AstraZeneca, which is working on the drug with Japan's Daiichi Sankyo 4568.T, said on Monday that Enhertu prolonged survival and slowed the progression of metastatic breast cancer with low levels of a protein known as HER2.

The improvement was "clinically meaningful" when compared with standard chemotherapy, it said, adding that detailed results of the late-stage trial would be presented at an as-yet undisclosed medical conference.

The company said it would reach out to regulatory agencies to enable a speedy review of a wider use for the drug.

While the study was limited to low-HER2 patients whose tumours had spread to other parts of the body, analysts have said a positive trial read-out could portend future use at an earlier stage of the disease with potentially hundreds of thousands of new eligible patients per year.

The read-out is set to bolster the Anglo-Swedish company's status among analysts as one of the world's fastest growing major pharma groups, thanks to a high success rate in cancer drug development.

It has forecast higher group revenues for 2022, despite a decline in sales of its widely used COVID-19 vaccine, driven by cancer drugs such Tagrisso against lung tumours and treatments for kidney disease and rare conditions.Read full story

The company's shares were up 1.4% at 8.921 pence at 0823 GMT, outperforming a 0.5% increase in the STOXX Europe 600 Health Care index .SXDP and extending a 5% gain over the previous four trading sessions.

"HER2 low is a large and previously unaddressed patient pool in breast cancer," analysts at Credit Suisse said in a note, adding they saw a 50% probability of $3 billion peak sales potential from the new patient group.

Enhertu belongs to a promising class of therapies called antibody drug conjugates, which are engineered antibodies that bind to tumour cells and then release cell-killing chemicals.

AstraZeneca secured rights to the Daiichi Sankyo compound three years ago in a deal worth up to $6.9 billion. Enhertu has since been shown to help women with metastatic breast cancer characterised by high levels of HER2 when compared to Kadcyla, the ADC drug from Switzerland's Roche ROG.S - the world's biggest cancer drug maker.

This led to initial market approval in late 2019. A further clearance was won for HER2-driven gastric cancer. The drug is also being tested against other forms of gastric, lung and colorectal cancers.