Doxazosin

Introduction Overview

Generic Name

doxazosin

Brand Name

Cardura, Cardura XL, Doxadura, Cascor, Carduran

Dosage Form

Immediate-release tablets, Extended-release tablets

Drug Class

Alpha blockers; Antiadrenergic agents, peripherally acting

Pharmacokinetics

Absorption:
Doxazosin is well absorbed from the gastrointestinal tract following oral administration, with bioavailability exceeding 65%. Peak plasma concentrations occur approximately 2 to 3 hours post-dose for immediate-release formulations, while extended-release forms reach peak levels at about 6 to 12 hours, providing sustained plasma concentrations for once-daily dosing. Food does not significantly affect the extent of absorption but may delay the time to peak concentration. The drug exhibits linear pharmacokinetics over the therapeutic dose range. Absorption is generally consistent in hypertensive patients and those with benign prostatic hyperplasia (BPH).

Distribution:
Doxazosin has a large volume of distribution, approximately 1.4 to 2 L/kg, indicating extensive tissue distribution. It is highly (~98%) bound to plasma proteins, primarily albumin, which influences its pharmacodynamic effects and drug interactions. The drug readily crosses membranes to reach target tissues such as vascular smooth muscle and prostate tissue. Limited data indicate minimal penetration into the central nervous system. Tissue binding contributes to the drug's prolonged duration of action.

Metabolism:
Doxazosin undergoes extensive hepatic metabolism primarily via cytochrome P450 isoenzymes CYP3A4 and CYP2D6. The metabolic pathways include oxidation and conjugation, forming inactive metabolites. Metabolism plays a critical role due to significant first-pass metabolism leading to moderate bioavailability. Hepatic impairment can alter the metabolism rate, requiring careful dose adjustment. No active metabolites with significant pharmacologic activity have been identified.

Elimination:
Doxazosin and its metabolites are excreted via both renal and fecal routes, with approximately 50% eliminated in urine and the remainder in feces, primarily as metabolites. The terminal elimination half-life ranges between 16 to 30 hours, enabling convenient once-daily dosing. Steady-state plasma levels are typically achieved within 3 to 5 days of consistent dosing. Renal impairment does not significantly affect elimination, but severe hepatic impairment may prolong half-life and increase plasma concentrations, necessitating monitoring and potential dosage modification.

Dosage Forms

Available Dosage Forms:
- Immediate-release oral tablets: 1 mg, 2 mg, 4 mg, and 8 mg
- Extended-release oral tablets (Cardura XL): 4 mg and 8 mg

Dosages for Adults and Adolescents:
For Hypertension:
- Initial dose: 1 mg orally once daily
- Maintenance dose: Titrated from 1 mg up to 16 mg orally once daily based on clinical response and tolerability

For Benign Prostatic Hyperplasia (BPH):
Immediate-release formulation:
- Initial dose: 1 mg orally once daily
- Maintenance dose: 1 to 8 mg orally once daily

Extended-release formulation (Cardura XL):
- Initial dose: 4 mg orally once daily with breakfast
- Maintenance dose: 4 to 8 mg orally once daily with breakfast; dosing may be titrated to 8 mg considering symptomatic response and tolerability with a recommended titration interval of 3 to 4 weeks

Specific Patient Populations:
- Geriatric patients (≥65 years):
Hypertension initial dosage may be reduced to 0.5 mg orally once daily to decrease risk of orthostatic hypotension

- Hepatic impairment: Cautious dosage adjustment advised due to altered metabolism and clearance

- Renal impairment: No dosage adjustment typically required, but monitoring is prudent for severe impairment

- Transitioning between formulations:
When switching from immediate-release to extended-release tablets, initiate extended-release dosing at the lowest dose of 4 mg once daily. Any prior immediate-release dose on the evening before starting extended-release should be omitted.
- If extended-release therapy is interrupted for several days, restarting at 4 mg once daily is recommended.

Mechanism Of Action

Doxazosin is a selective antagonist of alpha-1 adrenergic receptors, which are G protein-coupled receptors located primarily on vascular smooth muscle, prostate, and bladder neck tissues. By competitively blocking these receptors, doxazosin inhibits norepinephrine-induced vasoconstriction, leading to arterial and venous vasodilation. This vasodilatory effect reduces peripheral vascular resistance and venous return, effectively lowering blood pressure in hypertensive patients.

In the genitourinary system, doxazosin antagonizes alpha-1 receptors in the smooth muscle of the prostate and bladder neck, causing relaxation of these muscles. This muscle relaxation alleviates urinary obstruction and improves urine flow in patients with benign prostatic hyperplasia. The mechanism distinguishes doxazosin from other antihypertensives by directly targeting both vascular tone and urinary tract obstruction.

At the cellular level, alpha-1 receptor antagonism prevents activation of phospholipase C and subsequent intracellular calcium release, reducing smooth muscle contraction. Clinical studies have demonstrated that this effect leads to symptomatic improvement in lower urinary tract symptoms associated with BPH, as well as sustained blood pressure reduction without significant reflex tachycardia.

