(Reuters) - Idenix Pharmaceuticals Inc said it regained the worldwide rights to develop, market and license its drug candidates from Swiss partner and stakeholder Novartis.

Idenix, whose experimental treatments for hepatitis C have led to extensive investor interest, said Novartis's rights to license Idenix's current and future development-stage drugs for any disease has been terminated.

In exchange, Idenix will pay Novartis a royalty based on the worldwide sales of Idenix's future hepatitis C drugs, unless they are used in combination with Novartis' drugs.

Idenix will no longer receive royalty or milestone payments from Novartis on the worldwide sales of the hepatitis B treatment Tyzeka/Sebivo, which was jointly developed and launched by the two drugmakers.

Novartis, which currently holds a 31 percent stake in Idenix, will have the right to designate one member to Idenix's board, reduced from two, as long as it continues to own at least 15 percent stake in Idenix.

According to the original deal signed in May 2003, Novartis had the option to license any of Idenix's drug candidates after positive results from a proof-of-concept trial, as long as Novartis had at least 30 percent stake in Idenix.

Idenix shares closed at $11.02 on Monday on the Nasdaq.

(Reporting by Esha Dey in Bangalore; Editing by Supriya Kurane)