Carbonic anhydrase inhibitors

Pharmacodynamics and mechanism of action

The primary target of drugs in this class is the enzyme carbonic anhydrase. In the kidneys, it is located predominantly on the apical membrane of the proximal tubule epithelial cells. Normally, this enzyme catalyzes the crucial hydration and dehydration processes of carbonic acid.

When carbonic anhydrase is pharmacologically blocked by acetazolamide, the following occurs:

• The reabsorption of bicarbonate ions ($HCO_3^-$) and sodium ions ($Na^+$) from the renal tubule lumen into the bloodstream is impaired.
• The excretion of bicarbonates along with urine is sharply increased.
• As a result, the urine reaction shifts towards the alkaline side.
• In parallel, bicarbonate reserves in the blood and tissues are depleted, which inevitably leads to blood acidification—metabolic acidosis develops.

Specifics of use as a diuretic

Historically, acetazolamide was prescribed for edema syndrome associated with heart failure, liver cirrhosis, or renal failure. However, today it is used extremely rarely as a classical diuretic due to low efficacy and an inconvenient dosing regimen.

The main problem with the drug is the tolerance phenomenon, which is directly related to its mechanism of action. The diuretic effect of acetazolamide is short-lived and strictly dependent on the presence of bicarbonate ions in the body. As soon as their reserves are depleted and metabolic acidosis sets in, the drug simply stops working.

To restore the drug's efficacy, it is necessary to replenish the alkaline reserve of the blood. For this purpose, acetazolamide is prescribed exclusively in intermittent courses: for example, the patient takes the tablets for 5 days, followed by a mandatory 2-day break.

Extrarenal effects and application in ophthalmology

Since carbonic anhydrase is present not only in the kidneys, inhibiting this enzyme causes a number of significant extrarenal effects. These are what determine the modern clinical significance of the group.

Effect on the visual organ (main application): In the ciliary body of the eye, the drug inhibits carbonic anhydrase, leading to a decrease in intraocular fluid production. This causes a drop in intraocular pressure.

• Glaucoma: Acetazolamide is prescribed orally (per os) for the rapid management of acute attacks, as well as part of preoperative preparation.
• Local alternative: For regular glaucoma treatment, another inhibitor is used—dorzolamide (Trusopt), which is available as eye drops.

Effect on the central nervous system: In the CNS, blockage of the enzyme leads to a reduction in cerebrospinal fluid (CSF) production. This property allows the drug to be successfully used for elevated intracranial pressure.

Pharmacokinetics and other indications

Acetazolamide has good absorption when taken orally. Its action begins within 40–60 minutes, peaking after 2–4 hours. The total duration of the clinical effect ranges from 10 to 12 hours.

In addition to glaucoma, edema, and intracranial hypertension, the drug is used in the following situations:

1. Altitude sickness (accelerates acclimatization to high altitudes).
2. Epilepsy (specifically, absence seizures in pediatric practice).
3. Correction of metabolic alkalosis (since the drug itself purposefully induces acidosis, it is logical to use it to compensate for excessive blood alkalinization).

Class and exam questions

What else to study on this topic

• The role of the carbonic anhydrase enzyme in the physiology of the renal proximal convoluted tubules.
• The pathogenesis of metabolic acidosis development upon bicarbonate loss.
• The mechanisms of mountain sickness development and the sites of action of acetazolamide in this pathology.
• Comparative pharmacokinetics of systemic (acetazolamide) and local (dorzolamide) inhibitors.
• The rationale for using carbonic anhydrase inhibitors in pediatric absence seizures.

Other topics in the section "Diuretics"

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