Kyowa Hakko Kogyo Co., Ltd said its wholly owned US subsidiary, Kyowa Pharmaceutical, received a not approvable letter from the US Food and Drug Administration (FDA) for istradefylline (KW-6002), its investigational drug for the treatment of Parkinson's disease.

In the letter, FDA expressed concern if the efficacy findings support clinical utility of istradefylline (KW-6002). FDA requested an overall summary of nonclinical mineralisation findings. Additionally, FDA asked for clinical pharmacology follow-up information as a phase III commitment.

"After closely examining the FDA's response, Kyowa Hakko will work closely with FDA and conduct a comprehensive discussion to determine a path forward for istradefylline (KW-6002)," said Dr. Yuzuru Matsuda, President, Kyowa Hakko Kogyo.

Istradefylline (KW-6002) has a novel mechanism of action that antagonizes the adenosine A2A receptor. It has been under FDA review as adjunctive therapy to levodopa/carbidopa for the treatment of idiopathic Parkinson's disease to improve motor function in patients who experience motor response complications.