AT THE 1996 world AIDS conference in Vancouver last month the optimism was palpable, a mood swing brought about by results flooding in from clinical trials testing new anti-HIV drugs called protease inhibitors. These drugs, which knock out one of the enzymes the virus uses to replicate itself, dramatically reduce the amount of HIV in patients鈥� blood, especially when used in combination with other drugs. Upbeat headlines appeared in The Washington Post and the Wall Street Journal. Some AIDS researchers and activists at the meeting even bandied around the word 鈥渃ure鈥�.
The casual observer could be forgiven for thinking that AIDS researchers had proved that the new drugs extend the lives of people with HIV. But although recent developments probably do hold out new hope, the truth is far more complex. For a start, the data on survival are not available yet, not by a long chalk, since key studies are only just getting off the ground.
For the protease inhibitors to fulfill their promise, a new batch of clinical trials-some of which look like no other HIV study that has gone before-must show that reducing the amount of virus in the blood actually leads to long-term improvements in patients鈥� health. That that will happen is no sure thing.
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On the face of it, 鈥渧iral load鈥� measurements have logic on their side-knock out the obvious traces of HIV and perhaps you can reduce the severity of the disease it causes. But so-called 鈥渟urrogate markers鈥�, which are designed to indicate the severity of a disease, have misled medical science before, and some AIDS researchers fear that viral load could lead them astray, too.
Until the advent in 1992 of the viral load tests-which have only become available in the easy kit form this summer-the only way to tell how well people with HIV were responding to drugs was either to monitor their symptoms, or to use rough and ready surrogate markers such as estimates of the number of immune cells in the blood.
Each method had its disadvantages. AIDS symptoms appear late in the game, and only after the immune system is almost totally shot, so showing that a treatment works by symptoms alone takes years. If the drug fails to lengthen the time that someone remains healthy, science is well served but it is a disaster for the individual.
In theory, surrogate markers get around those problems by monitoring how drugs slow the progress of the disease, long before symptoms appear. But in practice, all surrogate markers for HIV, including the number of immune cells called CD4 cells that the virus destroys, have proved insensitive, unreliable indicators of how effective any drug really is.
Viral load could be the exception. It is an actual measure of the pathogen that causes the disease, rather than an indirect measure like the number of CD4 cells. Using PCR, or another gene amplification technique called branch chain DNA, researchers can measure viral load with great accuracy.
Viral load also correlates well with different stages of the disease. It peaks soon after infection, and then it is suppressed by the immune system until the immune system is destroyed, at which point viral load soars again. What is more, unlike previous attempts to monitor the amount of virus in the blood by culturing samples in a test tube, viral load is far less variable from person to person.
Viral load is also a good predictor of how long someone with HIV is likely to live. Some 70 per cent of people infected by HIV whose blood contains less than 10 200 viral particles per millilitre survive 10 years, compared with only 30 per cent of people whose blood contained more than that amount, according to a study from a team led by John Mellors at the University of Pittsburgh. The load for a person with HIV at the midstage of the disease is usually 10 000 to 100 000 viral particles per millilitre.
Unfortunately, Mellors鈥� study had one big problem: it did not take into account whether people took anti-HIV drugs, and, if so, which ones and when. 鈥淚t could be that people who have naturally low viral loads have stronger immune systems and live longer for that reason,鈥� says Edward King, editor of the monthly newsletter AIDS Treatment Update. 鈥淎rtificially reducing the viral load with drugs might not be the same thing.鈥�
So far the strongest evidence that reducing viral load with protease inhibitors improves health comes from two short trials of patients in the very last stages of the disease. These trials showed that protease inhibitors called ritonavir and saquinavir, sometimes used in combination with drugs such as zidovudine (AZT), both reduced the viral load and slashed the amount of AIDS-related death and disease by at least 50 per cent. The other 15 or so trials of protease inhibitors in people with and without AIDS have only shown that viral load plummets, not that health is improved or life span extended.
Lingering dangerously
What is more, none of the trials gives much of a clue about what the virus is doing in other parts of the body where it is not picked up by viral load tests. HIV can lurk in many parts of the body and wreak havoc, for example, in the cells of the nervous system where it produces HIV-related dementia.
The trials also give little indication of whether, ultimately, the side effects of the drugs could cause as much death and discomfort as the disease itself if the drugs are used for the long periods of time envisaged. The known side effects include liver and kidney damage, nausea and anaemia. Only long-term studies will show whether they cause other diseases such as cancer.
The notion that reducing viral load with protease inhibitors in patients in the earlier stages of asymptomatic disease will extend life makes some doctors nervous, because hard evidence for it is lacking. Relying on untested if sensible-sounding surrogate markers in other diseases in the past has proved disastrous. The heart drugs encainide and flecainide are the classic examples.
In the 1980s, doctors reasoned that because these drugs could prevent heart arrhythmias, which are a risk factor for cardiac arrest, then they should also prevent cardiac arrest. The drugs were widely prescribed before they were tested in clinical trials. Despite the drugs鈥� impressive impact on arrhythmias, they tripled the death rate because of previously unnoticed side effects.
Still, the encainide-flecainide debacle and other similar examples have convinced relatively few AIDS researchers to stop relying on viral load to test anti-HIV drugs. Unlike arrhythmia HIV infection almost invariably means death, so waiting until there is definite proof that reducing viral load will save lives is not seen as an option. 鈥淲e don鈥檛 have the luxury of waiting that long,鈥� says Mike Youle, who coordinates clinical trials at the Chelsea and Westminster Hospital in London. Viral load may not be perfect, he says, but it 鈥渋s the best marker of any in existence鈥�.
