Pharmacology of P2X channels

Significant progress in understanding the pharmacological characteristics and physiological importance of homomeric and heteromeric P2X channels has been achieved in recent years. P2X channels, gated by ATP and most likely trimerically assembled from seven known P2X subunits, are present in a broad distribution of tissues and are thought to play an important role in a variety of physiological functions, including peripheral and central neuronal transmission, smooth muscle contraction, and inflammation. The known homomeric and heteromeric P2X channels can be distinguished from each other on the basis of pharmacological differences when expressed recombinantly in cell lines, but whether this pharmacological classification holds true in native cells and in vivo is less well-established. Nevertheless, several potent and selective P2X antagonists have been discovered in recent years and shown to be efficacious in various animal models including those for visceral organ function, chronic inflammatory and neuropathic pain, and inflammation. The recent advancement of drug candidates targeting P2X channels into human trials, confirms the medicinal exploitability of this novel target family and provides hope that safe and effective medicines for the treatment of disorders involving P2X channels may be identified in the near future.

Pharmacological pleiotropism of the human recombinant alpha1A-adrenoceptor: implications for alpha1-adrenoceptor classification

1. Three fully-defined alpha1-adrenoceptors (alpha1A, alpha1B and alpha1D) have been established in pharmacological and molecular studies. A fourth alpha1-adrenoceptor, the putative alpha1L-adrenoceptor, has been defined in functional but not molecular studies, and has been proposed to mediate contraction of human lower urinary tract tissues; its relationship to the three fully characterized alpha1-adrenoceptors is not known. 2. In the present study, binding affinities were estimated by displacement of [3H]-prazosin in membrane homogenates of Chinese hamster ovary (CHO-K1) cells stably expressing the human alpha1A-, alpha1B- and alpha1D-adrenoceptors and were compared with affinity estimates obtained functionally in identical cells by measuring inhibition of noradrenaline (NA)-stimulated accumulation of [3H]-inositol phosphates. 3. For the alpha1A-adrenoceptor, binding studies revealed a pharmacological profile typical for the classically defined alpha1A-adrenoceptor, such that prazosin, RS-17053, WB 4101, 5-methylurapidil, Rec 15/2739 and S-niguldipine all displayed subnanomolar affinity. A different profile of affinity estimates was obtained in inositol phosphates accumulation studies: prazosin, WB 4101, 5-methylurapidil, RS-17053 and S-niguldipine showed 10 to 40 fold lower affinity than in membrane binding. However, affinity estimates were not 'frameshifted', as tamsulosin, indoramin and Rec 15/2739 yielded similar, high affinity estimates in binding and functional assays. 4. In contrast, results from human alpha1B- and alpha1D-adrenoceptors expressed in CHO-K1 cells gave antagonist affinity profiles in binding and functional assays that were essentially identical. 5. A concordance of affinity estimates from the functional (inositol phosphates accumulation) studies of the alpha1A-adrenoceptor in CHO-K1 cells was found with estimates published recently from contractile studies in human lower urinary tract tissues (putative alpha1L-adrenoceptor). These data show that upon functional pharmacological analysis, the cloned alpha1A-adrenoceptor displays pharmacological recognition properties consistent with those of the putative alpha1L-adrenoceptor. Why this profile differs from that obtained in membrane binding, and whether it explains the alpha1L-adrenoceptor pharmacology observed in many native tissues, requires further investigation.

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.

Purinoceptors as therapeutic targets for lower urinary tract dysfunction

Lower urinary tract symptoms (LUTS) are present in many common urological syndromes. However, their current suboptimal management by muscarinic and alpha(1)-adrenoceptor antagonists leaves a significant opportunity for the discovery and development of superior medicines. As potential targets for such therapeutics, purinoceptors have emerged over the last two decades from investigations that have established a prominent role for ATP in the regulation of urinary bladder function under normal and pathophysiological conditions. In particular, evidence suggests that ATP signaling via P2X(1) receptors participates in the efferent control of detrusor smooth muscle excitability, and that this function may be heightened in disease and aging. ATP also appears to be involved in bladder sensation, via activation of P2X(3) and P2X(2/3) receptors on sensory afferent neurons, both within the bladder itself and possibly at central synapses. Such findings are based on results from classical pharmacological and localization studies in non-human and human tissues, knockout mice, and studies using recently identified pharmacological antagonists--some of which possess attributes that offer the potential for optimization into candidate drug molecules. Based on recent advances in this field, it is clearly possible that the development of selective antagonists for these receptors will occur that could lead to therapies offering better relief of sensory and motor symptoms for patients, while minimizing the systemic side effects that limit current medicines.

