MARLBOROUGH, MA—Boston Scientific Corp. has agreed to buy rival BTG Corp. for $4.2 billion to expand its offering of medical devices to treat cancer and other disorders. Boston Scientific makes stents that hold open damaged blood vessels. BTG makes medical technology for physicians, such as cryoablation products to freeze and destroy diseased cells, and radiotherapy that delivers radiation straight to tumors. The company also makes medicines and antidotes against snake venom for emergency care.

The acquisition “will augment our capabilities in important areas of unmet need” such as cancer and harmful blood clots, according to Boston Scientific Chief Executive Officer Michael Mahoney. The transaction would be funded with a combination of cash on hand and proceeds from debt financing. 

The purchase is Boston Scientific’s biggest since 2005, when it acquired device maker Guidant for $25.2 billion, according to data compiled by Bloomberg.