https://www.slideshare.net/nileshkucha/chapter-23-topoisomerase-inhibitors

1. Presentation
on
Reverse transcriptase inhibitor and their
restances
Submitted by-
Samad Muhammad Abdus
ID-AM202318003
School of Medicine
Jeju National University
For-
Kang Hee-Kyung
Professor
School of Medicine
Jeju National University

2. Reverse-transcriptase inhibitors (RTIs) are a class of antiretroviral drugs used to treat HIV infection or AIDS, and
in some cases hepatitis B. RTIs inhibit activity of reverse transcriptase, a viral DNA polymerase that is required
for replication of HIV and other retroviruses.
When there is worry about a patient's risk for contracting HIV, reverse transcriptase inhibitors have also been
utilized for post-exposure prophylaxis. Finally, the transmission of HIV from mother to child during pregnancy,
labor, and delivery is being curbed by the use of reverse transcriptase inhibitors. Antiretroviral therapy based on
zidovudine is the preferred medication for treating HIV in the mother during pregnancy.
Types:
RTIs come in three forms:
• Nucleoside analog reverse-transcriptase inhibitors (NARTIs or NRTIs)
• Nucleotide analog reverse-transcriptase inhibitors (NtARTIs or NtRTIs)
• Non-nucleoside reverse-transcriptase inhibitors (NNRTIs)

3. Nucleoside analog reverse-transcriptase inhibitors (NARTIs or NRTIs):
The first class of developed antiretroviral medications consists of nucleoside analog reverse-transcriptase
inhibitors (NARTIs or NRTIs). NRTIs must be activated in the cell by the addition of three phosphate groups to
their deoxyribose moiety to create NRTI triphosphates in order to be integrated into the viral DNA. Cellular kinase
enzymes carry out this phosphorylation process. NRTIs can cause mitochondrial dysfunction, which can result in a
variety of negative side effects, such as symptomatic lactic acidosis.

4. Nucleoside analog reverse-transcriptase inhibitors (NARTIs or NRTIs):
Example:
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, also called ddI, with the trade names Videx and Videx EC, was the second FDA-approved
antiretroviral drug. It is an analog of adenosine. Stavudine, also called d4T, has trade names Zerit and Zerit XR.
Lamivudine, also called 3TC, has the trade name Zeffix and Epivir. It is approved for the treatment of both HIV
and hepatitis B.
Abacavir, also called ABC, has the trade name Ziagen, is an analog of guanosine.
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, also called ETV, is a guanosine analog used for hepatitis B under the trade name Baraclude. It is not
approved for HIV treatment.
Truvada, made of emtricitabine and tenofovir disoproxil fumarate, is used to treat and prevent HIV. It is approved
for HIV prevention in the US and manufactured by Gilead.
Azvudine, also called RO-0622. It has been investigated as a possible treatment of AIDS, hepatitis C, and Sars-
Cov-2

5. Mechanism of action of ‘NARTIs’ or ‘NRTIs’:
NRTIs are non-functional until the substance is changed into its active di- or tri-phosphate metabolite form.
However, the amounts of activated NRTI derivatives and endogenous dNTP levels are extremely important for
NRTI transport and metabolism. The transport, cellular metabolism, and catabolism of NRTIs are all discussed in
this section.

6. Mechanism of action of ‘NARTIs’ or ‘NRTIs’ (cont.):
In a typical situation, HIV reaches immune system cells in the body. T cells or CD4 cells are what these cells are
known as. HIV starts to replicate once it gets inside CD4 cells. It must convert its RNA—the genetic code of the
virus—into DNA in order to achieve this. Reverse transcriptase is an enzyme that is needed for this process, which
is also known as reverse transcription.
Reverse transcriptase of the virus cannot correctly convert its RNA into DNA when NRTIs are present. Without
DNA, HIV is unable to replicate itself.
Chemical substances known as nucleotide base analogues make up the NRTI family of antiretroviral medications.
When the DNA chain is extended during the reverse transcription process, which is carried out by HIV reverse
transcriptase, they serve as chain-terminators. The NRTI compounds allow for accurate base pairing and inclusion
into the DNA chain, but a crucial hydroxyl group needed for the insertion of the next nucleotide has been replaced
with a non-reactive chemical group.
With the exception of Tenofovir, NRTIs do not naturally contain phosphate groups, hence before they can be
utilized as substrates by reverse transcriptase, they must first be converted into the triphosphate form of the base
analogue by the addition of three phosphate groups. Tenofovir only has one phosphonate group, and it takes two
phosphate groups to turn it into the active substance. Enzymes (kinases) from the host cell do the extra
phosphorylation.

