"We missed the immunology wagon in recent years but we are committed to catching up and we have a significant number of assets," Jorge Insuasty, Sanofi's global head of development, told journalists on Wednesday.

Sanofi is working on 10 new cancer medicines. Only one of them, however - immunotherapy agent Cemiplimab, submitted along with U.S partner Regeneron and designed to treat cutaneous squamous cell carcinoma - has made it to a review process in the United States and the European Union.

Cancer immunotherapy has become the fastest-growing part of the $100 billion-a-year cancer drug market, with sales expected to top $25 billion by 2021, according to analyst forecasts compiled by Thomson Reuters.

Such prospects have led to relentless competition and experts warn many drugmakers are often following the same targets. For investors, this translates into greater scepticism and a need for ever more thorough analysis.

Because many firms are using similar molecular pathways in immunotherapy, the challenge is now to come up with smart combinations of drugs to succeed in fighting cancer.

This has proven a challenge so far. Earlier this year, a study showed an experimental drug developed by U.S Incyte had failed to bolster the effectiveness of Merck & Co's blockbuster Keytruda to treat skin cancer patients.

Switzerland's Roche also failed a key combination trial last month in colorectal cancer.

(Reporting by Matthias Blamont; Editing by Sarah White and Louise Heavens)