In vivo receptor binding of novel alpha1-adrenoceptor antagonists for treatment of benign prostatic hyperplasia

Characterization of bladder selectivity of antimuscarinic agents on the basis of in vivo drug-receptor binding

The in vivo muscarinic receptor binding of antimuscarinic agents (oxybutynin, solifenacin, tolterodine, and imidafenacin) used to treat urinary dysfunction in patients with overactive bladder is reviewed. Transdermal administration of oxybutynin in rats leads to significant binding of muscarinic receptors in the bladder without long-term binding in the submaxillary gland and the abolishment of salivation evoked by oral oxybutynin. Oral solifenacin shows significant and long-lasting binding to muscarinic receptors in mouse tissues expressing the M₃ subtype. Oral tolterodine binds more selectively to muscarinic receptors in the bladder than in the submaxillary gland in mice. The muscarinic receptor binding of oral imidafenacin in rats is more selective and longer-lasting in the bladder than in other tissues such as the submaxillary gland, heart, colon, lung, and brain, suggesting preferential muscarinic receptor binding in the bladder. In vivo quantitative autoradiography with (+)N-[¹¹C]methyl-3-piperidyl benzilate in rats shows significant occupancy of brain muscarinic receptors with the intravenous injection of oxybutynin, solifenacin, and tolterodine. The estimated in vivo selectivity in brain is significantly greater for solifenacin and tolterodine than for oxybutynin. Imidafenacin occupies few brain muscarinic receptors. Similar findings for oral oxybutynin were observed with positron emission tomography in conscious rhesus monkeys with a significant disturbance of short-term memory. The newer generation of antimuscarinic agents may be advantageous in terms of bladder selectivity after systemic administration.

α1-Adrenoceptors and muscarinic receptors in voiding function - binding characteristics of therapeutic agents in relation to the pharmacokinetics

In vivo and ex vivo binding of α(1)-adrenoceptor and muscarinic receptors involved in voiding function is reviewed with therapeutic agents (α(1)-adrenoceptor antagonists: prazosin, tamsulosin and silodosin; and muscarinic receptor antagonists: oxybutynin, tolterodine, solifenacin, propiverine, imiafenacin and darifenacin) in lower urinary tract symptoms. This approach allows estimation of the inhibition of a well-characterized selective (standard) radioligand by unlabelled potential drugs or direct measurement of the distribution and receptor binding of a standard radioligand or radiolabelled form of a novel drug. In fact, these studies could be conducted in various tissues from animals pretreated with radioligands and/or unlabelled novel drugs, by conventional radioligand binding assay, radioactivity measurement, autoradiography and positron emission tomography. In vivo and ex vivo receptor binding with α(1)-adrenoceptor antagonists and muscarinic receptor antagonists have been proved to be useful in predicting the potency, organ selectivity and duration of action of drugs in relation to their pharmacokinetics. Such evaluations of drug-receptor binding reveal that adverse effects could be avoided by the use of new α(1)-adrenoceptor antagonists and muscarinic receptor antagonists for the treatment of lower urinary tract symptoms. Thus, the comparative analysis of α(1)-adrenoceptor and muscarinic receptor binding characteristics in the lower urinary tract and other tissues after systemic administration of therapeutic agents allows the rationale for their pharmacological characteristics from the integrated viewpoint of pharmacokinetics and pharmacodynamics. The current review emphasizes the usefulness of in vivo and ex vivo receptor binding in the discovery and development of novel drugs for the treatment of not only urinary dysfunction but also other disorders.

α(1)-Adrenoceptors in the urinary tract

α(1)-Adrenoceptors have been identified and characterized extensively by functional, radioligand-binding, and molecular biological techniques. Molecular clones have been isolated for three α(1)-subtypes (α(1a), α(1b), and α(1d)), and these subtypes are also functionally characterized. α(1)-Adrenoceptors are present in the prostate, urethra, bladder (urothelium, smooth muscle, and afferent nerves), ureter, vas deferens, peripheral ganglia, nerve terminals, vascular tissues, and central nervous system (CNS), and they could all potentially influence overall urinary function and contribute to both the therapeutic and adverse effects of α(1)-adrenoceptor antagonists in patients with benign prostatic hyperplasia (BPH) and lower urinary tract symptoms (LUTS). This review aimed to discuss the relevant physiological and pharmacological roles and molecular biology of α(1)-adrenoceptor subtypes in the prostate, urethra, bladder, ureter, and CNS.

