Discovery Pharmaceuticals Ltd

4.3         Contraindications

Doxazosin is contraindicated in

-        Patients with a known hypersensitivity to quinazolines (e.g. prazosin, terazosin, doxazosin), or any of the excipients

-        Patients with a history of orthostatic hypotension

-        Patients with benign prostatic hyperplasia and concomitant congestion of the upper urinary tract, chronic urinary tract infection or bladder stones.

-        Patients with a history of gastro-intestinal obstruction, oesophageal obstruction, or any degree of decreased lumen diameter of the gastro-intestinal tract ¹

-        During lactation (please see section 4.6)²

-        Patients with hypotension³

Doxazosin is contraindicated as monotherapy in patients with either overflow bladder, or anuria with or without progressive renal insufficiency.

^[1] For patients taking the prolonged release tablets only.

² For the hypertension indication only

³ For the benign prostatic hyperplasia indication only

4.4         Special warnings and precautions for use

Information to be given to the Patient: Patients should be informed that doxazosin tablets should be swallowed whole. Patients should not chew, divide or crush the tablets.

For some prolonged-release formulations the active compound is surrounded by an inert, non absorbable coating that is designed to control the release of the drug over a prolonged period. After transit through the gastrointestinal tract, the empty tablet shell is excreted. Patients should be advised not to be concerned if they occasionally observe remains in their stools that look like a tablet.

Abnormally short transit times through the gastrointestinal tract (e.g. following surgical resection) could result in incomplete absorption. In view of the long half life of doxazosin the clinical significance of this is unclear.

Initiation of Therapy: In relation with the alpha-blocking properties of doxazosin, patients may experience postural hypotension evidenced by dizziness and weakness, or rarely loss of consciousness (syncope), particularly with the commencement of therapy. Therefore, it is prudent medical practice to monitor blood pressure on initiation of therapy to minimise the potential for postural effects. The patient should be cautioned to avoid situations where injury could result should dizziness or weakness occur during the initiation of doxazosin therapy.

Use in patients with Acute Cardiac Conditions: As with any other vasodilatory anti-hypertensive agent it is prudent medical practice to advise caution when administering doxazosin to patients with the following acute cardiac conditions:

- pulmonary oedema due to aortic or mitral stenosis

- heart failure at high output

- right-sided heart failure due to pulmonary embolism or pericardial effusion

- left ventricular heart failure with low filling pressure.

Use in Hepatically Impaired Patients: As with any drug wholly metabolised by the liver, doxazosin should be administered with particular caution to patients with evidence of impaired hepatic function. Since there is no clinical experience in patients with severe hepatic impairment use in these patients is not recommended.

Use with PDE-5 inhibitors: Concomitant use of phosphodiesterase-5 inhibitors (e.g. sildenafil, tadalafil, vardenafil) and doxazosin may lead to symptomatic hypotension in some patients. In order to minimise the risk for developing postural hypotension the patient should be stable on the alpha-blocker therapy before initiating use of phosphodiesterase-5-inhibitors.

Use in patients undergoing cataract surgery: The ‘Intraoperative Floppy Iris Syndrome’ (IFIS, a variant of small pupil syndrome) has been observed during cataract surgery in some patients on or previously treated with tamsulosin. Isolated reports have also been received with other alpha-1 blockers and the possibility of a class effect cannot be excluded. As IFIS may lead to increased procedural complications during the cataract operation current or past use of alpha-1 blockers should be made known to the ophthalmic surgeon in advance of surgery.

Laboratory data

Doxazosin may influence the plasma renin activity and urinary excretion of vanillylmandelic acid. This should be considered when analysing laboratory data.

4.5        Interactions with other medicinal products and other forms of interaction

Concomitant use of phosphodiesterase-5 inhibitors (e.g. sildenafil, tadalafil, vardenafil) and doxazosin may lead to symptomatic hypotension in some patients (see section 4.4).

Most (98%) of plasma doxazosin is protein bound. In vitro data in human plasma indicate that doxazosin has no effect on protein binding of digoxin, warfarin, phenytoin or indometacin.

Conventional doxazosin has been administered without any adverse drug interaction in clinical experience with thiazide diuretics, furosemide, beta-blockers, non-steroidal anti-inflammatory drugs, antibiotics, oral hypoglycaemic drugs, uricosuric agents, and anticoagulants. However, data from formal drug/drug interaction studies are not present.

Doxazosin potentiates the blood pressure lowering activity of other alpha-blockers and other antihypertensives.

In an open-label, randomized, placebo-controlled trial in 22 healthy male volunteers, the administration of a single 1 mg dose of doxazosin on day 1 of a four-day regimen of oral cimetidine (400 mg twice daily) resulted in a 10% increase in mean AUC of doxazosin, and no statistically significant changes in mean Cmax and mean half-life of doxazosin. The 10% increase in the mean AUC for doxazosin with cimetidine is within intersubject variation (27%) of the mean AUC for doxazosin with placebo.

Non-steroidal antirheumatics or estrogens may reduce the antihypertensive effect of doxazosin.

Sympathomimetics may reduce the antihypertensive effect of doxazosin; doxazosin may reduce blood pressure and vascular reactions to dopamine, ephedrine, epinephrine, metaraminol, methoxamine and phenylephrine.

There are no studies available concerning interactions with agents influencing hepatic metabolism.

4.6        Pregnancy and lactation

For the hypertension indication:

As there are no adequate and well controlled studies in pregnant women, the safety of doxazosin during pregnancy has not been established. Accordingly, during pregnancy, doxazosin should be used only if the potential benefit outweighs the risk. Although no teratogenic effects were seen in animal testing, reduced foetal survival was observed in animals at high doses (see Section 5.3: Preclinical Safety Data).

Doxazosin is contraindicated during lactation as the drug accumulates in milk of lactating rats and there is no information about the excretion of the drug into the milk of lactating women.

Alternatively, mothers should stop breast-feeding when treatment with doxazosin is necessary (Please see section 5.3).

For the benign prostatic hyperplasia indication:

This section is not applicable.

4.7         Effects on ability to drive and use machines

The ability to engage in activities such as operating machinery or operating a motor vehicle may be impaired, especially when initiating therapy.

4.8         Undesirable effects

Frequencies used are as follows: Very common ≥ 1/10, Common ≥ 1/100 and < 1/10, Uncommon ≥ 1/1,000 and < 1/100, Rare ≥ 1/10,000 and < 1/1,000, Very rare < 1/10,000, not known (cannot be estimated from the available data).

4.9       Overdose

Should overdosage lead to hypotension, the patient should be immediately placed in a supine, head down position. Other supportive measures should be performed if thought appropriate in individual cases. Since doxazosin is highly protein bound, dialysis is not indicated.

Toxicity

There is limited data on the effect of overdoses. Syncope occurred in a fasting adult who had taken doxazosin 16 mg. A 13-year-old experienced moderate intoxication following a maximum dose of doxazosin 40 mg.

Symptoms:

Headache, dizziness, unconsciousness, syncope, dyspnoea, hypotension, palpitations, tachycardia, arrhythmia. Nausea, vomiting. Possibly hypoglycaemia, hypokalaemia.

Treatment:

Ventricle emptying and charcoal if required. In cases of hypotension: lower the head position, provide intravenous fluids and if needed vasopressors (for instance noradrenaline or ephedrine). Provide symptomatic treatment as needed.