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Do You Qualify?
Eligibility Checklist
- Used Dupixent (dupilumab) for any FDA-approved or off-label condition
- Diagnosed with cutaneous T-cell lymphoma (CTCL), mycosis fungoides, or Sézary syndrome after starting Dupixent
- Diagnosed with another form of T-cell lymphoma or blood cancer after Dupixent use
- Able to document Dupixent treatment dates through medical records or pharmacy records
- Cancer diagnosis occurred during or after Dupixent treatment (no minimum duration required)
How Dupixent (Dupilumab) May Cause Cutaneous T-Cell Lymphoma
In Plain Language
Dupixent (dupilumab), a monoclonal antibody manufactured by Regeneron Pharmaceuticals and Sanofi, works by blocking interleukin-4 (IL-4) and interleukin-13 (IL-13) signaling pathways. While effective for atopic dermatitis, asthma, and other inflammatory conditions, emerging evidence suggests that this immune pathway modification may impair the body's natural cancer surveillance mechanisms, potentially enabling the development or unmasking of cutaneous T-cell lymphoma (CTCL). A 2024 study of 19,612 patients found a 4.5-fold increased risk of CTCL among Dupixent users compared to controls.
IL-4/IL-13 Pathway Blockade Disrupts Immune Surveillance
Dupilumab binds to the IL-4 receptor alpha subunit, simultaneously blocking IL-4 and IL-13 signaling. These cytokines play critical roles in type 2 immune responses, but they also participate in broader immune regulation. By suppressing the Th2 pathway, dupilumab shifts the immune balance toward Th1/Th17 responses while potentially reducing the immune system's ability to detect and eliminate abnormal T-cell clones in the skin. This altered immune microenvironment may create permissive conditions for malignant T-cell proliferation.
Unmasking of Pre-Existing CTCL Misdiagnosed as Eczema
Early-stage cutaneous T-cell lymphoma (mycosis fungoides) presents with erythematous, scaly patches that are clinically and histopathologically similar to chronic eczema. Patients with undiagnosed early-stage CTCL may be prescribed Dupixent for presumed refractory atopic dermatitis. When Dupixent suppresses the inflammatory component of the skin disease but fails to address the underlying malignancy, the CTCL progresses and becomes clinically apparent. This unmasking phenomenon means some CTCL cases attributed to Dupixent may represent delayed diagnosis enabled by the drug's partial symptomatic control.
Disruption of Anti-Tumor T-Cell Clonal Surveillance
The IL-4/IL-13 axis influences dendritic cell maturation, antigen presentation, and the activation of cytotoxic T-cells that normally eliminate malignant cells. By blocking these pathways, dupilumab may impair the skin's resident immune cells from mounting an effective anti-tumor response against emerging malignant T-cell clones. Published case reports describe CTCL emerging in patients with no prior history of lymphoma after months to years of Dupixent therapy, suggesting a direct immunosuppressive contribution rather than simple unmasking.
Chronic Immune Modulation Alters Skin Microenvironment
Long-term IL-4/IL-13 blockade fundamentally alters the cytokine milieu in the skin. The resulting shift in T-cell subsets, reduced eosinophil recruitment, and modified dendritic cell function create a persistently altered dermal immune environment. Over months or years of continuous therapy, this modified microenvironment may favor the survival and expansion of aberrant T-cell populations that would otherwise be eliminated by normal immune surveillance mechanisms.
Danger Factors
- Long-Term Continuous Use in Immunologically Vulnerable Patients: Dupixent is prescribed as an ongoing maintenance therapy, often for years. Patients with severe atopic dermatitis frequently have baseline immune dysregulation, making them particularly susceptible to the consequences of additional immune pathway modification. The cumulative effect of prolonged IL-4/IL-13 blockade on cancer surveillance has not been adequately studied in long-term clinical trials.
- Diagnostic Confusion Between Atopic Dermatitis and Early CTCL: The clinical overlap between refractory eczema and early mycosis fungoides is well-documented in dermatology literature. Patients placed on Dupixent for treatment-resistant eczema may actually harbor undiagnosed CTCL, and the drug's anti-inflammatory effects can mask disease progression until the lymphoma reaches an advanced stage.
- Rapid Expansion of Approved Indications Without Proportionate Safety Monitoring: Dupixent has been approved for progressively broader indications — atopic dermatitis, asthma, chronic rhinosinusitis with nasal polyps, eosinophilic esophagitis, prurigo nodularis, and COPD — exposing millions of patients to long-term IL-4/IL-13 blockade. The post-market safety surveillance infrastructure has not kept pace with this rapid expansion.
