ABT-866, a novel alpha(1A)-adrenoceptor agonist with antagonist properties at the alpha(1B)- and alpha(1D)-adrenoceptor subtypes

Identification of a Novel Subtype-Selective α1B-Adrenoceptor Antagonist

α1A-, α1B-, and α1D-adrenoceptors (α1-ARs) are members of the adrenoceptor G protein-coupled receptor family that are activated by adrenaline (epinephrine) and noradrenaline. α1-ARs are clinically targeted using antagonists that have minimal subtype selectivity, such as prazosin and tamsulosin, to treat hypertension and benign prostatic hyperplasia, respectively. Abundant expression of α1-ARs in the heart and central nervous system (CNS) makes these receptors potential targets for the treatment of cardiovascular and CNS disorders, such as heart failure, epilepsy, and Alzheimer's disease. Our understanding of the precise physiological roles of α1-ARs, however, and their involvement in disease has been hindered by the lack of sufficiently subtype-selective tool compounds, especially for α1B-AR. Here, we report the discovery of 4-[(2-hydroxyethyl)amino]-6-methyl-2H-chromen-2-one (Cpd1), as an α1B-AR antagonist that has 10-15-fold selectivity over α1A-AR and α1D-AR. Through computational and site-directed mutagenesis studies, we have identified the binding site of Cpd1 in α1B-AR and propose the molecular basis of α1B-AR selectivity, where the nonconserved V19745.52 residue plays a major role, with contributions from L3146.55 within the α1B-AR pocket. By exploring the structure-activity relationships of Cpd1 at α1B-AR, we have also identified 3-[(cyclohexylamino)methyl]-6-methylquinolin-2(1H)-one (Cpd24), which has a stronger binding affinity than Cpd1, albeit with reduced selectivity for α1B-AR. Cpd1 and Cpd24 represent potential leads for α1B-AR-selective drug discovery and novel tool molecules to further study the physiology of α1-ARs.

Activation of α-adrenoceptors depresses synaptic transmission of myelinated afferents and inhibits pathways mediating primary afferent depolarization (PAD) in the in vitro mouse spinal cord

Somatosensory afferent transmission strength is controlled by several presynaptic mechanisms that reduce transmitter release at the spinal cord level. We focused this investigation on the role of α-adrenoceptors in modulating sensory transmission in low-threshold myelinated afferents and in pathways mediating primary afferent depolarization (PAD) of neonatal mouse spinal cord. We hypothesized that the activation of α-adrenoceptors depresses low threshold-evoked synaptic transmission and inhibits pathways mediating PAD. Extracellular field potentials (EFPs) recorded in the deep dorsal horn assessed adrenergic modulation of population monosynaptic transmission, while dorsal root potentials (DRPs) recorded at root entry zone assessed adrenergic modulation of PAD. We found that noradrenaline (NA) and the α1-adrenoceptor agonists phenylephrine and cirazoline depressed synaptic transmission (by 15, 14 and 22%, respectively). DRPs were also depressed by NA, phenylephrine and cirazoline (by 62, 30, and 64%, respectively), and by the α2-adrenoceptor agonist clonidine, although to a lower extent (20%). We conclude that NA depresses monosynaptic transmission of myelinated afferents onto deep dorsal horn neurons via α1-adrenoceptors and inhibits interneuronal pathways mediating PAD through the activation of α1- and α2-adrenoceptors. The functional significance of these modulatory actions in shaping cutaneous and muscle sensory information during motor behaviors requires further study.

