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How Does It Work?
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| How might microbicides work? | How do vaccines work? | How do antiviral/antiretroviral drugs work? |
How might microbicides work?1-3
Microbicides are gels, creams, or other products that can be applied topically to reduce the transmission of HIV and other sexually transmitted infections (STIs) during vaginal sex. Some microbicides may also prevent pregnancy, though not all are “dual action.” Although a microbicide may be effective in protecting against more than one disease, it may not be effective against all STIs.
The first wave of microbicides were developed in the 1990s and do not specifically target HIV. These formulations should offer some protection against HIV. The next wave of products target HIV and should provide greater effectiveness in blocking transmission of the virus. There are currently about 30 microbicides in development, with at least 10 in clinical trials.
Microbicides work in one of four ways – two approaches target the virus before it penetrates the vaginal lining and two approaches act once the virus has penetrated the skin:
- Killing or disrupting viruses by breaking down their outer coating or envelope before the viruses can enter epithelial cells in the genital tract.
- Strengthening the body’s defenses by preserving healthy levels of microflora (i.e., the normal balance of bacteria, fungi, and other microorganisms) to maintain the vagina’s natural acidic environment (pH = 3.8 – 4.5).4 This acidic environment would inactivate HIV, as inactivation has been reported to occur between pH 4.0 and 5.8, depending on the study.5, 6
HIV prefers a less acidic environment – with optimal pH around 7.0 (neutral), similar to the pH of semen. Following ejaculation, the pH of the vagina increases to between 5.5 and 7.0 for at least two hours. This allows HIV to infect the vaginal epithelial cells. A microbicide that maintained the vagina’s natural pH level would greatly reduce the ability of HIV to infect the epithelial cells.
If the virus manages to cross the epithelium or layer of vaginal skin:
- Inhibiting viral entry by preventing attachment of the virus (also called binding or fusion) to the cells targeted by the virus or preventing entry into the cells (also called adsorption).
- Inhibiting viral replication. In the case of retroviruses (viruses with RNA as their genetic material), HIV replication can be suppressed by a combination of antiretroviral drugs and a microbicide that targets reverse transcriptase, which is an enzyme required for HIV viral reproduction can suppress replication of HIV.
What Microbicides Could Do
 Reprinted by permission from Macmillan Publishers Ltd: [Nature Reviews Microbiology] (Shattock R, Moore JP. Inhibiting sexual transmission of HIV-1 infection.
Nat Rev Microbiol 2003; 1(1):25-34 | website
While condoms are currently more effective barriers to HIV transmission, microbicides offer some protection for women who are unable or afraid to ask their partner to use a condom or for women whose partners refuse to use condoms.
Though much of the research is focused on developing a vaginal microbicide, there are efforts to develop a microbicide that would offer some protection during anal sex. The risk of HIV transmission via tissue tears is greater during anal sex than during vaginal sex, as the vaginal lining is thicker than the anal lining. Development of an anal microbicide would be protective for women who engage in anal sex and for men who have sex with men.7
How do vaccines work?8-11
For more than 200 years, vaccines have been used to prevent viral (e.g., rabies and influenza) and bacterial (e.g., diphtheria and tetanus) diseases. In 1796, Edward Jenner developed the first vaccine – using cowpox to immunize people against the deadly smallpox virus. Since then, vaccines have been developed against many microbes.
There are two types of vaccines: preventive and therapeutic. Most vaccines are preventive – to prevent infection and illness. Therapeutic vaccines offer an innovative approach to treating an already infected person.
- Preventive vaccines are given prior to infection, as a way to prime the immune system. By exposing the immune system to the virus or bacteria, it allows the certain cells of the immune system (B-cells and T-cells) to identify it as a foreign body and remember it. Then, when the body is exposed to the virus or bacteria again, the humoral (B-cell antibody-related) and cell-mediated (T-cell-related) immune responses will be stronger and faster. This enhances the body’s efforts in fighting the infection.
