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REGN fell 10%. The numbers tell a complicated story.

Regeneron (REGN) just took the kind of hit that makes your watchlist feel heavy, and this one has a few layers worth unpacking before you do anything.

On May 15, 2026, Regeneron disclosed that its Phase 3 trial of fianlimab (a LAG-3 inhibitor) combined with cemiplimab (its own PD-1 drug, Libtayo) did not reach statistical significance for the primary endpoint – progression-free survival (PFS) versus pembrolizumab (Keytruda) – in patients with first-line unresectable or metastatic melanoma. The trial enrolled 1,546 patients. By Monday’s session, shares had fallen roughly 10% to around $620, erasing close to $9 billion in market value in a single day.

Here’s what’s frustrating about this one. The data wasn’t bad. It just wasn’t enough.

The high-dose fianlimab arm posted a median PFS of 11.5 months. Keytruda came in at 6.4 months. That is a 5.1-month numeric improvement in a disease where every month matters. But the hazard ratio didn’t clear the statistical significance bar, which means regulators won’t act on it, and Wall Street immediately started removing the program from valuation models. At least 10 brokerages cut their price targets. Citi downgraded to Neutral from Buy and slashed its target from $900 to $700. RBC lowered its target to $707, having previously projected peak probability-adjusted sales of $1.6 to $1.8 billion for fianlimab in this indication – all of that is now gone.

Why this stings more than a typical miss

This isn’t happening in isolation. Three weeks earlier, REGN dropped 7.1% after Q1 2026 earnings showed adjusted operating income of $642.9 million – a 32.4% miss versus analyst expectations. Operating margin contracted to 17.8% from 19.5% a year ago. Eylea sales fell 10% in the quarter, already under pressure from biosimilar competition expected to intensify in the second half of 2026. The fianlimab melanoma result was supposed to be the pipeline story that offset those concerns.

Slight tangent, but it matters: LAG-3 as a mechanism has been largely disappointing across the industry. BofA noted internally that it had been cautious on a positive outcome specifically because the LAG-3 class has a thin track record, and slow event accrual in the trial had already forced multiple protocol amendments before this readout. The market wasn’t completely blindsided – but the size of the numeric improvement made people hopeful anyway. That hope is expensive when it unravels.

What matters now is context. Regeneron still has Dupixent, co-developed with Sanofi, which continues to expand into new indications and has been the clearest growth engine in the portfolio. BofA reiterated its Buy at $860, and Jefferies held its Buy at $870 – both keeping fianlimab’s melanoma revenue at zero in their models and still finding the stock attractive at current levels. That’s actually useful information.

So where I’m at: the 10% drop doesn’t automatically make this cheap. It removes a specific future revenue stream that some models were pricing in, and it raises a harder question about whether management can rebuild confidence across the oncology pipeline after what BMO called “back-to-back key pipeline misses” in the first half of 2026.

Three things worth watching from here:

• The separate Phase 3 trial comparing fianlimab head-to-head against Bristol Myers’ Opdualag is still ongoing – at least two brokerages have low confidence in that outcome now, but it’s not over
• Eylea HD’s biosimilar pressure in H2 2026 is the real slow-burn risk that doesn’t get enough attention relative to the trial headlines
• Dupixent’s continued label expansion is the floor under this stock – how wide that floor holds is the question you actually need to answer before adding shares

REGN is down about 19% year to date and is trading roughly 22% below its 52-week high of $812. The P/E sits around 17x – moderate for a large-cap biotech with Dupixent’s growth profile. Whether that multiple makes sense without a credible oncology pipeline story is what the next 12 months will answer.

Full breakdown worth your time before you decide anything.