U.S. regulators have granted a priority review to AstraZeneca's ovarian cancer drug Lynparza as a treatment for breast cancer, putting it on track for potential approval in the new disease area during the first quarter of 2018.

The medicine, which is being jointly developed and marketed with Merck under a deal struck in July, is the first poly ADP-ribose polymerase (PARP) drug to be considered for use outside ovarian cancer.

Lynparza is also being tested in prostate and pancreatic cancers, the two companies said on Wednesday.

The successful breast cancer submission to the U.S. Food and Drug Administration follows a clinical study showing advanced breast cancer patients with certain gene mutations experienced double the response rate and delayed disease progression when treated with Lynparza compared with chemotherapy.

Lynparza - abandoned at one stage by AstraZeneca but revived by CEO Pascal Soriot when he took over in 2012 - became the first PARP drug to reach the market when it won U.S. approval for ovarian cancer at the end of 2014.

It now faces competition from rival products made by Tesaro and Clovis Oncology that are also approved for ovarian cancer.

PARP inhibitors work by blocking enzymes involved in repairing damaged DNA, thereby helping to kill cancer cells.