Prothena Corporation plc : Prothena Announces Public Offering of Ordinary Shares

Cantor Fitzgerald & Co. is acting as the sole bookrunner for the offering.

The ordinary shares of Prothena described above are being offered only by means of a prospectus supplement and base prospectus forming part of an effective shelf registration statement filed with the Securities and Exchange Commission. Copies of the prospectus for this offering may be obtained, when available, by contacting Cantor Fitzgerald & Co., Attn: Capital Markets, 499 Park Ave., 5th Floor, New York, New York 10022, or by telephone at 212-829-7122, or by e-mail at prospectus@cantor.com.

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

About Prothena

Prothena Corporation plc is a global, late-stage clinical biotechnology company with a clinical pipeline of novel therapeutic antibodies. Prothena is establishing a fully integrated research, development and commercial focus and has advanced several drug candidates into clinical studies while pursuing discovery of additional novel therapies. Our pipeline of antibody-based product candidates targets a number of potential indications including AL amyloidosis (NEOD001), Parkinson's disease and other related synucleinopathies (PRX002), inflammatory diseases, including psoriasis and psoriatic arthritis (PRX003), and ATTR amyloidosis (PRX004).

Forward-Looking Statements

This press release contains forward-looking statements. These statements relate to, among other things, the timing and size of the proposed offering, the conditions affecting the capital markets, general economic, industry, or political conditions, and the satisfaction of customary closing conditions related to the proposed offering. These statements are based on estimates, projections and assumptions that may prove not to be accurate, and actual results could differ materially from those anticipated due to known and unknown risks, uncertainties and other factors, including but not limited to the risks, uncertainties and other factors described in the "Risk Factors" sections of Prothena's Annual Report on Form 10-K filed with the Securities and Exchange Commission (SEC) on February 27, 2017 and in the preliminary prospectus supplement related to the proposed offering to be filed with the SEC on or about the date hereof. Prothena undertakes no obligation to update publicly any forward-looking statements contained in this press release as a result of new information, future events or changes in Prothena's expectations.