Shares in Lundbeck have fallen on the news that US regulators have asked for more information about the Danish drugmaker’s schizophrenia treatment Serdolect.

The firm has received a complete response letter from the US Food and Drug Administration which includes a request for additional data “to best understand the appropriate patient population for which Serdolect (sertindole) could be made available”. Lundbeck said that it is currently evaluating the feedback from FDA and will work with the agency “to resolve these outstanding questions and discuss appropriate next steps required to bring Serdolect safely to market”.

The letter is particularly disappointing for Lundbeck because it comes a couple of months after the FDA’s Psychopharmacologic Drugs Advisory Committee voted 8-2, with three abstentions, that certain patients, possibly those do not respond adequately to current therapies, could benefit from taking Serdolect. The committee also unanimously agreed that the antipsychotic is effective but voted 12 to 1 that Serdolect is not "acceptably safe" for the general population who suffer from schizophrenia.

Lundbeck said that in the coming months it is committed to addressing the CRL “and will work with the agency to expedite completion of its review”. The original application for US approval was filed in 1995 but Serdolect was withdrawn globally in 1998 after it was linked to cardiac arrhythmias and arrests. However it returned to the European market in 2006 on the basis of fresh clinical data but also new safeguards were put in place, including a dose reduction from 24mg to 20mg once-daily.

Investors are now worried about the likelihood of Serdolect reaching the US market, especially if the agency demands more trials. Lundbeck’s share price fell 3.9% to 99.25 Danish kroner.