EXHIBIT 10.90(c) [SBC Warburg Dillon Read Letterhead] September 18, 1997 Thomas F. Farb Executive Vice President , Finance and Chief Financial Officer Interneuron Pharmaceuticals, Inc. One Ledgemont Center 99 Hayden Avenue, Suite 340 Lexington, MA 02173 Dear Mr. Farb: Reference is made to the five call option transactions ("Transactions") entered into between Swiss Bank Corporation, London Branch ("SBC") and Interneuron Pharmaceuticals, Inc. ("Interneuron") relating to the stock of Interneuron Pharmaceuticals, Inc., each as evidenced by a confirmation dated May 12, 1997, SBC Reference Numbers 1255576, 1255577, 1255578, 1255579, 1255580, and 1255581 (collectively, the "Confirmations"). SBC and Interneuron agree that the Expiration Date, Strike Price and Cap Price in each of the Confirmations is hereby amended as provided in Exhibit 1. As consideration for this amendment, SBC will pay $500,000 to Interneuron on September 23, 1997. Except as expressly modified herein, all other terms of the Transactions shall continue to be and shall remain in full force and effect as set forth in the Confirmations. This Letter Agreement shall be governed by and construed in accordance with the laws of New York without giving effect to conflict of law principles. Please confirm that the foregoing correctly sets forth the terms of our agreement by executing the copy of this letter enclosed for that purpose and returning it to us. Yours sincerely, SWISS BANK CORPORATION, LONDON BRANCH By: /s/ Victoria Harkness Sotgiu By: /s/ John Marshall Name: Victoria Harkness Sotgiu Name: John Marshall Title: Authorized Signatory Title: Authorized Signatory Accepted and agreed as of the 29th day of September, 1997 Interneuron Pharmaceuticals, Inc. By: /s/ Thomas F. Farb Name: Thomas F. Farb Title: Treasurer Exhibit 1 SBC Reference Original Amended Original Amended Original Amended Number Expiration Date Expiration Date Strike Price Strike Price Cap Price Cap Price - ------- --------------- --------------- ------------ ------------ --------- --------- CONTRACT A 1255580 September 24, 1997 December 31, 1997 $17.75 $14.50 $26.00 $22.50 1255579 March 9, 1998 June 9, 1998 $17.75 $14.50 $34.00 $29.50 1255577 May 21, 1998 September 21, 1998 $17.75 $14.50 $38.00 $32.50 1255578 August 24, 1998 January 11, 1999 $17.75 $14.50 $40.00 $34.50 CONTRACT B 1255576 May 21, 1999 December 30, 1999 $40.30 $36.00 1255581 May 24, 1999 December 31, 1999 $40.30 $36.00