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Prothena Corporation plc (PRTA) fair value: what the stock is really worth

As of Sep 23, 2026: fair value of Prothena Corporation plc $5.14, price $8.68, upside -40.8%, quality 37 out of 100. Calculated from audited financials with 26 valuation models and 37 quality factors, updated daily.

  1. Fair value above price? No
  2. Good quality? No
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Healthcare · US · ISIN USG728001086

PC Prothena Corporation plc logo Thin data Sep 23, 2026

Prothena Corporation plc

PRTA · US

Weakest SetupStrongly overvalued and low quality.

!Fair value $5.14 · Strongly overvalued (−41%)
!Quality 37/100
!Weak Growth (revenue 5y +62.6 %/yr)
!Low debt · negative free cash flow
!Mixed vs. peers (5/10)
!Narrow moat 32/100
!Evidence only low, so the estimate is less certain
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What runs behind every stock

26 valuation models 8 families, DCF to NCAV 37 quality factors margins, balance sheet, cashflow, debt 6 AI factors moat, what no ratio measures 2,000+ data points verified, per stock, median

69 individual criteria per stock, every one traceable See the method →

Price vs Fair Value

$78.89 $4.58 Fair Value $5.14 Jun 2021 Sep 2026

White line = price, green steps = our fair value per fiscal year, dashed = 300-day average. As of Sep 23, 2026.

How to read this chart

60‑month range $4.58 – $78.89 · fair‑value band $3.39 – $6.42 · the $8.68 price screens above the $5.14 fair value. Dashed = 300-day average. As of Sep 23, 2026.

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Company profile

Prothena Corporation plc, a late-stage clinical biotechnology company, focuses on discovery and development of novel therapies to treat diseases caused by protein dysregulation.

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Prothena Corporation plc, a late-stage clinical biotechnology company, focuses on discovery and development of novel therapies to treat diseases caused by protein dysregulation. Its product pipeline includes Prasinezumab, an investigational humanized monoclonal antibody which is in Phase 3 clinical trial for the treatment of parkinson's disease and other synucleinopathies. The company also develops Coramitug, an investigational antibody, which is in Phase 3 clinical trial for the treatment of transthyretin amyloidosis; BMS-986446, an anti-tau antibody which is in Phase 2 clinical trial to treat Alzheimer's disease; PRX019, an investigational antibody which is in Phase 1 clinical trial for the treatment of neurodegenerative diseases; and PRX123, a Dual Aß-Tau Vaccine, which is in preclinical trial for treating Alzheimer's disease. In addition, it develops discovery- and late-preclinical-stage programs include TDP-43 CYTOPE for the treatment of amyotrophic lateral sclerosis; PRX012, a next-generation anti-Aß antibody which is in phase 1 clinical trial to treat Alzheimer's disease; and PRX012-TfR, a preclinical program for the treatment of Alzheimer's disease. It has a license, development, and commercialization agreement with F. Hoffmann-La Roche Ltd. and Hoffmann-La Roche Inc. to develop and commercialize antibodies that target a-synuclein, including prasinezumab; and master collaboration agreement with Bristol Myers Squibb to develop and commercialize antibodies targeting tau, TDP-43. Prothena Corporation plc was incorporated in 2012 and is based in Dublin, Ireland.

Stock analysis

Prothena Corporation plc (PRTA) currently trades at $8.68, while our model-based Fair Value estimate is $5.14, implying the stock looks roughly 68.9% overvalued today.

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Valuation

How firm this estimate is: it rests on 1 models at a data quality of 95/100, which puts the evidence level at low.

Scenario range: $3.39 (bear) to $6.42 (bull), the price of $8.68 sits above it. Bear and bull are the same models on cautious and optimistic assumptions, a range, not a price target.

Quality & growth

The Quality Score stands at 37/100 (below-average quality), in the Healthcare sector.

Weak Growth: Revenue growth is inconsistent: the periods point in different directions, so there is no trend to rely on.

Prothena Corporation plc reported revenue of $9.7M in FY2025 versus $201M in FY2021, a compound −53.1%/yr. Reported net income was −$244M in FY2025.

Key figures

Market cap $467M · P/S ratio 8.07 · EPS (TTM) $−2.81 · Net margin −261% · Return on equity −40.3% · Return on assets (EBIT) −23.5% · Operating margin 50.5% · Revenue growth (YoY) +17.1%.

Competitive moat

Our AI-assisted moat analysis scores the competitive advantage at 28 out of 100 (medium confidence).

What moves the price

The share trades about 25% below its 52-week high and 10% above its 52-week low, currently below its 200-day average.

