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Antiviral Drugs
One key strategy to block viral infection is to target steps of replication of the viral genome.
Another is to target the integration of viral DNA into the host genome.
A third strategy is to target steps involved in the maturation of the virus, assembly of the viral capsid or packaging of the viral genome into the capsid.
The following Table lists several drugs that are effective against different kinds of viruses. All except one affect the replication of the viral genome, either the duplication of RNA or the reverse transcription of RNA into a DNA. The last one blocks the activity of a protease required to cleave a polyprotein into individual viral proteins
Drug: Viruses: Chemical Type: Target:
Vidarabine Herpesviruses Nucleoside analogue Virus polymerase
Acyclovir Herpes simplex (HSV)
Nucleoside analogue Virus polymerase
Gancyclovir and Valcyte ™ (valganciclovir)
Cytomegalovirus (CMV)
Nucleoside analogue
Virus polymerase (needs virus UL98 kinase for activation)
Nucleoside-analog reverse transcriptase inhibitors (NRTI): AZT (Zidovudine), ddI (Didanosine), ddC (Zalcitabine), d4T (Stavudine), 3TC (Lamivudine)
Retroviruses (HIV)
Nucleoside analogue
Reverse transcriptase
Non-nucleoside reverse transcriptase inhibitors (NNRTI): Nevirapine, Delavirdine
Retroviruses (HIV)
Nucleoside analogue
Reverse transcriptase
Protease Inhibitors: Saquinavir, Ritonavir, Indinavir, Nelfinavir HIV Peptide
analogue HIV protease
Historically, the discovery of antiviral drugs has been largely fortuitous. Spurred on by success with antibiotics, drug companies launched huge blind-screening programs - with relatively little success. Lead compounds were modified by chemists in an attempt to improve bioactivity. Solubility, stability, availability and activity are all important in the overall effectiveness of any given antiviral agent.
The most effective agents for blocking replication are nucleoside or nucleotide analogs. They are useful against viruses with RNA genomes, DNA genomes or retroviruses (viruses that have an RNA genome that must be converted into a DNA intermediate for viral replication). These analogs can compete with the normal ribonuclelotides that bind to the catalytic pocket of RNA dependent RNA polymerase enzyme, or they can compete with the deoxynucleotide binding pocket of reverse transcriptase, or they can compete with the deoxynucleotide binding pocket of a DNA dependent DNA polymerse.
Shown below are nucleoside analogs that have strong antiviral activity. One is a pyrimidine analog, the other is purine analog.
Zidovudine
Didanosine
Let us now consider the use of some of the antiviral agents in the context of HIV. HIV has an RNA genome which also serves the messenger for the translation of viral proteins. Following infection, the RNA genome can be translated into important viral proteins. One of these proteins is the reverse transcriptase that converts the viral RNA first into single stranded DNA and then into double stranded DNA. The reverse transcriptase is produced as part of a larger protein which is cleaved at appropriate positions by the viral protease. The viral protease is packaged into the virus particle and is delivered to the cell during infection. Following reverse transcription, the double stranded viral DNA is integrated into the genome with the help of the integrase protein.
The integrated viral genome can be transcribed into RNA which serves as the message for the gag‐pol polyprotein and the envelope protein (see Figure 1 below)
Figure 1. The translation of the HIV mRNA results in the gag‐pol poly protein and the envelope protein. The viral protease cleaves this protein at multiple points to yield the indicated proteins.
The proteins p17, p24 and p7 contribute to the structure of the virion, and help in packaging the viral RNA. As pointed out, the functions of the protease, reverse transcriptase and the integrase are in processing the gag‐pol protein, making a
cDNA copy of the viral RNA and integrating the DNA into the host genome, respectively.
Combating HIV proliferation: Highly active anti‐retroviral therapy (HAART) is a combination therapy that utilizes two reverse transcriptase inhibitors and a protease inhibitor. There are two advantages to combination therapy. First, they help the efficacy of the treatment through synergistic effects. Second, and perhaps more importantly, they minimize the chances of viral mutations that confer resistance to the drugs. The reverse transcriptase, unlike DNA polymerases responsible for duplicating host genomes, is an error prone enzyme. They lack the sophisticated proof‐reading mechanisms responsible for the high fidelity of DNA replication. As a result mutations arise in viruses at a fairly high frequency. Some of these mutations are deleterious and the mutant genomes carrying them will be eliminated from the population. However, another subset of mutations may confer resistance to a certain drug giving the particular mutant an advantage. The resistant virus will proliferate and infect new T cells of the host making the treatment with that particular drug ineffective. By recombination between viral genomes, mutations can be acquired in a combinatiorial fashion, thus increasing the chances of drug resistance. The probability of acquiring mutations that induce viral resistance to multiple drugs would be quite low, close to zero. Hence, the multi‐drug treatment regimen is quite successful in keeping HIV replication and sustained re‐infection of T cells under control.
