Exploring novel dilazep derivatives as hENT1 inhibitors and potentially covalent molecular tools.

The human equilibrative nucleoside transporter 1 (SLC29A1, hENT1) is a solute carrier that modulates the passive transport of nucleosides and nucleobases, such as adenosine. This nucleoside regulates various physiological processes, such as vasodilation and -constriction, neurotransmission and immune defense. Marketed drugs such as dilazep and dipyridamole have proven useful in cardiovascular afflictions, but the application of hENT1 inhibitors can be beneficial in a number of other diseases. In this study, 39 derivatives of dilazep's close analogue ST7092 were designed, synthesized and subsequently assessed using [3H]NBTI displacement assays and molecular docking. Different substitution patterns of the trimethoxy benzoates of ST7092 reduced interactions within the binding pocket, resulting in diminished hENT1 affinity. Conversely, [3H]NBTI displacement by potentially covalent compounds 14b, 14c, and 14d resulted in high affinities (Ki values between 1.1 and 17.5 nM) for the transporter, primarily by the ability of accommodating the inhibitors in various ways in the binding pocket. However, any indication of covalent binding with amino acid residue C439 remained absent, conceivably as a result of decreased nucleophilic residue reactivity. In conclusion, this research introduces novel dilazep derivatives that are active as hENT1 inhibitors, along with the first high affinity dilazep derivatives equipped with an electrophilic warhead. These findings will aid the rational and structure-based development of novel hENT1 inhibitors and pharmacological tools to study hENT1's function, binding mechanisms, and its relevance in (patho)physiological conditions.

Species differences and mechanism of action of A3 adenosine receptor allosteric modulators.

Activity of the A3 adenosine receptor (AR) allosteric modulators LUF6000 (2-cyclohexyl-N-(3,4-dichlorophenyl)-1H-imidazo [4,5-c]quinolin-4-amine) and LUF6096 (N-{2-[(3,4-dichlorophenyl)amino]quinolin-4-yl}cyclohexanecarbox-amide) was compared at four A3AR species homologs used in preclinical drug development. In guanosine 5'-[γ-[35S]thio]triphosphate ([35S]GTPγS) binding assays with cell membranes isolated from human embryonic kidney cells stably expressing recombinant A3ARs, both modulators substantially enhanced agonist efficacy at human, dog, and rabbit A3ARs but provided only weak activity at mouse A3ARs. For human, dog, and rabbit, both modulators increased the maximal efficacy of the A3AR agonist 2-chloro-N 6-(3-iodobenzyl)adenosine-5'-N-methylcarboxamide as well as adenosine > 2-fold, while slightly reducing potency in human and dog. Based on results from N 6-(4-amino-3-[125I]iodobenzyl)adenosine-5'-N-methylcarboxamide ([125I]I-AB-MECA) binding assays, we hypothesize that potency reduction is explained by an allosterically induced slowing in orthosteric ligand binding kinetics that reduces the rate of formation of ligand-receptor complexes. Mutation of four amino acid residues of the human A3AR to the murine sequence identified the extracellular loop 1 (EL1) region as being important in selectively controlling the allosteric actions of LUF6096 on [125I]I-AB-MECA binding kinetics. Homology modeling suggested interaction between species-variable EL1 and agonist-contacting EL2. These results indicate that A3AR allostery is species-dependent and provide mechanistic insights into this therapeutically promising class of agents.

Affinity and kinetics study of anthranilic acids as HCA2 receptor agonists.

Structure-affinity relationship (SAR) and structure-kinetics relationship (SKR) studies were combined to investigate a series of biphenyl anthranilic acid agonists for the HCA2 receptor. In total, 27 compounds were synthesized and twelve of them showed higher affinity than nicotinic acid. Two compounds, 6g (IC50=75nM) and 6z (IC50=108nM) showed a longer residence time profile compared to nicotinic acid, exemplified by their kinetic rate index (KRI) values of 1.31 and 1.23, respectively. The SAR study resulted in the novel 2-F, 4-OH derivative (6x) with an IC50 value of 23nM as the highest affinity HCA2 agonist of the biphenyl series, although it showed a similar residence time as nicotinic acid. The SAR and SKR data suggest that an early compound selection based on binding kinetics is a promising addition to the lead optimization process.

