Ipsen Acquires Kartos Therapeutics for $450M to Develop Navtemadlin, an MDM2 Inhibitor for Myelofibrosis
Key Insights
Ipsen announced the acquisition of Kartos Therapeutics (search) for $450 million upfront, with up to $1.3 billion in milestone payments, to gain navtemadlin, a Phase III oral MDM2 (search) inhibitor for myelofibrosis (search).
Navtemadlin is designed as an add-on therapy to ruxolitinib for patients with suboptimal response, where median overall survival after discontinuation drops to approximately 1–2 years.
Phase Ib/II data showed 42% of ruxolitinib non-responders achieved at least a 25% spleen volume reduction at Week 24, with 71% showing a ≥20% reduction in driver variant allele frequency.
Ipsen and Kartos Therapeutics (search) announced on June 29, 2026, that they have entered into a definitive merger agreement under which Ipsen will acquire Kartos Therapeutics for $450 million upfront. Kartos shareholders are also eligible to receive additional milestone payments of up to $1.3 billion, including a significant regulatory approval milestone and sales-based milestones, bringing the total potential deal value to $1.75 billion. The transaction is expected to close by the end of Q3 2026, subject to customary closing conditions including the expiration of the waiting period under the Hart-Scott-Rodino Antitrust Improvements Act.
The centerpiece of the acquisition is navtemadlin, an investigational oral MDM2 (search) inhibitor designed to restore the natural tumor-suppressing function of p53, a critical tumor-suppressor in myelofibrosis (search). The drug is being developed as an add-on therapy to ruxolitinib for patients with intermediate and high-risk TP53 wild-type (wt) myelofibrosis who have a suboptimal response to standard of care.
The Unmet Need in Myelofibrosis (search)
Myelofibrosis (search) is a myeloproliferative neoplasm frequently linked to alterations in the JAK/STAT pathway, characterized by bone marrow failure, fibrosis, splenomegaly, and a high symptom burden including fatigue and night sweats. The condition affects approximately 1.5 per 100,000 people in the U.S. and Europe, with a median age at diagnosis of approximately 67–69 years. Approximately 75–89% of patients are intermediate- or high-risk at diagnosis, and more than 95% are TP53wt.
Ruxolitinib, a JAK inhibitor, has anchored first-line treatment since 2011. However, an estimated 50%–75% of patients discontinue treatment after three years, and roughly 40% develop a suboptimal response. After discontinuation, median overall survival collapses to approximately one to two years.
“As a treating clinician who cared for more than a thousand patients with myelofibrosis (search), I have seen first-hand the significant care gap for patients with myelofibrosis who remain symptomatic or have persistent splenomegaly despite ruxolitinib treatment,” said Srdan Verstovsek, MD, PhD, Chief Medical Officer of Kartos Therapeutics (search).
Phase Ib/II Clinical Evidence
The clinical rationale for navtemadlin builds on data from the Phase Ib/II trial (KRT-232-109), presented at the European Hematology Association Congress in 2023. In patients with a suboptimal response to ruxolitinib (n=19), navtemadlin demonstrated clinically meaningful activity at Week 24: 42% achieved at least a 25% reduction in spleen volume, 32% achieved at least a 35% reduction in spleen volume, and 32% achieved a total symptom score improvement of at least 50%.
Notably, the data also suggested disease-modifying potential. Among evaluable patients (n=7), 71% achieved a ≥20% reduction in driver variant allele frequency, and 57% showed an improvement in bone marrow fibrosis by Central Review of ≥1 Grade by Week 24. This biological signal—reduction in the malignant clone and improvement in bone marrow architecture—distinguishes navtemadlin from purely symptom-directed approaches.
“The clinical rationale for combining navtemadlin with ruxolitinib is very compelling, and the emerging data suggest the synergistic potential to deepen responses and address the underlying biology of the disease,” said John Mascarenhas, Professor of Medicine at the Icahn School of Medicine at Mount Sinai and Director of the Center of Excellence for Blood Cancers and Myeloid Disorders.
The Phase III POIESIS Trial
Navtemadlin is currently being evaluated in the global Phase III POIESIS trial, designed to enroll more than 600 patients across more than 250 sites—making it the largest trial ever conducted in myelofibrosis (search). The study evaluates navtemadlin as an add-on therapy to ruxolitinib versus ruxolitinib alone in patients with intermediate and high-risk TP53wt myelofibrosis who have a suboptimal response to ruxolitinib.
“The Phase III POIESIS trial has the potential to redefine how we treat patients with myelofibrosis (search),” said Dr. Pankit Vachhani, Associate Professor of Medicine and Director of Clinical Research Unit at the University of Alabama at Birmingham, and Global Principal Investigator of POIESIS. “It is the largest trial conducted in this disease and uniquely designed to reflect real-world clinical practice.”
Top-line data from POIESIS are expected in 2027, positioning a potential regulatory submission in the 2027-to-2028 window. Ipsen projects commercial accretion no earlier than 2029.
Strategic Rationale and Competitive Landscape
Navtemadlin’s mechanism is deliberately orthogonal to JAK inhibition. Rather than replacing ruxolitinib, it is positioned as an add-on to pull suboptimal responders into a clinical response. This strategy does not directly displace other approved JAK inhibitors—fedratinib, pacritinib, and momelotinib—but introduces a new competitive layer for the ruxolitinib-maintained patient population, which remains the largest segment in the myelofibrosis (search) market.
“This acquisition further strengthens our late-stage oncology pipeline and reflects our continued focus on bringing transformational treatments to people living with cancer,” said David Loew, CEO of Ipsen. “We are excited by the potential of navtemadlin to define a new treatment paradigm for patients with myelofibrosis (search) who have a suboptimal response to current standard of care, addressing a critical care gap and offering the potential for a new therapeutic option as early as 2028.”
The single number worth watching as POIESIS matures is bone marrow fibrosis improvement by central review. The Phase Ib/II signal—57% achieving a grade or better reduction at Week 24—is suggestive but based on just seven evaluable patients. Confirmation of that histologic endpoint at scale would substantially strengthen both the regulatory and commercial case for navtemadlin.
Orrick Herrington & Sutcliffe LLP is acting as legal counsel to Ipsen. Goldman Sachs & Co. LLC and PJT Partners (UK) Ltd are serving as financial advisors to Kartos Therapeutics (search), with DLA Piper LLP serving as legal counsel.