Regeneron LAG-3 Phase 3 Failure Crashes Stock 11%; Supreme Court Cements IRA

Regeneron's LAG-3 inhibitor fianlimab crashes out in Phase 3 melanoma, sending the stock down over eleven percent and putting the entire checkpoint combination class on notice.

The Supreme Court locks in IRA drug pricing, AstraZeneca lands a first-in-class hypertension approval, and Merck's China-licensed ADC delivers a Phase 3 win in endometrial cancer. Full Monday ahead.

Welcome to The Pharma Closeout for Monday, May 18th, 2026. I'm Alex Mercer.

And I'm Maya Patel. Let's get into it.

Top story. Regeneron's Phase 3 trial of fianlimab — their LAG-3 inhibitor — combined with Libtayo versus Keytruda monotherapy in advanced melanoma missed its primary endpoint. No statistically significant improvement in progression-free survival. This was Regeneron's bid to break into frontline melanoma and challenge Merck's dominant franchise, and it fell short. Results were announced Thursday. The market is delivering its verdict today.

What makes this particularly painful — the numerical trend reportedly favored the combination arm. But numerical trends without statistical significance don't get you a label change. You go head-to-head against pembrolizumab in melanoma, you're challenging the undisputed standard of care. You need unambiguous separation. The confidence intervals didn't deliver it, and in a registrational setting, that closes the door.

REGN dropped 11.1% today — down $77.25 from a prior close of $698.25, landing around $619. Intraday range: $605 to $630. That's roughly eight billion in market cap erased in a single session. BMO Capital's Evan David Seigerman called this, quote, "the defining catalyst of 1H26, with share sentiment inextricably tied to this release." Price target cuts came immediately — BMO, Citi, RBC Capital, Evercore ISI, Leerink Partners. All before lunch.

I want to push back on the reflex to call this a full class failure. There's a scenario where LAG-3 co-blockade works in a biomarker-defined subset — where co-expression patterns on tumor-infiltrating lymphocytes predict response — but this trial wasn't enriched for that. The open question: does anyone have the risk tolerance to run a biomarker-selected registration study after back-to-back Phase 3 misses in the class?

Probably not. Pharmaphorum framed this as "another LAG-3 setback." FierceBiotech reported analysts "shaking heads." The pattern is getting hard to dismiss — these combinations keep failing to separate from PD-1 monotherapy in adequately powered trials.

The preclinical signal on exhausted T-cell rescue was the entire premise for this class. Dual checkpoint blockade was supposed to reinvigorate those populations beyond what PD-1 inhibition achieves alone. What we're seeing clinically is that the preclinical models may have been showing us the ceiling, not the floor. That rewrites the investment thesis.

Regeneron pivoted fast. Same morning, they announced a $125 million collaboration with Parabilis Medicines — $50 million upfront, $75 million equity commitment, up to $2.2 billion in potential milestones — to develop antibody-peptide conjugates against historically undruggable targets. The timing speaks for itself.

Crisis-mode IR management. The Parabilis platform may have real legs — constrained peptide conjugates are a legitimate modality — but you don't drop a preclinical-stage collaboration at dawn on the day your stock is in freefall unless you're managing the narrative. The deeper question for Regeneron: can the oncology franchise be rebuilt around a different biological hypothesis, or does fianlimab's failure narrow the long-term value story back to Dupixent and Eylea?

That's the overhang Regeneron carries forward. Shifting to the deal side — a very different morning at Merck. Their ADC sac-TMT, sacituzumab tirumotecan, licensed from China's Kelun-Biotech, hit both co-primary endpoints — overall survival and progression-free survival — in the Phase 3 TroFuse-005 trial in advanced or recurrent endometrial cancer. Patients had progressed after platinum-based chemo and anti-PD-1 immunotherapy. And per Endpoints News, the trial read out ahead of schedule.

Per Merck's own announcement, sac-TMT is the first TROP2 ADC to demonstrate statistically significant improvements in both OS and PFS versus chemotherapy in this patient population. Dual-endpoint wins in endometrial cancer are uncommon — and this validates the Kelun licensing strategy at exactly the moment Merck needs post-Keytruda pipeline credibility. The ahead-of-schedule readout also shortens the runway to a regulatory filing.

