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Report of the Research Briefing Panel on Prevention and Treatment of Viral Diseases

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Research Briefing Panel on Prevention and Treatment of Viral Diseases Wolfgang K. Joklik (Chairman), Duke University Medical Center, Durham, N.C. Seymour S. Cohen, State University of New York at Stony Brook (retired), Woods Hole, Mass. William Haseltine, Dana Farber Cancer Institute, Boston, Mass. Maurice R. Hilleman, Merck Institute for Therapeutic Research, West Point, Pa. Joseph L. MeInick, Baylor College of Medicine, Houston, Tex. Thomas C. Merigan, Ir., Stanford University School of Medicine, Stanford, Calif. Roland K. Robins, ICN Pharmaceuticals Inc., Costa Mesa, Calif. 50 Bernard Roizman, University of Chicago, Chicago, Ill. Julius S. Youngner' University of Pittsburgh School of Medicine, Pittsburgh, Pa. Staff Roy WidJus, Director, Division of International Health, Institute of Medicine Mark Feinberg, Consultant Alian R. Hoffman, Executive Director, Committee on Science, Engineering, and Public Policy

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Report of the Research Briefing Panel on Prevention and Treatment of Viral Diseases INTRODUCTION Viruses are segments of genetic material, either RNA or DNA, encased in protein sheds and often further wrapped in lipid-contain- ing envelopes. Viruses multiply only within living cells, commandeering the host cell to synthesize their proteins and also their nu- cleic acids. Infection profoundly affects the host cell, bestowing on it characteristics that distin- guish it from uninfected cells. Viruses dis- rupt or kill infected cells or transform them into tumor ceils, thereby causing disease. Viral diseases vary in severity from mild and transitory infections to illnesses that termi- nate in death. Persistent viral infection, which for years may not be accompanied by symp- toms, can eventually cause chronic degen- erative disease with fatal outcome. Viruses cause a wide variety of cancers in animals, and the epidemiologic and laboratory evi- dence is very strong that viruses also cause human cancer. Minimizing the harmful effects of viral in- fections has long been a major goal of med- ical and veterinary science. The two major approaches to achieving this goal are pre- venting the onset of viral diseases by im 51 munization (vaccination) and treating viral diseases by arresting and curing infections once they have started. PREVENTION OF VIRAE DISEASES Prevention of viral diseases is based on the fact that viruses usually elicit the for- mation of protective or "neutralizing" an- tibodies, cell-mediated immunity, or both. It is therefore possible to protect against viral infection by immunization raising a host's immune defenses ahead of time so that when the disease-causing virus enters the host, it is quickly neutralized by antibodies. In conjunction with other components of the host's immune system, these antibodies inactivate, destroy, and eliminate the virus. The immunizing agent may be active virus in the form of a harmless (avirulent) variant, inactivated (killed) virus, or the viral pro- teins that elicit the formation of neutralizing antibody (subunit vaccine). The earliest form of immunization undoubtedly was variola- tion, invented many centuries ago by the Chinese, in which persons were exposed to skin scabs from others who had survived smallpox infections. The rationale was that in such cases the disease had been caused

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by a less virulent form of the variola or smallpox virus. The practice was danger- ous, with a fatality rate of up to ~ percent, but it did afford a measure of protection. The first example of a "killed" vaccine was the rabies vaccine developed by Pasteur. As for the subunit approach, many such vac- cines have been devised, but none was widely used in human beings until a hep- atitis B virus subunit vaccine was licensed in the United States in 1981. Prevention of viral diseases has had some outstanding successes. Smallpox, one of the most devastating of all infectious agents in human beings, has been eradicated globally through the use of energetically executed vaccination programs. Yellow fever virus has also essentially been eliminated in some parts of the world. Effective control has been es- tablished over poliomyelitis, measles, mumps, and rubella, which until the midctle of the twentieth century infected millions of children annually in this country, causing many deaths and disabling even larger numbers. TREATMENT OF VIRAE DISEASES Successful treatment of viral diseases re- quires interruption of virus multiplication by specifically inhibiting the functioning of virus-encoded proteins and nucleic acids. This strategy primarily entails the identifi- cation of analogues of nucleic acid and pro- tein components capable of inhibiting virus- specified reactions to a greater extent than reactions essential for host cell multiplica- tion and survival. The approach used so far has been some- what empirical: effective compounds are ei- ther identified by the use of screening programs, or progressively more active an- alogues are clevised in organic synthesis programs. Numerous drugs have been found that are capable of inhibiting the multipli- cation of a wide variety of viruses in cul- lured cells