Binding characteristics of naftopidil and α1-adrenoceptor antagonists to prostatic α-adrenoceptors in benign prostatic hypertrophy

Thermoanalytical investigation of terazosin hydrochloride

PURPOSE

Thermal analysis (TGA, DTG and DTA) and differential scanning calorimetry (DSC) have been used to study the thermal behavior of terazosin hydrochloride (TER).

METHODS

Thermogravimetric analysis (TGA/DTG), differential thermal analysis (DTA) and differential scanning calorimetry (DSC) were used to determine the thermal behavior and purity of the used drug. Thermodynamic parameters such as activation energy (E*), enthalpy (∆H*), entropy (∆S*) and Gibbs free energy change of the decomposition (∆G*) were calculated using different kinetic models.

RESULTS

The purity of the used drug was determined by differential scanning calorimetry (99.97%) and specialized official method (99.85%) indicating to satisfactory values of the degree of purity. Thermal analysis technique gave satisfactory results to obtain quality control parameters such as melting point (273 ºC), water content (7.49%) and ash content (zero) in comparison to what were obtained using official method: (272 ºC), (8.0%) and (0.02%) for melting point, water content and ash content, respectively.

CONCLUSION

Thermal analysis justifies its application in quality control of pharmaceutical compounds due to its simplicity, sensitivity and low operational costs. DSC data indicated that the degree of purity of terazosin hydrochloride is similar to that found by official method.

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.

Naftopidil for the treatment of urinary symptoms in patients with benign prostatic hyperplasia

Naftopidil, approved only in Japan, is an α1-adrenergic receptor antagonist (α1-blocker) used to treat lower urinary tract symptoms (LUTS) suggestive of benign prostatic hyperplasia (BPH). Different from tamsulosin hydrochloride and silodosin, in that it has higher and extremely higher affinity respectively, for the α1A-adrenergic receptor subtype than for the α1D type, naftopidil has distinct characteristics because it has a three times greater affinity for the α1D-adrenergic receptor subtype than for the α1A subtype. Although well-designed large-scale randomized controlled studies are lacking and the optimal dosage of naftopidil is not always completely determined, previous reports from Japan have shown that naftopidil has superior efficacy to a placebo and comparable efficacy to other α1-blockers such as tamsulosin. On the other hand, the incidences of ejaculatory disorders and intraoperative floppy iris syndrome induced by naftopidil may be lower than for tamsulosin and silodosin having high affinity for the α1A-adrenergic receptor subtype. However, it remains unknown if the efficacy and safety of naftopidil in Japanese is applicable to white, black and Hispanic men having LUTS/BPH in western countries.

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.

Prediction of alpha1-adrenoceptor occupancy in the human prostate from plasma concentrations of silodosin, tamsulosin and terazosin to treat urinary obstruction in benign prostatic hyperplasia

Alpha1-adrenoceptor antagonists are clinically useful for the improvement of urinary obstruction due to benign prostatic hyperplasia (BPH), and their therapeutic effects are mediated through the blockade of prostatic alpha(1)-adrenoceptors. The present study was undertaken to predict the magnitude and duration of alpha(1)-adrenoceptor occupancy in the human prostate after oral alpha(1)-adrenoceptor antagonists. Prostatic alpha(1)-adrenoceptor-binding parameters of silodosin were estimated by measuring specific [(3)H]prazosin binding in rat prostate after oral administration of this drug. The plasma concentration of silodosin after oral administration in rats and healthy volunteers was measured using a high-performance liquid chromatographic method. The alpha(1)-adrenoceptor-binding affinities (K(i)) of silodosin, tamsulosin, and terazosin in the human prostate and plasma concentrations of tamsulosin and terazosin were obtained from the literature. Using the alpha(1)-adrenoceptor binding parameters of silodosin in rat prostate, alpha(1)-adrenoceptor occupancy in the human prostate was estimated to be around 60-70% at 1-6 h after oral administration of silodosin at doses of 3.0, 8.1, and 16.1 micromol. Thereafter, the receptor occupancy was periodically decreased, to 24% (8.1 micromol) and 54% (16.1 micromol) 24 h later. A similar magnitude and time course of alpha(1)-adrenoceptor occupancy by silodosin in the human prostate were estimated using alpha(1)-adrenoceptor-binding affinities (K(i)) in the human prostate. Despite about two orders of differences in the plasma unbound concentrations after clinically effective oral dosages of silodosin, tamsulosin, and terazosin, there was a comparable magnitude of prostatic alpha(1)-adrenoceptor occupancy by these drugs. In conclusion, the prediction of alpha(1)-adrenoceptor occupancy in the human prostate by alpha(1)-adrenoceptor antagonists may provide the rationale for the optimum dosage regimen of these drugs in the therapy of BPH.

