HIV vaccines under study

Transcription

1 Dermatologic Therapy, Vol. 22, 2009, Printed in the United States All rights reserved HIV vaccines under study Parisa Ravanfar*, Natalia Mendoza*, Anita Satyaprakash* & Bilal I. Jordan *Center for Clinical Studies, Houston, Texas, Universidad El Bosque, Bogota, Colombia, and Saint Louis University School of Medicine, St. Louis, Missouri 2009 Wiley Periodicals, Inc. DERMATOLOGIC THERAPY ISSN ABSTRACT: Human immunodeficiency virus (HIV) infection is a worldwide epidemic, with over 42 million people currently infected. Since the discovery of HIV as the causative agent of the acquired immune deficiency syndrome (AIDS), many potential vaccines have been created. The first of these vaccines presented disappointing results; however, that has not deterred researchers from continuing to develop more potential HIV vaccines. This article will review the various current HIV vaccine candidates under study. KEYWORDS: AIDS, HIV, HIV vaccine, vaccination Introduction Over one million people living in the United States are human immunodeficiency virus (HIV) positive, and an estimated 40,000 people become newly infected each year. Although the life expectancy for HIV-positive individuals has drastically increased because of advancements in highly active antiretroviral therapy, the treatments are not without considerable side effects and are very costly. Regardless of treatment, it is only a matter of time until the virus eventually overcomes the host s immune system, causing severe disease and eventual mortality. It is therefore of paramount importance that an effective HIV vaccine be created to prevent transmission and thus contain the HIV epidemic. In spite of many failed HIV vaccine candidates, there are still countless more currently under study. Examples of such vaccines will be further discussed below. HIV epidemiology Over 42 million people have been infected with HIV since the beginning of the epidemic in the 1980s. In Address correspondence and reprint requests to: Parisa Ravanfar, MD, MBA, MS, Center for Clinical Studies, Houston, TX 77030, or pravanfar@ccstexas.com 2003, there were an estimated five million newly infected people and three million deaths (1). By the end of 2003, approximately 1,039,000 1,185,000 persons in the United States were living with HIV/ acquired immune deficiency syndrome (AIDS); of which 24 27% were undiagnosed and unaware of their HIV-positive status (2). The Centers for Disease Control and Prevention (CDC) estimated that approximately 56,300 people were newly infected with HIV in 2006 (2). The collective number of cases of AIDS through 2006 in the United States and dependent areas was estimated as 1,014,797 cases by the CDC (2). Over 22 million people have died from AIDS. Moreover, of the over 42 million people currently living with HIV/AIDS; 74% live in sub-saharan Africa (3). There are approximately 14,000 newly infected individuals daily; 95% of whom live in developing countries. Half of the five million new annual HIV infections occur among people ages years (3). The United Nations estimates that there are currently 14 million AIDS orphans, and that number is estimated to be 25 million by HIV virology AIDS was first recognized in the United States in HIV was identified in 1983 and was clearly 158

2 HIV vaccines FIG. 1. Structure of the human immunodeficiency virus (HIV) (71). FIG. 2. The business of binding. The initial event in human immunodeficiency virus (HIV)-1 infection is the stable tethering of HIV-1 to a host cell. This interaction is mediated by a binding event between the glycoprotein (gp) 120 molecule that protrudes from the virus and a CD4 molecule on the host cell. (An accessory host cell molecule, CCR5, is also involved in the binding event.) Stable binding leads to infection (lefthand panel). The binding reaction can be blocked by neutralizing antibodies, but only two antibodies are known to bind gp120 stably (middle panel). The majority of antibodies fail to bind gp120, and therefore do not neutralize infection. A recent study shows that gp120 is refractory to the binding of these impotent antibodies because the molecule is unusually flexible, and the binding regions of the antibodies are specific to a particularly flexible part of it (right-hand panel) (72). demonstrated to be the causative agent of AIDS in HIV is a human retrovirus and belongs to the family Retroviridae and the subfamily lentivirus. HIV-1 and HIV-2 