Antihistamine

Antihistamines

Antihistamines primarily act to block histamine effects that occur in an immediate (type I) hypersensitivity reaction, commonly called an allergic reaction. They’re available alone or in combination products by prescription or over-the-counter.

Histamine-1 receptor antagonists

The term antihistamine refers to drugs that act as histamine-1 (H₁) receptor antagonists; that is, they compete with histamine for binding to H₁-receptor sites throughout the body. However, they don’t displace histamine already bound to the receptor.

It’s all about chemistry

Based on chemical structure, antihistamines are categorized into five major classes:

• Ethanolamines include clemastine fumarate, dimenhydrinate, and diphenhydramine hydrochloride.
• Alkylamines include brompheniramine maleate, chlorphenir-amine maleate, and dexchlorpheniramine maleate.
• Phenothiazines include promethazine hydrochloride.
• Piperidines include azatadine maleate, cetirizine hydrochloride, cyproheptadine hydrochloride, desloratadine, fexofenadine hydrochloride, loratadine, and meclizine hydrochloride.
• Miscellaneous drugs, such as hydroxyzine hydrochloride and hydroxyzine pamoate, also act as antihistamines.

Pharmacokinetics (how drugs circulate)

H₁-receptor antagonists are well absorbed after oral or parenteral administration. Some can also be given rectally.

Distribution

With the exception of loratadine and desloratadine, antihistamines are distributed widely throughout the body and central nervous system (CNS).

Less penetration, fewer effects

Fexofenadine, desloratadine, and loratadine, which are nonsedating antihistamines, minimally penetrate the blood-brain barrier so that little of the drug is distributed in the CNS, producing fewer effects there than other antihistamines.

Memory jogger

Anti- is a familiar prefix meaning “opposing.” That’s exactly what antihistamines do: They oppose histamine effects (or allergic reactions).

Metabolism and excretion

Antihistamines are metabolized by liver enzymes and excreted in urine; small amounts appear in breast milk. Fexofenadine, mainly excreted in stool, is an exception. Cetirizine undergoes limited hepatic metabolism.

Pharmacodynamics (how drugs act)

H₁-receptor antagonists compete with histamine for H₁ receptors on effector cells (the cells that cause allergic symptoms), blocking histamine from producing its effects.

Antagonizing tactics

H₁-receptor antagonists produce their effects by:

• blocking the action of histamine on the small blood vessels
• decreasing dilation of arterioles and engorgement of tissues
• reducing the leakage of plasma proteins and fluids out of the capillaries (capillary permeability), thereby lessening edema
• inhibiting most smooth-muscle responses to histamine (in particular, blocking the constriction of bronchial, GI, and vascular smooth muscle)
• relieving symptoms by acting on the terminal nerve endings in the skin that flare and itch when stimulated by histamine
• suppressing adrenal medulla stimulation, autonomic ganglia stimulation, and exocrine gland secretion, such as lacrimal and salivary secretion.

Straight to the head

Several antihistamines have a high affinity for H₁ receptors in the brain and are used for their CNS effects. These drugs include diphenhydramine, dimenhydrinate, promethazine, and various piperidine derivatives.

No stomach for this

H₁-receptor antagonists don’t affect parietal cell secretion in the stomach because their receptors are H₂ receptors, not H₁

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Pharmacotherapeutics (how drugs are used)

Antihistamines are used to treat the symptoms of type I hypersensitivity reactions, such as:

• allergic rhinitis (runny nose and itchy eyes caused by a local sensitivity reaction)
• vasomotor rhinitis (rhinitis not caused by allergy or infection)
• allergic conjunctivitis (inflammation of the membranes of the eye)
• urticaria (hives)
• angioedema (submucosal swelling in the hands, face, and feet).

Now I get it!

How chlorpheniramine stops an allergic response

Although chlorpheniramine can’t reverse symptoms of an allergic response, it can stop the progression of the response. Here’s what happens.

Release the mediators

When sensitized to an antigen, a mast cell reacts to repeated antigen exposure by releasing chemical mediators. One of these mediators, histamine, binds to histamine-1 (H₁) receptors found on effector cells (the cells responsible for allergic symptoms). This initiates the allergic response that affects the respiratory, cardiovascular, GI, endocrine, and integumentary systems.

The first one there wins

Chlorpheniramine competes with histamine for H₁-receptor sites on the effector cells. By attaching to these sites first, the drug prevents more histamine from binding to the effector cells.

Not just for allergies

Antihistamines can have other therapeutic uses:

• Many are used primarily as antiemetics (to control nausea and vomiting).
• They can also be used as adjunctive therapy to treat an anaphylactic reaction after the serious symptoms are controlled.
• Diphenhydramine can help treat Parkinson’s disease and drug-induced extrapyramidal reactions (abnormal involuntary movements).
• Because of its antiserotonin qualities, cyproheptadine may be used to treat Cushing’s disease, serotonin-associated diarrhea, vascular cluster headaches, and anorexia nervosa.

Drug interactions

Antihistamines may interact with many drugs, sometimes with life-threatening consequences:

• They may block or reverse the vasopressor effects of epinephrine, producing vasodilation, increased heart rate, and very low blood pressure.
• They may mask the toxic signs and symptoms of ototoxicity (a detrimental effect on hearing) associated with aminoglycosides or large dosages of salicylates.
• They may increase the sedative and respiratory depressant effects of CNS depressants, such as tranquilizers or alcohol.
• Loratadine may cause serious cardiac effects when taken with macrolide antibiotics (such as erythromycin), fluconazole, ketoconazole, itraconazole, miconazole, cimetidine, ciprofloxacin, and clarithromycin.

Warning!

Adverse reactions to antihistamines

The most common adverse reaction to antihistamines (with the exceptions of fexofenadine and loratadine) is central nervous system (CNS) depression. Other CNS reactions include:

• dizziness
• lassitude and fatigue
• disturbed coordination
• muscle weakness.

Gut reactions

GI reactions may include:

• epigastric distress
• loss of appetite
• nausea and vomiting
• constipation
• diarrhea
• dryness of the mouth, nose, and throat.

They can get the heart racing

Cardiovascular reactions may include:

• hypotension
• hypertension
• rapid heart rate
• arrhythmias.

A sensitive issue

Sensitivity reactions can also occur.