Ipsen to buy Kartos Therapeutics for up to $1.75 billion

Ipsen's $1.75 billion bet on a late-stage MDM2 inhibitor targets a critical care gap in myelofibrosis, where half of patients stop responding to standard therapy within three years.

Ipsen agreed to acquire Kartos Therapeutics for as much as $1.75 billion, adding navtemadlin, a Phase III oral MDM2 inhibitor designed to restore p53 tumor-suppressor function in patients with myelofibrosis who have a suboptimal response to standard-of-care ruxolitinib. The French drugmaker will pay $450 million upfront, with Kartos shareholders eligible for up to $1.3 billion in regulatory and sales-based milestone payments.

"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, chief executive officer of Ipsen, in a statement. "We are excited by the potential of navtemadlin to define a new treatment paradigm for patients with myelofibrosis 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."

Ruxolitinib, a JAK inhibitor from Incyte and Novartis, is the first-line standard for myelofibrosis, a rare myeloproliferative neoplasm affecting about 1.5 per 100,000 people in the U.S. and Europe. While the drug improves splenomegaly and disease-related symptoms, an estimated 50 percent to 75 percent of patients discontinue treatment within three years. Outcomes after discontinuation are poor, with median overall survival of roughly one to two years. Navtemadlin is being evaluated in the global Phase III POIESIS trial, designed to enroll more than 600 patients across over 250 sites, as an add-on to ruxolitinib in intermediate- and high-risk TP53 wild-type myelofibrosis patients. Top-line data are expected in 2027.

The deal underscores a broader push by large pharma to acquire late-stage oncology assets with defined regulatory pathways rather than early-stage bets. Ipsen, which generated about 3.3 billion euros in 2025 revenue from its oncology, rare disease, and neuroscience franchises, expects the transaction to be accretive to core operating income from 2029, with limited dilution to its 2026 full-year guidance. The acquisition is expected to close by the end of the third quarter of 2026, subject to U.S. antitrust review under the Hart-Scott-Rodino Act.

Clinical Data Support the Add-On Strategy

Data from a Phase Ib/II trial (KRT-232-109) presented at the European Hematology Association Congress in 2023 showed that at Week 24, 42 percent of patients with a suboptimal response to ruxolitinib achieved at least a 25 percent reduction in spleen volume, while 32 percent achieved at least a 35 percent reduction. A total of 32 percent also reported a total symptom score improvement of at least 50 percent. The data also suggested potential disease-modifying activity: 71 percent of evaluable patients showed a 20 percent or greater reduction in driver variant allele frequency, and 57 percent demonstrated an improvement in bone marrow fibrosis of at least one grade by central review.

"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.

Deal Advisors and Structure

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, with DLA Piper LLP providing legal counsel to the target company. Kartos Therapeutics, a clinical-stage biopharmaceutical company based in Redwood City, California, has focused its pipeline on harnessing the p53 pathway for myeloproliferative neoplasms. More than 95 percent of myelofibrosis patients are TP53 wild-type, making them eligible for MDM2 inhibitor therapy.

The transaction is the latest in a series of oncology-focused acquisitions by European drugmakers seeking to replenish pipelines ahead of patent expiries. Ipsen's bet on navtemadlin carries binary risk tied to the POIESIS readout, but the deal structure — a modest upfront with back-loaded milestones — limits downside exposure if the drug fails to meet endpoints.

This article is for informational purposes only and does not constitute investment advice.