Patrick McGee, Senior Editor
The HIV/AIDS pandemic began in 1981 and the first drug to treat it, AZT, followed in 1986. Since then, scientists have worked to keep up with a constantly evolving virus.
Late in the afternoon on Friday, November 16, 1984, Marty St. Clair was in the virology lab at Burroughs Wellcome in Research Triangle Park, N.C. She had been working on an assay to determine the activity of compounds against a new, then-mysterious disease that would go on to kill 25 million people worldwide, more than 500,000 of them Americans. Late in 1983, researchers determined that the disease, Acquired Immune Deficiency Syndrome (AIDS), was caused by a virus, later named Human Immunodeficiency Virus (HIV).
“We were doing what today would be considered an absolutely archaic assay where we actually infected cells with the virus,” St. Clair recalls. “The cells were stuck to the bottom of Petri dishes and we infected those cells with virus. Every place where a virus infected cells, they would die and fall off the plate, and so what you would have was a hole in the cell sheet. You would count the holes and that would indicate the number of viruses that were replicating.”
On that November Friday, St. Clair was counting plates as she had for the last several months as researchers in the lab tested numerous compounds that might work against HIV, one of which was zidovudine, also called AZT. “I came to a set of 16 plates that had all been treated with AZT and none of them had any plaques at all. It was absolutely eye-opening, totally different from anything I had seen before,” St. Clair says, describing it as “absolutely a eureka moment.”
But she had her doubts. After showing the plates to her supervisor, she wondered aloud if she had forgotten to put the virus in those 16 wells. “My supervisor said, ‘Of course not. Out of 350 plates you forgot to put the virus in these 16? It makes no sense.'” In addition to her supervisor, St. Clair told a few other people about the findings before leaving for the weekend. “Monday morning when I came in to work, I think I had eight telephone messages: ‘Is it true, AZT works against HIV?’ People were very, very excited.”
Early, ominous clues
The first mention of what would become a global pandemic appeared in the Centers for Disease Control’s Morbidity and Mortality Weekly Report. On June 5, 1981, it described five previously healthy homosexual males ages 29 through 36 treated for biopsy-confirmed Pneumocystis carinii pneumonia at three different hospitals in Los Angeles. Two of the cases were fatal. An editorial appended to the article noted the rarity of such a cluster and underscored facts with alarming implications: “Pneumocystis pneumonia in the United States is almost exclusively limited to severely immunosuppressed patients. The occurrence of pneumocystis in these five previously healthy individuals without a clinically apparent underlying immunodeficiency is unusual. The fact that these patients were all homosexuals suggests an association between some aspect of a homosexual lifestyle or disease acquired through sexual contact and Pneumocystis pneumonia in this population.”
While described by some early on as a “gay” disease, HIV/AIDS has proven to be anything but, having gone on to infect more than 65 million men, women and children worldwide. It has been particularly deadly in poorer countries with limited healthcare infrastructures and insufficient funds for treatment. But the news has not been all bad. Soon after HIV was identified, AZT emerged, and in the ensuing years more drugs with increasingly sophisticated mechanisms of action and efficacy were approved.
The emergence of combination therapies and assays that measure viral load in the mid-1990s allowed for more effective treatment that could be tailored to particular cases. This greatly reduced mortality, transforming a diagnosis of HIV/AIDS from a literal death sentence in the early days of the disease to a chronic illness which today can, in the vast majority of cases, be managed. But the work of pharmaceutical and biotechnology companies is not finished, as there is a constant battle to continually develop new therapies with new mechanisms of action to combat a virus that is constantly mutating.
“We’re in an era where HIV therapy has come from essentially nothing,” says Dale Kempf, PhD, distinguished research fellow in antivirals at Abbott Laboratories, Abbott Park, Ill. “We now have quite a few drugs, and over the next several years we’ll probably have more than 30 drugs to treat the infection. The therapy will be more and more driven by convenience and tolerability than by other factors. There will always be some need for agents that will treat resistant viruses, but with the drugs we have today, if one gets newly infected with HIV, in most cases those drugs can be very effective. Drug resistance is still an issue and the incidence of transmission of drug resistant viruses is on the rise, so that will eventually impact things as well.”
Jeffrey Murray, MD, MPH, head of the US Food and Drug Administration’s division of antiviral products, agrees that a great deal of progress has been made, but he sees a pressing need for newer drugs. “I think it would be nice to get a couple of totally new classes of drugs out in close proximity so people who have burned through the other classes would have a shot at a full regimen and gaining complete suppression and complete control of their virus. In the beginning, a lot of people started being treated with mono therapy, then with dual therapy and then finally triple therapy, and the people who started out taking one drug at a time burnt through drugs. Now, I think initially we get people under better control right away because they start out with three drugs.”