Label-free detection of prostaglandin transporter (SLCO2A1) function and inhibition: insights by wound healing and TRACT assays.

The prostaglandin transporter (PGT, SLCO2A1) mediates transport of prostanoids (a.o. prostaglandin E2 (PGE2)) into cells and thereby promotes their degradation. Overexpression of PGT leads to low extracellular PGE2 levels and has been linked to impaired wound healing of diabetic foot ulcers. Inhibition of PGT could thus be beneficial, however, no PGT inhibitors are currently on the market and drug discovery efforts are hampered by lack of high-through screening assays for this transporter. Here we report on a label-free impedance-based assay for PGT that measures transport activity through receptor activation (TRACT) utilizing prostaglandin E2 receptor subtype EP3 and EP4 that are activated by PGE2. We found that induction of PGT expression on HEK293-JumpIn-SLCO2A1 cells that also express EP3 and EP4 leads to an over 10-fold reduction in agonistic potency of PGE2. PGE2 potency could be recovered upon inhibition of PGT-mediated PGE2 uptake with PGT inhibitors olmesartan and T26A, the potency of which could be established as well. Moreover, the TRACT assay enabled the assessment of transport function of PGT natural variants. Lastly, HUVEC cells endogenously expressing prostanoid receptors and PGT were exploited to study wound healing properties of PGE2 and T26A in real-time using a novel impedance-based scratch-induced wound healing assay. These novel impedance-based assays will advance PGT drug discovery efforts and pave the way for the development of PGT-based therapies.

Development of subtype-selective covalent ligands for the adenosine A2B receptor by tuning the reactive group.

Signalling through the adenosine receptors (ARs), in particular through the adenosine A2B receptor (A2BAR), has been shown to play a role in a variety of pathological conditions, ranging from immune disorders to cancer. Covalent ligands for the A2BAR have the potential to irreversibly block the receptor, as well as inhibit all A2BAR-induced signalling pathways. This will allow a thorough investigation of the pathophysiological role of the receptor. In this study, we synthesized and evaluated a set of potential covalent ligands for the A2BAR. The ligands all contain a core scaffold consisting of a substituted xanthine, varying in type and orientation of electrophilic group (warhead). Here, we find that the right combination of these variables is necessary for a high affinity, irreversible mode of binding and selectivity towards the A2BAR. Altogether, this is the case for sulfonyl fluoride 24 (LUF7982), a covalent ligand that allows for novel ways to interrogate the A2BAR.

Targeting the Kv11.1 (hERG) channel with allosteric modulators. Synthesis and biological evaluation of three novel series of LUF7346 derivatives.

We synthesized and evaluated three novel series of substituted benzophenones for their allosteric modulation of the human Kv11.1 (hERG) channel. We compared their effects with reference compound LUF7346 previously shown to shorten the action potential of cardiomyocytes derived from human stem cells. Most compounds behaved as negative allosteric modulators (NAMs) of [3H]dofetilide binding to the channel. Compound 9i was the most potent amongst all ligands, remarkably reducing the affinity of dofetilide in competitive displacement assays. One of the other derivatives (6k) tested in a second radioligand binding set-up, displayed unusual displacement characteristics with a pseudo-Hill coefficient significantly distinct from unity, further indicative of its allosteric effects on the channel. Some compounds were evaluated in a more physiologically relevant context in beating cardiomyocytes derived from human induced pluripotent stem cells. Surprisingly, the compounds tested showed effects quite different from the reference NAM LUF7346. For instance, compound 5e prolonged, rather than shortened, the field potential duration, while it did not influence this parameter when the field potential was already prolonged by dofetilide. In subsequent patch clamp studies on HEK293 cells expressing the hERG channel the compounds behaved as channel blockers. In conclusion, we successfully synthesized and identified new allosteric modulators of the hERG channel. Unexpectedly, their effects differed from the reference compound in functional assays on hERG-HEK293 cells and human cardiomyocytes, to the extent that the compounds behaved as stand-alone channel blockers.

Crystal Structure and Subsequent Ligand Design of a Nonriboside Partial Agonist Bound to the Adenosine A2A Receptor.

