Amgen ( AMGN) said late Thursday it is seeking to raise $2.5 billion in a convertible debt offering. The biotech firm said it intends to use proceeds from the offering to repurchase up to $650 million of its own common stock, and for general corporate purposes, including acquisitions. Amgen is in the midst of completing a $16 billion acquisition of Immunex ( IMNX), which includes payment of about $2.5 billion in cash. The deal is expected to close in the second half of the year. Amgen had $2.6 billion in cash on hand at the end of 2001. The offering calls for the sale of 30-year, zero-coupon senior notes convertible into Amgen common shares.