are cytopathic viruses, with the majority of HIV disease, both worldwide and in the United States, caused by HIV-1. The mature HIV virion has a spherical morphology of nm in diameter and consists of a dense nucleocapsid (core) that contains the genomic RNA molecules, the viral protease, reverse transcriptase (RT), integrase (IN), Vpu, Vif, Vpr, and Nef, and cellular factors, which are all surrounded by a lipid bilayer membrane (4) (FIG. 1). The HIV-1 genome is composed of two identical 9.2 kb singlestranded RNA molecules within the virion; however, within infected cells, the persistent form of the HIV-1 genome is double-stranded DNA (5). HIV infection starts with the attachment of the virion to the cell surface, which is achieved by highaffinity binding between the extracellular domain of HIV-1, namely the glycoprotein (gp) 120 protein, and host cell receptors (5,6) (FIG. 2). CD4 is the major host cell receptor for HIV-1 and HIV-2, and the chemokine receptors CCR5 and CXCR4 are the main HIV-1 coreceptors (7). After binding, viral and cellular membranes fuse, allowing release of the virus into the host cell cytoplasm. Viral uncoating involves cellular factors and the viral proteins MA, Nef, and Vif. The viral RNA genome is subsequently retrotranscribed into a full-length double-stranded DNA by the viral RT (5). Reverse transcription is achieved through the presence of the cellular protein APOBEC3G (or CEM15). HIV-1 Vif counteracts the antiretroviral effect of the cellular protein APOBEC3G by reducing its expression and incorporation into progeny virions (5,8 11). The preintegration complex (12) attaches to the nuclear membrane directed by HIV-1 Vpr (13) and enters the nucleus through the nuclear pore. The DNA translocates into the host nucleus where it becomes integrated into the host cell chromosomes by the viral enzyme IN. HIV clades are taxonomic subgroups of HIV that are divided by geographical region. Each clade has genetic similarities and markers. Many of these clades are distinct from one another, thus contributing to the difficulty of developing an HIV vaccine. HIV is notorious for its capability to rapidly mutate its envelop proteins and evading the host s immune system, which also adds to the challenge of creating a vaccine. There are two major HIV clades: clade M, or Main, and clade O, or Outgroup. The M clade causes the majority of HIV infections, 159

3 Ravanfar et al. lymphoid tissues. A clinically latent phase ranging from weeks to years is followed by a chronic and persistent phase of viral replication. The destruction of CD4 lymphocytes leads to decreasing immunity, thus allowing for opportunistic infections as well as increased risk of malignancy. The HIV epidemic has created a surge of opportunistic diseases that were previously not often seen in the medical field, creating further challenges associated with HIV. HIV vaccines FIG. 3. Immune responses to human immunodeficiency virus (HIV) infection, showing plasma HIV levels, HIV-specific CD8+ T cells, and HIV-neutralizing antibodies (65). whereas the O HIV clade is less common. The M clade is composed of eight different subtypings, lettered A through H. Certain geographic regions tend to be associated with a particular clade. For instance, clades A and D are the most common in East Africa, clade B is the most common in Europe and the Americas, and clade C is the most common in East Asia. Viruses in each clade vary in degrees of virulence and response to different treatments. HIV pathophysiology HIV is transmitted though contact with blood and blood-related products, which includes parenteral and sexual transmission. HIV disease is characterized by profound immunodeficiency resulting from a progressive quantitative and qualitative deficiency in the subset CD4+ T lymphocytes. As the number of CD4+ T cells fall, the patient s risk of becoming infected with opportunistic diseases increases. In primary HIV infection, the virus undergoes significant replication in the CD4 T cells prior to the start of an HIV-specific immune response, allowing for an explosion of viremia and spread of the virus to various organ tissues (14). Approximately half of primary infected individuals experience an acute HIV syndrome, which is associated with very high levels of viremia, measured in millions of copies of HIV RNA per milliliter, lasting for weeks (14) (FIG. 3). This acute viremia is correlated with the mononucleosis-like symptoms that occur with the