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The development of antiviral drugs is still in its infancy with rapid changes and progressive milestones encountered almost daily. By the time this book is distributed, new drugs may have already been added. This is particularly true for the anti- retroviral drugs, which seem to be growing in number expo- nentially to the casual observer trying to keep abreast of recent advances in this ﬁeld. As this book is going to press, the United States Food and Drug Administration granted acceler- ated approval of Epzicom and Truvada. Epzicom is a ﬁxed- dose combination of the antiretroviral drugs Ziagen (abacavir sulfate) and Epivir (lamivudine). Truvada is a ﬁxed-dose com- bination of Emtriva (embricitabine) and Viread (tenofovir dis- oproxil fumarate). In keeping pace with these advances, the book will survey the latest in antiretroviral drugs, general antiviral therapies, the antiviral vaccines, and immunothera- pies used for treatment, and prophylaxis of viral infections.
The book begins with a review of the current state of anti- viral management (therapy and prophylaxis) and discussion of the challenges for the future. The second chapter discusses the major categories as well as the indications, adverse reac- tions, and drug interactions of each speciﬁc medication of the
antiretroviral drugs. Chapter Three delves into the treat- ments available for other viral infections, such as herpes sim- plex virus, varicella zoster virus, cytomegalovirus, human papilloma virus, chronic viral hepatitis, and others. The book then concludes with a discussion of the vaccines that are cur- rently available and being developed and gives an overview of the use of immunoglobulins and monoclonal antibodies for antiviral therapy.
The last two decades have been the most dynamic in the history of viral infections and their management. During this time the eradication of the epidemic form of the most deadly viral infection known to medicine, smallpox, was announced. Ironically, this landmark achievement was followed almost immediately by the observation of a new viral pandemic that currently infects 46 million people, i.e., HIV/AIDS. Within the past decade several new emerging viral diseases, e.g., West Nile virus, SARS, avian inﬂuenza, etc., have challenged our ability to recognize and manage these infections. Unfortu- nately, antiviral drugs have been effective for only a few groups of viruses up until now. Most antiviral drugs do not produce a cure, but rather allow control of the infection. An exception to this observation has recently been seen with the combined use of pegylated interferon alpha and ribavirin, which allows virologic cures for the majority of hepatitis C patients who successfully complete therapy. However, the lim- itations of antiviral therapy, including the high costs of drugs, make the need for prevention even more urgent. The most cost effective means of prevention are public health measures, such as proper sanitation/clean drinking water, mosquito con- trol, testing blood/blood products, not sharing needles, and safer sex/condom use. In addition, vaccines provide the most effective and cost-efﬁcient means of preventing infectious dis- eases. The greatest success story in medical history was the eradication of epidemic smallpox, which was due to a com- bined effort of public health measures and an effective vac- cine. For such combined efforts to eradicate other viral diseases, such as measles and polio, the challenges are not only to reach the susceptible populations but also to overcome unfounded prejudice against vaccines. At the same time, new
technologies will lead to the development of new prophylactic vaccines, particularly for infections such as HIV, human pap- illomaviruses, and herpes simplex viruses, ushering in a whole new set of arsenals in the ﬁght against viral infections. It is my hope that Antiviral Agents, Vaccines, and Immuno- therapies will serve as a valuable tool for the clinician and the basic scientist in better understanding the current manage- ment protocols of viral diseases as well as greater possibilities for the future.
Stephen K. Tyring, MD, PhD, MBA
The efforts of Nancy Bell, PhD, for all aspects of the editing and production process are deeply appreciated. I wish to thank my wife, Patricia Lee, MD, for her support, encourage- ment, and dedication throughout the writing and publication of this book. In addition, I wish to thank my mentor (and a pioneer in antiviral research), Samuel Baron, MD, Professor (and Chairman Emeritus) of Microbiology/Immunology and Internal Medicine at The University of Texas Medical Branch, Galveston, Texas, for his guidance, suggestions, and wisdom over the past 25 years. Without such outstanding people, the publication of Antiviral Agents, Vaccines, and Immunothera- pies would not have been possible.
Public health measures should always be the ﬁrst line of defense against viral infections and should include clean drinking water, proper sewage disposal, vector control, testing of blood and blood products, nonsharing of needles, hand wash- ing and use of disposable gloves, and safer sex/condom usage/ abstinence (Fig. 1.1). Global travel has made the rapid imple- mentation of these measures paramount. The rapid spread of the SARS (severe acute respiratory syndrome) corornavirus and avian inﬂuenza are recent examples of the problem of viral globalization. In a similar manner, the West Nile virus and monkeypox virus made their appearance in North America. In addition, new viruses are being described for previously recognized diseases, such as the role of human metapneumo- virus as the second most common cause of infant respiratory infections. For viral diseases for which they are available, vac- cines can be added to the list of measures to prevent viral dis- eases and should be used in combination with the other interventions. Antiviral drugs, however, can be considered a
second line of defense against viral infections because they are generally not viracidal, because they are much more expen- sive than other forms of intervention, and because they are usually most beneﬁcial when used acutely (which limits the time in which the patient can obtain them).