From leukemia to AIDS
The action of AZT was detected not long after HIV was identified as the cause of AIDS and before it was fully understood how HIV attacks and replicates itself in immune system cells. AZT was synthesized as a potential treatment for leukemia in 1964 by Jerome Horowitz at the Michigan Cancer Foundation, but after it proved toxic in studies involving mice, it was put on the shelf. Later research showed it could inhibit replication in retroviruses, but there was little interest because retroviruses were then unknown in humans. That changed in 1983.
Burroughs Wellcome, which has morphed over the years into Glaxo Wellcome and then GlaxoSmithKline (GSK), was able to move quickly on this news because they had recently determined the mechanism of action of acyclovir, at that time one of the first effective antivirals for herpes. St. Clair had worked on retroviruses as an undergraduate and was working on a PhD in virology at Duke University when she left to take a job in a new virology lab at Burroughs Wellcome. She had worked on acyclovir, so when the company announced it was establishing a program to attack HIV/AIDS, she eagerly volunteered.
Following the positive results of her finding on November 16, she repeated her initial assay using concentrations that were 1,000 times lower. The results two weeks later showed an almost complete inhibition of virus replication. “It was just like a whirlwind,” St. Clair says. “We were going to develop this drug. We had to do everything possible to get things going quickly to get the rest of the tox data, to get pharmacokinetic information and then to get it into HIV-positive humans.” A 24-week randomized, double-blind clinical trial with 282 patients began in January, 1986. After 16 weeks, only one of the 145 patients receiving AZT had died, compared to 16 patients receiving placebo, leading the independent monitoring board to halt the trial. AZT was released in October, 1986.
HIV works by forming viral DNA to take over host cells and replicate inside them; the enzyme that transcribes RNA into a DNA copy inside the cell is reverse transcriptase. AZT works by inhibiting reverse transcriptase, and once it is incorporated into the viral DNA chain, it prevents its completion. “The minute AZT is inserted, all replication stops. . . . AZT terminates the chain,” St. Clair says. Because of its action, AZT and the subsequent drugs in its class came to be called nucleoside analogue reverse transcriptase inhibitors (NRTIs).
While hailed as a wonder drug, AZT was far from perfect and later classes of drugs such as protease inhibitors would improve upon it. “AZT and many of the other drugs of that class are just not as potent as protease inhibitors,” says Abbott’s Kempf. “And although AZT was shown to prolong life, it soon became evident that the virus could become resistant to AZT and those patients would die with resistant viruses in their blood.” Another issue with AZT was considerable toxicity that could result, in some cases, in anemia and myopathy.
Off the sidelines
Given the reception that AZT received, it is no surprise that other pharmaceutical companies, who had until then largely been standing on the sidelines, decided to start development programs for HIV/AIDS. In 1991, Bristol Myers Squibb came out with Videx, the second NRTI to be approved, and the company added another, Zerit, in 1994. “They are very potent compounds that have successfully been used since their time of approval and are typically part of combination therapy, typically your triple combinations that might include two nucleosides as a backbone with a primary drug that could either be a nonnucleoside or a protease inhibitor,” says Steven Schnittman, MD, vice president of global clinical research in HIV/virology at the BMS Pharmaceutical Research Institute in Wallingford, Conn.
Other companies have introduced a number of other NRTIs, including one, Emtriva (FTC, emtricitabine) from Gilead Sciences Inc., Foster City, Calif., as recently as 2003. But it became clear very soon after AZT came on the market that resistance was, and would continue to be, a vexing issue. “When we first saw the nucleosides, the issue of resistance was less clear because the nucleosides were good drugs, but the potency that they had was fairly limited. So when we eventually saw the emergence of resistance it was fairly subtle because the activity against which you are working is fairly limited,” says Miklos Salgo, MD, PhD, clinical science leader in HIV at Hoffmann-LaRoche, Nutley, N.J.
Due to an extremely high rate of replication and a polymerase that is very prone to error, HIV mutates easily and often. “We learned early on that if you were on one drug or two drugs, this resistance developed very rapidly,” says Schnittman.
Due to a number of factors, this began to change in the mid-1990s, which is regarded by many as the golden age for the treatment of HIV/AIDS, an age that began in 1995. The FDA approved GSK’s Epivir (lamivudine, 3TC) plus Retrovir (AZT), two medications taken at the same time, as a dual HIV/AIDS therapy. A study by the National Institute of Allergy and Infectious Diseases demonstrated that combining drugs already on the market slowed HIV’s high mutation rates. The era of combination therapy had begun and the phrase “drug cocktail” entered the public lexicon.
Interfering with enzymes
The year 1995 marked the beginning of another era, that of the protease inhibitors, the first of which was Invirase (saquinavir mesylate, SQV) from Roche, which was approved only 97 days after being submitted to the FDA. Protease inhibitors (PIs) work by interfering with the function of a key enzyme, protease, essential to the multiplication of HIV. Salgo says PIs emerged out of the growing knowledge of the life cycle of HIV in the late 1980s.
