Fingerprinting heterocellular ß-adrenoceptor functional expression in the brain using agonist activity profiles.

Noradrenergic projections from the brainstem locus coeruleus drive arousal, attentiveness, mood, and memory, but specific adrenoceptor (AR) function across the varied brain cell types has not been extensively characterized, especially with agonists. This study reports a pharmacological analysis of brain AR function, offering insights for innovative therapeutic interventions that might serve to compensate for locus coeruleus decline, known to develop in the earliest phases of neurodegenerative diseases. First, ß-AR agonist activities were measured in recombinant cell systems and compared with those of isoprenaline to generate Δlog(Emax/EC50) values, system-independent metrics of agonist activity, that, in turn, provide receptor subtype fingerprints. These fingerprints were then used to assess receptor subtype expression across human brain cell systems and compared with Δlog(Emax/EC50) values arising from ß-arrestin activation or measurements of cAMP response desensitization to assess the possibility of ligand bias among ß-AR agonists. Agonist activity profiles were confirmed to be system-independent and, in particular, revealed ß2-AR functional expression across several human brain cell types. Broad ß2-AR function observed is consistent with noradrenergic tone arising from the locus coeruleus exerting heterocellular neuroexcitatory and homeostatic influence. Notably, Δlog(Emax/EC50) measurements suggest that tested ß-AR agonists do not show ligand bias as it pertains to homologous receptor desensitization in the system examined. Δlog(Emax/EC50) agonist fingerprinting is a powerful means of assessing receptor subtype expression regardless of receptor expression levels or assay readout, and the method may be applicable to future use for novel ligands and tissues expressing any receptor with available reference agonists.

Locus Coeruleus and Noradrenergic Pharmacology in Neurodegenerative Disease.

Adrenoceptors (ARs) throughout the brain are stimulated by noradrenaline originating mostly from neurons of the locus coeruleus, a brainstem nucleus that is ostensibly the earliest to show detectable pathology in neurodegenerative diseases such as Alzheimer's and Parkinson's diseases. The α1-AR, α2-AR, and ß-AR subtypes expressed in target brain regions and on a range of cell populations define the physiological responses to noradrenaline, which includes activation of cognitive function in addition to modulation of neurometabolism, cerebral blood flow, and neuroinflammation. As these heterocellular functions are critical for maintaining brain homeostasis and neuronal health, combating the loss of noradrenergic tone from locus coeruleus degeneration may therefore be an effective treatment for both cognitive symptoms and disease modification in neurodegenerative indications. Two pharmacologic approaches are receiving attention in recent clinical studies: preserving noradrenaline levels (e.g., via reuptake inhibition) and direct activation of target adrenoceptors. Here, we review the expression and role of adrenoceptors in the brain, the preclinical studies which demonstrate that adrenergic stimulation can support cognitive function and cerebral health by reversing the effects of noradrenaline depletion, and the human data provided by pharmacoepidemiologic analyses and clinical trials which together identify adrenoceptors as promising targets for the treatment of neurodegenerative disease.

The discovery and development of gefapixant.

Gefapixant is the approved generic name for a compound also known as MK-7264, and prior to that AF-219 and RO-4926219. It is the first-in-class clinically developed antagonist for the P2X3 subtype of trimeric ionotropic purinergic receptors, a family of ATP-gated excitatory ion channels, showing nanomolar potency for the human P2X3 homotrimeric channel and essentially no activity at related channels devoid of P2X3 subunits. As the first P2X3 antagonist to have progressed into clinical studies it has now progressed to the point of successful completion of Phase 3 investigations for the treatment of cough, and the NDA application is under review with US FDA for treatment of refractory chronic cough or unexplained chronic cough. The molecule was discovered in the laboratories of Roche Pharmaceuticals in Palo Alto, California, but clinical development then continued with the formation of Afferent Pharmaceuticals for the purpose of identifying the optimal therapeutic indication for this novel mechanism and establishing a clinical plan for development in the optimal patient populations selected. Geoff Burnstock was a close collaborator and advisor to the P2X3 program for close to two decades of discovery and development. Progression of gefapixant through later stage clinical studies has been conducted by the research laboratories of Merck & Co., Inc., Kenilworth, NJ, USA (MRL; following acquisition of Afferent in 2016), who may commercialize the product once authorization has been granted by regulatory authorities.

