Antiviral Chemotherapy

Antiviral Chemotherapy
There is a need for antiviral drugs active against viruses for which vaccines are not available or not highly effective. Antivirals are needed to reduce morbidity and economic loss due to viral infections and to treat increasing numbers of immunosuppressed patients who are at increased risk of infection.
Types of antiviral drugs:
Nucleoside Analogs
The majority of available antiviral agents are nucleoside analogs. They inhibit nucleic acid replication by inhibition of polymerases for nucleic acid replication. In addition, some analogs can be incorporated into the nucleic acid and block further synthesis or alter its function.
Nucleotide Analogs
Nucleotide analogs differ from nucleoside analogs in having an attached phosphate group. Their ability to persist in cells for long periods of time increases their potency. Cidofovir is an example.
Nonnucleoside Reverse Transcriptase Inhibitors
Nevirapine was the first member of the class of nonnucleoside reverse transcriptase inhibitors. It does not require phosphorylation for activity and does not compete with nucleoside triphosphates. It acts by binding directly to reverse transcriptase and disrupting the enzyme s catalytic site.
Protease Inhibitors
Saquinavir was the first protease inhibitor to be approved for treatment of HIV infection. Such drugs inhibit the viral protease that is required at the late stage of the replicative cycle to cleave the viral polypeptide precursors to form the mature virion core and activate the reverse transcriptase that will be used in the next round of infection. Protease inhibitors include indinavir and ritonavir .
Fusion Inhibitor
Fuzeon is a large peptide that blocks the virus and cellular membrane fusion step involved in entry of HIV-1 into cells.

Other Types of Antiviral Agents
Amantadine and Rimantadine
These synthetic amines specifically inhibit influenza A viruses by blocking viral uncoating.
Foscarnet (Phosphonoformic Acid, PFA)
Foscarnet, an organic analog of inorganic pyrophosphate, selectively inhibits viral DNA polymerases and reverse transcriptases at the pyrophosphate-binding site.
Methisazone
Methisazone is of historical interest as an inhibitor of poxviruses. It blocked a late stage in viral replication, resulting in the formation of immature, noninfectious virus particles.
Table 1. Examples of Antiviral Compounds Used for Treatment of Viral Infections.

Drug Nucleoside Analog Mechanism of Action Viral Spectrum

Acyclovir Yes Viral polymerase inhibitor Herpes simplex, varicella-zoster
Amantadine No Blocks viral uncoating Influenza A
Cidofovir No Viral polymerase inhibitor Cytomegalovirus, herpes simplex, polyomavirus
Didanosine (ddI) Yes Reverse transcriptase inhibitor HIV-1, HIV-2
Foscarnet No Viral polymerase inhibitor Herpesviruses, HIV-1, HBV
Fuzeon No HIV fusion inhibitor (blocks viral entry) HIV-1
Ganciclovir Yes Viral polymerase inhibitor Cytomegalovirus
Indinavir No HIV protease inhibitor HIV-1, HIV-2
Lamivudine (3TC) Yes Reverse transcriptase inhibitor HIV-1, HIV-2, HBV
Nevirapine No Reverse transcriptase inhibitor HIV-1
Ribavirin Yes Perhaps blocks capping of viral mRNA Respiratory syncytial virus, influenza A and B, Lassa fever, hepatitis C, others
Ritonavir No HIV protease inhibitor HIV-1, HIV-2
Saquinavir No HIV protease inhibitor HIV-1, HIV-2
Stavudine (d4T) Yes Reverse transcriptase inhibitor HIV-1, HIV-2
Trifluridine Yes Viral polymerase inhibitor Herpes simplex, cytomegalovirus, vaccinia
Valacyclovir Yes Viral polymerase inhibitor Herpesviruses
Vidarabine Yes Viral polymerase inhibitor Herpesviruses, vaccinia, HBV
Zalcitabine (ddC) Yes Reverse transcriptase inhibitor HIV-1, HIV-2, HBV
Zidovudine (AZT) Yes Reverse transcriptase inhibitor HIV-1, HIV-2, HTLV-1




Interferons
Interferons (IFNs) are host-coded proteins that are members of the large cytokine family and which inhibit viral replication. They are produced very quickly (within hours) in response to viral infection or other inducers and are one of the body s first responders in the defense against viral infection. Interferons are central to the innate antiviral immune response. They also modulate humoral and cellular immunity and have broad cell growth regulatory activities.
Properties of Interferons
There are multiple species of interferons that fall into three general groups as follow:
Table 2. Properties of Human Interferons.


