Dupixent Lawsuit

From the record

A large-scale observational study analyzing 19,612 dupilumab-treated patients matched against a control cohort found a statistically significant 4.5-fold increased risk of cutaneous T-cell lymphoma (CTCL) among Dupixent users. The study controlled for confounding variables including baseline atopic dermatitis severity, age, sex, and prior immunosuppressive therapy use. Investigators noted that the risk signal persisted after sensitivity analyses designed to account for potential diagnostic unmasking, suggesting that the association may reflect a true pharmacological effect rather than simply improved detection of pre-existing disease.

From the record

The FDA's Adverse Event Reporting System has accumulated over 300 reports associating dupilumab use with cutaneous T-cell lymphoma diagnoses, including mycosis fungoides, Sezary syndrome, and other T-cell lymphoproliferative disorders. Reports span the period from initial commercial availability in 2017 through 2025, with a notable acceleration in reporting frequency beginning in 2022. Many reports describe CTCL diagnoses occurring 6 to 36 months after Dupixent initiation in patients who were being treated for presumed atopic dermatitis. The reports include submissions from dermatologists, oncologists, and patients, indicating awareness of the safety signal across multiple clinical stakeholder groups.

From the record

Between 2021 and 2025, at least 25 published case reports and case series in peer-reviewed dermatology journals documented CTCL diagnoses in patients receiving dupilumab therapy. Cases include both new-onset mycosis fungoides in patients with no prior lymphoma history and rapid progression of previously occult CTCL that became clinically apparent after Dupixent initiation. Several reports describe a characteristic pattern: patients with long-standing eczema who showed partial improvement on Dupixent before developing treatment-resistant patches that, upon biopsy, revealed malignant T-cell infiltrates diagnostic of CTCL. The published case literature spans multiple countries and healthcare systems, arguing against a localized diagnostic artifact.

From the record

What researchers found when they systematically analyzed the FDA Adverse Event Reporting System database was a pattern that should have alarmed Regeneron and Sanofi years before it alarmed the FDA. Over 300 adverse event reports linked Dupixent to lymphoma and blood cancers — with cutaneous T-cell lymphoma representing the most frequently reported malignancy. The FAERS signal was not subtle: the reporting odds ratio for CTCL in Dupixent users was significantly elevated compared to other biologic drugs used for inflammatory conditions. The signal had been building since 2019, two years after Dupixent's launch, but the pace of reports accelerated sharply in 2023 and 2024 as the drug's patient population expanded past 30 million prescriptions. FAERS data is inherently limited — it captures only voluntarily reported events and likely underestimates true incidence by a factor of 5 to 10. That means the 300+ reports may represent only the tip of a much larger iceberg. The FDA placed Dupixent on its quarterly drug safety watchlist in March 2025 based on this FAERS analysis and escalated to a formal investigation in September 2025 when it determined the signal warranted regulatory action. The investigation remains ongoing as of April 2026. For plaintiffs' attorneys, the FAERS timeline is critical: if discovery reveals that Regeneron's own pharmacovigilance team identified the CTCL signal in FAERS data before the FDA did — and failed to report it proactively or update the drug label — it would support claims of fraudulent concealment and potentially trigger punitive damages.

From the record

When independent researchers went back to reanalyze the pivotal clinical trial data that supported Dupixent's FDA approval, they found something that should have been caught during the original review process. The SOLO 1, SOLO 2, and CHRONOS trials — which collectively enrolled over 2,100 patients — were designed to measure eczema symptom improvement over 16 to 52 weeks. They were not designed, powered, or monitored for cancer outcomes. The trials excluded patients over age 75, patients with significant comorbidities, and patients with a history of malignancy — precisely the populations that might be most susceptible to immune-mediated cancer promotion. Follow-up periods were too short to capture malignancies with long latency periods. And the trial monitoring protocols did not include systematic skin biopsy surveillance that would have been necessary to detect early-stage CTCL. The reanalysis identified several patients in the active treatment arms who developed unexplained dermatologic events — worsening skin lesions, treatment-resistant patches, and atypical skin findings — that were classified as adverse events related to atopic dermatitis rather than investigated for malignancy. Some of these events, when reviewed retrospectively with current knowledge about Dupixent and CTCL, are consistent with early-stage mycosis fungoides that was misclassified as eczema flare. The researchers concluded that the clinical trial program was structurally incapable of detecting the CTCL signal, and that Regeneron's post-market pharmacovigilance should have compensated for this gap far earlier than it did.