The document discusses the central role of vaccines in controlling virus infections, noting their limitations with certain viruses such as influenza and retroviruses, which led to the development of antiviral drugs. It highlights various classes of antiviral agents, their mechanisms of action, and the challenges related to resistance and efficacy. Additionally, the document outlines specific antiviral compounds in use today, their pharmacological profiles, and the implications for treating infections in immunocompromised patients.

1. Antiviral Agents
Atsabina E.W.

2. Introduction
Vaccines have, to date, occupied the central position in
attempts to control virus infections.
Vaccines are relatively cheap and safe and the immunity is
often life-long.
However, some viruses, for some reasons, are not fully
amenable to this approach, such as influenza, retroviruses,
herpesviruses, the slow viruses, rhinoviruses and
arboviruses. Obstacles to the use of vaccines include
(1) Multiplicity of serotypes e.g. Rhinoviruses, togaviruses
(2) Antigenic change e.g. influenza, retroviruses
(3) Latent infections.

3. •Only relatively recently have notable successes on a large
scale been achieved with antiviral drugs such as acyclovir
and AZT, in situations where no vaccine is available.
•However. acyclovir and AZT do not approach penicillin in
their spectra of activity or degree of inhibition.
•They are more analogous to some of the first antibacterial
agents such as salvarsan.
•No antiviral compound tested has been able to inhibit
completely the replication of any virus and a proportion of
viral particles always seems to be able to circumvent the
drug-induced blockade.

4. The Chemistry of Antiviral Compounds
They vary greatly in complexity and include natural products of plants,
synthetic oligonucleotides, oligosaccharides, simple inorganic and
organic compounds and nucleoside analogues.
Examples of antiviral compounds in current use include:
1.Nucleoside analogues - thousands of analogues of naturally
occurring nucleosides have now been synthesized and tested in the
laboratory, initially as herpesvirus inhibitors and many now are retested
as anti-HIV agents. In addition to purine and pyrimidine nucleosides,
ara-, amino, aza-nucleosides or nucleotides have been synthesized.
Even single atomic substitution may change an active to an inactive
molecule.
2.Pyrophosphate analogues - forscarnet is an example of a
pyrophosphate analogue. This specifically inhibits herpesvirus DNA
polymerase at the pyrophosphate binding sites and it also has anti-HIV
activity.
3.Amantidine molecules - amantidine is licensed for the treatment of
influenza A infection. Addition of a methyl grouping (rimantidine) alters
the pharmacological distribution of the drug and prevents entry to the
brain, thus reducing the side effect described as "jitteriness"

5. Resistance of viruses to inhibitors
•A disappointing feature of antiviral chemotherapy has been
the failure so far of any antiviral molecule to inhibit virus
replication completely.
•Antiviral activity tends to produce a 100 to 1000 fold
reduction in virus titre which, although significant, still allows
some infective particles to survive.
•This may have important consequences in
immunocompromized patients who may be unable to
eradicate any residual virus.
•It is not known for certain whether these virions are drug-
resistant mutants or with different biologically or genetically
from the major portions of the virus population.

6. Points of action of antiviral molecules
1. Cell-free virus
Few antiviral compounds inhibit or inactivate extracellular virus in vivo.
An exception is the series of WIN compounds which bind to the external
proteins of picornaviruses and thereby stabilizing the particle and prevent
uncoating.
2. Virus Adsorption
In the case of HIV, which binds specifically to CD4 receptors, short
peptides have been synthesized to correspond with the sequence of the
receptor binding site of CD4 molecule and with the binding protein gp120.
These peptides should block the interaction of the receptor region and
gp120 without interrupting other receptor functions of the CD4.

