What they do
Prothena discovers antibodies for diseases caused by misfolded proteins and licenses them to large drug companies. Its four partnered programs are prasinezumab with Roche (Parkinson's), coramitug with Novo Nordisk (a heart disease caused by amyloid), and two programs with Bristol Myers Squibb: BMS-986446 in Alzheimer's disease and PRX019, which has finished its first trial.
Why we picked it
- 01$288M
Cash covers about two thirds of the market value. Prothena held $288M of cash and no debt at 30 Jun 2026, against a market value of $429M at the 16 Oct close. That leaves about $140M for four partnered programs at Roche, Novo Nordisk and Bristol Myers Squibb. The company guides to about $259M of cash at the end of 2026, counting buybacks through July.
- 02$8.8M
Spending has been cut hard since its own late-stage drug, birtamimab, was dropped in May 2025. R&D was $8.8M in the June 2026 quarter, against $40.5M a year earlier and $50.7M in the September 2024 quarter. The company guides to only $18M to $23M of net cash used in all of 2026, helped by a $50M milestone from Novo Nordisk received in the first half.
- 03-4.6%
The company is buying back its own stock. It bought 2,243,888 shares for about $22.3M in the first half of 2026 under a $100M program that runs to the end of the year, and shares outstanding are down 4.6% in a year, from 53.8M to 51.3M.
- 04$10.9M
A 10% holder keeps buying. William P. Scully, who reported a 10.3% stake in May, has since bought 1.25M more shares in the open market for about $10.9M, about $6.7M of it between 18 Sep and 14 Oct, none of it under a trading plan. Separately, Bristol Myers Squibb is expected to decide by the end of 2026 whether to advance PRX019, which would pay Prothena a $55M milestone that is not in its guidance.
The risk
The catch: Prothena has no product of its own, so every milestone depends on Roche, Novo Nordisk or Bristol Myers Squibb, and the two Phase 3 trials are not expected to finish until 2029. The steady buyer is an outside holder, not management, and the company's buyback in the first half was done at about $9.93 a share, above today's price. Without new milestones the cash shrinks: before the one-off $50M Novo Nordisk payment, operating cash use was about $110M over the last four reported quarters, and the company says it may need to raise money beyond the next twelve months.
The full analysis
Prothena ($PRTA): two thirds cash, a buyback, and a 10% holder who keeps buying
Our view: bullish, because Prothena's cash covers about two thirds of its market value, it has cut its spending hard and is buying back stock, and a 10% holder has put about $10.9M into the stock in the open market since May.
The thesis
Prothena discovers antibodies for diseases caused by misfolded proteins and licenses them to large drug companies. Its four partnered programs are prasinezumab with Roche in Parkinson's disease, coramitug with Novo Nordisk in a heart disease caused by amyloid, and two programs with Bristol Myers Squibb: BMS-986446 in Alzheimer's disease and PRX019, which has finished its first trial. After its own late-stage drug, birtamimab, was dropped in May 2025, the company cut its costs and turned into a lean partnership business. We think the market is paying little for those partnerships, and that the cash, the buyback and the steady buying by a large holder give the story time while the partners run the trials.
The evidence
Cash covers about two thirds of the market value. Prothena held $288M of cash and no debt at 30 Jun 2026, against a market value of $429M at the 16 Oct close, which leaves about $140M for the four programs. Total liabilities were only $15.7M (10-Q, Aug 2026).
Spending has been cut hard. R&D was $8.8M in the June 2026 quarter, against $40.5M a year earlier and $50.7M in the September 2024 quarter, and general and administrative costs fell to $10.9M from $15.9M (10-Qs). The company guides to only $18M to $23M of net cash used in all of 2026, helped by a $50M milestone from Novo Nordisk received in the first half, and to about $259M of cash at year-end, counting buybacks through July (Q2 release, Aug 2026).
The company is buying back its own stock. It bought 2,243,888 shares for about $22.3M in the first half of 2026 under a $100M program that runs to 31 Dec 2026, and shares outstanding are down 4.6% in a year, from 53.8M to 51.3M (10-Q covers).
A 10% holder keeps buying. William P. Scully reported a 10.3% stake in May (13G/A, May 2026) and has since bought 1.25M more shares in the open market for about $10.9M, about $6.7M of it between 18 Sep and 14 Oct, none of it under a trading plan (Forms 4, May to Oct 2026).
What the market is missing
The partnerships carry large milestone schedules, and one decision is close. Bristol Myers Squibb is expected to decide by the end of 2026 whether to advance PRX019, which would pay Prothena a $55M clinical milestone that is not in its guidance (Q2 release). Bristol Myers Squibb's Phase 2 trial of BMS-986446 in early Alzheimer's disease is expected to reach primary completion in the first half of 2027. With cash at two thirds of the market value, the rest of the price is a modest bet on four programs run and paid for by much larger companies.
