Desmond-Hellmann, who until October was also on the board of the social networking company Facebook Inc, said she was stepping down from the Gates Foundation as of February 2020 to be able to "care properly for myself and my family."

Desmond-Hellmann was previously Chancellor of the University of California, San Francisco, the first woman to hold that position, and had worked at the pharmaceutical firm Genentech - now owned by Roche - where she helped develop some of the first gene-targeted cancer drugs, Avastin and Herceptin.

"This was without a doubt the toughest decision of my career. But... one of my mantras is: take your own pulse first. Over the last few weeks and months, I have done just that and concluded that I need to slow down," Desmond-Hellmann said in a letter to Foundation employees, a copy of which was shared with Reuters.

The Gates Foundation, led by Microsoft co-founder Bill Gates and his wife, Melinda, is one of the world's largest funders of global health programmes to combat disease and poverty. It has around $50 billion in assets.

Desmond-Hellmann's successor, Mark Suzman, joined the Gates Foundation in 2007 and is currently its chief strategy officer.

(This story corrects spelling of CEO's surname in headline, first sentence and throughout story to Desmond-Hellmann, not Desmond-Hellman. In penultimate paragraph, deletes "former" in describing Gates as company co-founder)

(Editing by Christina Fincher)

By Kate Kelland