Property Type
Alpha Beta Gamma
Current nomenclature IFN-? IFN-? IFN-?
Former designation Leukocyte Fibroblast Immune interferon
Type designation Type I Type I Type II
Number of genes that code for family 20 1 1
Principal cell source Most cell types Most cell types Lymphocytes
Inducing agent Viruses; dsRNA Viruses; dsRNA Mitogens
Stability at pH 2.0 Stable Stable Labile
Introns in genes No No Yes
Chromosomal location of genes 9 9 12
Size of secreted protein (number of amino acids) 165 166 143
Chromosomal location of IFN receptor genes 21 21 6



Viral Vaccines
The purpose of viral vaccines is to utilize the immune response of the host to prevent viral disease. Several vaccines have proved to be effective at reducing the incidence of viral disease Vaccination is the most cost-effective method of prevention of serious viral infections.
Mucosal immunity (local IgA) is important in resistance to infection by viruses that replicate exclusively in mucosal membranes (rhinoviruses, influenza viruses, rotaviruses). Viruses that have a viremic mode of spread (polio, hepatitis, measles) are controlled by serum antibodies. Cell-mediated immunity also is involved in protection against systemic infections (measles, herpes).
Table 3 Comparison of Characteristics of Killed and Live Viral Vaccines

Characteristic Killed Vaccine Live Vaccine
Number of doses Multiple Single
Need for adjuvant Yes No
Duration of immunity Shorter Longer
Effectiveness of protection (more closely mimics natural infection) Lower Greater
Immunoglobulins produced IgG IgA and IgG
Mucosal immunity produced Poor Yes
Cell-mediated immunity produced Poor Yes
Residual virulent virus in vaccine Possible No
Reversion to virulence No Possible
Excretion of vaccine virus and transmission to nonimmune contacts No Possible
Interference by other viruses in host No Possible
Stability at room temperature High Low


Oncogenic viruses (Human cancer viruses)
Viruses are considered to be factors in the development of several types of human tumors. The viruses that have been strongly associated with human cancers are listed in Table 4. They include human papillomaviruses, Epstein-Barr virus, human herpesvirus 8, hepatitis B virus, hepatitis C virus, and two human retroviruses plus several candidate human cancer viruses. Many viruses can cause tumors in animals, either as a consequence of natural infection or after experimental inoculation.
Table 4. Association of Viruses with Human Cancers.1


Virus Family Virus Human Cancer
Papillomaviridae Human papillomaviruses Genital tumors
Squamous cell carcinoma
Oropharyngeal carcinoma
Herpesviridae EB virus Nasopharyngeal carcinoma
Burkitt s lymphoma
Hodgkin s disease
B cell lymphoma
Human herpesvirus 8 Kaposi s sarcoma
Hepadnaviridae Hepatitis B virus Hepatocellular carcinoma
Retroviridae HTL virus Adult T cell leukemia
Human immunodeficiency virus AIDS-related malignancies
Flaviviridae Hepatitis C virus Hepatocellular carcinoma


1Candidate human tumor viruses include additional types of papillomaviruses and polyomaviruses SV40, JC, and BK.
EB, Epstein-Barr; HTL, human T-cell lymphoma.

General Features of Viral Carcinogenesis
These features are summarized in Table 5


Table 5 Features of Viral Carcinogenesis.