A prototype successful vaccine was the use of vaccinia to eliminate epidemic smallpox from the world. Starting with Jenner’s use of cowpox to prevent smallpox in 1797 through the last epidemic case of smallpox, treated in 1977, this accomplishment could easily be considered the single greatest achievement in medical history. Even in the 20th century, hundreds of millions of persons died from smallpox and hun- dreds of millions more suffered marked morbidity, such as blindness. Thus, by extrapolation into the 21st century and beyond, an inﬁnite number of lives and dollars will have been saved by this landmark event. It is important to remember that no antiviral drug was approved for smallpox and that the elimination of this deadly disease was accomplished via public health measures plus an effective vaccine.
For the 150 years after Jenner, only one other viral vac- cine was developed—the rabies vaccine by Pasteur in 1885. In the 50 years between 1945 and 1995, however, 11 more vac- cines were approved (Fig. 1.2). Although the rotavirus vaccine
Dates for smallpox and rabies vaccines are for the first published results of vaccine usage. Remaining dates are for FDA approval of vaccine.
Fig. 1.2 Timeline of virus vaccine development.
was approved by the U.S. Food and Drug Administration (FDA) in 1998, it was taken off the market after a few months due to a relatively small but statistically signiﬁcant increase in the number of infants developing intussusception. There- fore, the medical-legal environment in the United States forced the removal of a vaccine from the market that could have saved millions of infants around the world from dying of diarrhea. Hopefully, potentially safer versions of the rotavirus vaccine, currently being tested, will be available in a few years. It may be asked whether the smallpox vaccine, i.e., vac- cinia, would have reached the market and saved hundreds of millions of lives and billions of dollars if it had been required to pass the extremely rigorous standards of the 21th century.
Although vaccinia has not been routinely administered (and not available) to the public for more than 20 years, it is now being offered to certain populations considered to be at high risk in the event that the smallpox virus is used for bio- terrorism. In contrast to other available vaccines, there is the potential for morbidity, and even mortality, with vaccinia. This potential, however, is very low if the vaccinated population does not include immunosuppressed individuals or persons with certain skin conditions, such as atopic dermatitis, that compromise the epidermal barrier. Potentially safer alterna- tives to live vaccinia for smallpox vaccination, however, are being studied in clinical trials.
Other marked success stories with vaccines include those to prevent poliomyelitis and to prevent measles. Polio vaccines (plus public health measures) have eliminated polio from North America and from most of the remainder of the world. Unfortunately, the goal of global elimination of polio by the beginning of the 21st century has not been met, primarily due to the difﬁculty of reaching susceptible individuals in a few war-torn parts of the world. Although the distinct advantage exists of polio having an effective oral vaccine, polio has proven more difﬁcult to eradicate than smallpox because of the numerous subclinical infections (1). The measles vaccine has been almost as successful as the polio vaccine in the United States, considering that fewer than 100 cases have been reported annually in this country for the past few years.
In many parts of the world, however, measles is a major source of mortality as well as morbidity. Approximately 36 million cases of measles occurred globally in 2003. More than one mil- lion children die of measles annually, mostly in Third World countries where there is marked malnutrition and lack of vaccines.
Biological criteria that are essential for a disease to be considered a reasonable candidate for global elimination are as follows:
• The disease is speciﬁcally human, with no animal reservoir.
• The disease is acute, self-limiting, and infectious for other persons for about only a week (two exceptions exist: inclusion body encephalitis and subacute scle- rosing panencephalitis).
• An effective method of intervention exists, e.g., a vac- cine (2).
Measles, like smallpox, meets these criteria, but measles appears more difﬁcult to eradicate, partly because it is more infectious than smallpox. Also, there is a period of vulnerabil- ity between passive immunity due to maternal antibodies (and concomitant resistance to measles vaccination) and the age of 12 months, the youngest age at which the vaccine is known to be effective.
Because measles vaccine is usually given along with the rubella and mumps vaccines, these viral diseases should be candidates for eradication as well. This possibility is espe- cially important because of the severity of congenital rubella. The difﬁculty in diagnosis of rubella (and mumps) makes the surveillance of the eradication of these viruses more difﬁcult. Because measles is the most infectious of the three diseases and the clinical manifestations are most easily recognized, a good surveillance system for measles potentially can serve as an effective marker for surveillance of rubella and mumps in judging the efﬁcacy of vaccination campaigns.
Hepatitis B is also a speciﬁcally human pathogen and the vaccine is safe and effective. Indeed, hepatitis B produces extensive morbidity and mortality worldwide. Unlike other viruses that are reasonable candidates for global eradication, hepatitis B is a disease in which many persons become chron- ically infected early in life and become persistent or recurrent excretors of the virus. Therefore, surveillance becomes much more difﬁcult and would involve large numbers of people who are infected but are not ill.
Some viral diseases that have effective vaccines are less likely to be eradicated because of animal reserves. Therefore, control is more reasonable than eradication. In the case of yel- low fever, mosquito control is extremely important and proved very effective even before the vaccine became available. Another important factor in the control of yellow fever is the fact that monkeys constitute a jungle reservoir of this virus. Likewise, intervention in the spread of rabies involves control and containment of animal reservoirs.