primary infection. Furthermore, the initial viremia results in the dissemination of the virus to It was soon after HIV was discovered to be the causative agent of AIDS that enormous scientific energy was devoted to the development of a vaccine. A successful vaccine must be able to prevent infection in HIV-negative individuals, or at least reduce the viral load in those infected with HIV. There are multiple HIV vaccine classes currently under study, such classes include canarypox vectors, DNA plasmid vaccines, fowlpox vector vaccines, lipopeptides, live attenuated vesicular stomatitis virus vectors, modified vaccinia Ankara (MVA) vector vaccines, nonreplicating adenoviral vector vaccines, peptides, proteins, and Venezuelan equine encephalitis virus (VEE) vectors. Fowlpox vector vaccines Poxviruses such as canarypox, vaccinia, and fowlpox are the most commonly used live vectors for HIV vaccines under study (15). This is due to the fact that poxviruses are able to accommodate large amounts of foreign DNA and are capable of infecting mammalian cells, therefore allowing the expression of a significant amount of foreign protein (15). The fowlpox virus (FPV) belongs to the Poxviridae family, genus Avipoxvirus. FPV can only infect avain cells (15); however, mammalian cells inoculated with avipox-based recombinants have demonstrated expression of foreign genes, as well as induction of protective immunity (16,17). Various studies have demonstrated that mammalian species immunized by recombinant FPV can result in both humoral and cell-mediated immunity to the expressed transgene product without any resulting local or systemic adverse effects (15,18 22). Safety. FPV vaccines have been administered to various mammalian species without any reported local or systemic adverse effects (23 26). There 160

4 HIV vaccines is very minor concern for potential systemic infection due to the fact that fowlpox cannot replicate in nonavian species, thus allowing for better safety compared with replication-competent vaccine vectors when infection is of concern. Multiple clinical studies have been performed utilizing fowlpox-based vaccines. Such vaccines have been administered to hundreds of patients to date and have supported their safety (15). Immunogenicity. FPV vaccines have been utilized in various animal models for a variety of different infectious agents (16,17,19,22,25). Many of these studies have demonstrated protective immunogenicity from infectious agents, including immunodeficiency viruses and plasmodium parasites. Immunization series, consisting of a DNA vaccine primer followed by a booster recombinant fowlpox-based vaccine, have been shown to provide some degree of protection in animal primates after challenging them with an immunodeficiency virus (15). Nonreplicating adenoviral vector vaccines Nonreplicating viral vector vaccines are prepared by deleting one or more genes from a virus capable of entering human cells (27).The deleted genes are replaced by inserted segments of DNA encoding HIV proteins, referred to as transgenes, which results in a viral vector that is incapable of replication. The vectors enable the HIV transgenes to enter into cells, thus allowing for expression of the HIV peptides or proteins (27). Through this mechanism, nonreplicating viral vector vaccines carry HIV antigens into the cytoplasm of antigen-presenting cells without the possibility of the host actually being infected with HIV. The viral antigens are thus processed with MHC class I to be presented on the cell surface to CD8 cells. This produces an HIV-specific cytotoxic T cell (CTL) response (27). Viral vectors in the form of adenoviruses with gene deletions, termed adenovectors, are currently under study as potential HIV vaccines. Adenovectors were originally produced to be used as delivery vehicles for gene therapy (27). The vectors produced from adenovirus type 5 (Ad5) are incapable of replication due to the inactivation of the E1 gene. The Ad5 vector was studied in animal and early clinical trials as possible HIV vaccines. Adenovector HIV vaccines are significant in that they have been shown to elicit both high-titer antibody and high-frequency CTL responses in animal models (28). More importantly, they have shown protection in studies that have challenged primates to HIV exposure (28). Some phase I studies using an adenoviral vector vaccine containing the HIV gag gene, administered as a series of injections or as a booster