to a greater extent than they inhibit the multiplication of the host cells them 52 selves. Several such drugs (idoxurctine, ri- bavirin, and Acyclovir) have been licensed for human use. Empirical approaches are not generally designed to yield drugs that are specific in- hibitors of the functions of viral proteins and nucleic acids. Until very recently, most of the drugs examined have been general in- hibitors of protein and nucleic acid synthe- sis, and a few happen to inhibit certain v~rus- encoded enzymes somewhat more effec- tively than analogous host cell functions. However, investigation of viral multiplica- tion cycles at the molecular level has greatly expanded knowledge of the virus-specific processes and structures that could serve as targets for antiviral chemotherapy. TARGET VIRAE DISEASES Many viral diseases still present grave problems. The virus that causes acquired immune deficiency syndrome (AIDS) is a recently recognized and serious problem, but many other viruses have long been known to cause a wicle spectrum of diseases, both acute and chronic, involving all organ sys- tems of the human body. Diarrheal and re- spiratory disease viruses are probably the major global cause of morbidity and mor- tality in children, especially in developing countries. These include rotavirus, parain- fluenza viruses, coronaviruses, and respi- ratory syncytial virus. Influenza virus afflicts all age groups but is most life threatening in the elderly. Other important viruses in- clude hepatitis B. cytomegalovirus, and herpes simplex. Viruses causing hemor- rhagic fevers or encephalitis are often insect- borne and include dengue virus (prevalent in the Caribbean and Southeast Asia) and Japanese B encephalitis virus (prevalent in the Far East). Poliomyelitis, measles, mumps, and rubella are still widespread in devel- aping countries. Viruses are suspected of involvement in several slow degenerative diseases; exam- ples include Creutzfel~t-lakob disease, Alz

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PREVENTION AND TREATMENT OF VIRAL DISEASES heimer's disease, and juvenile diabetes mellitus. The same is true for a variety of human malignancies including cervical car- cinoma (certain human papillomaviruses); Burkitt's lymphoma and nasopharyngeal carcinoma (Epstein-Barr virus); liver cancer (hepatitis B virus); and certain human leu- kemias (HTLV-I and -II). Many of these hu- man tumor viruses were discovered in the past 10 years, and it is expected that more will be found before long. In addition to the viruses that affect hu- man beings, there are many others that cause disease in animals and plants, with enor- mous economic loss to agriculture. In summary, viruses remain among the major scourges of mankind. They cause an enormous burden of illness with resultant economic loss; they kill and permanently disable millions annually both in this coun- try and throughout the world. The objective of this report is to illustrate that recent ad- vances in virology and molecular biology have pointed the way to new strategies for preventing and treating viral diseases. RECENT ADVANCES IN MOLECULAR VIROLOGY The advent of two technologies during the past 15 years has led to greater knowI- ecige of the nature of viruses and of their multiplication cycles. These are recombi- nant DNA (gene-splicing) technology, which permits the isolation and detailed molecular characterization of DNA or RNA, and the technology for producing monoclonal anti- bodies, which provides reagents not only for specific proteins but also for specific an- tigenic sites on specific proteins. Applica- tion of these methods has provided new insights into the structure of virus particles and surface proteins and into the regulation of virus multiplication. Each of these areas offers different opportunities for combating viral diseases. The more detailed descrip- tion, below, of advances in each area is fol- lowed for each by identification of the specific 53 research opportunities that could be pur- sued. THE STRUCTURE OF VIRUS PARTICLES Great progress has been made in recent years with respect to knowledge of virus structure. The three-dimensional structures of several plant and animal viruses, includ- ing some human pathogens (e.g., polio- myelitis virus and rhinovirus) have been de- termined. These studies are extremely im- portant in the elucidation of how virus particles interact with host cells and anti- bodies. These studies on whole virus particles have been paralleled by studies of individual components of virus particle surfaces. Two types of such components are of funda- mental importance. The first includes the viral cell attachment proteins, which are the proteins that recognize receptors on suscep- tible cells. For instance, certain viruses have proteins Termed hemagglutinins) that rec- ognize receptors on erythrocytes (red blood celIs) and cause their agglutination. Attach- ment proteins have been identified for a va- riety of viruses, and more recently cellular receptors also are being identified. An im- portant question is whether these cellular receptors have some other essential cellular function, in addition to the recognition of viral attachment proteins. Studies of this question and of the role of these receptors during virus contact by and penetration of host ceils are under way. They will provide a basis for investigations to determine if the interaction of viruses with their receptors could be prevented, inhibited, or termi- nated so as to protect from infection. The second class of viral surface features important to the interaction of viruses and organisms is their antigenic sites, termed epitopes. Here the primary question is which epitopes elicit the formation of protective or neutralizing antibodies. The surface com- ponents of many types of virus particles have been elucidated and the genes that encode

