Betahistine dihydrochloride

Mechanism

Pharmacological Class and Group

Histamine analogue. Weak agonist of histaminergic H1 receptors and potent antagonist of H3 receptors of the vestibular and cochlear system.

Mechanism of Action

Betahistine acts by modifying the hemodynamics of the microcirculation of the inner ear and regulating the electrical activity of the vestibular nuclei of the central nervous system:

Molecular Mechanism in the Vascular Stria and Vestibular Nuclei

1. Blockade of Presynaptic H3 Receptors: By acting as an antagonist of H3 receptors, betahistine blocks the negative feedback loop, increasing the synthesis and local release of histamine in the neuronal endings of the inner ear and vestibular nuclei.

2. Postsynaptic H1 Stimulation (Local Vasodilation): The released histamine stimulates the H1 receptors in the capillaries of the stria vascularis of the inner ear. This induces the activation of the enzyme nitric oxide synthase (NOS), raising local levels of nitric oxide (NO). NO relaxes endothelial cells and precochlear capillary pericytes, increasing microvascular blood flow and decreasing hydrostatic pressure of the membranous labyrinth, correcting endolymphatic hydrops.

↑ Cochlear blood flow Resolution of endolymphatic hydrops in Menière's Syndrome

3. Regulation of Activity in the Vestibular Nuclei: Reduces the disparity of basal electrical discharges between the left and right labyrinth through a dose-dependent inhibitory effect on the lateral and medial vestibular nuclei, accelerating the process of vestibular compensation after an acute loss of function.

Pharmacokinetics

High Resolution Pharmacokinetics

• Absorption and Bioavailability: Rapid and complete gastrointestinal absorption after oral intake. However, the systemic bioavailability of betahistine is less than 1% due to extremely rapid and efficient hepatic first-pass metabolism.
• Distribution: Low binding to circulating plasma proteins (<5%). Moderate apparent volume of distribution.
• Metabolism: It is rapidly converted by the enzyme monoamine oxidase type B (MAO-B) into the inactive, fast-clearing metabolite 2-pyridylacetic acid (2-PAA). Plasma concentrations of unchanged betahistine are virtually undetectable under standard therapeutic doses.
• Excretion: 90% of the 2-PAA metabolite is eliminated via the kidneys (urine) within 24 hours.
• Half-life (t_1/2): The plasma elimination half-life of the reference active/inactive metabolite is approximately 3 to 4 hours.

Indicators and dose

Clinical Indications and Off-Label Uses

• Menière's disease: Prevention of recurrent episodes of vertigo, tinnitus and fluctuating hearing loss.
• Peripheral Vestibular Vertigo: Symptomatic relief of vertigo associated with vestibular neuronitis, acute labyrinthitis or after chemical labyrinthectomy.
• Vertebrobasilar insufficiency (off-label): Adjuvant management in vertigo of vascular origin in elderly patients with cervical arterial stenosis.

Clinical Pearl: Dosage and Escalation in Menière

The efficacy of betahistine in preventing vertiginous attacks of Menière's disease is highly dose-dependent. In cases refractory to the standard initial dose of 24 mg/day, escalation to high doses (such as 48 mg divided into three doses or even 96 mg/day in exceptional cases under strict monitoring) has been shown to achieve a superior clinical response without substantially increasing gastrointestinal ADRs, as long as it is administered after main meals.

Dosage and Clinical Adjustment

Administration Scheme in Adults:

• Usual Starting Dose: 8 mg to 16 mg three times a day, administered with meals.
• Optimized Maintenance Dose: 24 mg twice daily orally (48 mg/day total).
• Duration of Treatment: A minimum continuous period of 2 to 3 months is required to observe the maximum prophylactic benefit of the drug in Menière's Syndrome, maintaining it in the long term according to the evolution of the vertiginous episodes.

Adjustment in Renal or Hepatic Failure: No specific dosage adjustment is required in mild to moderate renal or hepatic failure due to the rapid conjugation and clearance of the inactive metabolite.

Security

Absolute and Relative Contraindications

Absolute Contraindications

• Known hypersensitivity to betahistine or the components of the formulation.
• Pheochromocytoma: Being a histamine analog, it can indirectly induce the release of catecholamines from the adrenal tumor, causing severe hypertensive crises that are difficult to control.
• Active peptic ulcer with active bleeding.

Relative Contraindications

• Active bronchial asthma or history of severe bronchospasm (can induce mild bronconstriction by stimulation of H1 receptors in bronchial smooth muscle).
• Chronic inactive peptic ulcer or severe gastroesophageal reflux.

Adverse Effects (ADR) and Specific Toxicity

• Frequent (1%-10%): Gastric dyspepsia, nausea, heartburn, diffuse abdominal pain (significantly attenuated when the drug is administered together with food), mild tension headache.
• Uncommon (0.1%-1%): Skin hypersensitivity reactions (pruritus, urticaria, morbilliform rash, mild angioedema), dry mouth.
• Rare (<0.1%): Moderate thrombocytopenia of immune origin, transient palpitations due to mild peripheral vasodilation.

Drug Interactions of Clinical Relevance

• H1 antihistamines (Diphenhydramine, Cetirizine, Bilastine): Direct pharmacodynamic antagonistic interaction. They can neutralize the peripheral stimulatory effect of betahistine in the stria vascularis, nullifying the clinical benefit in the inner ear.
• Monoamine Oxidase Inhibitors (MAO Inhibitors, e.g., Selegiline): May block MAO-B-mediated hepatic clearance of betahistine, increasing its plasma concentrations and the risk of hypotension or bronchospasm.

Safety in Pregnancy and Breastfeeding

• Pregnancy: There are no clinical exposure data available in pregnant women. Experimental studies in animals do not show direct reproductive toxicity, but it is advisable to avoid its prescription during pregnancy unless it is strictly essential.
• Breastfeeding: Not recommended. It is unknown whether betahistine or the 2-PAA metabolite are excreted in human breast milk.