mechanisms of action and resistance Lamivudine triphosphate potently inhibits the DNA polymerase/reverse transcriptase of HBV. Lamivudine has enhanced antiviral activity against hepadnaviruses when combined with adefovir or penciclovir. Point mutations in the HBV DNA polymerase markedly reduce sensitivity. Lamivudine resistance confers cross-resistance to agents such as emtricitabine and is often associated with an additional mutation that confers cross-resistance to famciclovir. Lamivudine-resistant HBV retains susceptibility to adefovir and partially to entecavir. Viruses bearing certain mutations are less replication competent than wildtype HBV, but lamivudine resistance is associated with elevated HBV DNA levels, decreased likelihood of HbeAg loss or seroconversion, hepatitis exacerbations, and progressive fibrosis and graft loss in transplant recipients.
ABSORPTION, DISTRIBUTION, AND ELIMINATION
Following oral administration, lamivudine is absorbed rapidly with a bioavailability of ~80% in adults. Lamivudine is distributed widely in a volume comparable with total-body water. The plasma t/2 of elimination averages ~9 hours, and ~70% of the dose is excreted unchanged in the urine. About 5% is metabolized to an inactive trans-sulfoxide metabolite. In HBV-infected children, doses of 3 mg/kg/day provide drug levels comparable with those in adults receiving 100 mg daily. Dose reductions are indicated for creatinine clearance <50 mL/min.
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