7. Resistance Mechanism of ‘NARTIs’ or ‘NRTIs’:

8. Resistance Mechanism of ‘NARTIs’ or ‘NRTIs’ (cont.):
There are two ways in which NRTI resistance can be achieved:
1) Discrimination pathway:
These resistance mutations, which affect the amino acids in reverse transcriptase's main
structure, improve the enzyme's capacity to preferentially incorporate natural nucleotides over
NRTI-triphosphate.
There are two ways in which discrimination can be achieved:
Reduce the NRTI-triphosphate's binding affinity to the natural nucleotide in the reverse
transcriptase active site (for example, M184V and V75T). Reverse transcriptase carrying the
M184V mutation was shown to bind the 3TC-triphosphate with a reduced affinity over the wild-
type enzyme, resulting in a 48.8 fold increase in resistance, in experiments measuring the
binding affinity for the compound.
Reduce the rate at which NRTI-triphosphate is incorporated over natural nucleotides (such as
K65R, K70E, L74V, and Q151M). The mutant reverse transcriptase was observed to lower
the rate of incorporation of 3TC-triphospate, resulting in a 13.1 fold increase in resistance, in
studies using wild-type and mutant reverse transcriptase containing the K65R mutation
evaluating the polymerisation activity for 3TC-triphosphate.

9. Resistance Mechanism of ‘NARTIs’ or ‘NRTIs’ (cont.):
2) Excision pathway:
These resistance mutations, which include the amino acid alterations M41L, D67N, K70R, L210W, T215F/Y, and
K219Q, make it easier to remove the chain-terminators NRTI-triphosphate from the 3' end of the DNA chain after
they have been inserted.
Because the excision route requires adenosine triphosphate (ATP) or pyrophosphate, mutations that improve
reverse transcriptase's affinity for ATP or speed up the elimination of the analog complex are preferred. The
excision mechanism can be improved by alterations in the template/primer dissociation rate from the enzyme as
well as the translocation capacity of residues from the active site (N-site) to the post-translocation site (P-site).

10. Nucleotide analog reverse-transcriptase inhibitors (NtARTIs or NtRTIs):
As mentioned before, host cells convert nucleoside analogs to nucleotide analogs by phosphorylation. In order
to provide the intended antiviral impact and medicine toxicity/side effects, the latter act as poison building
blocks (chain terminators) for both viral and host DNA. The first stage of phosphorylation is directly skipped
when using phosphonate nucleotide analog reverse-transcriptase inhibitors (NtARTIs or NtRTIs), but host
enzymes still need to phosphorylate the phosphonate nucleotide analogue to the phosphonate-diphosphate
state for anti-viral action.
Example:
Tenofovir, commonly known as TDF, is a so-called "prodrug" that has a molecular side chain that dissolves in
the body, deactivating the active ingredient and enabling a low dose of tenofovir to reach the site of desired
action.
Adefovir also goes by the commercial names Preveon and Hepsera. It is also known as ADV or bis-POM
PMPA. Due to toxicity concerns, the FDA has not authorized it for the treatment of HIV, however hepatitis B
can be treated with a lesser dosage.

11. Mechanism of action of ‘NtARTIs’ or ‘NtRTIs’:

12. Mechanism of action of ‘NtARTIs’ or ‘NtRTIs’ (cont.):
HIV cannot attach to or fuse with target cells when it is inhibited by entry inhibitors or fusion inhibitors,
respectively. Depending on which receptor or complex formation is inhibited, a different mechanism may be at
work: blocking the interaction of CCR5 with viral gp120 protein (CCR5 antagonists), inhibiting viral gp120
(attachment inhibitors), or inhibiting the conformational changes of the CD4/gp120 complex that allow the
binding to co-receptors (CCR5 or CCR4) (post-attachment inhibitors).
NtRTIs, which can be either nucleoside or nucleotide analogues, work by preventing reverse transcriptase, a
viral enzyme that converts viral RNA into DNA in freshly infected cells, from synthesising DNA. They do this by
functioning as fake nucleotides.
PIs also block the maturation of viral proteins into their functional conformations and the production of new
viral particles by binding to the viral protease enzyme's active site.

13. Resistance mechanism of ‘NtARTIs’ or ‘NtRTIs’:
For the treatment of hepatitis B and HIV infection, a novel family of chemicals that are analogues of
nucleotides rather than nucleosides has been created. Adefovir dipivoxil, one of the two original drugs, and 9-
[2-(R-phosphonomethoxy)propyl]adenine, or PMPA, a more recent drug, have both been demonstrated to be
potent HIV inhibitors in vitro and in early clinical studies.
Nucleotide discrimination. Mutations in pol cause steric hindrance at the pol active site, excluding certain
drugs, for example 3TC, from being incorporated during reverse transcription. Both examples yield a complex
competent for polymerization. Yellow circle with the letter A and three phosphates, ATP; black circles with three
phosphates, dNTPs; red circle with the letter Z and the N3 azido group, AZT-MP; blue circle with three
phosphates, 3TC-triphosphate; P, phosphate group. RNA is depicted with white circles; DNA is depicted with
black circles.

14. NNRTIs, also known as non-nucleoside reverse transcriptase inhibitors, are the third family of
antiretroviral medications that have been created. In all circumstances, patents are still in effect after
2007. At the Rega Institute for Medical Research in Belgium, this family of medications was initially
described.
Example:
Efavirenz has the trade names Sustiva and Stocrin.
Nevirapine has the trade name Viramune.
Delavirdine, currently rarely used, has the trade name Rescriptor.
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.

