(Reuters) – An experimental cancer drug developed by AstraZeneca and Daiichi Sankyo kept metastatic breast cancer at bay for months in women who had exhausted other treatment options, its first clinical study showed.

Patients on trastuzumab deruxtecan, also known as DS-8201, who had already undergone roughly six prior treatment courses, were saw no further progression for a median of 16.4 months.

Jose Baselga, AstraZeneca’s head of oncology research and development, said this patient group would normally encounter further cancer deterioration after about six months.

“This is a totally unprecedented clinical benefit,” he said of the so-called antibody-drug conjugate, which links powerful cell toxins to antibodies that cling to cancer cells, sparing the healthy ones that would be damaged in normal chemotherapy.

AstraZeneca signed a licensing and collaboration deal in March which committed the British drugmaker to paying up to $6.9 billion to its Japanese partner Daiichi.

Analysts have said the deal, and a $3.5 billion share issue to fund it, will require AstraZeneca to focus on cash flow.

Baselga added that as patients exhaust treatment options each new approach – or line of treatment – that is tried normally brings a lower clinical benefit than the one before.

The study dubbed DESTINY-Breast01 is the first of an envisaged 28 trials with trastuzumab deruxtecan.

Later trials will tackle breast cancer in earlier lines of treatment and also lung and gastric cancer.

“If this benefit is observed in this line of therapy I think that this drug can be incredibly beneficial in patients with earlier disease,” said Baselga.

Daiichi and AstraZeneca, which said in May that the main goal of the trial had been met, have already put in a request for approval with the U.S. Food and Drug Administration.

The full results are due to be presented at the San Antonio Breast Cancer Symposium this week.

(Reporting by Pushkala Aripaka in Bengaluru and Ludwig Burger in Frankfurt; Editing by Gopakumar Warrier/Keith Weir/Alexander Smith)

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