Pharmacological properties of 2-((R-5-chloro-4-methoxymethylindan-1-yl)-1H-imidazole (PF-3774076), a novel and selective alpha1A-adrenergic partial agonist, in in vitro and in vivo models of urethral function

2-((R-5-chloro-4-methoxymethyl-indan-1-yl)-1H-imidazole (PF-3774076) is a central nervous system (CNS) penetrant, potent, selective, partial agonist at the human alpha1(A)-adrenoceptor, demonstrating efficacy and selectivity in a range of binding and functional assays. In vivo, PF-3774076 increases peak urethral pressure in anesthetized female dogs in a dose-dependent manner, inducing changes in both the proximal and distal portions of the urethra via a central mechanism of action. The profile of this compound suggests that a CNS penetrant partial agonist at the alpha1(A)-adrenoceptor may offer significant benefit in stress urinary incontinence (SUI). However, despite partial agonism at the alpha1(A)-adrenoceptor and selectivity over alpha1(B)- and alpha1(D)-adrenoceptors, PF-3774076 did not offer the necessary degree of separation over cardiovascular events when assessed in in vivo models of cardiovascular function. This may be due to activation of both peripheral and central alpha1(A)-adrenoceptors. These data indicate that although central, partial alpha1(A)-agonists may offer significant benefit in the treatment of SUI, it may not be possible to achieve the desired level of urethral selectivity over cardiovascular events with this class of agent.

Mechanism of decongestant activity of alpha 2-adrenoceptor agonists

The vascular bed in nasal mucosa of different species, including human, is highly vascularized and an extensive sinusoidal network of large capacitance vessels is present deep within the submucosa. When this network of venous sinusoids is engorged with blood, the swollen mucosa reduces the size of the airway lumen and congestion ensues. Nasal vasculature tone is strongly influenced by the sympathetic nervous system and the only drugs approved specifically to relieve vascular nasal obstruction are alpha-adrenoceptor sympathomimetic agents. Due to their vasoconstrictor action, the sympathomimetic decongestants oppose vasodilation, reducing nasal airway resistance and thus facilitating nose breathing. However, standard decongestants that are non-selective alpha-adrenoceptor agonists are associated with the potential for side-effect liabilities including hypertension, stroke, insomnia and nervousness. We propose than a selective alpha 2-adrenoceptor agonist, by acting preferentially on nasal venous capacitance vessels, will elicit decongestion with a reduced side-effect liability. In the present study, we evaluated the effects of the selective alpha 2-adrenoceptor agonist BHT-920 in a real-time tissue contractility assay using isolated pig nasal explants and in an in vivo cat model of congestion. The vasoconstrictor and decongestant effects of BHT-920 were compared to the non-selective alpha-adrenoceptor agonist epinephrine and the standard decongestant oxymetazoline. Our results showed that the alpha 2-adrenoceptor agonist BHT-920 preferentially contracts venous sinusoids confirming previous observations [Corboz MR, Varty LM, Rivelli MA, Mutter JC, Mingo G, McLeod R, et al. Effects of an alpha 2-adrenoceptor agonist in nasal mucosa. Arch Physiol Biochem 2003;11: 335-6, Corboz MR, Rivelli MA, Varty LM, Mutter J, Cartwright M, Rizzo CA, et al. Pharmacological characterization of postjunctional alpha-adrenoceptor in human nasal mucosa. Am J Rhinol 2005;19: 495-502] and displays decongestion without affecting blood pressure. Therefore, an alpha 2-adrenoceptor agonist, by causing constriction in the capacitance vessels of nasal mucosa, can produce nasal decongestion without the effects on blood pressure observed with the standard selective alpha 1-adrenoceptor and non-selective alpha-adrenoceptor sympathomimetic decongestants.

