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Antihistamines are a class of drugs that block or reduce the effects of histamine, a chemical released by the immune system during allergic reactions. Histamine binds to specific receptors in the body, leading to symptoms like itching, swelling, runny nose, watery eyes, and hives(ulticaria). Antihistamines work by blocking histamine receptors, thus relieving these symptoms.
1. H₁-Antihistamines:
Uses for Allergic conditions such as hay fever, urticaria (hives), allergic rhinitis, conjunctivitis, and insect bites.
Mechanism: Block histamine at H₁ receptors, which are primarily found in the skin, lungs, and blood vessels.
Examples:
First generation drugs: Diphenhydramine, Chlorpheniramine (sedating)
Second generation drugs: Loratadine, Levocetirizine, Cetirizine, Fexofenadine (non-sedating)
2. H₂-Antihistamines:
Used for Gastric conditions such as acid reflux, ulcers, and Zollinger–Ellison syndrome.
Mechanism: Block histamine at H₂ receptors, found mainly in the stomach lining, reducing gastric acid secretion.
Examples: Ranitidine, Famotidine, Cimetidine
| Feature | First-Generation | Second-Generation |
|---|---|---|
| Sedation | High (cross blood-brain barrier) | Low or none |
| Duration of action | Short (requires multiple doses/day) | Longer-lasting |
| CNS side effects | Common (drowsiness, dizziness) | Rare |
Key functional groups:
| Peak (cm⁻¹) | Assignment |
|---|---|
| 3432 | O–H / N–H stretching (broad) |
| 3044–3023 | Aromatic C–H stretching |
| 1740 | C=O stretching (carboxyl) |
| 1602–1457 | Aromatic C=C & C–Cl stretches |
| 1318, 1185, 1077, 1055 | C–O / C–N stretches |
| 846–758 | Aromatic C–H bending |
Cetirizine hydrochloride standard:
HPLC :
Key functional groups:
| Peak (cm⁻¹) | Assignment |
|---|---|
| 3400 | N–H stretching |
| 2950 | Aliphatic C–H stretching |
| 1740 | Carbonyl (C=O) stretch |
| 1600 | Aromatic C=C stretch |
| 1320 | C–N stretching (amine/piperazine ring) |
| 1135 | C–O stretching |
Levocetrizine dichloride Standard:
Key functional groups:
| Wavenumber (cm⁻¹) | Assignment | Interpretation |
|---|---|---|
| ~3035 | Aromatic C–H stretching | Indicative of diphenyl aromatic ring hydrogens |
| ~2950–2890 | Aliphatic C–H stretching (–CH₃, –CH₂–) | Signals from N,Ndimethylaminoethyl moiety |
| 2400–2700 | Broad band from protonated tertiary amine (–N⁺HCH₃) | Characteristic of hydrochloride salt form |
| ~1592, 1468, 1455 | Aromatic C=C stretching and CH₂ bending | Skeletal vibrations and methylene bending |
| ~1399–1375 | Methyl CH bending | Common in –N(CH₃)₂ group |
| ~1282, 1171, 1104 | C–N stretching (amines), C–O–C (ether linker) | Reflects diphenylmethoxy and dimethylamino groups |
| ~1017–1021 | C–N stretching | Additional evidence of tertiary amine bonding |
| ~991 | Aromatic C–H deformation | Out-of-plane ring modes |
| ~712–755 | Aromatic C–H out-of-plane bending | Characteristic of mono-substituted phenyl rings |
Diphenhydramine Hydrochloride standard:
Key functional groups:
| Peak (cm⁻¹) | Functional Group / Assignment |
|---|---|
| ~3365 | Broad O–H stretching (carboxylic and tertiary alcohol) |
| 2935–2940 | Aliphatic C–H stretch (–CH₂–/–CH₃ groups) |
| 1705–1712 | Carbonyl (C=O) stretch of carboxylic acid |
| 1460–1495 | Aromatic C=C stretch (phenyl rings) |
| ~1400 | OH in-plane deformation of tertiary alcohol |
| 1279–1280 | C–N stretch (tertiary amine)/C–O (tertiary alcohol) |
| ~1168 | C–O stretching (tertiary alcohol group) |
Fexofenadine hydrochloride standard:
Bronchodilators are medications used to relax the smooth muscles of the airways (bronchi), leading to dilation (widening) of the air passages in the lungs. This improves airflow and makes breathing easier, especially in conditions like asthma, chronic obstructive pulmonary disease (COPD), and chronic bronchitis.
Based on method of action & duration of action classified into
1. β2-Adrenergic Agonists (β2-agonists)
Used to stimulate β2-receptors in bronchial muscles, causing relaxation.
Examples: Salbutamol (Albuterol), Levalbuterol, Salmeterol, Formoterol, Vilanterol
2. Anticholinergics (Muscarinic Antagonists)
They block acetylcholine at muscarinic receptors, preventing bronchoconstriction.
Examples: Ipratropium bromide, Tiotropium, Aclidinium
3. Methylxanthines:
These are phosphodiesterase inhibitors that increase cAMP, causing bronchodilation. Second line drugs for asthma or COPD condition.
Examples: Aminophylline, Theophylline
Key functional groups
| Peak (cm⁻¹) | Functional Group | Assignment |
|---|---|---|
| ~3400–3500 | O–H stretching | Alcohol and phenol hydroxyl groups |
| ~2950–2850 | C–H stretching | Aliphatic chains (–CH₃, –CH₂–) |
| ~1600–1510 | Aromatic C=C stretching | Phenyl ring |
| ~1250–1020 | C–O stretching | Ether/alcohol C–O bond |
| ~1380–1375 | C–N stretching | Tertiary amine (in the –NH–tert-butyl) |
Salbutamol sulphate (albuterol) Standard:
HPLC:
