FOSAMPRENAVIR (Telzir, Lexiva)

WHAT IS FOSAMPRENAVIR?
Fosamprenavir is a drug used as part of antiretroviral therapy (ART). It is also called Lexiva?. In Europe it is called Telzir. It is manufactured by GlaxoSmithKline. Fosamprenavir is a pro-drug. The drug you swallow is not active against HIV. It breaks down in the body to produce the active form, which is the drug amprenavir (see Fact Sheet 445). Fosamprenavir was formerly called GW433908, or 908.

Fosamprenavir is a protease inhibitor. These drugs prevent the protease enzyme from working. HIV protease acts like a chemical scissors. It cuts the raw material for HIV into specific pieces needed to build a new virus. Protease inhibitors "gum up" these scissors.
WHO SHOULD TAKE IT?
Fosamprenavir was approved in 2003 as an antiretroviral drug (ARV) for people with HIV infection. In 2007 a liquid version was approved for patients ages 2 to 18.

There are no absolute rules about when to start ART. You and your health care provider should consider your CD4 (T-cell) count, your viral load, any symptoms you are having, and your attitude about taking HIV medications. Fact Sheet 404 has more information about guidelines for the use of ART.

If you take fosamprenavir with other ARVs, you can reduce your viral load to extremely low levels, and increase your CD4 counts. This should mean staying healthier longer.
WHAT ABOUT DRUG RESISTANCE?
Many new copies of HIV are mutations. They are slightly different from the original virus. Some mutations can keep multiplying even when you are taking an ARV. When this happens, the drug will stop working. This is called "developing resistance" to the drug. See Fact Sheet 126 for more information on resistance.

Sometimes, if your virus develops resistance to one drug, it will also have resistance to other ARVs. This is called "cross-resistance." Fosamprenavir might not be cross-resistant with other protease inhibitors. Resistance can develop quickly. It is very important to take ARVs according to instructions, on schedule, and not to skip or reduce doses.
HOW IS IT TAKEN?
Fosamprenavir is taken by mouth as a tablet. The normal adult dose is 1400 milligrams (mg) two times a day. The tablets are 700 mg, so you will take 2 tablets at a time.

Fosamprenavir is also approved in combinations with ritonavir, as follows:

  • 1400 mg of fosamprenavir plus 200 mg of ritonavir, taken once daily
  • 700 mg of fosamprenavir plus 100 mg of ritonavir, taken twice daily

For patients who have already used other protease inhibitors, the only recommended dosing is 700 mg of fosamprenavir plus 100 mg of ritonavir, taken twice daily.

If you have liver problems or damage, talk to your health care provider. You may need an adjusted dose of fosamprenavir.

Fosamprenavir is available as 700 mg tablets. It can be taken with or without food. It should be stored at room temperature. If you take fosamprenavir with ritonavir, be sure to keep the ritonavir in your refrigerator, or for up to 30 days at room temperature (below 77 degrees F, or 25 degrees C). Unlike the original version of amprenavir, fosamprenavir does not contain any Vitamin E.
WHAT ARE THE SIDE EFFECTS?
The most common side effects are nausea, diarrhea, vomiting, rash, and headache. Some patients also felt numbness around the mouth, and abdominal pain. Less than 1% of people get serious skin reactions, including Stevens-Johnson syndrome. No other side effects seem to be very serious. The diarrhea in most cases can be controlled with over-the-counter medications.

Fosamprenavir can increase triglycerides (a blood fat). See Fact Sheet 123 for more information on blood fats. However, fosamprenavir might cause less of an increase in cholesterol than other protease inhibitors.

Fosamprenavir is a sulfa drug. If you are allergic to sulfa drugs, be sure to tell your health care provider.
HOW DOES IT REACT WITH OTHER DRUGS?
Fosamprenavir can interact with other drugs or supplements that you are taking. These interactions can change the amount of each drug in your bloodstream and cause an under- or overdose. New interactions are being identified all the time.

Drugs to watch out for include other ARVs, drugs to treat tuberculosis (see fact sheet 518), for erectile dysfunction (such as Viagra), for heart rhythm (antiarrhythmics), and for migraine headaches. Interactions are also possible with several antihistamines (allergy medications), sedatives, drugs to lower cholesterol, and anti-fungal drugs. Make sure that your health care provider knows about ALL drugs and supplements you are taking.

Do not combine fosamprenavir and Kaletra. Blood levels of both drugs are reduced. More side effects were observed.

Delavirdine should not be combined with fosamprenavir.

Some birth control pills may not work if you are taking fosamprenavir. Talk to your health care provider about how to prevent an unwanted pregnancy.

Fosamprenavir causes major increases in blood levels of drugs like Elavil and Tofranil. These drugs are sometimes used to treat peripheral neuropathy (see fact sheet 555). However, Fosamprenavir decreases blood levels of Paxil (paroxetine), an antidepressant. Discuss the use of antidepressants with your health care provider.

Fosamprenavir levels do not appear to be affected by the use of antacids.

Taking fosamprenavir with methadone can lower blood levels of both drugs. Watch for signs of excessive sedation if you take fosamprenavir with buprenorphine.

The herb St. John's Wort (See Fact Sheet 729) lowers the blood levels of some protease inhibitors. Do not take it while taking fosamprenavir.