PROTHENA CORP PUBLIC LTD CO (PRTA) stock intrinsic value fair value estimate & owner earnings analysis

Prothena is a clinical-stage biotechnology company developing antibodies for neurodegenerative and related diseases. It has no products on the market. Its income comes from partners that license its drugs: Roche (prasinezumab, Parkinson's disease), Bristol Myers Squibb (BMS-986446 and PRX019) and Novo Nordisk (coramitug, in the Phase 3 CLEOPATTRA trial). What the company actually earns. We measure earning power as operating cash flow, less swings in working capital, less stock compensation, less the equipment spending needed to stay in business. On that basis the company lost money in six of the last seven years, between about $(1.95) and $(3.71) per share. The exception was 2021, when partner payments produced revenue of about $200 million and roughly +$1.50 per share. Since then revenue has been lumpy ($54 million in 2022, $91 million in 2023, $135 million in 2024, $10 million in 2025) and has not covered the cost of research. Stock compensation of $38 to $46 million a year is treated as a real cost. In 2025 a $43 million non-cash tax charge explains most of the gap between the $244 million net loss and the $164 million cash outflow from operations. Because the typical year is a loss, the record supports no positive earnings level to build on. Bear case: $0 per share. The partnered programs do not reach approval, no further milestones arrive, and the company cannot fund its own pipeline without asking shareholders for more money. Operations used about $164 million of cash in 2025 against about $308 million of cash at year-end, which management says covers at least the next twelve months. The company has no debt, so we do not assign a value below zero. Base case: $0 per share. We can name the contracts, but every dollar of future milestones (up to $563 million on BMS-986446, up to $617.5 million on PRX019, up to $1.13 billion from Novo Nordisk) depends on trial outcomes and approvals that have not happened, so we do not count them. Royalties described as high single digit to high teen percentages likewise only matter after approval. With no proven, repeating source of profit, we assign no value to the pipeline. Bull case: $2.00 per share. BMS-986446 is approved (we assume around 2032) and BMS pays the regulatory milestones of up to $187.5 million, worth about $125 million today, or about $2.30 per share; we use $2.00. Royalties and sales milestones are excluded because there is no sales record to measure them against. The one concrete sign of progress is the FDA Fast Track designation BMS obtained for BMS-986446 in October 2025. Roche's Phase 3 PARAISO trial of prasinezumab began in the fourth quarter of 2025, but the earlier Phase 2b PADOVA trial missed statistical significance on its primary endpoint (hazard ratio 0.84, p=0.0657), so we do not credit it. What would make this wrong: a clearly positive PARAISO result, or a new partner paying a large option fee, would put value well above our bull case. Partner cash has also arrived before without approvals: BMS paid option fees of $80 million in 2021, $55 million in 2023 and $80 million in 2024, and Roche milestones and Novo Nordisk payments have been part of revenue too. The company holds about $308 million of cash and no debt, so shareholders own more than nothing, but that cash is being consumed at roughly $164 million a year. Our values reflect earning power only, and the cash is not added because the current operating model spends it.