Daiichi Sankyo and Plexxikon’s investigational rare cancer drug PLX3397 has lead to prolonged tumour regressions in its Phase 1 trial.

PLX3397 is a CSF-1R inhibitor for the treatment of tenosynovial giant cell tumor (TGCT) – a rare type of cancer that affects the joints or tendon sheath and can lead to loss of mobility. There is currently no systematic therapy approved for TGCT.

The trial results, published in the New England Journal of Medicine, showed that 52% of patients achieved a partial response and 30% had stable disease.

"TGCT can be a very difficult disease to manage, with treatment options largely limited to surgery to remove as much of the tumor as possible,” says Dr William Tap, lead author of the study and chief of the Sarcoma Medical Oncology Service at Memorial Sloan Kettering Cancer Center.

“Despite the best surgical intervention, recurrence of diffuse TGCT is high and the disease may advance to the point where surgery is no longer an option.

"These preliminary results demonstrate that by targeting CSF-1R, PLX3397 may inhibit tumor growth in some patients with TGCT, potentially offering those patients an alternative non-surgical treatment option."

Daiichi Sankyo bought Plexxikon for $805 million back in 2011, securing its melanoma drug Zelboraf (vemurafenib), which is co-developed with Roche. The drug has since faced stiff competition from Novartis’ combination of Tafinlar (dabrafenib) and Mekinist (trametinib), though.