Animal models in urological disease and sexual dysfunction

There are several conditions associated with dysfunction of the lower urinary tract or which result in a reduction in the ability to engage in satisfactory sexual function and result in significant bother to sufferers, partners and/or carers. This review describes some of the animal models that may be used to discover safe and effective medicines with which to treat them. While alpha adrenoceptor antagonists and 5-alpha-reductase inhibitors deliver improvement in symptom relief in benign prostatic hyperplasia sufferers, the availability of efficacious and well-tolerated medicines to treat incontinence is less well served. Stress urinary incontinence (SUI) has no approved medical therapy in the United States and overactive bladder (OAB) therapy is limited to treatment with muscarinic antagonists (anti-muscarinics). SUI and OAB are characterised by high prevalence, a growing ageing population and a strong desire from sufferers and physicians for more effective treatment options. High patient numbers with low presentation rates characterizes sexual dysfunction in men and women. The introduction of Viagra in 1998 for treating male erectile dysfunction and the success of the phosphodiesterase type 5 inhibitor class (PDE5 inhibitor) have indicated the willingness of sufferers to seek treatment when an effective alternative to injections and devices is available. The main value of preclinical models in discovering new medicines is to predict clinical outcomes. This translation can be established relatively easily in areas of medicine where there are a large number of drugs with different underlying pharmacological mechanisms in clinical usage. However, apart from, for example, the use of PDE5 inhibitors to treat male erectile dysfunction and the use of anti-muscarinics to treat OAB, this clinical information is limited. Therefore, current confidence in existing preclinical models is based on our understanding of the biochemical, physiological, pathophysiological and psychological mechanisms underlying the conditions in humans and how they are reflected in preclinical models. Confidence in both the models used and the pharmacological data generated is reinforced if different models of related aspects of the same disorder generate confirmatory data. However, these models will only be fully validated in retrospect once the pharmacological agents they have helped identify are tested in humans.

Phenylpropanolamine constricts mouse and human blood vessels by preferentially activating alpha2-adrenoceptors

Phenylpropanolamine (dl-norephedrine) was one of the most widely used therapeutic agents to act on the sympathetic nervous system. Because of concerns regarding incidents of stroke, its use as a nasal decongestant was discontinued. Although considered an alpha1-adrenergic agonist, the vascular adrenergic pharmacology of phenylpropanolamine was not fully characterized. Unlike most other circulations, the vasculature of the nasal mucosa is highly enriched with constrictor alpha2-adrenoceptors. Therefore, experiments were performed to determine whether phenylpropanolamine activates vascular alpha2-adrenoceptors. Mouse tail and mesenteric small arteries and human small dermal veins were isolated and analyzed in a perfusion myograph. The selective alpha1-adrenergic agonist phenylephrine caused constriction of tail and mesenteric arteries and human veins. The selective alpha2-adrenergic agonist UK14,304 [5-bromo-N-(4,5-dihydro-1H-imidazol-2-yl)-6-quinoxalinamine] caused constriction in tail arteries and in human veins, but not mesenteric arteries. The lack of constriction to UK14,304 was also observed in endothelium-denuded mesenteric arteries. Phenylpropanolamine constricted both types of artery but was 62-fold more potent in tail arteries. In mesenteric arteries, constriction to phenylpropanolamine was not affected by the selective alpha2-adrenergic antagonist, rauwolscine (10(-7) M) but was abolished by the selective alpha1-adrenergic antagonist, prazosin (3 x 10(-7) M). In contrast, constriction to phenylpropanolamine in tail arteries and in human veins was inhibited by rauwolscine but not prazosin. Therefore, phenylpropanolamine is a preferential alpha2-adrenergic agonist. At low concentrations, it constricts blood vessels that express functional alpha2-adrenoceptors, whereas at much higher concentrations, phenylpropanolamine also activates vascular alpha1-adrenoceptors. This action likely contributed to phenylpropanolamine's therapeutic activity, namely constriction of the nasal vasculature.

Alpha-1D adrenoceptors are involved in reserpine-induced supersensitivity of rat tail artery