In vitro alpha1-adrenoceptor pharmacology of Ro 70-0004 and RS-100329, novel alpha1A-adrenoceptor selective antagonists

It has been hypothesized that in patients with benign prostatic hyperplasia, selective antagonism of the alpha1A-adrenoceptor-mediated contraction of lower urinary tract tissues may, via a selective relief of outlet obstruction, lead to an improvement in symptoms. The present study describes the alpha1-adrenoceptor (alpha1-AR) subtype selectivities of two novel alpha1-AR antagonists, Ro 70-0004 (aka RS-100975) and a structurally-related compound RS-100329, and compares them with those of prazosin and tamsulosin. Radioligand binding and second-messenger studies in intact CHO-K1 cells expressing human cloned alpha1A-, alpha1B- and alpha1D-AR showed nanomolar affinity and significant alpha1A-AR subtype selectivity for both Ro 70-0004 (pKi 8.9: 60 and 50 fold selectivity) and RS-100329 (pKi 9.6: 126 and 50 fold selectivity) over the alpha1B- and alpha1D-AR subtypes respectively. In contrast, prazosin and tamsulosin showed little subtype selectivity. Noradrenaline-induced contractions of human lower urinary tract (LUT) tissues or rabbit bladder neck were competitively antagonized by Ro 70-0004 (pA2 8.8 and 8.9), RS-100329 (pA2 9.2 and 9.2), tamsulosin (pA2 10.4 and 9.8) and prazosin (pA2 8.7 and 8.3 respectively). Affinity estimates for tamsulosin and prazosin in antagonizing alpha1-AR-mediated contractions of human renal artery (HRA) and rat aorta (RA) were similar to those observed in LUT tissues, whereas Ro 70-0004 and RS-100329 were approximately 100 fold less potent (pA2 values of 6.8/6.8 and 7.3/7.9 in HRA/RA respectively). The alpha1A-AR subtype selectivity of Ro 70-0004 and RS-100329, demonstrated in both cloned and native systems, should allow for an evaluation of the clinical utility of a 'uroselective' agent for the treatment of symptoms associated with benign prostatic hyperplasia.

Human cloned alpha1A-adrenoceptor isoforms display alpha1L-adrenoceptor pharmacology in functional studies

The recombinant alpha1A-adrenoceptor displays a distinct pharmacological profile ('classical alpha1A-adrenoceptor') in homogenate binding assays, but displays the properties of the so-called alpha1L-adrenoceptor in functional studies in whole cells at 37 degrees C. As three splice variants of the human alpha1A-adrenoceptor have been described previously (alpha1A-1, alpha1A-2 and alpha1A-3), we have compared their functional pharmacological profiles, when expressed stably in Chinese hamster ovary (CHO-K1) cells (antagonist inhibition of noradrenaline-stimulated [3H]inositol phosphates accumulation). A fourth, novel isoform (alpha1A-4) has also been studied: alpha1A-4 mRNA predominates in several human tissues including prostate, liver, heart and bladder. In homogenate binding studies, all four isoforms displayed essentially identical affinity profiles, with prazosin (1-(4-amino-6,7-dimethoxy-2-quinazolinyl)-4-(2-furoyl)piperazine), tamsulosin (5-[2-[[2-(2-ethoxyphenoxy)ethyl]-amino]propyl]-2-methoxybenzen esulfonamide), RS-17053 (N-[2-(2-cyclopropylmethoxyphenoxy)ethyl]-5-chloro-alpha,alphad imethyl-1H-indole-3-ethanamine hydrochloride), WB 4101 ((2,6-dimethoxyphenoxyethyl)aminomethyl-1,4-benzodioxane hydrochloride) and 5-Me-urapidil (5-methyl-6[[3-[4-(2-methoxyphenyl)-1-piperazinyl]propyl]amino]-1,3-d imethyuracil) all displaying subnanomolar affinities. In functional studies, noradrenaline accelerated [3H]inositol phosphates production with potencies (p[A]50) of between 5.8 and 6.6. The affinities of prazosin, RS-17053, WB 4101 and 5-Me-urapidil, at antagonizing responses to noradrenaline, were reduced by approximately 10-fold (cf. binding data), while those for tamsulosin and indoramin (N-[1-[2-(1H-indol-3-yl)ethyl]-4-piperidinyl]benzamide) remained constant or increased, consistent with the previously described alpha1L-adrenoceptor. Thus, all four human recombinant alpha1A-adrenoceptor isoforms display the pharmacology of the alpha1L-adrenoceptor when studied in functional assays, consistent with the hypothesis that the putative alpha1L-adrenoceptor represents a functional phenotype of the alpha1A-adrenoceptor.

