Antiviral Agents

Antiviral Agents

INTRODUCTION

Antiviral agents are substances used in the treatment and prophylaxis of diseases caused by viruses. Viral diseases include influenza, rabies, yellow fever, poliomyelitis, ornithosis, mumps, measles, ebola, human immuno deficiency virus (HIV), herpes, warts, and small pox. Viruses are not proper living things, but consists of a genome; they are smaller in size with simple chemical composition, sometimes a few enzymes stored in a capsule made up of protein and rarely covered with a lipid layer. The viruses only replicate within the host cell and the viral replication depends primarily on the metabolic processes of the invaded cell. Viruses does not possess cell wall, but they have RNA or DNA enclosed in a shell of protein known as capsid. The capsid is composed of several subunits known as capsomers. In certain cases, capsid may be surrounded by an outer protein or lipoprotein envelope. One group of RNA virus that deserves special mention are reteroviruses. They are responsible for acquired immuno deficiency syndrome (AIDS) and T-leukaemias. Reteroviruses contain reverse transcriptase (RT) enzyme activity that makes a DNA copy of the viral RNA template. Then, the DNA copy is integrated into the host genome, at which it is referred to as provirus and is transcribed into both the genomic RNA and mRNA for translocation into the viral proteins, giving generation to new virus particles. Viral life cycle varies according to the species, but they all share a general pattern that can be sequenced as follows (Fig. 7.1):

·Adsorption: Attachment of the virus to the host cell.

·Penetration: Penetration of virus into the cell.

·Uncoating: The genetic material or viral genome (DNA or RNA) passes into the host cell leaving the capsid covering outside the host cell.

·Transcription: Production of the viral mRNA from the viral genome.

·Translation: The viral genome enters the cytoplasm or the nucleoplasma and directs or utilizes the host nucleic acid machinery for the synthesis of the new viral protein and for the production of more viral genome. The viral protein modifies the host cell and allows the viral genome to replicate by using host and viral enzyme. This is often the stage at which the cell is irreversibly modified and eventually killed.

·Assembly of the viral particle: New viral coat protein assembles into capsid and viral genomes.

·Release of the mature virus from the cell and the budding process or rupture of the cell and repeat of the process, in a fresh host cell.

Since the host cell machinery is totally utilized for the production of new virions, the normal cell function is affected. Antiviral agents have been developed to act at various stages in the viral replication cycle, such as attachments, replication, and release of the virus.

Some virus types together with diseases that they cause are listed in Table 7.1.

Table 7.1 Examples of viruses with diseases.