Alpha1-, alpha2- and beta-adrenoceptors in the urinary bladder, urethra and prostate

1 We have systematically reviewed the presence, functional responses and regulation of alpha(1)-, alpha(2)- and beta-adrenoceptors in the bladder, urethra and prostate, with special emphasis on human tissues and receptor subtypes. 2 Alpha(1)-adrenoceptors are only poorly expressed and play a limited functional role in the detrusor. Alpha(1)-adrenoceptors, particularly their alpha(1A)-subtype, show a more pronounced expression and promote contraction of the bladder neck, urethra and prostate to enhance bladder outlet resistance, particularly in elderly men with enlarged prostates. Alpha(1)-adrenoceptor agonists are important in the treatment of symptoms of benign prostatic hyperplasia, but their beneficial effects may involve receptors within and outside the prostate. 3 Alpha(2)-adrenoceptors, mainly their alpha(2A)-subtype, are expressed in bladder, urethra and prostate. They mediate pre-junctional inhibition of neurotransmitter release and also a weak contractile effect in the urethra of some species, but not humans. Their overall post-junctional function in the lower urinary tract remains largely unclear. 4 Beta-adrenoceptors mediate relaxation of smooth muscle in the bladder, urethra and prostate. The available tools have limited the unequivocal identification of receptor subtypes at the protein and functional levels, but it appears that the beta(3)- and beta(2)-subtypes are important in the human bladder and urethra, respectively. Beta(3)-adrenoceptor agonists are promising drug candidates for the treatment of the overactive bladder. 5 We propose that the overall function of adrenoceptors in the lower urinary tract is to promote urinary continence. Further elucidation of the functional roles of their subtypes will help a better understanding of voiding dysfunction and its treatment.

Uroselectivity in male dogs of silodosin (KMD-3213), a novel drug for the obstructive component of benign prostatic hyperplasia

AIMS

Our main aim was to compare the prostatic selectivity of silodosin with that of other alpha(1)-adrenoceptor (AR) antagonists.

METHODS

We examined uroselectivities in two sets of experiments namely, in vitro and in vivo functional studies using male dogs. In the in vitro study, after evaluating the inhibitory effects of silodosin on noradrenaline (NA)-induced contractions in the isolated prostate and isolated carotid artery using the Magnus method, we calculated prostatic selectivity. In the in vivo study, we examined the effects of drugs on the hypogastric nerve stimulation (HNS)-induced increase in intraurethral pressure (IUP) and on blood pressure. The uroselectivity of silodosin was compared with those of tamsulosin and naftopidil.

RESULTS

In vitro, all drugs antagonized NA-induced contraction in both prostate and carotid artery. The prostatic selectivity of silodosin (79.4) was much higher than those of tamsulosin (1.78), naftopidil (0.55), BMY 7378 (0.115), and prazosin (0.01). In vivo, intravenously (i.v.) administered silodosin dose-dependently inhibited the HNS-induced increase in IUP with much less hypotensive effect than either tamsulosin or naftopidil, the uroselectivity (ED(15)/ID(50)) of silodosin (237) being significantly higher than those of tamsulosin (1.21) and naftopidil (2.65).

CONCLUSIONS

Our results clearly demonstrate that silodosin is a potent and highly selective alpha(1A)-AR antagonist. A selective alpha(1A)-AR antagonist such as silodosin may have good potential as a less-hypotensive drug for the treatment of urinary dysfunction in benign prostatic hyperplasia patients.