In general, the risk / beneﬁt ratio of approved vaccines is extremely favorable. In fact, the chance of major morbidity (or mortality) from these vaccines is on the order of a millionfold less than that of the diseases they are designed to protect against. Many persons in industrialized countries have reached adulthood in recent decades without knowing anyone who has suffered from measles, rubella, hepatitis B or polio. Thus, they sometimes do not believe there is sufﬁcient justiﬁ- cation to have their children vaccinated. Moreover, unfounded claims have surfaced linking the measles/mumps/rubella (MMR) vaccine to autism as well as the hepatitis B vaccine to certain autoimmune diseases. Extensive investigations by such independent agencies as the FDA and the Centers for Disease Control and Prevention (CDC), however, have demon- strated that there is no cause-and-effect relationship between these vaccines and the alleged diseases. Likewise, in some Third World countries, some persons refuse government- sponsored vaccination programs because they fear that politi- cians do not have their interests in mind. For example, some groups believe that vaccine is a form of population control and that it might make them sterile.
While many persons do not become vaccinated because they do not feel they are at risk, this false sense of security becomes even more complicated when vaccines to prevent sexually transmitted diseases (STDs) and parental consent are involved. Currently, only one vaccine to prevent an STD is available—the hepatitis B vaccine. Because hepatitis B can also be transmitted by nonsexual routes, however, it is not perceived by the public as an STD vaccine. The safety and efﬁcacy of vaccines to prevent herpes simplex virus (HSV)-2 and human papillomavirus (HPV)-16 indicate that these vaccines could be marketed by the end of the decade (3,4). If these vaccines are not given before a girl becomes sexually active, the chance that they would beneﬁt her is markedly decreased. On the other hand, convincing the parents that their daughter will need a vaccine against an STD is often difﬁcult. The medical need for vaccination against HPV-16—prevention of cervical cancer—is obvious to the medical community.
The added difﬁculty that advocates of this vaccine face is that this relationship between HPV and cervical cancer is unknown to most of the lay public.
Although the beneﬁts of vaccination over therapy are numerous, many persons do not become vaccinated for one reason or another and there are vaccines against only 13 viral diseases. Sometimes the cost of the vaccine is cited as a reason not to vaccinate. The cost of not vaccinating, however, is almost always greater than the cost of vaccinating.
In the past three years there have been eight major short- ages of vaccines. In the 2003–2004 “ﬂu” season, the shortage of inﬂuenza vaccine resulted from the early start of the inﬂuenza outbreak as well as the severity of the infections. In fact, inﬂu- enza vaccines have been in short supply for three of the past four years. Since the beginning of 2000, there have also been shortages of vaccines for varicella and measles as well as for such bacterial diseases as diphtheria and tetanus. A report by the Institute of Medicine (IOM), a branch of the National Academy of Sciences, noted that there has been a steady ero- sion in the number of vaccine producers over the past three decades. In the 1970s, there were 25 vaccine makers, but in 2004 there are only ﬁve manufacturers. Most of the decline is due to slim proﬁt margins and legislative and liability issues. Due to such small number of producers, shortages can develop quickly as a result of manufacturing problems or underesti- mating the expected demand.
Approximately 150 million people are considered at high risk for inﬂuenza in the United States, especially children, persons over 50 years of age and those suffering from chronic diseases. One of the big uncertainties in forecasting demand for vaccines is that only 70 million to 80 million people are vaccinated annually, leaving a large number who might panic and seek vaccination once a severe outbreak begins, as occurred during the 2003–2004 season.
Although vaccines have been available for inﬂuenza viruses for a number of years, these viruses also mutate rap- idly or undergo antigenic drift. Therefore, development of an effective vaccine each year usually has limited beneﬁt for sub- sequent years. Because time is needed for development and manufacture of the appropriate inﬂuenza vaccine at the beginning of each “ﬂu” season and approximately two weeks are needed for optimal immunity after receiving the vaccine, each year vaccine manufacturers must make an educated “guess” regarding the appropriate strain of virus for which the vaccine must be made. Even more difﬁcult is the task of deter- mining the quantity of vaccine to manufacture.
Distribution is often a problem with inﬂuenza vaccines, because some states are usually hit worse than others. Trans- ferring vaccines between states is a moderate problem, but during the 2003–2004 inﬂuenza season, there was a nation- wide shortage which necessitated the United States to buy inﬂuenza vaccine from other countries.
Four anti-inﬂuenza medications are available for those persons who do not receive the vaccine or if the vaccine strain is signiﬁcantly different than the infecting strain. These agents include amantadine, ramantadine, oseltamivir, and zanamivir. Whereas some studies suggest that such agents may help protect a person from the symptoms of inﬂuenza if started before or soon after exposure to the virus, most clinical investigators have conﬁrmed that these drugs can shorten the duration of symptoms of inﬂuenza if they are initiated within the ﬁrst one to two days of the ﬁrst symptom.