Two more items to flag. Enhertu locked down dual FDA approvals in early breast cancer — we flagged this Friday, and the labels are now official. And Bristol-Myers Squibb is evaluating Houston for a potential major manufacturing campus — a substantial facility expected to create hundreds of jobs, per a state filing. BMS made a significant commitment to U.S. manufacturing expansion last year, and this is the latest concrete signal of execution. With tariff policy still unsettled and political pressure to onshore intensifying, domestic manufacturing capacity is becoming a strategic asset, not just a cost center — and the companies building early will have leverage when the rules finally settle.

And on the regulatory front, the biggest structural development today wasn't an approval — it was the Supreme Court. The Court declined to hear challenges to the IRA's Medicare drug price negotiation provisions brought by several major pharmaceutical companies including AstraZeneca, Boehringer Ingelheim, Bristol Myers Squibb, Janssen, Novartis, and Novo Nordisk, per STAT News. All turned away.

That's the end of the legal road. The industry's best shot at blocking Medicare negotiated pricing through the courts just died. CMS now has effective judicial blessing to proceed with expanding price negotiations. Every company with a high-revenue drug approaching that negotiation window is recalculating long-term revenue projections tonight.

This is where the budget conversation gets real. With that legal uncertainty gone, negotiated pricing isn't a temporary headwind — it's permanent infrastructure. Revenue models that assumed a court intervention would eventually limit CMS authority need to be rebuilt from the ground up. The discount curve just steepened for every large-cap product nearing that window.

On the approval side, the FDA has approved AstraZeneca's Baxfendy — baxdrostat — a first-in-class aldosterone synthase inhibitor for hypertension in adults not adequately controlled on existing therapy. Novel mechanism, massive therapeutic area, and the first and only approved drug in this class. This gives AstraZeneca a meaningful cardiovascular foothold in an area where they've been overshadowed by their oncology and respiratory franchises.

A correction from Friday. We discussed Sara Høeg's departure from CDER as an expected transition. She has since confirmed on X — her exact words — "I was fired." After six months as acting CDER head, she's out. Michael Davis replaces her. Combined with Makary's earlier departure, the FDA leadership vacuum is real and widening. When the acting CDER head publicly states she was terminated, that's not orderly succession — it's institutional turbulence, and it raises pointed questions about review continuity and whether the drug review apparatus can maintain its operational tempo.

One safety signal worth tracking. Amgen is standing by Tavneos — avacopan — after reports that 20 patients have died in Japan following treatment. Per Reuters, Amgen's Japanese partner Kissei Pharmaceutical issued a safety notice citing serious liver dysfunction. Previously, multiple deaths had been linked to the drug.

Hepatotoxicity at this magnitude was not the expected safety profile from C5a receptor inhibition. Whether this is an off-target liver effect specific to avacopan or something inherent to the mechanism carries very different implications for the broader complement inhibitor class. That's what regulators in Tokyo and Washington need to untangle.

Looking ahead this week. Three items. First, FDA leadership — with Høeg fired and Makary gone, CDER's direction is genuinely uncertain. Watch for signals on permanent appointments or further departures, because the conversation about regulatory philosophy and review pace hinges entirely on who's at the helm. Second, Merck's filing timeline for sac-TMT. A dual-endpoint Phase 3 win ahead of schedule positions them for an accelerated submission — how fast they move tells us how urgently they're assembling the post-Keytruda narrative. And third, the LAG-3 landscape. Every other program in this class is reassessing trial design and patient selection after fianlimab's miss. If biomarker enrichment becomes the only credible path forward, several development timelines just got meaningfully longer.

And keep Tavneos on your radar. Twenty deaths activates formal regulatory machinery. Expect Japan's PMDA to move first, then watch whether the FDA opens a parallel safety review stateside. If that hepatotoxicity signal is confirmed as drug-related, label implications extend well beyond Japan.

That is your Pharma Closeout for Monday, May 18th. Regeneron's LAG-3 thesis takes a body blow in melanoma, the Supreme Court cements IRA drug pricing as settled law, AstraZeneca wins its first-in-class hypertension approval, and Merck's China-licensed ADC strategy gets its strongest proof point yet. A lot to process on a Monday — and this week is just getting started. If this briefing sharpens your morning, follow us on Spotify and drop a rating — it helps other pharma professionals find the show. We are back tomorrow.

Big week starting. See you tomorrow.