α1-Adrenoceptor Subtype Selectivity and Organ Specificity of Silodosin (KMD-3213)

The selectivity of silodosin (KMD-3213), an antagonist of alpha(1)-adrenoceptor (AR), to the subtypes (alpha(1A)-, alpha(1B)- and alpha(1D)-ARs) was examined by a receptor-binding study and a functional pharmacological study, and we compared its subtype-selectivity with those of other alpha(1)-AR antagonists. In the receptor-binding study, a replacement experiment using [(3)H]-prazosin was conducted using the membrane fraction of mouse-derived LM (tk-) cells in which each of three human alpha(1)-AR subtypes was expressed. In the functional pharmacological study, the following isolated tissues were used as representative organs with high distribution densities of alpha(1)-AR subtypes (alpha(1A)-AR: rabbit prostate, urethra and bladder trigone; alpha(1B)-AR: rat spleen; alpha(1D)-AR: rat thoracic aorta). Using the Magnus method, we studied the inhibitory effect of silodosin on noradrenaline-induced contraction, and compared it with those of tamsulosin hydrochloride, naftopidil and prazosin hydrochloride. Silodosin showed higher selectivity for the alpha(1A)-AR subtype than tamsulosin hydrochloride, naftopidil or prazosin hydrochloride (affinity was highest for tamsulosin hydrochloride, followed by silodosin, prazosin hydrochloride and naftopidil in that order). Silodosin strongly antagonized noradrenaline-induced contractions in rabbit lower urinary tract tissues (including prostate, urethra and bladder trigone, with pA(2) or pKb values of 9.60, 8.71 and 9.35, respectively). On the other hand, the pA(2) values for antagonism of noradrenaline-induced contractions in rat isolated spleen and rat isolated thoracic aorta were 7.15 and 7.88, respectively. Selectivity for lower urinary tract was higher for silodosin than for the other alpha(1)-AR antagonists. Our data suggest that silodosin has a high selectivity for the alpha(1A)-AR subtype and for the lower urinary tract.

Recent advances in the identification of a 1- and a 2-adrenoceptor subtypes: therapeutic implications

The cloning of multiple subtypes of both alpha1- and alpha2-adrenoceptors has renewed interest in the therapeutic application of agents interacting with these receptors. Effort has primarily been directed towards the design of uroselective alpha1-adrenoceptor antagonists for the treatment of benign prostatic hyperplasia (BPH). Evidence is accumulating for the involvement of a novel alpha1-adrenoceptor, designated as alpha1L-adrenoceptor, in alpha1-adrenoceptor-mediated smooth muscle contraction in prostatic and other urogenital tissues. While several antagonists showing a high degree of uroselectivity in animal models have been identified, their clinical superiority over the currently available alpha1-adrenoceptor antagonists has not yet been demonstrated. It is possible that the interaction with alpha1-adrenoceptors, as yet uncharacterised subtypes, at non-prostatic sites contributes to the therapeutic activity of this drug class in BPH. The alpha1-adrenoceptor subtypes involved in the control of vascular tone are currently being evaluated, and the profile of interaction with the various alpha1-adrenoceptor subtypes may play a key role in the efficacy of cardiovascular drugs such as carvedilol. Alpha2-adrenoceptor agonists are now being employed for a variety of therapeutic applications, most involving actions on receptors within the central nervous system (CNS). These agents are useful in the treatment of hypertension, glaucoma, opiate withdrawal and attention deficit hyperactivity disorder (ADHD), and as analgesics and adjuncts to general anaesthesia. While subtype selectivity has not yet been applied to the design of new alpha2-adrenoceptor agonists for these applications, recent gene mutation/knock-out experiments have identified the alpha2-subtypes involved in some of these actions, and optimisation of a therapeutic profile may be possible. Furthermore, the design of agents combining affinities for multiple adrenoceptor subtypes, or the combination of a specific adrenoceptor affinity profile with another pharmacological action, may offer advantages over molecules selective for an individual adrenoceptor subtype.

