Ipsen said on Aug. 21 that it has completed the acquisition of Redwood City, California-based Kartos Therapeutics, the company behind navtemadlin, an oral MDM2 inhibitor in a Phase III trial as an add-on to ruxolitinib in myelofibrosis. The June 29 merger agreement set the price at $450 million at closing plus up to $1.3 billion in regulatory and sales milestones, and the deal closed inside the end-of-third-quarter window Ipsen had guided to.
- Closing date: Aug. 21, 2026, after a June 29, 2026, agreement.
- Terms: $450 million upfront and up to $1.3 billion more, including a regulatory approval milestone and sales-based milestones.
- Asset: navtemadlin, an investigational oral MDM2 inhibitor designed to restore p53 function.
- Trial: Phase III POIESIS, navtemadlin plus ruxolitinib versus ruxolitinib alone in patients with a suboptimal response, with top-line data expected in 2027.
- Financials: accretive to Ipsen core operating income from 2029, with what Ipsen called limited dilution to 2026 guidance.
- Advisers: Orrick Herrington & Sutcliffe for Ipsen; Goldman Sachs, PJT Partners and DLA Piper for Kartos.
A ruxolitinib add-on, built around p53
Navtemadlin is being developed for patients with intermediate- or high-risk, TP53 wild-type myelofibrosis whose response to first-line ruxolitinib falls short. Ipsen's June announcement described the drug as designed "to restore the natural tumor-suppressing function of p53," and the POIESIS study is testing whether adding it to the JAK inhibitor improves outcomes compared with ruxolitinib alone. Ipsen's own figures put 75% to 89% of patients in the intermediate- or high-risk categories at diagnosis, which is the population the trial is drawing from.
The commercial case rests on how ruxolitinib performs over time. Ipsen says a significant share of patients have an initial suboptimal response, that roughly 50% to 75% discontinue the drug within three years, and that median overall survival after discontinuation is typically one to two years. David Loew, Ipsen's CEO, said in June the company saw "the potential for a new therapeutic option as early as 2028."
What the trial investigators said
Pankit Vachhani, MD, associate professor of medicine at the University of Alabama at Birmingham and global principal investigator of POIESIS, called it "the largest trial conducted in this disease and uniquely designed to reflect real-world clinical practice." Srdan Verstovsek, MD, PhD, Kartos' chief medical officer, described the approach as moving "patients with a suboptimal response into a clinical responder group by optimizing their care," while "avoiding unnecessary over-treatment of those already responding well." John Mascarenhas, MD, of the Icahn School of Medicine at Mount Sinai, said the emerging data "suggest the synergistic potential to deepen responses and address the underlying biology of the disease." All three were quoted in Ipsen's June release. The completion announcement carried no new quotes or data.
Where it sits in Ipsen's oncology build-out
Ipsen's June headline framed the deal as expanding its hemato-oncology late-stage pipeline, and Kartos is the second acquisition the company has closed this summer, after Memo Therapeutics. The June release had set closing for the end of the third quarter, pending Hart-Scott-Rodino clearance, and completion came about six weeks ahead of that. Ipsen has not said whether the regulatory milestone in the $1.3 billion package is tied to a US or European approval, and the release gives no breakdown between that milestone and the sales-based tranches.
Why This Matters to the APO|APE Reader
Ipsen puts the TP53 wild-type share of myelofibrosis at more than 95%, so a label that follows the POIESIS population would exclude only a small mutated minority, yet it would still require a documented TP53 result before a patient on ruxolitinib is moved to the combination. With first data due in 2027 and Ipsen talking about 2028 availability, hematopathology and molecular labs have roughly two years to settle where that result comes from and how it is reported on the myelofibrosis workup.
