Biotech firm

Immunex

(IMNX)

said Friday that it plans to sell its cancer treatment Leukine to German pharmaceutical company

Schering AG

for $380 million in cash, plus the payment of additional cash consideration upon achievement of certain milestones.

In a press release, Immunex said that the sale comes in connection with its pending acquisition by

Amgen

(AMGN) - Get Report

. The company has been

searching for potential Leukine buyers since January, as the drug might be too similar to Amgen's blockbuster Neupogen cancer treatment for the acquisition to receive federal approval.

Schering AG said it made the purchase to help grow its U.S. business, and it saw potential in Leukine to treat other diseases beyond bone marrow cancer. The company plans to explore using Leukine, which generates sales of $108 million a year for Immunex, for the treatment of Crohn's disease, among others.

The closing of the Leukine sale is subject to approval by the Federal Trade Commission and the closing of the Amgen acquisition, expected by June.

Shares of Immunex closed at $26.01 Thursday.