The forefront for novel therapeutic agents based on the pathophysiology of lower urinary tract dysfunction: bladder selectivity based on in vivo drug-receptor binding characteristics of antimuscarinic agents for treatment of overactive bladder

We have reviewed the binding of antimuscarinic agents, used to treat urinary dysfunction in patients with overactive bladder, to muscarinic receptors in target and non-target tissues in vivo. Transdermal administration of oxybutynin in rats led to significant binding in the bladder without long-term binding in the submaxillary gland and the abolishment of salivation evoked by oral oxybutynin. Oral solifenacin showed significant and long-lasting binding to muscarinic receptors in mouse tissues expressing the M(3) subtype. Oral tolterodine bound more selectively to muscarinic receptors in the bladder than in the submaxillary gland in mice. The muscarinic receptor binding activity of oral darifenacin in mice was shown to be pronounced and long-lasting in the bladder, submaxillary gland, and lung. In vivo quantitative autoradiography using (+)N-[(11)C]methyl-3-piperidyl benzilate in rats showed significant occupancy of brain muscarinic receptors on intravenous injection of oxybutynin, propiverine, solifenacin, and tolterodine. The estimated in vivo bladder selectivity compared to brain was significantly greater for solifenacin and tolterodine than oxybutynin. Darifenacin occupied few brain muscarinic receptors. Similar findings were also observed with positron emission tomography in conscious rhesus monkeys. The newer generation of antimuscarinic agents may be advantageous in the bladder selectivity after systemic administration.

Effects of Silodosin (KMD-3213) on Phenylephrine-induced Increase in Intraurethral Pressure and Blood Pressure in Rats—Study of the Selectivity for Lower Urinary Tract—

The effects of silodosin, an alpha(1A)-adrenoceptor (AR) antagonist, and of other alpha(1)-AR antagonists on the phenylephrine (PE)-induced increase in intraurethral pressure (IUP) and on blood pressure (BP) were studied in anesthetized rats. The drugs were administered intravenously (i.v. study) or intraduodenally (i.d. study). IUP and BP were measured via catheters inserted into the prostatic urethra and common carotid artery, respectively. In the i.v. study, drugs were administered every 30 min for effects on BP, and 5 min before each PE-injection (30 microg/kg, every 60 min) with stepwise increases in dose for effects on IUP. In the i.d. study, one dose of drug was administered per rat, then IUP and BP were observed for 4 h [IUP being measured time-dependently following PE-injection (30 microg/kg)], and IUP and BP were expressed as a percentage of the values without any drugs. ID(50) for IUP and ED(15) for BP were calculated, and uroselectivity was determined as ED(15)/ID(50) for each drug. All drugs both inhibited the IUP increase and lowered BP, each effect being dose-dependent. The order of uroselectivities was silodosin (11.7)>tamuslosin (2.24)>naftopidil (0.133) in the i.v. study, and silodosin (26.0)>tamuslosin (3.82)>naftopidil (1.39) in the i.d. study. Selectivity for the lower urinary tract (LUT) was higher for silodosin than for tamsulosin (alpha(1A)/alpha(1D)-AR), naftopidil (alpha(1D)-AR), or prazosin (non-selective alpha(1)-AR). These results suggested that an alpha(1A)-AR selective antagonist like silodosin might be effective in the LUT without causing hypotension.

Advantages for transdermal over oral oxybutynin to treat overactive bladder: Muscarinic receptor binding, plasma drug concentration, and salivary secretion

To clarify pharmacological usefulness of transdermal oxybutynin in the therapy of overactive bladder, we have characterized muscarinic receptor binding in rat tissues with measurement of plasma concentrations of oxybutynin and its metabolite N-desethyl-oxybutynin (DEOB) and salivation after transdermal oxybutynin compared with oral route. At 1 and 3 h after oral administration of oxybutynin, there was a significant increase in apparent dissociation constant (Kd) for specific [N-methyl-3H]scopolamine ([3H]NMS) binding in the rat bladder, submaxillary gland, heart, and colon compared with control values. Concomitantly, submaxillary gland and heart showed a significant decrease in maximal number of binding sites (Bmax) for [3H]NMS binding, which lasted until 24 h. Transdermal application of oxybutynin caused dose-dependent increases in Kd values for specific [3H]NMS binding in rat tissues. The increment of Kd values by transdermal oxybutynin was dependent on the application time. Plasma concentrations of oxybutynin and DEOB peaked at 1 h after oral oxybutynin. In contrast, plasma concentrations of oxybutynin increased slowly, depending on the transdermal application time of this drug until 12 h. Suppression of pilocarpine-induced salivation in rats due to transdermal oxybutynin was significantly weaker and more reversible than that by oral oxybutynin, which abolished salivary secretion. The present study has shown that transdermal oxybutynin binds significantly to rat bladder muscarinic receptors without producing both long-lasting occupation of exocrine receptors and cessation of cholinergic salivation evoked by oral oxybutynin. Thus, the present study provides further pharmacological basis for advantage of transdermal over oral oxybutynin in the therapy of overactive bladder.