For context, the median of 10 Healthcare peers we cover trades at −5% fair-value upside, at −41%, PRTA screens richer than that median.

Fair Value models

Bear $3.39 Fair Value $5.14 Bull $6.42
Model ⓘEach model values the company its own way (discounted cash flow, earnings, assets, dividends). The fair value above is their evidence-weighted blend, not the output of a single model. Bear ⓘBear = the cautious scenario: the same model computed with conservative anchors (lower growth, margins and valuation multiples). Together with Bull it frames a plausible valuation range, not a price target.BaseBull ⓘBull = the optimistic scenario: the same model computed with favourable anchors (higher growth, margins and valuation multiples). Together with Bear it frames a plausible valuation range, not a price target. Evidence ⓘEvidence = how well-backed THIS model's estimate is for this stock (0–100): how complete and reliable its input data are, and how well the model fits the company. The final Fair Value is an evidence-weighted blend, so better-evidenced models count more. Green ≥70, amber 50–69, grey below 50.
Highest evidence
NCAV (Graham) $2.68 $3.59 $5.36 54
All 1 models by family
Asset-Based
NCAV (Graham) $2.68 $3.59 $5.36 54

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Quality Score breakdown

Overall quality 37/100

Of which business quality 34 · Market factors (momentum, volatility) 45

Profitability 1
Margins and returns on capital today
Quality Growth 0
Are margins and returns improving?
Cashflow 4
Earnings quality: real cash, not paper profit
Fin. Strength 71
Balance sheet, leverage, solvency risk
Investment 100
Disciplined investing over empire-building
Low Volatility 63
Calm price path (market factor)
Momentum 41
Price trend over the last 3–12 months (market factor)
52W Momentum 30
Distance to the 52-week high (market factor)
Net Issuance 58
Share count: buybacks or dilution?

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Revenue & earnings trend

Growth Quality ⓘGrowth is valuable only when reinvestment earns attractive returns (Aswath Damodaran): sustainable growth depends on return on capital, reinvestment and efficiency. Basis: total company revenue over the periods shown on the cards (3Y/5Y/all years), not per share, so a merger can show up as a growth jump; per-share earnings growth is in the value-creation card. 30/100
Revenue growth is inconsistent: the periods point in different directions, so there is no trend to rely on.
Revenue growth 1 year
−92.8%
Revenue growth 3 years ⓘMeasures the company's TOTAL revenue from reported annual statements: compound average growth per year (CAGR) from the fiscal year 3, 5 or N years back to the most recent comparable fiscal year. Not per share: buybacks do not change this number, and a merger can appear as a growth jump. Per-share growth (which does include buybacks) is in the value-creation card.
−43.6%
Revenue growth 5 years ⓘMeasures the company's TOTAL revenue from reported annual statements: compound average growth per year (CAGR) from the fiscal year 3, 5 or N years back to the most recent comparable fiscal year. Not per share: buybacks do not change this number, and a merger can appear as a growth jump. Per-share growth (which does include buybacks) is in the value-creation card.
+62.6%
Start year 2020 (pandemic). Over 10 years: +19.7% a year
Revenue growth 15 years ⓘMeasures the company's TOTAL revenue from reported annual statements: compound average growth per year (CAGR) from the fiscal year 3, 5 or N years back to the most recent comparable fiscal year. Not per share: buybacks do not change this number, and a merger can appear as a growth jump. Per-share growth (which does include buybacks) is in the value-creation card.
+14.7%
Profit margin (trend) ⓘOperating margin then vs now, from reported annual statements. A falling margin after a boom year is the classic peak-earnings signal: the growth rate then leans on an inflated base.
−13,216.2% (2020) → −1,905.8% (2025)
What shareholders gained per year ⓘWe only publish this rate when it is defensible. Reason: fiscal 2025 is a loss year, no rate is defined from a loss
not computed

PRTA screens 69% overvalued. Compare with Vertex Pharmaceuticals Incorporated →

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Peer GroupⓘHow this stock ranks against its industry: the green marker is this stock, the band is the typical 25–75% peer range, and the tick is the median.Biotechnology · 651 stocks

Beats the industry median on 4/9 measures
A mixed picture versus its industry peers.
Valuation
Quality Score 35 · Below median
Fair Value upside −43% · Above median
Profitability
Return on assets −14% · Below median
Operating margin (TTM) 50% · Top 25%
Growth and dividend
Revenue growth 17% · Above median
Balance sheet
Debt / equity 0.13× · Above median

Valuation Multiplesvs Biotechnology median · lower = cheaper

P/B 1.73× · Pricier than median
P/S (TTM) 8.35× · Pricier than median
PEG 0.80× · Cheaper than median

Strength profile in five axes (Snowflake)

VALUEFUTUREPASTHEALTHDIVIDEND
This stock
VALUEFUTUREPASTHEALTHDIVIDEND
Sector peers
VALUE (fair-value potential)0 · sector 0
FUTURE (revenue growth)85 · sector 0
PAST (return on equity)0 · sector 0
HEALTH (low debt)94 · sector 97
DIVIDEND (yield)0 · sector 23

VALUE 0: the price sits above our fair-value range.

