Prothena Corporation plc (PRTA): A Bull Case Theory

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We came across a bullish thesis on Prothena Corporation plc on BiotechBonanza’s Substack by AnotherBio. In this article, we will summarize the bulls’ thesis on PRTA. Prothena Corporation plc's share was trading at $9.92 as of October 3rd. PRTA’s forward P/E was 33.90 according to Yahoo Finance.

Prothena (PRTA) is a well-capitalized biotech company entering a wind-down phase, with most of its pipeline partnered to major pharmaceutical companies. The company’s value is primarily driven by four partnered programs, two of which are advancing into Phase 3 studies with Roche and Novo, targeting multi-billion-dollar indications. Milestones and potential royalties from these programs alone could exceed Prothena’s current enterprise value.

Additional programs with BMY, including early-stage Alzheimer’s treatments, provide further optionality, while wholly owned assets are being de-prioritized or wound down. Prothena has signaled its intention to return capital to shareholders, supported by pro forma net cash of approximately $326 million by year-end 2025, reducing enterprise value to around $85 million and positioning the company as a highly capital-rich wind-down with substantial upside optionality. The recent corporate restructuring, including a 63% workforce reduction, and the decision not to continue its sole advanced wholly owned program independently, further underscore the transition toward a pass-through, milestone-driven model.

Key assets like Roche’s Prasinezumab program and Novo’s Coramitug program carry large potential payouts through development and approval milestones, while royalties could provide recurring value. Conservative estimates suggest the company’s remaining net cash and partnered assets could cover 60–150% of current valuation, with upside from positive trial outcomes potentially driving a 20–30x return over several years.

A planned share buyback could further enhance shareholder value, reducing free float and amplifying returns on successful milestone events. PRTA represents a rare combination of capital return potential, optionality on late-stage biotech assets, and upside leverage to Phase 3 readouts from Tier 1 pharmaceutical partners, offering a high-risk/high-reward investment opportunity in the biotech space.

Previously we covered a bullish thesis on Rocket Pharmaceuticals, Inc. (RCKT) by sharkbiotech in March 2025, which highlighted Phase 1 success for its Danon disease therapy, a large potential patient pool, and a strong balance sheet. The stock has depreciated approximately 63% since coverage. The thesis still stands as RCKT prepares pivotal readouts. AnotherBio focuses on Prothena Corporation plc (PRTA), emphasizing a wind-down, partnered milestones, and capital return potential.

Prothena Corporation plc is not on our list of the 30 Most Popular Stocks Among Hedge Funds. As per our database, 28 hedge fund portfolios held PRTA at the end of the second quarter which was 16 in the previous quarter. While we acknowledge the potential of PRTA as an investment, we believe certain AI stocks offer greater upside potential and carry less downside risk. If you're looking for an extremely undervalued AI stock that also stands to benefit significantly from Trump-era tariffs and the onshoring trend, see our free report on the best short-term AI stock.

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Disclosure: None.