Doxazosin. An update of its clinical pharmacology and therapeutic applications in hypertension and benign prostatic hyperplasia

Doxazosin for benign prostatic hyperplasia in primary care

The incidence of benign prostatic hyperplasia (BPH) is increasing, and an estimated one quarter of men older than 60 years have both BPH and hypertension. Medical management of BPH is now an effective alternative to surgery, and primary care physicians are treating more patients for this condition. Alpha-adrenergic blockers reduce the tone of prostatic smooth muscle, and doxazosin, a selective alpha-1 adrenergic blocker, is effective and well tolerated in the treatment of BPH. Doxazosin can be given in a convenient once-daily dose, and its efficacy is maintained in the long term. Patients with BPH who are already being treated for hypertension with doxazosin need no alteration in their dosage regimen, and doxazosin can be used as a single agent to treat both conditions. Selective alpha-adrenergic blockers are likely to be used increasingly by primary care physicians in the treatment of BPH.

Comparison of the antagonistic activity of tamsulosin and doxazosin at vascular alpha 1-adrenoceptors in humans

alpha 1-Adrenoceptor blockers such as prazosin and doxazosin are used to treat hypertension as well as benign prostatic hyperplasia (BPH), whereas the new alpha 1-adrenoceptor blocker tamsulosin is used only for BPH and does not reduce blood pressure at the doses used to relax prostatic smooth muscle. In contrast to prazosin, tamsulosin has a higher affinity for prostatic than vascular alpha 1-adrenoceptors in vitro. The functional correlate of this observation in humans is the subject of this study. The alpha 1-adrenoceptor blockade by oral tamsulosin (0.2 mg), doxazosin (1 mg) or placebo on finger tip vascular and dorsal hand venous alpha 1-adrenoceptors stimulated by cold treatment (immersion in ice water) and the alpha 1-adrenoceptor agonist phenylephrine, was thus studied in a 3-way crossover study in eight, healthy, male adults. Finger tip vasoconstriction after cold stimulation was assessed by laser Doppler flowmetry. A linear variable differential transformer was used to assess the drug effect on phenylephrine-induced venoconstriction. All study parameters were assessed at around 2 and 3.5 h after oral intake of doxazosin and tamsulosin respectively. The drug plasma levels were not significantly different. No significant differences were found for blood pressure or heart rate in the three treatments in supine and erect position. The reduction in finger tip blood flow after cold stimulation was significantly smaller after doxazosin treatment (P < 0.01) than after tamsulosin or placebo, whereas there was no significant difference between tamsulosin and placebo treatments. The infusion rate of phenylephrine producing a half-maximum venoconstriction was significantly larger after doxazosin than after tamsulosin (P < 0.05) or placebo (P < 0.01), whereas there was again no significant difference between tamsulosin and placebo treatments. The data suggest that, at doses producing equal plasma levels after single oral doses in human subjects, the blocking activity at vascular alpha 1-adrenoceptors is lower for tamsulosin than for doxazosin.

The alpha-adrenoceptor antagonist properties of the enantiomers of doxazosin in the human prostate

The alpha-adrenoceptor antagonist properties of doxazosin and its enantiomers were characterized using human prostate tissue and cell membranes isolated from rat-1 fibroblast expressing each of the cloned human alpha 1-adrenoceptor subtypes. In the alpha 1-adrenoceptor-binding studies on the human prostate with [3H]doxazosin and 2-{[beta-(3-[125I],4-hydroxyphenyl)ethyl]aminomethyl}-l-tetralone ([125I]HEAT), no significant differences were observed between racemic doxazosin, R-doxazosin and S-doxazosin (mean -log Ki (pKi) values were 8.60-8.63, 8.47-8.55 and 8.61-8.65, respectively), whereas the alpha 2-adrenoceptor-binding studies with [3H]rauwolscine and [3H]clonidine revealed that the alpha 2-adrenoceptor-binding affinity of S-doxazosin (pKi = 5.91-5.94) was slightly (3- or 4-fold), but significantly lower than that of R-doxazosin (pKi = 6.47-6.54). Studies in phenylephrine-contracted prostatic tissue showed no significant difference in alpha 1-adrenoceptor antagonist potency between racemic doxazosin, R-doxazosin and S-doxazosin (pA2 values were 8.43 +/- 0.28, 8.64 +/- 0.56 and 8.75 +/- 0.38, respectively). In the binding studies with cloned alpha 1-adrenoceptor subtypes using [3H]prazosin and [125I]HEAT, racemic doxazosin, R-doxazosin and S-doxazosin showed no selectivity for the alpha 1-adrenoceptor subtypes. The present study demonstrated that doxazosin and its enantiomers are highly selective alpha 1-adrenoceptor antagonists and that there is no evidence suggesting differential alpha 1-adrenoceptor antagonist effects of doxazosin and its enantiomers in the human prostate. Doxazosin, therefore, could be described as displaying balanced activity across all three alpha 1-adrenoceptor subtypes.