In this study, we determined the crystal structure of an engineered human adenosine A2A receptor bound to a partial agonist and compared it to structures cocrystallized with either a full agonist or an antagonist/inverse agonist. The interaction between the partial agonist, belonging to a class of dicyanopyridines, and amino acids in the ligand binding pocket inspired us to develop a small library of derivatives and assess their affinity in radioligand binding studies and potency and intrinsic activity in a functional, label-free, intact cell assay. It appeared that some of the derivatives retained the partial agonist profile, whereas other ligands turned into inverse agonists. We rationalized this remarkable behavior with additional computational docking studies.

Synthesis and Pharmacological Evaluation of Triazolopyrimidinone Derivatives as Noncompetitive, Intracellular Antagonists for CC Chemokine Receptors 2 and 5.

CC chemokine receptors 2 (CCR2) and 5 (CCR5) are involved in many inflammatory diseases; however, most CCR2 and CCR5 clinical candidates have been unsuccessful. (Pre)clinical evidence suggests that dual CCR2/CCR5 inhibition might be more effective in the treatment of such multifactorial diseases. In this regard, the highly conserved intracellular binding site in chemokine receptors provides a new avenue for the design of multitarget ligands. In this study, we synthesized and evaluated the biological activity of a series of triazolopyrimidinone derivatives in CCR2 and CCR5. Radioligand binding assays first showed that they bind to the intracellular site of CCR2, and in combination with functional assays on CCR5, we explored structure-affinity/activity relationships in both receptors. Although most compounds were CCR2-selective, 39 and 43 inhibited ß-arrestin recruitment in CCR5 with high potency. Moreover, these compounds displayed an insurmountable mechanism of inhibition in both receptors, which holds promise for improved efficacy in inflammatory diseases.

Development of Covalent Ligands for G Protein-Coupled Receptors: A Case for the Human Adenosine A3 Receptor.

The development of covalent ligands for G protein-coupled receptors (GPCRs) is not a trivial process. Here, we report a streamlined workflow thereto from synthesis to validation, exemplified by the discovery of a covalent antagonist for the human adenosine A3 receptor (hA3AR). Based on the 1 H,3 H-pyrido[2,1- f]purine-2,4-dione scaffold, a series of ligands bearing a fluorosulfonyl warhead and a varying linker was synthesized. This series was subjected to an affinity screen, revealing compound 17b as the most potent antagonist. In addition, a nonreactive methylsulfonyl derivative 19 was developed as a reversible control compound. A series of assays, comprising time-dependent affinity determination, washout experiments, and [35S]GTPγS binding assays, then validated 17b as the covalent antagonist. A combined in silico hA3AR-homology model and site-directed mutagenesis study was performed to demonstrate that amino acid residue Y2657.36 was the unique anchor point of the covalent interaction. This workflow might be applied to other GPCRs to guide the discovery of covalent ligands.

A new generation of adenosine receptor antagonists: from di- to trisubstituted aminopyrimidines.

New adenosine receptor ligands were designed as hybrid structures between previously synthesized substituted dicyanopyridines and aminopyrimidines, yielding two series of cyano-substituted diphenylaminopyrimidines. We were interested in assessing the effect of this substitution pattern on both affinity and intrinsic activity, as the dicyanopyridines comprised both agonists and inverse agonists, whereas the original aminopyrimidines were exclusively inverse agonists. It was found that the new compounds were generally selective for adenosine A(1) receptors, although affinity for the adenosine A(2A) receptor was also noticed for some of the compounds. In a cAMP second messenger assay the compounds behaved as inverse agonists rather than agonists. Among the more A(1) receptor-selective compounds were 5 (LUF6048), 27 (LUF6040) and 53 (LUF6056) with K(i) values of 8.1, 1.2 and 5.7nM, respectively.

Pyrrolone Derivatives as Intracellular Allosteric Modulators for Chemokine Receptors: Selective and Dual-Targeting Inhibitors of CC Chemokine Receptors 1 and 2.