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many of these proteins have been se- quenced, that is, deciphered for the infor- mation in their basic structural units. In both cases, studies have included viruses from all the major classes in these two broad cat- egories. The most intensively studied of these proteins is the hemagglutinin (HA) of influ- enza virus. Not only have the amino acid sequence and three-dimensional structure of the HA of an important influenza strain been determined, but variant strains (with different antigenic properties) also have been sequenced. As a result, it is now known where the important epitopes on the influ- enza virus HA are, what the nature of the antibodies synthesized in response to in- dividual epitopes is, and how single amino acid changes affect epitope function. Similar studies are in progress on several other vi- ruses and viral surface components. SPECIFIC STRATEGIES FOR PREVENTING VIRAL DISEASES Strategies for preventing viral diseases have as their goal the neutralization of virus particles before they can establish produc- tive infection. Among the most interesting approaches are those ctiscussed below. GENET~cA~Y Modified LIVE Viruses The most successful vaccines are those containing attenuated (weakened) live virus particles. Attenuation is generally achieved by serially culturing the virulent virus in the cells or tissues of some other host, which often gives rise to variants that have lost virulence for human beings. Suitable var- iant strains that still will grow well enough in the human host to elicit the formation of neutralizing antibodies against the disease- causing strain, but that cause neither dis- ease nor untowarc! reactions, are then se- lected. It has recently become clear that genetic engineering techniques could be used to provide improved attenuated vaccine 54 strains much more rapidly than the slow and relatively uncontrollable process of se- rial culturing. The genetic material of many viruses has now been sequenced, and the genes re- sponsible for specifying the interactions of the virus with the host organism are being mapped. These inclucle the genes respon- sible for determining affinity for particular host tissues, ability of the virus to spread from one location in the organism to an- other, virulence and nature of cytopathic ef- fects, capacity to establish latent or persistent infection, and immunogenicity. A wicle va- riety of genetic techniques are available to identify anc! manipulate such genes. Once the genes governing virulence or other fac- tors have been identified and characterized, they can be altered or inactivated relatively simply. In this manner, it should be possible to provide a new generation of acceptably safe vaccine virus strains. THE USE OF VIRUS VECTORS The genes for many proteins capable of eliciting the formation of neutralizing anti- bodies have been isolated. Such genes de- rived from virulent disease-causing virus can be inserted into avirulent vectors such as vaccinia virus or adenovirus. When these vectors are used to infect hosts (without causing disease), the inserted foreign genes are expressed, and the host develops anti- bodies and immunity to the virus from which they were derived. The feasibility of this ap- proach has been demonstrated: a vector car- ~ing the major rabies virus glycoprotein has been used successfully to protect foxes against challenge with wild virus. Work is needed to optimize the safety and efficiency of the vectors. PURIFIED VIRAE PROTEINS AS ANTIGENS FOR VACCINES When individual proteins that elicit the formation of neutralizing antibodies were

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PREVENTION AND TREATMENT OF VIRAL DISEASES first identified, attempts were begun to use them as subunit vaccines. Until recently the major difficulty was the inability to isolate sufficiently large amounts of the proteins in a state of sufficient purity. Molecular clon- ing techniques have greatly improved the feasibility of this approach. Genes for viral proteins (such as those for protective anti- gens) can now be inserted into a variety of prokaryotic and eukaryotic expression vec- tors. These vectors can be introduced into bacterial, yeast, or mammalian ceils where they can be made to induce the synthesis of large amounts of the specific proteins ac- cording to instructions coded by the in- serted viral gene. These viral proteins could then be purified in large quantities and used as safe and specific vaccines. DEVELOPMENT OF TECHNOLOGIES FOR ENHANc~NG IMMuNoGEN~c~TY An important aspect of antibody forma- tion is the optimal presentation of antigens to the immune system. Recent advances in immunology have suggested several new ways of improving such presentation. Among possible approaches are the use of protein conjugates or aggregates, lipo- somes, or immunostimulatory complexes produced with plant