15. Among reverse transcriptase inhibitors, NNRTIs are the another class. The main mode of action involves the
NNRTI binding to the reverse transcriptase and the formation of a hydrophobic pocket close to the active site.
By rearranging the substrate-binding site's spatial arrangement, this pocket lowers the polymerase's overall
activity. DNA synthesis is generally slowed down by changing the configuration. HIV-2 reverse transcriptase is
ineffective against NNRTI due to its non-competitive inhibitory activity.
The late stages of HIV-1 replication have been shown to be inhibited by some NNRTIs, such as
efavirenz, by interfering with the processing of the HIV-1 Gag-Pol polyprotein, whereas other NNRTIs,
like the pyrimidinediones, have been shown to inhibit both HIV-1 RT-mediated reverse transcription and
HIV-1/HIV-2 viral entry.

17. The RT enzyme is bound non-competitively by the NNRTIs (figure). The three-dimensional structure of the
enzyme is altered by the binding, resulting in the creation of the NNIBP.[4][5] Tyr-181 and Tyr-188's rotamer
conformation alterations are what cause the p66 thumb domain to become hyper extended when NNRTI bind
to HIV-1 RT.[13] By blocking the polymerase active site of the RT's p66 subunit, this reduces the enzyme's
catalytic activity and prevents HIV-1 replication.
Additionally, the global conformational shift weakens the enzyme's attachment to its nucleic acid template and
decreases its capacity to bind nucleotides.[15] The viral replication rate decreases as a result of the inhibition
of viral RNA transcription.[4] Numerous investigations have shown that this is the main mode of action, even
though the precise molecular mechanism is still speculative.
It has been demonstrated that the NNRTIs have other mechanisms of action and interfere with different stages
of the reverse transcriptase process in addition to this postulated core mechanism of action.[5] Reverse
transcription may be inhibited by NNRTIs due to their impact on RT Rnase H activity and/or template/primer
binding, according to certain theories. By preventing the last step of HIV-1 replication, several NNRTIs prevent
the processing of the Gag-Pol polyprotein.

18. The NNRTIs have selective anti-HIV-1 action but not anti-HIV-2 or anti-other retroviral activity. They bind to
reverse transcriptase's catalytic site noncompetitively and interfere with it. Drug resistance to NNRTIs can
emerge quickly following the start of an NNRTI as part of a nonsuppressive regimen since it only takes a single
mutation that does not seem to influence viral fitness for resistance to emerge. Cross-resistance is more
frequent with the NNRTIs efavirenz (EFV), nevirapine (NVP), and rilpivirine (RPV); etravirine (ETR) is less
frequently associated with cross-resistance.

19. The majority of NNRTI-resistant HIV strains have mutations in and around the NNIBP that either directly
disrupt NNRTI binding by changing the size, shape, and polarity on various pockets, or indirectly
influence access to the pocket.[9] These mutations are mainly found in the p66 subunit's domains that
span amino acids 98–108, 178–190, or 225-238. NNRTIs bind to the p66 subdomain rather than the
polymerase's active site in a less conserved pocket close to the active site. Mutations brought on by
NNRTIs reduce the drug's affinity for this pocket.
NNRTI resistance mostly occurs through three methods.
In the First-NRTI mutations prevent the inhibitor from making certain interactions with the NNRTI
binding pocket. K103N and K101E, which are present at the pocket's entrance and prevent the
medication from entering or binding, serve as an illustration of this.
Secondly-disrupting crucial interactions on the interior of the pocket is a second method. Examples of
mutations that cause the deletion of significant aromatic rings involved in NNRTI binding include Y181C
and Y188L.
Thirdly-changes in the overall conformation or the size of the NNRTI binding pocket are the results of
the third category of mutations. G190E is one instance where the steric mass it forms in the pocket
prevents an NNRTI from securely binding.

20. Reference:
1. https://encyclopedia.pub/entry/33577
2. https://www.immunopaedia.org.za/?s=NtRTI
3. https://www.sciencedirect.com/topics/medicine-and-dentistry/reverse-transcriptase-inhibitor
4. https://www.ncbi.nlm.nih.gov/books/NBK551504/
5. https://www.hiv.uw.edu/go/antiretroviral-therapy/evaluation-management-virologic-failure/core-
concept/all
6. https://www.ncbi.nlm.nih.gov/books/NBK2249/
7. https://www.sciencedirect.com/topics/neuroscience/non-nucleoside-reverse-transcriptase-
inhibitors
8.https://bio.libretexts.org/Bookshelves/Microbiology/Microbiology_(Boundless)/13%3A_Antimicrobia
l_Drugs/13.07%3A_Antiviral_Drugs/13.7C%3A_Nucleotide_and_Nonnucleotide_Reverse_Transcript
ase_Inhibitors

21. Thank You