Many vaccines, such as those for tetanus and varicella, have only a single supplier in the U.S. market. The inﬂuenza vaccine, including the recently available nasal spray, has only three manufacturers. At least for some vaccines, stockpiling is one solution to reducing shortages, but the CDC notes that only three vaccines (of the 10 vaccines targeted for stockpil- ing) were stockpiled in 2002. The vaccines that were stock- piled were for measles, mumps, and rubella, but a small amount of polio vaccine is also in storage.
Stockpiling, however, is expensive and the CDC has been conservative about developing stockpiles to minimize the ﬁnancial risk. Because of seasonal strain variations, the inﬂu- enza vaccine cannot be stockpiled.
One reason for manufacturers’ decreased interest in vac- cines is the fact that the U.S. government buys slightly more than 50% of the vaccines in the United States and keeps prices low, primarily through the Vaccines for Children pro- gram run by the CDC. Because the inﬂuenza vaccines are given to many more adults than children, the government buys a lower percentage of these vaccines. The CDC negotiates a discounted price with the manufacturer under the Vaccines for Children program. Then it allocates to each state a credit balance, which states can use to buy vaccines from the man- ufacturer at the discounted price. The program offers free vaccines to uninsured children under 18 years of age or to those eligible for Medicaid or care from federally funded qual- iﬁed health centers.
The IOM reports that health-care providers such as phy- sicians and clinics face unusual difﬁculties in carrying out vaccination programs and notes that “reimbursements for vaccines and administrative fees barely cover the costs of vac- cine purchase. In many cases, providers lose money on immu- nization.” In addition, the cost of immunizing children and vulnerable adults is escalating rapidly, as new, expensive rec- ommended vaccines are FDA-approved. The cost of immuniz- ing children (adjusted for inﬂation) has risen to $385 per child in 2001 from $10 per child in 1975, and may triple to more than $1000 per child by 2020.
The IOM concluded that the price squeeze, coupled with a heavy regulatory burden, has discouraged investment and driven drug companies out of the vaccine business. In addi- tion, the manufacture of most vaccines involves the complex transformation of live organisms into pure, active, safe, and stable vaccines. Many vaccines must remain in a narrow tem- perature range during storage and delivery, called the cold chain. In addition, each lot must be tested and approved before release.
One possible attractive and potentially inexpensive alter- native to vaccination by injection is the ingestion of transgenic plants expressing recombinant vaccine immunogens. Such edible plants can be grown locally and easily distributed with- out special training or equipment. For example, the full- length HPV-L1 protein has been expressed and localized to the nuclei of potatoes. The plant-expressed L1 self-assembles into VLPs (Virus-Like Particles) with immunological proper- ties comparable to those of native HPV virions. In mice, inges- tion of the transgenic potatoes induced a humoral response similar to that induced by parenteral administration of HPV- L1 VLPs (5). Thus, the potential exists for such vaccines to become available for use in resource-poor areas of the world, where most cervical cancer is found.
There is also the problem of legal liability. Supposedly, manufacturers are protected from lawsuits regarding pediat- ric vaccines, but plaintiffs’ attorneys have found ways around that insulation.
All available vaccines are prophylactic; there are no approved viral vaccines that are therapeutic once symptoms develop. Some vaccines, however, can be effective if given shortly after exposure to the virus, although it is always pref- erable to administer the vaccine before exposure to assure that sufﬁcient immunity develops. Vaccination after symp- toms of a disease are manifested is rarely effective. This strat- egy, however, has been studied for the therapy of human immunodeﬁciency virus (HIV), herpes simplex virus (HSV) and human papillomavirus (HPV) diseases, but with limited success. A possible exception is the varicella zoster virus (VZV) vaccine, which is normally given to prevent primary varicella (i.e., chickenpox). This vaccine is currently being studied to determine whether it can be given to older individ- uals to prevent the reemergence of this virus in the form of shingles. Preliminary evidence is encouraging thus far.
Although therapeutic viral vaccines are not yet available, certain antiviral drugs can help prevent infection (Fig. 1.2). The ﬁrst routine use of an antiviral drug to help prevent infec- tion was the administration of antiretroviral agents to preg- nant HIV-seropositive women to help reduce transmission of HIV to their infants before delivery. It is logical to consider that similar use of antiretroviral agents should reduce sexual transmission of HIV as well. In fact, investigators have reported a 60% per partnership reduction in risk of HIV infec- tion following the widespread use of highly active antiretrovi- ral therapy (HAART) by HIV-seropositive persons in San Francisco. Unfortunately, however, study participants dou- bled their rate of unprotected receptive anal intercourse, which offset the beneﬁcial effects of HAART (6). Another potential means of preventing infection following occupational exposure to HIV is postexposure prophylaxis (PEP). After occupational exposure to blood, empirical treatment with two or more antiretroviral drugs not part of the source patient’s current regimen (i.e., PEP) should be provided unless infor- mation such as HIV test results in the source patient or a detailed description of the exposure indicate that PEP is not necessary. A third drug, such as a protease inhibitor, is a rec- ommended addition to the regimen if other factors such as deep puncture, high viral load, etc., suggest an increased risk of HIV. The source patient should be evaluated to determine the probability of HIV infection (in accordance with state and local laws and policies). The use of a quick HIV test can reduce the time needed to rule out HIV infection to a few hours or less. A useful resource for discussing treatment options and obtaining advice regarding the management of adverse effects of drugs is the U.S. National Clinicians’ Post-Exposure Prophylaxis Hotline (PEPline, 888-448-4911).