P2X3 receptor antagonism reduces the occurrence of apnoeas in newborn rats.

Hyperreflexia of the peripheral chemoreceptors is a potential contributor of apnoeas of prematurity (AoP). Recently, it was shown that elevated P2X3 receptor expression was associated with elevated carotid body afferent sensitivity. Therefore, we tested whether P2X3 receptor antagonism would reduce AoP known to occur in newborn rats. Unrestrained whole-body plethysmography was used to record breathing and from this the frequency of apnoeas at baseline and following administration of either a P2X3 receptor antagonist - AF-454 (5 mg/kg or 10 mg/kg s.c.) or vehicle was derived. In a separate group, we tested the effects of AF-454 (10 mg/kg) on the hypoxic ventilatory response (10 % FiO2). Ten but not 5 mg/kg AF-454 reduced the frequency of AoP and improved breathing regularity significantly compared to vehicle. Neither AF-454 (both 5 and 10 mg/kg) nor vehicle affected baseline respiration. However, P2X3 receptor antagonism (10 mg/kg) powerfully blunted hypoxic ventilatory response to 10 % FiO2. These data suggest that P2X3 receptors contribute to AoP and the hypoxic ventilatory response in newborn rats but play no role in the drive to breathe at rest.

Action of MK-7264 (gefapixant) at human P2X3 and P2X2/3 receptors and in vivo efficacy in models of sensitisation.

BACKGROUND AND PURPOSE: The P2X3 receptor is an ATP-gated ion channel expressed by sensory afferent neurons and is used as a target to treat chronic sensitisation conditions. The first-in-class, selective P2X3 and P2X2/3 receptor antagonist, the diaminopyrimidine MK-7264 (gefapixant), has progressed to Phase III trials for refractory or unexplained chronic cough. We used patch clamp to elucidate the pharmacology and kinetics of MK-7264 and rat models of hypersensitivity and hyperalgesia to test its efficacy on these conditions. EXPERIMENTAL APPROACH: Whole-cell patch clamp of 1321N1 cells expressing human P2X3 and P2X2/3 receptors was used to determine mode of MK-7264 action, potency, and kinetics. The analgesic efficacy was assessed using paw withdrawal threshold and limb weight distribution in rat models of inflammatory, osteoarthritic, and neuropathic sensitisation. KEY RESULTS: MK-7264 is a reversible allosteric antagonist at human P2X3 and P2X2/3 receptors. Experiments with the slowly desensitising P2X2/3 heteromer revealed concentration- and state-dependency to wash-on, with faster rates and greater inhibition when applied before agonist compared to during agonist application. The wash-on rate (τ value) for MK-7264 at maximal concentrations was much lower when applied before compared to during agonist application. In vivo, MK-7264 displayed efficacy comparable to naproxen in inflammatory and osteoarthritic sensitisation models and gabapentin in neuropathic sensitisation models, increasing paw withdrawal threshold and decreasing weight-bearing discomfort. CONCLUSIONS AND IMPLICATIONS: MK-7264 is a reversible and selective P2X3 and P2X2/3 antagonist, exerting allosteric antagonism via preferential activity at closed channels. Its efficacy in rat models supports its clinical investigation for chronic sensitisation conditions.

Novel compounds lowering the cellular isoform of the human prion protein in cultured human cells.

PURPOSE: Previous studies showed that lowering PrP(C) concomitantly reduced PrP(Sc) in the brains of mice inoculated with prions. We aimed to develop assays that measure PrP(C) on the surface of human T98G glioblastoma and IMR32 neuroblastoma cells. Using these assays, we sought to identify chemical hits, confirmed hits, and scaffolds that potently lowered PrP(C) levels in human brains cells, without lethality, and that could achieve drug concentrations in the brain after oral or intraperitoneal dosing in mice. METHODS: We utilized HTS ELISA assays to identify small molecules that lower PrP(C) levels by ≥30% on the cell surface of human glioblastoma (T98G) and neuroblastoma (IMR32) cells. RESULTS: From 44,578 diverse chemical compounds tested, 138 hits were identified by single point confirmation (SPC) representing 7 chemical scaffolds in T98G cells, and 114 SPC hits representing 6 scaffolds found in IMR32 cells. When the confirmed SPC hits were combined with structurally related analogs, >300 compounds (representing 6 distinct chemical scaffolds) were tested for dose-response (EC50) in both cell lines, only studies in T98G cells identified compounds that reduced PrP(C) without killing the cells. EC50 values from 32 hits ranged from 65 nM to 4.1 µM. Twenty-eight were evaluated in vivo in pharmacokinetic studies after a single 10 mg/kg oral or intraperitoneal dose in mice. Our results showed brain concentrations as high as 16.2 µM, but only after intraperitoneal dosing. CONCLUSIONS: Our studies identified leads for future studies to determine which compounds might lower PrP(C) levels in rodent brain, and provide the basis of a therapeutic for fatal disorders caused by PrP prions.

