Diphenhydramine pharmacology
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Diphenhydramine Mechanism of Action and Receptor Pharmacology
Diphenhydramine is a first-generation antihistamine that works by competitively blocking histamine H1 receptors. This action prevents histamine from binding to its receptors on bronchial smooth muscle, capillaries, and gastrointestinal smooth muscle, thereby reducing symptoms such as bronchoconstriction, vasodilation, increased capillary permeability, and GI smooth muscle spasms 78. In addition to its primary action on H1 receptors, diphenhydramine has been shown to have similar binding affinity to H1 receptors as orphenadrine, but it has lower affinity for NMDA receptors, which may explain differences in their pharmacological effects .
Pharmacokinetics: Absorption, Distribution, and Elimination
Diphenhydramine exhibits variable oral absorption across species. In dogs, oral bioavailability is low (about 7.8%), but increases to 22% when administered as dimenhydrinate, a salt form that enhances absorption and prolongs half-life . In horses, oral bioavailability is even lower, with less than 1% at lower doses and up to 6% at higher doses, making oral administration unlikely to achieve therapeutic plasma levels . After intravenous administration, diphenhydramine is rapidly distributed and eliminated, but the terminal elimination half-life can vary significantly depending on the formulation and species 14.
Protein Binding and Use in Special Populations
Diphenhydramine is highly bound to plasma proteins, particularly albumin. This high protein binding limits its removal by dialysis, which is important for patients with end-stage kidney disease (ESKD). There is limited data on the safety and dosing of diphenhydramine in ESKD, and its use should be approached with caution due to the risk of accumulation and side effects .
Drug Interactions and Safety Considerations
Diphenhydramine is a potent inhibitor of the CYP2D6 enzyme. This can lead to dangerous drug-drug interactions, especially with medications metabolized by CYP2D6, such as hydrocodone. Inhibition of this enzyme can impair the metabolism of hydrocodone, leading to increased levels and risk of toxicity or fatal overdose, as highlighted in a case report .
Additional Pharmacological Effects
Beyond its antihistaminic effects, diphenhydramine has been shown to modulate the effects of other drugs. For example, it can increase the therapeutic window of platinum-based chemotherapy drugs by sensitizing tumor cells to treatment while protecting normal cells from toxicity. This is achieved by affecting drug transport mechanisms and reducing the activity of efflux pumps in cancer cells .
Conclusion
Diphenhydramine is a widely used first-generation antihistamine with well-established H1 receptor antagonism, but its pharmacokinetics can vary greatly depending on the route of administration and species. Its high protein binding and CYP2D6 inhibition present important considerations for use in special populations and in combination with other drugs. While primarily used for allergic symptoms, diphenhydramine also has emerging roles in cancer therapy modulation and requires careful attention to dosing and potential interactions.
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