7. 3. Virus entry and uncoating
•Viruses such as influenza and certain flaviviruses enter by viropexis or
engulfment.
•Immediately afterwards while in a cytoplasmic endosome (vacuole), the virus
catalyses fusion between the viral lipid-containing membrane and the membrane
of the intracellular vacuole.
•The fusion is mediated by a sequence of hydrophobic amino acids or one of the
glycoproteins of the virus.
•In the case of influenza A, the fusion sequence on the HA molecule can only act
after a structural 3- dimensional rearrangement of the HA molecule.
•Amantidine appear to inhibit influenza A replication in part by raising the pH of the
cytoplasmic vacuole, thus preventing virus-induced fusion and hence virus
uncoating.
•Other enveloped viruses such as paramyxoviruses and HIV, enter cells by virus-
induced fusion with the plasma membrane of the cell.
•This "fusion from without" may be susceptible to short peptides which may act on
the fusion sequence extracellularly.

8. 4. Transcription and translation of viral nucleic acids and release of
virus
Most of the antiviral drugs now known act by inhibiting the replication or
transcription of viral nucleic acids.
1.a. Inhibitors of herpes DNA polymerase - by far the most amenable
target for antiviral drugs is the herpes simplex DNA polymerase. The most
successful antiviral compound yet developed is acyclovir inhibits the
function of this enzyme. The ideal antiviral drug should (1) be taken up only
into infected cells (2) the actual inhibitory molecule should be generated
inside the infected cell by enzymatic activity (3) the inhibitor should have a
selective effect on a virus enzyme. Acyclovir demonstrates all of the above
characteristics.
2.b. Inhibitors of viral reverse transcriptase - AZT and the majority of
other compounds act as chain terminators. AZT triphosphate binds to and
inhibit virus RT more effectively than normal cellular DNA polymerases and
so some antiviral specificity is achieved. However, the compound is
certainly not comparable to acyclovir in terms of antiviral specificity. This is
reflected in the toxicity of AZT in clinical practice. This cellular toxicity may
be partly explained by the fact that normal cellular enzymes phosphorylate
AZT and is thus activated in both infected and uninfected cells.

9. 5. Translation
•It may be possible to interfere with the viral mRNA itself.
•Small anti-sense oligonucleotides can be constructed which are
complementary to specific genes, such as the rev gene.
•Fomivirsen (Vitravene) is a 21-base anti-sense oligonucleotide
complementary to the early region 2 mRNA of CMV.
•It is approved for the local treatment of CMV retinitis in AIDS patients.
6. Assembly
•HIV protease is required for the cleavage of the gag-pol fusion protein.
•Inhibitors of this enzyme may therefore block the assembly of HIV.

10. Some Commonly Used Antiviral Agents
1. Acyclovir
•Acyclovir is a synthetic guanine nucleoside analogue.
•The initial step of phoshorylation to ACV monophosphate is
preferentially carried out by viral thymidine kinase rather than cellular
kinases.
•The monophosphate cannot leave infected cells so that more non-
phosphorylated compound enters to make up for the depleted
intracellular concentration, only to be converted to the monophosphate.
•In this manner, the drug accumulates in the herpes-infected cells rather
than in the uninfected counterparts.

11. •The monophosphate is then phosphorylated to the di and
tri-phosphate forms by cellular enzymes.
•ACV triphosphate is the pharmacologically active form of
the drug. It inhibits herpes DNA polymerase with little
effect on the host cell DNA polymerase.
•It also has some chain termination activity and thereby it
behaves as a "suicide inhibitor"

12. •Acyclovir resistant strains of HSV have mutations in either
the viral thymidine kinase gene or the viral DNA
polymerase.
•Acyclovir also has antiviral activity against other
herpesviruses such as VZV, CMV and EBV, although the
mechanism is not so well understood in these cases.
•Forscarnet is the preferred drug in the treatment of
acyclovir-resistant strains.

13. 2. Valacyclovir
•Valacylovir is an ester of acyclovir that is well-absorbed.
•Its bioavailibility is 2-5* greater than acyclovir.
•It is used for the treatment and suppression of genital herpes
infection.

14. 3. Famciclovir
•Famiciclovir is the prodrug of penciclovir which is the active
form and a guanosine analog.
•It has a very high bioavailabiity of 77%. It is converted into
penciclovir by a two step process.
•The first step occurs in the gut and the second step in the
liver.
•It has a long half life in the gut.
•It has a higher affinity for HSV thymidine kinase than
acyclovir but a lower affinity for HSV DNA polymerase than
acyclovir.
•It acts as an inhibitor of viral DNA polymerase and also as a
chain terminator.
•At present famciclovir is licensed for the treatment of
shingles and the dosage is 250mg tds.
•It is also used for the treatment and suppression of genital
herpes infection.

