Shares in France's Ipsen have taken a bashing after two late-stage clinical trials for its experimental haemophilia drug, which is being developed with the USA's Inspiration Biopharmaceuticals, were put on hold.

Inspiration, in which Ipsen has a 44% stake, has been notified by the US Food and Drug that both studies (one in adults, one in children) evaluating IB1001 for the treatment and prevention of bleeding episodes in people with haemophilia B, have been placed on clinical hold. The drug is an investigational intravenous recombinant factor IX (rFIX) therapy.

Ipsen said that "during the course of routine laboratory evaluations", a trend was noted towards a higher proportion of IB1001-treated individuals developing antibodies to proteins from the Chinese hamster ovary (CHO)  host cells used to manufacture the therapy than was expected based on earlier study data. The Paris-based firm noted that small amounts of host cell protein are expected in recombinant therapeutic products of all types, but the higher-than-expected rate of anti-CHO antibody has led Inspiration to initiate "a thorough investigation".

Some 86 people with haemophilia B have received IB1001 in the studies and, to date, no adverse events related to the development of antibodies to CHO protein have been reported. Furthermore, Ipsen added, no relationship has been demonstrated between the development of antibodies to CHO protein and the development of any antibodies to factor IX.

The company added that "while this finding may be a potential safety concern, no evidence suggests a change in the current overall clinical benefit and risk profile of IB1001". Nevertheless, investors are spooked and Ipsen shares fell 11.4% to 17.70 euros.

Inspiration chief executive John Butler said "we have already made progress in identifying potential options to reduce antibody formation and will continue to work with the FDA on plans to move the IB1001 programme forward". IB1001 was filed for approval in Europe in December 2011 and in the USA in April this year but the clinical trials hold has put back any chance of approvals in the near future.