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1 Filed Pursuant to Rule 424(b)(3) and (c) File Number 333-85339 PROSPECTUS SUPPLEMENT DATED APRIL 26, 2000 to Prospectus Dated November 12, 1999 and supplemented December 15, 1999 and February 11, 2000 $345,000,000 IDEC PHARMACEUTICALS CORPORATION LIQUID YIELD OPTION(TM) NOTES DUE 2019 (ZERO COUPON - SUBORDINATED) AND COMMON STOCK ISSUABLE UPON CONVERSION OF THE LYONS This prospectus supplement supplements the prospectus dated November 12, 1999 and supplemented December 15, 1999 and February 11, 2000 (the "prospectus") of IDEC Pharmaceuticals Corporation ("IDEC Pharmaceuticals" or the "Company" or "we," "our" or "us") relating to the sale by certain of our securityholders or by their pledgees, donees, transferees or other successors in interest that receive any of these securities as a gift, partnership distribution or other non-sale related transfer (the "Selling Securityholders") of up to $345,000,000 principal amount of Liquid Yield Option(TM) Notes due 2019 (the "LYONs") and up to 4,646,460 shares of common stock to be issued upon conversion of the LYON, as adjusted to reflect the two-for-one stock split of the Company's common stock effected in December, 1999. You should read this prospectus supplement in conjunction with the prospectus, and this prospectus supplement is qualified by reference to the prospectus except to the extent that the information in this prospectus supplement supersedes the information contained in the prospectus. Capitalized terms used in this prospectus supplement and not otherwise defined herein have the meanings specified in the prospectus. SELLING SECURITYHOLDERS The following table provides certain information with respect to the principal amount of LYONs at maturity beneficially owned by an entity who was identified in the prospectus as a Selling Securityholder and has increased the principal amount of LYONs that may be offered by such Selling Securityholder pursuant to the prospectus; the percentage of outstanding LYONs this represents; and the number of shares of common stock issuable upon conversion of such LYONs to be registered for sale hereby. Distributions may be made by certain of the Selling Securityholders listed below to individuals other than those listed in the prospectus. The table of Selling Securityholders in the prospectus is hereby amended to amend the information provided for "Warburg Dillon Read LLC" and "All other Holders of LYONs or future transferee, pledgee, donee or successor of any such Holders" in the table of Selling Securityholders in the prospectus. 2 - ---------- (1) This prospectus supplement shall also cover any additional shares of common stock which become issuable in connection with the shares registered for sale hereby by reason of any stock dividend, stock split, recapitalization or other similar transaction effected without the receipt of consideration which results in an increase in the number of the Selling Securityholders' outstanding shares of common stock. The number of shares of common stock listed in this table have been adjusted to reflect the two-for-one stock split of the Company's common stock effected in December, 1999. (TM) Trademark of Merrill Lynch & Co.
Aggregate Number of Principal Amount Shares of of LYONs at Percentage Common Maturity That of LYONs Stock That Name of Selling Securityholder May be Sold Outstanding May be Sold(1) - ------------------------------ ---------------- ----------- ------------ Warburg Dillon Read LLC $ 37,610,000 10.9% 506,531 All other Holders of LYONs or $ 14,755,000 4.3% 198,720 future transferee, pledgee, donee, or successor of any such Holders.