Anti histamines

Antihistamines

H₁ Antagonists

  • Definitions:

    • Antihistamines: Medications that block the action of histamine at H1 receptors.

    • First-Generation H1 Antagonists: Generally more sedating due to ability to cross the blood-brain barrier (BBB).

    • Second-Generation H1 Antagonists: Less sedating, with reduced ability to cross BBB.

  • Chemical Structures:

    • Phenbenzamine and Cetirizine are examples of H1 antagonists.

    • H2 antagonists include Cimetidine and Proton Pump Inhibitors like Omeprazole.

Histamine: An Immune Signal

  • Release Mechanism:

    • Histamine is released from mast cells and basophils in response to allergen crosslinking with IgE antibodies.

    • Histamine binds to cell surface receptors triggering immune responses.

  • Physiological Functions:

    • Allergic Inflammatory Response: H1 agonist activity, linked to G-Protein Coupled Receptor (GPCR) signaling.

    • Regulates Gastric Acid Secretion: H2 agonist activity, also linked to GPCR signaling.

Basic Chemistry of Histamine

  • Structure:

    • Imidazole Heterocycle: Contains aromatic structure with 6 pi electrons.

    • Basic Nitrogen: Presence of amine that can be protonated.

    • pKa Values:

    • pKa 6.0 (ionization occurs)

    • pKa 9.8 (further ionization at basic pH).

  • Physicochemical Properties:

    • Histamine is ionized at physiological pH (pH 7.4).

Immune Sensitivity Reactions

  • IgE-Mediated Hypersensitivity:

    • Triggered by environmental allergens (e.g., pollen), insect allergens (e.g., bee stings), and food allergens (e.g., shellfish).

Histamine Action on Target Cells

  • Signal Transduction:

    • Histamine serves as an agonist for GPCRs leading to signal cascade initiation.

    • Influences gene transcription through activated cellular pathways.

  • Histamine Binding Sites:

    • Extensive hydrogen bonding in ligand-binding pockets of H1 GPCRs involves asparagine, tyrosine, aspartate, and threonine.

Downstream Effects of Histamine Action

  • Physiological Responses:

    • Vasodilation

    • Edema (fluid retention)

    • Flushing

    • Itching, hives (urticaria)

    • Wheal and flare (skin eruptions)

    • Bronchoconstriction (H1)

    • GI smooth muscle constriction (H2)

Antihistamine Classification

  • First-Generation Antihistamines:

    • Common Issues: Enhanced sedation due to BBB permeability.

    • Examples: Diphenhydramine (Benadryl), Hydroxyzine, Cetirizine.

  • Second-Generation Antihistamines:

    • Benefits: Non-sedating, minimal interactions with CNS, safer profile. Examples include Desloratadine and Cetirizine.

First-Generation H1 Antagonists

  • Classes:

    • Ethylenediamines

    • Ethanolamine ethers

    • Piperazines

    • Propylamines

    • Phenothiazines

    • Dibenzoheptanes

  • Physicochemical Properties:

    • Halogen substitution on aromatic rings increases potency.

    • General structure includes bulky aromatic groups.

Side Effects and Toxicities

  • Common Side Effects:

    • Sedation

    • Antiserotonergic effects

    • Anticholinergic effects (e.g., dry mouth, urinary retention, constipation)

  • Enhanced Polarity: Higher structure-functional interactions lead to variable side effects.

H2 Antagonists - Mechanism and Applications

  • Mechanism of Action:

    • H2 antagonists target gastric H+/K+ ATPase pumps reducing acid production.

  • Therapeutic Applications:

    • Management of peptic ulcers and gastroesophageal reflux disease (GERD).

    • Reduces symptoms of indigestion and aids in H. pylori eradication.

Proton Pump Inhibitors (PPIs)

  • Mechanism:

    • Inhibit gastric proton pumps (H+/K+ ATPase).

  • Key Examples:

    • Omeprazole: First PPI, enteric-coated for stability in acidic environments, requires activation under acidic conditions.

    • Esomeprazole: S-enantiomer of omeprazole, higher bioavailability, less variability in response.

    • Lansoprazole and Dexlansoprazole: Broad therapeutic applications in acid-related disorders.

Drug Development Insights

  • General Observations:

    • Drug modifications can lead to enhanced efficacy or reduced toxicity (e.g., Cimetidine's less toxic derivatives - Ranitidine, Nizatidine).

  • Metabolism Insights:

    • The role of CYP450 enzymes in the metabolism of these drugs, influencing both pharmacokinetics and drug interactions.

Readiness Assessments

  • Identify and characterize substances based on their structures and pharmacological properties:

    • Assess sedative effects across different classes.

    • Determine engagement in specific receptor activities (e.g., antiserotonin).

    • Identify metabolites and their roles in therapeutic efficacy.

Conclusion

  • Antihistamines have a crucial role in treating allergic reactions and acid-related conditions. The understanding of chemical structures, mechanisms of action, and side effects guide effective therapeutic use.