1. Viruses can cause cancer in animals and humans.
2. Tumor viruses frequently establish persistent infections in natural hosts.
3. Host factors are important determinants of virus-induced tumorigenesis.
4. Viruses are seldom complete carcinogens.
5. Virus infections are more common than virus-related tumor formation.
6. Long latent periods usually elapse between initial virus infection and tumor appearance.
7. Viral strains may differ in oncogenic potential.
8. Viruses may be either direct- or indirect-acting carcinogenic agents.
9. Oncogenic viruses modulate growth control pathways in cells.
10. Animal models may reveal mechanisms of viral carcinogenesis.
11. Viral markers are usually present in tumor cells.
12. One virus may be associated with more than one type of tumor.


Tumor Viruses Are of Different Types
Like other viruses, tumor viruses are classified among different virus families according to the nucleic acid of their genome and the biophysical characteristics of their virions. Most recognized tumor viruses either have a DNA genome . DNA tumor viruses are classified among the papilloma-, polyoma-, adeno-, herpes-, hepadna-, and poxvirus groups. DNA tumor viruses encode viral oncoproteins that are important for viral replication but also affect cellular growth control pathways.
Most RNA tumor viruses belong to the retrovirus family. Retroviruses carry an RNA-directed polymerase (reverse transcriptase) that constructs a DNA copy of the RNA genome of the virus. The DNA copy (provirus) becomes integrated into the DNA of the infected host cell, and it is from this integrated DNA copy that all proteins of the virus are translated.
Multistep Carcinogenesis
Carcinogenesis is a multistep process, ie, multiple genetic changes must occur to convert a normal cell into a malignant one. Intermediate stages have been identified and designated by terms such as "immortalization," "hyperplasia," and "preneoplastic." Tumors usually develop slowly over a long period of time. It appears that a tumor virus usually acts as a cofactor, providing only some of the steps required to generate malignant cells.
Interactions of Tumor Viruses with Their Hosts
Persistent Infections
The known tumor viruses establish long-term persistent infections in humans. Because of differences in individual genetic susceptibilities and host immune responses, levels of virus replication and tissue tropisms may vary among persons.
Host Immune Responses
Viruses that establish persistent infections must avoid detection and recognition by the immune system that would eliminate the infection. Different viral evasion strategies have been identified, including :infection of sites relatively inaccessible to immune responses (human papillomavirus in the epidermis); mutation of viral antigens that allows escape from antibody and T cell recognition (human immunodeficiency virus); modulation of host major histocompatibility complex class I molecules in infected cells (adenovirus, cytomegalovirus); inhibition of antigen processing (Epstein-Barr virus); and infection and suppression of essential immune cells (human immunodeficiency virus).
Mechanisms of Action by Human Cancer Viruses
Tumor viruses mediate changes in cell behavior by means of a limited amount of genetic information. There are two general patterns by which this is accomplished: The tumor virus introduces a new "transforming gene" into the cell (direct-acting), or the virus alters the expression of a preexisting cellular gene or genes (indirect-acting). In either case, the cell loses control of normal regulation of growth processes. DNA repair pathways are frequently affected, leading to genetic instability and a mutagenic phenotype.
Cellular transformation may be defined as a stable, heritable change in the growth control of cells in culture. No set of characteristics invariably distinguishes transformed cells from their normal. Transformation to a malignant phenotype is recognized by tumor formation when transformed cells are injected into appropriate test animals.
Indirect-acting tumor viruses are not able to transform cells in culture.
Cell Susceptibility to Viral Infections
At the cellular level, host cells are either permissive or nonpermissive for replication of a given virus. Permissive cells support viral growth and production of progeny virus; nonpermissive cells do not. Especially with the DNA viruses, permissive cells are not transformed unless the viral replicative cycle that normally results in death of the host cell is blocked in some way; nonpermissive cells may be transformed. In contrast, a characteristic property of RNA tumor viruses is that they are not lethal for the cells in which they replicate. Cells that are permissive for one virus may be nonpermissive for another.
Some viruses are associated with a single tumor type, whereas others are linked to multiple tumor types. These differences reflect the tissue tropisms of the viruses.
In some viral systems, virus-transformed cells may release growth factors that affect the phenotype of neighboring uninfected cells, thereby contributing to tumor formation. It is also possible that as tumor cells collect genetic mutations during tumor growth, the need for the viral genes that drove tumor initiation may become unnecessary and will be lost from some cells.
Retroviruses
Retroviruses contain an RNA genome and an RNA-directed DNA polymerase (reverse transcriptase). RNA tumor viruses in this family mainly cause tumors of the reticuloendothelial and hematopoietic systems (leukemias, lymphomas) or of connective tissue (sarcomas).
Important properties of the retroviruses are listed in Table 6
Table 6. Important Properties of Retroviruses.