The following year Abbott’s Norvir (ritonavir, ABT-538) was approved. Kempf says the company’s effort, like that of many other companies, grew out of work already being done with inhibitors of other proteases of the same class. The most notable of these was renin, a human enzyme involved in blood pressure maintenance. “When HIV protease was discovered, people immediately started testing their renin inhibitors for activity against HIV. Some of the protease inhibitors can be traced directly back to those efforts,” Kempf says. Abbott used testing from its renin program and combined it with information on the three-dimensional structure of HIV protease to guide the design of its inhibitors, engineering them to match the particular structural characteristics of the enzyme. “The HIV protease inhibitors were one of the first examples of a rational, structure-based approach.”
As Abbott researchers were working with Norvir and finding that it did indeed inhibit the replication of HIV, they discovered a second property that could be potentially troubling, the fact that it also inhibited cytochrome P450, an enzyme that clears drugs from the body. Paradoxically, this was something that would be used to enhance the performance of PIs, which are cleared very rapidly from the body. The inhibition of cytochrome P450 meant the drug stayed in the body longer, making it more effective.
This was seen with Abbott’s second PI, Kaletra (lopinavir, ritonavir), which came on the market in 2000. “One of the things we discovered between Norvir and Kaletra was that you could co-administer Norvir with almost any other protease inhibitor and you could increase the concentrations of that other inhibitor in the blood many fold,” Kempf says. They chose to combine lopinavir with ritonavir because the former was “supercharged” by the latter, he adds. The concept of “boosting” the effect of one drug by combining it with another emerged early in 1995, and today “virtually all” protease inhibitors are used in this manner whether they’re co-formulated or not, Kempf says. “That understanding of the relationship of drug action, drug pharmacokinetics and resistance led to another transforming event that allows us to really prevent resistance from happening in many cases.”
The emergence of a third class of drugs at about the same time gave patients and clinicians even more options. Viramune (nevirapine, BI-RG-587) from Boehringer
Ingelheim Corp., Ridgefield, Conn., was the first nonnucleoside reverse transcriptase inhibitor (NNRTI), a class which today consists of three drugs which work in a manner similar to NRTIs. Sustiva (efavirenz), an NNRTI from BMS approved in 1998, has gone on to become one of the most commonly used drugs in first-line therapies, Schnittman says. “It’s very easy for patients to take because it’s only once a day, and because of its good potency and tolerability, patients can take that now for several years or more and still be successful on it.” The practice of combining these three classes of drugs came to be referred to as highly active antiretroviral therapy (HAART).
Some of the first data on the effectiveness of combination therapies emerged in 1996, much of it at that year’s International AIDS conference in Vancouver, Canada. For the first time since the start of the pandemic 13 years before, there seemed to be real reasons for hope. That same year, the FDA approved the Amplicor HIV-1 Monitor test from Roche Diagnostics, Basel, Switzerland, enabling clinicians to measure down to 400 HIV RNA copies per milliliter, thus allowing them to better monitor their patients’ progress and adjust medications accordingly. “The concurrence of the test and PIs is frequently not appreciated for its importance and just plain good fortune, because what we observed enabled the rationalized, mathemetized,” treatment of HIV/AIDS, says John Leonard, MD, vice president of global pharmaceutical R&D at Abbott Laboratories. “It sounds trivial in some respects, but it was fundamental.”
This also had a huge impact on how clinical studies were conducted, Salgo says. Earlier studies required 500 or 600 patients and took two to three years to track disease progression or time to death. “Once we got the viral load, we were able to do much smaller studies and to do them more rapidly.”
A dramatic impact
The combination of HAART and the ability to monitor viral loads had an almost-immediate and dramatic impact. In the United States, the number of AIDS-related deaths dropped from just under 45,000 in 1993 to just over 17,000 in 1998. Between 1996 and 1997 alone, AIDS-related deaths dropped by 42%. But despite those impressive numbers and the hope they inspired, there were still challenges to be overcome. Michael Rodgers, PhD, vice president of the division of viral diseases at GSK, says one of these was that while HAART was very effective, it involved very complex drug regimens. “It may sound trivial if you have such a serious disease, but for patients trying to be adherent on these regimens, taking lots of pills was a big issue and adherence became a major barrier to survival.” Over the years, combination pills have simplified regimens so much that instead of taking dozens of pills over the course of a day, patients can take as little as two pills twice a day.
