Reverse transcriptase inhibitor
Encyclopedia
Reverse-transcriptase inhibitors (RTIs) are a class of antiretroviral drug
used to treat HIV
infection, tumors, and cancer
. RTIs inhibit activity of reverse transcriptase
, a viral DNA polymerase enzyme
that retroviruses need to reproduce.
genome into a double-stranded viral DNA
. The viral DNA is then integrated into the host chromosomal DNA, which then allows host cellular processes, such as transcription and translation to reproduce the virus. RTIs block reverse transcriptase's enzymatic function and prevent completion of synthesis of the double-stranded viral DNA, thus preventing HIV from multiplying.
A similar process occurs with other types of viruses. The hepatitis B virus, for example, carries its genetic material in the form of DNA, and employs a RNA-dependent DNA polymerase to replicate. Some of the same compounds used as RTIs can also block HBV replication; when used in this way they are referred to as polymerase inhibitors.
The mode of action of NRTIs and NtRTIs is essentially the same; they are analogues of the naturally occurring deoxynucleotides
needed to synthesize the viral DNA and they compete with the natural deoxynucleotides for incorporation into the growing viral DNA chain. However, unlike the natural deoxynucleotides substrates, NRTIs and NtRTIs lack a 3'-hydroxyl group on the deoxyribose moiety. As a result, following incorporation of an NRTI or an NtRTI, the next incoming deoxynucleotide cannot form the next 5'-3' phosphodiester bond needed to extend the DNA chain. Thus, when an NRTI or NtRTI is incorporated, viral DNA synthesis is halted, a process known as chain termination
. All NRTIs and NtRTIs are classified as competitive substrate inhibitors.
In contrast, NNRTIs have a completely different mode of action. NNRTIs block reverse transcriptase by binding at a different site on the enzyme, compared to NRTIs and NtRTIs. NNRTIs are not incorporated into the viral DNA but instead inhibit the movement of protein domains of reverse transcriptase that are needed to carry out the process of DNA synthesis. NNRTIs are therefore classified as non-competitive inhibitors
of reverse transcriptase.
groups to their deoxyribose moiety, to form NRTI triphosphates. This phosphorylation
step is carried out by cellular kinase
enzymes.
Zidovudine : Zidovudine
, also called AZT, ZDV, and azidothymidine, has the trade name Retrovir. Zidovudine was the first antiretroviral drug approved by the FDA for the treatment of HIV.
Didanosine : Didanosine
, also called ddI, with the trade names Videx and Videx EC, was the second FDA-approved antiretroviral drug. It is an analog of adenosine.
Zalcitabine : Zalcitabine, also called ddC and dideoxycytidine, has the trade name Hivid. This drug has been discontinued by the manufacturer.
Stavudine : Stavudine
, also called d4T, has trade names Zerit and Zerit XR.
Lamivudine : Lamivudine
, also called 3TC, has the trade name Zeffix and Epivir. It is approved for the treatment of both HIV and hepatitis B.
Abacavir : Abacavir
, also called ABC, has the trade name Ziagen, is an analog of guanosine.
Emtricitabine : Emtricitabine
, also called FTC, has the trade name Emtriva (formerly Coviracil). Structurally similar to lamivudine, it is approved for the treatment of HIV and undergoing clinical trials for hepatitis B.
Entecavir : Entecavir
, also called ETV, is a guanine analog that has the trade name Baraclude. Though not currently approved as an HIV drug, entecavir is approved for the treatment of hepatitis B.
Apricitabine : Apricitabine, also called ATC. As of 2009, this drug is undergoing Phase-III evaluation, and if successful may achieve FDA approval in 2011.
Tenofovir : Tenofovir, also known as TFV 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 USA for the treatment of both HIV and hepatitis B.
Adefovir : Adefovir
, also known as bis-POM PMPA, has trade names Preveon and Hepsera. It was 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.
(Belgium
)
Efavirenz : Efavirenz
has the trade names Sustiva and Stocrin.
Nevirapine : Nevirapine has the trade name Viramune.
Delavirdine : Delavirdine
, currently rarely used, has the trade name Rescriptor.
Etravirine : Etravirine
has the trade name Intelence, and was approved by the FDA in 2008.
Rilpivirine : Rilpivirine
has the trade name Edurant, and was approved by the FDA in May 2011.
s".
