William P. Scully, identified as a 10% owner of Prothena Corp Public Ltd Co (PRTA), purchased 103,500 shares in an open-market transaction on September 28, 2026, according to an SEC Form 4 summary. The summary lists a price of $8.60 per share and a transaction value of $889,872.

The SEC item was published on September 29, 2026, at 21:37:37 UTC. That is the publication timestamp supplied for the filing report; the provided material does not separately specify the filing’s submission time. The purchase date and the report’s publication date therefore refer to different events.

Purchase details and a numerical caveat

The supplied summary classifies Scully as a 10% owner. It does not identify him as a director or executive, and that ownership designation alone does not establish either role.

The reported figures also warrant a precision caveat. Multiplying 103,500 shares by the displayed $8.60 price produces $890,100, which is $228 above the stated transaction value. The supplied material does not explain that difference or provide a more precise per-share figure. Accordingly, $889,872 is the reported value, rather than a total independently reconciled from the displayed price.

The summary says the purchase was not made under a Rule 10b5-1 plan. That establishes the reported plan status, but does not explain Scully’s decision to buy or the considerations behind the transaction.

What the disclosure establishes

The reported facts describe one owner’s purchase of Prothena shares. They do not establish his complete holdings after the transaction, his intended holding period, or whether additional purchases are planned. No explanation from Scully accompanies the supplied material.

Analysis: The transaction provides a concrete data point about an identified owner’s buying activity. Its value as an investment signal is limited by the absence of broader ownership context and any stated rationale; it does not establish a change in Prothena’s operating performance or prospects.

What to watch

Further filing detail could clarify the price-and-value discrepancy and Scully’s resulting holdings. Subsequent disclosures would be needed to establish any additional transactions.