Ljubljana/Currenttreatment

Current Disease treatment

HIV is a typical retrovirus that upon entry into the host cell reverse transcribes its RNA to DNA, which integrates into the host genome. The viral genes are expressed as polyprotein that is cleaved into functional proteins by HIV protease. Viral coat is composed of lipids and two predominant proteins, gp41 and gp120, which are essential for binding onto the host cell receptor, CD4. Several drugs were developed which target crucial stages of HIV entry into the host cell, its reverse transcription, integration and processing of the polyprotein. Current standard therapy consists of taking a combination of at least three different antiretroviral drugs.

Viral Life Cycle

HIV (human immunodeficiency virus) is a retrovirus, Its genome is composed of two single stranded RNA molecules. It has a gag/pol/env organization; gag genes (group specific antigen) code for structural proteins, env for proteins that build viral envelope, while pol genes are responsible for viral reproduction (they contain genes for reverse transcriptase, integrase and HIV protease).

The HIV envelope consists of lipids and viral glycoproteins gp120 and gp41, which are crucial for binding of HIV to the host cell membrane and for entering into the cell. Inserted into the lipid bilayer are also other glycoproteins that guarantee firmness and protective function of the viral envelope. Gp120 binds to receptors (CD4) on the host cell surface, but additional co-receptors like chemokine receptors (CCR5, CxCR4) are also required for successful entry of HIV. Mutations in co-receptor genes can cause immunity – if HIV cannot enter host cells, HIV infection is prevented, and AIDS cannot develop.

The characteristic retroviral enzyme is reverse transcriptase, which transcripts viral RNA into DNA. Only DNA can integrate into the host cell genome – this is the crucial step in expressing viral proteins that are needed for assembly of new viral particles. Viral gag and gag/pol genes are expressed as polyprotein; until this polyprotein is cut into functional units, it exerts no biological function. Polyprotein clipping is done by HIV protease. The resulting polyprotein fragments represent functional enzymes and structural proteins.

Transcription of viral RNA into DNA and processing of the viral polyprotein are the two most important steps in the HIV replication cycle. These are thus obvious targets for HIV therapeutics. Inhibitors of reverse transcriptase and HIV protease are currently used to treat acute HIV infection.

Current Disease Treatment

One of the first AIDS therapeutics were nucleotide or nucleoside analogues (NRTI – nucleoside-analogue reverse transcriptase inhibitors) – pseudosubstrates, which are during reverse transcription integrated into viral DNA instead of nucleosides during reverse transcription and thus block the transcription. These drugs were superseded by non-nucleoside inhibitors (NNRTI) that could inhibit reverse transcriptase by binding into the allosteric site of the enzyme. The third type of drugs are a family of HIV protease inhibitors. In most cases specific inhibitors are very similar to protease substrates - the only difference is that because they cannot be cut, they block the active site by remaining bound into it.

The weakness of all these therapeutics is that they are very sensitive to HIV mutations – HIV can easily mutate and thus become drug resistant. A combination of drugs is used to minimize HIV's potential to develop resistance to each individual drug in the mixture. Some of the drugs induce mutations that have negative effect on the virulence and such drugs can be used in spite of developed resistance.

We still do not have a cure for AIDS that would be insensitive to HIV mutations. Our project presents new ways of potential AIDS therapeutics. Our approaches can be considered independent of HIV mutations. We have set up a few of biological ambushes; if HIV manages to avoid them, we presume that it would not be able to infect the cell anyway.

Classes of Antiretroviral Drugs

Antiretroviral drugs are mostly inhibitors of different stages in the HIV life cycle. They are targeted at different enzymes or events that are typical for HIV infection – entry of the virus into the cell, reverse transcription, polyprotein cleavage... and are divided into seven main classes (Drugs Used in the Treatment of HIV Infection, 2007):

HAART

Current AIDS treatment is based on HAART – highly active antiretroviral therapy. It is a mixture of at least three different antiretroviral drugs. By disturbing the viral replication cycle at different stages it is much harder for HIV to mutate and become drug-resistant. Synergistic enhancers are added; these are drugs that are either inappropriate for monotherapy or are enhancers of metabolism of other antiretroviral drugs. Although HAART can provide a longer and better life for patients with AIDS, it cannot totally eliminate HIV from a patient's body because it cannot excise latent viral DNA that is already integrated into the host genome and therefore has to be taken for life.

There are two more problems with HAART – its side effects and cost. We can try to minimize the side effects but we will probably never find a cure for AIDS that would be completely harmless. Because HAART must be taken continuously, its price makes it impossible to use widely in areas with the highest percentage of infected population. Ideally, vaccination could limit the AIDS epidemic because vaccines usually cost less than drugs for continuous treatment and could be affordable for developing countries. After many years of intense research we still do not have a vaccine for HIV and because of its high mutation rate HIV is a difficult target.

Alternative Therapies Under Development

• siRNA targeted against virus (LTR, Tat, integrase...) or host proteins (knock down of surface receptors...)
• gene therapy - decrease of the surface cellular receptors, addition of decoy soluble receptors
• intrabodies, ribozymes directed against viral RNA...