Oseltamivir phosphate is an ethyl ester prodrug. Oseltamivir carboxylate has an antiviral spectrum and potency similar to that of zanamivir. It inhibits amantadine- and rimantadine-resistant influenza A viruses and some zanamivir-resistant variants.
mechanisms of action and resistance Influenza neuraminidase cleaves terminal sialic acid residues and destroys the receptors recognized by viral hemagglutinin, which are present on the cell surface, in progeny virions, and in respiratory secretions, which is essential for virus release from infected cells. Oseltamivir carboxylate causes a conformational change in neu-raminidase's active site and inhibits its activity, leading to viral aggregation at the cell surface and reduced virus spread within the respiratory tract.
Influenza variants selected in vitro for resistance to oseltamivir carboxylate contain hemagglu-tinin and/or neuraminidase mutations. The most commonly recognized resistant variants have reduced infectivity and virulence in animal models. Outpatient oseltamivir therapy has been associated with recovery of resistant variants in ~0.5% of adults and 5.5% of children; a higher frequency (~18%) occurs in hospitalized children.
ABSORPTION, DISTRIBUTION, AND ELIMINATION
Oral oseltamivir phosphate is absorbed rapidly (Table 49-3) and cleaved to the active carboxylate by esterases in the GI tract and liver. The bioavailability of the carboxylate is ~80%; food does not decrease bioavailability but reduces GI symptoms. The carboxylate has a volume of distribution similar to extracellular water. Both prodrug and active metabolite are eliminated primarily unchanged by the kidney.
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