- Therapeutic vaccines expose an already infected person to an additional viral or bacterial stimulus. The goal of this approach is to provoke a stronger and faster response from the immune system and slow the progression of the disease.
Several strategies are used to develop vaccines. Vaccination against some diseases uses a combined approach – using one strategy for the primary inoculation and another for a booster shot. The ability to sequence microbial DNA offers opportunities to understand how specific microbes work and a new avenue for vaccine development.
- Killed microbes. This strategy uses the whole microbe, which has been inactivated by chemicals, irradiation, or some other technique. The injected polio and influenza vaccines use this approach.
- Attenuated or weakened microbes. This strategy uses microbes that have been modified or grown in culture for several generations so they cannot cause disease. Smallpox, oral polio, mumps, measles, and rubella vaccines use this approach.
- Subunit vaccines. This strategy uses only part of the microbe to produce an immune response. Typhoid, hepatitis B, and pertussis (whooping cough) vaccines use this approach.
- Conjugate and virus-like particle vaccines. The conjugate vaccine strategy tricks the body into an immune response by inserting proteins or inactivated toxins from one bacterium into the outer coat of another bacterium. These vaccines are particularly effective in infants, whose immature immune systems are not able to recognize the outer coat of some bacteria, such as pneumonia and meningitis. By inserting the components of harmful bacteria into an outer coat that is recognized as foreign, infants can be immunized against the harmful bacteria. Haemophilus influenzae type b (Hib), which is a cause of pneumonia, uses this approach. Similarly, the virus-like particle vaccine approach uses a non-infectious microbe that contains some proteins from a virus. This approach is being used to develop an HIV vaccine.
- Peptide and DNA vaccines. This strategy uses DNA that encodes viral proteins or chemically synthesized proteins (peptides) to stimulate immunity. HIV vaccines using these approaches are being tested.
- Recombinant vaccines. These strategies alter the genetic structure of a microbe – removing genes that cause disease or adding a gene to the microbe’s genetic material. Unlike the other strategies, recombinant vaccines could potentially confer life-long protection, as the modified microbial DNA would be taken up by cells and incorporated into the host genome. Testing is underway on recombinant vaccines to malaria, influenza, and HIV.
Vaccines and HIV/AIDS. There are currently no preventive or therapeutic vaccines for HIVAIDS, though research is being conducted on both. The existence of many HIV subtypes complicates efforts to develop a vaccine, as vaccines may need to be specific to subtypes and/or geographic regions. Although there are several vaccine strategies under investigation, they do not use killed or attenuated viruses, as there are safety concerns about using these strategies.
- Preventive HIV vaccines are complex because of the relatively high mutation rate of the virus and the virus’s ability to hide and remain latent in the T-cells and macrophages of the immune system.
- Given the ability of the virus to mutate within an individual, a preventive vaccine would need to completely block infection, which is very challenging.
- Studies using animal models suggest that a preventive vaccine may be able to slow the progression of the disease.
- Therapeutic vaccines have been largely ineffective, though there are some promising signs that a therapeutic vaccine could slow the progression of the disease.
More about vaccines will be added later.
How do antiviral (or antiretroviral) drugs work?9, 12-14
Antiviral drugs target specific viruses at key stages of their replication. Antiretroviral drugs are used to treat antiretroviral infections, such as HIV/AIDS. These drugs are used after infection or the emergence of clinical symptoms of disease and are rarely used for preventive purposes.
HIV Structure15
 Note: gp120, gp41, p17, and p24, are surface proteins on the virus. Protease and
reverse transcriptase are viral enzymes needed for reproduction.
The ways in which antiviral drugs work include:
- Viral attachment to the cell – fusion/entry inhibitors. In order for the virus to enter the cell, viral proteins need to bind to receptors and proteins on the surface of the cell. If that link cannot be established, the virus cannot enter the cell. Therefore, attachment can be prevented by using a drug to mimic the virus or using a drug to mimic a cell’s receptor.
- Mimicking the virus – the drug binds to the cell surface receptors to which the virus would usually bind. Thus, the virus has no place to attach to the cell – if the virus cannot bind, it cannot enter the cell and replicate.
