NEW alpha 1-ADRENOCEPTOR ANTAGONIST, JTH-601, SHOWS MORE THAN 10 TIMES HIGHER AFFINITY FOR HUMAN PROSTATES THAN ARTERIES

The pharmacology of α1-adrenoceptor subtypes

This review examines the functions of α₁-adrenoceptor subtypes, particularly in terms of contraction of smooth muscle. There are 3 subtypes of α₁-adrenoceptor, α_1A- α_1B- and α_1D-adrenoceptors. Evidence is presented that the postulated α_1L-adrenoceptor is simply the native α_1A-adrenoceptor at which prazosin has low potency. In most isolated tissue studies, smooth muscle contractions to exogenous agonists are mediated particularly by α_1A-, with a lesser role for α_1D-adrenoceptors, but α_1B-adrenoceptors are clearly involved in contractions of some tissues, for example, the spleen. However, nerve-evoked responses are the most crucial physiologically, so that these studies of exogenous agonists may overestimate the importance of α_1A-adrenoceptors. The major α₁-adrenoceptors involved in blood pressure control by sympathetic nerves are the α_1D- and the α_1A-adrenoceptors, mediating peripheral vasoconstrictor actions. As noradrenaline has high potency at α_1D-adrenceptors, these receptors mediate the fastest response and seem to be targets for neurally released noradrenaline especially to low frequency stimulation, with α_1A-adrenoceptors being more important at high frequencies of stimulation. This is true in rodent vas deferens and may be true in vasopressor nerves controlling peripheral resistance and tissue blood flow. The α_lA-adrenoceptor may act mainly through Ca²⁺ entry through L-type channels, whereas the α_1D-adrenoceptor may act mainly through T-type channels and exhaustable Ca²⁺ stores. α₁-Adrenoceptors may also act through non-G-protein linked second messenger systems. In many tissues, multiple subtypes of α-adrenoceptor are present, and this may be regarded as the norm rather than exception, although one receptor subtype is usually predominant.

Distribution of alpha1L-adrenoceptors in canine prostate: characterization by quantitative autoradiography

Our aim was to determine the distribution of alpha1L-adrenoceptors in canine prostate by an autoradiographic technique using [3H]JTH-601 (an alpha1L-adrenoceptor antagonist) and [3H]JTH-601-G1 (an active metabolite of JTH-601). Prostates were removed from three male beagle dogs. Several slices of the specimens were incubated with 5 nM of [3H]JTH-601, [3H]JTH-601-G1 and [3H]tamsulosin (an alpha1A-adrenoceptor antagonist). For macroscopic autoradiography, visualization was performed using an imaging plate and image-analyser. To examine microscopic localization of binding sites, preparations were exposed, developed and fixed. Specific binding of [3H]JTH-601 and [3H]JTH-601-G1 was observed diffusely throughout the entire interstitium on macroscopic autoradiography. Specific binding of [3H]tamsulosin was also recognized although the binding was weaker than that of [3H]JTH-601. On microscopic autoradiograms, the grains of each ligand were mainly distributed on smooth muscle. These results indicate morphologically that specific binding sites of JTH-601 and JTH-601-G1 exist in canine prostate, suggesting the distribution of alpha1L-adrenoceptors in this tissue, in addition to alpha1A-adrenoceptors.

Cloning of rabbit alpha(1b)-adrenoceptor and pharmacological comparison of alpha(1a)-, alpha(1b)- and alpha(1d)-adrenoceptors in rabbit

We have isolated a cDNA clone of the rabbit alpha(1b)-adrenoceptor which has an open reading frame of 1557 nucleotides encoding a protein of 518 amino acids. The sequence shows higher identity to those of hamster, human, and rat alpha(1b)-adrenoceptors than to those of rabbit alpha(1a)- and alpha(1d)-adrenoceptors. The pharmacological binding properties of this clone expressed in Cos-7 cells showed a characteristic profile as alpha(1b)-adrenoceptor; high affinity for prazosin (pK(i)=10.3), relatively high affinity for tamsulosin (9.5) and low affinity for (-)-(R)-1-(3-hydroxypropyl)-5-[2-[[2-[2-(2,2, 2-trifluoroethoxy)phenoxy]ethyl]amino]propyl]indoline-7-carboxamid e (KMD3213) (8.5), 2-(2,6-dimethoxy-phenoxyethyl)-aminomethyl-1, 4-benzodioxane hydrochloride (WB4101) (8.7), and 8-[2-[4-(2-methoxy-phenyl)-L-piperazinyl]-8-azaspiro[4,5]decane-7, 9-dione dihydrochloride (BMY7378) (7.3). We have compared the levels of mRNA expression of three alpha(1)-adrenoceptor subtypes in rabbit tissues using the competitive reverse transcription/polymerase chain reaction (RT/PCR) assay. In most rabbit tissues except heart, alpha(1a)-adrenoceptor mRNA was expressed 10 folds more than the other two subtypes. However, binding experiments with [3H]prazosin and [3H]KMD3213 in rabbit tissues revealed a poor relationship between binding density and mRNA level. Especially, alpha(1b) binding sites were exclusively predominant in spleen, whereas the alpha(1b) subtype was minor at the mRNA level. These results indicate a high identity of structural and pharmacological profiles of three distinct alpha(1)-adrenoceptor subtypes between rabbit and other species, but there are species differences in their distribution.

