Solaris Energy Infrastructure (NYSE: SEI) joins S&P SmallCap 600 on July 15 as Catalyst Pharmaceuticals exits on Angelini Pharma deal

Solaris Energy Infrastructure Inc. (NYSE: SEI) will join the S&P SmallCap 600 before the opening bell on Wednesday, July 15, 2026, replacing Catalyst Pharmaceuticals Inc. (NASD: CPRX). The slot opens because Angelini Pharma S.p.A. is acquiring Catalyst, which disqualifies the pharma company from index membership once the transaction closes. S&P Global announced the reconstitution on July 9, leaving less than a week for the market to price in the mechanics before the effective date.

The mechanics behind the slot

This is not a discretionary selection based on Solaris's financials. Catalyst Pharmaceuticals exits the index because an acquisition by Angelini Pharma S.p.A. removes it from the eligible pool of publicly traded domestic equities. Solaris fills the resulting vacancy. That sequencing matters for the tape: SEI enters the index because CPRX is leaving it, and the trigger is a deal, not a committee judgment on either company's merits.

Funds benchmarked to the S&P SmallCap 600 are required to hold the index's constituents in line with its composition. Those vehicles need SEI on their books before Wednesday's open.

What the setup looks like

Mechanical demand arrives with the reconstitution. Passive managers tracking the index add SEI before July 15, regardless of the price at which they execute. The announcement does not provide a figure for total assets benchmarked to the index, so the aggregate scale of that buying is not quantifiable from this source.

For Catalyst Pharmaceuticals shareholders, the index exit is a byproduct of the Angelini Pharma S.p.A. transaction rather than a separate event. The operative question for CPRX holders remains when the acquisition closes, since that completion is what makes the index removal permanent.

What to watch

July 15, prior to the open, is the confirmable milestone already on the calendar. In the session that follows, the watch point for SEI is whether the stock holds any pre-announcement level once passive buying completes. The Angelini Pharma S.p.A. deal close is the final event that settles both the acquisition and Catalyst's exit from the S&P SmallCap 600.