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 deal includes a drug approved in the U.S. and the EU for systemic mastocytosis, a rare immunology disease, as well as an early-stage immunology pipeline, the companies said.

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