Prothena Reports Fourth Quarter and Full Year 2015 Financial Results; Provides Financial Guidance and R&D Update

Prothena Reports Fourth Quarter and Full Year 2015 Financial Results; Provides Financial Guidance and R&D Update

DUBLIN, Ireland - February 18, 2016 - Prothena Corporation plc (NASDAQ:PRTA), a late-stage clinical biotechnology company focused on the discovery, development and commercialization of novel protein immunotherapies, today reported financial results for the fourth quarter and full year ended December 31, 2015. In addition, the Company provided 2016 financial guidance and an update on its R&D programs.

"Prothena had an exceptional 2015, driven by clinical advances in each of our three lead protein immunotherapy programs and increased financial stability," said Dale Schenk, PhD, President and Chief Executive Officer of Prothena. "We continue to enroll patients in The VITAL Amyloidosis Study, our Phase 3 registrational trial of NEOD001, and announced PRONTO, a Phase 2b study to potentially accelerate our pathway to patients with AL amyloidosis. We reported encouraging clinical results from trials of both NEOD001 and PRX002, while increasing awareness of both programs through presentations at major international medical meetings and a peer-reviewed publication. We also initiated clinical development of PRX003 and advanced a series of antibodies in preclinical development against diseased forms of the transthyretin protein, known as TTR."

Dr. Schenk continued, "We look forward to the year ahead with milestones expected in each of our clinical stage programs, advancing our objective to bring novel disease-modifying therapies to patients."

Full Year 2015 and Recent Highlights:

NEOD001 is a monoclonal antibody for the potential treatment of AL amyloidosis:

PRX002 is a monoclonal antibody for the potential treatment of Parkinson's disease and related synucleinopathies, and is the primary focus of Prothena's worldwide collaboration with Roche:

PRX003 is a monoclonal antibody for the potential treatment of inflammatory diseases, including psoriasis:

Preclinical Programs:

Upcoming Research and Development Milestones

Prothena's clinical development pipeline includes three lead protein immunotherapy programs.

Fourth Quarter and Full Year of 2015 Financial Results and 2016 Financial Guidance

Prothena reported a net loss of $24.2 million and $80.6 million for the fourth quarter and full year of 2015, respectively, as compared to a net loss of $13.1 million and $7.2 million for the fourth quarter and full year of 2014, respectively. Net loss per share for the fourth quarter and full year of 2015 was $0.76 and $2.66, respectively, as compared to a net loss per share for the fourth quarter and full year of 2014 of $0.48 and $0.29, respectively, on a fully diluted basis.

Prothena reported total revenue of $0.3 million and $1.6 million for the fourth quarter and full year of 2015, respectively, as compared to total revenue of $2.0 million and $50.9 million for the fourth quarter and full year of 2014, respectively. The decrease in revenue for the fourth quarter and full year was primarily due to lower revenue from Prothena's collaboration agreement with Roche.

Research and development (R&D) expenses totaled $17.9 million and $58.4 million for the fourth quarter and full year of 2015, respectively, as compared to $10.1 million and $38.5 million for the fourth quarter and full year of 2014, respectively. The increase in R&D expenses was primarily due to increased external expenses related to clinical trials and increased personnel cost. R&D expenses included non-cash share-based compensation expense of $1.3 million and $4.3 million for the fourth quarter and full year of 2015, respectively, as compared to $0.6 million and $2.3 million for the fourth quarter and full year of 2014, respectively.

General and administrative (G&A) expenses totaled $6.6 million and $23.1 million for the fourth quarter and full year of 2015, respectively, as compared to $5.0 million and $19.1 million for fourth quarter and full year of 2014, respectively. The increase in G&A expenses for the fourth quarter and full year was primarily due to increases in personnel costs and legal expenses. G&A expenses included non-cash share-based

compensation expense of $1.9 million and $6.1 million in the fourth quarter and full year of 2015, respectively, as compared to $0.8 million and $3.3 million in the fourth quarter and full year of 2014, respectively.

