Pharmacology: Loratadine like other antihistamines acts via competitive blocking of H1-histaminic receptors. Loratadine penetrates very poorly into CNS. Therefore, it is devoid of significant CNS depressant effects. Loratadine is highly selective antagonist of H1-histaminic receptors. Loratadine has very weak affinity for α-adrenoreceptors or acetylcholine receptors. In animal models, Loratadine has very high affinity for H1-histaminic receptors, which is greater than that of the terfenadine or astemizole. The antihistaminic effect persists for 18-24 hrs. In human subjects, Loratadine suppresses wheal response to intradermally injected histamine. This action starts within 1-2 hrs of oral administration and maximum inhibitory effect evident in 2-4 hrs. The magnitude and duration of effects are dependent upon Loratadine dose. On long-term administration, the drug retains its effectiveness without developing tachyphylaxis. In animal models, Loratadine has also shown anti-serotonin activity. It also suppresses mast cell mediator release.
In addition, Loratadine also possess anti-allergic activity which has been demonstrated in humans by skin prick and nasal challenge tests.
In human volunteers, Loratadine did not reduce psychomotor performance as measured by sleep latency, symbol copying, digit symbol substitution, dynamic visual activity test or driving performance. In addition, Loratadine does not potentiate the effects of centrally acting drugs eg, diazepam, ethanol and barbiturates.
Pharmacokinetics: Absorption: In humans, Loratadine is rapidly absorbed after oral administration. There is a linear relationship between Loratadine dose and both Cmax and AUC. The 10, 20 and 40 mg of Loratadine produce peak plasma concentrations of 5, 11, 28 mcg/L, respectively. Time to reach maximum concentration occurs between 1-1.5 hrs. After repeated dosing steady-state plasma concentrations occur by 5th day.
Metabolism: Loratadine is metabolized to descarboethoxyloratadine (DCL) which is pharmacologically active.
Distribution: Loratadine has extensive distribution in the body. The distribution t½ of Loratadine and DCL is 1.02 and 3.8 hrs, respectively.
Loratadine is 97-99% bound to plasma proteins whereas DCL is 73-76% bound to plasma proteins. Small amount of both Loratadine and DCL is secreted in breast milk, but the amount is minimal.
Elimination: Loratadine undergoes extensive biotransformation. The elimination t½ of Loratadine after single dose is 8-11 hrs and that of DCL is 17-24 hrs. After multiple dosing, elimination t½ of Loratadine is 4.4 hrs and that of DCL is 8.7 hrs.
Pharmacokinetics of Loratadine and that of DCL is not affected in patients with renal disease and in plasma patients on hemodialysis. In elderly patients too, disposition kinetics of Loratadine and DCL is not affected. In patients of alcoholic liver disease, the t½ of Loratadine and that of DCL are longer but still within the range observed in a population of normal subjects.
Use exactly as directed on the label, or as prescribed by your doctor. Do not use in larger or smaller amounts or for longer than recommended.
Loratadine is usually taken once per day. Follow your doctor's instructions.
Do not crush, chew, or break the regular Loratadine tablet. Swallow the pill whole.
Measure liquid medicine with a special dose-measuring spoon or medicine cup, not with a regular table spoon. If you do not have a dose-measuring device, ask your pharmacist for one.
To take Loratadine orally disintegrating tablet (Loratadine, Alavert):
Keep the tablet in its blister pack until you are ready to take the medicine. Open the package and peel back the foil from the tablet blister. Do not push a tablet through the foil or you may damage the tablet.
Using dry hands, remove the tablet and place it on your tongue. It will begin to dissolve right away.
Do not swallow the tablet whole. Allow it to dissolve in your mouth without chewing.
Swallow several times as the tablet dissolves. If desired, you may drink water to help swallow the dissolved tablet.
Call your doctor if your symptoms do not improve.
Store at room temperature away from moisture and heat.
May be administered without regard to meals.
Dispersible tablet: Place in mouth and allow to dissolve. Swallow with or without water.
Loratadine is a long acting second generation antihistamine that is similar in structure to cyproheptadine and azatadine. The pharmacology of Loratadine is similar to other antihistamines, but unlike other H1-blockers, loratidine is shown to exhibit competitive, specific, and selective antagonism of H1 receptors. The exact mechanism of this interaction is unknown, but disposition of the drug suggests that Loratadine's prolonged antagonism of histamine may be due to the drug's slow dissociation from the receptor or the formation of the active metabolite, desloratadine. Loratadine does not penetrate the CNS effectively and has a low affinity for CNS H1-receptors.
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Information checked by Dr. Sachin Kumar, MD Pharmacology