Context: sector, industry, market

Values & ESG

Does this company touch areas you may want to avoid? The classification is inferred from sector and industry.

None of the checked exposures detected

Similar stocks

10 more Biotechnology stocks, each showing price versus our Fair Value estimate.

Stock Price Fair Value vs Fair Value
Vertex Pharmaceuticals Incorporated VRTX $514.99 $566.49 +10%
Regeneron Pharmaceuticals, Inc REGN $803.90 $1,252 +56%
argenx SE ARGX $963.79 $918.60 −5%
Samsung Biologics Co 207940 1,366,000 KRW 1,502,600 KRW +10%
CSL Limited CSL A$179.24 A$197.16 +10%
Alnylam Pharmaceuticals, Inc ALNY $251.65 $218.36 −13%
Royalty Pharma plc RPRX $58.52 $24.22 −59%
Celltrion, Inc 068270 177,400 KRW 74,519 KRW −58%
BeOne Medicines AG ONC $368.08 $280.94 −24%
BioNTech SE BNTX $100.87 $63.62 −37%

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Cite: Fair Value Calculator (2026). "Prothena Corporation plc Fair Value". https://www.fairvalue-calculator.com/stock/PRTA

Frequently asked questions

Is Prothena Corporation plc (PRTA) overvalued or undervalued?
As of Sep 23, 2026, our model estimates a fair value of $5.14 versus a price of $8.68, about −41% upside (overvalued).
What is the fair value of PRTA?
Our model-based fair value for Prothena Corporation plc is $5.14 (as of Sep 23, 2026), built from audited fundamentals. The current price: $8.68.
What is the quality score of PRTA?
Prothena Corporation plc has a Quality Score of 37/100. It measures business quality (profitability, growth, cash flow, balance-sheet strength, investment discipline, share issuance). Market factors such as price momentum and volatility do not enter the number; they are shown separately in the detail view.
What is the price target for Prothena Corporation plc (PRTA)?
Our model-based price target is the fair value of $5.14 (as of Sep 23, 2026) from 1 valuation models. Cautious scenario $3.39, optimistic scenario $6.42. It is a calculation from audited fundamentals, not an analyst target.
What is the Prothena Corporation plc stock forecast for 2026?
Our models put fair value at $5.14, about −41% upside versus a price of $8.68 (overvalued). Cautious scenario $3.39, optimistic scenario $6.42. The calculation is refreshed regularly with new filings.
What is the intrinsic value of Prothena Corporation plc (PRTA)?
Intrinsic value and fair value mean the same thing: what the business is worth on its numbers, independent of the share price. For Prothena Corporation plc it is $5.14 per share (as of Sep 23, 2026), against a price of $8.68. It is the blended result of 1 valuation models (cash flow, earnings, asset, dividend).
Is Prothena Corporation plc stock overvalued or undervalued in 2026?
As of Sep 23, 2026, PRTA trades above its calculated fair value: price $8.68, fair value $5.14, a gap of about −41% (overvalued). The calculation is refreshed with each new quarterly report, so the verdict can change during the year.
Is fair value the same as the market price of PRTA?
No. The price is what the market pays today ($8.68); the fair value is what the company's own numbers justify ($5.14). For Prothena Corporation plc the two are $3.54 per share apart. That gap is exactly why we show both numbers side by side.
How much is Prothena Corporation plc worth?
The market values Prothena Corporation plc at about $467M (market capitalisation, as of Sep 23, 2026). Per share that is $8.68; our models calculate a fair value of $5.14 per share.
What do the bullish and bearish scenarios say about PRTA?
Our models span a range for Prothena Corporation plc: cautious scenario $3.39, base $5.14, optimistic $6.42 per share (as of Sep 23, 2026, price $8.68). The range comes from different growth and margin assumptions, not from analyst opinions.
What is the PEG ratio of PRTA?
The PEG ratio of Prothena Corporation plc is 0.80 (P/E divided by earnings growth, as of Sep 23, 2026). That is below 1, so growth is priced more cheaply than the earnings multiple alone suggests.
How solid is the balance sheet of Prothena Corporation plc (PRTA)?
Balance-sheet figures for Prothena Corporation plc (as of Sep 23, 2026): return on equity −40.3%, debt of 0.13 per unit of equity. They feed the Quality Score of 37/100, which measures business quality independently of the share price.
How far is PRTA from its 52-week high?
Prothena Corporation plc trades at $8.68, about 25% below its 52-week high of $11.51 and 10% above the low of $7.91 (as of Sep 23, 2026). Distance from the high says nothing about value: that is what the fair value of $5.14 is for.
Which stocks are comparable to Prothena Corporation plc?
From the same area (Healthcare) we also value Vertex Pharmaceuticals Incorporated, Regeneron Pharmaceuticals, Inc, argenx SE, Samsung Biologics Co, among others. Each of them has its own fair-value calculation on this site using the same models, so the comparison is like for like.
Is Prothena Corporation plc stock attractive at the current price?
The data as of Sep 23, 2026: price $8.68, calculated fair value $5.14 (−41%), Quality Score 37/100, from 1 models. Whether that fits your horizon and risk tolerance is your call. We provide the calculation, not investment advice.
How is the fair value of PRTA calculated?
We run Prothena Corporation plc through 1 models from four families: discounted cash flow, earnings models, asset and balance-sheet models, and dividend models. Every model gets the same audited fundamentals; the result is the weighted average of $5.14, with the spread shown as a cautious and an optimistic scenario.
Is it still worth investing in stocks now?
The global stock market currently sits 14.1 % above its aggregate fair value. Prothena Corporation plc itself currently trades above fair value. An expensive market is no reason to stay out and no reason to buy everything at once: keep a broad savings plan running, buy single stocks only with money you can do without, and hold part of your money liquid while the market is high. The full picture including sectors is on is it worth investing now.
What is the share price of Prothena Corporation plc (PRTA)?
The closing price on Sep 23, 2026 was $8.68. Our model-based fair value is $5.14, about −41% upside (overvalued). We carry closing prices from our data provider, not a live tick.
What should I pay attention to with Prothena Corporation plc right now?
The price sits above even our optimistic bull case ($6.42). The favourable scenario is already priced in. Weak quality (37/100) and above fair value at the same time, the margin of safety is missing on both counts. The large gap to fair value rests on thin data (low evidence): fewer applicable models and a shorter history. Read it with extra caution. A fairly wide model range ($3.39 to $6.42) leaves room in how you read the outcome.

