The purchase is too small relative to market cap to be a strong standalone signal, but the insider's 10% owner status and recent buying pattern warrant monitoring. Watch for additional insider purchases or a material catalyst (e.g., pipeline update, partnership) that could confirm the bullish view. The next catalyst to watch is the upcoming earnings or clinical data readout.
Price Chart
Executive Summary
William P. Scully, a 10% owner of Prothena, purchased 27,000 shares on the open market for approximately $256K at $9.47/share. This is a standalone conviction buy from a major insider, representing about 0.05% of the company's $497M market cap, which is below the 0.1% threshold for materiality. The purchase is modest in size relative to market cap, but the insider's status as a 10% owner and the recent cluster of insider buys ($1.0M over 30 days) add some weight, though historical performance on PRTA insider-buy signals has been mixed with a 50% win rate at T+20.
Key Financial Metrics
Key Facts
- 10% owner William P. Scully purchased 27,000 shares at $9.47/share for $256K on 2026-09-02.
- The purchase represents ~0.05% of PRTA's $497M market cap, below the 0.1% materiality threshold.
- This is a single insider buy, not a cluster; the broader 30-day insider activity shows $1.0M in total buys.
- Historical PRTA insider-buy signals have a 50% win rate at T+20 with an average alpha of +0.21% vs SPY.
Financial Impact
$256K open-market purchase by 10% owner, representing ~0.05% of market cap
Risk Factors
- Purchase is modest at 0.05% of market cap, limiting its signaling power.
- Historical insider-buy signals on PRTA have been mixed, with a 50% win rate at T+20.
- No accompanying material corporate event (e.g., earnings, pipeline update) to contextualize the buy.
Market Snapshot
Investment Themes
Documents Analyzed
This report is based on 1 SEC document filed with EDGAR.
| Document | Accession Number |
|---|---|
| INSIDER-BUY Data (Synthetic) | 0001045463-26-000015 |
Filters
| Type | Now | ||||
|---|---|---|---|---|---|
Sep 3, 2026 28d ago | Insider Buy | $9.82 $8.86 | ▼ −9.78% | ▼ −8.57% | $8.44 (−14.05%) |
Aug 21, 2026 6w ago | Insider Buy | $9.45 $9.33 | ▼ −1.27% | ▼ −1.74% | $8.44 (−10.69%) |
Aug 10, 2026 7w ago | Insider Buy | $9.12 $8.88 | ▼ −2.63% | ▼ −2.58% | $8.44 (−7.46%) |
Jul 31, 2026 9w ago | Insider Cluster | $8.12 $8.76 | ▲ +7.88% | ▲ +4.37% | $8.44 (+3.94%) |
Jun 18, 2026 15w ago | Insider Buy | $8.93 $9.27 | ▲ +3.81% | ▲ +5.47% | $8.44 (−5.49%) |
Jun 12, 2026 15w ago | Insider Buy | $8.15 $9.15 | ▲ +12.27% | ▲ +13.65% | $8.44 (+3.56%) |
May 14, 2026 20w ago | 3 | $9.76 $9.83 | ▲ +0.72% | ▼ −0.25% | $8.44 (−13.52%) |
May 7, 2026 21w ago | 8-K | $10.73 $9.86 | ▼ −8.11% | ▼ −9.57% | $8.44 (−21.34%) |
Mar 27, 2026 26w ago | DEFA14A | $9.25 $10.36 | ▲ +12.00% | ▲ +7.70% | $8.44 (−8.76%) |
Mar 3, 2026 30w ago | EFFECT | $9.36 $9.77 | ▲ +4.38% | ▲ +4.86% | $8.44 (−9.83%) |
US Market Status
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