Compared to other alpha-1 blockers such as prazosin and terazosin, doxazosin has a longer half-life, allowing once-daily dosing and improved patient compliance. Research, including the ALLHAT trial, has provided important safety data regarding its use in hypertension, influencing current clinical recommendations to reserve doxazosin primarily for BPH and select hypertensive patients. Ongoing studies continue to evaluate combination therapies incorporating doxazosin to optimize management of cardiovascular and urologic conditions.

Pharmacological Actions

Selective alpha-1 adrenergic receptor antagonism leading to vasodilation
Reduction of peripheral vascular resistance to lower blood pressure
Relaxation of smooth muscle in the prostate and bladder neck to improve urinary flow
No significant blockade of alpha-2 adrenergic receptors, preserving central sympathetic tone
Improvement in lower urinary tract symptoms associated with benign prostatic hyperplasia
Hemodynamic stability with minimal reflex tachycardia compared to non-selective alpha blockers
Prolonged duration of action permitting once-daily dosing for chronic conditions
Potential mild diuretic effect secondary to vasodilation and decreased renal vascular resistance
Reduction of afterload on the heart by reducing systemic vascular resistance
Decreased venous return through venous dilation, potentially lowering preload
Antihypertensive effect with gradual titration minimizing orthostatic hypotension
Indirect benefits on endothelial function by improving arterial compliance
Improved symptom control in BPH without compromising sexual function significantly
Compatibility with combination antihypertensive regimens including diuretics and beta-blockers
Minimal central nervous system penetration leading to low incidence of CNS side effects

Therapeutic Or Clinical Uses

Treatment of essential hypertension to lower elevated blood pressure
Management of lower urinary tract symptoms associated with benign prostatic hyperplasia (BPH)
Alleviation of urinary obstruction symptoms by relaxing prostate and bladder neck smooth muscle
Adjunct therapy in patients with combined hypertension and BPH to address both conditions
Use in hypertensive patients intolerant to other classes due to contraindications or side effects
Extended-release formulation (Cardura XL) specifically for BPH symptom management
Off-label use in certain cases of Raynaud's syndrome due to peripheral vasodilation
Facilitation of improved urine flow in patients experiencing bladder outlet obstruction
Reduction of blood pressure variability through sustained alpha-1 blockade
Potential improvement in quality of life by reducing nocturia and urinary frequency in BPH
Treatment of hypertension in geriatric patients with dose adjustments for safety
Supportive therapy in congestive heart failure patients with hypertension, though caution advised
Complementary agent in multidrug regimens for resistant hypertension
Symptomatic control of BPH delaying or preventing invasive procedures such as surgery
Management of hypertension secondary to sympathetic overactivity

Side Or Adverse Effects

Orthostatic hypotension causing dizziness or fainting, especially after initial doses
Drowsiness and somnolence related to central alpha-1 blockade
Headache occurring during titration or blood pressure lowering
Fatigue and general weakness reported by some patients
Nasal congestion due to vasodilation of nasal mucosa
Palpitations or reflex tachycardia occasionally observed
Rare but serious allergic reactions including rash, urticaria, and angioedema
Potential for priapism characterized by painful and prolonged erections
Gastrointestinal disturbances such as nausea or abdominal pain in some cases
Peripheral edema from vasodilation and fluid retention
Blurred vision or changes in pupil behavior relevant to cataract surgery patients
Dry mouth reported in some patients during therapy
Postural dizziness, particularly on sudden standing or during hot weather
Less commonly, exacerbation of heart failure symptoms in susceptible individuals
Increased risk of falls particularly in elderly during early treatment phases

Precautions

Use caution in patients with history of orthostatic hypotension or volume depletion
Monitor blood pressure closely during initiation and dose titration to avoid severe hypotension
Exercise caution in patients with liver disease due to altered drug metabolism
Inform healthcare providers prior to cataract surgery due to potential intraoperative floppy iris syndrome
Advise patients to arise slowly from sitting or lying positions to minimize dizziness
Avoid abrupt discontinuation without consulting a healthcare provider
Careful use in elderly patients with frequent monitoring for falls and hypotension
Use with caution in patients with congestive heart failure due to potential fluid retention
Assess for possible drug interactions including concomitant antihypertensives and PDE5 inhibitors
Pregnancy safety not well established; recommend use only if clearly needed
Breastfeeding not advised as excretion into milk is unknown and potential risks exist
Advise limiting alcohol intake to prevent additive hypotensive effects and dizziness
Patients should report any symptoms of syncope or severe dizziness immediately
Screen and manage constipation or gastrointestinal obstruction prior to initiating therapy
Advise patients to avoid prolonged standing and overheating during exercise or hot weather

