Fusion inhibitors (FIs)


Fusion inhibitors (FIs) are a type of antiretroviral medication that helps prevent HIV (human immunodeficiency virus) from entering and infecting human cells. Although they are not curative, these drugs have shown to effectively slow the replication of the virus in the body when taken in combination with other antiretrovirals.
Currently, the U.S. Food and Drug Administration (FDA) has approved one fusion inhibitor, enfuvirtide (FuzeonĀ®), also called T-20. Clinical trials have demonstrated that enfuvirtide-based therapy is an effective treatment for patients who have taken other types of antiretrovirals in the past. Enfuvirtide should not be taken in patients who have never received antiretroviral drugs before.
HIV primarily targets the body's CD4 T-cells, which are white blood cells that help coordinate the immune system's response to infection and disease. Therefore, HIV patients are vulnerable to opportunistic infections. These infections occur in patients who have weakened immune systems.
Antiretrovirals are able to suppress HIV, which boosts the patient's immune system. These drugs have also shown to dramatically slow the progression of opportunistic infections. These infections can be potentially fatal and may lead to the development of AIDS, if not properly treated.
HIV patients typically receive a combination of antiretroviral drugs because a single patient may have several different strains (types) of the virus circulating in the blood. The different strains of the virus may respond differently to specific types of drugs. Therefore, highly active antiretroviral therapy (HAART), which is a combination of drugs from at least two different classes, is recommended. There are four major classes of antiretrovirals: fusion inhibitors, protease inhibitors, nucleoside reverse transcriptase inhibitors (NRTIs), and non-nucleoside reverse transcriptase inhibitors (NNRTIs). Each drug class disrupts different stages of HIV's life cycle.

Related Terms

Acquired immune deficiency syndrome, acquired immunodeficiency syndrome, AIDS, antiretrovirals, antiretroviral therapy, ART, CD4 cells, immune, immune defense system, immunocompromised, immunodeficiency, HAART, highly active antiretroviral therapy, HIV, human immunodeficiency virus, non-nucleoside reverse transcriptase inhibitors, nucleoside reverse transcriptase inhibitors, OI, opportunistic infections, protease inhibitors, reverse transcriptase inhibitors, suppressed virus, viral infection, viral suppression, virus, weakened immune system, white blood cells.


Enfuvirtide (FuzeonĀ®), also called T-20, is the only fusion inhibitor available for the treatment of HIV. This antiretroviral prevents HIV from fusing with outer surface (cell membrane) of the host's cells. When the virus cannot penetrate the cell membrane, it is unable to infect and destroy the patient's cells.
Fusion inhibitors, first introduced in 2003, are taken in combination with other antiretroviral drugs to suppress the replication of HIV. When the virus is suppressed, the body's immune system is restored and the patient is less susceptible to infections and diseases.
Some HIV patients who are receiving a combination of antiretrovirals may achieve undetectable levels of the virus. This does not mean that the patient is cured. It simply means that the number of viral particles in the blood (viral load) is lower than the test can detect.

side effects

Common side effects: The most common side effect associated with enfuvirtide is a skin rash where the medication was administered. It is estimated that up to 98% of patients experience some sort of skin reaction, which may cause bruising, cysts, itching, bumps, pain, and redness.
Other common side effects include cough, headache, pain or tenderness around the eyes and cheekbones, shortness of breath, wheezing, bad taste in the mouth, flu-like illness, fatigue, herpes simplex infection, loss of appetite, weight loss, muscle pain, nervousness, sinus problems, enlarged lymph nodes, difficulty breathing, runny or stuffy nose, tightness of the chest, dizziness, as well as burning, numbness, tingling, or pain in the arms, legs, hands, or feet. These side effects generally subside or disappear as treatment is continued.
Less common side effects: Less common side effects include bloating, chills, constipation, darkened urine, dry or itchy eyes, excessive tearing, eye discharge, rapid heartbeat, fever, indigestion, stomach pain, loss of appetite, swelling of the eye, vomiting, jaundice (yellowing of the eyes or skin), as well as itching, swelling, or warmth near the injection site.
Serious side effects: Severe allergic reactions have been reported with the use of enfuvirtide. These side effects may include chills, fever, nausea and vomiting, kidney problems, low blood pressure, paralysis, severe rash, dizziness, loss of consciousness, and difficulty breathing. Patients who experience any of these symptoms should seek immediate medical treatment because severe drug reactions may be fatal.
Severe infections at injection sites have also been reported.
Some patients who use needle-free devices to administer enfuvirtide have experienced nerve pain lasting up to six months, as well as bruising and internal bleeding. Patients should tell their healthcare providers if they have experienced any of these side effects.
In clinical trials, patients who took enfuvirtide were more likely to develop bacterial pneumonia than patients who took an inactive treatment (placebo). It remains unknown if enfuvirtide increases the risk of pneumonia, but patients should consult their healthcare providers if they experience symptoms of pneumonia including chills, high fever, sweating, chest pain, and cough that produces thick phlegm.


Allergies: Patients should tell their healthcare providers if they have a history of allergic reactions to enfuvirtide. There have been reports of severe allergic reactions to enfuvirtide. Symptoms of a severe allergic reaction may include difficulty breathing, hives, chest pain, nausea, low blood pressure, or loss of consciousness. Patients who develop these symptoms should seek immediate medical treatment.
Co-existing medical conditions: Patients should tell their healthcare providers if they have any other medical conditions because they may increase the risk of developing serious side effects associated with enfuvirtide. Patients should tell their healthcare providers if they smoke, inject illegal drugs (such as cocaine and heroin), or have a family history of lung disease because they may have an increased risk of developing pneumonia.
Pregnancy and breastfeeding: Enfuvirtide has not been studied in pregnant women. Although the drug has not shown to cause birth defects or other medical problems in animal studies, this drug should be use cautiously in patients who are pregnant, might be pregnant, or are thinking about becoming pregnant. Patients should discuss the potential health risks and benefits of treatment with their healthcare providers.
It remains unknown whether enfuvirtide can pass into breast milk. However, HIV/AIDS patients should not breastfeed their babies because the virus may be transmitted through the breast milk.
Proper administration: Enfuvirtide should not be taken in patients who have never received antiretroviral drugs before.
The drug should be injected under the skin into the fatty layer beneath the surface. It is recommended that patients alternate injecting the drug into the abdomen, upper arm, and front of the thigh. Enfuvirtide should not be injected in areas where large nerves are close to the skin, such as the tops of the hands. The drug should not be injected into a vein or muscle. Enfuvirtide should not be injected into moles, scars, tattoos, burns, open cuts, blisters, or sores on the skin.
Patients should not share needles or other injection equipment with others.
Patients should not alter their doses or stop taking enfuvirtide without first consulting their healthcare providers.
Enfuvirtide may make some patients dizzy. Patients who feel dizzy or lightheaded after treatment should not drive, operate heavy machinery, or engage in potentially dangerous activities.