Shares in both Santarus and Pharming have taken a knock after US regulators refused to accept the latter's application to market Rhucin for hereditary angioedema.

Rhucin (recombinant human C1 inhibitor) has been granted orphan drug designation for the treatment of acute attacks of HAE, a genetic disorder characterised by unpredictable and debilitating episodes of intense swelling of the extremities, face, trunk, genitals, abdomen and upper airway.

But the US Food and Drug Administration has sent Pharming a 'refusal to file' letter indicating that the Biologics License Application is "not sufficiently complete to enable a critical medical review" the firm said.

According to the regulator, the studies contained within the application assessing Rhucin failed to provide data for a sufficient number of subjects to support the proposed dose of 50 U/kg, and also lacked a validation of the scale used to gage the clinical effects of the drug.

In addition, the FDA has also requested data of the Phase IIIb clinical study be included in any future application for the drug, which received a green light in Europe last November.

Santarus licensed certain exclusive rights from Pharming to sell Rhucin in North America for acute attacks of HAE and other future indications, and as per the terms of the deal the Dutch biotech stands to receive a $5 million dollar milestone payment from its US upon FDA acceptance of the BLA. 

Both companies said they intend to meet with the agency "at the earliest opportunity" to discuss the issues raised the letter, but the FDA's decision could significantly delay Rhucin's path to market in the US, and investors are clearly not impressed. Santarus' stock had slipped 8% on the Nasdaq by early afternoon Monday, but Dutch group Pharming's shares dived nearly 19% in Amsterdam.