15. 4. Ganciclovir
•Ganciclovir is a guanine nucleoside chemically related to acyclovir.
•It acts as a chain terminator and subsequent termination of viral DNA
replication.
•The active form is thought to be the triphosphate.
•CMV does not specify TK and the initial phosphorylation of ganciclovir is
thought to be mediated by other cellular enzymes.
•Ganciclovir has potent in vitro activity against all herpesviruses, including
CMV.
•It has some activity against other DNA viruses such as vaccinia and
adenovirus.
•Ganciclovir is more active against CMV than acyclovir.
•Ganciclovir has been shown to be of value in treating severe CMV
infections in the immunocompromized, especially in conjunction with
hyperimmune immunoglobulin.

16. •Reversible neutropenia is the most frequent adverse
reaction.
•Ganciclovir resistance has been reported in
immunocompromised patients being treated for CMV
disease and is thought to be due to lack of phosphorylation
of the drug by CMV infected cells.
•A recent prospective study estimated that 8% of patients
receiving ganciclovir for more than 3 months developed
resistant CMV.

17. 5. Ribavirin
•Ribavirin is a synthetic triazole nucleoside and the active form
is ribavirin triphosphate.
•It is not incorporated into the primary structure of DNA or
RNA during cellular synthesis of nucleic acids.
•In the case of influenza viruses, it inhibits the 5' capping of
viral mRNAs.
•It has also been shown to inhibit influenza viral RNA
polymerase complex.
•It has further been postulated that ribavirin triphosphate
inhibits several steps in viral replication and this phenomenon
may explain the failure to detect viral isolates that are resistant
to ribavirin.

18. •Ribavirin has been shown to possess activity against both
DNA and RNA viruses in infected cells.
•It has been found to have activity against adenoviruses,
herpesviruses, CMV. vaccinia. influenza A and B,
parainfluenza 1, 2, 3, measles, mumps, RSV, rhinovirus.
•Ribavirin has made a major contribution to the therapy of
children infected with RSV where is given as an aerosol in
hospital.
•It has also been shown to be effective against influenza A
and B.
•It has also been reported to be of value in the treatment of
Lassa fever, hantavirus disease and hepatitis C.

19. 6. Zidovudine (AZT)
•AZT is a synthetic analogue of thymidine.
•It requires conversion to the triphosphate form by cellular enzymes.
•It inhibits viral reverse transcriptase by acting as a chain terminator.
•Viral reverse transcriptase is 100 times more susceptible to inhibition by
zidovudine triphosphate than host cellular DNA polymerase.
•Once incorporated into the viral DNA chain, viral DNA synthesis is
terminated as no more phosphodiester bonds could be formed.
•AZT is active in vitro against many human retroviruses, including HTLV-
I and HIV.
•AZT is currently indicated for the management of patients with HIV
infection who have impaired immunity. (T4 cell count of 400- 500 or less)
AZT has been clearly shown to prolong the life of individuals infected
with HIV.
•It has also been shown to be of benefit for the treatment of
symptomless individuals although this is controversial.

20. 7. Lamivudine
•Lamivudine is a potent reverse transcriptase inhibitor.
•It is generally well tolerated by patients.
•It now usually forms an essential component in the
combination therapy of HIV patients.
•Recently, it had been approved for the treatment of chronic
hepatitis B.

21. 8. Forscarnet
•Forscarnet is a pyrophosphate analog and unlike nucleoside
analogues, forscarnet does not need to be activated by cellular or viral
kinases.
•Forscarnet binds directly to the pyrophoshate- binding sites of RNA or
DNA polymerases.
•Forscarnet is difficult to use as it must be given continuously
intravenously via an infusion pump.
•It is used for the treatment of CMV retinitis in AIDS patients receiving
AZT therapy, as it does not have overlapping toxicity with AZT.
•It is also used in the treatment of AZT resistant HSV infections. Its major
adverse effect is on renal function.

