Arrowhead Raises $6.2 Million through Registered Direct Offering of Common Stock and Warrants

Pasadena, CA — August 13, 2012 — Arrowhead Research Corporation (NASDAQ: ARWR) today announced that it has entered into a securities purchase agreement with institutional and individual investors to raise gross proceeds of approximately $6.2 million. Investors include several existing investors in the Company, including James Mellon, and a new fund dedicated to health care investments. The placement consisted of the sale of 2.3 million shares of its common stock, and warrants to purchase 1.7 million shares of common stock. The stock and warrants will be sold in units, at a price of $2.76 per unit, with each unit consisting of one share of common stock and a warrant to purchase 0.75 shares of common stock. The warrants have an exercise price of $3.25 per share of common stock, with a term of four years from the date of issuance and are exercisable commencing 6 months following the closing of the offering.

Rodman & Renshaw, LLC is acting as the exclusive placement agent for the offering. Use of proceeds includes general corporate, research and development, and clinical trial expenditures. The closing is expected to take place on or about August 16, 2012, subject to the satisfaction of customary closing conditions.

The securities will be issued under Arrowhead’s shelf registration statement filed on Form S-3 (File No. 333-176159), which was declared effective by the Securities and Exchange Commission on August 17, 2011. The final prospectus supplement related to this offering will be filed with the Securities and Exchange Commission.

This press release does not constitute an offer to sell or a solicitation of an offer to buy the securities referred to herein, nor shall there be any sale of these securities in any state in which such an offer, solicitation or sale would be unlawful prior to registration or qualification under the securities laws of any such state.

The prospectus, forming part of the effective registration statement, and prospectus supplement relating to this offering contain this and other information about the Company. The offering is being made only by means of the prospectus and the related prospectus supplement, copies of which may be obtained from Rodman & Renshaw, LLC.;. by calling 212-201-8064 or by email at placements@rodm.com.

About Arrowhead Research Corporation

Arrowhead Research Corporation is a clinical stage targeted therapeutics company with development programs in oncology, obesity, and infectious disease. The company leverages its platform technologies to design and develop peptide-drug conjugates (PDCs) which specifically home to cell types of interest while sparing off-target tissues, creates targeted drugs based on the gene silencing RNA interference (RNAi) mechanism, and works with partners to create improved versions of traditional small molecule drugs.

For more information please visit http://www.arrowheadresearch.com, or follow us on Twitter @ArrowRes. To be added to the Company’s email list to receive news directly, please send an email to ir@arrowres.com