Naftopidil, a novel alpha1-adrenoceptor antagonist, displays selective inhibition of canine prostatic pressure and high affinity binding to cloned human alpha1-adrenoceptors

The pharmacological profiles of the alpha1-adrenoceptor antagonists naftopidil, tamsulosin and prazosin were studied in an anesthetized dog model that allowed the simultaneous assessment of their antagonist potency against phenylephrine-mediated increases in prostatic pressure and mean blood pressure. The intravenous administration of each of these compounds dose-dependently inhibited phenylephrine-induced increases in prostatic pressure and mean blood pressure. To further assess the ability of the three compounds to inhibit phenylephrine-induced responses, the doses required to produce a 50% inhibition of the phenylephrine-induced increases in prostatic and mean blood pressure and the selectivity index obtained from the ratio of those two doses were determined for each test compound. Forty minutes after the intravenous administration of naftopidil, the selectivity index was 3.76, and those of tamsulosin and prazosin were 1.23 and 0.61, respectively. These findings demonstrated that naftopidil selectively inhibited the phenylephrine-induced increase in prostatic pressure compared with mean blood pressure in the anesthetized dog model. The selectivity of naftopidil for prostatic pressure was the most potent among the test compounds. In addition, using cloned human alpha1-adrenoceptor subtypes, naftopidil was selective for the alpha1d-adrenoceptor with approximately 3- and 17-fold higher affinity than for the alpha1a- and alpha1b-adrenoceptor subtypes, respectively. The selectivity of naftopidil for prostatic pressure may be attributable to its high binding affinity for alpha1a- and alpha1d-adrenoceptor subtypes.

Pharmacology of tamsulosin: saturation-binding isotherms and competition analysis using cloned alpha 1-adrenergic receptor subtypes

BACKGROUND

alpha 1-adrenergic receptors (alpha 1 ARs) are important in the dynamic component of benign prostatic hyperplasia (BPH). Currently, several alpha 1AR antagonists are being used in the treatment of BPH.

METHODS

In order to more fully characterize the pharmacology of the alpha 1AR antagonist tamsulosin, we utilized saturation-binding isotherms with [3H] tamsulosin to determine the Kd of this compound at all three cloned alpha 1AR subtypes stably expressed in rat-1 fibroblasts. To confirm these results, we performed competition binding experiments, displacing [125I]HEAT with increasing concentrations of alfuzosin, doxazosin, 5-methyl-urapidil, prazosin, tamsulosin, terazosin, and (+)YM617 (stereoisomer of tamsulosin) in the same clonal cell lines.

RESULTS

[3H]tamsulosin binds to cloned alpha 1AR subtypes with a rank order of affinity of alpha 1a = alpha 1d > alpha 1b. Competition experiments confirmed the relative nonselectivity of alfuzosin, doxazosin, and prazosin, but revealed slight alpha 1b = alpha 1d > alpha 1a selectivity for terazosin, and clear alpha 1a = alpha 1d > alpha 1b for (+)YM617 and tamsulosin([-]YM617); alpha 1a > alpha 1d > alpha 1b selectivity for 5-methyl-urapidil was confirmed.

CONCLUSIONS

We conclude that tamsulosin displays selectivity for alpha 1a and alpha 1d ARs. This selectivity may contribute to the tamsulosin efficacy reported in several recent clinical studies in patients with BPH.

Drugs for treatment of benign prostatic hyperplasia: affinity comparison at cloned alpha 1-adrenoceptor subtypes and in human prostate

1. We have previously shown that among alpha 1-adrenoceptor antagonists used or investigated for the treatment of benign prostatic hyperplasia, tamsulosin discriminates alpha 1-adrenoceptor subtypes in rat tissues whereas alfuzosin and naftopidil do not. We now expand these studies to additional drugs (doxazosin, terazosin) being used and/or investigated for this purpose, and have evaluated all of these drugs at cloned subtypes and in human prostate. 2. Competition binding studies were performed with [3H]-prazosin in membrane samples from rat spleen, kidney and cerebral cortex and human prostate and with cloned alpha 1-adrenoceptors expressed in COS cells. Doxazosin and terazosin did not discriminate alpha 1-adrenoceptor subtypes in rat kidney and cerebral cortex. In contrast, the subtypes present in the tissues were well discriminated by the alpha 1A-adrenoceptor-selective reference drug WB 4101. 3. Alfuzosin, doxazosin, naftopidil and terazosin did not discriminate cloned alpha 1-adrenoceptor subtypes transiently expressed in COS cells whereas tamsulosin and WB 4101 did. 4. In human prostate, alfuzosin, doxazosin, naftopidil and terazosin did not discriminate the alpha 1-adrenoceptor subtypes present in this tissue whereas tamsulosin and the alpha 1A-adrenoceptor-selective reference drugs WB 4101, phentolamine and 5-methylurapidil did. Based on data with the alpha 1A-adrenoceptor-selective drugs, human prostate contains alpha 1A- and alpha 1B-adrenoceptors in an approximate 70:30% ratio. 5. We conclude that tamsulosin, in common with WB 4101, but in contrast to alfuzosin, doxazosin, naftopidil, and terazosin is selective for alpha 1A-adrenoceptors which appear to dominate in the human prostate; the therapeutic relevance of this selectivity remains to be assessed in clinical studies.