And Michael Saag of the University of Alabama in Birmingham points out that when researchers look back at the large clinical trials testing the earlier anti-HIV drugs, a drug-induced reduction in viral load does seem to correlate with longer life. Of course, none of this is proof positive that if you reduce viral load with protease inhibitors it will also save lives, but as Saag says, 鈥渋n aggregate, the data is overwhelming鈥ayer that over your intuition, and there is no argument鈥�.
That sense of certainty has led Saag and other clinicians to break rank and use viral load, not only to test experimental drugs, but also to tailor individual drug therapy. If viral load suddenly soars, it is likely that the virus is no longer susceptible to the drugs. So it seems to makes sense to measure viral load every few months, and then use it to work out when to shift to new combinations of drugs.
Measured therapy
In the June edition of Nature Medicine, Saag and his colleagues wrote that viral load gives 鈥渋mportant information for patient management, including鈥he degree of initial anti-retroviral effect achieved and when a drug regimen is failing鈥�. They went on to recommend how to use viral load measurements in clinics.
The US Food and Drug Administration, on the other hand, has refused to approve viral load kits for tailoring drug therapy in clinics on the grounds that there is insufficient evidence that it works. In lieu of that FDA seal of approval, at least one drugs company that manufactures viral load kits, Roche Diagnostics, ordered 50 000 reprints of the Nature Medicine article, and distributed many of them in Vancouver.
Others are far more cautious about the merits of embracing viral load tests in either the clinic or the clinical trial. 鈥淰iral load tests give us information that no one knows how to interpret,鈥� warns Tony Pinching, professor of immunology at St Bartholomew鈥檚 Medical School in London. He claims that no one is able to say with any certainty how much viral load has to fall by, or for how long, for people to benefit. He also warns that measuring viral load in individuals encourages people to change between drugs they are taking, heightening the risk that drug-resistant HIV strains will emerge, and leaving no drugs in reserve for when they are really needed.
But to many researchers the potential advantages of combining individual viral load measurements with protease inhibitors and other drugs, in a high-tech assault on the virus, outweigh the risks. To settle the argument once and for all, Saag and other researchers are setting up clinical trials to test that approach.
Saag and his colleagues at the National Institute of Allergy and Infectious Diseases in Bethesda in Maryland are putting the finishing touches to a large 鈥渟trategy鈥� trial. This clinical trial will not compare effects of two different drug regimens. Instead, unlike any previous HIV trial, it will compare the health and survival of a group of people with HIV whose viral load is undetectable with that of a group whose viral load is kept below 5000 viral particles per millilitre of blood.
鈥淚 can鈥檛 say exactly what drug regimens we鈥檙e going to use. That鈥檚 not the point. We鈥檙e going to use the combinations necessary to get viral load down,鈥� says Saag. The trial should show whether it is better to do whatever it takes to keep the virus completely at bay-or hold some drugs in reserve, in case an individual becomes ill.
Meanwhile, David Ho and Martin Markowitz at the Aaron Diamond AIDS Research Center in New York City are going for broke in an experiment designed to knock out the virus with super aggressive drugs treatment from the very earliest days of infection. They hope that by keeping viral replication low from the start, they can prevent the emergence of the mutations that make the virus drug resistant, and possibly eradicate the virus from the body. Ho and Markowitz have recruited 21 volunteers to date. Each one receives three drugs, including the protease inhibitor ritonavir, from within three months of infection. The first recruit signed up in July last year. So far the viral load of all 21 remains undetectable. It was in connection with this study that a 鈥渃ure鈥� was mentioned in Vancouver.
But Markowitz, who鈥檚 painfully aware of the rising optimism and the crashing disappointments that have punctuated AIDS research in the past, says that it鈥檚 far too soon to even consider the possibility of a cure. For example, it will be at least a year before his team has had a chance to study each patients鈥� lymph nodes to see how their immune systems are bearing up. At the moment, they have no idea whether or not HIV outside of the blood stream is still causing damage.
Like so much else in AIDS research, 鈥渢his is a work in progress鈥�, he says. 鈥淲e don鈥檛 claim to have won the war. This is the beginning, not the end.鈥�
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Chipping away at mutations
DOCTORS would like to keep tabs on the strains of HIV that infect each individual, so as the virus mutates its way out of the clutches of one drug they can make sure their man or woman receives the new drugs that are most likely to work.
Now, researchers at Affymetrix, a biotech company in Santa Clara, California, have developed a technique that can monitor on a single glass chip every known genetic variation (and many of the thousands of unknown ones) in two key HIV genes in a matter of hours.
Thomas Gingeras of Affymetrix and his colleagues have already used the DNA-on-a-chip technique to show that the gene that codes for an essential HIV enzyme called protease is far more genetically diverse than anyone ever imagined.
Using the chip, the researchers sequenced the protease gene in 102 Californians infected with HIV 鈥淏鈥�-the subtype of HIV-1 that predominates in the US and in Europe. From the results, they calculated that almost half the 99 amino acids that make up the enzyme can vary, according to their report in the July issue of Nature Medicine. Even some HIV-infected people who had never taken protease inhibitors, the anti-HIV drugs that attack the protease enzyme, carried one or more of the dozen or so known mutations that help make the virus resistant to those drugs.
Affymetrix is now marketing a single chip that can measure all the genetic variation in both the HIV protease gene and in the reverse transcriptase gene, the target of drugs like AZT. At the moment, the chip is intended for research use, but in the AIDS arena research and therapy tend to go hand-in-hand, so it could become a bestseller.
Virologist Brendan Larder of the Glaxo Wellcome Research Laboratories in Stevenage says the chip technology will speed up the identification of the numerous mutations that make the virus resistant to each of the ten or more anti-HIV drugs. Once they鈥檝e been identified, he says, the chip 鈥渃ould hopefully be used to make treatment decisions鈥�.