The risks
Prothena has no product of its own, so every milestone depends on Roche, Novo Nordisk or Bristol Myers Squibb, and the two Phase 3 trials, PARAISO and CLEOPATTRA, are not expected to finish until 2029. The steady buyer is an outside holder, not management, and the company's first-half buyback was done at about $9.93 a share, above today's price. The Chief Development Officer left on 9 Oct to join one of the company's partners. Without new milestones the cash shrinks: before the one-off $50M Novo Nordisk payment, operating cash use was about $110M over the last four reported quarters, and the company says it may need to raise money beyond the next twelve months.
What would change our mind
Bristol Myers Squibb declining to advance PRX019, a partner handing back a program, the year-end cash coming in well below the guided $259M, or a share sale that reverses the buyback.
Sources: Prothena Q2 2026 10-Q and results release (6 Aug 2026); 10-K for 2025 (27 Feb 2026); 10-Q and 10-K filings, Nov 2025 to Aug 2026 (quarterly R&D); buyback 8-K (27 Feb 2026); 8-K on the Chief Development Officer (2 Oct 2026); proxy statement (27 Mar 2026); William P. Scully's 13G/A (11 May 2026) and Forms 4 (26 May to 15 Oct 2026); close from Yahoo Finance and Nasdaq, 16 Oct 2026.
By the numbers
| Quarter | Revenue | Gross margin | Net income | Cash | Debt | Diluted shares | Filing |
|---|---|---|---|---|---|---|---|
| Q2 2630 June 2026 | $1.0M | n/a | -$18.6M | $288.2M | n/a | 52.1M | 10-Q, 6 August 2026 |
| Q1 2631 March 2026 | $51.1M | n/a | $32.7M | $329.5M | n/a | 54.1M | 10-Q, 7 May 2026 |
| Q4 25*31 December 2025 | $21K | n/a | -$21.6M | $307.5M | n/a | n/a | 10-K, 27 February 2026 |
| Q3 2530 September 2025 | $2.4M | n/a | -$36.5M | $330.8M | n/a | 53.8M | 10-Q, 6 November 2025 |
| Q2 2530 June 2025 | $4.4M | n/a | -$125.8M | $371.4M | n/a | 53.8M | 10-Q, 6 August 2026 |
| Q1 2531 March 2025 | $2.8M | n/a | -$60.2M | $417.9M | n/a | 53.8M | 10-Q, 7 May 2026 |
| Q4 24*31 December 2024 | $2.1M | n/a | -$58.0M | $471.4M | n/a | n/a | 10-K, 27 February 2026 |
| Q3 2430 September 2024 | $970K | n/a | -$59.0M | $519.3M | n/a | 53.8M | 10-Q, 6 November 2025 |
Source: the company's 10-Q and 10-K filings on SEC EDGAR, as tagged in their XBRL data. Each quarter uses the most recent filing that reports it, so restated quarters compare cleanly. Quarters are labelled by the calendar quarter they end in. Debt is current plus long-term debt; net income is as reported, including any discontinued operations. *Fourth quarter: the full year in the 10-K less the first nine months. n/a: not reported for that quarter on its own.
Insiders
Directors and executive officers as a group (14 persons), including options exercisable within 60 days own 10.4% of the shares (CEO Gene Kinney 4.3%; William P. Scully, an outside 10% holder, is separate (10.3% per his May 2026 13G/A)), according to the DEF 14A 2026 filed 27 March 2026, as of 2 March 2026.
| Date | Who | Shares bought | Price | Value | Filing |
|---|---|---|---|---|---|
| 14 October 2026 | William P Scully10% owner | 10,000 | $8.46 | $85K | Form 4 |
| 13 October 2026 | William P Scully10% owner | 100,000 | $8.35 | $835K | Form 4 |
| 8 October 2026 | William P Scully10% owner | 90,000 | $8.57 | $771K | Form 4 |
| 7 October 2026 | William P Scully10% owner | 125,000 | $8.40 | $1.1M | Form 4 |
| 2 October 2026 | William P Scully10% owner | 100,000 | $8.57 | $857K | Form 4 |
| 29 September 2026 | William P Scully10% owner | 162,000 | $8.68 | $1.4M | Form 4 |
| 28 September 2026 | William P Scully10% owner | 103,500 | $8.60 | $890K | Form 4 |
| 18 September 2026 | William P Scully10% owner | 100,000 | $8.43 | $843K | Form 4 |
Open-market purchases (Form 4, transaction code P) over the last twelve months. Sales, grants and option exercises are not shown.
What to watch
- By year-end 2026
Bristol Myers Squibb's decision on advancing PRX019; if it does, Prothena earns a $55M clinical milestone that is not in its 2026 guidance. Q2 2026 results release (6 Aug 2026)
- 31 Dec 2026
The $100M buyback program expires; about $22.3M was used in the first half. 8-K (27 Feb 2026); Q2 2026 10-Q
- First half of 2027
Expected primary completion of Bristol Myers Squibb's Phase 2 TargetTau-1 trial of BMS-986446 in early Alzheimer's disease. Q2 2026 results release (6 Aug 2026)
Sources: Q2 2026 10-Q and results release (6 Aug 2026), Forms 4 (May to Oct 2026), close 16 Oct 2026.
Market value: shares outstanding on the latest SEC filing cover page, times the latest price.