Drug company Pfizer Inc together with the US Food and Drug Administration (FDA) warned yesterday, Monday, that its HIV drug Viracept (nelfinavir mesylate) may be contaminated with a potential carcinogen and asked doctors not to start pregnant women or children on the drug.

Children who are already on the drug and are stable may continue to take Viracept, but pregnant women should be switched to an alternative antiretroviral said Pfizer and the FDA.

The drug company issued a Dear Healthcare Professional letter about the presence of an impurity, ethyl methanesulfonate (EMS), that has entered the Viracept formula during the manufacturing process. The letter also gives guidelines on the use of Viracept in pediatric patients and pregnant women.

Although no human studies exist, EMS is considered a potential human carcinogen: animal studies indicate it to be teratogenic (causing birth defects), mutagenic (causing mutation through DNA damage) and carcinogenic (causing cancer).

In the meantime the FDA has asked Pfizer to limit the amount of EMS in its US Viracept products by re-engineering the manufacturing process to new specifications.

The agency and the drug company have agreed that pediatric patients who are stable on a drug regimen that contains Viracept may continue to take it because the risk to benefit ratio should remain favourable. However, pediatric patients who are just starting out on HIV treatments should not be offered Viracept until further notice.

Pregnant women who are on Viracept and for whom there is a viable alternative, should be transferred onto an alternative antiretroviral until the new manufacturing specifications are implemented and the FDA and Pfizer say it is safe again.

For pregnant women for whom there is no viable alternative treatment, the FDA and Pfizer agree that the risk to benefit ratio remains favourable for them to continue on Viracept. Pregnant women starting out on antiretroviral treatment should not be offered Viracept however.

The issue of possible contamination arose in June 2007 when another drug company that also manufactures Viracept, Roche Ltd, detected excess levels of EMS in the drug. Roche recalled all its Viracept products in the European Union (EU).

The FDA then asked Pfizer to implement a new specification to limit the amount of EMS in the Viracept products it makes that are marketed in the US. When it tested its own Viracept products Pfizer said levels of EMS "substantially lower than those associated with the Roche EU recall" were detected.

The drug company said there is no change in the recommended use of Viracept for other patients and refer professionals to the full prescribing information that accompanies the letter.

The Pfizer letter asks health professionals, when considering treatment for patients, to be aware that many HIV antiretrovirals show up as carcinogenic in animal studies, and some HIV antiretrovirals are mutagenic or teratogenic. As with all drugs it is a matter of weighing up the benefits versus the risks, and in the case of HIV-1 antiretrovirals the benefit of taking them outweight the risk of completely stopping them.

Viracept is an antiretroviral drug for treating patients with HIV in combination with other medications. In the US it is approved for use in adults and children over 2 who are infected with HIV-1 (human immunodeficiency virus, the virus that causes AIDS, acquired immune deficiency syndrome).

Most adults who are on Viracept take the 625mg tablet while children are usually prescribed powder or the 250mg tablet.

The active ingredient of Viracept is a a protease inhibitor, nelfinavir mesylate. It blocks an enzyme that helps to multiply the HIV-1 virus, preventing the virus from reproducing normally and slowing down the spread of infection in the body.

Click here to read the full Dear Healthcare Professional letter from the drug company (PDF).

Click here for FDA drug information page on Viracept.

: Catharine Paddock

View drug information on VIRACEPT.