Antiprion compounds that reduce PrP(Sc) levels in dividing and stationary-phase cells.

During prion diseases, a normally benign, host protein, denoted PrP(C), undergoes alternative folding into the aberrant isoform, PrP(Sc). We used ELISA to identify and confirm hits in order to develop leads that reduce PrP(Sc) in prion-infected dividing and stationary-phase mouse neuroblastoma (ScN2a-cl3) cells. We tested 52,830 diverse small molecules in dividing cells and 49,430 in stationary-phase cells. This led to 3100 HTS and 970 single point confirmed (SPC) hits in dividing cells, 331 HTS and 55 confirmed SPC hits in stationary-phase cells as well as 36 confirmed SPC hits active in both. Fourteen chemical leads were identified from confirmed SPC hits in dividing cells and three in stationary-phase cells. From more than 682 compounds tested in concentration-effect relationships in dividing cells to determine potency (EC50), 102 had EC50 values between 1 and 10 µM and 50 had EC50 values of <1 µM; none affected cell viability. We observed an excellent correlation between EC50 values determined by ELISA and Western immunoblotting for 28 representative compounds in dividing cells (R(2)=0.75; p <0.0001). Of the 55 confirmed SPC hits in stationary-phase cells, 23 were piperazine, indole, or urea leads. The EC50 values of one indole in stationary-phase and dividing ScN2a-cl3 cells were 7.5 and 1.6 µM, respectively. Unexpectedly, the number of hits in stationary-phase cells was ~10% of that in dividing cells. The explanation for this difference remains to be determined.

Biaryl amides and hydrazones as therapeutics for prion disease in transgenic mice.

The only small-molecule compound demonstrated to substantially extend survival in prion-infected mice is a biaryl hydrazone termed "Compd B" (4-pyridinecarboxaldehyde,2-[4-(5-oxazolyl)phenyl]hydrazone). However, the hydrazone moiety of Compd B results in toxic metabolites, making it a poor candidate for further drug development. We developed a pharmacophore model based on diverse antiprion compounds identified by high-throughput screening; based on this model, we generated biaryl amide analogs of Compd B. Medicinal chemistry optimization led to multiple compounds with increased potency, increased brain concentrations, and greater metabolic stability, indicating that they could be promising candidates for antiprion therapy. Replacing the pyridyl ring of Compd B with a phenyl group containing an electron-donating substituent increased potency, while adding an aryl group to the oxazole moiety increased metabolic stability. To test the efficacy of Compd B, we applied bioluminescence imaging (BLI), which was previously shown to detect prion disease onset in live mice earlier than clinical signs. In our studies, Compd B showed good efficacy in two lines of transgenic mice infected with the mouse-adapted Rocky Mountain Laboratory (RML) strain of prions, but not in transgenic mice infected with human prions. The BLI system successfully predicted the efficacies in all cases long before extension in survival could be observed. Our studies suggest that this BLI system has good potential to be applied in future antiprion drug efficacy studies.

Optimization of Arylamides as Novel, Potent and Brain-penetrant Antiprion Lead Compounds.

