Private equity firms are mulling over a $10 billion plan to buy and merge older drug brands of Britain's GlaxoSmithKline and France's Sanofi, the Financial Times reported on Sunday, citing sources.

U.S.-based KKR and Warburg Pincus were among the firms considering making bids for assets owned by GSK and Sanofi, the newspaper quoted several people familiar with the matter as saying.

GSK Chief Executive Andrew Witty in April said the drugmaker was reviewing its portfolio of mature products and wanted to dispose of off-patent drugs marketed in North America and Western Europe.

GSK in May invited firms to consider bidding on the portfolio that has annual sales of around 1 billion pounds ($1.7 billion). According to an internal document seen by Reuters in July, Sanofi held talks with listed and private equity firms in relation to the sale of a 6.3 billion euro ($8.5 billion) portfolio of mature drugs.

The FT quoted people close to the matter as saying that Blackstone, Advent, Apollo and Bain Capital were among the other private equity players to have shown interest in either or both of the portfolios.

Reuters reported in July that private equity firms looking at GSK products could be deterred from making bids as the company did not plan to sell the factories need to make the medicines as well as the sales forces required to sell them.

Officials for GSK and Warburg Pincus declined to comment, while all the other parties concerned could not immediately be reached for comment. – Reuters