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Syncope and "First-dose" Effect
Doxazosin, like other alpha-adrenergic blocking agents, can cause marked hypotension, especially in the upright position, with syncope and other postural symptoms such as dizziness. Marked orthostatic effects are most common with the first dose but can also occur when there is a dosage increase, or if therapy is interrupted for more than a few days. To decrease the likelihood of excessive hypotension and syncope, it is essential that treatment be initiated with the 1 mg dose. The 2, 4, and 8 mg tablets are not for initial therapy. Dosage should then be adjusted slowly (see DOSAGE AND ADMINISTRATION), with evaluations and increases in dose every two weeks to the recommended dose. Additional antihypertensive agents should be added with caution.
Patients being titrated with doxazosin should be cautioned to avoid situations where injury could result should syncope occur, during both the day and night.
In an early investigational study of the safety and tolerance of increasing daily doses of doxazosin in normotensives beginning at 1 mg/day, only 2 of 6 subjects could tolerate more than 2 mg/day without experiencing symptomatic postural hypotension. In another study of 24 healthy normotensive male subjects receiving initial doses of 2 mg/day of doxazosin, seven (29%) of the subjects experienced symptomatic postural hypotension between 0.5 and 6 hours after the first dose, necessitating termination of the study. In this study, 2 of the normotensive subjects experienced syncope. Subsequent trials in hypertensive patients always began doxazosin dosing at 1 mg/day, resulting in a 4% incidence of postural side effects at 1 mg/day with no cases of syncope.
In multiple-dose clinical trials in hypertension involving over 1500 hypertensive patients with dose titration every one to two weeks, syncope was reported in 0.7% of patients. None of these events occurred at the starting dose of 1 mg, and 1.2% (8/664) occurred at 16 mg/day.
In placebo-controlled clinical trials in BPH, 3 out of 665 patients (0.5%) taking doxazosin reported syncope. Two of the patients were taking 1 mg doxazosin, while one patient was taking 2 mg doxazosin when syncope occurred. In the open-label, long-term extension follow-up of approximately 450 BPH patients, there were 3 reports of syncope (0.7%). One patient was taking 2 mg, one patient was taking 8 mg, and one patient was taking 12 mg when syncope occurred. In a clinical pharmacology study, one subject receiving 2 mg experienced syncope.
If syncope occurs, the patient should be placed in a recumbent position and treated supportively as necessary.
Rarely (probably less frequently than once in every several thousand patients), alphai antagonists, including doxazosin, have been associated with priapism (painful penile erection, sustained for hours and unrelieved by sexual intercourse or masturbation). Because this condition can lead to permanent impotence if not promptly treated, patients must be advised about the seriousness of the condition (see PATIENT INFORMATION).
Prostate Cancer: Carcinoma of the prostate causes many of the symptoms associated with BPH and the two disorders frequently co-exist. Carcinoma of the prostate should therefore be ruled out prior to commencing therapy with CARDURA (doxazosin mesylate) .
Cataract Surgery: Intraoperative Floppy Iris Syndrome (IFIS) has been observed during cataract surgery in some patients on or previously treated with alpha1 blockers. This variant of small pupil syndrome is characterized by the combination of a flaccid iris that billows in response to intraoperative irrigation currents, progressive intraoperative miosis despite preoperative dilation with standard mydriatic drugs, and potential prolapse of the iris toward the phacoemulsification incisions. The patient's surgeon should be prepared for possible modifications to their surgical technique, such as the utilization of iris hooks, iris dilator rings, or viscoelastic substances. There does not appear to be a benefit of stopping alphai blocker therapy prior to cataract surgery.
Orthostatic Hypotension: While syncope is the most severe orthostatic effect of CARDURA (doxazosin mesylate) , other symptoms of lowered blood pressure, such as dizziness, lightheadedness, or vertigo can occur, especially at initiation of therapy or at the time of dose increases.
These symptoms were common in clinical trials in hypertension, occurring in up to 23 % of all patients treated and causing discontinuation of therapy in about 2%.
In placebo-controlled titration trials in hypertension, orthostatic effects were minimized by beginning therapy at 1 mg per day and titrating every two weeks to 2, 4, or 8 mg per day.
There was an increased frequency of orthostatic effects in patients given 8 mg or more, 10%, compared to 5% at 1-4 mg and 3% in the placebo group.
Benign Prostatic Hyperplasia
In placebo-controlled trials in BPH, the incidence of orthostatic hypotension with doxazosin was 0.3% and did not increase with increasing dosage (to 8 mg/day). The incidence of discontinuations due to hypotensive or orthostatic symptoms was 3.3% with doxazosin and 1 % with placebo. The titration interval in these studies was one to two weeks.
Patients in occupations in which orthostatic hypotension could be dangerous should be treated with particular caution. As alphai antagonists can cause orthostatic effects, it is important to evaluate standing blood pressure two minutes after standing, and patients should be advised to exercise care when arising from a supine or sitting position.
If hypotension occurs, the patient should be placed in the supine position and, if this measure is inadequate, volume expansion with intravenous fluids or vasopressor therapy may be used. A transient hypotensive response is not a contraindication to further doses of CARDURA (doxazosin mesylate) .
