Mechanism of action
Loratadine acts at the effector stage of an allergic reaction. It is a competitive antagonist of histamine and selectively blocks $H_1$-histamine receptors. Unlike some other agents, the drug has high selectivity and does not affect other receptor subtypes (e.g., $H_2$ receptors responsible for Lewis's triple response).
By binding to $H_1$ receptors, loratadine prevents the initiation of the intracellular calcium cascade, which blocks smooth muscle contraction and the development of other histamine-mediated reactions.
Pharmacokinetics and metabolism
The drug is administered enterally (orally). A crucial feature of loratadine as a second-generation representative is that it practically does not cross the blood-brain barrier (BBB). This is due to two factors:
- Low lipophilicity of the molecule.
- Active efflux of the drug from the BBB endothelium back into the bloodstream by the transport pump protein — P-glycoprotein.
Loratadine is a prodrug. Upon entering the liver, it is metabolized by the cytochrome P450 isoenzyme — CYP3A4. This results in the formation of an active metabolite — desloratadine. It is this transformation that ensures the prolonged duration of the drug's effect, which ranges from 12 to 24 hours.
Pharmacological effects and clinical use
The drug belongs to agents affecting immune processes and suppressing inflammation. In terms of antihistamine activity gradation, loratadine demonstrates a moderate effect (inferior to first-generation drugs such as promethazine and clemastine).
Due to its prolonged effect, it is convenient to use — prescribed only once a day. In pediatric practice, loratadine is considered one of the drugs of choice for treating allergies in children.
Side effects and drug interactions
Since loratadine does not penetrate the central nervous system, it lacks the pronounced sedative effect characteristic of the first generation (e.g., diphenhydramine). It also lacks anticholinergic (atropine-like) or antiemetic activity.
The main clinical issue lies in drug interactions at the hepatic level. Since metabolism occurs via CYP3A4, concurrent administration of inhibitors of this enzyme (ketoconazole, erythromycin, as well as grapefruit juice) slows down the breakdown of loratadine. This leads to an increase in its blood concentration and the risk of adverse reactions.
Prescription writing rules
The drug is available in 0.01 g (10 mg) tablets. It is prescribed orally before meals, 1 tablet once a day.
Prescription example: Rp.: Tab. Loratadini 0,01 D.t.d. N. 10 S. Orally, 1 tablet 1 time a day before meals.
Briefly: answers to topical questions
What specific diseases and conditions are indications for prescribing loratadine?
Source-confirmed indications/clinical settings for loratadine use include:
- Allergic rhinitis — in tuberculosis patients receiving chemotherapy.
- Allergic conjunctivitis — in tuberculosis patients receiving chemotherapy.
- Dermatitis — in complications of scabies, as part of systemic desensitizing therapy.
- Microbial eczema — in complications of scabies, as part of systemic desensitizing therapy.
- Unspecified urticaria — prolonged outpatient use of second-generation non-sedating antihistamines is recommended; loratadine belongs to second-generation drugs.
Loratadine — The Lazy Histamine Catcher: works long (24 hours), stays out of the brain (no drowsiness), waits for liver help (prodrug).
Class and exam questions
Why doesn't loratadine cause drowsiness, unlike diphenhydramine?
Loratadine belongs to the 2nd generation. It has low lipophilicity and is actively pumped out of cerebral vessel endothelium by P-glycoprotein, which is why it practically does not cross the blood-brain barrier.
Why does loratadine need to pass through the liver if it is already a blocker?
Loratadine itself is a prodrug. In the liver, under the action of the CYP3A4 enzyme, it turns into an active metabolite (desloratadine), which provides the prolonged antihistamine effect (12–24 hours).
Can loratadine be washed down with grapefruit juice?
No. Grapefruit juice, like erythromycin or ketoconazole, inhibits the CYP3A4 isoenzyme. This will slow down the drug's metabolism and increase its concentration in the blood.
What else to study on this topic
- Comparative characteristics of 1st and 2nd generation $H_1$ blockers.
- The role of P-glycoprotein in pharmacokinetics.
- Cytochrome P450 system: clinical significance of CYP3A4 inducers and inhibitors.
- Mechanism of mast cell degranulation and targets of antiallergic drugs.
- Active metabolites of antihistamines (transition from loratadine to desloratadine).
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