-p, and (3) 2-amino-5-bromo-3-benzoyl-4-phenylthiophenes, 10a-h. An in vitro assay employing the A(1)AR agonist [(125)I]ABA and membranes from CHO-K1 cells stably expressing the hA(1)AR measured an index of AE activity, the ability of a candidate AE to stabilize the agonist-A(1)AR-G protein ternary complex, scored as the percentage of ternary complex remaining after 10 min of dissociation initiated by CPX and GTPgammaS. The AE activity score of 2-amino-4,5-dimethyl-3-(3-trifluoromethylbenzoyl)thiophene (PD 81,723), which was 19%, served as a standard for comparison. Two 3-carboxythiophene 3-trifluoromethylbenzyl esters, 3d (49%) and 3f (63%), had substantial AE activity. The 3-(1-naphthoyl) substituent of 7e (52%) also supported AE activity. Compounds in series 3 tended to be more potent, 10a and 10c having scores of 91 and 80%, respectively. The activity of 2-amino-5-bromo-3-ethoxycarbonyl-4-(3-nitrophenyl)thiophene, 10h (26%), is an exception to the rule that a 3-ethoxycarbonyl substituent cannot support AE activity.

2-Amino-3-aroyl-4,5-alkylthiophenes: agonist allosteric enhancers at human A(1) adenosine receptors

2-Amino-3-benzoylthiophenes are allosteric enhancers (AE) of agonist activity at the A(1) adenosine receptor. The present report describes syntheses and assays of the AE activity at the human A(1)AR (hA(1)AR) of a panel of compounds consisting of nine 2-amino-3-aroylthiophenes (3a-i), eight 2-amino-3-benzoyl-4,5-dimethylthiophenes (12a-h), three 3-aroyl-2-carboxy-4,5-dimethylthiophenes (15a-c), 10 2-amino-3-benzoyl-5,6-dihydro-4H-cyclopenta[b]thiophenes (17a-j), 14 2-amino-3-benzoyl-4,5,6,7-tetrahydrobenzo[b]thiophenes (18a-n), and 15 2-amino-3-benzoyl-5,6,7,8-tetrahydro-4H-cyclohepta[b]thiophenes (19a-o). An in vitro assay employing the A(1)AR agonist [(125)I]ABA and membranes from CHO-K1 cells stably expressing the hA(1)AR measured, as an index of AE activity, the ability of a candidate AE to stabilize the agonist-A(1)AR-G protein ternary complex. Compounds 3a-i had little or no AE activity, and compounds 12a-h had only modest activity, evidence that AE activity depended absolutely on the presence of at least a methyl group at C-4 and C-5. Compounds 17a-c lacked AE activity, suggesting the 2-amino group is essential. Polymethylene bridges linked thiophene C-4 and C-5 of compounds 17a-j, 18a-n, and 19a-o. AE activity increased with the size of the -(CH(2))(n)()- bridge, n = 3 < n = 4 < n = 5. The 3-carbethoxy substituents of 17a, 18a, and 19a did not support AE activity, but a 3-aroyl group did. Bulky (or hydrophobic) substituents at the meta and para positions of the 3-benzoyl group and also 3-naphthoyl groups greatly enhanced activity. Thus, the hA(1)AR contains an allosteric binding site able to accommodate 3-aroyl substituents that are bulky and/or hydrophobic but not necessarily planar. A second region in the allosteric binding site interacts constructively with alkyl substituents at thiophene C-4 and/or C-5.