Aspartate residues 110, 185, and 186 in the reverse transcriptase polymerase domain are important in the binding and incorporation of nucleotides. The side chains of residues K65, R72, and Q151 interact with the next incoming nucleotide. Also important is L74, which interacts with the template strand to position it for base pairing with the nucleotide. Mutation of these key amino acids results in reduced incorporation of the analogs.
The second mechanism is the excision or the hydrolytic removal of the incorporated drug or pyrophosphorlysis
. This is a reverse of the polymerase reaction in which the pyrophosphate/PPI released during nucleotide incorporation reacts with the incorporated drug (monophosphate) resulting in the release of the triphosphate drug. This ‘unblocks’ the DNA chain, allowing it to be extended, and replication to continue. Excision enhancement mutations, typically M41L, D67N, K70R, L210W, T215Y/F, and K219E/Q, are selected for by thymidine analogs AZT and D4T; and are therefore called thymidine analog mutations (TAMs). Other mutations including insertions and deletions in the background of the above mutations also confer resistance via enhanced excision.
There are three main mechanisms of NNRTI resistance. In the first NRTI mutations disrupt specific contacts between the inhibitor and the NNRTI binding pocket. An example of this is K103N and K101E which sit at the entrance of the pocket, blocking the entrance/binding of the drug. A second mechanism is the disruption of important interactions on the inside of the pocket. For example Y181C and Y188L result in the loss of important aromatic rings involved in NNRTI binding. The third type of mutations result in changes in the overall conformation or the size of the NNRTI binding pocket. An example is G190E, which creates a steric bulk in the pocket, leaving little of no room for an NNRTI to tightly bind.
Antiretroviral drug
Antiretroviral drugs are medications for the treatment of infection by retroviruses, primarily HIV. When several such drugs, typically three or four, are taken in combination, the approach is known as Highly Active Antiretroviral Therapy, or HAART...
used to treat HIV
HIV
Human immunodeficiency virus is a lentivirus that causes acquired immunodeficiency syndrome , a condition in humans in which progressive failure of the immune system allows life-threatening opportunistic infections and cancers to thrive...
infection, tumors, and cancer
Cancer
Cancer , known medically as a malignant neoplasm, is a large group of different diseases, all involving unregulated cell growth. In cancer, cells divide and grow uncontrollably, forming malignant tumors, and invade nearby parts of the body. The cancer may also spread to more distant parts of the...
. RTIs inhibit activity of reverse transcriptase
Reverse transcriptase
In the fields of molecular biology and biochemistry, a reverse transcriptase, also known as RNA-dependent DNA polymerase, is a DNA polymerase enzyme that transcribes single-stranded RNA into single-stranded DNA. It also helps in the formation of a double helix DNA once the RNA has been reverse...
, a viral DNA polymerase enzyme
Enzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...
that retroviruses need to reproduce.
Mechanism
When HIV infects a cell, reverse transcriptase copies the viral single stranded RNARNA
Ribonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....
genome into a double-stranded viral DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
. The viral DNA is then integrated into the host chromosomal DNA, which then allows host cellular processes, such as transcription and translation to reproduce the virus. RTIs block reverse transcriptase's enzymatic function and prevent completion of synthesis of the double-stranded viral DNA, thus preventing HIV from multiplying.
A similar process occurs with other types of viruses. The hepatitis B virus, for example, carries its genetic material in the form of DNA, and employs a RNA-dependent DNA polymerase to replicate. Some of the same compounds used as RTIs can also block HBV replication; when used in this way they are referred to as polymerase inhibitors.
Types
RTIs come in three forms:- NucleosideNucleosideNucleosides are glycosylamines consisting of a nucleobase bound to a ribose or deoxyribose sugar via a beta-glycosidic linkage...
analog reverse-transcriptase inhibitors (NARTIs or NRTIs) - NucleotideNucleotideNucleotides are molecules that, when joined together, make up the structural units of RNA and DNA. In addition, nucleotides participate in cellular signaling , and are incorporated into important cofactors of enzymatic reactions...
analog reverse-transcriptase inhibitors (NtARTIs or NtRTIs) - Non-nucleoside reverse-transcriptase inhibitors (NNRTIs)
The mode of action of NRTIs and NtRTIs is essentially the same; they are analogues of the naturally occurring deoxynucleotides
Nucleotide
Nucleotides are molecules that, when joined together, make up the structural units of RNA and DNA. In addition, nucleotides participate in cellular signaling , and are incorporated into important cofactors of enzymatic reactions...
