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Pharmacology: Pharmacodynamics: HBV polymerase (reverse transcriptase, rt): base priming, reverse transcription of the negative strand from the pregenomic messenger rna, and synthesis of the positive strand of HBV DNA. Upon activation by kinases, the drug can be incorporated into the DNA which has the ultimate effect of inhibiting the HBV polymerase activity.
Pharmacokinetics: Entecavir is absorbed from the gastrointestinal tract after doses. Peak plasma concentrations occur 30 to 90 minutes after a dose and steady state concentration after 6 to 10 days of treatment. Bioavailability of the tablet formulation is equal to that of the oral solution and they may be given interchangeably. Binding of entecavir to plasma proteins is about 13% in vitro. Entecavir is not metabolized by the cytochrome P450 system. It is mainly eliminated by the kidneys by glomerular filtration and active tubular secretion, with a terminal elimination half-life of 128 to 149 hours. Minor amounts of glucuronide and sulfate conjugate metabolites occur. Entecavir is partially removed by hemodialysis.
Microbiology: Antiviral Action: Entecavir is phosphorylated intracellularly to the active triphosphate form which competes with deoxyguanosine triphosphate, the natural substrate of hepatitis B virus reverse transcriptase, thereby inhibiting every stage of the enzyme's activity.