following a primer DNA vaccine have been found to induce significant Gag-specific CD8 responses in humans (27,29). Safety. Adenovectors have been used for numerous gene therapy studies in multiple different human diseases such as cancer, cystic fibrosis, and cardiovascular disease. The adenovectors have also been administered via various routes such as aerosol, intradermal, intramyocardial, intrapleural, and intratumoral (27,30 39). Many studies have been conducted to evaluate adenovectors as gene therapy agents, with the majority employing vectors based on Ad5. Data from such studies illustrated that side effects were mild, local, or absent in the majority of cases when the agents were administered intradermally or intramuscularly, with no significant toxicities caused by the vector (27,30 39). A dose-escalation trial of an HIV-1 Gag-Ad5 vaccine in humans found that transient adverse events were more common at higher doses of the vaccine as well as in subjects without preexisting neutralizing antibody Ad5. Such adverse events consisted of moderate reactions at the injection site, malaise, and myalgias (27,29). There has been at least one study in which mortality was related to the use of an adenovector. The study was composed of 19 patients diagnosed with ornithine transcarbamylase deficiency. The patients were administered adenovector particles infused directly into the hepatic circulation at doses ranging from to (27). Virtually, all study subjects experienced at least one systemic symptom such as fever, myalgia, nausea, or vomiting. Almost all patients displayed a mild and transient thrombocytopenia, without corresponding coagulation abnormalities. The subjects who received higher dose levels were observed to demonstrate abnormal liver function studies (40). Of the subjects, there was an 18-year-old patient who received the highest study dose and subsequently died after the administration of adenovector particles directly into the hepatic artery (40). The National Institutes of Health (NIH) report, after review of the clinical data and the fatal case, concluded that the death was presumably caused by a systemic adenovector-induced shock syndrome, leading to a cytokine cascade, causing disseminated intravascular coagulation, acute respiratory distress, and multiorgan failure (41). 161

5 Ravanfar et al. Although there have been no reports of serious reactions with intramuscular administration at lower doses, systemic administration of a highdose adenoviral vector has caused one associated fatality. The highest adenovector dose currently used in HIV vaccines is over 300 times lower than the dose administered in the previous fatal case and has thus far been safe in phase I trials (29). The adenovectors currently under investigation in HIV vaccine studies have deletions in the E1 region and some in the E3 and/or E4 regions. These deletions allow for the production of a vector that is incapable of acquiring the ability to replicate. However, all adenovectors are initially tested for cytopathic effects in tissue culture prior to use in human trials (27). There is always the theoretical possibility that an adenovector vaccine could undergo recombination with a wild-type adenovirus that is concurrently infecting a vaccine (27). Immunogenicity. Studies in primates and in humans have demonstrated the ability of adenovectors to induce strong CD8 T cell responses to encoded HIV antigens, as well as strong antibody responses (27,28). Adenovirus serotype 5 is endemic in many areas. Neutralizing antibodies to Ad5 have been detected in 30 70% of vaccine trial participants in the United States (27,29). The seroprevalence of neutralizing antibodies to Ad5 is almost 90% in sub- Saharan Africa (27). A prior immunity to the Ad5 vector may lower the host s immune response to an Ad5 vaccine (27). This attenuation may be resolved by increasing the vaccine dosage, using a heterologous primeboost approach, or both (27 29). In nonhuman primate trials, boosting with adenovectors based on alterative serotypes (such as Ad24) has been associated with an improvement in cellular immune responses after administration of an Ad5- adenovector primer (27,29). Another similar study showed that an Ad5-adenovector primer with a subsequent canarypox vector booster (ALVAC vcp 205) resulted in greater cellular immune responses to HIV than simply using a booster with the same adenovector (27). DNA plasmid vaccines DNA vaccines are usually