Gene dose effect reveals no Gs-coupled A2A adenosine receptor reserve in murine T-lymphocytes: studies of cells from A2A-receptor-gene-deficient mice

Agonist binding to extracellular A2A adenosine receptors (A2ARs) inhibits the activation of virtually all tested functions of T-cells and can induce apoptosis in thymocytes. The evaluation of levels of expression of these immunosuppressive receptors is expected to clarify whether the absence of spare A2ARs (no 'receptor reserve') might be one of the mechanisms of attenuation of the effects of extracellular adenosine on T-cells. A2A transcript is found in T-cells and functional receptors can be demonstrated, but the density of receptor on T-cells is too low to be detected by radioligand binding. Studies of direct radioligand binding to murine brain with the selective A2AR agonist [3H]CGS21680 (2-(4-[(2-carboxyethyl)-phenyl]ethylamino)-5'-N-ethylcarboxamidoadenosine) established that striata levels of A2AR are virtually absent from A2A knock-out mice. Mice that are heterozygous (A2AR+/-) for the A2AR express significantly decreased levels of A2AR. To test for the presence of spare receptors in T-cells we took advantage of this gene dose effect and examined whether the decrease in the number of receptors in thymocytes from A2AR+/- mice was proportionately reflected in a decrease in the functional cAMP response of T-cells to adenosine. cAMP accumulation and apoptosis induced by adenosine and by A2AR agonist are of a lower magnitude in T-cells from A2AR+/- heterozygous mice than in T-cells from A2AR+/+ littermate control mice. These results indicate that there is no A2AR reserve in murine T-cells. Strongly decreased adenosine-triggered cAMP increases were detected in thymocytes from A2AR-/- mice, suggesting that A2B adenosine receptors cannot fully compensate for the loss of A2ARs in murine T-cells. We conclude that the number of A2ARs is the limiting factor in determining the maximal cAMP response of T-lymphocytes to extracellular adenosine, thereby minimizing the immunosuppressive effects of extracellular adenosine.

Characterization of human A(2B) adenosine receptors: radioligand binding, western blotting, and coupling to G(q) in human embryonic kidney 293 cells and HMC-1 mast cells

Recombinant human A(2B) adenosine receptors (A(2B)ARs) and receptors extended on the amino terminus with hexahistidine and the FLAG epitope, DYKDDDDK (H/F-A(2B)) were stably overexpressed (to >20,000 fmol/mg protein) in human embryonic kidney 293 cells (HEK-A(2B)). By Western blotting, the H/F-A(2B) receptor runs as a 34.8-kDa glycoprotein. Pharmacological properties of A(2B)ARs were characterized with (125)I-3-aminobenzyl-8-phenyl-(4-oxyacetic acid)-1-propylxanthine (K(D), 36 nM). In competition binding assays, the affinity of agonists is reduced by substitution on either the N(6)- or the C-2 position of the adenine ring, whereas 5'-substitutions increase affinity, resulting in the potency order: 5'-N-ethylcarboxamidoadenosine (NECA) >> N(6)-aminobenzyl-NECA approximately 2-chloroadenosine > 2-[4-(2-carboxyethyl)phenethylamino]-NECA (CGS21680) > N(6)-aminobenzyladenosine. The A(2B)AR is potently blocked by the A(2A)-selective antagonist 4-(2-[7-amino-2-[2-furyl][1,2, 4]triazolo-[2,3-a][1,3,5] triazin-5-yl-amino]ethyl)phenol (ZM241385; K(I), 32 nM for A(2B), 1.4 nM for A(2A)) and the A(1) selective antagonist 8-cyclopentyl-1,3-dipropylxanthine (K(I), 50.5 nM for A(2B); 2.5 nM for A(1)). The K(I) values for the antiasthmatic xanthines, theophylline (7.8 microM) and enprofylline (6.4 microM), are below their therapeutic plasma concentrations (20 to 50 microM), and agree with K(I) determinations for inhibition of NECA-stimulated cAMP accumulation in HEK-A(2B) cells. NECA or N(6)-(2-iodo)benzyl-5'-N-methylcarboxamidodoadenosine (IB-MECA) stimulate inositol trisphosphates and calcium accumulation in HEK-A(2B) or HEK-A(3) cells, respectively, but only the A(3) response is prevented by pertussis toxin. In human HMC-1 mast cells, A(2B)AR activation stimulates calcium mobilization and cAMP accumulation. We conclude that HEK-A(2B) cells and HMC-1 mast cells possess A(2B)AR glycoproteins that are coupled to both G(q/11) and G(s).