The USA's Xenoport is threatening to end its alliance with GlaxoSmithKline for the restless legs syndrome drug Horizant, after claiming the drugs major is not doing enough to market the treatment.

Horizant (gabapentin enacarbil) extended-release tablets were approved in the USA in April 2011 for the treatment of moderate-to-severe primary restless legs syndrome in adults. Under the terms of an agreement signed in November 2010, GSK is responsible for "the post-marketing requirements and commitments with respect to Horizant" for RLS, plus seeking approval of a supplemental New Drug Application for the therapy, which is a modified version of Pfizer’s epilepsy treatment Neurontin, for post-herpetic neuralgia and other indications.

However, in a filing with the US Securities and Exchange Commission, XenoPort says GSK has "materially breached its contractual obligation to use commercially reasonable efforts to maximise the sales of Horizant in an expeditious manner and achieve the sales milestones set forth in the agreement". The company is eligible to receive $312.5 million, of which $120.0 million has been received to date, and says it is entitled to an additional $290.0 million upon the achievement of specified sales levels.

XenoPort is claiming that "unless GSK has cured any such breach or default", it can terminate the agreement 90 days after the receipt of written notice, which will expire on April 24. The company believes it would regain certain Horizant product rights if that happens, plus "specified transition assistance" from GSK.

GSK has yet to respond but the XenoPort filing notes that the former "may terminate the agreement in its entirety for any reason and at any time". The dispute comes a week after XenoPort's partner in Asia, Astellas Pharma, announced that the drug has received marketing approval in Japan, where it will be sold as Regnite.