Glucose tolerance and postprandial glucose and insulin kinetics in rats with short- and long-term alpha 1-adrenergic blockade

The present study was designed to evaluate the acute and chronic effects of the alpha 1-adrenergic antagonist prazosin (Praz) on glucose tolerance and on postprandial plasma glucose and insulin kinetics. Rats were fed a high sucrose diet for 3 weeks, to which Praz (3 mg/kg/day) was added or not. They were then accustomed to ingest a meal 1 h after a single intraperitoneal (i.p.) injection of saline or Praz (1 mg/kg). Plasma levels of glucose and insulin were recorded at various times after meal intake. In addition, a fasting intravenous glucose tolerance test (IVGTT) was performed. In the chronic control cohort, pre-IVGTT plasma glucose and insulin levels were slightly but significantly higher after acute Praz than after acute saline administration. Factorial analysis of variance (ANOVA) showed that the increase in plasma glucose and insulin after intravenous (i.v.) glucose administration was also slightly greater in the animals that received acute Praz than in their saline counterparts. In contrast, after chronic treatment with Praz, pre- and post-IVGTT plasma glucose and insulin concentrations were identical in groups acutely injected with saline or Praz. In the chronic control cohort, preprandial plasma glucose and insulin measured in a sample collected before acute injection of saline or Praz were similar in both groups. The postprandial increase in plasma glucose and insulin was potentiated by the acute administration of Praz. Fasting plasma glucose and insulin concentrations were also similar in both groups chronically treated with Praz, and acute administration of the blocker still potentiated the increase in plasma glucose and insulin that followed meal intake. Chronic treatment alone did not affect postprandial glucose and insulin concentrations, and acute injection of Praz had comparable effects whether rats were chronically treated with the blocker or not. Therefore, potentiation of the glucose and insulin response to meal intake by Praz persisted after chronic treatment but required the acute presence of the blocker. However, repeated exposure to acute bouts of postprandial hyperinsulinemia induced by Praz did not lead to deterioration of insulin sensitivity or of the capacity of the pancreas to secrete insulin, as suggested by a normal response of glucose and insulin to IVGTT.

Safety and efficacy of doxazosin in benign prostatic hyperplasia: a pooled analysis of three double-blind, placebo-controlled studies

OBJECTIVES

To present the results of a pooled analysis of three double-blind, placebo-controlled studies of doxazosin in benign prostatic hyperplasia (BPH). Heterogeneous symptom and bother score data collected using different symptom indices were transformed to enable a comparison of the data and to conduct a pooled, in-depth analysis.

METHODS

Urinary flow rates, and symptom and bother score data were shown by analysis of covariance methods to give consistent estimates of the efficacy of doxazosin across different studies, thus confirming the validity of pooling the results. Prior to analysis, symptom and bother score data were transformed so that all scales started from zero (least symptoms or bother) and were expressed as a percentage of the maximum score.

RESULTS

Doxazosin produced a significantly greater improvement than placebo in peak urinary flow rate (P = 0.0017), symptom severity (P < 0.0001), and bother caused by symptoms (P < 0.0001). Stratification showed that a greater improvement was obtained during doxazosin treatment by those with more severe symptoms at baseline (P = 0.0001). Stratification by age showed that age did not affect the capacity to benefit from treatment. Analysis of the pooled peak flow-rate data showed that doxazosin produced a consistently greater increase in flow compared with placebo. Doxazosin was well tolerated, with 10% of patients having withdrawn due to adverse events versus 4% with placebo (P < 0.05).

CONCLUSIONS

Doxazosin is well tolerated and effective in the treatment of BPH. Pooling of data has enabled more extensive and robust conclusions to be drawn than was possible for each one of the individual three studies.