The guidelines for nonoccupational HIV postexposure prophylaxis (NPEP) are less well deﬁned, but a registry exists at http://www.hivpepregistry.org. The NPEP should never be given as a substitute for primary prevention, i.e., reduction of risky behavior. When prevention efforts have failed (e.g., con- dom breakage) or were not possible (e.g., sexual assault), NPEP can be an important second line of defense. Ideally, PEP or NPEP should be started within one hour, but at least within 72 hours, of exposure.
An early limitation in knowing who should receive pos- texposure prophylaxis was the fact that traditional labora- tory tests for HIV took days or weeks to produce results. In November, 2002, the OraQuick Rapid HIV-1 test was approved. Although the test only took 20 minutes, it required whole blood. In December, 2003, the Uni-Gold test was FDA-approved as the ﬁrst rapid-test product for testing blood serum, plasma and whole blood. This test, which takes only 10 minutes, had already been approved by the World Health Organization (WHO) for HIV testing in Africa. Preapproval testing demon- strated that the Uni-Gold test was 100% accurate in detecting known HIV-positive specimens and 99.7% accurate for conﬁrm- ing negative specimens. On March 25, 2004, the FDA approved the ﬁrst rapid test for HIV in oral ﬂuids. Therefore, these tests will guide physicians in the use of postexposure prophylaxis as well as in the use of antiretroviral agents in HIV-seropositive pregnant women to prevent transmission of the virus.
The ﬁrst FDA-approved use of an antiviral agent to reduce sexual transmission of a virus came in 2003 when val- acyclovir was approved to reduce the risk of transmission of genital herpes. This reduction was suspected based on the marked decline in asymptomatic viral shedding of HSV-2 in persons taking nucleoside analogs (acyclovir, famciclovir or valacyclovir) (7). In the study that led to FDA approval, 1484 immunocompetent, heterosexual, monogamous couples were enrolled: one person with clinically symptomatic genital HSV-2 and one HSV-2 seronegative partner. The partners with HSV-2 infection were randomly assigned to receive either 500 mg of valacyclovir once daily or placebo for eight months. Both part- ners were counseled on safer sex and were offered condoms at each visit. The susceptible partners were closely monitored for signs or symptoms of genital herpes as well as for seroconver- sion. The study demonstrated that daily use of valacyclovir by the source partner resulted in a 77% reduction in clinical gen- ital herpes and a 48% reduction in HSV-2 seroconversion in the susceptible partner (8). Thus, valacyclovir was capable of reducing transmission of genital herpes with safer sex coun- seling and condom use.
There is little data to support the use of antiherpes med- ication as a “morning after pill.” Although nucleoside analogs used for herpes treatment are generally very safe, they all require activation by viral thymidine kinase. Therefore, if a person is not already infected with a herpes virus, it is unlikely that the nucleoside analog would be active and thus able to prevent infection.
Although there is only one vaccine to prevent a herpesvi- rus infection—the varicella vaccine—numerous agents are available to treat HSV-1, HSV-2, VZV, and cytomegalovirus (CMV). Interferon-alpha (IFN-a) was approved to treat Kaposi’s sarcoma before the etiology was found to be HHV-8. Therefore, IFN-a alpha is not technically approved to treat HHV-8, but it is clear that part of its mechanism of action is antiviral. Systemic use of such nucleoside analogs as acyclovir, famciclovir, and valacyclovir can be safe and effective for acute therapy of herpes labialis, herpes genitalis, primary varicella, or herpes zoster. Suppressive daily use of these agents is also safe and effective for preventing most outbreaks of herpes labialis or herpes genitalis. Topical acyclovir and penciclovir are approved for therapy of herpes labialis and are very safe, but they are minimally effective. Triﬂuridine is available for optical use only. Although originally proposed to have antivi- ral properties, n-docosanol was approved as a nonprescription drug to treat herpes labialis. A number of agents are now available to treat cytomegalovirus (CMV) infections, primarily in immunocompromised patients. These drugs include ganci- clovir, valganciclovir, foscarnet, cidofovir, and fomivirsen. In general, agents used to treat herpes simplex viruses and vari- cella virus are safe and effective, but usually only beneﬁt the patient while the drug is being taken. Antiviral drugs used for therapy of CMV infections, on the other hand, can have dose- limiting toxicities, primarily renal, and should be reserved for those patients for whom there is a documented indication.
Acyclovir-resistant HSV-1, HSV-2, or VZV is usually the result of a mutation or a deﬁciency of viral thymidine kinase (TK), the enzyme necessary to phosphorylate acyclovir. Because famciclovir, penciclovir, and valacyclovir also must be activated by TK, acyclovir resistance usually translates into resistance to all members of this class of nucleoside analogs. Foscarnet is FDA-approved for therapy of acyclovir-resistant HSV and is frequently used for acyclovir-resistant VZV infec- tions. Although not FDA-approved for this indication, cido- fovir is also recommended by the CDC for acyclovir-resistant HSV and VZV infections.
