Pfizer opened the floodgates with a clean Phase 2 readout. Sanofi just became the first runner to stumble in the IL-4/13 bispecific gold rush — and the field is moving fast enough that nobody is waiting to find out if it is alone.
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Pfizer's trispecific antibody tilrekimig achieved statistically significant Phase 2 efficacy in atopic dermatitis with a 51.9% placebo-adjusted EASI-75 response rate and a clean safety profile, positioning it for Phase 3 this year. The readout has triggered a competitive land rush in the IL-4/13/TSLP space, with Sanofi's bispecific lunsekimig posting mixed results (asthma/sinusitis success but eczema miss) and J&J terminating its $1.25 billion acquisition JNJ-5939 after an interim analysis revealed it failed to meet internal efficacy thresholds.
Something changed in the IL-4 and IL-13 immunology space, and it happened fast. Five bispecific drug candidates targeting these pathways have hit the news in the past month — a pace that looks less like coincidence and more like a land rush. The trigger was Pfizer's announcement in early March that its trispecific antibody tilrekimig posted positive Phase 2 results in atopic dermatitis, clearing an efficacy bar that the field had been watching. Within weeks, Sanofi had mixed Phase 2 data on its candidate, J&J had quietly stopped a $1.25 billion asset in the same indication, and smaller companies from the US to China were publicly advancing their own versions.
The pattern is worth sitting with. When a large pharmaceutical company posts a clean Phase 2 readout in a validated but crowded mechanism, it tends to do two things: convince investors that the target is still commercially viable, and prompt every other company with a similar drug to talk about its own data before the window closes. That is what appears to be happening here.
Pfizer's tilrekimig is the clearest data point. The drug simultaneously targets IL-4, IL-13, and TSLP — three pathways implicated in the Type 2 inflammation that drives atopic dermatitis, asthma, and COPD. Phase 2 results showed statistically significant EASI-75 responses across all doses tested, with the middle dose delivering a 51.9 percent placebo-adjusted response rate. The safety profile was clean, adverse events were comparable to placebo, and notably the rate of conjunctivitis — a known side effect of IL-4 receptor inhibitors — was lower than with existing drugs. Pfizer is moving the candidate into Phase 3 for atopic dermatitis this year, with Phase 2 work ongoing in asthma and COPD.
That positive result appears to have sharpened the competitive focus across the field. Sanofi reported mixed Phase 2 data for its bispecific lunsekimig: the candidate hit endpoints in asthma and chronic sinusitis but missed in eczema. The company is still advancing it in atopic dermatitis with additional studies planned — but the eczema miss is a real crack in what the company had hoped would be a cleaner profile.
J&J's stoppage is the most concrete signal that the race has real attrition built into it. The company began a Phase 2b trial for JNJ-5939, a bispecific targeting IL-4 and IL-31, in late February 2025. Ten months later it terminated the study after an interim analysis showed the asset was not meeting the high efficacy bar J&J had set internally. The $1.25 billion upfront J&J paid Numab Therapeutics for the asset in 2024 is now a write-down of sorts. J&J has other candidates in its pipeline — including a bispecific acquired from Proteologix that targets IL-13 and TSLP — but the failure of JNJ-5939 is a reminder that mechanism validation in theory does not always survive contact with Phase 2 data.
The competitive landscape the Pfizer result crystallized spans companies far smaller than the top-five names. Aclaris Therapeutics has ATI-052, an anti-TSLP and IL-4R bispecific, in Phase 1b for asthma, with Phase 2b planned for the second half of 2026. Akeso's AK139 — an IL-4R and ST2 bispecific — was cleared for seven Phase 2 studies in China across respiratory and autoimmune indications. The company called it the first IL-4R/ST2 bispecific to enter clinical development anywhere. Separately, Earendil Labs announced a collaboration with Sanofi worth up to $2.5 billion to discover additional bispecific antibodies for autoimmune diseases, using AI-driven research to fuel the pipeline.
What connects all of these programs is Dupixent — the Sanofi and Regeneron drug that established IL-4 and IL-13 inhibition as one of the most commercially productive mechanisms in immunology. Dupixent posted $14.15 billion in global sales in 2024 and continues to grow. The commercial success validated the target and opened a pathway for anything that could demonstrably improve on it. The bet that bispecific and trispecific antibodies can be that improvement is what is driving the current concentration of programs.
The question the field has not answered is whether hitting two or three targets simultaneously produces meaningfully better clinical results than hitting one, and at what cost in safety and manufacturing complexity. J&J's failure suggests that adding a second target does not automatically improve outcomes — the mechanism still has to clear the bar the company sets for itself. Pfizer's positive result suggests that adding a third target, or choosing the right combination, might. That distinction has not stopped the field from moving fast in both directions.
The race is on. The outcome is not determined. What is clear is that the first positive Phase 2 result in a validated mechanism can mobilize an entire drug development ecosystem in weeks — and that not everyone who enters the race will finish it.
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Research completed — 10 sources registered. Pfizer tilrekimig (PF-07275315) trispecific positive Phase 2 atopic dermatitis March 2026: EASI-75 responses 38.7-51.9% placebo-adjusted across doses,
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