Warnings

Increased risk of congestive heart failure as demonstrated in the ALLHAT trial when used for hypertension
Not recommended as first-line therapy for hypertension due to cardiovascular risk concerns
Potential for intraoperative floppy iris syndrome during cataract surgery; inform ophthalmologist prior to surgery
Risk of severe hypotension and syncope, especially with first dose or dose escalation
Avoid use in patients with known hypersensitivity to doxazosin or other alpha-1 blockers
May cause dizziness impacting activities requiring alertness including driving or operating machinery
Use cautiously when co-administered with other antihypertensives and vasodilators to prevent additive hypotension
Sudden discontinuation may exacerbate hypertension or urinary symptoms; tapering advised
Potential for priapism necessitating urgent medical attention if it occurs
Elderly patients at higher risk for adverse events including falls and orthostatic hypotension
Use during pregnancy only if benefits outweigh risks due to insufficient safety data
Breastfeeding not recommended while taking doxazosin due to unknown effects on nursing infants
Hypotension risk heightened in volume-depleted patients or those taking diuretics
Concurrent use with PDE5 inhibitors such as sildenafil may potentiate hypotensive effects
Overdose symptoms include extreme dizziness and fainting requiring emergency care

Contraindication

Known hypersensitivity or allergy to doxazosin or other alpha-1 adrenergic blockers such as prazosin, terazosin, alfuzosin, tamsulosin, or silodosin
Patients with a history of severe orthostatic hypotension or syncope related to alpha-blocker therapy
Caution or avoidance in patients with severe hepatic impairment due to altered metabolism and risk of toxicity
Not advised for use in patients undergoing cataract surgery unless specifically managed due to risk of intraoperative ocular complications
Absolute contraindication during episodes of acute hypotension or shock
Contraindicated in patients with documented hypersensitivity reactions presenting as angioedema or anaphylaxis
Caution in patients who have experienced priapism related to previous alpha-1 blocker therapy
Avoid use in patients with symptomatic hypotension unresponsive to dose adjustments
Contraindication with concurrent use of other potent alpha-1 blockers without proper medical supervision
Not recommended in pediatric populations due to lack of established safety and efficacy
Use contraindicated in patients with partial or complete gastrointestinal obstruction
Avoidance recommended in patients with severe constipation impacting gastrointestinal motility
Not suitable for use as monotherapy in congestive heart failure due to increased risks
Avoid in patients with unstable cardiovascular conditions without close monitoring
Contraindicated during breastfeeding due to unknown drug excretion into breast milk and potential infant risk

Drug Interactions

Additive hypotensive effects when combined with other antihypertensive agents including diuretics, beta-blockers, and calcium channel blockers
Concomitant use with phosphodiesterase type 5 inhibitors (e.g., sildenafil) may precipitate symptomatic hypotension
CYP3A4 and CYP2D6 inhibitors may increase plasma concentrations of doxazosin, enhancing effects and side effects
Drugs that cause CNS depression may potentiate drowsiness and dizziness associated with doxazosin
Potential interaction with antidepressants, particularly tricyclics and monoamine oxidase inhibitors, enhancing hypotensive effects
Antiretrovirals used in HIV/AIDS therapy may alter doxazosin metabolism and plasma levels
Concurrent use with antibiotics such as macrolides or azoles that inhibit CYP enzymes require monitoring
Alcohol amplifies dizziness and hypotension related to doxazosin
Nonsteroidal anti-inflammatory drugs (NSAIDs) may reduce antihypertensive effects of doxazosin
Possible increased risk of orthostatic hypotension when taken with narcotics or sedatives
Immunosuppressants metabolized by CYP enzymes may alter doxazosin clearance
Caution with drugs that prolong QT interval as hypotension-related syncope risk may increase
Interactions with herbal supplements (e.g., ginseng, St. John’s Wort) may affect drug efficacy or metabolism
Use with lithium requires monitoring for changes in serum lithium levels
Careful monitoring recommended when initiating or discontinuing interacting medications to prevent adverse events

Preparations Of The Drug

Immediate-release oral tablets available in strengths of 1 mg, 2 mg, 4 mg, and 8 mg
Extended-release oral tablets marketed as Cardura XL available in 4 mg and 8 mg formulations
Tablets contain inactive ingredients such as cellulose derivatives, magnesium stearate, and lactose for tablet integrity and bioavailability
Extended-release formulation designed for once-daily dosing with controlled drug release profile
Available in blister packs or bottles depending on manufacturer and pharmacy dispensing practices
Packaging includes patient information leaflets detailing dosing, precautions, and potential side effects
No intravenous or parenteral formulations available; only oral administration recommended
Storage conditions require room temperature away from moisture and heat to maintain stability
Extended-release tablets should not be split or crushed to preserve prolonged release mechanism
Manufactured under Good Manufacturing Practice (GMP) standards ensuring quality and efficacy
Various generic formulations approved by regulatory authorities accessible depending on region
Compatibility with standard pill dispensers and blister packaging for patient compliance
Patient assistance programs and coupons available through some manufacturers to enhance affordability
Disposal guidelines advise against flushing medications; returning unused medication to take-back programs recommended
Labeling consistent with regulatory requirements including lot number, expiration date, and dosing instructions