Eyebrows have been raised after Astellas Pharma decided to end its collaboration with Ambit Biosciences Corp to develop the latter's acute myeloid leukaemia treatment quizartinib.

Back in 2009, the companies entered into a collaboration focusing on FMS-like tyrosine kinase-3 (FLT-3) inhibitors, including quizartinib. Now, the Japanese drugmaker has exercised its right to terminate the agreement, citing "strategic reasons", effective September 3, 2013.

Over the months ahead, the companies say they will work together to transfer current development activities to privately-owned Ambit, which will regain all rights and continue with the quizartinib clinical trial programme. No financial details have been disclosed.

Astellas' decision has come as something of a surprise, given the impressive Phase II data presented at the American Society of Hematology meeting in Atlanta in December. Quizartinib is designed to 'turn off' the mutated FLT3 enzyme, a "power switch" that leukaemia cells use to spread more aggressively.

Ambit chief executive Michael Martino said that based on the Phase II data, "we and members of the medical community continue to be excited about quizartinib and its potential to meet a significant, unmet need in AML patients." He added that "we are fully committed to moving forward with the Phase III clinical trial plan".

Ambit is developing a companion diagnostic with Genoptix Medical Laboratory, a unit of Novartis, to identify FLT3-ITD-positive patients. The company, which recently filed an initial public offering to raise $57.5 million, also is exploring quizartinib in other AML therapeutic settings, irrespective of FLT3-ITD status.

Astellas chief executive Yoshihiko Hatanaka said "while our decision is based on strategic reasons, we are proud of our collaborative work with Ambit, and we are committed to working… on a smooth transition". He added that  "we remain committed to the field of oncology as a major area of focus for the company".