Merck's rare tumour drug Pimjetmi claims FDA approval

Merck KGaA's oral CSF-1R inhibitor Pimjetmi has been cleared in the US for tenosynovial giant cell tumour (TGCT), adding a third FDA-approved treatment option for patients with the rare disorder.

Pimjetmi (pimicotinib) has been greenlit in the US for use in adults with symptomatic TGCT where surgery could cause worsening functional limitations or severe morbidity, adding to its earlier approvals in China, Canada, and Switzerland.

The new drug joins Daiichi Sankyo's CSF1R, KIT, and FLT3 inhibitor Turalio (pexidartinib) and Ono's CSF-1R inhibitor Romvimza (vimseltinib) on the US market, which have been available since 2019 and 2025, respectively.

TGCT is a rare non-cancerous tumour that causes inflammation and destruction of joints, leading to pain and reduced mobility, and is most commonly diagnosed in adults between the ages of 30 and 50 years. Because symptoms like joint pain and swelling are often mistaken for other conditions, patients can go for years before they get a definitive diagnosis.

The approval is based on the results of the phase 3 MANEUVER trial in 94 TGCT patients, which compared Pimjetmi to placebo over 24 weeks, achieving a 56% objective response rate (ORR) based on the RECIST v1.1 scale compared to 3% in the control group.

Treatment was also associated with statistically significant improvements on secondary endpoints relevant to patients' daily lives, such as relative range of motion, physical functioning, and pain. After 12 months' follow-up, 77% of patients who benefitted from treatment maintained the response through 12 months.

For Merck, Pimjetmi is a key part of a push into rare tumours that also drove its $3.9 billion takeover of SpringWorks last year, giving it MEK inhibitor Ezmekly (mirdametinib) for neurofibromatosis type 1 and gamma-secretase inhibitor Ogsiveo (nirogacestat) for desmoid tumours.

Merck licensed rights to pimicotinib from China's Abbisko Therapeutics in 2023, initially taking rights to the drug in China, Hong Kong, Macau, and Taiwan in a deal that involved an upfront payment of $70 million. Last year, Merck exercised an option for global rights to the drug, sparking another $85 million in an agreement that could be worth up to $605.5 million.

"People living with TGCT have long faced difficult treatment decisions as they navigate a disease that can significantly affect their daily lives, including their mobility," said Merck's healthcare chief executive, Danny Bar-Zohar.

"With the approval of Pimjetmi in the US, we strengthen our leadership position and growing portfolio in rare tumours and bring real hope to patients and their families."

Sales estimates for Pimjetmi vary widely, from a few tens of millions of dollars per year up to $1 billion-plus, depending on pricing in the US, its ability to capture market share from the two rival therapies, and potential expansion into additional indications, which could include pancreatic cancer and chronic graft-versus-host disease (cGvHD).