Why are antiviral drugs difficult to develop?
Viruses are non living outside the body. They contain genome DNA or RNA but cannot grow independently. Instead, they need a host organism for its survival as they use genetic instructions of host organism.
1. They can only reproduce in host cells and use same mechanisms as the host cells. Therefore, it is difficult to find drugs that target the virus and do not damage the host cell.
2. As viruses mutate quickly, therefore effective vaccines against a virus are difficult to produce.
3. Viruses hide inside host cells and thus difficult for the immune system to produce antibodies.
4. Antigenic drift is the main reason for the virus to escape from immune response of host. For example, people can get the flu again and again because of antigenic drift.
In case of bacteria, antibiotics are possible because bacteria has their own pathways for synthesis and thus antibiotics are produced that affects bacteria and not host synthesis machinery. Antibiotics that target the growth machinery of bacteria kill inhibit only that particular bacteria.
Viruses enter the host cells and use their machinery for their replication. Therefore, antiviral drugs work against the host cells also.
Antiviral drug must be designed that will be effective against viral proteins such as surface proteins by which virus attached to the host cell. Example: In case of influenza virus, antiviral vaccine interfere with enzyme called neuraminidase and it keeps the virus to infect a neighboring cell.