Total non-cash share-based compensation expense was $3.3 million and $10.4 million for the fourth quarter and full year of 2015, respectively, as compared to $1.4 million and $5.6 million for the fourth quarter and full year of 2014, respectively.

As of December 31, 2015, Prothena had $370.6 million in cash and cash equivalents, which does not include the $128.6 million in net proceeds received through the January 2016 equity offering, and no debt.

As of February 12, 2016, Prothena had approximately 34.3 million ordinary shares outstanding, which includes the issuance of approximately 2.6 million ordinary shares as part of the January 2016 offering.

The Company expects the full year 2016 net cash burn from operating and investing activities to be $105 to $115 million, ending the year with approximately $388 million in cash (mid-point). The estimated full year 2016 net cash burn from operating and investing activities is primarily driven by an estimated net loss of $132 to $149 million, which includes an estimated $18 million of non-cash share-based compensation expense.

Upcoming Investor Conferences

Members of the senior management team will present and participate in investor meetings at the following upcoming investor conferences:

A live webcast of the presentations can be accessed through the investor relations section of the Company's website at www.prothena.com. Following the live presentations, replays of the webcast will be available on the Company's website for at least 90 days following the presentation date.

Prothena Corporation plc is a late-stage clinical biotechnology company focused on the discovery, development and commercialization of novel protein immunotherapies for the potential treatment of diseases that involve amyloid or cell adhesion. The Company is developing antibody-based product candidates that target a number of potential indications including AL amyloidosis (NEOD001), Parkinson's disease and other related synucleinopathies (PRX002), and psoriasis and other inflammatory diseases (PRX003). For more information, please visit the Company's web site at www.prothena.com.

Conference Call Details

Prothena management will discuss these results and its 2016 outlook in a live audio webcast and conference call today, Thursday, February 18, 2016, at 4:30 p.m. ET. The webcast will be made available on the Company's website at www.prothena.com under the Investors tab in the Events and Presentations section. Following the live audio webcast, a replay will be available on the Company's website for 90 days.

To access the call via dial-in, please dial (877) 887-5215 (U.S. toll free) or (315) 625-3069 (international) five minutes prior to the start time and refer to conference ID number 33151950. A replay of the call will be available until February 25, 2016 via dial-in at (855) 859-2056 (U.S. toll free) or (404) 537-3406 (international), Conference ID Number 33151950.

Forward-looking Statements

This press release contains forward-looking statements. These statements relate to, among other things, whether our cash position can fund advancement of our clinical pipeline; whether we can advance on our objective of bringing disease-modifying therapies to patients; the timing of reporting results from the expansion cohort of our Phase 1/2 study, completing enrollment in The VITAL Amyloidosis Study, and reporting results from the PRONTO trial, all for NEOD001; the timing of reporting data from the Phase 1 multiple ascending dose study for PRX002; the timing of reporting data from the Phase 1 multiple ascending dose study and initiating a Phase 1 multiple ascending dose proof-of-biology study for PRX003; our anticipated net cash burn from operating and investing activities for 2016 and expected cash balance at the end of 2016; and our estimated net loss and non-cash share-based compensation expense for 2016. 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 our Annual Report on Form 10-K filed with the Securities and Exchange Commission (SEC) on March 13, 2015, our subsequent Quarterly Reports on Form 10-Q filed with the SEC and our Annual Report on Form 10-K to be filed with the SEC for our fiscal year 2015. 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.

PROTHENA CORPORATION PLC

CONSOLIDATED STATEMENTS OF OPERATIONS

(unaudited - amounts in thousands except per share data)

PROTHENA CORPORATION PLC

CONSOLIDATED BALANCE SHEETS

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Investors: Tran Nguyen, CFO

650-837-8535, IR@prothena.com

Media: Ellen Rose, Head of Communications

650-922-2405, ellen.rose@prothena.com