(Adds BTG reaction)
NEW YORK, Jan 18 (Reuters) - A U.S. administrative court invalidated a Johnson & Johnson patent on its blockbuster prostate cancer drug Zytiga, bringing rivals closer to selling generic versions and hitting shares in its British partner BTG Plc.
The Patent Trial and Appeal Board said a Johnson & Johnson patent describing a method of administering Zytiga should not have been granted because the process it described was obvious.
Wednesday's ruling was a victory for U.S. generic drug company Argentum Pharmaceuticals LLC, which had asked the board to invalidate the patent in the hope of being able to bring its own version to market.
Johnson & Johnson said it strongly disagreed with the decision and might ask a federal appeals court to reverse it.
Argentum did not immediately return a request for comment.
The patent is due to expire in 2027. Argentum and other generic drug makers have been blocked from launching their own versions of the cancer drug until its expiration date.
Zytiga generated nearly $2.3 billion in sales for Johnson & Johnson in 2016.
The drug is also a money-spinner for healthcare group BTG, which earns royalties on sales of a medicine that was originally developed by cancer researchers in Britain. BTG shares fell 5 percent on Thursday on fears for this revenue stream.
A BTG spokesman said its previous guidance on Zytiga royalties was unchanged and the group maintained that the earliest likely U.S. generic entry would be October 2018. (Reporting by Michael Erman and Jan Wolfe in New York and Ben Hirschler in London; Editing by Tom Brown and Alexander Smith)