Anti-sense nucleotides

Anti-sense nucleotides are short pieces of RNA which are specifically created to bind onto parts of HIV's genetic material and prevent them from fulfilling their functions.

Three anti-sense drugs, called GEM 91, GEM 92 and HGTV43, have been tested in early human trials. Development of GEM 91 was ceased due to toxicity and patchy results, but researchers have come up with GEM 92, a second generation, anti-sense oligonucleotide directed against the gag gene in HIV. Both GEM 91 and 92 bind onto part of HIV's messenger RNA and prevent the production of proteins from which new virus particles can be assembled. HGTV43 has shown promising results in test-tube experiments.

RNA interference

RNA interference (RNAi) is a mechanism used by plants and invertebrates to protect themselves against infection by RNA viruses. When double-stranded RNA is detected, 'Dicer' enzymes chop up the double-stranded RNA, producing RNA fragments called 'short interfering RNAs' (siRNAs). These are used to bind onto identical sequences of RNA still remaining in the cell, blocking the production of new viral proteins.

In humans the presence of double-stranded RNA normally stimulates an interferon-based immune response rather than RNAi. However, recent laboratory experiments have shown that it is possible to stimulate RNAi in human cells to silence the rev, tat, vif and nef genes, and to silence the cellular gene for the human CD4 receptor. Furthermore, it appears that HIV itself contains RNA sequences that will produce the double-stranded shape required to stimulate RNAi in human cells. Unfortunately, however, the viral gene Tat suppresses the cell's attempts to silence HIV, thereby escaping the human cell's RNAi activity. Finding ways to block Tat's activity may be necessary to tip the balance back into the cell's favour and allow RNAi to be used to prevent HIV replication (Bennasser 2005).

Test tube studies have shown that inserting snippets of HIV RNA into cells in the test tube can significantly reduce HIV replication in these cells, as can introduction of RNA that targets the human enzyme cyclin T1, which is essential for HIV replication (Han 2004; Li 2005; Lieberman 2003). This has provided proof of concept for RNAi as a potential HIV treatment strategy, despite the anti-RNAi activity of Tat.

However, finding ways to get double-stranded RNA into cells is proving difficult, often requiring the use of altered viruses or complex systems to deliver the RNA into cells. Consequently, RNAi is likely to be more useful as a research tool than as a direct therapy, allowing researchers to pinpoint exactly which gene products need to be inhibited in order to stop HIV replication. Some researchers are interested in silencing the gene that codes for the CCR5 coreceptor, used by HIV alongside the CD4 receptor to gain entry to cells. See Preventing viral attachment or fusion in Ways of attacking HIV for further information on CCR5 inhibition.

Other strategies

The final stage in HIV's life-cycle which can be targeted with anti-viral therapies is when new virus particles are assembled at the surface of the infected cell, forming their envelope from the cell membrane studded with viral proteins before being released into the body. Interferon alfa has been reported to inhibit the production and release of new virions.

Increasing the levels of the lipid ceramide in cells has also been reported to inhibit HIV replication, possibly by re-arranging the receptors to which HIV binds. Drugs that can increase ceramide levels include fenretinide (Finnegan 2004).

The mechanism of action of some anti-HIV drugs remains unknown. Compound Q is said to selectively inhibit HIV replication and to kill HIV-infected cells, although it has considerable toxic side-effects. A group of compounds called pyridine N-oxide derivatives have also shown anti-HIV activity in the test tube. While some of these compounds act as reverse transcriptase inhibitors, others prevent the expression of HIV's genes (Balzarini 2005).

There are also some experimental therapeutic approaches that aim to kill HIV directly. Extracorporeal photopheresis uses a combination of a drug and ultra-violet light; hyperthermia aims to kill the virus by heating up the blood; ozone is said to disrupt viral particles, as well as inhibiting reverse transcriptase and viral binding to human cells. None of these therapies has been clearly shown to have benefits for people with HIV and all run the risk of serious side-effects.

References

Balzarini J et al. Pyridine N-oxide derivatives: unusual anti-HIV compounds with multiple mechanisms of antiviral action. J Antimicrob Chemother 55: 135-138, 2005.

Bennasser Y et al. Evidence that HIV-1 encodes an siRNA and a suppressor of RNA silencing. Immunity 22: 607-619, 2005.

Finnegan CM et al. Ceramide, a target for antiretroviral therapy. Proc Natl Acad Sci U S A 101: 15452-15457, 2004.

Han W et al. Inhibition of human immunodeficiency virus type 1 replication by siRNA targeted to the highly conserved primer binding site. Virology 330: 221-232, 2004.

Li Z et al. Specific inhibition of HIV-1 replication by short hairpin RNAs targeting human cyclin T1 without inducing apoptosis. FEBS Lett 579: 3100-3106, 2005.

Lieberman J. Some steps toward moving RNA interference to the clinical. Tenth Conference on Retroviruses and Opportunistic Infections, Boston, abstract 51, 2003.

Yoshinaga T et al. S-1360: in vitro activity of a new HIV-1 integrase inhibitor in clinical development. Ninth Conference on Retroviruses and Opportunistic Infections, Seattle, abstract 13161, 2002.

Zhy K et al. Irreversible inhibition of HIV type 1 integrase by dicaffeoylquinic acid. J Virol 73: 3309-3316, 1999.