Eli Lilly, now facing one of the worst patent cliffs in its history, could find $15 billion in sorely needed relief if it beats the odds and wins a closely watched patent battle with generic drugmaker Teva Pharmaceutical Industries.

While a Lilly victory is not widely expected, a number of patent attorneys and industry analysts say the patent being challenged by Teva, which began Monday in Indianapolis federal court, will pass legal muster.

The U.S. Court of Appeals in Washington last summer upheld the validity of the basic patent on the chemical structure of Alimta. That protected Lilly's $2.6 billion-a-year lung cancer drug from generics until January 2017, including a six month patent extension granted by U.S. regulators for testing the drug in children.

The Indianapolis court will weigh the merits of a separate "method-of-use" patent on the way Alimta is administered. If the court upholds the patent, Lilly would be able to fend off generic alternatives to Alimta until 2022.

The so-called '209patent covers administration of two nutrients - folic acid and vitamin B12 - to patients before they receive Alimta, to prevent side effects of the drug. Alimta's package insert label instructs doctors to administer the nutrients prior to and during use of the medicine.

For a generic to win approval, it usually has to copy the language of the branded drug's label, Ben Hsing, a partner in the law firm of Kaye Scholer in New York, said in a recent interview.

But generics could have a hard time doing so because of Lilly's patented descriptions of the need to take the nutrients and how to do so, said Hsing, who last year successfully defended Roche Holding AG's Tarceva lung cancer drug from patent challenges by generic drugmaker Mylan Inc.

"This is the first time I've heard of a company going down this route to defend a patent, by adding nutrients" to a drug regimen, said Les Funtleyder, a healthcare strategist at investment firm Poliwogg. "So the likely outcome of this case is not clear cut."

The majority view, however, is that Teva will prevail over Lilly, according to Morningstar analyst Damien Conover, who said most method of use patents do not stand up in court.

Generic drugmakers Teva and Fresenius have challenged the validity of the '209 patent.

Lilly has said it expects cheaper copycats to flood the market in 2017, after Alimta's basic patent lapses. But the drugmaker on Monday said it plans to vigorously defend the method-of-use patent.

"We believe this patent is valid and enforceable and we are prepared to defend our intellectual property," said company spokesman Ed Sagabiel. "The significant scientific research that Lilly performed in support of the vitamin dosage regimen patent deserves intellectual property protection."

Sagabiel declined to comment further, including whether Lilly had changed its view and now expects to prevail in the case.

Lilly badly needs to hold onto whatever revenue it can, and to launch new products, to offset plunging sales of other drugs that have been hit by generics or soon will be.

Its Zyprexa schizophrenia treatment, with one-time annual sales of $4.5 billion, lost patent protection in late 2011, and its current flagship product, $5 billion-a-year antidepressant Cymbalta, goes generic in December. Adding to the pain, copycat forms of its blockbuster Evista osteoporosis treatment are due to arrive in early 2014.

Aside from assuring continued blockbuster sales of Alimta, a patent victory in Indianapolis would enable Lilly to use the drug as a calling card to introduce new cancer therapies to doctors, Funtleyder said, including a promising lung cancer medicine called necitumumab now in late-stage trials at Lilly.