Nucleoside reverse transcriptase inhibitors (NRTIs), sometimes called "nucleoside analogues" or "nukes," contain faulty versions of the building blocks (nucleotides) used by reverse transcriptase to convert RNA to DNA. When reverse transcriptase uses these faulty building blocks, the new DNA cannot be built correctly. In turn, HIV's genetic material cannot be incorporated into the healthy genetic material of the cell and prevents the cell from producing new virus.
Reverse transcriptase (RT) plays an essential role in the HIV-1 replication cycle. It is responsible for the process of reverse transcription, which converts the positive-sense single-stranded RNA genome into double-stranded DNA before integration into the host cell genome. RT performs two enzymatic functions: it is a DNA polymerase and a ribonuclease H (RNase H). As a DNA polymerase, RT copies either an RNA or a DNA template into a complementary DNA sequence. As an RNase H, RT cleaves and degrades the RNA template during and after synthesis of the first DNA strand.
Due to its critical role in the HIV replication cycle, RT is a principal target for anti-HIV drug development. RT inhibitors targeting DNA polymerase function are categorized into two groups: NRTIs and NNRTIs. NRTIs are called chain terminators and are synthetic analogs of the natural dNTP substrates. The nucleoside and nucleotide inhibitors require phosphorylation in vivo by cellular kinases and compete with normal nucleotides (dNTPs) for binding to the dNTP binding site. These inhibitors inhibit replication by terminating DNA chain elongation because they lack the 3’hydroxyl group needed for incorporation of additional nucleotides. These inhibitors include zidovudine (AZT), didanosine (ddl), zalcitabine (ddC), stavudine (d4T), lamivudine (3TC), abacavir (ABC), and tenofovir (TFV). NNRTIs are allosteric inhibitors that bind to a site 10 Å distant from the substrate-binding site. The spatial and functional association between the two binding sites has been exploited by combination therapies where both categories of inhibitors are included to increase effectiveness of the treatment. NNRTIs are noncompetitive inhibitors that bind a hydrophobic pocket of the p66 subunit adjacent to the polymerase active site. Unlike nucleoside or nucleotide analogs, NNRTIs do not require activation by intracellular phosphorylation.
NRTI are structurally similar to the building blocks of nucleic acids (RNA, DNA) but differ from their natural analogues by the replacement of the hydroxy group in the 3' position by another group that is unable to form the 5' to 3' phosphodiester linkage that is essential for DNA elongation. NRTI block reverse transcriptase activity by competing with the natural substrates and incorporating into viral DNA and so to act as chain terminators in the synthesis of pro-viral DNA. However, NNRTI bind directly and non-competitively to the enzyme reverse transcriptase. Although the drugs differ chemically from each other, they all bind to the same site which is distinct from the substrate binding site. They block DNA polymerase activity by causing a conformational change and disrupting the catalytic site of the enzyme. Unlike nucleoside analogues, NNRTIs do not require phosphorylation to become active and are not incorporated into viral DNA within the cell.
Nucleoside RT inhibitors (NRTIs) represent the first FDA approved class of therapeutics for treatment of HIV infection. As substrate analogs, NRTIs are competitive inhibitors administered as prodrugs that must be transported into the cell and phosphorylated to the metabolically active triphosphate in order to exert their therapeutic effect. In the case of thymidine analogs such as AZT, the first phosphorylation is catalyzed by thymidine kinase . Subsequent phosphorylations are performed by thymidylate kinase and nucleoside diphosphate kinase to yield the active triphosphate metabolite. All currently approved NRTIs lack a 3'-hydroxyl group required to perform nucleophilic attack on an incoming dNTP, therefore AZT and other NRTIs elicit their antiviral effect by halting primer extension.
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