Sanofi will extend its fruitful partnership with Regeneron into the promising area of cancer immunotherapies with an investment worth up to $2.17 billion.

The two companies will jointly develop a programmed cell death protein (PD-1) inhibitor currently in Phase I testing. Some of the leading cancer immunotherapies are PD-1 inhibitors, with notable examples including Merck’s Keytruda (pembrolizumab) and Bristol-Myers Squibb’s Opdivo (nivolumab).

Sanofi and Regeneron say that they also plan to initiate clinical trials for other candidates in 2016.

"The field of immuno-oncology has shown the potential to dramatically improve outcomes for patients with certain types of cancer – however, the field is still in its very early days," says George Yancopoulos, chief scientific officer of Regeneron and president of Regeneron Laboratories. "We believe the approaches most likely to deliver the best results to patients will combine multiple innovative therapies acting on different pathways and targets both in the tumor and the body's immune response – and precisely target these medicines to the right patient."

Sanofi will make an upfront payment of $640 million to Regeneron. The companies will also jointly invest $1 billion for discovery of immune-oncology candidates through proof of concept development – with Sanofi contributing  $750 million and Regeneron $250 million – as well as equally funding $650 million for the development of the REGN2810 PD-1 inhibitor.

Regeneron will receive a milestone payment of $375 million if any PD-1 product produced by the partnership exceeds $2 billion in sales over a 12-month period.

The companies’ collaborations have paid off in the past – last week their Praluent (alirocumab) became the first PCSK9 inhibitor for high cholesterol approved in the US.