June 4 (Reuters) – Gilead Sciences Inc’s (GILD.O) Trodelvy extended by 1.5 months, or 34%, the length of time women with advanced stages of a common type of breast cancer lived without their disease worsening, according to trial data presented on Saturday.The Phase III study compared Trodelvy to chemotherapy in 543 patients with hormone-sensitive tumors that test negative for a receptor called HER2 and who had stopped responding to at least two earlier lines of therapy.The data, presented in Chicago at the annual meeting of the American Society of Clinical Oncology (ASCO), show progression-free survival for the Trodelvy group of 5.5 months compared with 4 months for chemotherapy patients.”Some will argue is that really a clinically meaningful difference?” Dr. Julie Gralow, ASCO’s chief medical officer, told Reuters. “The real point here is we have proof of principle this drug works … I’m sure it will be used.”Trodelvy, also known as sacituzumab govitecan, uses a tumor-targeting antibody to deliver an anti-cancer drug.At the study’s first analysis, Trodelvy patients lived a median 13.9 months, compared to 12.3 months for the chemotherapy group – a difference that was not statistically significant.”We have to wait for that to mature,” Gilead Chief Executive Daniel O’Day told Reuters. “The final analysis for overall survival will most likely be in 2024 … but we’ll see if we take a look at it earlier.”He said Gilead intends to seek U.S. approval based on the recent data, pending routine discussions with the Food and Drug Administration.Patients with hormone-sensitive, HER2-negative metastatic breast cancer are treated with endocrine therapy combined with targeted drugs. As the disease becomes resistant, treatment is limited to chemotherapies with increasingly short durations of benefit.Gilead in March said the study met its main goal of reducing the risk of cancer progression or death, but did not disclose any details. Shortly afterward, the company wrote down by $2.7 billion the value of its 2020 acquisition of the drug’s developer, Immunomedics.”If data mature and a clearer overall survival benefit emerges, then we think there could be a viable path forward,” BMO Capital Markets analyst Evan Seigerman said in a research note on Saturday.But he expressed caution about potential competition from Enhertu – an AstraZeneca Plc (AZN.L) drug that is being studied for breast cancer patients with low levels of HER2.The most common adverse side effects reported for Trodelvy patients were low white blood cell counts and diarrhea.Trodelvy is already FDA-approved for previously treated metastatic triple-negative breast cancer and bladder cancer. It is being studied for use in a number of cancer types.O’Day said Gilead expects to begin in the second half of this year a trial of Trodelvy as an initial treatment for hormone receptor-positive, HER2-negative breast cancer.The drug has a U.S. list price of $2,188 per vial, but Gilead did not provide details of the average dose or duration of treatment.Wall Street analysts, on average, expect annual sales of the drug will top $2 billion 2026, according to Refinitiv.