Antiviral drugs are available to treat only a few viral diseases. The reason for this is the fact that viral replication is so intimately associated with the host cell that any drug that interferes significantly with viral replication, is likely to be toxic to the host.
Two useful antivirals are:
the nucleoside analogues and
the interferons
but there are other targets in the different stages of intracellular viral
growth which show potential for antiviral chemotherapy.
Diseases for which effective therapy is available:
Herpes Simplex virus (Acyclovir)
Varicella-Zoster virus (Acyclovir)
Cytomegalovirus (Gancyclovir, Foscarnet)
AIDS (Zidovudine, Lamivudine[3TC], Protease inhibitors; in
combination)
Respiratory Syncitial virus (Ribavirin)
Influenza (Amantadine)
These are synthetic compounds which resemble nucleosides, but have
an incomplete or abnormal deoxy-ribose /or ribose group.
These compounds are phosphorylated to the tri-phosphate form within
the infected cell. In this form, the drug competes with normal
nucleotides for incorporation into viral DNA or RNA. Incorporation into
the growing nucleic acid chain results in irreversible association with the
viral polymerase and chain termination.
There are three classes: alpha- beta- and gamma-
The alpha and
beta Interferons
are cytokines which are secreted by virus infected cells.
They bind to specific receptors on adjacent cells and protect them from infection
by viruses.
They form part of the immediate protective host response to invasion
by viruses.
In addition to these direct antiviral effects, alpha and beta interferon
also enhance the expression of class I and class II MHC molecules on
the surface of infected cells, in this way, enhancing the presentation of
viral antigens to specific immune cells. Their presence can be demonstrated
in body fluids during the acute phase of virus infection.
Recombinant alpha and beta interferons are now available and have
been used for the treatment of
Chronic hepatitis B and C virus infections.
However, side effects such as fever, malaise and weight loss have limited the use.
gamma Interferon
(immune interferon)
is a cytokine secreted by TH1 CD4 cells.
Its function is to enhance specific T cell mediated immune responses.
Passive immunisation is the transfer of immunity to a host by means of immunoglobulins (preformed antibodies). These immunoglobulins are typically prepared by cold ethanol fractionation as a 16% solution of gammaglobulin from large pools of serum obtained from the blood donations of at least 1000 donors.
Immunoglobulin from immune individuals can be used as prophylaxis to prevent viral infections in exposed, but non immune individuals. It works by binding to extra-cellular virions and preventing them from attaching to and entering susceptible cells. The protective effect is short lived (up to three months) because the antibodies are metabolised by the host.
"Normal" Immune globulin
This is a pooled product, prepared from the serum of normal blood donors.
It contains low titres of antibody to a wide range of human viruses. It is
mainly used as prophylaxis against:
Hyper-immune globulin
Immunoglobulin may be prepared from the serum of selected individuals who
have high titres of antibody to particular viruses.
Examples include:
Illustrations and layout by Linda Stannard, November 1999.