High-affinity specific [3H]tamsulosin binding to alpha 1-adrenoceptors in human prostates with benign prostatic hypertrophy

The binding of a novel radioligand, [3H]tamsulosin, to human prostatic membranes with benign prostatic hypertrophy (BPH) has been characterized. [3H]Tamsulosin rapidly associated with its binding sites in human prostatic membranes with BPH, and the binding reached steady state by 30 min at 25 degrees C. The rate constants for association and dissociation of [3H]tamsulosin binding were calculated to be 0.21 +/- 0.05/nM per minute and 0.01 +/- 0.004/min, respectively. The specific binding of [3H]tamsulosin in human prostatic membranes was saturable and of high affinity (Kd = 0.04 +/- 0.01 nM). The density of [3H]tamsulosin-binding sites (Bmax) was 409 +/- 28 fmol/mg protein. The Kd and Bmax values for [3H]tamsulosin binding in human prostates were significantly lower than those for [3H]prazosin binding. [3H]tamsulosin binding was remarkable for its significantly lower degree of nonspecific binding. Six alpha-adrenoceptor antagonists competed with [3H]tamsulosin for the binding sites in the rank order: tamsulosin > WB4101 > prazosin > S-(+)-isomer > naftopidil > yohimbine. The binding affinities (pKi) of these antagonists for [3H]tamsulosin binding in human prostates closely correlated with their pharmacological potencies (pA2) in prostates. In conclusion, [3H]tamsulosin selectively labels alpha 1-adrenoceptors in human prostates, and thus may become a useful radioligand for the further analysis of these receptors.

Alpha 1-adrenoceptors in human prostate: characterization and binding characteristics of alpha 1-antagonists

The role of alpha 1-adrenoceptors in the mediation of autonomic functions, particularly in the control of the cardiovascular system, is widely known. It has been shown that alpha 1-adrenoceptors localized in human prostate mediate the contraction of prostatic smooth muscles which produces an increase in the intraurethral pressure and thus, these receptors are important in the regulation of bladder outlet resistance. Alpha 1-antagonists such as prazosin relieve the symptoms of bladder outlet obstruction in men with symptomatic benign prostatic hypertrophy (BPH) by blocking alpha 1-adrenoceptors, thereby decreasing prostatic tone and urethral resistance. Thus, alpha 1-adrenergic stimulation may be one of the most important factors in the development of urinary obstruction in BPH. Alpha 1-adrenoceptors in human prostate have been identified and characterized extensively by functional, radioligand binding and molecular biological techniques. These studies provide evidence in support of the concept that the alpha 1C-subtype forms the majority of alpha 1-adrenoceptors in human prostatic smooth muscles. It has been shown that YM617 (tamsulosin) and naftopidil have higher affinities to alpha 1-adrenoceptors in the prostate than in the aorta. Some alpha 1-antagonists, such as prazosin and terazosin, are not selective with respect to alpha 1-adrenoceptor subtypes, while others, such as 5-methylurapidil and indoramin, show higher potencies for alpha 1C-adrenoceptors and much lower potencies for alpha 1A- and alpha 1B-subtypes. In conclusion, the recent findings from pharmacological and molecular biological studies indicate that selective antagonists of alpha 1C-adrenoceptors could be effective in the treatment of urinary obstruction in symptomatic BPH with fewer cardiovascular side effects.