Over 100 types of HPV have been described. These infec- tions cause either benign or malignant lesions of the skin or mucous membranes. Benign lesions can be treated with cytodestructive measures or with surgery, but often recur due to latent or subclinical HPV in clinically normal-appearing tis- sues. Oncogenic HPV can result in neoplasia such as squa- mous cell carcinoma, especially in the cervix and other anogenital tissues. Cervical cancer is the second most common cause of cancer death of women worldwide. Regular Pap smears have reduced cervical cancer in most industrialized countries by the detection of abnormal cytology and subse- quent cytodestructive/surgical intervention. The sensitivity and speciﬁcity of Pap smears have increased in recent years due to the concomitant use of HPV typing. Therapy of benign and dysplastic lesions due to HPV has improved during the past decade due to the use of immune response modiﬁers (IRMs), such as imiquimod. Our understanding of the mecha- nism of action of imiquimod is partly based on the activities of IFN-a, the ﬁrst antiviral agent approved for therapy of HPV infections. Interferon-alpha has antiviral action, modulates the immune system, is antiproliferative, causes phenotypic reversion, downregulates oncogenes, and upregulates antion- cogenes. Exogenous interferon was effective but had many negative features, such as the necessity of administration via injection, as well as systemic side effects. Imiquimod stimu- lates the production of many TH1 cytokines in addition to IFN-a. Thus it has potential to be more effective than exoge- nous IFN. In addition, imiquimod is applied topically and has no systemic side effects. Use of IRMs as monotherapy, or in combination with traditional treatment, has led to marked clearances and has signiﬁcantly reduced the recurrence rate of these lesions. The clinical effect of IRMs originates from cytokine-induced activation of the immune system. This is the initial event in an immunological cascade resulting in the stimulation of the innate immune response and the cell- medicated pathway of acquired immunity. This immune mod- iﬁcation mediates the indirect antiviral, antiproliferative, and antitumor activity of imiquimod in vivo.
Whereas VLP vaccines against oncogenic HPV will prob- ably be available before the end of this decade (4), much remains to be learned about how HPV causes cancer. The oncogenic HPV is considered necessary but not sufﬁcient to result in a carcinoma. The role of cofactors such as helper viruses, immunity, cigarette smoking, diet, and genetics are under active investigation. A better understanding of these factors should lead to better prevention and management of HPV infections (9).
Respiratory syncytial virus (RSV) is a common childhood infection, but no vaccine is yet available. Ribavirin, however, is an approved therapy and is administered to the affected patient via an aerosol. In addition, a monoclonal antibody, palivizumab, is widely used to prevent RSV infections in high- risk infants.
Approximately 46 million HIV-seropositive persons are estimated by the WHO to be living in the world at the begin- ning of 2004. Another 16,000 people acquire the virus every day, and millions of orphans have lost one or both parents to the virus. Over one-third of the adult populations of some sub- Saharan countries are infected and unable to work, resulting in collapsed economies. Therefore, the need for a safe and effective vaccine to prevent HIV infection is paramount. The results of the ﬁrst phase-III vaccine trial to prevent HIV, how- ever, failed to show efﬁcacy. The reasons for the failure of this recombinant gp120 vaccine are not completely understood, but possible explanations include its inability to induce cellu- lar immunity to the virus, although it did induce antibodies to gp120. General problems with developing an effective vaccine against HIV are the ability for the virus to mutate rapidly and the existence of many clades of HIV throughout the world. A number of more antigenic vaccines are being developed that involve recombinant HIV proteins being associated with attenuated carrier viruses such as adenovirus, vaccinia, or canarypox.
Antiretroviral agents are now available to block replica- tion of HIV at three different steps: 1) fusion of the virus and target cell; 2) reverse transcription; and 3) assembly of viral proteins. There are three types of reverse transcriptase inhibi- tors: 1) nucleoside inhibitors (e.g., zidovudine, lamivudine, zal- citabine, didanosine, stavudine, abacavir, and emtricitabine);
2) nucleotide inhibitors (e.g., tenofovir); and 3) nonnucleo- side inhibitors (e.g., nevirapine, efavirenz, and delavirdine). Protease inhibitors interfere with viral assembly and include saquinavir, ritonavir, indinavir, lopinavir, nelﬁnavir, ampre- navir, fosamprenavir, and atazanavir. The newest class of medication to be approved is the fusions inhibitors (e.g., enfu- virtide). When used in certain combinations, these agents compose highly active antiretroviral therapy (HAART). Although HAART has resulted in marked reductions in mor- bidity and mortality in those industrialized areas of the world where patients or third-party payers can afford the cost of approximately $20,000 per year to treat one patient, most HIV- seropositive persons live in Third World countries. Therefore, approximately 99% of the world’s AIDS patients cannot afford HAART. Although some of these persons have received medica- tion through donations from pharmaceutical companies, WHO activities, and inexpensive generic substitutes, distribution remains a problem. Most of the antiretroviral drugs have toxi- cities (e.g., gastrointestinal, hepatic, neurologic, pancreatic, etc.) in some individuals, while other persons have developed mutant HIV stains that do not respond to these agents. Phar- macogenetic laboratory tests, such as with single-nucleotide polymorphisms (SNPs), now allow many of these mutations to be detected before the patient initiates therapy, thus allowing alternative treatments (10).
