AstraZeneca (AZN) and Daiichi Sankyo said Monday the European Medicines Agency has validated the type II variation marketing authorization application for their drug, Enhertu, as a first-line treatment for patients with HER2-positive metastatic breast cancer.

The validation was based on a 1,157-patient phase 3 trial result that showed Enhertu in combination with pertuzumab "significantly improved" progression-free survival, compared with current first-line standard of care in HER2-positive inoperable or metastatic breast cancer, the companies said.

The validation launches the scientific review process for the indication by Europe's Committee for Medicinal Products for Human Use.

Enhertu was discovered by Daiichi Sankyo and is being jointly developed and commercialized by the company and AstraZeneca.

Shares of AstraZeneca were down over 3% in recent premarket activity Tuesday.