Pfizer Inc., the U.S. maker and distributor of Viracept (nelfinavir), alerted doctors on Monday that a manufacturing impurity could increase the risk of cancer in children and in fetuses during pregnancy. In response, Pfizer is cautioning pregnant women to avoid taking Viracept and is recommending that children not start taking the protease inhibitor.  

In June 2007, the European manufacturer of Viracept, Roche Ltd., announced that they had found dangerous levels of ethyl methanesulfonate (EMS) in Viracept distributed there. EMS is an impurity that occurs during the manufacturing of Viracept. Because EMS has been found to cause cancer in animals, Roche recalled Viracept from all European Union markets and lost its license to market it there until they show that EMS has been lowered to safe levels.

When Pfizer subsequently checked U.S. samples at the request of the Food and Drug Administration (FDA), EMS was found, though at lower levels than in Europe. Pfizer has agreed to continue checking newly manufactured Viracept and will only release those batches with a level of EMS that is not expected to increase the risk of cancer in adults who take it.

In order to minimize the risk of cancer in babies and children going forward, Pfizer is now urging pregnant women not to start Viracept or to switch it for a comparable drug if they’re currently taking it. The alert further states that children who are stable on a regimen that includes Viracept may continue to take it, but children who’ve not yet taken Viracept should avoid it until further notice. There is no change in the recommended use of Viracept for all other patients.