plasmids with a gene encoding the target antigen that is transcriptionally controlled by a promoter region active in human cells. DNA vaccines are usually administered intramuscularly (IM). DNA vaccines were initially tested in humans with HIV infection (42) and, subsequently, in uninfected people as HIV preventive vaccines (43,44). Although immune responses to DNA alone have been weak in humans, combination with adjuvants or with recombinant viral vectors in prime-boost approaches have produced considerable HIV-specific CD8 responses and have produced protective responses in primate models (44). Safety. Studies indicate that DNA vaccines rarely integrate into cellular DNA; however, as vectors are modified or adjuvanted in order to increase immunogenicity, the chances of integration could potentially increase. The primary concern is that an integrated vaccine could result in insertional mutagenesis by activation of oncogenes or inactivation of tumor suppressor genes (44). Furthermore, an integrated plasmid DNA vaccine could, in theory, result in chromosomal instability through the induction of chromosomal breaks or rearrangements (44). There is also the concern of developing vaccineassociated autoimmunity; however, to date, there has been no convincing data linking DNA vaccines with the development of autoimmunity (44). The production of DNA plasmids includes the selection of bacterial cells carrying the plasmid. Selection is achieved by culturing the cells in the presence of an antibiotic to which resistance is enabled by a gene in the plasmid. This leads to the concern that resistance to the same antibiotic might be acquired by vaccinees who receive the plasmid that is used (44). However, this concern is muted by the fact that the antibiotic resistance genes contained in vaccine plasmids replicate through a bacterial origin of replication sequence rather than a mammalian one, and are therefore unable to be expressed outside of bacterial cells (44). Furthermore, the antibiotic that is selected is often not an antibiotic usually used to treat human infections (44). Immunogenicity. The immune response to DNA vaccines results from uptake of plasmids into cells, including dendritic and muscle cells, where the target antigen gene is expressed. This results in proteins that are processed as intracytoplasmic antigens and thus peptides that bind to class I MHC molecules. The presentation of these peptides on the cell surface induces a CD8 T lymphocyte response. The plasmid-encoded proteins also appear to stimulate an antibody response, pointing toward B lymphocyte stimulation. In this manner, DNA vaccines mimic viral infection by stimulating both cellular and humoral immune responses (44). 162

6 HIV vaccines Primate vaccine studies that subsequently challenge with an immunodeficiency virus have suggested that DNA vaccines provide a level of protection from HIV infection. These studies involve vaccines such as DNA plasmid primer with recombinant MVA boost (45), DNA plasmid with cytokine (interleukin-2) adjuvant (46), DNA plasmid with nonionic blocked copolymer adjuvant, with or without recombinant adenovirus booster (28), DNA plasmid primer with recombinant FPV booster (19,22,47,48), and DNA plasmid primer with cytokine (interleukin-12) adjuvant plus recombinant gp140 protein boost (49). However, there are other studies that have failed to achieve protection in primates challenged with simian immunodeficiency virus (SIV) or simian/ human immunodeficiency virus (SHIV) after vaccination with DNA vaccine-containing regimens (50,51). MVA vector vaccines As previously mentioned, poxviruses are the most common live-vector HIV vaccines currently under study. MVA is a highly attenuated strain of vaccinia virus that was created at the conclusion of the smallpox eradication (52). MVA lacks approximately 10% of the vaccinia genome and is incapable of efficiently replicating in primate cells (53). Regardless, MVA provides similar levels of recombinant gene expression to vaccinia viruses in human cells (54). Safety. MVA has been administered to various mammalian species (52,55) without any serious adverse events. The use of MVA as a recombinant HIV vaccine is being studied in various phase I studies (52). The vaccinia vaccine has some rare reported serious adverse events, such as myocarditis, pericarditis, and myopericarditis (52,56). Although MVA is an attenuated vaccinia virus and does not replicate in the human body as