Irbis HT 300/12.5

Indications

• Hypertension
• Edema associated with congestive heart failure
• Edema due to renal dysfunction

Dosage

Children: Refer to the BNF for Children for specific dosing information.

Adults: 2.5 mg daily, taken in the morning for hypertension; higher doses may be used for other indications.

Mechanism of action

Hydrochlorothiazide is transported into epithelial cells of the distal convoluted tubule via organic anion transporters OAT1, OAT3, and OAT4. It inhibits the sodium-chloride symporter (SLC12A3), preventing sodium reabsorption. This action reduces the concentration gradient that promotes water reabsorption, leading to increased urine production.

Pharmacodynamics

Hydrochlorothiazide increases the excretion of sodium and water, thereby promoting diuresis. It is effective in lowering blood pressure and has a wide therapeutic range, with doses typically ranging from 25 to 100 mg. The drug exhibits a greater antihypertensive effect at lower doses due to enhanced vasodilation, while higher doses primarily exert their effects through diuresis. Caution is advised in patients with renal or hepatic impairment.

Pharmacokinetics

Hydrochlorothiazide is well absorbed from the gastrointestinal tract, with peak plasma concentrations occurring within 1-2 hours post-administration. It has a half-life of about 6-15 hours, depending on individual patient factors. The drug is primarily excreted unchanged in the urine. Dosage adjustments may be necessary in patients with renal impairment due to the risk of accumulation.

Contra-indications

• History of hypersensitivity to sulfonamides
• Acute porphyrias
• Severe renal impairment

Adverse effects

• Angina pectoris
• Arrhythmias
• Arthralgia
• Asthenia
• Chest pain
• Confusion
• Dry mouth
• Haemolytic anaemia
• Hepatic disorders
• Hyperkalaemia
• Hypokalemia
• Nausea
• Rhabdomyolysis
• Syncope
• Vertigo

Interactions

• Potassium-sparing diuretics
• Other antihypertensives
• Lithium
• Non-steroidal anti-inflammatory drugs (NSAIDs)
• Corticosteroids

Precautions

• Diabetes mellitus
• Elderly patients
• Basal cell carcinoma
• Cutaneous lupus erythematosus
• Patients with hepatic impairment
• Monitoring of electrolytes

Pregnancy

Hydrochlorothiazide should be used during pregnancy only if the potential benefit justifies the potential risk to the fetus. It is advisable to avoid use, especially during the first trimester.

Breast-feeding

Hydrochlorothiazide is present in breast milk; however, the amount is probably too small to be harmful. Caution is advised as large doses may suppress lactation.

Storage

Store in a cool, dry place away from light. Keep out of reach of children.

Formulations

• Tablets: 12.5 mg, 25 mg
• Oral solution
• Combination products with amiloride

Indications

• Hypertension
• Diabetic nephropathy in patients with type 2 diabetes

Dosage

Children: Refer to BNF for Children for specific dosing information.

Adults: Initially 75–150 mg once daily, may be increased to 300 mg once daily if tolerated.

Mechanism of action

Irbesartan prevents angiotensin II binding to the AT1 receptor in tissues such as vascular smooth muscle and the adrenal gland. It binds to the AT1 receptor with over 8500 times more affinity than to the AT2 receptor. By blocking angiotensin II from binding, irbesartan induces vascular smooth muscle relaxation and inhibits aldosterone secretion, which together lower blood pressure. This action counteracts the vasoconstrictive and sodium-retaining effects of angiotensin II.

Pharmacodynamics

Irbesartan is an effective angiotensin receptor blocker that lowers blood pressure and has a wide therapeutic index. It is well-tolerated at doses up to 900 mg daily, although typical therapeutic doses range from 150 mg. Its long half-life allows for once-daily dosing, making it convenient for patient adherence.

Pharmacokinetics

Irbesartan is well absorbed following oral administration, with peak plasma concentrations occurring within 1-2 hours. It undergoes extensive hepatic metabolism, primarily by glucuronidation, and is excreted via the urine and feces. The elimination half-life is approximately 11 hours, which supports its once-daily dosing regimen. Renal impairment may necessitate cautious use, especially in patients with creatinine clearance less than 30 mL/min.