The recent crystal structures of CC chemokine receptors 2 and 9 (CCR2 and CCR9) have provided structural evidence for an allosteric, intracellular binding site. The high conservation of residues involved in this site suggests its presence in most chemokine receptors, including the close homologue CCR1. By using [3H]CCR2-RA-[ R], a high-affinity, CCR2 intracellular ligand, we report an intracellular binding site in CCR1, where this radioligand also binds with high affinity. In addition, we report the synthesis and biological characterization of a series of pyrrolone derivatives for CCR1 and CCR2, which allowed us to identify several high-affinity intracellular ligands, including selective and potential multitarget antagonists. Evaluation of selected compounds in a functional [35S]GTPγS assay revealed that they act as inverse agonists in CCR1, providing a new manner of pharmacological modulation. Thus, this intracellular binding site enables the design of selective and multitarget inhibitors as a novel therapeutic approach.

Structure-Affinity Relationships and Structure-Kinetics Relationships of Pyrido[2,1-f]purine-2,4-dione Derivatives as Human Adenosine A3 Receptor Antagonists.

We expanded on a series of pyrido[2,1-f]purine-2,4-dione derivatives as human adenosine A3 receptor (hA3R) antagonists to determine their kinetic profiles and affinities. Many compounds showed high affinities and a diverse range of kinetic profiles. We found hA3R antagonists with very short residence time (RT) at the receptor (2.2 min for 5) and much longer RTs (e.g., 376 min for 27 or 391 min for 31). Two representative antagonists (5 and 27) were tested in [35S]GTPγS binding assays, and their RTs appeared correlated to their (in)surmountable antagonism. From a kon-koff-KD kinetic map, we divided the antagonists into three subgroups, providing a possible direction for the further development of hA3R antagonists. Additionally, we performed a computational modeling study that sheds light on the crucial receptor interactions, dictating the compounds' binding kinetics. Knowledge of target binding kinetics appears useful for developing and triaging new hA3R antagonists in the early phase of drug discovery.

The role of the C-terminus of the human hydroxycarboxylic acid receptors 2 and 3 in G protein activation using Gα-engineered yeast cells.

In the present study we focused our attention on the family of hydroxycarboxylic acid (HCA) receptors, a GPCR family of three members, of which the HCA2 and HCA3 receptors share 95% high sequence identity but differ considerably in C-terminus length with HCA3 having the longest tail. The two receptors were expressed and analysed for their activation profile in Saccharomyces cerevisiae MMY yeast strains that have different G protein Gα subunits. The hHCA2 receptor was promiscuous in its G protein coupling preference. In the presence of nicotinic acid the hHCA2 receptor activated almost all G protein pathways except Gαq (MMY14). However, the Gα protein coupling profile of the hHCA3 receptor was less promiscuous, as the receptor only activated Gαi1 (MMY23) and Gαi3 (MMY24) pathways. We then constructed two mutant receptors by 'swapping' the short (HCA2) and long (HCA3) C-terminus. The differences in HCA2 and HCA3 receptor activation and G protein selectivity were not controlled, however, by their C-terminal tails, as we observed only minor differences between mutant and corresponding wild-type receptor. This study provides new insights into the G protein coupling profiles of the HCA receptors and the function of the receptor's C terminus, which may be extended to other GPCRs.

Allosteric Modulation of Kv11.1 (hERG) Channels Protects Against Drug-Induced Ventricular Arrhythmias.

BACKGROUND: Ventricular arrhythmias as a result of unintentional blockade of the Kv11.1 (hERG [human ether-à-go-go-related gene]) channel are a major safety concern in drug development. In past years, several highly prescribed drugs have been withdrawn for their ability to cause such proarrhythmia. Here, we investigated whether the proarrhythmic risk of existing drugs could be reduced by Kv11.1 allosteric modulators. METHODS AND RESULTS: Using [(3)H]dofetilide-binding assays with membranes of human Kv11.1-expressing human embryonic kidney 293 cells, 2 existing compounds (VU0405601 and ML-T531) and a newly synthesized compound (LUF7244) were found to be negative allosteric modulators of dofetilide binding to the Kv11.1 channel, with LUF7244 showing the strongest effect at 10 µmol/L. The Kv11.1 affinities of typical blockers (ie, dofetilide, astemizole, sertindole, and cisapride) were significantly decreased by LUF7244. Treatment of confluent neonatal rat ventricular myocyte (NRVM) monolayers with astemizole or sertindole caused heterogeneous prolongation of action potential duration and a high incidence of early afterdepolarizations on 1-Hz electric point stimulation, occasionally leading to unstable, self-terminating tachyarrhythmias. Pretreatment of NRVMs with LUF7244 prevented these proarrhythmic effects. NRVM monolayers treated with LUF7244 alone displayed electrophysiological properties indistinguishable from those of untreated NRVM cultures. Prolonged exposure of NRVMs to LUF7244 or LUF7244 plus astemizole did not affect their viability, excitability, and contractility as assessed by molecular, immunological, and electrophysiological assays. CONCLUSIONS: Allosteric modulation of the Kv11.1 channel efficiently suppresses drug-induced ventricular arrhythmias in vitro by preventing potentially arrhythmogenic changes in action potential characteristics, raising the possibility to resume the clinical use of unintended Kv11.1 blockers via pharmacological combination therapy.

