Designing drugs to fight disease-causing viruses is a relatively young science that poses special challenges. Viruses live and multiply inside cells in the body that perform vital, day-to-day functions. Thus, to be safe for human use, an antiviral drug must kill its target without harming the cells it infects.
Until recently, the problems inherent to developing compounds to selectively kill viruses inside cells seemed insurmountable. The new tools of biotechnology, however, have enabled scientists to amass staggering amounts of information about the molecular biology and life cycles of viruses and how they cause diseases. This knowledge lets researchers zero in on a virus' most vulnerable features and design drugs to disarm it.
Bringing a new drug to market requires three principal steps. First, the candidate drug must either be discovered in the laboratory or the natural environment or be designed from scratch, based on information known about the microbe. This step involves evaluating the activity of the candidate drug in test-tube and tissue culture experiments and in animal models that mimic the human disease. Then the candidate drug must be purified and produced on large enough amounts so that its safety in animals can be assessed. Finally its safety and effectiveness must be established in human clinical trials.
As the number of people infected with the human immunodeficiency virus type 1 (HIV-1) continues to grow, efforts by government, university and private industry scientists to find treatments that inhibit HIV infection or slow the progress of HIV disease have expanded markedly. The National Institute of Allergy and Infectious Diseases (NIAID), through its Division of AIDS (DAIDS), plays a major role in fostering the discovery of promising anti-HIV agents and assuring their development into drugs for use in people infected with the virus. NIAID also has assumed this role for agents showing promise against microorganisms that cause opportunistic infections (OIs) associated with AIDS.
Drug Screening Versus Drug Design
Therapeutics for HIV disease and its associated OIs are discovered through a variety of approaches including high-volume screening of drug libraries, targeted drug screening, evaluation of natural products, and rational drug design.
Nearly all approved drugs used to fight infections were discovered through random screening of naturally occurring and synthetic compounds. Natural products or compounds synthesized in the laboratory can be submitted to the National Cancer Institute (NCI) to be initially screened for anti-HIV activity. Some 12,000 compounds are screened through this program yearly. About one out of every 50 compounds tested shows some measurable activity against HIV.
DAIDS staff use chemical databases, or drug libraries, to identify promising anti-HIV compounds that have not been synthesized to tested. An advantage of these databases is that they allow researchers to examine the potential effects of specific chemical changes on the activity and pharmacokinetics of the drug molecule without having to do tissue culture or animal model experiments.
Many compounds that inhibit HIV in screening tests, however, also seriously damage human cells. These compounds may be modified to reduce their toxicity and then be submitted for retesting. Only those that inhibit the virus while causing minimal damage to HIV-infected cells are candidates for development into drugs for human use. The compounds that meet these criteria amount to less than one percent of those screened each year.
A growing number of HIV researchers are pursuing a newer approach to drug discovery: rational, or targeted, drug design. The goal is to identify and learn as much as possible about vulnerable features of the disease-causing microbe and use these features as targets of drugs designed to thwart it.
DAIDS supports targeted drug discovery through the National Cooperative Drug Discovery Groups for HIV (NCDDG-HIV) and OIs (NCDDG-OI) programs and through a limited number of grants to individual investigators. In addition, the Strategic Program for Innovative Research on AIDS (SPIRAT) supports research on state-of-the-art therapeutic strategies from the advanced preclinical stage through pilot clinical studies in humans.
The NCDDG and SPIRAT programs provide major support for researchers pursuing targeted approaches to the discovery of anti-HIV as well as anti-OI drugs. But to design such drugs scientists first must gather extensive information about how HIV works.
The Biology of HIV
Since 1983 when HIV was discovered as the cause of AIDS, the virus has received an extraordinary amount of scientific attention. In that short time, researchers worldwide have learned more about the biology of HIV than of any other disease-causing virus. Information about the life cycle, reproduction and disease-causing mechanisms of HIV has come from studies of the genetics, biochemistry and molecular biology of HIV and its behavior once it infects and establishes itself within the cells.
HIV only multiplies inside cells. The virus gains access to the cell's interior through special attachment sites, or receptors, on the cell surface. Once inside, the virus uses its own enzyme, reverse transcriptase, to transcribe its genetic material, which is in the form of RNA, into DNA. HIV DNA then enters the cell nucleus and becomes inserted into the cell's DNA. From here, HIV genes govern the reproduction of new virus.
HIV's regulatory genes encode proteins that control virus replication. When the regulatory signals are turned on, the HIV DNA is copied back into RNA molecules, which than may become genetic material for a new virus or may be used to make structural proteins of HIV such as those of its outer coat or core.