- Mimicking the cell’s receptor – the drug binds to the viral proteins that normally bind to the cell surface receptors. Thus, the virus attaches to the drug instead of the cell by monopolizing the receptors.
- Penetration and uncoating. This replication stage is one of the most difficult to target, as it is unclear how the process works and which components or proteins are needed to perform these functions.
- Drugs can change the environment, making the cell less amenable to the virus.
- Drugs can change the physical structure of the proteins. These changes can result in proteins that are less efficient, that are unable to link to other proteins to perform certain functions, or proteins that cannot fold into their normal shape.
- Genome replication. The viral replication stage is the most accessible to target. There are three ways in which viral replication can be interrupted.
- Once inside a cell, the virus’ genetic material needs to be copied. Certain viral enzymes or proteins are needed for the replication of viral DNA or RNA. Viruses with DNA use polymerase; viruses with RNA use reverse transcriptase.
In addition to replicating the viral genome, these enzymes ensure that the viral genes receive preferential treatment over the cell’s own DNA during the replication process.
Most currently available antiviral drugs are polymerase and reverse transcriptase inhibitors. Preventing replication of the viral genome prevents production of new copies of the virus.
- In retroviruses, viral RNA is copied into DNA (using reverse transcriptase) and then the viral DNA is inserted into the host cell’s own DNA. Integrase is the viral enzyme that integrates the viral DNA into the host cell’s DNA.
Integrase inhibitors are a new class of drugs and are in the early stages of development. For HIV and other retroviruses, integrase inhibitors offer a promising target for preventing viral production.
- Once the viral DNA has been copied, it needs to be transcribed into RNA. The RNA is then processed to become new viral proteins that form the new virus particles. Preventing the transcription of DNA to RNA prevents new virus formation.
Transcription inhibitors are the newest class of drugs under investigation.
- Gene expression. Once the host cell has completed transcription of DNA to RNA, the next stage of replication translates the RNA into protein. This stage is generally not targeted for antiviral drugs, as most of the work in this stage is done by the host cell’s machinery rather than the virus.
- Assembly, maturation and release. This stage of replication is sometimes a target for antiviral drugs, however the processes are not clearly understood for all viruses. In this stage, new viral proteins are packaged in a viral coat, the mature viral particles are activated, and the virus copies are released from the cell.
For HIV, this stage has been used to create antiretroviral drugs, as the protease enzyme offers an effective drug target. Viral proteins are generated in long strings that are cut into pieces by the enzyme protease. Protease inhibitors do not allow the viral protein copies to be cut. The long strings cannot be packaged in the outer coat – thus, the process of assembly is interrupted. Along with reverse transcriptase inhibitors, protease inhibitors are the major strategies to combat HIV/AIDS.
Maturation inhibitors are another class of drugs under experimentation for possible HIV medications. These drugs would prevent viral particles from becoming active viruses.
Replication Cycle of HIV16 Courtesy of: Christine H. Herrmann
Department of Molecular Virology and Microbiology
at Baylor College of Medicine | website
HIV/AIDS. Antiretroviral (ARV) therapy may include treatment with only one drug or a combination of drugs. Highly Active Antiretroviral Therapy (HAART) refers to the use of three or more ARVs in combination. These drugs are grouped into four categories, based on the manner in which they attack the virus:
- Fusion or entry inhibitors – these drugs interfere with the attachment of HIV surface proteins to the host cell receptors. Some entry inhibitors target the gp120 or gp41 proteins on the virus, while others target the cell surface proteins and receptors.
- Nucleoside/nucleotide reverse transcriptase inhibitors (NRTIs) – these drugs prevent the viral RNA from being transcribed into viral DNA in the host cell. Nucleotides are the building blocks of DNA and NRTIs contain modified/faulty building blocks. When the virus tries to make DNA using these building blocks, it cannot construct a proper strand of DNA.