extracts. STRATEGIES FOR THE TREATMENT OF VIRAL DISEASES Strategies for the treatment of viral dis- eases have as their aim the interruption of viral infections once they have started, re- sulting in the elimination of the virus from the body and a cure of the disease. This aim can be addressed best with detailed knowI- edge of the key reactions of viral multipli- cation cycles. Such reactions are catalyzed by virus-specified enzymes engaged in sub- tle interactions of viral and cellular proteins and nucleic acids. In the case of enveloped viruses, interactions of viral and cellular proteins with lipid membranes are also of 55 crucial importance. In recent years, techni- cal advances provided by recombinant DNA technology and the availability of mono- clonal antibodies have led to a dramatic in- crease in knowlecige of the processes involved in virus multiplication and the in- teraction of viruses with their host cells. The following is an outline of current knowledge about virus multiplication and of the op- portunities to inhibit it. THE VIRAL MULTIPLICATION CYCLE Viruses multiply by means of precisely regulated series of reactions. Typically, vi- ruses adhere to host cells via specific recep- tors and are internalized by a process of engulfment (phagocytosis). The viral ge- nome (its DNA or RNA) is then liberated from its protective protein coat, and the viral genetic information is expressed through messenger RNAs that- are translated into proteins. Viruses vary in their complexity; thus the number of proteins encoded by vi- ruses varies: some encode fewer than 10, others more than 100. Subsequently the viral nucleic acid replicates, and the newly rep- licated (progeny) viral genomes are en- closed within newly formed protein coats. The number of progeny virus particles formed in a cell may vary from a few hundred to more than 100,000. Progeny virus parti- cles are liberated either when the host cell disintegrates or when virus particles "bud" through the cell membrane, thereby acouir- ing their envelopes. Strategies for the Expression of Viral Genetic Information One of the key steps in viral multiplica- tion is the expression of the genetic infor- mation encoded in the viral genome. To achieve this, different viruses use different strategies. The genetic information of sin- gle-stranded RNA viruses is translated into proteins either directly or through the in- volvement of a second messenger strand of

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RNA complementary to the genome. The latter process involves a virus-encoded en- zyme present in the virus particle. RNA vi- ruses containing double-stranded RNA must also first be transcribed into messenger RNA by virus-encoded enzymes present in virus particles. RNA viruses also include the retrov~uses. Upon infection, their RNA is transcribed by a virus-encoded RNA-dependent DNA po- {ymerase (reverse transcriptase) into double- stranded DNA (the prov~rus). Another unique enzyme newly translated from the viral RNA integrates this Proverbs into the host cell nu- cleic acid. The integrated prov~rus directs host cell enzymes in the synthesis of viral proteins and viral RNA, from which new progeny v~- ruses are assembled. DNA-containing viruses can only express their genetic information by its being tran- scribed into messenger RNA. Some use host enzymes for this purpose; others specify their own DNA-dependent RNA polymerases. Strategies for the Replication of Viral Genetic Material Viruses employ various strategies for rep- licating their genetic material just as they do various strategies for expressing it. The rep- lication of all RNA viruses except retrovi- ruses involves virus-encoded enzymes because uninfected cells do not possess en- zymes capable of replicating RNA. The rep- lication of DNA genomes is accomplished either by host cell DNA polymerases, or by virus-encoded DNA polymerases. OPPORTUNITIES FOR INTERFERING WITH THE VIRAE GROWTH CYCLE Recent advances in molecular virology have led to the following picture of viruses. Their genomes comprise both regulatory re- gions and coding regions. The regulatory regions include sequences that serve to reg- ulate a variety of processes, such as nucleic acid and protein synthesis and virus assem 56 bly, in a variety of ways (e.g., recognition, initiation, promotion, enhancement, and termination). These regulatory regions of viral nucleic acids may function by inter- acting with proteins or by interacting with other nucleic acid sequences. The coding re- gions encode virus-specified proteins. Viral proteins are of three kinds: (~) structural components of virus particles, (2) enzymes, and (3) regulatory proteins that interact with nucleic acids or other proteins. Numerous opportunities exist for inhibit- ing virus multiplication. One approach is to inhibit the activity of some viral enzymes; an- other is to disrupt the action of some regu- lato~ protein; and a third is to interfere with the function of a regulatory nucleic acid se- quence. Such approaches are made possible by the fact that the nucleic acid of many vi- ruses has now been molecularly cloned and sequenced. As a result, not only are the se- quences of many viral proteins now known, but also the sequences of many regions of nucleic acid with regulatory functions. The following appear to be feasible strat- egies for inhibiting virus multiplication. Inhibition