GSK’s Combivir (lamivudine, zidovudine) was the first fixed-dose combination of two antiretroviral agents in one pill when it was approved in 1997. It is one pill taken twice a day with no food or fluid restrictions. Combivir is a preferred NRTI backbone in the US Department of Health and Human Services guidelines for the treatment of therapy-naïve patients and has proven to be very effective when taken in combination with an NNRTI or a PI. “Since the approval of Combivir, our company and other companies have introduced combination products, and that’s drastically improved adherence, which has led to better survival,” Rodgers says. Earlier this month, the FDA approved a once-a-day, single-pill drug. The drug, Atripla, combines BMS’s Sustiva and Gilead’s Truvada, which is a combination of Viread (tenofovir disoproxil fumarate) and Emtriva (FTC, emtricitabine).
But despite such advances, there is one inescapable fact: Scientists are continually playing a game of catch-up with a constantly mutating virus. This has led to the introduction of a number of new drugs in existing classes over the last few years and the emergence of one new class, with the promise of others to come. The first in the most recent class of drugs is Fuzeon (enfuvirtide, T-20) from Roche, which was approved in 2003. Fuzeon is called a fusion inhibitor, but it is in a larger class of drugs called entry inhibitors, Salgo says, and there is often confusion due to the nomenclature used. “Entry inhibitors stop any of the different processes associated with virus entry into the cells,” but they work in different ways.
Fuzeon was developed by Trimeris Inc., Morrisville, N.C., a small biotech company that based the concept for the drug on research done at Duke University. Researchers there were looking at peptides being produced by the virus and looking at certain peptides that were relatively unchanged from different viral species. “They realized that by taking these peptides produced by the virus, they could actually inhibit the viral growth. It became evident that what was happening was that when the virus comes towards the cell, it doesn’t just bump into the cell, it has to have a very unique coupling to get into the cell, and that coupling requires several very specific steps,” Salgo says, including the action of CD4 cell antigen and the CCR5 co-receptor. Fuzeon interferes with the entry of HIV into cells by inhibiting the merger of the virus with the cellular membrane, thus blocking HIV’s entry.
Another entry inhibitor in development is maraviroc (UK-427, 857), a chemokine receptor antagonist designed to prevent infection of CD4-T cells by blocking the CCR5 receptor. Maraviroc, which was developed by Pfizer Inc., is now in phase IIb/III clinical trials, and because it works via a different mechanism than other drugs on the market, it will likely be promising for HIV-positive patients who no longer respond to those drugs. GSK had a CCR5 entry inhibitor called aplaviroc (GW873140) in development, but was forced to end studies in treatment naïve patients due to reports of severe hepatotoxicity with elevated liver enzymes and total bilirubin. “From a scientific perspective, it’s a very interesting approach,” says GSK’s Rodgers. “But it’s also very complicated.”
Merck & Co. Inc., Whitehouse Station, N.J., is hard at work testing a new compound for yet another class of drugs, integrase inhibitors, so named because they prevent the virus from integrating into the genetic material of immune cells. Earlier this year, Merck presented phase II data on MK-0518 showing it had greater anti-retroviral activity than placebo when tested in 167 patients. Gilead is working on GS 9137, another integrase inhibitor. Earlier this year Gilead announced results of a phase IIb dose escalation study that showed GS 9137 induced significant reductions in viral loads in patients using it as a monotherapy and in combination with ritonavir as a boosting agent compared to placebo. Rodgers says early studies have shown these compounds to be “very potent” inhibitors.
Finally, Panacos Pharmaceuticals Inc., Watertown, Mass., is developing what could be the first in yet another new class of drugs, maturation inhibitors. Its lead candidate, bevirimat (PA-457), which prevents virus particles from being released from cells, thus making the virus non-infectious, has just entered phase IIb clinical trials. “The industry is being pushed to look at novel mechanisms because in theory they should offer significant benefit to patients because they could be used against all viral strains,” Rodgers says. “What science has done is they’ve broken down the products that the virus encodes within a cell and they look for places of attack, and there are a number of enzymes that could be targeted.”
While it has led to the introduction of dozens of new drugs, the emergence of HIV/AIDS has also had a fundamental effect on the industry because it has changed how things are done. “HIV certainly broke the conventional mold,” Rodgers says. In the early 1980s, industry, academia, regulatory agencies and patient communities were very isolated, but that changed out of necessity. “I can’t think of any other example where patients had such a big influence on drug discovery and pricing and many other issues. It was really HIV—and the sometimes difficult partnerships among industry, the patient community, regulatory agencies like FDA and the government—working together. I think a lot of therapeutic areas have started to follow this example, particularly breast cancer.”
Abbott’s Leonard says that while many drugs have been developed and had a significant impact on mortality in wealthier countries, work to develop cheaper drugs with simpler regimens needs to continue. “It’s a wonderful story. It’s something that science and our industry should be very proud of, but we haven’t figured out collectively how to solve the problem for the other 98% of people infected with HIV.”
This article was published in Drug Discovery & Development magazine: Vol. 9, No. 7, July, 2006, pp. 10-16.
Filed Under: Drug Discovery