By Ludwig Burger and Ariane Luthi

(Reuters) -Swiss pharmaceutical giant Novartis has agreed to acquire Anthos Therapeutics, a biopharma firm majority-owned by Blackstone's drug development unit, for up to $3.1 billion, to boost its cardiovascular business.

Anthos was founded by private equity firm Blackstone's Life Sciences business and by Novartis in 2019 to develop, manufacture, and commercialise abelacimab, a treatment to prevent strokes and the recurrence of blood clots.

The deal underlines the Swiss group's commitment to cardiovascular therapies, one of five medical areas that it is focusing on, as its best-selling heart failure drug Entresto loses patent protection this year.

The transaction, expected to close in the first half of this year, also concludes a high-profile development partnership between a drug major and a private equity firm, an emerging funding model that has been explored by the industry.

Novartis will pay $925 million upfront with additional payments of up to $2.15 billion, contingent on development achievements, the companies said in statements.

A Blackstone spokesperson said the deal was the largest sale to date of a majority-owned Blackstone Life Sciences company.

Reuters reported in December 2023 that Blackstone was exploring the sale of Anthos.

Abelacimab belongs to a novel class of drugs known as factor XI inhibitors, designed to replace established blood thinners Eliquis by Bristol Myers-Squibb and Pfizer as well as Xarelto by Johnson & Johnson and Bayer, which are billion dollar sellers.

Bristol-Myers Squibb, together with Johnson & Johnson, are working in late stages of clinical testing on drug candidate milvexian.

Merck & Co is also in the factor XI race with a mid-stage development candidate. Germany's Bayer in 2023 suffered a huge setback in its effort to advance its factor XI drug.

Nicholas Galakatos, chairman of Anthos' board of directors and global head of Blackstone Life Sciences, said the firm was proud to have launched and helped grow Anthos.

"We believe abelacimab has the potential to be a leader in the new class of Factor XI anticoagulants and are pleased to have Novartis as a committed partner to advance the development and commercialization of abelacimab as a potential treatment ... for the millions of patients at risk of strokes," he said.

Anthos is conducting several phase 3 clinical studies, with data from these trials expected in the second half of 2026, Blackstone said.

Novartis said it already holds a small minority equity interest in Anthos, but declined to detail the size of it.

(Reporting by Ludwig Burger, Ariane Luthi and Paolo LaudaniAdditional reporting by Paul ArnoldEditing by Friederike Heine, Barbara Lewis and Kim Coghill)