- Non-nucleoside reverse transcriptase inhibitors (NNRTIs) – NNRTIs attach to the HIV’s reverse transcriptase, rendering the enzyme unable to transcribe RNA to DNA.
- Protease inhibitors (PIs) – copies of HIV proteins are produced in a long strand, which is cut apart by protease. PIs prevent this process, which prevents new viral particles from forming.
Examples of Viruses and the Diseases They Cause12, 17
| Entry Inhibitor |
NRTI |
NNRTI |
PI |
| Enfuvirtide |
Abacavir |
Delaviridine |
(Fos)-Amprenavir |
|
Didanoside |
Efavirenz |
Atazanavir |
| Maraviroc |
Emtricitabine |
Neverapine |
Darunavir |
|
Lamivudine |
|
Indinavir |
|
Stavudine |
|
Lopinavir/ritonavir |
|
Tenofovir |
|
Nelfinavir |
|
Zalcitabine |
|
Ritonavir |
|
Zidovudine |
|
Saquinavir |
|
|
|
Tripanavir |
| 1 |
Global Campaign for Microbicides. (accessed June 25, 2007), Available from: www.global-campaign.org/economics.htm |
| 2 |
AIDS InfoNet. 2007. Microbicides. Fact sheet 157. Available from: http://aidsinfonet.org/factsheets/en/pdfs/157.pdf |
| 3 |
International Partnership for Microbicides. (accessed October 31, 2007), Available from: www.ipm-microbicides.org/ |
| 4 |
Shattock RJ, Moore JP. 2003. Inhibiting sexual transmission of HIV-1 infection. Nature Reviews Microbiology 1:25-34. |
| 5 |
Forbes A. Vaginal dreams. (accessed February 19, 2007), Available from: www.aidsinfonyc.org/hivplus/issue2/prevent/vaginal.html |
| 6 |
Wijgert Jvd, Coggins C. 2002. Microbicides to prevent heterosexual transmission of HIV: ten years down the road. AIDScience 2(1). |
| 7 |
Global Campaign for Microbicides. 2006. All about rectal microbicides. Available from: www.global-campaign.org/clientfiles/FS6-RectalMicrobicides-28Sept06.pdf |
| 8 |
Inform P. 2004. HIV vaccines and your immune system. Available from: www.projinf.org/pdf/vaccines.pdf |
| 9 |
Fan HY, Conner RF, Villareal LP. 2004. AIDS science and society. 4th ed. Boston: Jones and Bartlett Publishers. |
| 10 |
Centers for Disease Control. Evolution of vaccines. (accessed October 31, 2007), Available from: www.hhs.gov/nvpo/factsheets/fs_tableI_doc4.htm |
| 11 |
National Institute of Allergy and Infectious Disease. Design of HIV vaccines. (accessed October 31, 2007), Available from: www.niaid.nih.gov/factsheets/hivvacdesign.htm |
| 12 |
Simon V, Ho DD, Karim QA. 2006. HIV/AIDS epidemiology, pathogenesis, prevention, and treatment. Lancet 368:489-504. |
| 13 |
Cann A. Prevention and treatment of virus diseases: antiviral drugs. (accessed October 31, 2007), Available from: www.tulane.edu/ |
| 14 |
AIDSmeds.com. The HIV life cycle. (accessed October 31, 2007), Available from: www.aidsmeds.com/articles/hiv_life_cycle_5014.shtml |
| 15 |
Human Immunodeficiency Virus and HIV disease. (accessed October 31, 2007), Available from: http://uhavax.hartford.edu/bugl/hiv.htm |
| 16 |
Herrmann CH. HIV/AIDS. Department of Molecular Virology and Microbiology at Baylor College of Medicine. (accessed October 30, 2007), Available from: www.bcm.edu/molvir/eidbt/eidbt-mvm-hivaids.htm |
| 17 |
International AIDS Society. 2007 The 4th IAS conference on pathogenesis, treatment and prevention: new research and its implication for policy and practice. Available from: www.iasociety.org/Web/WebContent/File/IAS_SYDNEY%20REPORT.pdf |
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