Contra-indications

• Hypersensitivity to irbesartan or any of the excipients
• Severe hepatic impairment
• Bilateral renal artery stenosis or stenosis in a solitary kidney

Adverse effects

• Dizziness
• Fatigue
• Hypotension
• Angioedema
• Elevated blood urea or creatinine
• Hyperkalemia
• Gastrointestinal disturbances

Interactions

• Irbesartan and bulevirtide: Unknown (affects efficacy)
• Other antihypertensive agents may have additive effects
• Non-steroidal anti-inflammatory drugs (NSAIDs) may reduce the antihypertensive effect of irbesartan

Precautions

• Caution in patients with a history of angioedema
• Monitor renal function in patients with renal impairment
• Use with caution in patients with diabetes or heart failure

Pregnancy

Irbesartan is contraindicated during pregnancy due to potential harm to the fetus, particularly in the second and third trimesters.

Breast-feeding

Caution is advised as it is not known if irbesartan is excreted in human milk.

Storage

Store at room temperature, away from moisture and heat.

Formulations

• Irbesartan 75 mg tablets
• Irbesartan 150 mg tablets
• Irbesartan 300 mg tablets
• Oral suspension (special order)

Mechanism of action

Hydrochlorothiazide is transported from the circulation into epithelial cells of the distal convoluted tubule by the organic anion transporters OAT1, OAT3, and OAT4. From these cells, hydrochlorothiazide is transported to the lumen of the tubule by multidrug resistance associated protein 4 (MRP4). Normally, sodium is reabsorbed into epithelial cells of the distal convoluted tubule and pumped into the basolateral interstitium by a sodium-potassium ATPase, creating a concentration gradient between the epithelial cell and the distal convoluted tubule that promotes the reabsorption of water. Hydrochlorothiazide acts on the proximal region of the distal convoluted tubule, inhibiting reabsorption by the sodium-chloride symporter, also known as Solute Carrier Family 12 Member 3 (SLC12A3). Inhibition of SLC12A3 reduces the magnitude of the concentration gradient between the epithelial cell and distal convoluted tubule, reducing the reabsorption of water.

Pharmacodynamics

Hydrochlorothiazide prevents the reabsorption of sodium and water from the distal convoluted tubule, allowing for the increased elimination of water in the urine. Hydrochlorothiazide has a wide therapeutic window as dosing is individualized and can range from 25-100mg. Hydrochlorothiazide should be used with caution in patients with reduced kidney or liver function.

Mechanism of action

Irbesartan prevents angiotensin II binding to the AT₁ receptor in tissues like vascular smooth muscle and the adrenal gland. Irbesartan and its active metabolite bind the AT₁ receptor with 8500 times more affinity than they bind to the AT₂ receptor. Irbesartan's prevention of angiotensin II binding causes vascular smooth muscle relaxation and prevents the secretion of aldosterone, lowering blood pressure. Angiotensin II would otherwise bind to the AT₁ receptor, inducing vasoconstriction and aldosterone secretion, raising blood pressure. Angiotensin II is a potent vasoconstrictor formed from angiotensin I in a reaction catalyzed by angiotensin-converting enzyme (ACE, kininase II). Angiotensin II is the principal pressor agent of the renin-angiotensin system (RAS) and also stimulates aldosterone synthesis and secretion by adrenal cortex, cardiac contraction, renal resorption of sodium, activity of the sympathetic nervous system, and smooth muscle cell growth. Irbesartan blocks the vasoconstrictor and aldosterone-secreting effects of angiotensin II by selectively binding to the AT1 angiotensin II receptor. There is also an AT2 receptor in many tissues, but it is not involved in cardiovascular homeostasis. Irbesartan is a specific competitive antagonist of AT1 receptors with a much greater affinity (more than 8500-fold) for the AT1 receptor than for the AT2 receptor and no agonist activity. Blockade of the AT1 receptor removes the negative feedback of angiotensin II on renin secretion, but the resulting increased plasma renin activity and circulating angiotensin II do not overcome the effects of irbesartan on blood pressure. Irbesartan does not inhibit ACE or renin or affect other hormone receptors or ion channels known to be involved in the cardiovascular regulation of blood pressure and sodium homeostasis. Because irbesartan does not inhibit ACE, it does not affect the response to bradykinin; whether this has clinical relevance is not known.

Pharmacodynamics

Irbesartan is an angiotensin receptor blocker used to treat hypertension and diabetic nephropathy. It has a long duration of action as it is usually taken once daily and a wide therapeutic index as doses may be as low as 150mg daily but doses of 900mg/day were well tolerated in healthy human subjects.