Biopharmaceutical company Celgene ( CELG) is proposing to offer $300 million of convertible notes to qualified institutional buyers. The notes will be convertible into Celgene common stock at a price to be determined. Celgene said in a press release after the close Wednesday that the notes will be due in 2008. Shares of Celgene, which is based in Warren, N.J., ended regular Nasdaq trading at $32.31, down $1.16, or 3.5%. In Instinet trading after the closing bell, Celgene slipped $1.30, or 4%.