The most important limitation of HAART, however, is that it does not provide a cure. Although some persons may have viral loads below detectable levels for many years, dis- continuation of HAART will result in reappearance of HIV in the system. Another major limitation to HAART, in addition to antiviral resistance, is noncompliance. The reason that com- pliance is difﬁcult for many patients is that HAART requires a lifetime commitment and often necessitates daily ingestion of multiple medications, avoidance of certain foods, and con- stant awareness of the potential of negative interactions with other medications. An unexpected consequence of the success of HAART is that many high-risk individuals have become complacent about AIDS. Because they know of the availability of HAART and do not see large numbers of AIDS patients suf- fering horrible deaths due to HIV in industrialized countries, many younger individuals are returning to unprotected sex, intravenous drugs, etc.
With more than 20 antiretroviral drugs currently avail- able and many more in clinical trials, many questions remain to be answered, such as
• What is the best combination of antiretroviral agents?
• When is the best time to start therapy?
• What is the optimal sequence in which to use the anti- retroviral agents?
• What parameters should be used to deﬁne the success or failure of HAART?
Whereas the best antiretroviral agents usually depend on mul- tiple factors, such as the HIV strain involved and the individ- ual patient’s potential for adverse reactions to a given drug, it was recently conﬁrmed that the efﬁcacy of antiretroviral drugs depends on how they are combined (11). Robbins et al. (12) determined that the combination of zidovudine, lamivudine, and efavirenz was superior to other antiretroviral combina- tions used in the study. This combination not only works longer and better, but is also easier to take and suppresses HIV more quickly. In previous studies, a combination of three antiretro- viral drugs was superior to two agents, and a combination of two agents was better than one. Shafer et al. (13) reported, however, that a four-drug regimen was not signiﬁcantly differ- ent from two consecutive three-drug regimens.
The best time to initiate therapy depends on many fac- tors and is currently debated among HIV experts. Until recently, the standard of care was to start therapy at a low- plasma HIV RNA level and when the CD4 cell count decreased to below 500/mm3. In 2004, however, most clinicians are advo- cating initiation of therapy when the CD4 cell count falls below 350/mm3 or if the plasma HIV RNA level raises above
55,000 copies/ml. If symptoms of HIV and/or an opportunistic infection develop, however, therapy should be initiated before these laboratory landmarks are reached. Another factor is patients’ potential to be compliant with the medication. If they are not prepared to adhere to therapy, drug resistance can develop. In fact, some studies suggest that missing even 5% of antiretroviral medication can hasten drug resistance. In vitro resistance testing of the virus and pharmacogenetics can help prevent the use of antiretrovirals that may be resisted. New classes of medication, such as the fusion inhibitors, have allowed new regimens to be initiated, if resistance develops. Although new drugs are rapidly appearing on the market, new classes of drugs are developed more slowly. These new catego- ries of antiretroviral agents include viral adsorption inhibi- tors, viral entry inhibitors, viral assembly and disassembly inhibitors, integrase inhibitors, and inhibitors of viral mRNA transcription (transactivation) processes (14).
The most important criteria for successful antiretroviral therapy are the clinical parameters of reductions in morbidity and mortality. Improvements in morbidity may be measured by reductions in opportunistic infections or lessened drug tox- icities. Laboratory changes, such as increasing CD4 cell counts and /or decreasing viral loads, usually correlate with clinical improvements.
While the treatment of hepatitis A is symptomatic, a safe and effective vaccine exists and is recommended for persons at high risk for this virus. The risk of hepatitis A is dependent on the quality of food preparation procedures and on the sanitary habits of food service workers. Many persons are therefore unaware of their risk of hepatitis A. This fact became obvious when 555 persons became ill with hepatitis A and three died in November, 2003, after ingesting green onions at a restau- rant in Pennsylvania (15).
The vaccine to prevent hepatitis B was the ﬁrst recombi- nant vaccine and is recommended for health care personnel as well as for anyone at high risk for infection with this virus. Because of the severe health consequences of hepatitis B infection, it is now given to neonates as part of their recom- mended childhood vaccines (www.cdc.gov/nip). A series of three injections is needed for vaccination against hepatitis B, and a series of two injections is needed for hepatitis A. For persons who have not been vaccinated against either virus, however, a simpler alternative to ﬁve injections is to receive the combination of vaccines against both viruses, which only requires three shots. Approved therapies for hepatitis B include interferon alpha, lamivudine, and adefovir, although a number of other treatments are under study.
Hepatitis C is a major cause of morbidity and mortality in many parts of the world and infections are increasing rapidly. Unfortunately, because of the rapid mutation rate of this virus, vaccine development is progressing relatively slowly. Therapy for hepatitis C was only moderately effective with interferon alpha monotherapy, but improved with the addition of ribavirin. Most recently, clinical and virological results have been signiﬁ- cantly better with pegylated interferon combined with ribavirin.