1. We examined reserpine-induced chemical denervation supersensitivity with special reference to alpha-1 adrenoceptor (AR) subtypes. 2. Chronic treatment with reserpine for 2 weeks depleted noradrenaline in the tail artery and spleen of rats. Noradrenaline in the thoracic aorta was negligible before and after reserpine treatment. 3. The treatment with reserpine produced supersensitivity in the contractile responses of the rat tail artery to phenylephrine, 5-HT and KCl, resulting in leftward shift of concentration-response curves (11.6-, 2.5- and 1.1-fold at EC(50) value, respectively). These results suggest a predominant sensitization of the alpha-1 AR-mediated response by reserpine treatment. 4. BMY 7378 at a concentration (30 nm) specific for blocking the alpha-1D AR subtype, but not KMD-3213 at a concentration (10 nm) selective for blocking the alpha-1A AR subtype, inhibited the supersensitivity of the phenylephrine-induced response in the reserpine-treated artery. On the other hand, the response to phenylephrine in reserpine-untreated artery was selectively inhibited by the same concentration of KMD-3213, but not by BMY 7378. Prazosin, a subtype-nonselective antagonist, blocked the responses to phenylephrine with the same potency, regardless of reserpine treatment. 5. In the thoracic aorta and spleen, no supersensitivity was produced in the responses to phenylephrine by reserpine treatment. 6. In a tissue segment-binding study using [(3)H]-prazosin, the total density and affinity of alpha-1 ARs in the rat tail artery were not changed by treatment with reserpine. However, alpha-1D AR with high affinity for BMY 7378 was significantly detected in reserpine-treated tail artery, in contrast to untreated artery. Decreases in alpha-1A AR with high affinity for KMD-3213 and alpha-1B AR with low affinities for KMD-3213 and BMY 7378 were also estimated in reserpine-treated tail artery. 7. Alpha-1D AR mRNA in rat tail artery increased to three-folds by reserpine treatment, whereas the levels of alpha-1A and 1B mRNAs were not significantly changed. 8. The present results suggest that chronic treatment with reserpine affects the expression of alpha-1 AR subtypes of rat tail artery and that the induction of alpha-1D ARs with high affinity for catecholamines is in part associated with reserpine-induced supersensitivity.

Synthesis and Structure−Activity Studies on N-[5-(1H-Imidazol-4-yl)-5,6,7,8-tetrahydro-1-naphthalenyl]methanesulfonamide, an Imidazole-Containing α1A-Adrenoceptor Agonist

Structure-activity studies were performed on the alpha(1A)-adrenoceptor (AR) selective agonist N-[5-(1H-imidazol-4-yl)-5,6,7,8-tetrahydro-1-naphthalenyl]methanesulfonamide (4). Compounds were evaluated for binding activity at the alpha(1A), alpha(1b), alpha(1d), alpha(2a), and alpha(2B) subtypes. Functional activity in tissues containing the alpha(1A) (rabbit urethra), alpha(1B) (rat spleen), alpha(1D) (rat aorta), and alpha(2A) (rat prostatic vas deferens) was also evaluated. A dog in vivo model simultaneously measuring intraurethral pressure (IUP) and mean arterial pressure (MAP) was used to assess the uroselectivity of the compounds. Many of the compounds that were highly selective in vitro for the alpha(1A)-AR subtype were also more uroselective in vivo for increasing IUP over MAP than the nonselective alpha(1)-agonists phenylpropanolamine (PPA) (1) and ST-1059 (2, the active metabolite of midodrine), supporting the hypothesis that greater alpha(1A) selectivity would reduce cardiovascular side effects. However, the data also support a prominent role of the alpha(1A)-AR subtype in the control of MAP.

Emerging Pharmacologic Approaches for the Treatment of Lower Urinary Tract Disorders

Lower urinary tract disorders include disorders affecting continence (stress urinary incontinence, urge urinary incontinence, and benign prostatic hyperplasia) and male erectile dysfunction. Although none of these conditions are fatal, they affect overall quality of life. Throughout modern medicine the treatment of these conditions was limited to psychological counseling or surgical intervention. In recent years, research defining the physiological mechanisms of continence and male sexual function has aided in the pharmacologic design of approaches to these conditions. These agents can act both centrally or on the peripheral genitourinary smooth muscle to alleviate disease symptoms. Incontinence is primarily treated with agents that act directly on the bladder smooth muscle such as muscarinic antagonists. However, afferent blockade to attenuate the spinalbulbospinal reflex pathway including mixed norepinephrine/serotonin reuptake inhibitors may provide a key breakthrough. Erectile dysfunction treatment has been revolutionized via the discovery of the nitric oxide pathway and phosphodiesterase 5 inhibitors. New peripheral targets as well as centrally acting agents represent potential emerging therapies. In this review, the pharmacologic basis of treatment of these disorders is discussed with special emphasis on emerging new therapeutics.