- Inadequate Cancer-Specific Endpoints in Clinical Trials: Regeneron's pivotal clinical trials for Dupixent were designed with efficacy endpoints focused on symptom reduction, not long-term oncologic safety. Trial durations of 16-52 weeks were insufficient to detect malignancies with latency periods of years. The 19,612-patient observational study that identified the 4.5x CTCL risk was conducted by independent researchers, not by the manufacturer.
Scientific Consensus
- A 2024 observational study of 19,612 dupilumab-treated patients identified a 4.5-fold increased risk of cutaneous T-cell lymphoma compared to matched controls, representing the largest systematic analysis of this safety signal to date.
- The FDA added CTCL to the Dupixent safety monitoring watchlist in March 2025 following review of post-market adverse event reports and published literature.
- Multiple peer-reviewed case reports and case series published between 2021 and 2025 document CTCL diagnoses in patients receiving dupilumab therapy, with cases spanning both new-onset lymphoma and unmasking of previously occult disease.
- Dermatology professional societies have issued clinical guidance recommending that patients with treatment-resistant eczema undergo skin biopsy to rule out CTCL before initiating dupilumab therapy.
Why This Matters for Your Case
The Dupixent litigation centers on whether Regeneron and Sanofi knew or should have known about the CTCL risk and failed to adequately warn prescribers and patients. The existence of over 300 adverse event reports to the FDA, combined with the independent 19,612-patient study showing 4.5x increased risk, forms the evidentiary foundation for failure-to-warn claims. Plaintiffs allege that earlier and more prominent warnings would have prompted diagnostic workups before drug initiation, potentially preventing CTCL progression in misdiagnosed patients.
Were you diagnosed with T-cell lymphoma after taking Dupixent? Get a free case evaluation today.
Get Your Free Case Reviewor call 1-800-555-0100
Internal Documents & Evidence
19,612-Patient Observational Study: 4.5-Fold Increased CTCL Risk with Dupilumab
“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.”
Impact: This study is the single most important piece of evidence in Dupixent litigation. It provides population-level quantification of the CTCL risk that goes beyond individual case reports, establishing the kind of systematic evidence needed to support general causation arguments. The fact that the study was conducted by independent researchers — not by Regeneron or Sanofi — strengthens its credibility and raises questions about why the manufacturer's own pharmacovigilance function did not conduct a similar analysis earlier.
FDA Adverse Event Reporting System (FAERS): 300+ CTCL-Related Reports
“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.”
Impact: FAERS data establishes the breadth and duration of the CTCL safety signal as known to the FDA and, through mandatory manufacturer reporting requirements, to Regeneron and Sanofi. The accumulation of over 300 reports is significant given well-documented underreporting in passive adverse event systems, where estimates suggest only 1-10% of actual adverse events are reported. Plaintiffs will argue that the true incidence of Dupixent-associated CTCL substantially exceeds the reported count.
Published Case Reports and Case Series: CTCL Diagnoses During Dupilumab Therapy
“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.”
Impact: Published case reports serve as the foundational clinical evidence linking Dupixent to CTCL in individual patients. While case reports alone cannot establish population-level causation, they provide the clinical narratives that are essential for specific causation arguments in individual plaintiff cases. The growing body of published cases also demonstrates that the medical community recognized this safety signal in the peer-reviewed literature — information that Regeneron and Sanofi's medical affairs teams were obligated to monitor under FDA pharmacovigilance requirements.
Were you diagnosed with T-cell lymphoma after taking Dupixent? Get a free case evaluation today.
Get Your Free Case Reviewor call 1-800-555-0100
Regulatory Actions and Safety Signals Related to Dupixent (Dupilumab)
Dupixent has been the subject of escalating regulatory scrutiny since post-market surveillance began identifying a potential association between dupilumab use and cutaneous T-cell lymphoma. The FDA's progressive response — from adverse event accumulation to formal watchlist placement — reflects growing concern about an immunomodulatory drug prescribed to millions of patients.
FDA Approves Dupixent for Moderate-to-Severe Atopic Dermatitis
The FDA approved dupilumab (Dupixent) in March 2017 for adults with moderate-to-severe atopic dermatitis inadequately controlled by topical therapies. The approval was based on pivotal trials (SOLO 1, SOLO 2, CHRONOS) demonstrating significant improvement in skin clearance. The prescribing information included standard warnings about hypersensitivity and conjunctivitis but did not reference any risk of lymphoma or malignancy. Trial durations of 16-52 weeks were insufficient to detect long-latency oncologic safety signals.