The prion diseases caused by PrPSc, an alternatively folded form of the cellular prion protein (PrPC), are rapidly progressive, fatal, and untreatable neurodegenerative syndromes. We employed HTS ELISA assays to identify compounds that lower the level of PrPSc in prion-infected mouse neuroblastoma (ScN2a-cl3) cells and identified a series of arylamides. SAR studies indicated that small amides with one aromatic, or heteroaromatic ring, on each side of the amide bond are of modest potency. Of note, benzamide (7), with an EC50 of 2200 nM, was one of only a few arylamide hits with a piperazine group on its aniline moiety. The basic piperazine nitrogen can be protonated at physiologic pH, improving solubility, and therefore we wanted to exploit this feature in our search for a drug candidate. An SAR campaign resulted in several key analogs, including a set with biaryl groups introduced on the carbonyl side for improved potency. Several of these biaryl analogs have submicromolar potency, with the most potent analog 17 having an EC50 = 22 nM. More importantly, 17 and several biarylamides (20, 24, 26, 27) were able to traverse the BBB and displayed excellent drug levels in the brains of mice following oral dosing. These biarylamides may represent good starting points for further lead optimization for the identification of potential drug candidates for the treatment of prion diseases.

A high-throughput screening assay for determining cellular levels of total tau protein.

The microtubule-associated protein (MAP) tau has been implicated in the pathology of numerous neurodegenerative diseases. In the past decade, the hyperphosphorylated and aggregated states of tau protein have been important targets in the drug discovery field for the potential treatment of Alzheimer's disease. Although several compounds have been reported to reduce the hyperphosphorylated state of tau or impact the stabilization of tau, their therapeutic activities are remain to be validated. Recently, reduction of total cellular tau protein has emerged as an alternate intervention point for drug development and a potential treatment of tauopathies. We have developed and optimized homogenous assays, using the AlphaLISA and HTRF assay technologies, for the quantification of total cellular tau protein levels in the SH-SY5Y neuroblastoma cell line. The signal-to-basal ratios were 375 and 5.3, and the Z' factors were 0.67 and 0.60 for the AlphaLISA and HTRF tau assays, respectively. The clear advantages of these homogeneous tau assays over conventional total tau assays, such as ELISA and Western blot, are the elimination of plate wash steps and miniaturization of the assay into 1536-well plate format for the ultra-high-throughput screening of large compound libraries.

Discovery and Preliminary SAR of Arylpiperazines as Novel, Brainpenetrant Antiprion Compounds.

Prion diseases are a group of fatal neurodegenerative disorders that include Creutzfeldt-Jakob disease (CJD) and kuru in humans, BSE in cattle, and scrapie in sheep. Such illnesses are caused by the conversion and accumulation of a misfolded pathogenic isoform (termed PrPSc) of a normally benign, host cellular protein, denoted PrPC. We employed high-throughput screening (HTS) ELISAs to evaluate compounds for their ability to reduce the level of PrPSc in Rocky Mountain Laboratory (RML) prion-infected mouse neuroblastoma cells (ScN2a-cl3). Arylpiperazines were among the active compounds identified but the initial hits suffered from low potency and poor drug-likeness. The best of those hits, such as 1, 7, 13, and 19, displayed moderate antiprion activity with EC50 values in the micromolar range. Key analogs were designed and synthesized based on the SAR, with analogs 41, 44, 46, and 47 found to have sub-micromolar potency. Analogs 41 and 44 were able to penetrate the blood-brain barrier (BBB) and achieved excellent drug concentrations in the brains of mice after oral dosing. These compounds represent good starting points for further lead optimization in our pursuit of potential drug candidates for the treatment of prion diseases.

2-Aminothiazoles with improved pharmacotherapeutic properties for treatment of prion disease.

Recently, we described the aminothiazole lead (4-biphenyl-4-ylthiazol-2-yl)-(6-methylpyridin-2-yl)-amine (1), which exhibits many desirable properties, including excellent stability in liver microsomes, oral bioavailability of ∼40 %, and high exposure in the brains of mice. Despite its good pharmacokinetic properties, compound 1 exhibited only modest potency in mouse neuroblastoma cells overexpressing the disease-causing prion protein PrP(Sc) . Accordingly, we sought to identify analogues of 1 with improved antiprion potency in ScN2a-cl3 cells while retaining similar or superior properties. Herein we report the discovery of improved lead compounds such as (6-methylpyridin-2-yl)-[4-(4-pyridin-3-yl-phenyl)thiazol-2-yl]amine and cyclopropanecarboxylic acid (4-biphenylthiazol-2-yl)amide, which exhibit brain exposure/EC50 ratios at least tenfold greater than that of compound 1.

Pharmacokinetics and metabolism of 2-aminothiazoles with antiprion activity in mice.

