Eli Lilly is due to start the defense of its blockbuster lung cancer therapy Alimta in a US district court later today in a bid to keep exclusivity for the product until 2022.

The lawsuit hinges on a method-of-use patent which covers the co-administration of Alimta (pemetrexed) with two nutrients - folic acid and vitamin B12 - that protect against the side effects of the drug.

Lilly has a composition of matter patent on Alimta - an antifolate drug - that expires in 2017. The use of the nutritional supplements to guard against the toxicity is on the US labelling for Alimta, however, so if it can defend the method patent from the challenge it will gain another five years of patent protection.

The generic challengers in the case will argue that the use of the nutrients to help offset the toxicity of Alimta is obvious and so does not warrant any additional intellectual property. The latest court case will see Lilly defend its patent against a generic drugmakers, including Teva, Barr Laboratories and APP Pharmaceuticals.

The additional protection would be worth billions of dollars to the US drugmaker, as Alimta pulled in $2.6 billion last year and is expected to reach $3.5bn-plus at peak. Moreover, it would give Lilly some additional breathing space to rebuild its product portfolio, which has already been hit by the loss of patent protection on former $4.5 billion antipsychotic brand Zyprexa (olanzapine) in 2011.

Now, Lilly is facing the start of generic competition to $5 billion-a-year antidepressant Cymbalta (duloxetine) in the US from December this year, while $1bn breast cancer drug Evista (raloxifene) loses patent protection in 2014.

A US district court affirmed the validity of the main patent for Alimta in 2011, while a federal court dismissed an attempt to overturn the method patent in October 2012 for "obviousness-type double patenting", in other words a claim that could logically have been interpreted from an earlier patent.

Lilly also has protection in Europe until 2015 under a composition patent filed with the European Patent Office (EPO), according to the company. Also in Europe, a method-of-use patent was challenged by the EPO with a ruling issued in Lilly’s favour in the first instance, although this is in the process of being appealed.