efficiently as vaccinia, it is currently unknown whether MVA can induce the same side effects as vaccinia. Immunogenicity. MVA vaccines have demonstrated immunogenic and protective properties against various infectious agents, including immunodeficiency viruses, influenza, parainfluenza, measles virus, flaviviruses, plasmodium parasites, and smallpox in multiple animal models (52). Furthermore, combinations of viral vector vaccines have been successfully conducted. For example, studies in mice show that fowlpox-based and MVA-based vaccines used in combination induce immunity and protection against challenge with plasmodium parasites (57). Also, DNA-based HIV vaccines administered to macques can be effectively boosted with recombinant MVA-based vaccines expressing HIV antigen (45). Immunization regimens that combine primers consisting of DNA vaccines with recombinant MVA-based vaccine boosters have been shown to provide some protection in primates after challenging with an immunodeficiency virus. Unfortunately, vaccination did not prevent infection in these studies, but it did result in lower viral loads, increased CD4 counts, and reduced rates of morbidity and mortality in vaccinated animals versus controls (45,52,58 60). HIV vaccine trials Numerous HIV vaccine trials in humans have been and are still being conducted. In March 1999, a phase III HIV vaccine trial of bivalent B/E rgp120hiv vaccine (AIDSVAX B/E, VanGen Inc., Brisbane, CA) was conducted to determine whether immunization with AIDSVAX would protect against HIV-1 infection in high-risk populations (namely intravenous [IV] drug users), as well as confirm safety (1,61 63). The vaccine consisted of 300 mg MN rgp120 combined with 300 mg A244 rgp120 in alum adjuvant, and the placebo consisted of only adjuvant (62). Randomized subjects received AIDSVAX B/E or placebo at Months 0, 1, and 6, with booster doses at 12, 18, 24, and 30 months. All subjects had a 3-year follow-up period. The primary end point of the study was evidence of infection, which was determined by an enzymelinked immunosorbent assay and immunoblot (64). The secondary end point was extent and duration of viremia, measured by quantitative RNA polymerase chain reaction (PCR) (64). The subjects were divided into two cohorts: the intention-totreat (ITT) cohort, and the weighted immunized (WI) cohort. The vaccine efficacy in prevention of HIV infection was 0.1% (95% confidence interval (CI), to 23.8%) and statistically insignificant in the ITT cohort, and -7.5% (95% CI, to -20.8%) and not statistically significant in the WI cohort (1). Disappointedly, the vaccine failed to achieve efficacy in both primary and secondary end points. The vaccine failure was considered to be caused by the lack of inducing broadly neutralizing antibodies (65). However, this has not diminished the persistence in developing an efficacious HIV vaccine. 163

7 Ravanfar et al. FIG. 4. Composition and timing of administration of the V520 vaccine (top) and the vaccine developed by the NIH (bottom). For the Merck vaccine candidate, a replication-deficient adenovirus type 5 was engineered to contain the gag, pol, or nef genes of the human immunodeficiency virus (HIV).This vaccine was given to volunteers at 0, 1, and 6 months. For thevaccine Research Center (VRC) vaccine, a mixture of six DNA plasmids containing the gag, pol, nef, env A, env B, or env C genes is given to volunteers at 0, 1, and 2 months. At Month 6, one injection of a different replication-deficient adenovirus type 5 is given; this was engineered to contain the gag/pol, env A, env B, or env C genes (69). Another phase III clinical trial is currently being conducted in Thailand (66). It is a combination vaccination series that consists of priming doses of vcp1521, a live recombinant canarypox viral vector ALVAC-HIV, followed by boosting vaccine doses of combination ALVAC-HIV (vcp1521) with the previously mentioned AIDSVAX B/E. The concept behind this vaccine combination is to induce both T cells and antibodies. The ALVAC- HIV (vcp1521) is given to subjects at Weeks 0, 4, 12, and 24. Subsequently, booster doses of both ALVAC-HIV (vcp1521) AIDSVAX B/E are given at Weeks 12 and 24 (1). The follow-up period will be 3 years, with HIV testing every 6 months (1). This is the world s first efficacy trial of a HIV prime-boost vaccine combination (1). The gp120 is from clades A and E, and the primer is from clade B. Another vaccine that is in phase II is a