Predicting Binding Affinities for GPCR Ligands Using Free-Energy Perturbation.

The rapid growth of structural information for G-protein-coupled receptors (GPCRs) has led to a greater understanding of their structure, function, selectivity, and ligand binding. Although novel ligands have been identified using methods such as virtual screening, computationally driven lead optimization has been possible only in isolated cases because of challenges associated with predicting binding free energies for related compounds. Here, we provide a systematic characterization of the performance of free-energy perturbation (FEP) calculations to predict relative binding free energies of congeneric ligands binding to GPCR targets using a consistent protocol and no adjustable parameters. Using the FEP+ package, first we validated the protocol, which includes a full lipid bilayer and explicit solvent, by predicting the binding affinity for a total of 45 different ligands across four different GPCRs (adenosine A2AAR, ß1 adrenergic, CXCR4 chemokine, and δ opioid receptors). Comparison with experimental binding affinity measurements revealed a highly predictive ranking correlation (average spearman ρ = 0.55) and low root-mean-square error (0.80 kcal/mol). Next, we applied FEP+ in a prospective project, where we predicted the affinity of novel, potent adenosine A2A receptor (A2AR) antagonists. Four novel compounds were synthesized and tested in a radioligand displacement assay, yielding affinity values in the nanomolar range. The affinity of two out of the four novel ligands (plus three previously reported compounds) was correctly predicted (within 1 kcal/mol), including one compound with approximately a tenfold increase in affinity compared to the starting compound. Detailed analyses of the simulations underlying the predictions provided insights into the structural basis for the two cases where the affinity was overpredicted. Taken together, these results establish a protocol for systematically applying FEP+ to GPCRs and provide guidelines for identifying potent molecules in drug discovery lead optimization projects.

Synthesis and biological evaluation of negative allosteric modulators of the Kv11.1(hERG) channel.

We synthesized and evaluated a series of compounds for their allosteric modulation at the Kv11.1 (hERG) channel. Most compounds were negative allosteric modulators of [(3)H]dofetilide binding to the channel, in particular 7f, 7h-j and 7p. Compounds 7f and 7p were the most potent negative allosteric modulators amongst all ligands, significantly increasing the dissociation rate of dofetilide in the radioligand kinetic binding assay, while remarkably reducing the affinities of dofetilide and astemizole in a competitive displacement assay. Additionally, both 7f and 7p displayed peculiar displacement characteristics with Hill coefficients significantly distinct from unity as shown by e.g., dofetilide, further indicative of their allosteric effects on dofetilide binding. Our findings in this investigation yielded several promising negative allosteric modulators for future functional and clinical research with respect to their antiarrhythmic propensities, either alone or in combination with known Kv11.1 blockers.

Structure-Affinity Relationships (SARs) and Structure-Kinetics Relationships (SKRs) of Kv11.1 Blockers.

Kv11.1 (hERG) blockers with comparable potencies but different binding kinetics might display divergent pro-arrhythmic risks. In the present study, we explored structure-kinetics relationships in four series of Kv11.1 blockers next to their structure-affinity relationships. We learned that despite dramatic differences in affinities and association rates, there were hardly any variations in the dissociation rate constants of these molecules with residence times (RTs) of a few minutes only. Hence, we synthesized 16 novel molecules, in particular in the pyridinium class of compounds, to further address this peculiar phenomenon. We found molecules with very short RTs (e.g., 0.34 min for 37) and much longer RTs (e.g., 105 min for 38). This enabled us to construct a k on-k off-KD kinetic map for all compounds and subsequently divide the map into four provisional quadrants, providing a possible framework for a further and more precise categorization of Kv11.1 blockers. Additionally, two representative compounds (21 and 38) were tested in patch clamp assays, and their RTs were linked to their functional IC50 values. Our findings strongly suggest the importance of the simultaneous study of ligand affinities and kinetic parameters, which may help to explain and predict Kv11.1-mediated cardiotoxicity.