Post-Market Adverse Event Reports Begin Accumulating in FAERS
By 2021, the FDA Adverse Event Reporting System (FAERS) had received a growing number of reports linking dupilumab use to new diagnoses of cutaneous T-cell lymphoma, including mycosis fungoides and Sezary syndrome. These reports, submitted by healthcare professionals and patients, documented CTCL diagnoses occurring months to years after Dupixent initiation in patients with no prior lymphoma history. The accumulating signal attracted attention from pharmacovigilance researchers and independent dermatology investigators.
FDA Places Dupixent on CTCL Safety Monitoring Watchlist
In March 2025, the FDA formally added Dupixent to its safety monitoring watchlist for potential association with cutaneous T-cell lymphoma. This action was prompted by the cumulative weight of FAERS adverse event reports (exceeding 300 cases), published case reports in peer-reviewed dermatology journals, and the landmark 19,612-patient observational study demonstrating a 4.5-fold increased CTCL risk. The watchlist designation signals that the FDA is actively evaluating whether labeling changes or additional risk mitigation measures are warranted.
FDA Initiates Formal Safety Review of Dupilumab and Lymphoma Risk
In September 2025, the FDA announced a formal safety review to evaluate the association between dupilumab and cutaneous T-cell lymphoma. The review encompasses analysis of FAERS data, manufacturer-submitted periodic safety update reports, published epidemiological studies, and biological plausibility assessments. The FDA requested that Regeneron and Sanofi provide additional post-market data including long-term follow-up of clinical trial participants and any internal analyses of the CTCL signal. Outcomes of the review may include labeling changes, a Risk Evaluation and Mitigation Strategy (REMS), or a formal FDA Drug Safety Communication.
European Medicines Agency Requests Safety Update from Regeneron/Sanofi
The European Medicines Agency's Pharmacovigilance Risk Assessment Committee (PRAC) requested a cumulative safety review from Regeneron and Sanofi regarding the association between dupilumab and T-cell lymphoproliferative disorders. The request followed European adverse event reports consistent with the U.S. FAERS signal and the publication of the 19,612-patient epidemiological study. PRAC signaled that it would evaluate whether updates to the European Summary of Product Characteristics (SmPC) are needed.
Significance Legend
Key Takeaway
The regulatory trajectory for Dupixent reflects a classic post-market safety signal evolution: initial adverse event accumulation, independent epidemiological confirmation, formal watchlist placement, and regulatory investigation. With over 300 adverse event reports and a 4.5x increased risk identified in the largest independent study, the FDA's September 2025 formal safety review may result in significant labeling changes that would strengthen failure-to-warn claims in ongoing and future litigation.
Regeneron and Sanofi: Corporate Accountability for Dupixent Safety Failures
Dupixent (dupilumab) is jointly developed and commercialized by Regeneron Pharmaceuticals and Sanofi under a global collaboration agreement. The drug has become one of the best-selling biologics in pharmaceutical history, generating over $11 billion in annual revenue by 2024. As CTCL safety concerns mount, both companies face scrutiny over whether their post-market surveillance and prescriber communications adequately addressed emerging lymphoma signals that were visible in their own pharmacovigilance data.
Timeline: Regeneron Pharmaceuticals / Sanofi
FDA Approval and Commercial Launch of Dupixent
Regeneron and Sanofi launched Dupixent in March 2017 following FDA approval for moderate-to-severe atopic dermatitis. The companies invested heavily in direct-to-consumer advertising and physician outreach, positioning Dupixent as a breakthrough biologic therapy for patients who had exhausted topical treatments. The initial prescribing label did not contain any reference to lymphoma or malignancy risk.
Rapid Indication Expansion Drives Blockbuster Revenue Growth
Between 2018 and 2022, Regeneron and Sanofi obtained FDA approval for Dupixent in five additional indications: asthma (2018), chronic rhinosinusitis with nasal polyps (2019), eosinophilic esophagitis (2022), prurigo nodularis (2022), and pediatric atopic dermatitis (ages 6 months+). Each new indication expanded the patient population by millions, driving annual revenue from approximately $2 billion in 2018 to over $11 billion by 2024. The aggressive expansion timeline meant that cancer-specific long-term safety data lagged far behind the growing exposed population.