PURPOSE: To discover drugs lowering PrP(Sc) in prion-infected cultured neuronal cells that achieve high concentrations in brain to test in mouse models of prion disease and then treat people with these fatal diseases. METHODS: We tested 2-AMT analogs for EC50 and PK after a 40 mg/kg single dose and 40-210 mg/kg/day doses for 3 days. We calculated plasma and brain AUC, ratio of AUC/EC50 after dosing. We reasoned that compounds with high AUC/EC50 ratios should be good candidates going forward. RESULTS: We evaluated 27 2-AMTs in single-dose and 10 in 3-day PK studies, of which IND24 and IND81 were selected for testing in mouse models of prion disease. They had high concentrations in brain after oral dosing. Absolute bioavailability ranged from 27-40%. AUC/EC50 ratios after 3 days were >100 (total) and 48-113 (unbound). Stability in liver microsomes ranged from 30->60 min. Ring hydroxylated metabolites were observed in microsomes. Neither was a substrate for the MDR1 transporter. CONCLUSIONS: IND24 and IND81 are active in vitro and show high AUC/EC50 ratios (total and unbound) in plasma and brain. These will be evaluated in mouse models of prion disease.

Discovery of 6-substituted indole-3-glyoxylamides as lead antiprion agents with enhanced cell line activity, improved microsomal stability and low toxicity.

A series of highly potent indole-3-glyoxylamide based antiprion agents was previously characterized, focusing on optimization of structure-activity relationship (SAR) at positions 1-3 of the indole system. New libraries interrogating the SAR at indole C-4 to C-7 now demonstrate that introducing electron-withdrawing substituents at C-6 may improve biological activity by up to an order of magnitude, and additionally confer higher metabolic stability. For the present screening libraries, both the degree of potency and trends in SAR were consistent across two cell line models of prion disease, and the large majority of compounds showed no evidence of toxic effects in zebrafish. The foregoing observations thus make the indole-3-glyoxylamides an attractive lead series for continuing development as potential therapeutic agents against prion disease.

2-Aminothiazoles as therapeutic leads for prion diseases.

2-Aminothiazoles are a new class of small molecules with antiprion activity in prion-infected neuroblastoma cell lines (J. Virol. 2010, 84, 3408). We report here structure-activity studies undertaken to improve the potency and physiochemical properties of 2-aminothiazoles, with a particular emphasis on achieving and sustaining high drug concentrations in the brain. The results of this effort include the generation of informative structure-activity relationships (SAR) and the identification of lead compounds that are orally absorbed and achieve high brain concentrations in animals. The new aminothiazole analogue (5-methylpyridin-2-yl)-[4-(3-phenylisoxazol-5-yl)-thiazol-2-yl]-amine (27), for example, exhibited an EC(50) of 0.94 µM in prion-infected neuroblastoma cells (ScN2a-cl3) and reached a concentration of ∼25 µM in the brains of mice following three days of oral administration in a rodent liquid diet. The studies described herein suggest 2-aminothiazoles as promising new leads in the search for effective therapeutics for prion diseases.

AF-353, a novel, potent and orally bioavailable P2X3/P2X2/3 receptor antagonist.

BACKGROUND AND PURPOSE: Purinoceptors containing the P2X3 subunit (P2X3 homotrimeric and P2X2/3 heterotrimeric) are members of the P2X family of ion channels gated by ATP and may participate in primary afferent sensitization in a variety of pain-related diseases. The current work describes the in vitro pharmacological characteristics of AF-353, a novel, orally bioavailable, highly potent and selective P2X3/P2X2/3 receptor antagonist. EXPERIMENTAL APPROACH: The antagonistic potencies (pIC(50)) of AF-353 for rat and human P2X3 and human P2X2/3 receptors were determined using methods of radioligand binding, intracellular calcium flux and whole cell voltage-clamp electrophysiology. KEY RESULTS: The pIC(50) estimates for these receptors ranged from 7.3 to 8.5, while concentrations 300-fold higher had little or no effect on other P2X channels or on an assortment of receptors, enzymes and transporter proteins. In contrast to A-317491 and TNP-ATP, competition binding and intracellular calcium flux experiments suggested that AF-353 inhibits activation by ATP in a non-competitive fashion. Favourable pharmacokinetic parameters were observed in rat, with good oral bioavailability (%F = 32.9), reasonable half-life (t(1/2) = 1.63 h) and plasma-free fraction (98.2% protein bound). CONCLUSIONS AND IMPLICATIONS: The combination of a favourable pharmacokinetic profile with the antagonist potency and selectivity for P2X3 and P2X2/3 receptors suggests that AF-353 is an excellent in vivo tool compound for study of these channels in animal models and demonstrates the feasibility of identifying and optimizing molecules into potential clinical candidates, and, ultimately, into a novel class of therapeutics for the treatment of pain-related disorders.

