Prothena Reports First Quarter 2020 Financial Results and Provides R&D Update and Updated Financial Guidance Net cash used in operating and investing activities was $23.2 million in the first quarter; quarter-end cash an

Prothena Reports First Quarter 2020 Financial Results and Provides R&D Update and Updated Financial Guidance

DUBLIN, Ireland, May 6, 2020-- Prothena Corporation plc (NASDAQ:PRTA), a clinical-stage neuroscience company with expertise in protein misfolding, today reported financial results for the first quarter of 2020. In addition, the Company provided an update on its R&D programs and 2020 financial guidance.

"While the COVID-19 pandemic has brought significant challenges to people around the world and to our industry, Prothena has acted to protect the health of patients and our employees and the continuity of our programs," said Gene Kinney, Ph.D., President and Chief Executive Officer of Prothena. "The situation is fluid and continues to evolve and as such we have identified potential disruptions and activated mitigation plans designed to advance our programs even under these challenging circumstances. Our team continues to manage critical activities, exhibiting extraordinary resilience, focus and commitment to advancing our pipeline of novel therapeutics for devastating diseases. We were fortunate that COVID-19 related disruptions began after patients in the Phase 2 prasinezumab study (PASADENA) and the Phase 1 PRX004 study had completed assessments that comprise the primary objective in each study."

First Quarter 2020 and Recent Developments

Research and Development Updates and Upcoming Milestones

Prasinezumab (PRX002/RG7935), a potential treatment for Parkinson's disease, is a monoclonal antibody designed to target alpha-synuclein and is the focus of the worldwide collaboration with Roche

PRX004, a potential treatment for ATTR amyloidosis, is a monoclonal antibody designed to deplete the pathogenic, non-native forms of the TTR protein

Discovery and Preclinical Development: Prothena is advancing an early-stage pipeline of programs for a number of potential neurological indications

First Quarter 2020 Financial Results and Updated 2020 Financial Guidance

For the first quarter of 2020, Prothena reported a net loss of $23.6 million, as compared to a net loss of $20.9 million for the first quarter of 2019, which included a restructuring credit of $0.1 million in the first quarter of 2019 resulting from an adjustment in previously recorded employee termination benefits associated with the discontinuation of the

NEOD001 program. Net loss per share for the first quarter of 2020 was $0.59, as compared to a net loss per share of $0.52 for the first quarter of 2019.

Prothena reported total revenue, all from its collaboration with Roche, of $0.1 million for the first quarter of 2020, as compared to total revenue of $0.2 million for the first quarter of 2019.

Research and development (R&D) expenses totaled $15.2 million for the first quarter of 2020, as compared to $13.3 million for the first quarter of 2019. The increase in R&D expense for the first quarter of 2020 compared to the same period in the prior year was primarily due to higher collaboration expense with Roche, higher manufacturing costs (primarily related to the tau and A programs) and higher clinical trial costs (primarily associated with the PRX004 program). R&D expenses included non-cash share-based compensation expense of $2.0 million for the first quarter of 2020, as compared to $2.1 million for the first quarter of 2019.

General and administrative (G&A) expenses totaled $9.7 million for the first quarter of 2020, as compared to $9.9 million for first quarter of 2019. G&A expenses for the first quarter of 2020 decreased compared to the same period in the prior year primarily related to lower personnel costs (including share-based compensation expense), and lower legal and accounting fees, which was offset in part by higher costs for our director and officer insurance premiums. G&A expenses included non-cash share-based compensation expense of $3.5 million for the first quarter of 2020, as compared to $4.1 million for the first quarter of 2019.

Total non-cash share-based compensation expense was $5.5 million for the first quarter of 2020, as compared to $6.2 million for the first quarter of 2019.

As of March 31, 2020, Prothena had $355.4 million in cash, cash equivalents and restricted cash and no debt.

As of May 1, 2020, Prothena had approximately 39.9 million ordinary shares outstanding.

The Company is updating its projected full year 2020 net cash burn from operating and investing activities, and expects it to be $75 to $85 million (versus prior guidance of $60 to $76 million), and expects to end the year with approximately $299 million (midpoint) in cash, cash equivalents and restricted cash (cash position), representing a decrease of $11 million from prior guidance of $310 million (midpoint). This decrease in cash position is primarily driven by an increase in expenses related to further clinical development, planning activities and ongoing evaluation of data from the Phase 2 PASADENA study of prasinezumab, and is based on Prothena's 30% portion of the U.S. co-development expenses. The updated estimated full year 2020 net cash burn from operating and investing activities is primarily driven by an updated estimated net loss of $101 to $118 million (versus prior guidance of $84 to $106 million), which includes an estimated $23 million of non-cash share-based compensation expense.

Prothena Corporation plc is a clinical-stage neuroscience company with expertise in protein misfolding, focused on the discovery and development of novel therapies with the potential to fundamentally change the course of devastating diseases. Fueled by its deep scientific expertise built over decades of research, Prothena is advancing a pipeline of therapeutic candidates for a number of indications and novel targets for which its ability to integrate scientific insights around neurological dysfunction and the biology of misfolded proteins can be leveraged. Prothena's partnered programs include prasinezumab (PRX002/RG7935), in collaboration with Roche for the potential treatment of Parkinson's disease and other related synucleinopathies, and programs that target tau, TDP-43 and an undisclosed target in collaboration with Bristol-Myers Squibb for the potential treatment of Alzheimer's disease, amyotrophic lateral sclerosis (ALS), frontotemporal dementia (FTD) or other neurodegenerative diseases. Prothena's proprietary programs include PRX004 for the potential treatment of ATTR amyloidosis, and programs that target A (Amyloid beta) for the potential treatment of Alzheimer's disease. For more information, please visit the Company's website at www.prothena.com and follow the Company on Twitter @ProthenaCorp.

Forward-looking Statements

This press release contains forward-looking statements. These statements relate to, among other things, the sufficiency of our cash position to fund advancement of a broad pipeline; the treatment potential and proposed mechanisms of action of prasinezumab and PRX004; plans for the ongoing Phase 2 clinical study of prasinezumab and the ongoing Phase 1 clinical study of PRX004; the expected timing of reporting data from the Phase 1 clinical study of PRX004 and from the Phase 2 clinical study of prasinezumab; the continued advancement of our discovery and preclinical pipeline; the timing of IND-enabling activities from our tau and A programs; our anticipated net cash burn from operating and investing activities for 2020 and expected cash balance at the end of 2020; and our estimated net loss and non-cash share-based compensation expense for 2020. 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 effects on our business of the worldwide COVID-19 pandemic and 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 3, 2020, as well as discussions of potential risks, uncertainties, and other important factors in our subsequent filings with the Securities and Exchange Commission.. 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

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PROTHENA CORPORATION PLC

CONSOLIDATED BALANCE SHEETS

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Media and Investor Contact:

Ellen Rose, Head of Communications

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