PARIS (Reuters) - Ipsen said two late-stage clinical trials for its experimental hemophilia drug have been put on hold, potentially delaying the French drugmaker's entry into the blood disorders market by up to six months.

The setback sent Ipsen shares down 11.3 percent at 17.72 euros by 1001 GMT - the worst performer on the French SBF 120 index - while the index was up 1.1 percent.

Ipsen said a high percentage of patients in the trials developed antibodies to a protein used to make the bleeding disorder treatment, which it had hoped to launch in the first half of 2013.

Ipsen, which is developing the biological drug, code named IB1001, with privately held U.S. biotech Inspiration Biopharmaceuticals, filed for approval in Europe in December 2011 and in the U.S. in April 2012.

"We now assume (launch in) the second half of 2013 as the best case if regulatory fears can be assuaged, with the worst case being discontinuation," Cheuvreux analyst Marcel Brand said in a note to clients.

Inspiration Biopharmaceuticals has started an investigation into those patients who developed antibodies at a higher-than-expected rate even though no-one reported adverse events, Ipsen said in a statement.

"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," the company said.

Hemophilia is a bleeding disorder caused by low levels, or the absence of, a protein essential for blood clotting.

The annual market for hemophilia treatments is estimated at $8 billion worldwide.

(Reporting by Elena Berton; Editing by Mike Nesbit)