Discovery and optimization of RO-85, a novel drug-like, potent, and selective P2X3 receptor antagonist.

Despite the extensive literature describing the role of the ATP-gated P2X(3) receptors in a variety of physiological processes the potential of antagonists as therapeutic agents has been limited by the lack of drug-like selective molecules. In this paper we report the discovery and optimization of RO-85, a novel drug-like, potent and selective P2X(3) antagonist. High-throughput screening of the Roche compound collection identified a small hit series of heterocyclic amides from a large parallel synthesis library. Rapid optimization, facilitated by high-throughput synthesis, focusing on increasing potency and improving drug-likeness resulted in the discovery of RO-85.

Identification and SAR of novel diaminopyrimidines. Part 2: The discovery of RO-51, a potent and selective, dual P2X(3)/P2X(2/3) antagonist for the treatment of pain.

The purinoceptor subtypes P2X(3) and P2X(2/3) have been shown to play a pivotal role in models of various pain conditions. Identification of a potent and selective dual P2X(3)/P2X(2/3) diaminopyrimidine antagonist RO-4 prompted subsequent optimization of the template. This paper describes the SAR and optimization of the diaminopyrimidine ring and particularly the substitution of the 2-amino group. The discovery of the highly potent and drug-like dual P2X(3)/P2X(2/3) antagonist RO-51 is presented.

Identification and SAR of novel diaminopyrimidines. Part 1: The discovery of RO-4, a dual P2X(3)/P2X(2/3) antagonist for the treatment of pain.

P2X purinoceptors are ligand-gated ion channels whose endogenous ligand is ATP. Both the P2X(3) and P2X(2/3) receptor subtypes have been shown to play an important role in the regulation of sensory function and dual P2X(3)/P2X(2/3) antagonists offer significant potential for the treatment of pain. A high-throughput screen of the Roche compound collection resulted in the identification of a novel series of diaminopyrimidines; subsequent optimization resulted in the discovery of RO-4, a potent, selective and drug-like dual P2X(3)/P2X(2/3) antagonist.

Expression and function of rat urothelial P2Y receptors.

The control and regulation of the lower urinary tract are partly mediated by purinergic signaling. This study investigated the distribution and function of P2Y receptors in the rat urinary bladder. Application of P2Y agonists to rat urothelial cells evoked increases in intracellular calcium; the rank order of agonist potency (pEC(50) +/- SE) was ATP (5.10 +/- 0.07) > UTP (4.91 +/- 0.14) > UTPgammaS (4.61 +/- 0.16) = ATPgammaS (4.70 +/- 0.05) > 2-methylthio adenosine 5'-diphosphate = 5'-(N-ethylcarboxamido)adenosine = ADP (<3.5). The rank order potency for these agonists indicates that urothelial cells functionally express P2Y(2)/P2Y(4) receptors, with a relative lack of contribution from other P2Y or adenosine receptors. Real-time PCR, Western blotting, and immunocytochemistry confirmed the expression of P2Y(2) and to a lesser extent P2Y(4) in the urothelium. Immunocytochemical studies revealed expression of P2Y(2) staining in all layers of the urothelium, with relative absence of P2Y(4). P2Y(2) staining was also present in suburothelial nerve bundles and underlying detrusor smooth muscle. Addition of UTP and UTPgammaS was found to evoke ATP release from cultured rat urothelial cells. These findings indicate that cultured rat urothelial cells functionally express P2Y(2)/P2Y(4) receptors. Activation of these receptors could have a role in autocrine and paracrine signaling throughout the urothelium. This could lead to the release of bioactive mediators such as additional ATP, nitric oxide, and acetylcholine, which can modulate the micturition reflex by acting on suburothelial myofibroblasts and/or pelvic afferent fibers.