Filed Pursuant to Rule 425

Under the Securities Act of 1933

And Deemed Filed Pursuant to Rule 14a-12

Under the Securities Exchange Act of 1934

Filed by NPS Pharmaceuticals, Inc.

Subject Company: NPS Pharmaceuticals, Inc.

Enzon Pharmaceuticals, Inc.

Commission File No. 000-23272

The following letter was sent to employees of NPS Pharmaceuticals, Inc. (“NPS”) on February 20, 2003 in connection with the announcement of the proposed business combination between NPS and Enzon Pharmaceuticals, Inc.

Before the financial markets opened today, we made a public announcement that NPS and Enzon Pharmaceuticals intend to merge the operations of the two companies. We signed a definitive agreement last night. We expect to close in May or June of this year following receipt of regulatory and stockholder approval and satisfaction of other customary closing conditions.

I have talked to you previously about a merger or acquisition as one of our core strategies. We have looked long and hard for the right partner – a partner that could help us achieve our goal of becoming an enduring, worldwide company. Arthur Higgins, the CEO of Enzon, and I both feel that this is the right match. We believe that the combining of these two companies will bring together talents covering scientific, technical, and commercial expertise exceeding that which either of us has singly. It positions us to have the financial resources to support our basic science, and to realize commercial success. Of course, it does not guarantee success. We must work together to take advantage of the potential that this combination presents.

The burden of success rests squarely on our shoulders. Obviously, our company will undergo changes as we create this new entity. For example, it is appropriate that we rename ourselves. Lots of questions exist, and we are addressing them as fast as we can. We will be asking many of you for input on a number of the decisions that still must be made. Arthur and I will speak with you personally as soon and as often as possible to give you more details, and to answer questions.

I am proud of what we have accomplished together. We have the opportunity now to do even more in realizing our vision of creating new and important medicines. Change is always unsettling, but to the extent that it furthers our shared goals, it is welcome and exciting. I ask that you turn your collective energies to the success of this venture, and I look forward to creating our future together.

Cautionary Statement For The Purpose Of The “Safe Harbor” Provisions

Of The Private Securities Litigation Reform Act Of 1995

This letter contains forward-looking statements within the meaning of the “safe harbor” provisions of the Private Securities Litigation Reform Act of 1995. These statements are based on management’s current expectations and beliefs and are subject to a number of factors and uncertainties that could cause actual results to differ materially from those described in the

forward-looking statements. The forward-looking statements contained in this letter include statements about future financial and operating results and the proposed NPS/Enzon merger. These statements are not guarantees of future performance, involve certain risks, uncertainties and assumptions that are difficult to predict, and are based upon assumptions as to future events that may not prove accurate. Therefore, actual outcomes and results may differ materially from what is expressed herein. For example, if either of the companies do not receive required stockholder or governmental approvals or fail to satisfy other conditions to closing, the transaction will not be consummated. In any forward-looking statement in which NPS or Enzon expresses an expectation or belief as to future results, such expectation or belief is expressed in good faith and believed to have a reasonable basis, but there can be no assurance that the statement or expectation or belief will result or be achieved or accomplished. The following factors, among others, could cause actual results to differ materially from those described in the forward-looking statements: the risk that the NPS and Enzon businesses will not be integrated successfully; costs related to the proposed merger, failure of the NPS or Enzon stockholders to approve the proposed merger; and other economic, business, competitive and/or regulatory factors affecting NPS’ and Enzon’s businesses generally as set forth in NPS’ and Enzon’s filings with the SEC, including their Annual Reports on Form 10-K for their respective most recent fiscal years, especially in the Management’s Discussion and Analysis section, their most recent Quarterly Reports on Form 10-Q and their Current Reports on Form 8-K. NPS and Enzon are under no obligation to (and expressly disclaim any such obligation to) update or alter their forward-looking statements whether as a result of new information, future events or otherwise.

Additional Information And Where To Find It

In connection with the proposed NPS/Enzon merger, NPS, Enzon and Momentum Merger Corporation (which will be renamed by NPS and Enzon in connection with the proposed merger) intend to file a joint proxy statement/prospectus with the Securities and Exchange Commission (the “SEC”) in connection with the transaction described herein. INVESTORS AND SECURITY HOLDERS ARE URGED TO READ THE JOINT PROXY STATEMENT/PROSPECTUS WHEN IT BECOMES AVAILABLE BECAUSE IT WILL CONTAIN IMPORTANT INFORMATION ABOUT THE TRANSACTION DESCRIBED HEREIN. Investors and security holders may obtain a free copy of the joint proxy statement/prospectus (when it is available) and other documents filed by NPS and Enzon with the SEC at the SEC’s web site at www.sec.gov or by contacting NPS at 801-583-4939 and through NPS’ website at www.npsp.com, or by contacting Enzon at 908-541-8678 and through Enzon’s website at www.enzon.com.

NPS and its directors and executive officers may be deemed to be participants in the solicitation of proxies from the stockholders of NPS and Enzon in connection with the transaction described herein. Information regarding the special interests of these directors and executive officers in the transaction described herein will be included in the joint proxy statement/prospectus described above. Additional information regarding these directors and executive officers is also included in NPS’ proxy statement for its 2002 Annual Meeting of Stockholders, which was filed with the SEC on or about April 19, 2002. This document is

available free of charge at the SEC’s web site at www.sec.gov or by contacting NPS at 801-583-4939 and through NPS’ website at [www.npsp.com]

Enzon and its directors and executive officers also may be deemed to be participants in the solicitation of proxies from the stockholders of Enzon and NPS in connection with the transaction described herein. Information regarding the special interests of these directors and executive officers in the transaction described herein will be included in the joint proxy statement/prospectus described above. Additional information regarding these directors and executive officers is also included in Enzon’s proxy statement for its 2002 Annual Meeting of Stockholders, which was filed with the SEC on or about October 28, 2002. This document is available free of charge at the SEC’s web site at www.sec.gov or by contacting Enzon at 908-541-8678.