AIDS TREATMENT UPDATE, Issue 30, June 1995
Raffi Babakhanian

One of the main difficulties in treating HIV with the existing anti-viral agents is that HIV tends to mutate rapidly, creating strains which are resistant to a particular drug. Researchers believe that using more than one anti-HIV drug at a time may help to reduce or even eliminate the problem of HIV mutation. This is called combination therapy, and there are two different types.

Single-target combination therapy (also known as convergent combinations) uses two, three, or even four different drugs that target the same point of the HIV life-cycle. The hope is that by hitting one point with a combination of drugs, the virus won't be able to mutate simultaneously against all the components of the combination. This theory is currently being tested in trials combining AZT plus either ddI, ddC, and/or 3TC, among others.

However, AZT, ddI, ddC, 3TC and nearly all other anti-viral drugs being used or tested against HIV target only one enzyme used in the HIV life-cycle, called reverse transcriptase (RT). While RT is essential to HIV replication, there are several other steps in the intracellular life cycle that could be inhibited.

Multiple target (or divergent) combination therapy uses combinations of drugs that inhibit several different enzymes or regulatory proteins necessary for HIV infection and reproduction. Here, the hope is that the virus will be unable to mutate successfully against simultaneous attacks at different sites. To understand how this might work, it's important to have a general understanding of how HIV infects cells and then uses those cells as a kind of factory to produce more virus.

VIRAL GENES

HIV has nine genes which contain all the information to enable it to infect healthy human cells and then use the infected cells to create more virus. Like all life forms HIV exists to procreate, and the nine genes each serve a function in this process. Roughly speaking, the genes can be grouped into three categories.

First, there are the structural genes gag, pol, and env, which tend to govern the structure of the virus i.e. its external surface (the envelope) and its internal structures. Gag and pol also give rise to three enzymes which are essential for HIV replication inside cells: reverse transcriptase, protease and integrase.

Secondly there are the regulatory genes tat, rev, vpr (vpx for HIV-2) and nef. They regulate the speed and efficiency with which HIV reproduces itself, so the proteins they give rise to clearly play an essential role in the replication of HIV within cells.

Finally, there are what have been called the accessory genes, vpu and vif. At first these genes and their proteins were thought not to be essential to the replication of HIV because even when they are removed in the laboratory, HIV can flourish in the test tube. However recent research has shown that these genes do play an important, if not essential, role in the HIV life-cycle within the body. Scientists are just now beginning to understand the subtle and interconnected functions that they serve in the replication and survival of HIV within infected cells.

HIV specifically infects cells which have a molecule called CD4 on their surface. T4 lymphocyte cells, which help to orchestrate the immune response against any foreign agent that might cause infection or disease, have more CD4 on their surface than any other cell in the human body, so HIV infects these cells the most. Other immune system cells such as macrophages and monocytes also have some CD4 on their surfaces, so HIV can infect those cells too.

Once HIV breaks through the cell wall, it releases its contents into the cell. These contents include all of the virus's genetic information (the nine genes) as well as most of the enzymes and proteins needed to control the integration of HIV into the host cell and then to produce more HIV.

First, the virus needs to incorporate its genetic information within the genetic information of the host cell. This is the safest place within the cell, and HIV's genetic information can lie there undisturbed and unharmed (dormant) for years. However, genetic information can be in one of two forms - a single strand of genetic material called RNA, or a double strand called DNA. HIV keeps its genetic code in RNA but human cells keep their genetic information in DNA, so the virus has to convert its information from RNA to DNA. It does this using the enzyme reverse transcriptase (RT). Without RT HIV couldn't use the host cell to produce more virus, and would thus be much less infectious. This is why so much attention has been focused on RT inhibitors such as AZT, 3TC and loviride, which bind to parts of the RT enzyme and prevent it from doing its job.

OTHER VIRAL ENZYMES

In recent years, trials have begun to test drugs that target another enzyme essential for HIV replication, protease. Protease acts like a pair of scissors, cutting up 'raw' strands of enzymes and proteins created by the HIV RNA into smaller products that are used in the HIV reproductive process. Without protease HIV would only create long strands of protein that couldn't serve any function, again drastically reducing the virus' ability to reproduce and infect new cells. Early results indicate that protease inhibitors are very effective in suppressing HIV, but the virus very quickly mutates against some currently being tested.

Once the virus has converted its genetic information from RNA to DNA it needs to insert its DNA into the DNA of the host cell. To do this, it uses the enzyme integrase. Integrase works either by splicing the HIV DNA into the host cell DNA, or by attaching it to some point along the host cell DNA strand. Much work has gone into searching for an integrase inhibitor, but to date none has been found that inhibits integrase without also killing or harming the host cell.

But new research developments have greatly increased the chances of designing better inhibitors of these three enzymes. All three enzymes have been crystallized, meaning that their three-dimensional chemical structures are now known. Now scientists can use this information to design drugs that bind to specific parts of each enzyme, perhaps to sites which are so essential to the proper functioning of the enzyme that any mutation would render the enzyme ineffective. This advance has already led to the design of the protease inhibitors, and it promises to produce new RT inhibitors that work more effectively. Previous drugs were developed either through trial and error or at best through educated guesses.

In addition to these enzymes, the other regulatory and accessory HIV genes produce proteins which enable the virus to reproduce and spread in diverse and destructive ways. One of the most studied regulatory proteins is tat. It seems to play an important role in stimulating a point on the HIV gene strand that starts viral reproduction, like a spark plug or switch. It was thought that inhibiting tat would inactivate viral replication, and a tat inhibitor called Ro 24-7429 was developed. But while Ro 24-7429 does seem effective in inhibiting tat in the test-tube, trials found that it didn't have any effect on HIV replication or viral load in the human body, possibly because it was not properly absorbed into HIV-infected cells.

Much recent research has focussed on the other regulatory proteins, namely rev, vpr, nef, vpu and vif. Inhibiting these proteins may become an important part of treating and controlling HIV, although to date no inhibitors have been developed. With such rapid progress in basic research into the structure and function of HIV, it is important that the growing knowledge from laboratory research be quickly channelled into developing drugs which in combination may halt the progression of HIV to AIDS.

MORE INFORMATION

More detail about HIV's lifecycle and the role of regulatory proteins is contained in the new edition of NAM's HIV & AIDS Treatments Directory, available from July 1995.

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