A rather innovative drug design is not only to target the replication of the virus but also that of the cells in which the virus is replicating. The principal target cell for HIV infection is the CD4+ T lymphocytes. Virus cannot replicate in resting T cells, although it can remain in a latent state. When T cells are activated by encountering an antigen, they multiply and carry out their function, for example, fighting bacterial infection (see Figure 2). Some of these cells revert to a nearly resting state (memory T cells), and they may maintain a low level of cell division. These cells maintain the memory of the original infection (or the antigen that stimulated them), so that when a re‐infection occurs, they quickly enter their proliferative mode.
The proliferating CD4+ T lymphocytes are excellent hosts for HIV infection, integration and proliferation (Figure 2). Many of these cells die by the cytopathic effects of the virus as well as by host immune response. This is why active HIV infection depletes the T cell pool of the victim. Some of the actively infected cells escape the viral cytopathic effects or killing by the host immune system. They become resting cells providing a long‐term stable reservoir of the virus. Because in the resting state, there is little or no viral gene expression and the lack of viral antigens keeps them protected from the host immune response. These cells survive by flying under the radar of immunosurveillence.
Mycophenolic acid has been known to block the proliferative activity of activated lymphocytes. It is an immunosuppressive agent, and has been used to prevent rejection in patients undergoing kidney transplant. It acts by inhibiting inosine monophosphate dehydrogenase, an enzyme required for the de novo biosynthesis of guanosine nucleotides. The action of this enzyme converts inosone 5’ monophopshate to xanthine 5’ monophosphate, a committed step in the biosynthesis of guanosine nucleotide. The mechanism of inhibition is by MPA mimicking the nicotinamide portion of the NAD (nicotinamide adenine dinucleotide) cofactor and a water molecule required for the activity of this enzyme. Lymphocytes depend on the activity of inosine monophosphate dehydrogenase for de novo synthesis of purines whereas other cell types can also use what is called a salvage pathway. In the salvage pathway, the free base is converted to the nucleotide by a phsphoribosyl transferase activity.
Adenine phosphoribosyltransferase (HGPRT) catalyzes:
adenine + PRPP <-----> AMP + PPi Hypoxanthine-guanine phosphoribosyltransferase (HGPRT) catalyzes the following reactions:
hypoxanthine + PRPP <------> IMP + PPi
guannine + PRPP <--------> GMP + PPi
PRPP = 5’‐phopshoribisyl pyrophosphate.
By inhibiting guanine nucleotide synthesis, MPA can potentiate the effects of reverse transcriptase inhibitors. In patients undergoing HAART, MPA causes substantial reduction of proliferating CD4+ and CD8+ cells but they do not reduce the overall population size of T cells. Thus it is useful for fighting HIV infection without impairing the immune system as a whole.
Figure 2. Mycophenolic acid (MPA) blocks the proliferation of (a) activated CD4+ T cells, (2) memory CD4+ T cells and (c) post‐integration latent CD4+ T cells as indicated in the diagram. MPA can potentiate the activity of a reverse transcriptase inhibitor (RTI). These drugs can be used in combination with the protease inhibitor, PI.
HIV integrase inhibitors
Another potential target protein in anti‐HIV therapeutics is the viral integrase. A number of such inhibitors have been developed in recent years. There are at least three classes of such inhibitors. The first is modified nucleotide inhibitors. They bind to the integrase active site and inhibit its activity. The second class, called thiazolothiazepines were discovered as a result of a large scale screening of drugs
sponsored by the National Cancer Institute. They seem to bind to the integrase‐DNA complexes and prevent integration. A third class consists of hydroxylated aromatic compounds. They seem to act by chelating the metal ion in the active site of the integrase. The integrases coordinates a Mg++ or Mn++ ion for its activity through an active site motif D‐‐‐‐D‐‐‐‐E (Asp‐‐‐‐‐Asp‐‐‐‐Glu).
Figure 3. Class I inhibitors of HIV integrase. They bind to the integrase active site.
Figure 4. The thiazolothiazepine class of HIV integrase inhibitors appear to prevent integration by binding to integrase-DNA complexes.
Figure 5. The hydroxylated aromatic compounds are thought to chelate the metal ion present within the active site of integrase.