Before leaving the cell, HIV particles assemble into whole viruses and are released through the cell membrane, picking up additional cellular proteins and other molecular components for their outer coats. Once the new viruses are outside the cell, they can infect new cells.
Targets for New Drugs
Because progression of HIV infection to AIDS depends greatly on the virus multiplying in and destroying immune system cells, many attempts to halt advancing disease are aimed at preventing HIV from replicating. Scientists designing anti-HIV drugs study each step in HIV's life cycle, looking for places where drugs may disrupt it. Candidate drugs might block receptors, inhibit essential enzymes, suppress replication signals or interfere with viral protein processing and assembly. Combinations of drugs may be aimed at two or more different steps in the virus' life cycle.
Examples of HIV drug targets include the following:
o reverse transcriptase - translates viral genetic information from RNA to DNA;
o protease - cleaves viral proteins into their mature, active forms;
o gp120 - mediates entry of the virus into the host cell;
o integrase - inserts viral DNA into the host cell chromosome.
Other viral proteins may yet prove to be good targets for drug discovery and, through additional studies, their mechanisms of action will be understood.
To make copies of itself, HIV takes over the cell's normal machinery. Thus, researchers must be careful to target compounds to specific virus or cell components without disrupting the normal workings of the cell. Such accidental disruptions in cell function are a major cause of toxic side effects and a key obstacle to designing safe and effective drugs against HIV and other viruses.
Drug designers must understand the physical characteristics of anti-HIV drugs and how HIV interacts with various components of the human immune system. X-ray crystallography is invaluable for determining the three-dimensional shape of HIV proteins and cell proteins so that critical sites can be targeted or the shape of existing drugs can be improved to fit more precisely the target site. The three-dimensional structures of three viral proteins -- CD4 (the cell surface binding site for HIV), protease and reverse transcriptase -- have been determined and have been used to design candidate anti-HIV drugs.
Drawing on the findings of basic research, NCDDG scientists develop laboratory procedures that allow candidate drugs to be tested for activity against HIV and its associated opportunistic infections. These researchers develop cell culture assays, biochemical screening tests and immunotherapies, or try to isolate and characterize active compounds from natural sources. The development of several new biochemical screens for agents blocking important HIV enzymes now enables investigators to screen more than 20,000 potential drugs annually without having to work directly with HIV.
So far, NCDDG scientists have helped advance several anti-HIV drugs into clinical testing. These include the following:
o stavudine (d4T), which like zidovudine (AZT), slows virus replication by inhibiting revers transcriptase; approved for treatment of adults with advanced HIV infection who no longer respond to or who are intolerant of other antiviral drugs;
o oral and intravenous inhibitors of protease, the enzyme that cuts up the larger HIV proteins made first by an HIV-infected cell into smaller pieces that can reassemble into a new virus; in particular, early work on discovery of protease inhibitors at Abbott Laboratories; the company's marketing application for liquid and capsule versions of its drug, ritonavir, is soon to be reviewed by the U.S. Food and Drug Administration (FDA);
o genetically engineered CD4 and its derivatives, which act as decoys to bind free HIV and prevent its entry into cells;
o two novel BHAP compounds, which also inhibit reverse transcriptase activity but by a different mechanism.
The Drug Development Pipeline
Whether promising anti-HIV agents are discovered by scientists in government, academia or private industry, these compounds must be carefully evaluated before they can be licensed for use in people. Such an evaluation, consisting of test-tube, cell culture and animal model studies of a drug's safety and potential effectiveness, is called preclinical drug development.
To determine whether a drug is safe and effective enough to be tested in humans, results from certain preclinical studies are reviewed by the FDA. Turning promising agents into effective drugs depends upon strong, efficient partnerships between government agencies and private industry. NIAID facilitates this process by collaborating and consulting with drug sponsors to ensure that sufficient and appropriate preclinical data are generated for FDA review.
Drug development advances in steps, not all of which are required before a drug enters clinical testing. These steps include: cell culture tests to determine anti-HIV activity, safety studies in animals, efficacy evaluations in animals, investigations into mechanisms of drug action and chemical synthesis and formulation. Those studies required to obtain FDA permission to begin clinical studies are termed "critical path" steps. Other studies not required by the FDA also may be conducted to help determine a drug's appropriateness for clinical trials.
Critical path steps fall into three general areas: efficacy, chemistry and safety. After a compound had shown activity against HIV in the test tube and in cultured cells, NIAID scientific staff ad drug company representatives plan a drug development strategy that emphasizes critical path components. Initial studies are designed to confirm a drug's anti-HIV activity. The design of such studies is important because anti-HIV activity can be evaluated in many test systems, each of which provides different kinds of information. These confirmatory tests validate and sometimes duplicate the original observation.