Other hepatitis viruses (i.e., D, E, and G) have no speciﬁc antiviral therapies or vaccines. The ﬁrst line of defense against all these viruses, including those for which a vaccine is available, continues to be good public health procedures.
Although antiviral drugs and vaccines are widely used, the third classiﬁcation of medical intervention—passive immunity via immunoglobulins—is generally less common. Immunoglobulins (IG) are difﬁcult to produce in large qualti- ties, have the potential of microbial contamination, and have only transient beneﬁts. In certain cases, the use of IGs has largely been replaced by speciﬁc vaccines that provide lifelong immunity (e.g., hepatitis A vaccine). Virus-speciﬁc IGs are given to unvaccinated persons exposed to hepatitis B or rabies. Varicella-zoster IG (VZIG) is given to susceptible persons exposed to varicella who have a high risk for complications (e.g., immunocompromised patients and neonates). Cytomegalovirus IG is administered to seronegative transplant recipients of an organ from a CMV-positive donor. Vaccinia IG is indicated for the therapy of eczema vaccinatum, but this IG is in very limited supply. Nonspeciﬁc IG is given to unvaccinated, high-risk per- sons exposed to measles, hepatitis A, rubella, or varicella (if VZIG is not available). Perhaps the most widely used IG is the speciﬁc monoclonal antibody against respiratory syncytical virus, which is used as prophylaxis in high-risk infants (e.g., those with bronchopulmonary dysplasia or prematurity).
Although approval of agents against other families of viruses (e.g., rhinoviruses) is expected in the near future, most advances are being made in antiretroviral drugs. Today, one- half of all antiviral agents are antiretrovirals, but greater understanding of how to interfere with replication of retrovi- ruses will aid in the development of antiviral agents against other families of viruses.
Vaccines for a number of emerging viral diseases are in development and include vaccines for West Nile virus, Ebola virus, and the coronavirus responsible for severe acute respira- tory syndrome (SARS). Development of a Dengue vaccine is a particular challenge because a person with antibodies to one strain of Dengue usually develops more severe clinical manifes- tations when subsequently exposed to a different strain. The analogous situation could arise if the antibodies to the ﬁrst strain originated from vaccination. Therefore, the ideal vaccine against Dengue would elicit antibodies to all four common strains.
Development of future antiviral agents and vaccines will require enhanced knowledge of viral immunology and phar- macogenetics. Studies that led to the ﬁrst safe and effective herpes simplex vaccine revealed at least two surprises:
1. Neither studies of circulating antibodies to the recom- binant gD2 glycoproteins nor investigations of periph- eral blood mononuclear cells in vaccinees were predictive of the clinical efﬁcacy of the vaccine; and
2. Women seronegative for HSV-1 and HSV-2 were pro- tected by the vaccine, but men were not (3).
Because of the fact that in over two centuries of vaccine develop- ment, there have been no previous reports of one gender being protected by a vaccine that did not protect the other, current studies involving the herpes simplex vaccine are focusing on the component of immunity that may play a greater role in protect- ing a woman against an STD than a man, i.e., mucosal immu- nity. In addition to understanding this component of viral immunity, development of future vaccines and antiviral agents will need to focus on persons’ genetic ability to respond, i.e., on the ﬁeld of pharmacogenetics. Just as some persons are more susceptible to infection than others, some individuals are genet- ically more able to respond to prophylaxis or therapy than others. One method used to distinguish the smallest possible genetic differences between individuals and thus identify those who could best beneﬁt from a drug or a vaccine is the SNP. Alternatively, SNPs can also be used to determine who will suf- fer an adverse effect of a drug, as is already being done in HIV therapy. For example, SNPs are used to detect the 5% of the pop- ulation who inherit a predisposition to a potentially fatal side effect of abacavir. Ultimately, future vaccines and drugs may be designed for a speciﬁc patient and against a speciﬁc virus.
The future development of drugs and vaccines, however, will be increasingly expensive. According to one study, the average investment required to get one drug approved by the FDA and marketed in the United States has risen to approxi- mately $1.7 billion if one extrapolates from spending by phar- maceutical companies on the various stages of research and development during the 2000–2002 period. This ﬁgure is an increase from $1.1 billion from 1995–2000, when clinical trials cost less and drug companies were more productive in drug discovery. A 2001 study, however, placed the cost of bringing a new drug to market at $802 million, but this study did not include such commercialization costs as preparing marketing materials. For every 13 drugs that start out in animal testing, only one reached the market in 2003–2004, in contrast to one in eight during the 1995–2000 period.
In summary, public health measures should remain the ﬁrst line of defense against viral diseases, and should be com- bined with antiviral vaccines when available. Antiviral drugs and passive immunity with IGs provide a second line of defense, but are usually more expensive than vaccines and public health measures, and provide shorter duration of pro- tection. Although many more antiviral drugs will become available in the 21st century, the greatest need is for safe and effective vaccines against HIV, HPV, HSV, Dengue, rotavi- ruses, Ebola, West Nile virus, SARS, coronavirus, etc. (16).