What is Tenofovir Disoproxil Fumarate? Tenofovir Disoproxil Fumarate (TDF) is an orally active fumaric acid salt prodrug of tenofovir, a nucleotide reverse transcriptase inhibitor (NRTI). Approved by the US FDA in 2001 under the brand name Viread, it is a cornerstone antiretroviral drug. It is widely used in combination regimens for the treatment of HIV-1 infection and as a standalone agent for the treatment of chronic Hepatitis B virus (HBV) infection. TDF has played a vital role in global HIV management and prevention, serving as a primary component of pre-exposure prophylaxis (PrEP) regimens. While highly effective, long-term use requires monitoring of renal function and bone density due to class-specific toxicities. Mechanism of Action Tenofovir disoproxil fumarate is an acyclic nucleoside phosphonate diester analog of adenosine 5′-monophosphate. After oral administration, it is absorbed and converted to tenofovir, which is then phosphorylated by cellular kinases to the active metabolite, tenofovir diphosphate. Tenofovir diphosphate inhibits the replication of viral genomes by competing with the natural substrate, deoxyadenosine 5′-triphosphate, for incorporation into viral DNA. Once incorporated, tenofovir diphosphate causes DNA chain termination because it lacks the 3′-hydroxyl group required to form a phosphodiester bond with the next nucleotide. This mechanism inhibits both the HIV-1 reverse transcriptase enzyme and the Hepatitis B virus polymerase, halting replication of both viruses. Pharmacokinetics & Elimination Following oral administration of TDF to fasting patients, maximum plasma concentrations (Cmax) are achieved within 1 hour. The oral bioavailability of tenofovir from TDF is approximately 25%. Administration with a high-fat meal increases bioavailability, resulting in a 40% increase in tenofovir AUC and a 14% increase in Cmax. Tenofovir is widely distributed throughout body tissues, with low plasma protein binding (< 7.2%). The intracellular half-life of active tenofovir diphosphate exceeds 60 hours in peripheral blood mononuclear cells, supporting once-daily dosing. Tenofovir is eliminated primarily by the kidneys, through a combination of glomerular filtration and active tubular secretion. Approximately 70% to 80% of the dose is recovered as unchanged tenofovir in the urine within 72 hours. Because tenofovir is cleared renally, dose adjustments or alternative agents are required in patients with moderate to severe renal impairment (creatinine clearance < 50 mL/min). Clinical Indications & Dosage Tenofovir Disoproxil Fumarate is indicated for: HIV-1 Infection: In combination with other antiretroviral agents for the treatment of HIV-1 infection in adults and pediatric patients weighing at least 17 kg. Chronic Hepatitis B: For the treatment of chronic HBV infection in adults and pediatric patients 2 years of age and older weighing at least 10 kg. HIV-1 PrEP: In combination with Emtricitabine for pre-exposure prophylaxis to prevent sexual acquisition of HIV-1 in high-risk individuals. The standard dosage for adults is 300 mg taken orally once daily, with or without food. In patients with moderate renal impairment (CrCl 30–49 mL/min), the dosing interval is adjusted to 300 mg every 48 hours. It is contraindicated for PrEP in individuals with a creatinine clearance < 60 mL/min. Warnings & Safety Profile Long-term TDF therapy is associated with specific safety warnings: Renal Toxicity: Can cause renal impairment, including proximal renal tubulopathy (Fanconi syndrome) and acute kidney injury. Renal function (creatinine clearance, urine protein, and phosphorus) must be assessed prior to starting and monitored regularly. Bone Mineral Density Decrease: Associated with bone mineral density loss and increased bone turnover. Calcium and vitamin D supplementation should be considered for patients at risk. Hepatitis B Exacerbation: Discontinuation of TDF in patients co-infected with HIV and HBV can cause severe, acute exacerbations of Hepatitis B. Hepatic function must be monitored closely for several months after stopping treatment. Frequently Asked Questions What is the difference between TDF and TAF? Tenofovir Alafenamide (TAF) is a newer prodrug of tenofovir. TAF delivers the active drug more efficiently to target cells, allowing for a much lower dose (typically 25 mg vs. 300 mg of TDF). This results in lower blood levels of tenofovir, significantly reducing the risk of renal and bone side effects compared to TDF. Can TDF be taken during pregnancy? Yes, TDF is widely used during pregnancy for both HIV treatment and prevention of mother-to-child transmission of HIV and Hepatitis B. Large clinical registries have shown no increased risk of major birth defects associated with its use.