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Dec 12 (Reuters) - Alexion Pharmaceuticals Inc, which is investigating allegations regarding some of its sales practices over its flagship drug, said its chief executive officer and chief financial officer resigned, effective immediately.

Alexion's shares were down 13.3 percent at $114.80 in premarket trading on Monday.

The company said David Hallal had resigned as CEO and that it had named David Brennan, board member and former AstraZeneca Plc chief, as his interim replacement.

Alexion had disclosed last month that it was investigating allegations made by a former employee regarding sales practices involving its costly blood disorder drug, Soliris.

The investigation, which is nearing completion, had not identified any facts that require the company to update its previously reported historical results at this point in time, Alexion said on Monday.

Soliris, approved for rare blood disorders paroxysmal nocturnal hemoglobinuria and atypical hemolytic uremic syndrome, had sales of $2.59 billion in 2015.

The company also said Chief Financial Officer Vikas Sinha left to pursue other opportunities and would be succeeded by David Anderson, who was earlier CFO at Honeywell International Inc.

Hallal, who joined Alexion in 2006, had been appointed the company's CEO in April 2015.

Spencer Stuart has been engaged to search for a permanent CEO, Alexion said.

Alexion, which had delayed filing its quarterly report for the period ended Sept. 30 due to the investigation, said it expected to file its report by January.

(Reporting by Ankur Banerjee in Bengaluru; Editing by Sriraj Kalluvila and Anil D'Silva)