(Updates with FDA warning)
By Ben Hirschler
LONDON, May 18 (Reuters) - U.S. and European regulators said they were assessing evidence that GlaxoSmithKline's HIV drug dolutegravir might be linked to serious birth defects, casting a shadow over a medicine that has been a key profit driver in recent years.
Friday's move by the U.S. Food and Drug Administration (FDA) and the European Medicines Agency (EMA) follows preliminary data from a study in Botswana which found four cases of neural tube defects in babies born to mothers who became pregnant while taking the drug.
GSK said it was notifying doctors of the issue and working with healthcare authorities to better understand the potential risk.
"There is no known mechanism linking dolutegravir with these types of birth defects and there are no relevant findings in pre-clinical studies," the company said in a statement.
The EMA said dolutegravir should not be prescribed to women seeking to become pregnant and women who could become pregnant should use effective contraception while taking it.
Neural tube defects (NTDs), such as spina bifida, are birth defects that can occur early in pregnancy when the spinal cord, brain and related structures do not form properly.
Dolutegravir is a so-called integrase inhibitor found in the branded medicines Tivicay and Triumeq, which are sold by GSK's majority-owned ViiV Healthcare unit. Gilead Sciences makes a recently approved rival product called bictegravir.
The drugs work by blocking an HIV enzyme to prevent the virus from multiplying, thereby reducing the amount of virus in the body.
"Animal models are generally predictive of drug-induced birth defects and it is very unusual to see an effect in humans if no evidence is observed in animal models," GSK said.
The World Health Organisation already advises against using the drug during pregnancy or breast-feeding due to insufficient safety and efficacy data in this population, and this guidance remains unchanged.
The role of dolutegravir in the four birth defects in Botswana has yet to be established, since NTDs are also associated with risk factors like folic acid deficiency and obesity. (Reporting by Ben Hirschler Editing by Alexander Smith)