GlaxoSmithKline has returned the rights to the investigational Parkinson's disease drug Rytary to partner Impax Laboratories, citing regulatory delays.

The companies linked up in December 2010 to develop Rytary, an extended-release capsule of carbidopa and levodopa for PD also known as IPX066. However in January this year, the US Food and Drug Administration issued a complete response letter for Rytary, telling Impax it required a satisfactory re-inspection of the company’s Hayward facility as a result of a warning letter issued in May 2011.

Last month, Impax said the FDA had completed a re-inspection and had made 12 observations, "three of which are designated as repeat observations from inspections that occurred prior to the warning letter". Chief executive Larry Hsu said "we have committed significant resources in our efforts to meet FDA requirements and are clearly disappointed by this news".

Now GSK and Impax have ended their collaboration for developing the drug outside the USA and Taiwan. The decision has been reached "because of delays in the anticipated regulatory approval and launch dates in countries in which GSK has rights to commercialise the product".

Impax will take over development of IPX066 on a worldwide basis and seek to find a partner "looking to grow their non-US neurology franchise".  GSK added that it remains committed to "discovering, developing and delivering" innovative PD medicines.