FRANKFURT, March 20 (Reuters) - Swiss drugmaker Novartis on Friday agreed to buy a breast cancer drug candidate for up to $3 billion from U.S. biotech firm Synnovation Therapeutics, adding a targeted therapy to its pipeline of cancer drugs.

The company will pay $2 billion upfront and up to $1 billion that is contingent on further development achievements as part of the deal.

The experimental drug, SNV4818, belongs to a class of selective PI3Kα inhibitors, a new approach for the treatment of a type of breast cancer known as HR positive/HER2 negative and potentially other solid tumours.

The acquisition adds to a growing pipeline of targeted cancer therapies, including a radioligand therapy candidate, that Novartis is already testing. SNV4818 is currently in early-stage trials and has shown promising activity against tumors in lab studies, Novartis said.

Synnovation's drug targets only the mutated form of PI3Kα, an enzyme that often malfunctions in breast and other forms of cancer, while sparing the normal version found in healthy cells. And it aims to avoid the side effects seen with existing PI3Kα-inhibiting therapies.

"While mutated PI3Kα is a well-established driver in HR+/HER2- breast cancer, there remains a challenge in achieving effective pathway inhibition with a tolerable therapeutic profile," said Shreeram Aradhye, the drugmaker's chief medical officer.

Novartis expects the deal to close in the first half of this year.