CAMBRIDGE, Mass. (

TheStreet

) --

Idenix Pharmaceuticals

(IDIX)

shares are weak Monday morning following the loss in the first phase of a hepatitis C patent dispute with

Gilead Sciences

(GILD) - Get Report

.

The U.S. Patent and Trademark Office (USPTO) determined that Idenix filed its patent application later than Gilead. This makes Idenix the "junior party" and Gilead the "senior party" in the ongoing case. Friday's ruling reverses an earlier decision by the USPTO which declared Idenix the senior party.

Idenix shares are down 10% to $4.20 in early Monday trading.

The USPTO ruling raises the burden of proof required for Idenix to prove its patent case against Gilead, but the ultimate outcome is determined by which party was the first to invent. This second and more important phase of the patent dispute is expected to begin in the second quarter, Idenix said.

Idenix alleges that a patent issued to Gilead covering the chemical makeup of certain hepatitis C drugs violates one its patent applications. If Idenix prevails, Gilead could be forced to pay a small royalty on sales of its hepatitis C drug sofosbuvir.

The ruling in phase I of the patent case does not affect Idenix's ability to develop its own hepatitis C drugs, most notably IDX719 or its uridine nucleoside inhibitor. Previously, Idenix announced plans to begin a phase II study of IDX719 in combination with Johnson & Johnson's simeprevir in the first half of the year. The company will also seek FDA permission to begin human studies of the nucleotide inhibitor before mid year.

-- Reported by Adam Feuerstein in Boston.

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Adam Feuerstein writes regularly for TheStreet. In keeping with company editorial policy, he doesn't own or short individual stocks, although he owns stock in TheStreet. He also doesn't invest in hedge funds or other private investment partnerships. Feuerstein appreciates your feedback;

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