Key figures of Prothena Corporation plc

How large is the market capitalisation of Prothena Corporation plc (PRTA)?
The market capitalisation of Prothena Corporation plc is $467M. The stock-market value of the whole company: share price times number of shares. It is what the market asks for the entire business today.
What is the P/S ratio of Prothena Corporation plc (PRTA)?
The price-to-sales ratio of Prothena Corporation plc is 8.07 (last twelve months). Price to sales: market value relative to yearly revenue. Useful when profit is thin or distorted.
What are the earnings per share of Prothena Corporation plc (PRTA)?
Earnings per share at Prothena Corporation plc are $−2.81. Earnings per share over the last twelve months: total profit spread across every single share.
What is the net margin of Prothena Corporation plc (PRTA)?
The net margin of Prothena Corporation plc is −261% (last twelve months). How much of every unit of revenue ends up as profit. 10% means 10 cents of profit per dollar of sales.
What is the return on equity of Prothena Corporation plc (PRTA)?
The return on equity (ROE) of Prothena Corporation plc is −40.3% (last twelve months). Profit relative to shareholders' equity, how efficiently the company works with its owners' money.
What is the EBIT return on assets of Prothena Corporation plc (PRTA)?
On an EBIT basis the return on assets of Prothena Corporation plc is −23.5% (avg 5y). Operating profit (EBIT) relative to everything the company owns, as a multi-year average. It describes the business model rather than one good or bad year.
What is the operating margin of Prothena Corporation plc (PRTA)?
The operating margin of Prothena Corporation plc is 50.5% (last twelve months). Profit from the core business (before interest and taxes) relative to revenue.
How fast is revenue growing at Prothena Corporation plc (PRTA)?
Revenue at Prothena Corporation plc is growing +17.1% versus a year earlier (3y avg −43.6%). How much revenue grew versus a year earlier (YoY). The 3-year average next to it puts the single year in context.
How much free cash flow does Prothena Corporation plc (PRTA) generate?
The free cash flow of Prothena Corporation plc is −$164M (fiscal year 2025). The cash truly left after running and investing in the business, this is what pays dividends and buybacks.
How much net cash does Prothena Corporation plc (PRTA) hold?
Prothena Corporation plc holds more cash than debt, $294M net (fiscal year 2025). The company holds more cash than debt, a safety cushion.
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