Pfizer's already-strong sales of Celebrex will get a further push after the US Patent & Trademark Office extended protection by granting a 'reissue patent' on the blockbuster painkiller.

Pfizer has been granted the reissue patent covering methods of treating osteoarthritis and other approved conditions with celecoxib, the active ingredient in Celebrex. These methods were claimed by a prior patent that was invalidated by a federal appeals court in 2008 and the move will extend the period which the drug can be marketed without generic competition by 18 months - from May 30, 2014 to December 2, 2015.

Now that the patent has been reissued, Pfizer has filed suit against Teva, Actavis, Lupin and Apotex. Each of these generic companies previously filed an abbreviated New Drug Application in the USA seeking approval to market their Celebrex copies beginning in May next year.

Sales of Celebrex in 2012 reached $2.72 billion and $1.75 billion of that came from the USA. Analysts had expected those figures to be hit substantially in 2014 but the Patent Office move means the COX-2 inhibitor will remain a big earner for sometime yet.