of Virus-Encoded Enzymes Nucleic Acid Synthesis Inhibition of viral nucleic acid synthesis would interrupt viral multiplication and infection. The feasibility of this approach is confirmed by the fact that most successful antiviral compounds cur- rently licensed (e.g., Acyclovir) or under in- vestigation are nucleotide analogues. Because host and viral enzymes differ in their ability to use these compounds as substrates, they (or their metabolites) are preferentially in- corporated into viral nucleic acids and halt multiplication. Virus-encoded enzymes of nucleic acid synthesis-the RNA and DNA polymerases are, therefore, the most ob- vious targets for antiviral chemotherapy. Many, like the RNA-dependent RNA poly- merases, have no counterparts in unin- fected cells. Now that genes for many of the viral enzymes of nucleic acid synthesis have

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PREVENTION AND TREATMENT OF VIRAL DISEASES been cloned, it will soon be possible to pre- pare them in large amounts. When the structures of the enzymes' catalytic sites have been cletermined, it should be possible to design compounds, possibly nucleotide an- alogues, that irreversibly inhibit them spe- cifically. Proteases These enzymes catalyze reac- tions essential to viral multiplication. Many viral proteins are synthesized in the form of precursors, usually 20 to 50 percent larger than the functional proteins. Sometimes several proteins are synthesized linked to- gether in the form of a polyprotein. Precur- sors and polyproteins are cleaved by highly specific virus-encoded proteases to active individual proteins. Proteases are, there- fore, potential targets for antiviral chemo- therapy. Capping Enzymes Several viruses encode enzymes that form caps (modified single nucleotide additions) at the end of messen- ger RNA molecules that are essential to the efficient functioning of these molecules. The viral cap-synthesizing enzymes are analo- gous to corresponding host cell enzymes, but their amino acid sequences are likely to be quite different. Therefore, they represent a unique target for intervention. Integrases The genomes of several types of virus are inserted into host cell DNA as an essential part of the virus life cycle. Usu- ally, but not invariably, this is the first step of transforming normal cells into tumor cells. Some viruses use host cell enzymes for this purpose, but others, such as the retrovi- ruses, encode their own integrase enzyme for this purpose. These enzymes apparently are not capable of recognizing unique cel- Jular DNA sequences; but often the nucleic acid segments that are integratecl, such as retroviral proviruses, possess highly dis- tinctive features recognized by integrates. This recognition feature of integrates pre- sents a target for antiviral chemotherapy. Sequence-Specific Nucleases Virus-en- coded, sequence-specific nucleases perform essential functions during viral genome rep- lication; that is, they cut nucleic acids at pre- cisely specified positions. Again, these represent a selective target. Inhibition of Interactions of Viral and Host Cell Proteins with Regulatory Sequences in Viral Genomes Precise regulation is essential to the com- plex process of viral multiplication. Most, if not all, regulatory regions in viral genomes operate through interaction with proteins that have the ability to recognize and inter- act with specific nucleic acid sequences (i.e., they are sequence-specific binding pro- teins). Such highly specific nucleic acid-pro- tein interactions promise to be targets for antiviral chemotherapy, but present rudi- mentary knowledge of these interactions make this a long-term approach. Two strat- eg~es can be imagined. First, it is now be- coming feasible to identify the regions of proteins that bind to nucleic acids. From such proteins, it may be possible to isolate pep- tides that retain nucleic acid binding ability and to use either these peptides, or ana- logues that bind even more strongly, to sat- urate the regulatory sequences, thus rendering them unavailable to the func- tional viral proteins. The second approach would be to use short complementary nu- cleic acid sequences to saturate the regula- tory sequences. Interference with Messenger RNA Function Translation of viral genetic information, by means of messenger RNA (mRNA), into proteins is essential to viral multiplication, and for this the mRNA must be accessible to the protein-synthesizing machinery as a single-stranded RNA. Through genetic en- gineering techniques, it is possible to syn- thesize RNA segments that are complementary to mRNA and that bind to 57