needed to synthesize the viral DNA and they compete with the natural deoxynucleotides for incorporation into the growing viral DNA chain. However, unlike the natural deoxynucleotides substrates, NRTIs and NtRTIs lack a 3'-hydroxyl group on the deoxyribose moiety. As a result, following incorporation of an NRTI or an NtRTI, the next incoming deoxynucleotide cannot form the next 5'-3' phosphodiester bond needed to extend the DNA chain. Thus, when an NRTI or NtRTI is incorporated, viral DNA synthesis is halted, a process known as chain termination
Chain termination
Chain termination is any chemical reaction that ceases the formation of reactive intermediates in a chain propagation step in the course of a polymerization, effectively bringing it to a halt.- Mechanisms of Termination :...
. All NRTIs and NtRTIs are classified as competitive substrate inhibitors.
In contrast, NNRTIs have a completely different mode of action. NNRTIs block reverse transcriptase by binding at a different site on the enzyme, compared to NRTIs and NtRTIs. NNRTIs are not incorporated into the viral DNA but instead inhibit the movement of protein domains of reverse transcriptase that are needed to carry out the process of DNA synthesis. NNRTIs are therefore classified as non-competitive inhibitors
Non-competitive inhibition
Non-competitive inhibition is a type of enzyme inhibition where the inhibitor reduces the activity of the enzyme, by binding not to the active site on the enzyme, but to a different site...
of reverse transcriptase.
Nucleoside analog reverse-transcriptase inhibitors (NARTIs or NRTIs)
Nucleoside analog reverse-transcriptase inhibitors (NARTIs or NRTIs) compose the first class of antiretroviral drugs developed. In order to be incorporated into the viral DNA, NRTIs must be activated in the cell by the addition of three phosphatePhosphate
A phosphate, an inorganic chemical, is a salt of phosphoric acid. In organic chemistry, a phosphate, or organophosphate, is an ester of phosphoric acid. Organic phosphates are important in biochemistry and biogeochemistry or ecology. Inorganic phosphates are mined to obtain phosphorus for use in...
groups to their deoxyribose moiety, to form NRTI triphosphates. This phosphorylation
Phosphorylation
Phosphorylation is the addition of a phosphate group to a protein or other organic molecule. Phosphorylation activates or deactivates many protein enzymes....
step is carried out by cellular kinase
Kinase
In chemistry and biochemistry, a kinase is a type of enzyme that transfers phosphate groups from high-energy donor molecules, such as ATP, to specific substrates, a process referred to as phosphorylation. Kinases are part of the larger family of phosphotransferases...
enzymes.
Zidovudine : Zidovudine
Zidovudine
Zidovudine or azidothymidine is a nucleoside analog reverse-transcriptase inhibitor , a type of antiretroviral drug used for the treatment of HIV/AIDS. It is an analog of thymidine....
, also called AZT, ZDV, and azidothymidine, has the trade name Retrovir. Zidovudine was the first antiretroviral drug approved by the FDA for the treatment of HIV.
Didanosine : Didanosine
Didanosine
Didanosine is sold under the trade names Videx and Videx EC. It is a reverse transcriptase inhibitor, effective against HIV and used in combination with other antiretroviral drug therapy as part of highly active antiretroviral therapy .-History:The related pro-drug of didanosine,...
, also called ddI, with the trade names Videx and Videx EC, was the second FDA-approved antiretroviral drug. It is an analog of adenosine.
Zalcitabine : Zalcitabine, also called ddC and dideoxycytidine, has the trade name Hivid. This drug has been discontinued by the manufacturer.
Stavudine : Stavudine
Stavudine
Stavudine is a nucleoside analog reverse transcriptase inhibitor active against HIV.-History:...
, also called d4T, has trade names Zerit and Zerit XR.
Lamivudine : Lamivudine
Lamivudine
Lamivudine is a potent nucleoside analog reverse transcriptase inhibitor .It is marketed by GlaxoSmithKline with the brand names Zeffix, Heptovir, Epivir, and Epivir-HBV.Lamivudine has been used for treatment of chronic hepatitis B at a lower dose than for treatment of HIV...
