Zofran and Pregnancy: the FDA Risk Categories-For Now

In 1979, the FDA established pregnancy risk categories that are required to be stated on the package inserts for prescription drugs. Generally described, these categories are:

A: There is no risk to the fetus in the first trimester based upon adequate and well-controlled human studies.

B: There is no risk based upon animal studies, but there are no adequate and well-controlled human studies, OR animal studies have shown an adverse effect, but adequate and well-controlled human studies have failed to demonstrate a risk in any trimester.

C: Animal studies have shown an adverse effect on the fetus and there are no adequate and well-controlled human studies, but potential benefits may warrant use of the drug in pregnant women despite the risks.

D: There is positive evidence of human fetal risk based on adverse reaction data from investigational or marketing experience in studies in humans, but potential benefits may warrant use of the drug in pregnant women despite potential risks.

X: Studies in animals and humans have demonstrated fetal abnormalities and/or there is positive evidence of human fetal risk based on adverse reaction data from investigational or marketing experience, and the risks involved in use of the drug in pregnant women clearly outweigh potential benefits.

From the time of its initial FDA approval in 1991 to the present, Zofran has been in pregnancy category B. That is about to change on June 30 of this year. The FDA has recognized the many valid criticisms of these categories. In general, these categories have been described as simplistic and misleading, and creating confusion.

Some of the specific concerns about the current category systems are 1) a failure to distinguish fetal toxicities according to severity, incidence and type; 2) the implication that the degree of risk increases with each category, which is not necessarily correct; 3) that all drugs within a given category do not pose an equal risk of fetal developmental toxicity (which has been commonly assumed); 4) that the current criteria permit drugs with known risks and drugs with no known risk to be placed in the same category; and 5) the categories do not indicate whether risks are posed in degrees, according to the dosage and duration of use of a drug, or gestational age at the time of exposure.

The new FDA rule, which will be discussed in our next post, takes a much different approach, and requires drug makers to disclose the existence of registries (where adverse human data, for example, can be reported) and sufficient information to contact the registries in order to obtain their information or to enroll.

We await with interest the new Zofran label on pregnancy.


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