In vivo identification and characterization of binding sites for selective serotonin reuptake inhibitors in mouse brain

The present study was undertaken to identify and characterize in vivo binding sites of selective serotonin reuptake inhibitors (SSRIs) in the mouse brain by using [3H]paroxetine as radioligand. Relatively higher concentration of [3H]paroxetine was detected in the whole brain (minus cerebellum) than in the plasma of mice after the i.v. injection of the radioligand, and the half-life (t1/2) of elimination was much slower. The in vivo specific [3H]paroxetine binding in the mouse brain after the i.v. injection was defined as the difference of particulate-bound radioactivity between the whole brain and cerebellum, and it was dose-dependently attenuated by oral or intraperitoneal administration of fluoxetine (8.68-116 micromol/kg). Furthermore, oral administration of fluvoxamine, fluoxetine, paroxetine and sertraline at the pharmacologically relevant doses reduced significantly (25-94%) in vivo specific [3H]paroxetine binding in the cerebral cortex, striatum, hippocampus, thalamus and midbrain of mice, and their significant decreases were observed up to at least 8 h (fluvoxamine), 24 h (fluoxetine), and 12 h (paroxetine and sertraline) later. The value of area under the curve (AUC) for decrease in [3H]paroxetine binding vs. time in each brain region was largest for fluoxetine among these SSRIs, due to the relatively longer-lasting occupation of brain serotonin transporter. The AUC value in mouse brain after oral administration of each SSRI was 1.2-3.2 times greater in the thalamus and midbrain than in the cerebral cortex, striatum and hippocampus. Thus, the present study has revealed that [3H]paroxetine may be a suitable radioligand for in vivo characterization of brain binding sites and pharmacological effects of SSRIs.

α1-Adrenergic Receptors and Their Inhibitors in Lower Urinary Tract Symptoms and Benign Prostatic Hyperplasia

PURPOSE

We provide a comprehensive overview of the role of alpha1-adrenergic receptors (alpha1ARs) as critical mediators of lower urinary tract symptoms (LUTS) and pathophysiology in benign prostatic hyperplasia (BPH), and we review the pharmacological antagonists of alpha1ARs.

MATERIALS AND METHODS

A review was performed of pertinent studies in the literature relating to the pathophysiology of LUTS and BPH, focusing on the role of alpha1ARs, and of clinical trial and practice data evaluating the different agents that inhibit these receptors.

RESULTS

Further characterization of the alpha1AR gene family indicates that 3 receptor subtypes exist in humans. Their different distribution between urinary tract and cardiovascular tissues has provided a strategy for the development of improved therapeutic agents. Since excessive activity of the alpha1aAR and alpha1dAR subtypes appears to be a common feature in symptomatic BPH and alpha1aARs are enriched in prostatic tissue, drugs that demonstrate high alpha1aAR selectivity have attracted attention. Tamsulosin, which has high affinity for alpha1aAR and alpha1dAR subtypes but not for alpha1bAR, shows efficacy similar to the nonsubtype selective agents terazosin and doxazosin. It is associated with fewer cardiovascular side effects, although it has some ejaculatory side effects. The nonsubtype selective agent alfuzosin also demonstrates efficacy and offers an enhanced side effect profile, particularly minimizing hypotension. Other agents with super selective specificity for the alpha1aAR subtype are under investigation.

CONCLUSIONS

Further advances in the treatment of LUTS associated with BPH may depend not only on receptor subtype selectivity, but also on other pharmacokinetic and pharmacodynamic factors.