Characterization of alpha 1-adrenoceptor subtypes in prostate and prostatic urethra of rat, rabbit, dog and man

The alpha 1-adrenoceptor subtypes present in the smooth muscle of urethra and prostate of different animal species, including man, were characterized by using receptor binding techniques. In prostatic urethra and prostate membranes, [3H]prazosin labelled a single population of alpha 1-adrenoceptors (Hill coefficient not different from unity) with a high affinity in the range 0.21-0.51 nM. The number of specific [3H]prazosin binding sites was partially affected by chloroethylclonidine only in human and rat prostate membranes, whereas this agent proved practically devoid of activity in rabbit and dog prostate membranes as well as in the prostatic urethra membranes of all the animal species examined. These findings indicate that in prostatic and urethral membranes the alpha 1-adrenoceptors mainly belong to the alpha 1A subtype. The binding results were confirmed by in vitro functional studies on noradrenaline-induced contractions of rabbit and dog urethral preparations. The agonist-induced contractions were practically unaffected by preincubation of both tissues with chloroethylclonidine, but were sensitive to nifedipine. We found, moreover, a good correlation between the potency of different selective and non-selective alpha 1-adrenoceptor antagonists (WB-4101, 5-methylurapidil, phentolamine, spiperone, prazosin and urapidil) tested against the noradrenaline-induced contractions of rabbit urethra and their affinity for the alpha 1A-adrenoceptor subtype, no correlation with the affinity for the alpha 1B subtype, and a lower correlation with the affinity for the alpha 1C-adrenoceptor subtype.

Role of intracellular Ca2+ stores in smooth muscle contractions of the guinea pig vas deferens

Guinea pig vas deferens was used as an animal model for alpha-1 adrenoceptor (alpha 1-receptor) mediated contractions in human hyperplastic prostatic tissue. The selective alpha 1-receptor agonist, phenylephrine (PE), induced fully reversible, dose-dependent contractions antagonized by increasing concentrations of the alpha 1-receptor blockers prazosin (1-100 nM) and YM 617 (0.1-10 nM). Removal of extracellular Ca2+ reduced PE-evoked contractions in a time-dependent manner. Nifedipine (1-1000 nM), a blocker of voltage-dependent L-type Ca2+ channels (VDCC), inhibited the PE-induced response by up to 65%. Removal of extracellular Ca2+ abolished the alpha 1-agonist reactivity in a time-dependent fashion. To elucidate the participation of intracellular Ca2+ stores in alpha 1-receptor-mediated contractions, the tissue was pretreated with ryanodine (10 microM) or thapsigargin (0.1 microM), established inhibitors of Ca2+ release from intracellular pools. Both substances reduced the PE contractions by up to 80%. Nifedipine suppressed the remaining contractions completely. This provides evidence that Ca2+ influx through VDCC and Ca2+ release from intracellular stores contribute to alpha 1-receptor-mediated contractions in the guinea pig vas deferens and may be important in obstructive benign prostatic hyperplasia.

Identification of alpha 1-adrenoceptor subtypes in the dog prostate

The alpha 1-adrenoceptor subtypes of dog prostate were characterized in binding and functional experiments. In saturation experiments, [3H]prazosin bound to alpha 1-adrenoceptors with high affinity. In the displacement experiments, unlabelled prazosin and WB4101 biphasically inhibited the binding of 400 pM [3H]prazosin, suggesting the presence of at least two distinct affinity sites for prazosin or WB4101. The proportion of high-affinity sites was approximately 10%. HV723 also recognized two distinct affinity sites but the proportion of high-affinity sites was approximately 20%. From these results the presence of three distinct alpha 1-adrenoceptor subtypes was suggested: presumably subtypes alpha 1A (high affinity for prazosin and WB4101), alpha 1N (high affinity for only HV723) and alpha 1L (low affinity for the three antagonists) according to the recently proposed alpha 1-adrenoceptor subclassification. The density of subtype alpha 1L was much higher than that of subtypes alpha 1A and alpha 1N subtypes. In the functional experiments, prazosin, WB4101 and HV723 competitively antagonized the contractile response to noradrenaline with low affinities close to those estimated for the alpha 1L subtypes. These results suggest that the contractile response to noradrenaline in the dog prostate is mediated predominantly through alpha 1L subtype alpha-adrenoceptors.