* Patient enrolled in trial dies of blood clot
* Patient misdiagnosed with hemorrhage initially
* Alnylam says will resume trials as soon as possible (Adds details, analyst's comments)
Sept 7 (Reuters) - Alnylam Pharmaceuticals Inc said on Thursday it stopped giving doses of its experimental drug for a rare bleeding disorder to patients enrolled in its clinical studies after one patient died, sending its shares sharply down.
The U.S. drug developer's shares were down 14.1 percent at $73.87.
A patient suffering from hemophilia A enrolled in a mid-stage study of Alnylam's experimental treatment, fitusiran, died after developing a blood clot, the company said.
The study was testing fitusiran as a treatment for hemophilia A and hemophilia B patients.
Hemophilia is an inherited bleeding disorder in which the blood does not clot properly, leading to spontaneous bleeding and bleeding following minor injuries or surgery.
About 20,000 Americans are estimated to suffer from hemophilia, according to the U.S. Centers for Disease Control and Prevention.
The deceased patient was initially misdiagnosed with a hemorrhage not related to fitusiran, for which he was administered factor VIII concentrate, a blood clotter, several times a day, the company said.
However, an independent review later revealed the patient experienced a cerebral venous sinus thrombosis (CVST).
CVST occurs when a blood clot forms in the brain causing blood cells to burst and leak into brain tissues. It affects about 3 to 4 people per million, according to the National Institutes of Health.
Leerink analyst Paul Matteis said care for the patient appeared to be "uniquely poor", adding it was unclear if the thrombotic event would have been fatal had it been caught earlier.
The company said it would reevaluate the dosage and frequency of fitusiran being administered, following the patient's death.
The company also said it expects to resume dosing of the drug in its clinical studies potentially by late 2017.
"Even if the dosing and late-stage programs are resumed for fitusiran, the commercial opportunity may now be limited given the risk-averse nature of the hemophilia patient population," Matteis said.
French drugmaker Sanofi in November agreed to co-develop and co-commercialize fitusiran with Alnylam in the United States, Canada and Western Europe.
Alnylam also said on Thursday it would move forward a trial of its experimental drug for porphyria, a disorder affecting the nervous system and skin, to begin late this year. (Reporting by Tamara Mathias in Bengaluru; Editing by Sai Sachin Ravikumar and Shounak Dasgupta)