Next, researchers must learn whether a drug's anti-HIV activity observed in laboratory studies can be duplicated in animal models of AIDS. Because HIV does not infect animals other than humans (except specifically designed experimental animal models), candidate drugs are tested in animals that are infected with viruses that cause AIDS-like disease symptoms, such as weight loss, abnormal blood cells and diarrhea. These model diseases are caused by viruses closely related to HIV.
Currently, DAIDS has available animal models of AIDS-like diseases in mice, cats and monkeys. Before carrying out studies in animal models, scientists conduct laboratory experiments to determine a drug's activity against the virus that produces the model disease. Results of such tests establish whether or not the virus is as susceptible as HIV to the new drug, and they also help in interpreting any negative results.
Although the importance of animal model testing is widely accepted and recognized, it cannot be used alone in determining the drug's efficacy because it does not directly measure the drug's effect on HIV. Still, the FDA currently recommends animal model testing, if available, for drugs that will be used to treat people with HIV infection. Until a predictive model is established, however, the FDA does not require that the experimental drug cure the disease in the animal.
Even though the FDA does not require efficacy in an animal model before allowing clinical trials to begin, clinical researchers often use data from animal model tests as the basis for selecting which compounds will be given priority for entering clinical trials.
Drugs developed to combat OIs also take a critical path toward clinical trials. All people with AIDS are infected with HIV, but the virus is rarely the direct cause of death. The deterioration and eventual collapse of the immune system triggered by HIV infection enable common microbial organisms to flourish. These normally are present in the body but are held in check by a healthy immune system. In people infected with HIV, an OI can become life-threatening owing to the inability of the immune system to control the infection.
The second major critical path area of preclinical drug development is chemistry. The quantities of a newly synthesized drug required for initial test-tube studies are very small. Animal model testing, however, requires much larger quantities of drug, and early clinical safety trials require even more drug. Consequently, if test-tube studies show promising results, chemist begin producing quantities of drug that may be 1,000 times greater than the original amount made. Most academic chemical laboratories, which synthesize many potential anti-HIV drugs, do not have the capacity or expertise to make large quantities of drugs, and it can take six to twelve months to produce enough drug to supply all studies required by the FDA for approving a new agent. DAIDS currently has the resources to assist drug sponsors with producing large quantities of new drugs, particularly those drugs used to combat OIs.
After animal model testing is completed, candidate drugs may need to be refined with certain chemical changes in the original drug for better efficacy or stability into a form easily given to humans. Thus, at the same time that animal model studies proceed, and while larger quantities of drug are synthesized, other chemists search for ways to modify the drug to make it easily administered and of greater benefit to people. DAIDS also has the capacity to develop these formulations and to help manufacture them in quantities and purities that can be administered to patients.
The final step in the critical path development of a new drug is to establish the safety of the drug before giving it to people for the first time. These studies, carried out in animals, are carefully designed to answer critical questions about how the drug will be metabolized by humans, how long it will remain in the blood after administration, in which tissues will it be localized and what toxic side effects it may have. Based on the results of these studies, researchers can decide how best to administer the new drug and which doses to use. Because of the importance of these studies to the safety of patients, the FDA requires that these studies be conducted under stringent laboratory control and monitoring. DAIDS currently has the capacity to provide such safety studies to drug sponsors.
Although preclinical drug development obviously emphasizes the critical path steps, many ancillary studies take place simultaneously throughout the development and clinical testing of a new drug. These studies may identify the molecular target of the new drug; describe the mechanisms by which the drug inhibits HIV replication; evaluate the effects of the drug on healthy, non-HIV-infected cells or on viruses other than HIV; and determining how chemically related drugs affect HIV.
With its resources for chemistry, as well as its capacity to determine drug safety, DAIDS continues to exert a strong leadership role in seeing promising new chemicals through the preclinical drug development process into clinical trials.
If laboratory and animal studies show that a candidate drug is safe and likely to be effective, it is ready to be evaluated in humans in clinical trials. NIAID asks a committee of expert advisors to prioritize promising drugs for advance into clinical trials. Because a drug may act much differently in humans than it does in animals, evaluating the safety and effectiveness of a candidate drug in people must be done in discrete phases.
A drug's sponsor, which may be a drug company, a university or a government agency, must first file an investigational new drug (IND) application with the FDA. This application presents the results of test-tube, tissue culture and animal model studies for FDA review. If these studies indicate that the drug is safe enough to be given to people, the sponsor can then begin the first phase of clinical trials.