Effects of KMD-3213, a uroselective alpha 1A-adrenoceptor antagonist, on the tilt-induced blood pressure response in normotensive rats

KMD-3213 ((-)-1-(3-hydroxypropyl)-5-((2R)-2-[[2-([2-[(2,2,2-trifluoroethyl)oxy]phenyl]oxy)ethyl]amino]propyl)-2,3-dihydro-1H-indole-7-carboxamide), an alpha(1A)-adrenoceptor antagonist with potency similar to that of tamsulosin, is under development for the treatment of bladder outlet obstruction in patients with benign prostatic hypertrophy. In the present study, we investigated the effects of KMD-3213 on the tilt-induced blood pressure response in anesthetized normotensive rats. Male normotensive Sprague-Dawley rats were placed in the supine position on a board under cocktail anesthetization (alpha-chloralose, urethane and sodium pentobarbital). The arterial blood pressure was measured from the carotid artery. The animals were given consistent 45 degrees head-up tilt from the horizontal position, following the transient decrease in the blood pressure, and then recovery of the blood pressure to the normal level. Significant orthostatic hypotension was seen with intravenous administration of both prazosin and tamsulosin at doses over 3 micro g/kg, and these drugs completely blocked the tilt-induced blood pressure responses at 30 micro g/kg. On the other hand, these responses were still retained when KMD-3213 was administered intravenously at a dose up to 75 micro g/kg of KMD-3213. Moreover, KMD-3213 showed the highest uroselectivity of the test drugs. These results indicate that KMD-3213 is not likely to induce orthostatic hypotension and would be a useful compound for the treatment of urinary outlet obstruction in patients with benign prostatic hyperplasia.

An in vivo binding assay to determine central alpha(1)-adrenoceptor occupancy using [(3)H]prazosin

An alpha(1) adrenoceptor (alpha(1)-AdR) assay using [(3)H]prazosin binding in mouse brain is described which allows in vivo determination of central alpha(1)-AdR occupancy for ligands with alpha(1)-AdR affinity. Binding of [3H]prazosin in rat and mouse brain membranes in vitro was used to characterise the pharmacological profile of alpha(1)-AdRs in order to determine any potential species variations. Saturation and displacement studies yielded comparable affinity and pharmacological profile for [(3)H]prazosin binding in mouse and rat brain homogenates. These studies confirmed the absence of species variation for ligands in central alpha(1)-AdR pharmacology which is in good agreement with previous studies in rat brain. Subsequently, in vivo binding of [(3)H]prazosin in mouse whole brain was used to measure the occupancy of a number of AdR ligands. Timecourse studies revealed that a [3H]prazosin (5 mu Ci/mouse) pretreatment time of at least 20 min following intravenous (i.v.) administration was required for optimal specific binding. Ligands were administered systemically 40 min prior to i.v. administration of radiolabel. The alpha(1)-adrenoceptor ligands prazosin (ED(50)=0.15 mg/kg i.p.), benoxathian (0.52 mg/kg i.p.) and phentolamine (51 mg/kg i.p.) were all able to block in vivo [(3)H]prazosin binding from mouse brain. In addition, receptor occupancy values for a number of compounds including haloperidol (ED(50)=0.83 mg/kg s.c.), clozapine (2.2 mg/kg s.c.) and MDL-100907 [R(+)-alpha-(2,3-dimethoxyphenyl)-1-[2-(4-fluorophenylethyl)]-4-piperidine-methanol], (10 mg/kg s.c.)], which possess high to moderate affinity at alpha(1)-adrenoceptors, were also determined. These results suggest that in the mouse, [(3)H]prazosin binding can be used to measure in vivo receptor occupancy of ligands with affinity at central alpha(1)-adrenoceptors.

Muscarinic receptor binding characteristics in rat tissues after oral administration of oxybutynin and propiverine

Ex vivo muscarinic receptor binding of oxybutynin and propiverine, the most commonly used anticholinergic agents for the treatment in patients with urinary incontinence, was investigated in rat tissues. The oral administration of oxybutynin (50.8 and 127 micromol/kg) caused a significant increase in the apparent dissociation constant (Kd) for specific (-)-[3H]QNB binding in the rat bladder, prostate, submaxillary gland, heart and cerebral cortex, compared with each of the control values. Also, in the submaxillary gland of these rats, there was a reduction in the maximal number of binding sites (Bmax) for (-)-[3H]QNB binding. Similarly, oral administration of propiverine at doses of 74.3-297 micromol/kg brought about a significant increase in the Kd values for (-)-[3H]QNB binding in rat tissues including the bladder, and greater increase in Kd values was seen in the rat prostate, heart and submaxillary gland. On the other hand, oral administration of propiverine, unlike oxybutynin, resulted in very little reduction in the Bmax valules for (-)-[3H]QNB binding in the submaxillary gland. In conclusion, the present study has shown that oxybutynin and propiverine, after oral administration, bind significantly to muscarinic receptors in tissues such as the bladder, which is the target organ for the treatment of urinary incontinence, and that oxybutynin appears to exhibit long-term binding to muscarinic receptors in the salivary gland.

