DEFA14A

Cautionary Statement Regarding Forward-Looking Statements

Information set forth in this communication, including financial estimates and statements as to the expected timing, completion and effects of the proposed transaction between Pfizer Inc. (“Pfizer”), Aris Merger Sub, Inc. (“Merger Sub”) and Seagen Inc. (“Seagen”), constitute forward-looking statements within the meaning of the safe harbor provisions of the Private Securities Litigation Reform Act of 1995. Actual results or developments may differ materially from those projected or implied in these estimates and statements. Such estimates and statements include, but are not limited to, statements about the benefits of the transaction, including future financial and operating results, Seagen’s plans, objectives, expectations and intentions and other statements that are not historical facts. Such statements are based upon the current beliefs and expectations of the management of Seagen and are subject to significant risks and uncertainties outside of our control. Among the risks and uncertainties that could cause actual results to differ from those described in the forward-looking statements are the following: the occurrence of any event, change or other circumstances that could give rise to the termination of the merger agreement; the risk that Seagen stockholders may not approve the transaction; the risk that the necessary regulatory approvals may not be obtained or may be obtained subject to conditions that are not anticipated; risks that any of the other closing conditions to the proposed transaction may not be satisfied in a timely manner; risks related to potential litigation brought in connection with the proposed transaction; risks related to financial community and rating agency perceptions of Seagen and its business, operations, financial condition and the industry in which it operates; risks related to disruption of management time from ongoing business operations due to the proposed transaction; effects of the announcement, pendency or completion of the proposed transaction on the ability of Seagen to retain customers and retain and hire key personnel and maintain relationships with suppliers and partners, and on Seagen’s operating results and businesses generally; and risks related to the potential impact of general economic, political and market factors on the companies or the proposed transaction.

Discussions of additional risks and uncertainties are contained in Seagen’s filings with the U.S. Securities and Exchange Commission (“SEC”). Seagen is under no obligation, and expressly disclaims any obligation, to update, alter or otherwise revise any forward-looking statements, whether written or oral, that may be made from time to time, whether as a result of new information, future events or otherwise. Persons reading this announcement are cautioned not to place undue reliance on these forward-looking statements which speak only as of the date hereof.

Additional Information and Where to Find It

This communication may be deemed to be solicitation material in respect of the proposed transaction between Pfizer, Merger Sub, and Seagen. In connection with the proposed transaction, Seagen will file a proxy statement on Schedule 14A with the SEC as well as other relevant materials regarding the transaction. Following the filing of the definitive proxy statement, Seagen will mail the definitive proxy statement and a proxy card to its shareholders in connection with the transaction. INVESTORS AND SECURITY HOLDERS OF SEAGEN ARE URGED TO READ THE PROXY STATEMENT AND OTHER RELEVANT DOCUMENTS FILED OR TO BE FILED WITH THE SEC CAREFULLY WHEN THEY BECOME AVAILABLE BECAUSE THEY WILL CONTAIN IMPORTANT INFORMATION ABOUT PFIZER, SEAGEN, THE TRANSACTION AND RELATED MATTERS. Investors and security holders will be able to obtain copies of the proxy statement (when available) as well as other filings containing information about Seagen, without charge, at the SEC’s website, http://www.sec.gov.