Drug maker Merck & Co. (MRK) said it withdrew its initial filing for U.S. antitrust clearance of its proposed $41 billion purchase of Schering-Plough Corp. (SGP), but plans to re-file with additional information as soon as possible.

Merck, based in Whitehouse Station, N.J., still expects to close the acquisition by end of the year, spokesman David Caouette told Dow Jones Newswires. He declined to specify the additional information Merck will provide or what led to the withdrawal of the initial filing.

In April, Merck filed information about its proposed acquisition with the U.S. Federal Trade Commission under the Hart-Scott-Rodino Act, which is designed to give the government time to review deals and ensure they aren't anti-competitive. Sometimes the FTC requires asset divestitures if two companies have overlapping businesses.

A 30-day waiting period for Merck's filing was set to expire Monday, May 18, Caouette said. But Merck decided to withdraw the filing Monday and will re-file soon, which will trigger another 30-day waiting period, he said.

"We believe the re-filing for us will provide a productive review process," said Caouette. "Other parties have taken similar actions in past transactions. We decided to re-file as part of our efforts to cooperate with the FTC in their review of the transaction."

Even when the new 30-day waiting period expires, it could be followed by a so-called second request for information from the FTC, which is relatively routine. In April, Pfizer Inc. (PFE) received a request for more information about its proposed acquisition of Wyeth (WYE).

An FTC spokesman couldn't immediately be reached.

-Peter Loftus; Dow Jones Newswires; 215-656-8289; peter.loftus@dowjones.com