CTCL Case Reports Emerge in Published Literature
Beginning in 2021, peer-reviewed dermatology journals published an increasing number of case reports documenting CTCL diagnoses in Dupixent-treated patients. Cases described both new-onset mycosis fungoides and Sezary syndrome in patients with no prior lymphoma history, as well as rapid CTCL progression in patients whose disease had been misdiagnosed as refractory eczema. These publications were accessible to Regeneron and Sanofi's medical affairs and pharmacovigilance teams, who are obligated to monitor published safety literature under FDA regulations.
Landmark 19,612-Patient Study Quantifies 4.5x CTCL Risk
Independent researchers published a large-scale observational study analyzing 19,612 dupilumab-treated patients matched against controls. The study found a 4.5-fold increased risk of cutaneous T-cell lymphoma among Dupixent users — a statistically significant signal that exceeded what could be explained by diagnostic bias or disease confounding alone. This study was not sponsored by Regeneron or Sanofi, highlighting the gap between manufacturer-sponsored safety monitoring and independent epidemiological research.
FDA Watchlist Placement and Formal Safety Review
The FDA placed Dupixent on its CTCL safety monitoring watchlist in March 2025, followed by a formal safety review announcement in September 2025. Regeneron's stock experienced significant volatility following both announcements. In quarterly earnings calls, Regeneron executives characterized the CTCL signal as potentially reflecting diagnostic unmasking rather than true drug-induced lymphoma — a position contested by plaintiff experts and independent researchers.
Litigation Filings Accelerate Nationwide
Following the FDA's formal safety review and growing media coverage of the CTCL association, plaintiff attorneys began filing individual lawsuits and investigating potential class action claims against Regeneron and Sanofi. Complaints allege failure to warn about CTCL risk, inadequate post-market surveillance, and negligent promotion of Dupixent without appropriate cancer screening recommendations. Over 37 million Dupixent prescriptions have been dispensed in the United States, creating a large potential plaintiff class.
Prioritizing Revenue Growth Over Long-Term Safety Monitoring
Plaintiffs allege that Regeneron and Sanofi pursued aggressive indication expansion and direct-to-consumer marketing while failing to invest proportionate resources in post-market cancer surveillance. The 19,612-patient study that identified the 4.5x CTCL risk was conducted by independent researchers — not by the manufacturer — despite Regeneron and Sanofi having access to their own pharmacovigilance databases that contained the same safety signal.
- Regeneron and Sanofi expanded Dupixent to six indications between 2017 and 2024 without conducting manufacturer-sponsored long-term studies specifically designed to evaluate oncologic safety endpoints.
- Independent researchers, not the manufacturer, conducted the landmark 19,612-patient study that quantified the CTCL risk — raising questions about whether Regeneron's pharmacovigilance function was adequately analyzing its own adverse event data.
- Direct-to-consumer television advertising for Dupixent did not mention any cancer or lymphoma risk, despite accumulating FAERS reports that were known to both companies.
- Regeneron executives publicly characterized the CTCL signal as likely representing diagnostic unmasking rather than drug-induced lymphoma, a position that plaintiff experts argue minimizes a serious safety concern to protect commercial interests.
Key Takeaway
With over $11 billion in annual revenue and 37 million prescriptions dispensed, Dupixent is one of the most commercially significant drugs in Regeneron and Sanofi's portfolios. The emerging CTCL litigation directly challenges whether these companies prioritized revenue growth and indication expansion over the patient safety investment needed to detect and communicate a serious cancer risk that independent researchers ultimately identified.
Were you diagnosed with T-cell lymphoma after taking Dupixent? Get a free case evaluation today.
Get Your Free Case Reviewor call 1-800-555-0100
Sources & References
- Peer-reviewed study of 19,612 patients analyzing the association between dupilumab use and cutaneous T-cell lymphoma incidence — Medical Literature (2024-2025)
- FDA Drug Safety Communication — Dupixent (dupilumab) placed on safety watchlist for potential cancer risk, March 2025 — U.S. Food and Drug Administration (FDA)
- FDA investigation of 300+ adverse event reports related to lymphoma in Dupixent users, September 2025 — U.S. Food and Drug Administration (FDA)
- Giovanni Fraioli v. Regeneron Pharmaceuticals et al., filed January 2026, Southern District of Florida — U.S. District Court, S.D. Florida
- Chandra Richardson Wrongful Death Complaint, filed October 2025, Tennessee — Tennessee State Court