Shares of big pharma AbbVie ( ABBV ) nosedived Monday after the U.S. Patent and Trademark Office agreed to review a challenge to a patent on its best-selling drug. The Patent Trial and Appeal Board accepted a request for inter partes review (IPR) by generic-drug startup Coherus Biosciences ( CHRS ) on patent 8,889,135, or the “methods” patent, for rheumatoid-arthritis treatment Humira. The patent is due to expire in 2025, but if Coherus is successful, it could launch its biosimilar version of Humira earlier. That might not happen too soon, though, since the review normally takes a year and is then open to appeal. AbbVie stock, which had opened flat on the stock market today , abruptly dropped 4% after the news came out mid-morning, to near 60, and ended Tuesday’s session at 60.23, down 3.6%. Coherus’ more lightly traded stock jumped 16%, to 18.85. AbbVie is No. 1 in its Industry Group — find out more at IBD Stock Checkup . Humira is currently the top-selling drug in the world, with $14 billion in sales last year. Coherus is one of several companies developing biosimilars of Humira — others include privately held Boehringer Ingelheim, which has some outstanding IPR requests on several different patents, and Amgen ( AMGN ), whose IPR request on a different Humira patent was denied back in January. Coherus has also filed IPR requests on two other patents. “Keep in mind AbbVie also has methods patents which cover other approved Humira indications, such as Crohn’s (disease), and we would expect additional IPRs/litigation to commence prior to potential biosimilar approvals (Amgen’s Humira biosimilar could be approved later this year),” wrote Evercore ISI analyst Mark Schoenebaum in an email. “Thus, these IPRs will likely be the first of many legal actions against AbbVie’s methods patent estate.”