Prothena's Q3: Costs Fall, Partners Carry the Pipeline, and Cash Burn Stays Low

The quarter in numbers

Prothena reported a third-quarter 2026 net loss of $16.5 million, or $0.32 per share, against a net loss of $36.5 million, or $0.68 per share, a year earlier. Total revenue was $0.6 million, down from $2.4 million. Loss from operations narrowed to $18.8 million from $40.2 million, as total operating expenses fell to $19.4 million from $42.7 million.

The nine-month picture is distorted by one-off items on both sides. The net loss was $2.4 million, or $0.05 per share, versus $222.5 million, or $4.13 per share, in the first nine months of 2025. Total revenue was $52.7 million versus $9.7 million. Other income, net, was $2.3 million in the quarter, down from $3.3 million.

What drove revenue

The quarterly revenue decline and the nine-month jump have separate causes. The company said nine-month 2026 revenue was primarily a $50.0 million milestone payment from Novo tied to the ongoing Phase 3 trial of coramitug, plus collaboration revenue from Bristol Myers Squibb for partial performance of the PRX019 Phase 1 trial obligation. Nine-month collaboration revenue was $2.7 million, down from $9.6 million, and the milestone appears in the license and intellectual property line at $50.1 million.

In the quarter, no such milestone was booked. Revenue of $0.6 million was all collaboration revenue, down from $2.4 million in the prior-year quarter. Prothena has no product sales in this release, so its income depends on partner milestones that arrive in lumps.

Costs and margins: a much leaner company

R&D expenses were $8.4 million in the quarter versus $28.9 million, and $29.9 million for nine months versus $120.3 million. The company attributed the decline to lower clinical trial, personnel and consulting expenses. G&A fell to $10.6 million from $13.2 million in the quarter, and to $34.2 million from $46.7 million for nine months, on lower personnel and consulting costs.

Restructuring was a $0.4 million expense in Q3 versus $0.5 million, and an aggregate $1.4 million credit for nine months versus a $33.1 million expense a year earlier. The 2025 nine-month comparison also carried a $43.3 million income tax provision, against $5 thousand in 2026. Total non-cash share-based compensation was $6.4 million in the quarter, down from $7.2 million, and $21.2 million for nine months versus $30.6 million.

Margins are not meaningful with revenue this small. The relevant measure is that quarterly operating expenses are now roughly 46% of last year's level, by our calculation from the release's figures.

Cash, guidance and buybacks

Net cash used in operating and investing activities was $12.0 million in the quarter and $8.9 million for nine months, the latter helped by the Novo milestone. Quarter-end cash, equivalents and restricted cash was $272.2 million, with no debt.

The company continues to expect full-year 2026 net cash use of $18 million to $23 million and year-end cash of approximately $259 million at the midpoint. Guidance rests on an estimated net loss of $25 million to $30 million, including an estimated $26 million of non-cash share-based compensation. It excludes any cash used for share repurchases. Against the nine-month figures, our arithmetic implies a fourth-quarter net loss of roughly $22.6 million to $27.6 million and net cash use of roughly $9.1 million to $14.1 million, so Q4 spending should step up from the nine-month average.

Prothena repurchased 501,492 shares in Q3, and 2,745,380 shares in the first nine months for $26.6 million, excluding commissions and expenses. That is under an up to $100.0 million program that expires on December 31, 2026. About 51.3 million ordinary shares were outstanding at October 29, 2026.

Pipeline and regulatory catalysts

The company says its partnered programs have generated $650 million in upfront, milestone and equity payments, with potentially more than $2.25 billion in additional milestones plus royalties. Two programs are in Phase 3 with primary completions expected in 2029. Roche's PARAISO trial of prasinezumab in Parkinson's disease enrolls approximately 900 participants for a minimum of 104 weeks. Novo's CLEOPATTRA trial of coramitug in ATTR-CM enrolls approximately 1,280. The release says Roche has stated peak sales potential for prasinezumab of more than $3.5 billion (unadjusted).

On coramitug, Novo presented post hoc Phase 2 analyses at the HFSA meeting. At week 52, pooled dose groups (10 and 60 mg/kg) showed an approximately 8% decrease in hsCRP versus an approximately 33% increase on placebo (treatment ratio 0.66; nominal p=0.0269). Because the analyses are post hoc with a nominal p-value, they are supportive rather than confirmatory. Coramitug and moponetug each hold FDA Fast Track designation.

The nearest binary event is the Phase 2 TargetTau-1 readout for moponetug, run by Bristol Myers Squibb in approximately 310 patients with early Alzheimer's disease, expected in the first half of 2027. On Prothena's own preclinical work, PRX012-TfR and TDP-43 CYTOPE continue, and the company said multiple CYTOPE research collaborations will run over the next 12 to 24 months.

What to watch next

The first marker is Q4 spending against the guidance range, and whether the company changes its buyback pace before the program expires on December 31. The second is the TargetTau-1 readout in the first half of 2027, along with the mid-2027 primary completion of the moponetug Phase 1 study and the 2027 primary completion of the coramitug PET/CT study. Finally, the release names no new partnering terms for CYTOPE, so any announced deal would be the first test of the platform's commercial value.