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it, blocking translation. The present ques- tion regarding this approach is how to in- troduce the inhibitory RNA into cells. However, new ideas are constantly being conceptualized and evaluated experimen- tally. Thus, this may be a feasible long-term approach to antiviral chemotherapy. The Target-Cell Approach A major concern of antiviral chemother- apy is that even in the most severe diseases only a very small fraction of cells in an or- ganism are infected. Clearly it would be ad- vantageous to aim the antiviral agent at the infected cell rather than to introduce it into every cell. Attention has therefore been di- rected at various forms of a target-cell ap- proach to antiviral chemotherapy. At least three strategies are being explored. The first and most attractive is exemplified by a strat- egy that is feasible in cells infected with some, but not all, herpesviruses. These viruses en- code an enzyme, a deoxypyrimidine kinase, that phosphorylates nucleoside analogues (such as Acyclovir) not readily phosphoryI- ated by the analogous host cell enzyme (thy- midine kinase). Such phosphorylated nucleoside analogues are then incorporated into viral DNA and inhibit virus multipli- cation. (Phosphorylation of nucleosides is a prerequisite for incorporation into viral DNA.) The attractive feature of this ap- proach is that the antiviral nucleoside ana- logues are not phosphorylated in urunfected ceils and therefore only exert their inhibi- tory effects in infected ceils. The second target-cell approach takes ad- vantage of the fact that virus-encoded pro- teins are incorporated into the membranes of infected host cells soon after infection. Monoclonal antibodies against such pro- teins can be produced and coupled to in- hibitors of nucleic acid or protein synthesis that would normally not enter cells. When combined with antibody molecules, how- ever, they are internalized. Thus, nonspe- cific inhibitors of nucleic acid and protein 58 synthesis can be introduced specifically into infected cells. The third target-cell approach is predi- cated on the fact that soon after infection the permeability of the cell membrane often increases, potentially permitting the entry of compounds that would be excluded from uninfected cells. Several other strategies for inhibiting viral infections can be envisaged. Possible strat- egies include direct inhibition of virus-en- coded regulatory proteins, inhibition of the interaction of viruses with their cellular re- ceptors, inhibition of the budding process for the release of enveloped viruses from infected cells, inhibition of viral protein glycosylation (addition of sugar residues), and inhibition of the intracellular trans- port of viral proteins. The research nec- essary to determine the feasibility of these strategies can now be outlined in some de- tail. THE NEED FOR IMPROVED TECHNIQUES TO DIAGNOSE VIRAL INFECTIONS There is a pressing need for ways to rap- idly diagnose viral infections. Treatment with a specific antiviral agent cannot be selected before the infecting virus is identified. Re- cent advances in biotechnology such as nu- cleic acid probe techniques and monoclonal antibodies have enhanced capabilities in this area, and excellent progress has been made on some problems, such as determining ex- posure to the virus that causes AIDS. The likely availability of effective antiviral drugs should provide a stimulus to the develop- ment of diagnostic tools. SELECTED OPPORTUNITIES AMONG VIRAL DISEASES The following viral diseases are high- priority areas for research into their preven- tion and treatment.

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PREVENTION AND TREATMENT OF VIRAL DISEASES AcQuIRED IMMUNE DEFICIENCY SYNDROME (AIDS) The severity of the AIDS problem war- rants major efforts in prevention and treat- ment, but immediate prospects for either are not highly promising. In this circum- stance a number of approaches should be pursued, and research on pathogenesis of the disease should be actively continued. It is not yet clear how best to approach the cievelopment of a vaccine to prevent AIDS. Among questions to be answered are whether antibodies to the major viral sur- face protein can be protective, the signifi- cance of the genetic variability of the virus, and why natural infection does not elicit protective antibodies. Control of the persistent infection oc- curring with the AIDS virus is also prob- lematic; drugs designed against it might have to be taken for extended periods. The virus encodes three enzymes, known for a number of years to exist in other retro- viruses but not yet characterized: a reverse transcriptase, an integrate, and a pro- tease. All of these are obvious targets for rational drug design efforts along with two newly identified regulatory proteins. Once the AIDS provirus has been integrated into the host cell DNA, drugs directed against any of these targets might have to be taken for the lifetime of the patient to prevent the spread of the virus, if the virus were not eliminatec! from the body by the ctrug. It is likely that this approach would only be practical and feasible if a strategy of targeting infected cells were adopted. Measures to remove the AIDS provirus from the genome of infected cells cannot yet be formulated. Further specific recommendations will be made in the report of the National Academy of Sciences-Institute of Medicine Committee on a National Strategy for AIDS, planned for publication in September 1986. 