By Deena Beasley

April 21 (Reuters) - Gilead Sciences GILD.O said on Monday a late-stage study showed that its drug Trodelvy used in combination with Merck's MRK.N immunotherapy Keytruda significantly slowed disease progression in patients with an aggressive type of breast cancer.

The Phase 3 trial compared the Trodelvy-Keytruda combo to a standard initial treatment of chemotherapy and Keytruda in 443 previously untreated patients with advanced triple-negative breast cancer whose tumors express PD-L1 - the protein targeted by immune system checkpoint inhibitors such as Keytruda.

Triple-negative breast cancer is known to not respond to hormone treatments or genetically targeted therapies. It accounts for 10% to 15% of breast cancer diagnoses, according to the American Cancer Society.

Trodelvy is an antibody-drug conjugate, a class of treatments designed to deliver an anti-cancer drug more precisely to malignant cells, unlike conventional chemotherapy that targets healthy cells too, potentially reducing damage to normal cells.

Dietmar Berger, Gilead's chief medical officer, said that with current treatment, patients can expect five to seven months of progression-free survival - the time until the disease begins to worsen.

He said the Trodelvy-Keytruda combo study is showing a trend toward better overall survival, but longer term follow ups with patients are needed to confirm that.

The findings are "statistically significant and clinically meaningful. ... This is a study that could potentially change the standard of care," Berger said.

The company said it will present detailed results from the study at a future medical meeting and discuss them with regulatory authorities.

Gilead said the safety profile of Trodelvy plus Keytruda was consistent with what has been seen when the drugs are used separately.

Trodelvy's label carries a boxed warning for neutropenia, a condition among people receiving cancer treatments that results in lower-than-normal levels of certain white blood cells, and for severe diarrhea.

The drug is already approved for patients with advanced triple-negative breast cancer who have received two or more prior therapies, and for previously treated hormone-receptor-positive, HER2-negative metastatic breast cancer patients.

Gilead is conducting several other Trodelvy studies, including a trial of the drug as an initial treatment for triple-negative breast cancer patients who do not express PD-L1.

(Reporting By Deena Beasley; Editing by Leslie Adler and Shinjini Ganguli)

((deena.beasley@thomsonreuters.com; 213 955 6746; Reuters Messaging: deena.beasley.thomsonreuters.com@reuters.net))