canarypox vaccine with lipopeptides and is composed of gag, pol, nef, env that are from clade B, plus cytotoxic T lymphocyte epitopes (65). There is a phase II clinical trial that started in September of 2005 that also utilizes a vaccine primer, followed by a booster. The primer is a DNA vaccine consisting of proteins gag, pol, nef, and env, and the boosting vaccine is adenovirus vector with gag, pol, and env components (67). It targets clades A, B, and C. There are at least three additional trials that employ both primer and booster vaccines, with the primer as a DNA vaccine. The booster vaccines used are NYVAC-C, MVA-CMDR with env, gag, pol, or adenovirus vector with gag, pol + env (67). Numerous phase I HIV vaccination trials are currently active. One such trial conducted in China utilizes a vaccine that is a replicating smallpox vector with HIV gene inserts (67). Another of these phase I trials uses a modified MVA viral vector with an HIV protein component (67). A phase I Merck (Whitehouse Station, NJ) trial with an Ad5 HIV-1 gag vaccine resembling clade B demonstrated excellent immunogenicity results in animal safety trials; however, when that vaccine was taken to human trials, it was observed to be genetically instable, and the vector was modified for future studies. The new vector was termed MRKad5 and did not appear to be genetically instable. A phase II proof-of-concept trial was therefore conducted to evaluate the safety and efficacy of a threedose regimen with the Merck adenovirus serotype 5 HIV-1 gag/pol/nef vaccine (FIG. 4). Unfortunately, the results were incredibly disappointing as the 164

8 HIV vaccines vaccine was not only shown to lack efficacy but was also associated with a higher rate of HIV infection. Yet another vaccine in phase I trial is an anthraxderived polpeptide-hiv gag fusion protein, clade B (65). An additional vaccine candidate in phase I is a Venezuelan equine encephalitis viral replicon, composed of gag from clade C (65). Conclusion/future aspects HIV vaccine production has been one of the greatest challenges of our century. The global investment in HIV vaccine research and development was estimated to be $759 million in 2005; 88% of this investment was from governments, 10% was from commercial firms, and 2% was from philanthropy (68). The NIH spends approximately $600 million annually on HIV vaccine research (69). Although there have been multiple setbacks and disappointments in the development of an HIV vaccine, the scientific community remains optimistic and relentless in creating an HIV vaccine. In the meanwhile, we as health-care professionals must continue to stress the importance of safe sex practices to our patients. Furthermore, patients should be thoroughly educated on other aspects of HIV prevention, such as the importance of treatment for HIV-positive pregnant women, the risks of needle-sharing, and the reduced risk of HIV transmission in circumcised men (70). References 1. Pitisuttithum P. HIV-1 prophylactic vaccine trials in Thailand. Curr HIV Res 2005: 3 (1): Centers for Disease Control and Prevention. Cases of HIV infection and AIDS in the United States and dependent areas. Atlanta, GA: Centers for Disease Control and Prevention, WHO. Data and statistics. Available at: int/hiv/data/en/. Accessed November 1, Hirsch MS, Curran J. Human immunodeficiency viruses. In: Fields, ed. Viruses, 4th ed. Philadelphia, PA: Lippincott- Raven Publishers, Sierra S, Kupfer B, Kaiser R. Basics of the virology of HIV-1 and its replication. 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9 Ravanfar et al. 26. Yamanouchi K, Barrett T, Kai C. New approaches to the development of virus vaccines for veterinary use. Rev Sci Tech 1998: 17 (3): Peiperl L. Class: nonreplicating adenoviral vector vaccines, Available at: page=vc Accessed November 1, Shiver JW, Fu TM, Chen L, et al. Replication-incompetent adenoviral vaccine vector elicits effective antiimmunodeficiency-virus immunity. Nature 2002: 415 (6869): Emini E. Ongoing development and evaluation of a potential HIV-1 vaccine using a replication-defective adenoviral vector. Keystone Conference on HIV Vaccines, Banff, Canada, March 31, Harvey BG, Leopold PL, Hackett NR, et al. Airway epithelial CFTR mrna expression in cystic fibrosis patients after repetitive administration of a recombinant adenovirus. J Clin Invest 1999: 104 (9): Hay JG, McElvaney NG, Herena J, Crystal RG. 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