French drugmaker Ipsen is acquiring U.S. biotech Kartos for navtemadlin, an experimental pill that targets the tumor suppressor p53; pivotal Phase 3 data in patients with the rare blood cancer myelofibrosis who had a suboptimal response to Incyte's Jakafi are expected in 2027.
For the patient with myelofibrosis, a rare blood cancer in which scar tissue crowds out the bone marrow, the only pill approved to control the disease is a JAK inhibitor called Jakafi (ruxolitinib). When that drug stops working, the next step has been mostly guesswork. French drugmaker Ipsen is paying up to $1.75 billion, $450 million in cash and up to $1.3 billion in milestones, for a small U.S. biotech whose lead drug is built around an entirely different idea: rebooting a single tumor-suppressor protein that cancer cells have learned to silence.
Ipsen is acquiring Kartos Therapeutics, founded in 2018, and with it the experimental pill navtemadlin. The drug is an oral MDM2 inhibitor, a class of compounds designed to break the leash that the MDM2 protein puts on p53, the tumor-suppressor often called the "guardian of the genome" because it stops DNA-damaged cells from multiplying. In TP53-wild-type myelofibrosis, the subset of patients whose p53 gene is intact but is being muted by overactive MDM2, that reactivation could in theory push the brakes back on the disease even after JAK inhibition has lost its effect.
The bet is built around a Phase 3 trial called POIESIS, which is enrolling more than 600 intermediate- and high-risk myelofibrosis patients with a suboptimal response to a JAK inhibitor. Topline data are expected in 2027, and Ipsen CEO David Loew has said the drug could be available "as early as 2028," language that frames the timing as an aspiration rather than a regulatory commitment. The pivotal design, not the earlier signal, is what the deal's claims have to rest on.
That earlier signal is small. A 2023 Phase 1b/2 add-on study enrolled 19 prior-Jakafi patients with a suboptimal response, treated them with navtemadlin on top of ruxolitinib, and reported that 42% achieved at least a 25% reduction in spleen volume at week 24, 32% achieved at least a 35% reduction, and 32% saw at least a 50% improvement in total symptom score. Those are the kind of directional numbers that justify running a pivotal study, not the kind that prove a drug works. A single-arm trial of 19 patients can suggest an effect; it cannot confirm one.
What POIESIS is designed to test is whether navtemadlin can move the needle on the two endpoints regulators actually care about for this disease, spleen volume and symptom burden, in patients who did not get enough from a JAK inhibitor alone. The readout matters beyond Ipsen because it will be the first large-scale look at p53 reactivation in myelofibrosis, and a clean miss would push the whole MDM2-in-myelofibrosis thesis back to the drawing board.
The competitive frame sits one disease over. PMV Pharmaceuticals is running a related p53 reactivator called rezatapopt in ovarian cancer and last year reported a 43% response rate in a mid-phase trial. That is not directly comparable; different disease, different patient population, but it is the closest analogue for judging whether restoring p53 from a drug can translate into measurable tumor control outside of myelofibrosis.
For Incyte, which markets Jakafi in the U.S., and for Novartis, which markets it abroad, the part worth tracking is not the first-line fight but the next one. Navtemadlin is not going after the same patient as first-line Jakafi; it is going after the cohort Jakafi cannot fully cover. If the Phase 3 holds up, the standard-of-care conversation in myelofibrosis stops being "which JAK inhibitor first" and starts becoming "which add-on, when the first one fades." If it does not, the unmet need stays where it has been.
The purchase is also a continuation of Ipsen's recent oncology deal-making. The company has separately added assets from Foreseen Biotechnology and the China-based Simcere Zaiming, including antibody-drug conjugates. For a mid-size French pharma rebuilding a hemato-oncology pipeline that had been thinning out, a single Phase 3 asset with a defined patient population and a 2027 readout is a manageable center of gravity.
The next test is timing. The 2027 POIESIS topline is the only data point that can convert "up to $1.75 billion" from a deal headline into a treatment that actually reaches the patient who needs it.