Entecavir

BRAND NAMES

1. Baraclude – the most widely recognized brand, originally by Bristol-Myers Squibb 

2. Entacavir – generic formulations used globally.

MECHANISM OF ACTION

Entecavir is a nucleoside analogue reverse transcriptase inhibitor (NRTI) that targets the hepatitis B virus (HBV) polymerase. Once phosphorylated inside hepatocytes to its active triphosphate form, entecavir competes with natural nucleosides during viral DNA synthesis.

PHARMACOKINETICS

Absorption

Entecavir is rapidly absorbed after oral administration, with peak plasma concentrations typically reached within 0.5 to 1.5 hours in fasting individuals.

Distribution

Entecavir has a moderate volume of distribution, approximately 0.8–1.3 L/kg, indicating that it distributes well throughout body tissues, including the liver, which is the primary site of hepatitis B virus replication.

Metabolism

Entecavir undergoes minimal metabolism in the body. The majority of the drug remains in its active, unchanged form, which contributes to its predictable pharmacokinetics and consistent antiviral activity.

Elimination

Entecavir is primarily eliminated through renal excretion, with approximately 80–90% of the administered dose excreted unchanged in the urine. Its elimination half-life ranges from 128 to 149 hours in patients with normal renal function, allowing for once-daily dosing.

PHARMACODYNAMICS

Entecavir exhibits potent, selective antiviral activity against hepatitis B virus (HBV) by inhibiting viral DNA synthesis. Its active triphosphate form blocks multiple steps of HBV polymerase activity, including base priming, reverse transcription of the negative DNA strand, and synthesis of the positive DNA strand, leading to suppression of viral replication.

ADMINISTRATION

Entecavir is administered orally in tablet form, usually once daily, with or without food. Tablets are available in different strengths, and the specific dose depends on the patient’s age, weight, renal function, and prior antiviral treatment history.

DOSAGE AND STRENGTH

Entecavir is available in 0.5 mg and 1 mg oral tablets, with dosing tailored to the patient’s treatment history and renal function. For treatment-naïve adults with chronic hepatitis B, the usual dose is 0.5 mg once daily, while patients with lamivudine-resistant HBV typically receive 1 mg once daily.

DRUG INTERACTIONS

Entecavir has a relatively low potential for drug-drug interactions because it undergoes minimal metabolism and is primarily eliminated unchanged by the kidneys. However, co-administration with nephrotoxic drugs (e.g., aminoglycosides, amphotericin B, or high-dose NSAIDs) may increase the risk of renal toxicity, since impaired kidney function can reduce entecavir clearance.

FOOD INTERACTIONS

Entecavir can be taken with or without food, as food has minimal impact on its absorption or antiviral efficacy. While high-fat meals may slightly delay the time to reach peak plasma concentrations, the overall bioavailability of the drug remains essentially unchanged.

CONTRAINDICATIONS

Entecavir is contraindicated in patients with known hypersensitivity to entecavir or any component of the formulation. It should not be used in patients with decompensated liver disease without close monitoring, as abrupt discontinuation can lead to severe exacerbation of hepatitis B.

SIDE EFFECTS

• Headache 

• Fatigue 

• Dizziness 

• Nausea 

• Vomiting 

• Diarrhea

TOXICITY

Entecavir exhibits a low toxicity profile at therapeutic doses, making it generally well tolerated in adults and children. Toxic effects are rare but can occur with overdose or in patients with severe renal impairment.