Effect of JTH-601, a novel alpha(1)-adrenoceptor antagonist, on prostate function in dogs

We examined the effect of JTH-601 (3-¿N-[2-(4-hydroxy-2-isopropyl-5-methylphenoxy)ethyl]-N-methylaminom ethyl¿-4-methoxy-2,5,6-trimethylphenol hemifumarate), a new alpha(1L)-adrenoceptor antagonist, on prostatic function in isolated canine prostate and in anesthetized dogs. In the contraction study, phenylephrine and noradrenaline produced concentration-dependent contractions in canine prostate and carotid artery, respectively. In these tissues, JTH-601, prazosin (a non-selective alpha(1)-adrenoceptor antagonist), and tamsulosin (an alpha(1A)-adrenoceptor antagonist) competitively antagonized contraction in a concentration-dependent manner. The pA(2) (pK(B)) values with prostate were 8.49+/-0.07 for JTH-601, 7.94+/-0.04 for prazosin and 9.42+/-0.22 for tamsulosin. The ratio of pA(2) (carotid artery/prostate), i.e. prostatic selectivity, was 10.471 for JTH-601, 0.008 for prazosin and 0.371 for tamsulosin, respectively. In anesthetized dogs, JTH-601 (1 mg/kg, i.d.) significantly decreased urethral pressure by 15% without affecting blood pressure or heart rate. Tamsulosin (0.1 mg/kg, i.d.) decreased urethral pressure to the same extent as did JTH-601, but with a significant effect on blood pressure and heart rate. JTH-601 showed higher selectivity for canine prostate both in vitro and in vivo. In prostate, an important role of the alpha(1L)-adrenoceptor is suggested in the smooth muscle contraction mediated by alpha(1)-adrenoceptors. JTH-601 is expected to be an effective alpha(1)-adrenoceptor antagonist for the treatment of urinary outlet obstruction by benign prostatic hypertrophy with a minimum effect on the cardiovascular system.

Prostate selectivity of JTH-601-G1, an active metabolite of JTH-601, in dogs

OBJECTIVE

To evaluate the effect of JTH-601-G1, an active metabolite and glucuronide conjugate of JTH-601 (an alpha1-adrenoceptor antagonist), on smooth muscle contraction in canine prostate and artery, and to examine the effect of JTH-601-G1 on prostatic urethral pressure and blood pressure in anaesthetized dogs. Materials and methods Male beagle dogs were used in both an in vitro and an in vivo study. In the former, the prostate and right common carotid artery were isolated, and smooth muscle strips from the prostate and open-ring strips from the carotid artery prepared. The effects of JTH-601-G1 on phenylephrine- and noradrenaline-induced contraction were assessed in these tissues. In the in vivo study, four dogs were anaesthetized and the change in urethral pressure, blood pressure and heart rate measured continuously. Vehicle (saline) and JTH-601-G1 were then infused intravenously in increasing doses (0.33-3.3 microg/kg/min for 30 min). In three other dogs, the effect of JTH-601-G1 infusion at a higher rate (25 microg/kg/min for 3 h) on blood pressure was evaluated, and the plasma concentration of JTH-601-G1 measured using high-performance liquid chromatography-mass spectrometry.

RESULTS

Of the distinct metabolites of JTH-601, JTH-601-G1 had the most potent alpha1-adrenoceptor antagonistic effect in isolated canine prostate. JTH-601-G1 also antagonized alpha1-adrenoceptor agonist-induced contraction in common carotid artery, but the pA2 value in the artery was approximately 25 times higher than that in the prostate. In anaesthetized dogs, JTH-601-G1 decreased urethral pressure in a dose-dependent manner; at the highest dose, urethral pressure decreased by 24.5% and blood pressure by 7.0%. However, there was no significant change in heart rate at any dose. The plasma concentration of JTH-601-G1 increased with the dose of JTH-601-G1, but the concentration of both JTH-601 and other metabolites was below the detection limit. The higher JTH-601-G1 infusion rate caused blood pressure to decrease by only 6-10% even at JTH-601-G1 plasma concentrations of approximately 1500 ng/mL during the infusion. Although there was a negative correlation between mean blood pressure and plasma JTH-601-G1 concentration, the decrease in blood pressure was small compared with the reduction in urethral pressure.

CONCLUSION

JTH-601-G1 appears to be a major active metabolite of JTH-601 but with a higher selectivity for canine prostate than artery. The results also indicate that in addition to the alpha1A-adrenoceptor, the alpha1L-adrenoceptor plays an important prostatic selective role in smooth muscle contraction via the alpha1-adrenoceptor.

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.

Alpha 1-adrenoceptors: subtypes, signaling, and roles in health and disease

Alpha 1-adrenoceptors mediate some of the main actions of the natural catecholamines, adrenaline, and noradrenaline. They participate in many essential physiological processes, such as sympathetic neurotransmission, modulation of hepatic metabolism, control of vascular tone, cardiac contraction, and the regulation of smooth muscle activity in the genitourinary system. It is now clear that alpha 1-adrenoceptors mediate, in addition to immediate effects, longer term actions of catecholamines such as cell growth and proliferation. In fact, adrenoceptor genes can be considered as protooncogenes. Over the past years, considerable progress has been achieved in the molecular characterization of different alpha 1-adrenoceptor subtypes. Three main subtypes have been characterized pharmacologically and in molecular terms. Splice variants, truncated isoforms, and polymorphisms have also been detected. Similarly, it is now clear that these receptors are coupled to several classes of G proteins that, therefore, are capable of modulating different signaling pathways. In the present article, some of these aspects are reviewed, together with the distribution of the subtypes in different tissues and some of the known roles of these receptors in health and disease.

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.