The largest network in the United States conducting human clinical trials of experimental AIDS therapies is the NIAID-supported AIDS Clinical Trials Group (ACTG). The ACTG coordinates studies involving drug companies, university hospitals and other government agencies to conduct trials to determine the dosage, safety and side effects of candidate drugs, as well as their effectiveness in fighting HIV infection or the immune deficiencies and opportunistic infections associated with AIDS.
In addition to the ACTG, NIAID supports clinical trials performed in community-based clinics under the Terry Beirn Community Programs for Clinical Research on AIDS (CPCRA). By involving community physicians in clinical trials, this program makes promising, experimental drugs available to patients who may not have access to clinical trials taking place at university-based medical centers.
A third mechanism for conducting clinical trials, the Division of AIDS Treatment Research Initiative (DATRI), was instituted by NIAID in October 1991. DATRI allows NIAID to rapidly address critical questions about new therapies and therapeutic approaches.
The Institute also sponsors AIDS clinical trials at the research hospital on the campus of the National Institutes of Health. Several important studies have been conducted and published by these researchers.
A Phase I clinical trial is the first setting in which an experimental drug is given to humans. These trials are thus designed to answer initial questions about a drug's safety. Researchers look for information about the drug's side effects, how much of the drug can be given to a patient and how the drug is handled by the body. Such information usually can be gathered in less than one year. Phase I trials are conducted on a small number of people, usually fewer than 20, and all participants receive the drug. Participants in Phase I trials for HIV and OI drugs usually have HIV infection.
If results from Phase I trials show that a drug is safe, it can enter the second phase of drug testing. Phase II trials enroll larger numbers of patients, as many as a few hundred. In these studies, researchers begin to ask questions about whether the drug is effective against HIV infection, AIDS-related immune deficiencies or opportunistic infections. Phase II clinical trials may take one to two years to complete.
To best answer questions about a drug's effectiveness, researchers try to avoid introducing psychological or intuitive biases into their studies. One was they can try to control conditions is by dividing study participants into two groups and giving the candidate drug to only one group. The other patients receive another drug already approved for their disorder or, if no such drug is available, they may get an inactive agent called a placebo. Doctors compare the outcomes in the two groups to see if the people who got the experimental drug have fewer symptoms, stay healthy longer or have fewer side effects. Currently, very few clinical trials of experimental AIDS drugs use placebos.
Another method of controlling bias in a clinical trial is to ensure that neither the health care providers nor the patients know who is getting which drugs. This is called "double blinding." Drugs are disguised and given code numbers so they cannot be recognized. The codes are kept secret, known only to a small number of people who oversee the progress of the clinical trial, until the study has ended.
Phase III clinical trials continue to answer questions about the drug's effectiveness and also look for long-term side effects that may not show up in earlier testing. For that reason, they may take up to four years to complete. Phase III trials often enroll several hundred to a few thousand patients and are controlled and blinded.
Traditionally, drug approval has been based on clinical efficacy, that is, an improvement in the patient's condition. With AIDS, however, the FDA is provisionally accepting efficacy based on improvements of the patient's laboratory tests, such as the number of certain immune system cells in the patient's blood. A drug approved under these guidelines is then re-evaluated when the final clinical data are submitted. This strategy is designed to speed the approval process.
The FDA has modified other regulations to speed the testing and approval processes for drugs aimed at life-threatening diseases. Under these guidelines, Phases II and III may be combined. This means that large Phase III trials to determine a drug's effectiveness may begin alongside Phase II trials that continue to evaluate and experimental drug's safety.
When Phase III clinical trials are complete, drug sponsors present results from all laboratory, animal and human studies to the FDA for review in the form of a new drug application (NDA) for approval to market the drug.
The magnitude and urgency of the AIDS epidemic have led researchers to look closely at the drug evaluation process to determine how it might be streamlined without compromising health and safety. One option is to conduct some controlled trials that measure fewer parameters and include more patients. Such studies are being conducted by the Terry Beirn CPCRA. The Public Health Service also has instituted a mechanism to make some promising drugs more available at the same time that they are being studied in clinical trials. Under this "parallel track" system, patients who do not qualify for or do not have access to clinical trials and who have no other therapeutic option can receive certain experimental drugs once these drugs have entered clinical trials.
Information on HIV/AIDS drug trials being conducted across the nation can be obtained by calling the AIDS Clinical Trials Information Service, a service of the U.S. Department of Health and Human Services, Public Health Service. The telephone number is 1-800-TRIALS-A (1-800-874-2572), and the service is open Monday through Friday form 9:00 a.m. through 7:00 p.m. Eastern Time. Spanish-speaking information specialists are available.
Prepared by: Office of Communications National Institute of Allergy and Infectious Diseases National Institutes of Health Bethesda, Maryland 20892
Public Health Service U.S. Department of Health and Human Services