59 INFLUENZA VIRUSES The optional strategy for protection against influenza viruses would be a safe and ef- fective vaccine, and new approaches are constantly being tested. In addition, drugs against influenza virus would be desirable when new virus strains pathogenic for hu- mans appear, before adequate amounts of new vaccine can be prepared. Targets for anti-influenza virus chemotherapy are an RNA-dependent RNA polymerase and a unique "cap stealing" protein involvec! in nucleic acid synthesis. Similar considera- lions apply to disease caused by respiratory syncytial virus and the parainfluenzavi- ruses, which are also important human pathogens. HERPES SIMPLEX VIRUSES (HSV) ~ AND 2 A few groups are pursuing various (sub- unit and genetically engineered attenuated) approaches to development of a vaccine to prevent this disease. However, because la- tent infections can give rise to recurrence of symptoms even in the presence of antibocly, a vaccine may not prevent infection but rather reduce the severity of initial lesions, their recurrences, and possibly their frequency. Intensive, long-term research will be nec- essary to devise ways to eliminate latent vi- rus once infection has occurred. However, herpes simplex virus (HSV) presents nu- merous targets for antiviral drugs to treat the lesions and other symptoms that occur with the initial infection and its recurrences. The best is probably deoxypyrimidine ki- nase, which invites the target-cell approach described above; progressively more effec- tive nucleoside analogues (that function like Acyclovir) are constantly being synthesized. Other good targets are provicled by the DNA polymerase and ribonucleotide reductase that are encoded by these viruses. HSV ~ and 2 can serve as models for con- trolling human infections with other her- pesviruses (cytomegalovirus, Epstein-Barr

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virus, and varicelIa-zoster virus). However, each of these presents unique epidemio- logic, economic, and disease problems. Probably the most costly in terms of per- centage of total health costs are cytomega- lovirus infections in immunocompromised people, such as transplant recipients, and in pregnant women, leading to mental anct developmental retardation of their off- spring. Epstein-Barr virus in the United States is associated with disseminated mild to se- vere infections of young adults (mononu- cleosis), severe infections in immuno- compromised individuals, and fulminating lethal infections in a small number of chil- dren. Outside the United States it is asso- ciated with certain malignancies including nasopharyngeal cancer. VaricelIa-zoster vi- rus is the agent of chickenpox in children and shingles in adults. For all these herpesviruses, specific strat- e~es can be devised for vaccines or antivir- als; for example, a vaccine for variceLa-zoster should be genetically engineered so that the genes that enable it to initiate latent infec- tions (that may result in shingles) are re- moved. HEPATITIS B VIRUS (HBV) Worldwide, millions of people are carriers of hepatitis B virus (HBV); that is, they are persistently and chronically infected with this virus. Each year 800,000 persons, mostly in developing countries, die of the conse- quences of HBV infection (cirrhosis and liver cancer). For these persons, antivirals would be helpful, and HBV encodes a unique DNA polymerase that would be an excellent tar- get for chemotherapy. For those not yet in- fected, HBV is highly amenable to new vaccine development approaches; a plasma- derived vaccine is available but expensive. The surface antigen of HBV has been cloned and can be expressed in yeast cells. In highly purified form, it is being tested for its ability to elicit the formation of neutralizing anti- bodies. Another promising approach to the 60 prevention of HBV infection is the insertion of the gene for the HBV surface antigen into a vector virus such as vaccinia virus. Im- munization of newborns is the favored pre- vention strategy in areas of high incidence of disease. ROTAVIRUSES Rotaviruses are an important cause of diarrhea! disease and infant mortality worIdwicle. Both the antiviral drug ap- proach and the vaccine approach appear to be promising. The rotavirus multiplication cycle involves two different RNA-depen- dent RNA polymerases, which are therefore good targets for antiviral chemotherapy. Some rotavirus vaccine candiciates are in development, such as those based on bo- vine or rhesus rotaviruses; other candidates have been developer! through genetic reas- sortment techniques. However, the genes that encode rotavirus surface antigens have been cloned and are now being placed into expression systems. Thus, it may also be possible soon to prepare large amounts of the surface antigens for use as a better de- fined, highly specific, and safe rotavirus subunit vaccine. OTHER VIRUSES A large number of other viral diseases are known; it should soon be possible to de- velop vaccines or antivirals to prevent or treat many of them. Among the most im- portant are human papillomavirus, certain insect-borne viruses, hepatitis A virus, and adult T-cell leukemia virus. IMPLEMENTING STRATEGIES FOR DEVELOPMENT OF NEW VACCINES AND ANTIVIRAI~S Pursuing the above approaches to the de- sign of new viral vaccines and specific an- tiviral cirugs is a long-term program. Al