, also called 3TC, has the trade name Zeffix and Epivir. It is approved for the treatment of both HIV and hepatitis B.
Abacavir : Abacavir
Abacavir
Abacavir is a nucleoside analog reverse transcriptase inhibitor used to treat HIV and AIDS. It is available under the trade name Ziagen and in the combination formulations Trizivir and Kivexa/Epzicom...
, also called ABC, has the trade name Ziagen, is an analog of guanosine.
Emtricitabine : Emtricitabine
Emtricitabine
Emtricitabine , with trade name Emtriva , is a nucleoside reverse transcriptase inhibitor for the treatment of HIV infection in adults and children....
, also called FTC, has the trade name Emtriva (formerly Coviracil). Structurally similar to lamivudine, it is approved for the treatment of HIV and undergoing clinical trials for hepatitis B.
Entecavir : Entecavir
Entecavir
Entecavir INN , abbreviated ETV, is an oral antiviral drug used in the treatment of hepatitis B infection. It is marketed under the trade name Baraclude ....
, also called ETV, is a guanine analog that has the trade name Baraclude. Though not currently approved as an HIV drug, entecavir is approved for the treatment of hepatitis B.
Apricitabine : Apricitabine, also called ATC. As of 2009, this drug is undergoing Phase-III evaluation, and if successful may achieve FDA approval in 2011.
Nucleotide analog reverse-transcriptase inhibitors (NtARTIs or NtRTIs)
Normally, nucleoside analogs are converted into nucleotide analogs by the body. Taking nucleotide analog reverse-transcriptase inhibitors (NtARTIs or NtRTIs) directly allows conversion steps to be skipped.Tenofovir : Tenofovir, also known as TFV 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 USA for the treatment of both HIV and hepatitis B.
Adefovir : Adefovir
Adefovir
Adefovir dipivoxil, previously called bis-POM PMEA, with trade names Preveon and Hepsera, is an orally-administered nucleotide analog reverse transcriptase inhibitor .-Uses:...
, also known as bis-POM PMPA, has trade names Preveon and Hepsera. It was 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.
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 ResearchRega Institute for Medical Research
The Rega Institute for Medical Research is a Belgian scientific establishment that is part of the Catholic University of Leuven in central Belgium...
(Belgium
Belgium
Belgium , officially the Kingdom of Belgium, is a federal state in Western Europe. It is a founding member of the European Union and hosts the EU's headquarters, and those of several other major international organisations such as NATO.Belgium is also a member of, or affiliated to, many...
)
Efavirenz : Efavirenz
Efavirenz
Efavirenz is a non-nucleoside reverse transcriptase inhibitor and is used as part of highly active antiretroviral therapy for the treatment of a human immunodeficiency virus type 1....
has the trade names Sustiva and Stocrin.
Nevirapine : Nevirapine has the trade name Viramune.
Delavirdine : Delavirdine
Delavirdine
Delavirdine is a non-nucleoside reverse transcriptase inhibitor marketed by ViiV Healthcare. It is used as part of highly active antiretroviral therapy for the treatment of human immunodeficiency virus type 1. It is presented as the mesylate...
, currently rarely used, has the trade name Rescriptor.
Etravirine : Etravirine
Etravirine
Etravirine is a drug used for the treatment of HIV. Etravirine is a non-nucleoside reverse transcriptase inhibitor . Unlike the currently available agents in the class, resistance to other NNRTIs does not seem to confer resistance to etravirine. Etravirine is marketed by Tibotec, a subsidiary...
has the trade name Intelence, and was approved by the FDA in 2008.
Rilpivirine : Rilpivirine
Rilpivirine
Rilpivirine is a pharmaceutical drug, developed by Tibotec, for the treatment of HIV infection. It is a second-generation non-nucleoside reverse transcriptase inhibitor with higher potency, longer half-life and reduced side-effect profile compared with older NNRTIs, such as efavirenz.Rilpivirine...
has the trade name Edurant, and was approved by the FDA in May 2011.
Portmanteau inhibitors
Researchers have designed molecules which dually inhibit both reverse transcriptase (RT) and integrase (IN). These drugs are a type of "portmanteau inhibitorPortmanteau inhibitor
A portmanteau inhibitor is a drug that is a combination of two drug molecules, each of which is itself a type of inhibitor. The term was coined in 2007 by University of Minnesota researchers who designed and synthesized a combination HIV reverse transcriptase inhibitor and an integrase inhibitor....
s".