22. 9. Amantidine
•this compound inhibit the growth of influenza viruses in cell culture and
in experimental animals.
•Amantidine is only effective against influenza A, and some naturally
occurring strains of influenza A are resistant to it.
•The mechanism of action of amantadine is not known. It is thought to act
at the level of virus uncoating.
•The compound has been shown to have both therapeutic and
prophylactic effects. Amantidine significantly reduced the duration of
fever (51 hours as opposed to 74 hours) and illness.
•The compound also conferred 70% protection against influenza A when
given prophylactically.
•Amantidine can occasionally induce mild neurological symptoms such
as insomnia, loss of concentration and mental disorientation.

23. •However, these symptoms quickly developed in susceptible
individuals and cease when treatment is stopped.
•The therapeutic and prophylactic activity of amantidine is
now generally accepted and numerous analogues of this
compound have been prepared.
•Rimantadine is not as effective as amantadine but is less
toxic.
•One factor that limits the usefulness of amantidine and
rimantidine is the rapid development of resistance of these
molecules in 30% of patients.
•These resistant mutants have been reported to be as
capable of being transmitted and causing disease as the wild
virus.

24. 10. Zanamivir
•The rational approach to drug design has led to the design of several
potent inhibitors of influenza neuraminidase of which two, zanamivir and
oseltamivir are licensed for the treatment of influenza A and B infections.
•In clinical trials, both agents have demonstrated efficacy with minimal side
effects.
•Because of its poor bioavailibility, Zanamivir must be given by inhalation
whilst oseltamivir can be given orally.
•Because selection of drug-resistant mutants characterized by changes in
NA requires prolonged passage in tissue culture, development of
zanamivir-resistant viruses is not expected to occur readily in patients.
•The available information suggests that mutants may be less stable in
vivo. The significance of changes in hemagglutinin remains to be
evaluated.
•Overall the NA family of anti-influenza drugs is showing considerable
promise; resistant variants do not occur readily and may be biological
cripples.

25. 11. Immunoglobulins
•Immunoglobulins are available in three different formulations; intramuscular form,
IVIG, and hyperimmune globulins against individual viruses.
•Immunoglobulins are more effective when used prophylactically rather than
therapeutically.
•Currently, HNIG is used primarily for the prevention of hepatitis A. HNIG can also be
given to non- immunized contacts of measles.
•Hyperimmune globulins are used for the postexposure prevention of hepatitis B,
chickenpox and rabies. It has also been used in the treatment of arenavirus
infections, Crimean-Congo haemorrhagic fever and Rift valley fever.
•CMV Ig is given prophylactically to seronegative recipients of kidneys from
seropositive donors.
•The use of prophylactic CMV Ig in BMT patients is controversial. CMV IVIG is used
in conjunction with ganciclovir in the treatment of CMV pneumonitis.
• IVIG is also used in the treatment of chronic enteroviral meningoencephalitis in
children with agammablobinaemia.

26. Anti-Retroviral Agents
A. Nucleoside Reverse Transcriptase
Inhibitor
• Zidovudine (AZT)
• Didanosine (ddI)
• Zalcitabine (ddC)
• Emtricitabine (FTC)
• Lamivudine (3TC)
• Tenofovir (TDF)
• Stavudine (d4T)
• Abacavir (ABC)
B. Non-Nucleoside Reverse
Transcriptase Inhibitor
• Nevirapine (NVP)
• Delaviridine (DLV)
• Efavirenz (EFV)
C. HIV Protease Inhibitors
• Tipranavi (TPV)
• Amprenavir (APV)
• Indinavir (IDV)
• Saquinavir (SQV)
• Ritonavir (RTV)
• Atazanavir (ATV)
• Fosamprenavir (FPV)
• Nelfinavir (NFV)
D. HIV Fusion Inhibitors
• Enfuvirtid (T-20)
• Maraviroc (MVC)

27. •There are a number of combination preparations on the
market e.g.
CBV (AZT+3TC),
TZV (AZT+3TC+ABC),
TVD (FTC+TDF),
Kaletra (Lopinavir/ritonavir).
•The use of combination preparations will reduce the numbed
of tablets that need to be taken each time.

