1. Introduction

Lamivudine (3′-hydroxy-3′-thiacidopyrimidine, 3TC) is a nucleoside reverse transcriptase inhibitor (NRTI) and one of the most widely used antiviral agents in the management of HIV infection and chronic hepatitis B virus (HBV) disease. Its broad-spectrum activity against both HIV-1 and HIV-2, as well as HBV, makes it an essential component of combination antiretroviral therapy (cART) and monotherapy for HBV when other options are not available or tolerated.

2. Mechanism of Action

Lamivudine is a synthetic nucleoside analogue that, after intracellular phosphorylation to its active triphosphate metabolite (lamivudine triphosphate, L-TP), competitively inhibits viral reverse transcriptase. Incorporation of L-TP into nascent viral DNA by HIV reverse transcriptase or HBV polymerase results in premature chain termination due to the absence of a 3’-hydroxyl group. This mechanism is highly selective for viral polymerases over host DNA polymerases, minimizing cytotoxicity at therapeutic doses.

3. Indications

• HIV Infection:
  • Used as part of combination therapy in adults and children ≥3 months of age.
  • Recommended for both HIV-1 and HIV-2 (though resistance patterns may differ).
• Chronic Hepatitis B Virus (HBV) Infection:
  • First-line or alternative therapy when other agents are contraindicated, not tolerated, or unavailable.
  • Used as monotherapy in patients with HBV without concurrent HIV coinfection.

4. Dosage and Administration

Adults and Adolescents (≥25 kg)

• HIV:
  • 300 mg once daily OR 150 mg twice daily.
• Hepatitis B:
  • 100 mg once daily.

Children

• Ages 3 months to <25 kg:
  • Dose is weight-based, typically 3–5 mg/kg/day divided into 1–2 doses (max 100 mg/day).
  • Oral solution available for those unable to swallow tablets.
• Ages ≥25 kg:
  • Same as adults.

Special Populations

• Renal Impairment:
  • Dose adjustment required; monitor renal function regularly.
  • Consider reduced dosing in severe renal impairment (e.g., CrCl <30 mL/min).
• Pregnancy & Lactation:
  • Use only if clearly needed and benefit outweighs risk.
  • Avoid first trimester for chronic HBV treatment; use with caution during pregnancy.

Drug Combinations

• Lamivudine is often combined with zidovudine (AZT), abacavir, or tenofovir in HIV regimens.
• Avoid concurrent use with emtricitabine due to increased risk of mitochondrial toxicity and lactic acidosis.
• Caution with trimethoprim/sulfamethoxazole (risk of hypersensitivity reactions); monitor for signs of infection.

5. Pharmacokinetics

• Absorption: Good oral bioavailability (~50–80%).
• Distribution: Widely distributed in body water compartments.
• Metabolism: Minimal hepatic metabolism; primarily excreted unchanged renally.
• Half-life: ~5–6 hours (longer in renal impairment).

6. Adverse Effects

Common (≥1% of patients)

• Nausea, diarrhea
• Headache, fatigue, malaise
• Cough, upper respiratory symptoms
• Transient elevations in liver enzymes

Serious (less common but clinically significant)

• Lactic acidosis: Rare but potentially fatal; higher risk in patients with renal impairment, obesity, or diabetes.
• Severe hypersensitivity reactions: Including rash, fever, hepatotoxicity—especially when combined with other NRTIs.
• Pancreatitis: Rare but reported; discontinue if symptoms arise.

Management

• Monitor for signs of infection, especially in immunocompromised patients.
• Assess renal function regularly (baseline and periodically).
• Educate patients on early symptoms of lactic acidosis (e.g., muscle pain, abdominal pain, nausea/vomiting).

7. Drug Interactions

• Avoid concurrent use with emtricitabine due to increased risk of mitochondrial toxicity.
• Trimethoprim/sulfamethoxazole: Risk of hypersensitivity; monitor closely.
• Sorbitol-containing products: May cause gastrointestinal upset.
• Other NRTIs: Potential for additive toxicities (e.g., renal, hepatic).

8. Contraindications

• Hypersensitivity to lamivudine or any excipients.
• First trimester of pregnancy for chronic HBV therapy (use only if benefits outweigh risks).
• Concurrent use with emtricitabine is contraindicated due to increased risk of adverse effects.

9. Storage and Handling

• Store at 20–25°C (68–77°F); protect from moisture.
• Oral solution should be kept in original container, protected from light and heat.

10. Clinical Pearls for Pharmacists

• Adherence is critical to prevent resistance—emphasize importance of consistent dosing.
• Renal function monitoring: Essential in all patients, especially elderly or those with comorbidities.
• Pregnancy counseling: Discuss risks/benefits and alternatives if applicable.
• Drug interaction awareness: Review patient’s medication list for contraindicated combinations.
• Patient education: Teach recognition of lactic acidosis symptoms and when to seek care.

11. References & Guidelines

• DHHS Guidelines (2023): Recommend lamivudine as part of first-line regimens in HIV and HBV.
• WHO Guidelines on HIV (2022): Emphasize combination therapy with NRTIs including lamivudine.
• FDA Labeling: Lamivudine 300 mg Tablets, Epivir-HBV Oral Solution
• UpToDate: Management of HIV and chronic hepatitis B.

Summary:
Lamivudine remains a cornerstone in the management of HIV and HBV due to its efficacy, safety profile, and broad applicability. Pharmacists play a vital role in ensuring correct dosing, monitoring for adverse effects, managing drug interactions, and providing patient education to optimize outcomes and minimize complications.