LONDON, Aug 5 (Reuters) - AstraZeneca and Daiichi Sankyo have secured faster-than-expected U.S. approval for their drug, Enhertu, in patients with a particular form of breast cancer.

The new U.S. Food and Drug Administration (FDA) approval - for patients with so-called HER2-low breast cancer - has opened a large, new multibillion-dollar market for the companies.

Enhertu originally won U.S. approval in late 2019 as a third-line treatment for the 15% of breast cancer patients with HER2-positive disease.

A host of targeted therapies has greatly improved the prognosis for patients with advanced HER2-positive breast cancer. But more than half of women whose breast cancer has spread to other organs and express little or no HER2 - referred to as HER2-low status - have limited treatment options. (Reporting by Natalie Grover in London; Editing by Kirsten Donovan)