28. 2. Monitoring anti-HIV therapy
a. Viral8 Load
Initiation - viral load is now the preferred method of monitoring therapy.
There should be >= 1 log reduction in viral load, preferably to less than
10,000 copies/ml HIV-RNA within 2-4 weeks after the commencement of
treatment. If <0.5 log reduction in viral, or HIV-RNA stays above 100,000,
then the treatment should be adjusted by either adding or switching drugs.
Monitoring - viral load measurement should be repeated every 4-6
months if patient is clinically stable. If viral load returns to 0.3-0.5 log of
pre-treatment levels, then the therapy is no longer working and should be
changed.

29. b. CD4 count
1.Initiation - within 2-4 weeks of starting treatment, CD4
count should be increased by at least 30 cells/mm3
.
•If this is not achieved, then the therapy should be changed.
Monitoring - CD4 counts should be obtained every 3-6
months during periods of clinical stability, and more frequently
should symptomatic disease occurs.
•If CD4 count drops to baseline (or below 50% of increase
from pre-treatment), then the therapy should be changed.

30. c. Anti-HIV Drug Resistance Testing
1.Genotypic Assays - genotypic assays detect drug resistance mutations
that are present in the relevant viral genes (i.e. RT and protease). Some
genotyping assays involve sequencing of the entire RT and protease
genes, while others utilize oligonucleotide probes to detect selected
mutations that are known to confer drug resistance.
2.Phenotypic Assays - phenotypic assays measure the ability of viruses
to grow in various concentrations of antiretroviral drugs. Recombinant
phenotyping assays involve insertion of the RT and protease gene
sequences derived from patient plasma HIV RNA into a laboratory clone of
HIV. Replication of the recombinant virus at various drug concentrations is
monitored by expression of a reporter gene and is compared with
replication of a reference strain of HIV.
Use in clinical setting - resistance assays may be useful in the setting of
virological failure on antiretroviral therapy.

31. E. Interferons
•There are 3 classes of interferons: alpha, beta and gamma
•IFNs mediate their actions through specific receptors at hormone like
concentrations. Interferon inducible response elements in the cellular
genome are activated.
•There are 2 main types of IFN receptors, one for alpha and beta1 and the
other for gamma.
•IFNs are released form many cell types in response to virus infection,
dsRNA, endotoxin, mitogenic and antigenic stimuli. DsRNA appears to be a
particularly important inducer.
•Usually, good IFN inducers are viruses that multiply slowly and do not
block the synthesis of host protein early or markedly damage the cells.

32. 1. Mechanism of Action
The antiviral effects of IFNs are exerted through several pathways;-
(1) Increased expression of Class I and Class II MHC glycoproteins, thereby
facilitating the recognition of viral antigens by the immune system.
(2) Immunoregulatory effects - activation of cells with the ability to destroy virus-
infected targets; these include NK cells and macrophages. IFNs appear to drive a
shift from humoral to cellular immunity.
(3) Direct inhibition of viral replication: several mechanisms contribute to the
third pathway.

33. 1.production of specific inhibitory proteins eg. the Mx protein which has
specific anti-influenza action. It is likely that more specific inhibitory
proteins will be identified.
2. inhibition of viral processes such as penetration, uncoating and budding
from infected cells have been reported.
3. in vitro studies with extracts of IFN-treated cells show that the main
target of IFN action is translation, which is blocked by 2 mechanisms, both
requiring the presence of minute amounts of dsRNA;-
(i). activation of a dsRNA dependent protein kinase - this phosphorylates
and inactivates the translation initiation factor eIF-2.
(ii). activation of 2-5 oligo A synthetases ® synthesis of 2-5A ® activates
endonuclease L (itself induced by IFN) ® degradation of mRNA ®
inhibition of protein synthesis.
•Used in treatment of Togaviruses, hepatitis B and C

34. THANKS!!