U.S. biotech Momenta Pharmaceuticals Inc said its experimental biosimilar version of AbbVie Inc's top-selling autoimmune drug, Humira, met the main goal in a late-stage trial involving patients with a form of psoriasis.

Humira, considered the world's biggest-selling medicine, generates $15 billion in annual sales and treats rheumatoid arthritis, Crohn's disease and psoriasis, among other conditions.

Biologics including Humira are manufactured in living cells, then extracted and purified. As they are more complex than traditional medicines and cannot be copied with precision, their knock-off versions are called biosimilars, rather than generics.

Several companies are developing biosimilar versions of top-selling biotech medicines for rheumatoid arthritis and cancer.

In September, Amgen Inc's Humira biosimilar was approved by the U.S. Food and Drug Administration - the first one to win U.S. regulatory nod for the drug. But AbbVie is trying to block its launch, contending that Humira's patents should protect it until at least 2022.

In Momenta's trial, adult patients with moderate-to-severe chronic plaque psoriasis received up to 48 weeks of treatment with Momenta's drug, called M923, Humira itself, or Humira alternating with M923.

The percentage of patients that achieved at least a 75 percent reduction on an index of psoriasis severity was equivalent between M923 and Humira, meeting the main goal of the study.

Momenta developed M923 in collaboration with Baxalta which is now part of Shire Plc - but in September, Shire decided to end its biosimilars program, and is in the process of transitioning the drug to Momenta.

Momenta's shares were up 7.9 percent at $15.00 in premarket trading.