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PREVENTION AND TREATMENT OF VIRAL DISEASES though the required principles are known, much remains to be done in the course of developing these approaches to yield prac- tical vaccines or drugs. Their theoretical ba- sis, however, is firm and their prospects highly promising. Pursuit of prevention or treatment mo- dalities should not be regarded as mutually exclusive efforts. Useful cross-fertilization occurs between the two activities. Treat- ment may be needed to "backstop" even a highly successful prevention effort, or it may be a more rational approach for some dis- eases where a target population for vacci- nation is presently difficult to identify. In spite of the high likelihood of success in this area, there are several impediments to its realization. The major problems that need to be addressed are as follows. 1. The development of highly specific and potent antiviral drugs requires the colIabo- ration of scientists in several disciplines, among them protein chemists, enzyme ki- neticists, biophysicists, x-ray crystallogra- phers, organic chemists, virolog~sts, cell biologists, pharmacologists, toxicologists, and clinicians. Not all such scientists would have to interact at the same time; but in the initial phases of the work, protein chemists, enzyme kineticists, biophysicists, and or- ganic chemists will have to interact quite closely. A coordinated national effort is needed and should involve extensive col- laboration among components of the entire scientific community. Such collaboration may come about through the cooperation of sci- entists in various disciplines on the same university campus, among several univer- sity campuses, within private companies, between universities and companies, and between all these groups and government scientists. 2. To capitalize on the opportunities that exist, there is a need to invigorate public- private partnerships. Private industry is concerned about the confidentiality of the results that are obtained because confiden 61 tiality is essential for the protection of patent rights, which are needed to recoup high de- velopment costs. Close, long-term colIabo- ration between scientists in universities and their counterparts in industry would be highly desirable, but such cooperation will require very careful management and may also require the development of new mech- anisms of promoting and funding it. 3. A serious impediment is the threat of possible liability for inadvertent injuries at- tributed to vaccines. A system providing compensation to individuals who incur un- toward injury from vaccines that are cor- rectly manufactured and administered is essential, along with some means of defin- ing much more clearly than is currently the case the limits of manufacturers' liabilities.* 4. Another major impediment is current uncertainty about the legal limits of appli- cability of recombinant DNA research. A vo- cal minority persists in opposing any kind of innovation resulting from the application of recombinant DNA technology to human health. There is a need for more public ed- ucation on the nature, benefits, risks, and practical capabilities of recombinant DNA technology. Within their manciates, agen- cies should attempt to provide guidance on these issues to interested parties for ex- ample, potential manufacturers and "con- sumers" of products. In the final analysis, the usefulness of an- tiviral drugs and vaccines should be judged on the basis of the net benefits they provide; absolute safety should not necessarily and invariably be the goal. Evaluations should take into account the number of lives saved, the misery spared, and the economic ben- efits accrued as well as known and potential risks. See Vaccine Supply and Innovation, a report from the Institute of Medicine, National Academy of Sciences, published by the National Academy Press, Washing- ton, D.C., 1985.

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BENEFITS OF A NATIONAL EFFORT ON VACCINES AND ANTIVIRALS Control of poliomyelitis, measles, mumps, and rubella in the United States has pro- duced annual savings estimated in 1980 at $2 billion. The benefits of the strategies en- visaged above would be the further savings of many lives and the enormous reduction of misery and costs attributable to acute dis- ease or to persistent, latent, and chronic in- fechons that later cause degenerative diseases or cancer. Additional viral diseases should be eradicated following the example of smallpox. The capabilities developed in a program on human viral diseases would also be ap- plicable to viral diseases of livestock and poultry, where economic losses of produc- lion are enormous. SUMMARY AND CONCLUSIONS Recent advances in molecular virology have laid the foundation for combating many viral diseases through new vaccines or more rational approaches to the development of antiviral drugs. These new approaches uti- lize recent advances in the knowledge of viral surfaces and of unique processes en- coded by viral nucleic acid. A central feature of the approaches to antivirals is the selec- lion, as targets, of processes that are essen- lial to viral multiplication but for which no host cell counterpart exists. While this is a program area in which timely addition of emphasis and support would pay great div- idends, certain organizational difficulties (such as the need for large collaborative ef- forts) and policy issues (such as liability for vaccine injury) will need to be addressed to ensure the realization of the great health and economic benefits that these new tech . . no~ogles promise. ACKNOWLEDGMENT The committee grate- fully acknowledges the assistance of the follow- ing individuals in the preparation of this document: Donald S.~ Burke, Walter Reed Army Institute of Research; Joel M. Dalrymple, U.S. Army Institute for Infectious Diseases; Bernard Moss, National Institutes of Health; Michael Lai, University of Southern California; Stephen E. Straus, National Institutes of Health. 62