Acetazolamide

Side effects

Common adverse effects of acetazolamide include the following: paraesthesia, fatigue, drowsiness, depression, decreased libido, bitter or metallic taste, nausea, vomiting, abdominal cramps, diarrhea, black stool, polyuria, kidney stones, metabolic acidosis and electrolyte changes (hypokalemia, hyponatremia). Whereas less common adverse effects include Stevens–Johnson syndrome, anaphylaxis and blood dyscrasias.

Contraindications

Contraindications include:

• Hyperchloremic acidosis
• Hypokalemia (low blood potassium)
• Hyponatremia (low blood sodium)
• Adrenal insufficiency
• Impaired kidney function
• Hypersensitivity to acetazolamide or other sulphonamides.
• Marked liver disease or impairment of liver function, including cirrhosis because of the risk of development of hepatic encephalopathy. Acetazolamide decreases ammonia clearance.

Interactions

It is possible that it might interact with:

• Amphetamines, because it increases the pH of the renal tubular urine, hence reducing the clearance of amphetamines.
• Other carbonic anhydrase inhibitors—potential for additive inhibitory effects on carbonic anhydrase and hence potential for toxicity.
• Ciclosporin, may increase plasma levels of ciclosporin.
• Antifolates such as trimethoprim, methotrexate, pemetrexed and raltitrexed.
• Hypoglycemics, acetazolamide can both increase or decrease blood glucose levels.
• Lithium, increases excretion, hence reducing therapeutic effect.
• Methenamine compounds, reduces the urinary excretion of methenamines.
• Phenytoin, reduces phenytoin excretion, hence increasing the potential for toxicity.
• Primidone, reduces plasma levels of primidone. Hence reducing anticonvulsant effect.
• Quinidine, reduces urinary excretion of quinidine, hence increasing the potential for toxicity.
• Salicylates, potential for severe toxicity.
• Sodium bicarbonate, potential for kidney stone formation.
• Anticoagulants, cardiac glycosides, may have their effects potentiated by acetazolamide.

Mechanism of action

Acetazolamide is a carbonic anhydrase inhibitor, hence causing the accumulation of carbonic acid. Carbonic anhydrase is an enzyme found in red blood cells and many other tissues that catalyses the following reaction:

H₂CO₃ ⇌ H₂O + CO₂

hence lowering blood pH, by means of the following reaction that carbonic acid undergoes:

H₂CO₃ ⇌ HCO₃⁻ + H⁺

which has a pK_a of 6.3.

The mechanism of diuresis involves the proximal tubule of the kidney. The enzyme carbonic anhydrase is found here, allowing the reabsorption of bicarbonate, sodium, and chloride. By inhibiting this enzyme, these ions are excreted, along with excess water, lowering blood pressure, intracranial pressure, and intraocular pressure. A general side effect of carbonic anhydrase inhibitors is loss of potassium due to this function. By excreting bicarbonate, the blood becomes acidic, causing compensatory hyperventilation with deep respiration (Kussmaul breathing), increasing levels of oxygen and decreasing levels of carbon dioxide in the blood.

In the eye this results in a reduction in aqueous humour.

Bicarbonate (HCO₃⁻) has a pK_a of 10.3 with carbonate (CO₃²⁻), far further from physiologic pH (7.35–7.45), and so it is more likely to accept a proton than to donate one, but it is also far less likely for it to do either, thus bicarbonate will be the major species at physiological pH.

Under normal conditions in the proximal convoluted tubule of the kidney, most of the carbonic acid (H₂CO₃) produced intracellularly by the action of carbonic anhydrase quickly dissociates in the cell to bicarbonate (HCO₃⁻) and an H⁺ ion (a proton), as previously mentioned. The bicarbonate (HCO₃⁻) exits at the basal portion of the cell via sodium (Na⁺) symport and chloride (Cl⁻) antiport and re-enters circulation, where it may accept a proton if blood pH decreases, thus acting as a weak, basic buffer. The remaining H⁺ left over from the intracellular production of carbonic acid (H₂CO₃) exits the apical (urinary lumen) portion of the cell by Na⁺ antiport, acidifying the urine. There, it may join with another bicarbonate (HCO₃⁻) that dissociated from its H⁺ in the lumen of the urinary space only after exiting the proximal convoluted kidney cells/glomerulus as carbonic acid (H₂CO₃) because bicarbonate (HCO₃⁻) itself can not diffuse across the cell membrane in its polar state. This will replenish carbonic acid (H₂CO₃) so that it then may be reabsorbed into the cell as itself or CO₂ and H₂O (produced via a luminal carbonic anhydrase). As a result of this whole process, there is a greater net balance of H⁺ in the urinary lumen than bicarbonate (HCO₃⁻), and so this space is more acidic than physiologic pH. Thus, there is an increased likelihood that any bicarbonate (HCO₃⁻) that was left over in the lumen diffuses back into the cell as carbonic acid, CO₂, or H₂O.

In short, under normal conditions, the net effect of carbonic anhydrase in the urinary lumen and cells of the proximal convoluted tubule is to acidify the urine and transport bicarbonate (HCO₃⁻) into the body. Another effect is excretion of Cl⁻ as it is needed to maintain electroneutrality in the lumen, as well as the reabsorption of Na⁺ into the body.

Thus, by disrupting this process with acetazolamide, urinary Na⁺ and bicarbonate (HCO₃⁻) are increased, and urinary H⁺ and Cl⁻ are decreased. Inversely, serum Na⁺ and bicarbonate (HCO₃⁻) are decreased, and serum H⁺ and Cl⁻ are increased. H₂O generally follows sodium, and so this is how the clinical diuretic effect is achieved, which reduces blood volume and thus preload on the heart to improve contractility and reduce blood pressure, or achieve other desired clinical effects of reduced blood volume such as reducing edema or intracranial pressure.

History

An early description of this compound (as 2-acetylamino-1,3,4-thiadiazole-5-sulfonamide) and its synthesis has been patented in 1951.

Research

Smaller clinical trials have also shown promising results in the treatment of normal pressure hydrocephalus (NPH).

Veterinary use

Acetazolamide was originally used as a diuretic but has been largely supplanted by other drugs such as furosemide. Carbonic anhydrase inhibitors are no longer the preferred method of treating intraocular pressure in cases of glaucoma but when a carbonic anhydrase inhibitor is used methazolamide is preferred, although acetazolamide is still used. Acetazolamide is also used to increase the alkalinity of urine to manage urinary calculi, but requires bicarbonate supplementation. In horses it is used as a treatment for hyperkalaemic periodic paralysis.

Acetazolamide has largely been supplanted by other drugs and therapies due to side effects such as hypokalaemia and respiratory acidosis.