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.

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.

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.

Pharmacological characteristics of Ro 115-1240, a selective alpha1A/1L-adrenoceptor partial agonist: a potential therapy for stress urinary incontinence

OBJECTIVE

To describe the preclinical pharmacology of Ro 115-1240, a peripherally acting selective alpha1A/1L-adrenoceptor (AR) partial agonist, compared with the alpha1A/1L-AR full agonist amidephrine, as AR agonists have some utility in the treatment of stress urinary incontinence (SUI) but are limited by undesirable cardiovascular and central nervous system side-effects.

RESULTS

In radioligand-binding studies Ro 115-1240 had greater affinity for alpha1A than for alpha1B and alpha1D subtypes. The potency and intrinsic activity of amidephrine and Ro 115-1240 relative to noradrenaline were determined in native and cell-based assays using human recombinant alpha1-ARs; they acted as selective alpha1A/1L-AR full and partial agonists, respectively. In anaesthetized micropigs and rabbits, amidephrine and Ro 115-1240 produced non-selective, dose-dependent increases in intraurethral and arterial blood pressures but the magnitude of the pressure increases evoked by Ro 115-1240 were about a third of those with amidephrine. In conscious micropigs both agents produced dose-dependent increases in urethral tension. Again, the magnitude of the urethral response to Ro 115-1240 was about a third of that with amidephrine. More importantly, only amidephrine produced dose-dependent increases in blood pressure and decreases in heart rate. Ro 115-1240 produced a maximum increase in urethral tension with no effect on blood pressure or heart rate.

CONCLUSION

These results show that by combining selectivity for the alpha1A/1L-AR subtype with a reduction in intrinsic agonist efficacy, Ro 115-1240 has reduced haemodynamic effects while retaining to some degree the contractile effects on urethral smooth muscle. These studies indicate that Ro 115-1240 may be useful as a novel treatment for SUI.

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.

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.

Antagonism of ATP responses at P2X receptor subtypes by the pH indicator dye, Phenol red

1 Many types of culture media contain a pH-sensitive dye. One commonly occurring dye, Phenol red sodium (Na(+)) salt, was tested for blocking activity at rat P2X(1-4) receptors (P2X(1-4)Rs) expressed in Xenopus oocytes. 2 Phenol red Na(+)-salt antagonised adenosine 5'-triphosphate (ATP) responses at P2X(1)R (IC(50), 3 microM) and, at higher concentrations, also blocked P2X(2)R and P2X(3)R. Phenol red Na(+)-salt, purified of lipophilic contaminants, blocked P2X(1)R and P2X(3)R by acting as an insurmountable antagonist. 3 Two lipophilic extracts of Phenol red antagonised ATP responses at P2XRs. Extract A was a potent antagonist at P2X(1)R (IC(50), 1.4 microM), whereas extract B was a potent antagonist at P2X(3)R (IC(50), 4.1 microM). A bisphenolic compound (RS151030) found in these extracts was a potent antagonist at P2X(1)R (IC(50), 0.3 microM) and at P2X(3)R (IC(50), 2.4 microM). 4 Phenolphthalein base was a potent irreversible antagonist at P2X(1)R (IC(50), 1 microM), whereas Phenolphthalein K(+)-salt was 25-fold less potent here. 5 Phenolphthalein base was a reversible antagonist of ATP responses at rat P2X(4)R (IC(50), 26 microM), whereas Phenolphthalein K(+)-salt was inactive. 6 Dimethyl sulphoxide (DMSO), used to dissolve lipophilic extracts, showed pharmacological activity by itself at rat P2X(1)R and P2X(4)R. 7 Thus, Phenol red and related compounds are antagonists at rat P2X(1)R, but are also active at other rat P2XRs. Phenolphthalein base is a newly identified, low potency antagonist of ATP responses at P2X(4)R. Culture media containing these red dyes should be used cautiously in future pharmacological studies of P2XRs. Also, wherever possible, the solvent DMSO should be used with caution.

