France's Sanofi has agreed to buy U.S.-based Blueprint Medicines Corporation for up to $9.5 billion to boost its position in rare immunology diseases, in the biggest deal struck by a European healthcare company so far this year,

The pharma giant is betting that Blueprint, whose research revolves around a certain kind of tyrosine kinase, will be a boost to its immunology business.

Sanofi agreed to buy Blueprint Medicines Corp. for at least $9.1 billion as the French drugmaker expands further in rare immunological diseases.