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Antihistamines. Histamine receptors – H 1 - Allergic responses. Watery eyes, congestion, etc. from allergies. Anaphylaxis – bronchial larynx constriction. Skin allergic response – reddening, rashes, welts. Edema from injury.
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Antihistamines • Histamine receptors – • H1- Allergic responses. Watery eyes, congestion, etc. from allergies. • Anaphylaxis – bronchial larynx constriction. • Skin allergic response – reddening, rashes, welts. • Edema from injury. • H2 – Gastric secretion. Important for ulcer treatment and acid reflux • H3 – ? CNS receptors. There are also H1 receptors in the CNS. • Antihistamines are also used for motion sickness. In general their antimuscarinic effects are similar to that of scopolamine, although weaker.
Antihistamines – H1 Blockers Ethylenediamines
Antihistamine SAR Aminoalkylethers Piperazine/N-heterocycle Series
Stereo- and regioisomers Antihistamines are stereospecific. A number of new, single isomer drugs are being developed. Antihistamines are structurally similar To SSRIs and to antipsychotics. Notice The subtle difference between fluoxetine And several antihistamines.
Alkyl Amines Long Duration, less sedation than ethylenediamines, ethanolamines. The “next best thing” until the “2nd generation” were developed.
Rigid Analogs (I) A Cl at the 2-position weakens H1 activity relative to antimuscarinic activity and D2 antagonist activity
Rigid Analogs (II) Primethixene Loratadine (Claritin) Desloratadine (Clarinex)
Astemizole Hismanal was FDA approved in 1988 as an antihistamine for allergy and hay fever symptom relief. The FDA first warned consumers and healthcare providers of new safety information regarding Hismanal February 9, 1998 due to the risk of death, cardiovascular adverse events, anaphylaxis, and serious drug interactions. In addition, Hismanal labeling was changed to stress avoiding the use of Hismanal in combination with certain other medications and for liver disorder patients to completely avoid its' use. After a series of labeling changes and warnings Hismanal was recalled on June 21, 1999.
Antihistamines “related” to butyrophenones Terfenadine was discontinued when it became apparent that there was a high frequency of heart arrythmia associated with the drug. Fexofenadine is a metabolite and is the activated form responsible for antihistamine activity. In patients with compromised liver metabolism, or when the presence of other drugs limited the metabolism of terfenadine, persistent levels resulted in the observed arrythmias. Therefore, the fexofenadine replaced terfenadine (1997). Ventricular Arrythmias are not good!
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Structural Summary Linking the first generation with Non-sedative Antihistamines. Big Picture - Bottom Line
Anti-emetic Table Synthetic Tropane Alkaloids
2005 Nobel Prize Press Release: The 2005 Nobel Prize in Physiology or Medicine 3 October 2005 The Nobel Assembly at Karolinska Institutet has today decided to award The Nobel Prize in Physiology or Medicine for 2005 jointly to Barry J. Marshall and J. Robin Warren for their discovery of "the bacterium Helicobacter pylori and its role in gastritis and peptic ulcer disease”
H2 Histamine antagonists. Gastric receptors are pharmacologically distinct. The classic H1 antagonists don’t interact with H2 receptors. Antihistamines are an important treatment for gastric disorders; antacids, ulcer treatment, acid-reflux disease.
