Merck & Co says its biosimilar of Sanofi's blockbuster insulin treatment Lantus is about to enter late-stage trials.

The US drugs giant has expanded its collaboration with Samsung Bioepis (the joint venture set up between South Korean electronics giant Samsung and Biogen Idec) to cover Merck's version of Lantus (insulin glargine). The partners, which first teamed up in February last year to develop multiple biosimilar candidates, say that Phase III studies in patients with type 1 and type 2 diabetes will begin soon.

Lantus is a huge earner for Sanofi with fourth-quarter sales climbing 19.9% to 1.51 billion euros. About two-thirds of that comes from the USA but the treatment goes off-patent there in February next year.

As it faces up to competition, Sanofi has recently sued Eli Lilly alleging four cases of patent infringement for the Lantus copy the latter is developing with Boehringer Ingelheim and which is already being reviewed by the US Food and Drug Administration.

The lawsuit has triggering a 30-month automatic stay of approval by the FDA so Lantus will likely have the market to itself until the summer of 2016.