Drug/Laboratory Test Interactions
CARDURA (doxazosin mesylate) does not affect the plasma concentration of prostate-specific antigen in patients treated for up to 3 years. Both doxazosin, an alphai inhibitor, and finasteride, a 5-alpha reductase inhibitor, are highly protein-bound and hepatically metabolized. There is no definitive controlled clinical experience on the concomitant use of alphai inhibitors and 5-alpha reductase inhibitors at this time.
Impaired Liver Function
CARDURA (doxazosin mesylate) should be administered with caution to patients with evidence of impaired hepatic function, or to patients receiving drugs known to influence hepatic metabolism (see CLINICAL PHARMACOLOGY, Pharmacokinetics).
Analysis of hematologic data from hypertensive patients receiving CARDURA (doxazosin mesylate) in controlled hypertension clinical trials showed that the mean WBC (N=474) and mean neutrophil counts (N=419) were decreased by 2.4% and 1.0%, respectively, compared to placebo, a phenomenon seen with other alpha-blocking drugs. In BPH patients, the incidence of clinically significant WBC abnormalities was 0.4% (2/459) with CARDURA (doxazosin mesylate) and 0% (0/147) with placebo, with no statistically significant difference between the two treatment groups. A search through a data base of 2400 hypertensive patients and 665 BPH patients revealed 4 hypertensives in which drug-related neutropenia could not be ruled out and one BPH patient in which drug-related leukopenia could not be ruled out. Two hypertensives had a single low value on the last day of treatment. Two hypertensives had stable, non-progressive neutrophil counts in the 1000/mm3 range over periods of 20 and 40 weeks. One BPH patient had a decrease from a WBC count of 4800/mm3 to 2700/mm3 at the end of the study; there was no evidence of clinical impairment. In cases where follow-up was available, the WBCs and neutrophil counts returned to normal after discontinuation of CARDURA (doxazosin mesylate) . No patients became symptomatic as a result of the low WBC or neutrophil counts.
Carcinogenesis, Mutagenesis, Impairment of Fertility
Chronic dietary administration (up to 24 months) of doxazosin mesylate at maximally tolerated doses of 40 mg/kg/day in rats and 120 mg/kg/day in mice revealed no evidence of carcinogenic potential. The highest doses evaluated in the rat and mouse studies are associated with AUCs (a measure of systemic exposure) that are 8 times and 4 times, respectively, the human AUC at a dose of 16 mg/day.
Mutagenicity studies revealed no drug- or metabolite-related effects at either chromosomal or subchromosomal levels.
Studies in rats showed reduced fertility in males treated with doxazosin at oral doses of 20 (but not 5 or 10) mg/kg/day, about 4 times the AUC exposures obtained with a 12 mg/day human dose. This effect was reversible within two weeks of drug withdrawal. There have been no reports of any effects of doxazosin on male fertility in humans.
Teratogenic Effects, Pregnancy Category C. Studies in pregnant rabbits and rats at daily oral doses of up to 41 and 20 mg/kg, respectively (plasma drug concentrations 10 and 4 times human Cmax and AUC exposures with a 12 mg/day therapeutic dose), have revealed no evidence of harm to the fetus. A dosage regimen of 82 mg/kg/day in the rabbit was associated with reduced fetal survival. There are no adequate and well-controlled studies in pregnant women. Because animal reproduction studies are not always predictive of human response, CARDURA (doxazosin mesylate) should be used during pregnancy only if clearly needed.
Radioactivity was found to cross the placenta following oral administration of labelled doxazosin to pregnant rats.
Nonteratogenic Effects: In peri-postnatal studies in rats, postnatal development at maternal doses of 40 or 50 mg/kg/day of doxazosin (8 times human AUC exposure with a 12 mg/day therapeutic dose) was delayed, as evidenced by slower body weight gain and slightly later appearance of anatomical features and reflexes.
Studies in lactating rats given a single oral dose of 1 mg/kg of [2-14C]-CARDURA (doxazosin mesylate) indicate that doxazosin accumulates in rat breast milk with a maximum concentration about 20 times greater than the maternal plasma concentration. It is not known whether this drug is excreted in human milk. Because many drugs are excreted in human milk, caution should be exercised when CARDURA (doxazosin mesylate) is administered to a nursing mother.
The safety and effectiveness of CARDURA (doxazosin mesylate) as an antihypertensive agent have not been established in children.
The safety and effectiveness profile of CARDURA (doxazosin mesylate) in BPH was similar in the elderly (age ≥ 65 years) and younger (age < 65 years) patients.
Clinical studies of CARDURA (doxazosin mesylate) did not include sufficient numbers of subjects aged 65 and over to determine whether they respond differently from younger subjects. Other reported clinical experience has not identified differences in responses between the elderly and younger patients. In general, dose selection for an elderly patient should be cautious, usually starting at the low end of the dosing range, reflecting the greater frequency of decreased hepatic, renal or cardiac function, and of concomitant disease or other drug therapy.
Last reviewed on RxList: 11/22/2010
This monograph has been modified to include the generic and brand name in many instances.
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