MorphoSys ( MOR ) AG  is reportedly facing a burgeoning safety concern surrounding pelabresib, an experimental myelofibrosis treatment.

Myelofibrosis is a type of blood cancer characterized by the buildup of scar tissue, called “fibrosis,” in the bone marrow.

Physicians participating in MorphoSys’ Phase 3 study have observed instances when patients treated with pelabresib swiftly transitioned from myelofibrosis to acute myeloid leukemia (AML), a more aggressive form of blood cancer.

Citing sources familiar with the cases, STAT News noted physicians saw several instances of patients undergoing such rapid progression. 

Among 212 patients receiving pelabresib, six serious safety events of “AML transformation” were reported, compared to only two events among 214 patients in the control group.

Pelabresib has been the centerpiece of MorphoSys’ proposed 2.7 billion euro ($2,894,958,900) acquisition by Novartis ( NVS ) AG .

The deal is expected to close in the first half of 2024.

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In the deal press release, MorphoSys ( MOR ) CEO Jean-Paul Kress said, “Pelabresib — the investigational therapy at the forefront of our promising oncology pipeline — has the potential to shift the treatment paradigm in myelofibrosis and further expand into other indications. Novartis ( NVS ) will provide ample resources currently unavailable to MorphoSys ( MOR ) as a standalone biotech company to help accelerate the development opportunities and maximize the commercialization potential of pelabresib at a greater speed and scale.” 

The FDA was alerted about this imbalance in severe cancer advancements. It issued a cautionary letter to all physicians involved in the study and is closely monitoring the situation. 

Despite this setback, MorphoSys ( MOR ) affirmed its acquisition by Novartis ( NVS ) is still on track to finalize in the first half of this year.

“We generally do not discuss our interactions with regulatory authorities publicly. We remain confident in the benefit-risk profile of the combination of pelabresib and ruxolitinib,” a Morphosys ( MOR ) spokesperson said in an emailed response to Reuters.

“The planned acquisition by Novartis ( NVS ) is progressing steadily,” they added.

In November, the company released topline results from the Phase 3 MANIFEST-2 study investigating pelabresib in combination with the JAK inhibitor ruxolitinib compared with placebo plus ruxolitinib in JAK inhibitor-naïve patients with myelofibrosis.

MOR Price Action: MorphoSys shares are down 3.38% at $17.43 at publication Monday.

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