Ipsen bets up to $1.75bn on Kartos to strengthen myelofibrosis pipeline

Late-stage myelofibrosis candidate navtemadlin could reshape treatment for patients with suboptimal responses to Jakafi.

Ipsen has agreed to acquire US biotech Kartos Therapeutics in a deal worth up to $1.75 billion, strengthening its late-stage oncology pipeline with navtemadlin (KRT-232), an investigational therapy targeting patients with myelofibrosis who no longer respond adequately to current standard treatment.

The acquisition combines an upfront payment of $450 million with up to $1.3 billion in development and commercial milestone payments, highlighting Ipsen's confidence in the asset's long-term potential.

Targeting an unmet need in myelofibrosis

Myelofibrosis is a rare blood cancer characterised by scarring of the bone marrow, leading to impaired blood cell production, enlarged spleen size and debilitating symptoms.

Current treatment is largely centred around Jakafi (ruxolitinib), the JAK inhibitor developed by Incyte. While Jakafi has transformed care for many patients by reducing spleen enlargement and improving symptoms, a significant proportion eventually experience a suboptimal response or discontinue treatment altogether.

Navtemadlin has been designed to address this gap.

Unlike JAK inhibitors, the oral therapy works by inhibiting MDM2, restoring the activity of the tumour suppressor protein p53 in patients whose cancers retain wild-type TP53. Rather than replacing existing therapy, navtemadlin is being developed as an add-on treatment to improve outcomes for patients whose disease is no longer adequately controlled.

Phase III study underway

The lead programme is currently being evaluated in the Phase III POIESIS trial, which is enrolling patients with intermediate- and high-risk TP53 wild-type myelofibrosis who have experienced a suboptimal response to Jakafi.

Top-line data are expected during 2027.

Earlier Phase Ib/II data have provided encouraging signals. Among 19 patients receiving navtemadlin alongside Jakafi:

• 42% achieved at least a 25% reduction in spleen volume after 24 weeks.

• 32% achieved the more stringent 35% spleen volume reduction endpoint.

• 32% experienced at least a 50% improvement in total symptom score.
  

While based on a relatively small patient population, the results suggest the combination could improve outcomes in a setting where treatment options remain limited.

Building Ipsen's oncology franchise

The acquisition continues Ipsen's strategy of expanding its oncology portfolio through targeted business development.

Commenting on the deal, Ipsen CEO David Loew said navtemadlin has the potential to establish "a new treatment paradigm" for patients with myelofibrosis who have an inadequate response to current standard of care, with a potential commercial launch as early as 2028 if development progresses successfully.

The transaction also follows Ipsen's acquisition of ImCheck Therapeutics in October 2025, when the company agreed a deal worth up to €1 billion for the experimental immuno-oncology antibody ICT01.

What the acquisition means

For Ipsen, Kartos represents more than a pipeline expansion.

The company is investing in a late-stage asset targeting a clearly defined unmet clinical need within an established treatment market.

Rather than competing directly with Jakafi, navtemadlin aims to complement the current standard of care, potentially extending treatment benefit for patients who would otherwise have limited options.

If the ongoing Phase III trial confirms the earlier clinical findings, the acquisition could provide Ipsen with an important growth driver in haematological oncology while offering physicians a new therapeutic strategy for managing myelofibrosis beyond first-line JAK inhibition.

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