Tat Inhibitors, A New Approach: Interview with Olaf Kutsch, Ph.D.

by John S. James

Summary: An important potential target for antiretrovirals is the HIV protein Tat (produced by the virus and essential for infection, but is not used by the human body). Years ago, a Tat inhibitor worked well in the laboratory but failed in patients. Modern biotechnology may have shown why -- and how to screen for drugs more likely to work.

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Years ago Hoffmann-LaRoche (now Roche) developed an experimental drug that blocked the HIV protein Tat in laboratory tests. But it did not work against HIV in patients, for reasons that were then unknown. Though scientists considered this kind of antiretroviral particularly promising, industry largely abandoned it. Despite over 2,500 scientific publications on the HIV tat gene or Tat protein, the knowledge was not translated into practical drug development.

In 2004 GlaxoSmithKline awarded research grants to three scientists working on new treatment approaches. One award, about $83,000, went to Dr. Olaf Kutsch at the University of Alabama at Birmingham, who had helped develop a new laboratory test for Tat inhibitors. This test can tell how fast the inhibitor is working -- which is important, because HIV-infected T-cells do not survive long in the body. Dr. Kutsch suspects that this is one of the reasons the Roche Tat inhibitor worked in the laboratory but not in patients.

We interviewed Dr. Kutsch on January 12, 2005, about his current work and future directions.

AIDS Treatment News: Why would a Tat inhibitor be important?

Dr. Olaf Kutsch: Usually the preferred approach in developing antiretrovirals is to find a drug that specifically targets a part of the viral life cycle that is unique to the virus. The HIV Tat protein is essential for the virus to reproduce, but is not found at all in uninfected human cells. Tat was recognized early on as an important target, but so far no one has been able to identify a compound that effectively blocks Tat not only in the laboratory, but also in patients.

I became interested in developing cell-based systems for screening large numbers of compounds to look for potential antiretrovirals, based on work done by Dr. RM Anderson at the Imperial College in London. The idea is to look at instantaneous instead of cumulative outcome of inhibition.

Previous systems looked at cumulative inhibition after several days. But in the body, the half-life of an HIV-infected T-cell is less than two days -- though in the laboratory it can stay alive much longer. So in the laboratory, a compound that would suppress HIV after two or three days might look great at day six. But in the body, if it hasn't done its job in two days, history has passed over that inhibitor. It isn't going to work in patients.

We believe, and hope to prove some day, that our system will have higher predictive value for finding Tat inhibitors that actually work as antiretrovirals. It is impossible to prove this today, because so far there is no example of a Tat inhibitor that works in patients.

ATN: How does your system detect if a chemical inhibits Tat?

Dr. Kutsch: Once HIV has infected a cell, it starts to produce new viral parts that then assemble into new viruses. Imagine this virus production to be controlled by something like a dimmer switch that you use at home to control how bright your light is. This viral dimmer element is called a promoter. Tat is what turns the light on -- actually, really bright. We have taken a copy of this dimmer (promoter) and altered it such that it controls the expression of a green fluorescent protein in our cells. These cells also hold an active virus that produces Tat, which now in turn, turns on the expression of green fluorescent protein. In other words, EGFP [enhanced green fluorescent protein] fluorescence is a direct marker of Tat activity. If an inhibitor does not allow Tat to activate the promoter, you see a decrease in the fluorescence signal -- not instantly, but you can calculate back to what is happening with HIV expression. As Tat inhibitors are the only ones that interfere with the HIV promoter in our test, a strong decrease in the fluorescence signal tells us that we have identified a Tat inhibitor. With this test, if you put the old Roche inhibitor on the cells, for the first two days almost nothing happens. There may be a 20% decline in the signal -- not enough to control the virus. But if you look after 4 or 8 days, as people did previously, you get the same result they did, 80% to 90% inhibition, which would be fine. But then it is too late.

We have developed a system that can screen at least 10,000 chemical compounds in two days. We have access to a chemical library of 100,000 different compounds. If we can get substantial funding we want to test them all. The test itself is cheap compared to other tests, but infrastructure to do the work must be supported.

With the Glaxo grant and some additional funding, scientists here will be able to go through the 100,000 chemicals and pick a diversity set of 5,000 and screen that. This means that compounds will be chosen because they are representative of chemical groups. For every 20 related compounds we would look at one.

I am concerned that if we run only the 5,000 tests, we would have to be lucky to find something. Other work here has shown that extremely small changes in the molecule make all the difference whether it is going to inhibit Tat or not.

If we are lucky, or if we find the money to run the whole 100,000 chemicals in the library, I think we have a good chance to find something. Then the steps are the usual ones -- you would probably need to find an industry partner who is willing to put money on the compound [the Glaxo grant did not require assigning rights to the results, so other companies could be involved]. The toxicity testing and initial human trial evaluation is very expensive, and there is no mechanism to finance that through Federal funding.

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