also known as TDF is a so-called 'prodrug' with the active compound deactivated by a molecular side chain that dissolves in the human body allowing a low dose of tenofovir to reach the site of desired activity. One example of the prodrug form is tenofovir disoproxil fumarate with the trade name Viread (Gilead Sciences Inc USA). It is approved in the US for the treatment of both HIV and hepatitis B. Adefovir, also known as ADV or bis-POM PMPA, has trade names Preveon and Hepsera. It is not approved by the FDA for treatment of HIV due to toxicity issues, but a lower dose is approved for the treatment of hepatitis B. While often listed in chronological order, NRTIs/NtRTIs are nucleoside/nucleotide analogues of cytidine, guanosine, thymidine and adenosine: Thymidine analogues: zidovudine (AZT) and stavudine (d4T) Cytidine analogues: zalcitabine (ddC), lamivudine (3TC), and emtricitabine (FTC) Guanosine analogues: abacavir (ABC) and entecavir (ETV) Adenosine analogues: didanosine (ddI), tenofovir (TDF), and adefovir (ADV) Non-nucleoside reverse-transcriptase inhibitors (NNRTIs) Non-nucleoside reverse-transcriptase inhibitors (NNRTIs) are the third class of antiretroviral drugs that were developed. In all cases, patents remain in force until beyond 2007. This class of drugs was first described at the Rega Institute for Medical Research (Belgium). Efavirenz has the trade names Sustiva and Stocrin. Nevirapine has the trade name Viramune. Delavirdine, currently rarely used, has the trade name Rescriptor. Etravirine has the trade name Intelence, and was approved by the FDA in 2008. Rilpivirine has the trade name Edurant, and was approved by the FDA in May 2011. Doravirine (MK-1439), also called Pifeltro, is a non-nucleoside reverse transcriptase inhibitor developed by Merck & Co. for use in the treatment of HIV/AIDS. In August 2018, the FDA approved doravirine. It is also used in a combination tablet as doravirine/lamivudine/tenofovir disoproxil fumarate (Delstrigo). Nucleoside reverse transcriptase translocation inhibitor (NRTTIs) This is a new class of antivirals, MK-8591 or Islatravir being the first agent of this group. Islatravir was developed by Merck & Co. It is orally available, long acting antiviral, being tested as ART against HIV-1. Portmanteau inhibitors Researchers have designed molecules which dually inhibit both reverse transcriptase (RT) and integrase (IN). These drugs are a type of "portmanteau inhibitors". Mechanisms of resistance to reverse transcriptase inhibitors While NRTIs and NNRTIs alike are effective at terminating DNA synthesis and HIV replication, HIV can and eventually does develop mechanisms that confer the virus resistance to the