Ipsen buys Kartos Therapeutics for $450 million to boost oncology pipeline

Ipsen agreed to buy Kartos Therapeutics for $450 million upfront, adding the U.S. biotech’s lead cancer drug, navtemadlin, to its oncology pipeline and tying as much as $1.3 billion more to future regulatory and sales milestones. Ipsen expects the transaction to close in the third quarter of 2026, subject to antitrust clearance.

The deal gives Ipsen a late-stage asset in myelofibrosis, a rare blood cancer. Navtemadlin is an oral MDM2 inhibitor designed to restore p53 tumor-suppressor function. Kartos shareholders could receive up to $1.3 billion more if the program clears regulatory and sales hurdles.

The lead study for navtemadlin is the Phase 3 POIESIS trial, a randomized, double-blind add-on study. ClinicalTrials.gov lists it as enrolling 600 patients. The study began on June 3, 2024, with estimated primary completion on December 31, 2026, and estimated study completion on December 31, 2028. Top-line data are expected in 2027. The trial is testing navtemadlin plus ruxolitinib against placebo plus ruxolitinib in patients with myelofibrosis who have had a suboptimal response to ruxolitinib, focused on intermediate- and high-risk TP53 wild-type disease.

Kartos is based in Redwood City, California, and was founded in 2018. OrbiMed is among its investors. Kartos also had prior clinical work in myelofibrosis through the BOREAS program. Ipsen, headquartered in Paris and listed on Euronext Paris and via an ADR in the United States, bought ImCheck Therapeutics in December 2025 and, in February 2026, continued expanding its pipeline after a strong year.