Antivirals are drugs used to treat infections caused by viruses.
Viruses
Viruses are very small infectious agents. They cannot be considered microorganisms, since they consist only of proteins and a single type of nucleic acid (DNA or RNA, which can be both double-stranded and single-stranded); moreover, they lack their own motility.
For these reasons, viruses are defined "obligate cell parasites". Their structure is such as to guarantee protection from the external environment and such as to allow it to penetrate inside the host cells.
Viruses can be divided according to the nucleic acid that constitutes them, therefore, they are distinguished:
- DNA virus; among the viruses belonging to this class we remember; Parvoviruses, Papillomaviruses, Hepadnaviruses and Herpesviruses;
- RNA virus; some of the viruses belonging to this class are Caliciviruses, Flaviviruses and Retroviruses.
Antiviral therapy
The goal of antiviral therapy is to interfere in viral biological processes that are instead absent in the host cell (which is therefore not damaged).
The first antiviral drugs to have been discovered did not possess sufficient selectivity, were toxic already at therapeutic concentrations and had a rather reduced spectrum of action.
The antiviral drugs currently marketed are instead able to fight viruses, but often fail to completely eradicate the infection.
Furthermore, the symptoms of viral infection can manifest themselves only after the virus has completed its replication and only after its genome has been internalized within the host cell; this makes it even more difficult to identify and synthesize antiviral drugs that are truly effective and capable of completely destroying the virus. In fact, once the virus has invaded the host cell and has begun to replicate, the infection is difficult to cure. .
Virus vaccines are effective, but they are only effective against certain types of strains. For this reason, research aims at obtaining antivirals with a broader spectrum of action and greater efficacy.
The ideal antiviral drug should possess the following characteristics:
- Have a broad spectrum of action;
- Being able to totally inhibit viral replication;
- Also be effective against mutant strains;
- Reach the target organ without interfering with the biological processes of the host organism;
- Do not interfere with the host organism's immune system.
Classification of antiviral agents
The antiviral agents currently on the market can be classified into various groups, according to their mechanism of action.
These groups will be briefly listed below.
Inhibitors of the early stages of viral replication
Antiviral drugs belonging to this group exert their action by interfering with the early stages of viral replication, such as the adhesion of the virus to the receptors of the host cell, its penetration into it and the loss of the viral coating.
They are part of this group "amantadine and the rimantadine.
Neuraminidase inhibitors
Neuraminidase is an enzyme expressed mainly on the surface of influenza viruses and plays an important role in the activation of new viruses and their release from the host cell.
They belong to this group of antivirals zanamivir and the "oseltamivir.
Agents that interfere with viral nucleic acid replication
As you can guess from the name of this group, the drugs that belong to it are able to inhibit the synthesis of the nucleic acid of the virus (DNA).
These drugs have a chemical structure very similar to that of the nitrogenous bases that make up DNA and - thanks to their similarity - they are incorporated into the newly synthesized DNA strand thus blocking its growth.
The "acyclovir, the famciclovir, the cytarabine (also used as an anticancer drug), the ganciclovir, the trifluridine and the "idoxuridine.
Antiretroviral (or anti-HIV) agents
The HIV virus is responsible for human acquired immunodeficiency (better known as AIDS).
HIV is a Retrovirus, which is a particular type of RNA virus.
In order to replicate, this virus needs the conversion of its RNA into DNA; this conversion occurs thanks to a particular enzyme, the reverse transcriptase.
Some anti-HIV agents work by inhibiting reverse transcriptase. Among these inhibitors, we remember the zidovudine, the didanosine, the zalcitabine, the lamivudine and the stavudine.
Other antiretroviral agents work by inhibiting the protease HIV, an enzyme essential for the growth of the virus.
They are protease inhibitors the ritonavir and the "indinavir.
Protease inhibitors can be administered in combination with reverse transcriptase inhibitors to try to improve the efficacy of the therapy.
