Eli Lilly and Co. asked a U.S. trade agency on Thursday to block Hospira Inc. from selling a generic version of the cancer treatment Gemzar, saying the drug violates a patent on the process for making the active ingredient.

Lilly’s complaint to the U.S. International Trade Commission in Washington claims Hospira’s gemcitabine, Gemzar’s active ingredient, infringes a patent expiring in February 2014.

Global sales of Gemzar, used to treat lung, breast, pancreatic and ovarian cancer, fell about 14 percent to $905.8 million in the nine months ended in September, the Indianapolis-based company said Oct. 21.

Hospira began selling gemcitabine in November after Lilly lost an appeals court ruling that said a separate patent expiring in May 2013 was invalid.

The patented method is “the most commercially viable process available for manufacturing gemcitabine” and “involves less steps, higher yields and lower costs,” Lilly said in its complaint. Hospira has refused to provide records on the process it uses to make gemcitabine, according to the complaint.

Lilly is committed to protecting its intellectual property, Mark Taylor, a company spokesman, said in an e-mail Friday. Officials with Lake Forest, Ill.-based Hospira didn’t immediately return a message seeking comment.

The ITC has the power to bar imports of products that infringe U.S. patents and typically completes investigations in about 15 months.

Hospira sued Lilly in October. It’s seeking a court ruling that its process wouldn’t infringe the patent.

Lilly is facing generic competition for its three biggest products in the next two years: schizophrenia drug Zyprexa, antidepressant Cymbalta and the insulin Humalog. The three drugs generated $9.95 billion in 2009 sales, or about 46 percent of the company’s total revenue.