Discovery and synthesis of a novel and selective drug-like P2X1 antagonist

Although there is extensive literature to indicate that many different types of P2 purinoceptors are present in the lower urinary tract, the physiological role of these receptors in micturition is still uncertain. In part, this uncertainty has been caused by a lack of P2 subtype selective ligands. In this paper we report the discovery, gram scale synthesis, and binding results for 1, the first potent, drug-like, selective P2X(1) receptor antagonist described. Compound 1 was shown to be more than 30-fold selective over other purinergic receptor subtypes.

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.

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.

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.

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

The prion diseases caused by PrP^Sc, an alternatively folded form of the cellular prion protein (PrP^C), are rapidly progressive, fatal, and untreatable neurodegenerative syndromes. We employed HTS ELISA assays to identify compounds that lower the level of PrP^Sc 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.

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 PrP^Sc) of a normally benign, host cellular protein, denoted PrP^C. We employed high-throughput screening (HTS) ELISAs to evaluate compounds for their ability to reduce the level of PrP^Sc 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.

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.

Regulated expression of the rat recombinant P2X(3) receptor in stably transfected CHO-K1 tTA cells

In this report, the regulatable expression by tetracycline of the rat recombinant P2X(3) receptor in stably transfected Chinese hamster ovary (CHO-K1) expressing the tetracycline-controlled transactivator (tTA) is described. cDNA encoding the rat P2X(3)-receptor was subcloned into pTRE (a tetracycline-repressible expression vector) which was used to transfect stably CHO-K1 tTA cells. Using whole cell patch clamp techniques, 100 microM ATP evoked inward currents of 2.9+/-1.6 nA in transfected cells grown in the absence of tetracycline (tet-). The P2X(3) receptor protein was detectable by immunoblot as early as 24 h and protein expression levels continued to increase as much as 192 h following activation of tTA by the removal of the antibiotic. Saturation binding isotherms using [35S]ATP gamma S yielded a pK(d) of 8.2+/-0.1 and a B(max) of 31.9+/-3.5 pmol/mg protein in tet- cell membranes and a pK(d) of 8.1+/-0.1 and a B(max) of 5.8+/-0.8 pmol/mg protein in tet+ cell membranes. The agonist ligands 2MeSATP and alpha beta MeATP displaced the binding of [35S]ATP gamma S in tet- cell membranes with very high affinity, yielding pIC(50) values of 9.4+/-0.2 and 7.5+/-0. 2, respectively. In tet+ cell membrane, displacement of [35S]ATP gamma S by 2MeSATP and alpha beta MeATP was of much lower affinity (pIC(50) values of 7.8 and 6.2, respectively). ATP, ADP and UTP showed similar displacement of [35S]ATP gamma S binding in tet- and tet+ cell membranes. In other experiments, cytosolic Ca(2+) was monitored using the fluorescent indicator, fluo-3. Increases in cytosolic Ca(2+) were elicited by 100 nM alpha beta MeATP in tet- cells while no increases in cytosolic Ca(2+) were detected below 100 microM alpha beta MeATP in either tet+ cells or untransfected cells. These calcium responses to alpha beta MeATP had a pEC(50) of 6.7 and were transient, returning to baseline within 120 s. Suramin produced concentration-dependent, parallel, dextral shifts of E/[A] curves to alpha beta MeATP yielding a pK(B) of 5.6. PPADS produced non-parallel, dextral shifts of E/[A] curves to alpha beta MeATP which were insurmountable. These results show for the first time, expression of a functional, homomeric recombinant rat P2X(3) receptor which is under regulated expression in a stably transfected mammalian cell line.

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 β-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.

Fingerprinting heterocellular β-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, β-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 β-arrestin activation or measurements of cAMP response desensitization to assess the possibility of ligand bias among β-AR agonists. Agonist activity profiles were confirmed to be system-independent and, in particular, revealed β2-AR functional expression across several human brain cell types. Broad β2-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 β-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.