Antiviral drug
Antiviral drug
Antiviral drugs are a class of medication used specifically for
treating viral infections Like antibiotics for bacteria, specific
antivirals are used for specific viruses. Unlike most antibiotics,
antiviral drugs do not destroy their target pathogen; instead they
inhibit their development.
Antiviral drugs are one class of antimicrobials, a larger group
which also includes antibiotic, antifungal and antiparasitic drugs.
They are relatively harmless to the host, and therefore can be used
to treat infections. They should be distinguished from viricides,
which are not medication but destroy virus particles.
Most of the antivirals now available are designed to help deal
with HIV, herpes viruses (best known for causing cold sores and
genital herpes, but actually causing a wide range of diseases), the
hepatitis B and C viruses, which can cause liver cancer, and
influenza A and B viruses. Researchers are working to extend the
range of antivirals to other families of pathogens.
Designing safe and effective antiviral drugs is difficult,
because viruses use the host's cells to replicate. This makes it
difficult to find targets for the drug that would interfere with
the virus without also harming the host organism's cells.
The emergence of antivirals is the product of a greatly expanded
knowledge of the genetic and molecular function of organisms,
allowing biomedical researchers to understand the structure and
function of viruses, major advances in the techniques for finding
new drugs, and the intense pressure placed on the medical
profession to deal with the human immunodeficiency virus (HIV), the
cause of the deadly acquired immunodeficiency syndrome (AIDS)
pandemic.
Almost all anti-microbials, including anti-virals, are subject
to drug resistance as the pathogens mutate over time, becoming less
susceptible to the treatment. For instance, a recent study
published in Nature Biotechnology emphasized the urgent need for
augmentation of oseltamivir (Tamiflu) stockpiles with additional
antiviral drugs including zanamivir (Relenza) based on an
evaluation of the performance of these drugs in the scenario that
the 2009 H1N1 'Swine Flu' neuraminidase (NA) were to acquire the
tamiflu-resistance (His274Tyr) mutation which is currently
widespread in seasonal H1N1 strains. Mechanism of actionAmantadine
and rimantadine were the first generation of influenza antiviral
agents.1These compounds specificallyblock the ion channel function
of the M2 protein of influenza A virus, thus interfering with
corresponding specific steps in the viral life cycle. The
neuraminidase inhibitors are novel drugs, designed on the basis of
the three-dimensional structure of the influenza A and B
neuraminidase. The mechanisms of action of the four available
specific anti-influenza viral drugs are summarized in.
Figure 1 The mechanism by which antiviral drugs interrupt the
replicative cycle of influenza is illustrated. M2 inhibitors
prevent the M2-mediated acidification of the interior of the virus
while it resides in endosomes and the subsequent uncoating of the
viral genome, thus inhibiting viral replication. Neuraminidase
inhibitors (NAIs) prevent cleavage of sialic acid residues and thus
newly formed virus cannot be released from the cell surface to
infect adjacent cells; also, virus particles remain associated to
one another.
The M2 channel inhibitors amantadine and rimantadine
At high concentrations (>15 g/ml), amantadine and rimantadine
non-specifically raise the pH within cellular endosomes, thus
inhibiting or retarding the acid-induced conformational change in
the viral HA. However, the required concentrations of the drugs are
not generally attainedin vivo. At low, pharmacologically relevant
concentrations (250 cells/L or men with CD4 counts of >400
cells/L, unless the benefit clearly outweighs the risk. Monitor
liver tests closely for the first 16 weeks of treatment.
Rilpivirine Insomnia
Depression
Elevations in liver function tests
Elevations in serum creatinine
May be less potent if baseline HIV RNA >100,000 copies/mL
Requires acidic gastric environment for adequate absorption:
Must be taken with food.
Contraindicated with proton pump inhibitors.
Other antacid medications and H2 blockers require specific
timing if used by persons taking rilpivirine.
Drug reaction
Antiviral drug interactions are a particular problem among
immuno-compromised patients because these patients are often
receiving multiple different drugs, i.e. antiretroviral drugs and
drugs effective against herpesvirus. The combination of zidovudine
and other antiretroviral drugs with different adverse event
profiles, such as didanosine, zalcitabine and lamivudine, appears
to be well tolerated and no relevant pharmacokinetic interactions
have been detected. The adverse effects of didanosine and
zalcitabine (i.e. peripheral neuropathy and pancreatitis) should be
taken into account when administering these drugs with other drugs
with the same tolerability profile. Coadministration of zidovudine
and ganciclovir should be avoided because of the high rate of
haematological intolerance. In contrast, zidovudine and foscarnet
have synergistic effect and no pharmacokinetic interaction has been
detected. No major change in zidovudine pharmacokinetics was seen
when the drug was combined with aciclovir, famciclovir or
interferons. However, concomitant use of zidovudine and ribavirin
is not advised. Although no pharmacokinetic interaction was
documented when didanosine was first administered with intravenous
ganciclovir, recent studies have shown that concentration of
didanosine are increased by 50% or more when coadministered with
intravenous or oral ganciclovir. The mechanism of this interaction
has not been elucidated. Lack of pharmacokinetic interaction was
demonstrated between foscarnet and didanosine or ganciclovir.
Clinical trials have shown that zidovudine can be administered
safely with paracetamol (acetaminophen), nonsteroidal
anti-inflammatory drugs, oxazepam or codeine. Inhibition of
zidovudine glucuronidation has been demonstrated with fluconazole,
atovaquone, valproic acid (valproate sodium), methadone, probenecid
and inosine pranobex; however, the clinical consequences of this
have not been fully investigated. No interaction has been
demonstrated with didanosine per se but care should be taken of
interaction with the high pH buffer included in the tablet
formulation. Drugs that need an acidic pH for absorption
(ketoconazole, itraconazole but not fluconazole, dapsone,
pyrimethamine) or those that can be chelated by the ions of the
buffer (quinolones and tetracyclines) should be administered 2
hours before or 6 hours after didanosine. Very few interaction
studies have been undertaken with other antiviral drugs.
Coadministration of zalcitabine with the antacid 'Maalox' results
in a reduction of its absorption. Dapsone does not influence the
disposition of zalcitabine. Cotrimoxazole
(trimethoprim-sulfamethoxazole) causes an increase in lamivudine
concentrations by 43%. Saquinavir, delavirdine and atevirdine
appeared to be metabolised by cytochrome P450 and interactions with
enzyme inducers or inhibitors could be anticipated. Some studies
showed that interferons can reduce drug metabolism but only a few
studies have evaluated the pathways involved. Further studies are
required to better understand the clinical consequences of drug
interactions with antiviral drugs. Drug-drug interactions should be
considered in addition to individual drug clinical benefits and
safety profiles.
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