Evaluation of (4-Arylpiperidin-1-yl)cyclopentanecarboxamides As High-Affinity and Long-Residence-Time Antagonists for the CCR2 Receptor.

Animal models suggest that the chemokine ligand 2/CC-chemokine receptor 2 (CCL2/CCR2) axis plays an important role in the development of inflammatory diseases. However, CCR2 antagonists have failed in clinical trials because of a lack of efficacy. We previously described a new approach for the design of CCR2 antagonists by the use of structure-kinetics relationships (SKRs). Herein we report new findings on the structure-affinity relationships (SARs) and SKRs of the reference compound MK-0483, its diastereomers, and its structural analogues as CCR2 antagonists. The SARs of the 4-arylpiperidine group suggest that lipophilic hydrogen-bond-accepting substituents at the 3-position are favorable. However, the SKRs suggest that a lipophilic group with a certain size is desired [e.g., 3-Br: Ki =2.8 nM, residence time (t(res))=243 min; 3-iPr: Ki =3.6 nM, t(res) =266 min]. Alternatively, additional substituents and further optimization of the molecule, while keeping a carboxylic acid at the 3-position, can also prolong t(res); this was most prominently observed in MK-0483 (Ki =1.2 nM, t(res) =724 min) and a close analogue (Ki =7.8 nM) with a short residence time.

Binding kinetics of ZM241385 derivatives at the human adenosine A2A receptor.

Classical drug design and development rely mostly on affinity- or potency-driven structure-activity relationships (SAR). Thus far, a given compound's binding kinetics have been largely ignored, the importance of which is now being increasingly recognized. In the present study, we performed an extensive structure-kinetics relationship (SKR) study in addition to a traditional SAR analysis at the adenosine A2A receptor (A2A R). The ensemble of 24 A2A R compounds, all triazolotriazine derivatives resembling the prototypic antagonist ZM241385 (4-(2-((7-amino-2-(furan-2-yl)-[1,2,4]triazolo[1,5-a][1,3,5]triazin-5-yl)amino)ethyl)phenol), displayed only minor differences in affinity, although they varied substantially in their dissociation rates from the receptor. We believe that such a combination of SKR and SAR analyses, as we have done with the A2A R, will have general importance for the superfamily of G protein-coupled receptors, as it can serve as a new strategy to tailor the interaction between ligand and receptor.

Molecular mechanism of allosteric modulation at GPCRs: insight from a binding kinetics study at the human A1 adenosine receptor.

BACKGROUND AND PURPOSE: Many GPCRs can be allosterically modulated by small-molecule ligands. This modulation is best understood in terms of the kinetics of the ligand-receptor interaction. However, many current kinetic assays require at least the (radio)labelling of the orthosteric ligand, which is impractical for studying a range of ligands. Here, we describe the application of a so-called competition association assay at the adenosine A1 receptor for this purpose. EXPERIMENTAL APPROACH: We used a competition association assay to examine the binding kinetics of several unlabelled orthosteric agonists of the A1 receptor in the absence or presence of two allosteric modulators. We also tested three bitopic ligands, in which an orthosteric and an allosteric pharmacophore were covalently linked with different spacer lengths. The relevance of the competition association assay for the binding kinetics of the bitopic ligands was also explored by analysing simulated data. KEY RESULTS: The binding kinetics of an unlabelled orthosteric ligand were affected by the addition of an allosteric modulator and such effects were probe- and concentration-dependent. Covalently linking the orthosteric and allosteric pharmacophores into one bitopic molecule had a substantial effect on the overall on- or off-rate. CONCLUSION AND IMPLICATIONS: The competition association assay is a useful tool for exploring the allosteric modulation of the human adenosine A1 receptor. This assay may have general applicability to study allosteric modulation at other GPCRs as well.