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 drugs. HIV-1 RT does not have proof-reading activity, this combined with selective pressure from the drug leads to mutations in reverse transcriptase that make the virus less susceptible to NRTIs and NNRTIs.Aspartate residues 110, 185, and 186 in the reverse transcriptase polymerase domain are important in the binding and incorporation of nucleotides. The side chains of residues K65, R72, and Q151 interact with the next incoming nucleotide. Also important is L74, which interacts with the template strand to position it for base pairing with the nucleotide. Mutation of these key amino acids results in reduced incorporation of the analogs.
NRTI resistance
There are two major mechanisms of NRTI resistance. The first being reduced incorporation of the nucleotide analog into DNA over the normal nucleotide. This results from mutations in the N-terminal polymerase domain of the reverse transcriptase that reduce the enzyme’s affinity or ability to bind to the drug . A prime example for this mechanism is the M184V mutation that confers resistance to lamivudine (3TC) and emtricitabine (FTC). Another well characterized set of mutations is the Q151M complex found in multi-drug resistant HIV which decreases reverse transcriptase’s efficiency at incorporating NRTIs, but does not affect natural nucleotide incorporation. The complex includes Q151M mutation along with A62V, V75I, F77L, and F116Y. A virus with Q151M alone is intermediately resistant to zidovudine (AZT), didanosine (ddI), zalcitabine (ddC), stavudine (d4T), and slightly resistant to abacavir (ABC). A virus with Q151M complexed with the other four mutations becomes highly resistant to the above drugs, and is additionally resistant to lamivudine (3TC) and emtricitabine (FTC) .The second mechanism is the excision or the hydrolytic removal of the incorporated drug or pyrophosphorlysis
Pyrophosphate
In chemistry, the anion, the salts, and the esters of pyrophosphoric acid are called pyrophosphates. Any salt or ester containing two phosphate groups is called a diphosphate. As a food additive, diphosphates are known as E450.- Chemistry :...
. This is a reverse of the polymerase reaction in which the pyrophosphate/PPI released during nucleotide incorporation reacts with the incorporated drug (monophosphate) resulting in the release of the triphosphate drug. This ‘unblocks’ the DNA chain, allowing it to be extended, and replication to continue. Excision enhancement mutations, typically M41L, D67N, K70R, L210W, T215Y/F, and K219E/Q, are selected for by thymidine analogs AZT and D4T; and are therefore called thymidine analog mutations (TAMs). Other mutations including insertions and deletions in the background of the above mutations also confer resistance via enhanced excision.
NNRTI resistance
NNRTIs do not bind to the active site of the polymerase but in a less conserved pocket near the active site in the p66 sudomain. Their binding results in a conformational change in the reverse transcriptase that distorts the positioning of the residues that bind DNA, inhibiting polymerization. Mutations in response to NNRTIs decrease the binding of the drug to this pocket. Treatment with a regimen including efavirenz (EFV) and nevirapine (NVP) typically results in mutations L100I, Y181C/I, K103N, V106A/M, V108I, Y188C/H/L and G190A/S.There are three main mechanisms of NNRTI resistance. In the first NRTI mutations disrupt specific contacts between the inhibitor and the NNRTI binding pocket. An example of this is K103N and K101E which sit at the entrance of the pocket, blocking the entrance/binding of the drug. A second mechanism is the disruption of important interactions on the inside of the pocket. For example Y181C and Y188L result in the loss of important aromatic rings involved in NNRTI binding. The third type of mutations result in changes in the overall conformation or the size of the NNRTI binding pocket. An example is G190E, which creates a steric bulk in the pocket, leaving little of no room for an NNRTI to tightly bind.
See also
- Discovery and development of non-nucleoside reverse-transcriptase inhibitors
- Protease inhibitors
- Discovery and development of nucleoside and nucleotide reverse-transcriptase inhibitorsDiscovery and development of nucleoside and nucleotide reverse-transcriptase inhibitorsDiscovery and development of nucleoside and nucleotide reverse-transcriptase inhibitors began in the 1980s when the AIDS epidemic hit Western societies. NRTIs inhibit the reverse transcriptase , an enzyme that controls the replication of the genetic material of the human immunodeficiency virus ....