Comparison of alpha 1-adrenoceptor agonists in canine urethral pressure profilometry and abdominal leak point pressure models.

PURPOSE: We describe and compare the usefulness of 3 minimally invasive dog urethral function models to demonstrate the efficacy potential of alpha 1 agonists for stress urinary incontinence. From this overall composite dataset the efficacy profiles of the alpha 1A selective agonist A-61603 and the active metabolite of midodrine ST-1059 were specifically compared. MATERIALS AND METHODS: Isoflurane anesthetized multiparous female beagle dogs were used in all studies. Intraurethral pressure was measured using a 7Fr balloon catheter. Profilometry was performed using an 8Fr Millar transducer catheter. Bladder pressure required to produce leakage in response to external abdominal ballottements of increasing intensity was measured using a 5Fr transurethral catheter. Agonist responses were measured before and after increasing cumulative intravenous doses of each compound in each test. RESULTS: Agonist induced increases in maximal urethral closure and leak point pressure strongly correlated in linear fashion (R(2) = 0.94), as did measurements of agonist induced increases in proximal intraurethral pressure using the microtransducer or balloon catheter technique (R(2) = 0.87). A dose of 0.01 to 1 nmol./kg. A-61603 and 10 to 1,000 nmol./kg. ST-1059 intravenously each caused dose dependent increases in maximum urethral closure, leak point and intraurethral pressure. CONCLUSIONS: While the dose range for which alpha 1 agonists affect urethral pressure was adequately predicted by any of the 3 methods used, the leak point pressure assay described has the advantage of being a dynamic test that directly evaluates efficacy to protect against leakage caused by increases in abdominal pressure. This leak point pressure test appears be useful for the preclinical evaluation of compounds used to treat stress urinary incontinence.

Actions of A-131701, a novel, selective antagonist for alpha-1A compared with alpha-1B adrenoceptors on intraurethral and blood pressure responses in conscious dogs and a pharmacodynamic assessment of in vivo prostatic selectivity.

A-131701 (3-[2-((3aR,9bR)-cis-6-methoxy-2,3,3a,4,5,9b, hexahydro-[1H]-benz[e]isoindol-2-yl)ethyl]pyrido [3',4': 4,5]thieno[3,2-d]pyrimidine-2,4(1H,3H)-dione) is a novel compound previously shown to be selective for alpha-1a sites compared with alpha-1b adrenoceptors in radioligand binding studies and isolated tissue bioassays and to block canine urethral pressure (IUP) responses to exogenous alpha-1 adrenergic agonists to a greater extent than blood pressure responses. In conscious dogs in which IUP and mean arterial blood pressure (MABP) responses were measured periodically up to 24 hr, A-131701 blocked phenylephrine (PHE)-induced increases in IUP to a greater extent than MABP responses, and the blockade of the IUP effects of PHE was significantly different from control for up to 12 hr after doses greater than 0.3 mg/kg p.o., whereas blood pressure effects were of a lesser extent and duration. In addition to the weak antagonism of PHE-induced blood pressure responses, A-131701 also exhibited minimal effects on basal blood pressure in the dog, unlike terazosin, doxazosin or tamsulosin. Pharmacokinetic analysis of plasma samples from dogs indicated that A-131701 had a half-life of 0.4 to 0.8 hr and a bioavailability of 30 to 50% in dogs. Somewhat longer half-lives were observed in rat and monkey, with bioavailability values in the 25 to 30% range. Evidence of nonlinearity of pharmacokinetics was obtained in dogs and monkeys. Pharmacodynamic analysis revealed differences between A-131701 and nonselective alpha-1 adrenoceptor antagonists in selectivity for prostatic versus vascular alpha-1 adrenoceptors based on either extent or duration of blockade, which were either similar to or superior to compounds such as tamsulosin or REC 15/2739. These data demonstrate that A-131701 selectively blocks canine prostatic alpha-1 adrenoceptors for prolonged periods compared with MABP responses in vivo. Therefore, A-131701 should have clinical utility in the pharmacotherapy of benign prostatic hyperplasia.

Relationships between pharmacokinetics and blockade of agonist-induced prostatic intraurethral pressure and mean arterial pressure in the conscious dog after single and repeated daily oral administration of terazosin.

The purpose of this study was to determine the potency and selectivity of the alpha-1 adrenergic receptor antagonist terazosin based on relationships between plasma concentrations and blockade of intraurethral pressure (IUP) and mean arterial pressure (MAP) responses after single dosing and to determine cumulative effects after repeated dosing. To this end, the relationships between plasma concentrations and blockade effects of terazosin on phenylephrine (PE)-induced IUP and MAP were evaluated in conscious male beagle dogs after single (0.1, 0.3 and 1 mg/kg) and repeated (0.3 and 1 mg/kg) daily oral dosing of terazosin. Blockade effects and plasma concentrations were evaluated at selected times for periods of < or = 24 hr. Terazosin produced dose-dependent antagonism of PE-induced IUP and MAP responses. When IUP and MAP blockade effects were plotted against terazosin plasma concentration, direct relationships were observed that were well described by the sigmoidal maximal effect model and resulted in IUP and MAP IC50 values of 48.6 and 12.2 ng/ml, respectively. Repeated daily dosing resulted in little accumulation of terazosin in plasma and demonstrated consistent blockade responses over 7 days. MAP blockade was observed up to 23 hr after terazosin administration, whereas IUP blockade returned to control levels before 23 hr. Combined pharmacokinetic/pharmacodynamic analysis showed no selective antagonism of IUP by terazosin but may provide a useful way to show uroselectivity of novel agents.

Effects of selective and nonselective alpha-1-adrenoceptor antagonists on intraurethral and arterial pressures in intact conscious dogs.

In this study, we used a novel conscious dog model to evaluate the uroselectivity of selected alpha 1-antagonists either approved for human use or in clinical development for the treatment of symptomatic benign prostatic hyperplasia (BPH) and compared those results to their in vitro binding and functional affinities at alpha 1A, alpha 1B and alpha 1D receptor subtypes. Conscious dogs were instrumented acutely with a balloon catheter for the measurement of changes in prostatic intraurethral pressure (IUP) and chronically with implantable telemetry devices for the measurement of arterial pressure. The pressor effects of the alpha 1-agonist phenylephrine (PE) on IUP and mean arterial pressure (MAP) were compared before and at various time points after oral doses of either terazosin, doxazosin, tamsulosin or Rec 15/2739 (SB 216469). At submaximal doses, terazosin and doxazosin blocked PE-induced increases in MAP to a greater extent than increases in IUP. Tamsulosin blocked both parameters equally at the lowest and highest doses; however, at the intermediate dose, IUP was blocked more than MAP. Rec 15/2739 at each dose always blocked IUP to a greater extent than MAP. While the in vivo uroselectivity of these agents was predicted by radioligand binding and in vitro functional selectivity for the alpha 1A subtype over alpha 1B and alpha 1D subtypes, results from conscious dog experiments indicate that estimates of in vivo uroselectivity also depend upon dose and the time after administration. Our conscious canine model provides the basis for frequent and repeated evaluation of uroselectivity parameters over many hours, thus providing a pharmacological profile of compound effects perhaps more relevant to clinical practice.