Mebhydrolin (Diazolin): 1st generation $H

Classification and place in therapy

Mebhydrolin (known as Diazolin) is a representative of the group of agents that affect immune processes and suppress inflammation. In pharmacological classification, it belongs to antihistamines ($H_1$-receptor blockers) of the first generation.

This same subgroup includes diphenhydramine, clemastine, and quifenadine (phencarol). It is important to clearly distinguish them from second-generation drugs (e.g., cetirizine), which have a higher selectivity for $H_1$ receptors and do not cause pronounced sedative effects.

Pharmacodynamics and Cellular Mechanism

The action of mebhydroline is based on interference with the function of histamine receptors, which are structurally coupled to G proteins.

Normally, the pathogenesis of an allergic reaction at the cellular level proceeds as follows: free histamine excites the $H_1$ receptor $\rightarrow$ the enzyme phospholipase C is activated $\rightarrow$ the intracellular concentration of calcium ions ($Ca^{2+}$) increases sharply.

Mebhydroline has specificity: it selectively blocks $H_1$ receptors specifically (rather than all types of histamine receptors simultaneously, and it does not affect $H_2$, $M_1$, or $\alpha_1$ receptors). By occupying the receptor, the drug prevents histamine from triggering the cascade described above involving phospholipase C and calcium.

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Pharmacological effects

By blocking the intracellular release of $Ca^{2+}$, mebhydroline prevents the development of the tissue effects of histamine. Primarily, it:

Prevents smooth muscle contraction (which is characteristic of allergic spasms).
Prevents increased vascular permeability (which underlies the development of allergic edema).

Thus, the drug effectively suppresses inflammation. However, it is characterized by a short duration of action, which is a typical feature of first-generation antihistamines.

Side Effects and Characteristics

The main pharmacokinetic feature of mebhydroline is its ability to cross the blood-brain barrier (BBB).

Penetration into the central nervous system causes a sedative effect. This is a key side effect of all first-generation drugs, which must be considered when prescribing (especially to patients whose activities require high concentration).

Formulation and administration

According to the prescription reference guide, Diazolinum is available in a solid dosage form as coated tablets (dragées).

Dosages: 0.05 g and 0.1 g.
Route of administration: strictly orally.
Therapeutic range: from 0.05 to 0.2 g per single dose.

Briefly: answers to topical questions

Which type of G proteins are $H_1$ histamine receptors coupled to?

$H_1$ histamine receptors are coupled with G proteins; the specific type of G protein is not specified in the cited sources. Their stimulation leads to the activation of phospholipase C and an increase in the intracellular concentration of calcium ions. The receptors are localized predominantly on the postsynaptic membrane; activation of this cascade causes smooth muscle contraction and increased vascular permeability.

How to memorize

1st generation = Crosses the blood-brain barrier = Sedation. Mebhydrolin is short-acting: the patient sleeps, the allergy sleeps.

Class and exam questions

How does mebhydrolin differ from cetirizine?

Mebhydrolin belongs to the 1st generation, crosses the blood-brain barrier, has a short duration of action, and causes a sedative effect. Cetirizine is a 2nd-generation drug; it is more selective and does not have a pronounced sedative effect.

Which intracellular mechanism does mebhydrolin interrupt?

It blocks $H_1$ receptors coupled to G proteins, preventing the activation of phospholipase C and the subsequent increase in intracellular $Ca^{2+}$ ion concentration.

Does the drug block all types of histamine receptors?

No, antihistamines are specific. Mebhydrolin predominantly blocks $H_1$ receptors responsible for allergic reactions without affecting other types.

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What else to study on this topic

Comparative characteristics of 1st and 2nd generation antihistamines.
The role of phospholipase C and calcium ions in smooth muscle contraction.
Mechanisms of increased vascular permeability in allergic inflammation.
Features of drug penetration across the blood-brain barrier (BBB).
Rules for prescribing coated tablets.

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Mebhydrolin