Tenofovir

Tenofovir disoproxil is an antiretroviral medication used to prevent and treat HIV/AIDS and to treat chronic hepatitis B. The active substance is tenofovir, while tenofovir disoproxil is a prodrug that is used because of its better absorption in the gut.

The drug is on the World Health Organization's List of Essential Medicines, the most important medications needed in a basic health system. It is marketed by Gilead Sciences under the trade name Viread (as the fumarate, TDF). As of 2015 the cost for a typical month of medication in the United States is more than 200 USD.

Medical uses

 * HIV-1 infection: Tenofovir is indicated in combination with other antiretroviral agents for the treatment of HIV-1 infection in adults and pediatric patients 2 years of age and older. This indication is based on analyses of plasma HIV-1 RNA levels and CD4 cell counts in controlled studies of tenofovir in treatment-naive and treatment-experienced adults.
 * Tenofovir is indicated for the treatment of chronic hepatitis B in adults and pediatric patients 12 years of age and older.

HIV risk reduction
A Cochrane review examined the use of tenofovir for prevention of HIV before exposure. It found that both tenofovir alone and the tenofovir/emtricitabine combination decreased the risk of contracting HIV.

The U. S. Centers for Disease Control and Prevention (CDC) conducted a study in partnership with the Thailand Ministry of Public Health to ascertain the effectiveness of providing people who inject drugs illicitly with daily doses of the antiretroviral drug tenofovir as a prevention measure. The results of the study were released in mid-June 2013 and revealed a 48.9%-reduced incidence of the virus among the group of subjects who received the drug, in comparison to the control group who received a placebo. The principal investigator of the study stated: "We now know that pre-exposure prophylaxis can be a potentially vital option for HIV prevention in people at very high risk for infection, whether through sexual transmission or injecting drug use."

Adverse effects
The most common side effects associated with tenofovir include nausea, vomiting, diarrhea, and asthenia. Less frequent side effects include hepatotoxicity, abdominal pain, and flatulence. Tenofovir has also been implicated in causing renal toxicity, particularly at elevated concentrations.

Tenofovir can cause acute renal failure, Fanconi syndrome, proteinuria, or tubular necrosis. These side effects are due to accumulation of the drug in proximal tubules. Tenofovir can interact with didanosine by increasing didanosine's concentration. It also decreases the concentration of atazanavir sulfate.

Mechanism of action
Tenofovir is a defective adenosine nucleotide that selectively inhibits the action of reverse transcriptase, while only weakly interfering with mammalian DNA polymerases α, β, and mitochondrial DNA polymerase γ. Tenofovir prevents the formation of the 5′ to 3′ phosphodiester linkage essential for DNA chain elongation. A phosphodiester bond cannot be formed because the tenofovir molecule lacks an —OH group on the 3′ carbon of its deoxyribose sugar. Once incorporated into a growing DNA strand, tenofovir causes premature termination of DNA transcription. The drug is classified as a nucleotide analogue reverse transcriptase inhibitor (NtRTI). Reverse transcriptase is a crucial viral enzyme in retroviruses such as human immunodeficiency virus (HIV) and in hepatitis B virus infections.

History
Tenofovir was initially synthesized by Antonín Holý at the Institute of Organic Chemistry and Biochemistry of the Academy of Sciences of the Czech Republic in Prague. The patent filed by Holý in 1984 makes no mention of the potential use of the compound for the treatment of HIV infection, which had only been discovered one year earlier.

In 1985, De Clercq and Holý described the activity of PMPA against HIV in cell culture. Shortly thereafter, a collaboration with the biotechnology company Gilead Sciences led to the investigation of PMPA's potential as a treatment for HIV infected patients. In 1997 researchers from Gilead and the University of California, San Francisco demonstrated that tenofovir exhibits anti-HIV effects in humans when dosed by subcutaneous injection.

The initial form of tenofovir used in these studies had limited potential for widespread use because it was not absorbed when administered orally. A medicinal chemistry team at Gilead developed a modified version of tenofovir, tenofovir disoproxil. This version of tenofovir is often referred to simply as "tenofovir". In this version of the drug, the two negative charges of the tenofovir phosphonic acid group are masked, thus enhancing oral absorption.

Tenofovir disoproxil was approved by the U.S. FDA on October 26, 2001, for the treatment of HIV, and on August 11, 2008, for the treatment of chronic hepatitis B.

Drug forms
Tenofovir disoproxil is a prodrug form of tenofovir. It is also marketed under the brand name Reviro by Dr. Reddy's Laboratories. Tenofovir is also available in a fixed-dose combination with emtricitabine in a product with the brand name Truvada for once-a-day dosing. Efavirenz/emtricitabine/tenofovir disoproxil (brand name Atripla) — a fixed-dose triple combination of tenofovir, emtricitabine, and efavirenz, was approved by the FDA on 12 July 2006 and is now available, providing a single daily dose for the treatment of HIV.

Therapeutic drug monitoring
Tenofovir may be measured in plasma by liquid chromatography. Such testing is useful for monitoring therapy and to prevent drug accumulation and toxicity in people with kidney or liver problems.