Subtype selective alpha1-adrenoceptor antagonists for the treatment of benign prostatic hyperplasia

Benign prostatic hyperplasia (BPH) is highly prevalent in the male population beyond the age of 60. Impairment of urinary flow due to prostate enlargement gives rise to symptoms of 'prostatism' that have a detrimental impact on the quality of life. The current trend in the management of symptomatic BPH favours pharmacotherapy as a first line option, while the number of surgical procedures being performed has experienced a steady decline during the last ten years. Among the pharmacological treatments, the use of alpha1-adrenoceptor blockers has demonstrated to be an effective treatment option for BPH. These agents reduce the adrenergic tone to the prostate and increase urinary flow, with a concomitant reduction of lower urinary tract symptoms. The alpha1-blockers currently approved include compounds such as alfuzosin, terazosin and doxazosin, originally developed for the treatment of hypertension, and more recently tamsulosin, an alpha1-subtype selective drug. The blockade of alpha1-adrenoceptors present in vascular smooth muscle is largely responsible for the most prominent side effects of current drugs, which can be severe and require patients dose titration. The limitation imposed by side effects naturally raises the possibility that complete blockade of prostatic alpha1 receptors is not attained at the maximum tolerated dose. The extensive efforts by the pharmaceutical industry towards the development of uroselective alpha1-blockers, is the subject of this review. Advances in the molecular cloning of genes encoding three alpha1-adrenoceptors led to the identification of the alpha1A-subtype as the predominant receptor responsible for the contraction of prostate smooth muscle. In preclinical animal models, selective alpha1A-antagonists have consistently been found to have minimal cardiovascular effects, thus providing a pharmacological rationale for uroselectivity. It has also become apparent, however, that uroselectivity can emerge in a poorly understood manner from the pharmacodynamic properties of compounds without alpha1A-subtype selectivity. Clinical experience with tamsulosin, an alpha1A/alpha1D selective drug, has failed to demonstrate a significant improvement in efficacy beyond that demonstrated for non-subtype selective alpha1-blockers, and gives support to the notion that alpha1A-selective antagonists might achieve greater efficacy for the treatment of BPH. Given the demonstrated uroselectivity of alpha1A-selective antagonists in preclinical models, it is anticipated that third generation alpha1-blockers will exhibit improved urinary flow efficacy and be better tolerated than tamsulosin. The extent to which the improvement in urinary flow will translate to the relief of symptoms of prostatism, however, remains to be demonstrated in randomised placebo-controlled clinical trials of alpha1A-selective antagonists.

Effect of JTH-601, a novel alpha1-adrenoceptor antagonist, on the function of lower urinary tract and blood pressure

In the present study, we investigated the effect of JTH-601 (3-{N-[2-(4-hydroxy-2-isopropyl-5-methylphenoxy)ethyl]-N-methylaminomethyl}-4-methoxy-2,5,6-trimethylphenol hemifumarate), a novel alpha1-adrenoceptor antagonist, in vitro and in vivo. JTH-601 (10(-9)-3 x 10(-8) M) competitively antagonized phenylephrine-induced contraction in lower urinary tract tissues (prostate, urethra and bladder trigon) in a concentration-dependent manner. The mean pA2 values for JTH-601 were 8.59+/-0.14, 8.74+/-0.09 and 8.77+/-0.11 for prostate, urethra and bladder trigon, respectively. In anesthetized rabbits, intraduodenal administration of JTH-601 (0.3-3 mg/kg), prazosin (0.03-0.3 mg/kg) and tamsulosin (0.03-0.3 mg/kg) dose dependently inhibited the phenylephrine-induced increase in urethral pressure for 3 h. Although these drugs also decreased mean blood pressure, JTH-601 was less potent than prazosin or tamsulosin. In conscious rabbits, administered JTH-601 (0.01-1 mg/kg, i.v.) had a tendency to augment orthostatic hypotension, but dose dependency was not evident. Prazosin (0.01-1 mg/kg) and tamsulosin (0.001-1 mg/kg) dose dependently augmented orthostatic hypotension. These results indicate that JTH-601 antagonized alpha1-adrenoceptor-mediated contractile responses more potently than prazosin or tamsulosin in rabbit lower urinary tract both in vitro and in vivo. JTH-601 is therefore expected to be effective in the treatment of urinary outlet obstruction in benign prostatic hypertrophy.