(Reuters) - Coherus BioSciences Inc said on Monday the U.S. Patent and Trademark Office denied institution of its petition for a review of the formulation of AbbVie Inc's top-selling Humira rheumatoid arthritis treatment.

Shares of Coherus, which specializes in developing biosimilars or copies of biotech drugs, fell 16 percent to $23.70 in afternoon trading on Monday.

AbbVie's shares were up 5.7 percent at $59.25.

Such reviews, known as Inter Partes Reviews, allow anyone to challenge the validity of a patent far more cheaply and quickly than going through the U.S. federal court system.

Humira, also the world's biggest-selling drug, generates $15 billion annually, but investors are getting increasingly anxious with the drug's slowing sales and the expiry of its main U.S. patent in December.

Sales of the drug, also used to treat Crohn's disease and psoriasis, increased 11.3 percent in the third quarter, compared with growth of 14.9 percent and 17.4 percent, respectively, in the first and second quarters of 2016.

Denial of Coherus's petition on the Humira patent is a major positive for AbbVie and neutralizes the bear thesis that Coherus could launch in 2018, Jefferies analysts wrote in a note.

(Reporting by Ankur Banerjee in Bengaluru; Editing by Sriraj Kalluvila)