Virion: Spherical, 80–110 nm in diameter, helical nucleoprotein within icosahedral capsid
Composition: RNA (2%), protein (about 60%), lipid (about 35%), carbohydrate (about 3%)
Genome: Single-stranded RNA, linear, positive-sense, 7–11 kb, diploid; may be defective; may carry oncogene
Proteins: Reverse transcriptase enzyme contained inside virions
Envelope: Present
Replication: Reverse transcriptase makes DNA copy from genomic RNA; DNA (provirus) integrates into cellular chromosome; provirus is template for viral RNA
Maturation: Virions bud from plasma membrane
Outstanding characteristics:
Infections do not kill cells
May transduce cellular oncogenes, may activate expression of cell genes
Proviruses remain permanently associated with cells and are frequently not expressed
Many members are tumor viruses

Tumor Suppressor Genes
Human Retroviruses
Only a few retroviruses are linked to human tumors. The human T-lymphotropic (HTLV) group of retroviruses has probably existed in humans for thousands of years. HTLV-1 has been established as the causative agent of adult T cell leukemia-lymphomas (ATL) as well as a nervous system degenerative disorder called tropical spastic paraparesis. It does not carry an oncogene. A related human virus, HTLV-2, has been isolated but has not been conclusively associated with a specific disease. HTLV-1 and HTLV-2 share about 65% sequence homology and display significant serologic cross-reactivity.
The human lymphotropic viruses have a marked affinity for mature T cells. HTLV-1 is expressed at very low levels in infected individuals. It appears that the viral promoter-enhancer sequences in the long terminal repeat may be responsive to signals associated with the activation and proliferation of T cells. If so, the replication of the viruses may be linked to the replication of the host cells—a strategy that would ensure efficient propagation of the virus.
The human retroviruses are transregulating (Figure 43–2). They carry a gene, tax, whose product alters the expression of other viral genes. Transactivating regulatory genes are believed to be necessary for viral replication in vivo and may contribute to oncogenesis by also modulating cellular genes that regulate cell growth.
There are several genetic subtypes of HTLV-1, with the major ones being subtypes A, B, and C (these do not represent distinct serotypes).
The virus is distributed worldwide, with an estimated 10 to 20 million infected individuals. Clusters of HTLV-associated disease are found in certain geographic areas (southern Japan, Melanesia, the Caribbean, Central and South America, and parts of Africa) (Figure 43–5). Although fewer than 1% of people worldwide have HTLV-1 antibody, more than 10% of the population in endemic areas are seropositive, and antibody may be found in 50% of relatives of virus-positive leukemia patients.
ATL is poorly responsive to therapy. The 5-year survival rate for patients with this cancer is < 5%.
Transmission of HTLV-1 seems to involve cell-associated virus. Mother-to-child transmission via breast feeding is an important mode. Efficiency of transmission from infected mother to child is estimated at 15–25%. Such early-life infections are associated with the greatest risk of ATL. Blood transfusion is an effective means of transmission, as are sharing blood-contaminated needles (drug abusers) and sexual intercourse.
Seroepidemiology has linked infection with HTLV-1 to a syndrome called HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). The primary clinical feature is development of progressive weakness of the legs and lower body. The patient s mental faculties remain intact. HAM/TSP is described as being of the same magnitude and importance in the tropics as is multiple sclerosis in Western countries.
A group of human retroviruses has been established as the cause of AIDS; see Chapter 44. The viruses are cytolytic and nontransforming and are classified as lentiviruses. However, AIDS patients are at elevated risk of several types of cancer because of the immune suppression associated with human immunodeficiency virus infection. These cancers include lymphomas and cervical cancer.
The simian foamy viruses from the Spumavirus genus are highly prevalent in captive nonhuman primates. Humans occupationally exposed to the primates can be infected with foamy viruses, but these infections have not resulted in any recognized disease.

