1. Clinical Overview
A fixed-dose combination (FDC) of two nucleoside/nucleotide reverse transcriptase inhibitors (NRTIs) used primarily for the treatment of HIV-1 infection and for pre-exposure prophylaxis (PrEP) against HIV. Emtricitabine is a cytosine analogue, and tenofovir disoproxil fumarate (TDF) is an acyclic nucleoside phosphonate diester analogue of adenosine monophosphate. This combination is a cornerstone of first-line antiretroviral therapy (ART) in India and globally, known for its high efficacy, tolerability, and once-daily dosing.
| Onset | Duration | Bioavailability |
|---|---|---|
| Antiviral effect begins within hours of administration, but significant virological suppression (reduction in plasma HIV-1 RNA) is typically observed within 2-4 weeks of initiating therapy. | Approximately 24 hours, supporting once-daily dosing. The intracellular half-lives of the active metabolites are long (Emtricitabine-TP: ~39 hours; Tenofovir-DP: >150 hours). | Emtricitabine: ~93%. Tenofovir disoproxil fumarate: ~25% (fasted). Bioavailability of TDF increases to ~39% with a high-fat meal. |
2. Mechanism of Action
The combination inhibits the reverse transcriptase enzyme of HIV-1, a critical enzyme for viral replication. Emtricitabine and tenofovir are both analogues of natural nucleosides/nucleotides. They are phosphorylated intracellularly by cellular kinases to their active diphosphate (Emtricitabine 5'-triphosphate, FTC-TP) and diphosphate (Tenofovir diphosphate, TFV-DP) forms, respectively. These active metabolites compete with the natural substrates (deoxycytidine 5'-triphosphate for FTC-TP and deoxyadenosine 5'-triphosphate for TFV-DP) for incorporation into the growing viral DNA chain by HIV-1 reverse transcriptase. Once incorporated, they act as chain terminators because they lack the 3'-hydroxyl group needed to form the next phosphodiester bond, thereby halting viral DNA synthesis and replication.
3. Indications & Uses
- Treatment of HIV-1 infection in adults and pediatric patients (weighing at least 17 kg) in combination with other antiretroviral agents.
- Pre-exposure Prophylaxis (PrEP) to reduce the risk of sexually acquired HIV-1 in at-risk adults and adolescents (weighing at least 35 kg).
4. Dosage & Administration
Adult Dosage: One tablet (containing Emtricitabine 200mg + Tenofovir disoproxil fumarate 300mg) orally, once daily, with or without food.
Administration: Swallow the tablet whole with water. Can be taken with or without food, but a high-fat meal increases tenofovir absorption. For PrEP, adherence is critical for effectiveness. Must be taken daily, not just before/after sexual activity.
5. Side Effects
Common side effects may include:
- Headache
- Diarrhea
- Nausea
- Fatigue
- Dizziness
- Rash
- Abdominal pain
- Flatulence
- Insomnia
- Abnormal dreams
6. Drug Interactions
| Drug | Effect | Severity |
|---|---|---|
| Didanosine (ddI) | Increased didanosine concentrations and potential for didanosine-related adverse effects (pancreatitis, neuropathy). Tenofovir may increase ddI levels. | Major |
| Adefovir dipivoxil | Additive nephrotoxicity potential. | Major |
| Other nephrotoxic drugs (e.g., Aminoglycosides, Amphotericin B, Foscarnet, Ganciclovir, Vancomycin, NSAIDs like Ibuprofen) | Increased risk of renal impairment. | Major |
| Probenecid, Cidofovir | May increase tenofovir concentrations by competing for renal tubular secretion. | Moderate |
| Ledipasvir/Sofosbuvir or other HCV regimens containing Velpatasvir | May increase tenofovir exposure; monitor for tenofovir-associated adverse effects, especially renal. | Moderate |
| Rifampin, Rifabutin | May decrease tenofovir concentrations slightly; not considered clinically significant for efficacy. | Minor |
| Atazanavir (boosted with Ritonavir) | Increases tenofovir exposure. Monitor for renal toxicity. | Moderate |
7. Patient Counselling
- DO take exactly as prescribed, once daily. Adherence is crucial for success.
- DO get regular blood tests as advised by your doctor (HIV viral load, CD4 count, kidney function, bone health).
- DO inform all your healthcare providers you are taking this medicine.
- DO use additional barrier protection (condoms) to prevent HIV and other STIs, even while on PrEP.
- DONT stop taking this medicine without consulting your doctor, especially if you have Hepatitis B.
- DONT share your medication with anyone else.
- DONT take other medicines, including herbal supplements, without informing your doctor.
8. Toxicology & Storage
Overdose: Limited data. Highest dose studied: Emtricitabine 1200mg + Tenofovir DF 600mg single dose. Symptoms may be consistent with the known side effect profile, particularly nausea, vomiting, dizziness, and potentially renal impairment.
Storage: Store below 30°C. Keep in the original blister pack or bottle to protect from moisture. Keep out of reach and sight of children.