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OCR for page 49
Report of the
Research Briefing Panel on
Prevention and Treatment of Viral Diseases
OCR for page 50
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
OCR for page 51
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
OCR for page 52
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
OCR for page 53
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
OCR for page 54
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
OCR for page 55
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
OCR for page 56
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
OCR for page 57
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
OCR for page 58
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.
OCR for page 59
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
OCR for page 60
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
OCR for page 61
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.
OCR for page 62
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
Representative terms from entire chapter:
nucleic acid