Polyomaviruses
Important properties of polyomaviruses are listed in Table 7.
Table 7. Important Properties of Polyomaviruses.


Virion: Icosahedral, 45 nm in diameter
Composition: DNA (10%), protein (90%)
Genome: Double-stranded DNA, circular, 5 kbp, MW 3 million
Proteins: Three structural proteins; cellular histones condense DNA in virion
Envelope: None
Replication: Nucleus
Outstanding characteristics:
Stimulate cell DNA synthesis
Viral oncoproteins interact with cellular tumor suppressor proteins
Important model tumor viruses
Human viruses can cause human neurologic and renal disease
May cause human cancer


Papillomaviruses
Important properties of papillomaviruses are listed in Table 8
Table 8. Important Properties of Papillomaviruses.1


Virion: Icosahedral, 55 nm in diameter
Composition: DNA (10%), protein (90%)
Genome: Double-stranded DNA, circular, 8 kbp, MW 5 million
Proteins: Two structural proteins; cellular histones condense DNA in virion
Envelope: None
Replication: Nucleus
Outstanding characteristics:
Stimulate cell DNA synthesis
Restricted host range and tissue tropism
Significant cause of human cancer, especially cervical cancer
Viral oncoproteins interact with cellular tumor suppressor proteins

Adenoviruses
The adenoviruses comprise a large group of agents widely distributed in nature. They are medium-sized, nonenveloped viruses containing a linear genome of double-stranded DNA (26–45 kbp). Replication is species-specific, occurring in cells of the natural hosts. Adenoviruses commonly infect humans, causing mild acute illnesses, mainly of the respiratory and intestinal tracts.
Herpesviruses
These large viruses (diameter 125–200 nm) contain a linear genome of double-stranded DNA (125–240 kbp) and have a capsid with icosahedral symmetry surrounded by an outer lipid-containing envelope. Herpesviruses typically cause acute infections followed by latency and eventual recurrence in each host, including humans.
In humans, herpesviruses have been linked to several specific types of tumors. Epstein-Barr (EB) herpesvirus causes acute infectious mononucleosis when it infects B lymphocytes of susceptible humans. EB virus is etiologically linked to Burkitt s lymphoma, a tumor most commonly found in children; to nasopharyngeal carcinoma (NPC), more common to posttransplant lymphomas; and to Hodgkin s disease. These tumors usually contain EB viral DNA .
Kaposi s sarcoma-associated herpesvirus, also known as human herpesvirus 8 (KSHV/HHV8). It is suspected of being the cause of Kaposi s sarcoma, primary effusion lymphoma, and a particular lymphoproliferative disorder. KSHV has a number of genes that may stimulate cellular proliferation and modify host defense mechanisms.
Poxviruses
Poxviruses are large, brick-shaped viruses with a linear genome of double-stranded DNA (130–375 kbp). Very little is known about the nature of these proliferative diseases, but the poxvirus-encoded growth factor that is related to epidermal growth factors and to transforming growth factor may be involved.
Hepatitis B Virus & Hepatitis C Virus
Hepatitis B virus a member of the Hepadnaviridae family, is characterized by 42-nm spherical virions with a circular genome of double-stranded DNA (3.2 kbp). One strand of the DNA is incomplete and variable in length.
In addition to causing hepatitis, hepatitis B virus is a risk factor in the development of liver cancer in humans and the development of hepatocellular carcinoma.
Hepatitis C virus a member of the Flaviviridae family, contains a genome of single-stranded RNA 9.4 kb in size. It appears that the majority of infections become persistent, even in adults. Chronic infection with hepatitis C virus is also considered to be a causative factor in hepatocellular carcinoma. Most probably, hepatitis C virus acts indirectly in the development of hepatocellular carcinoma.