Sofosbuvir is used to treat Hepatitis C, a viral infection affecting the liver. Left untreated, the infection can lead to liver cirrhosis or liver cancer and has a high mortality rate.

Last week, the IPO rejected Gilead’s patent application covering the metabolites of sofosbuvir, although the main patent application covering sofosbuvir is still pending.

Gilead said it will approach the Intellectual Property Appellate Board (IPAB) or file a writ petition in the Delhi High Court. The legal battle could be protracted as the one involving Novartis’ blood cancer drug Gleevec in India.

“We believe the Patent Office’s Section 3(d) decision to be improper,” Gregg Alton, Executive Vice President, Corporate & Medical Affairs, Gilead Sciences, said in a statement.

“Gilead strongly defends its intellectual property. The company will be appealing the decision as well as exploring additional procedural options,” he said.