Breast cancer experts are hailing what could be some of the biggest advances in more than a decade - two new medicines that significantly delay the time until women with very advanced cases get worse.
In a large international study, an experimental drug from Genentech called pertuzumab held cancer at bay for an average of 18 months when given with standard treatment, as opposed to 12 months for others given only the usual treatment.
It also strongly appears to be improving survival, and follow-up is continuing to see if it does.
"You don't see that very often. ... It's a spectacular result," said one study leader, Dr Sandra Swain, medical director of Washington Hospital Centre's cancer institute.
In a second study, another drug long used in organ transplants but not tried against breast cancer - everolimus, sold as Afinitor by Novartis AG - kept cancer in check for an average of seven months in women whose disease was worsening despite treatment with hormone-blocking drugs. A comparison group that received only hormonal medicine had just a 3-month delay in disease progression.
Afinitor works in a novel way, seems "unusually effective" and sets a new standard of care, said Dr Peter Ravdin, breast cancer chief at the UT Health Science Centre in San Antonio, Texas. He has no role in the work or ties to drug makers.
Most patients have tumours like those in this study - their growth is fuelled by oestrogen.
Results were released at the San Antonio Breast Cancer Symposium and some were published online by the New England Journal of Medicine. They come a few weeks after US government approval was revoked for another Genentech drug, Avastin, that did not meaningfully help breast cancer patients. It still is sold for other tumour types.
The new drugs are some of the first major developments since Herceptin came out in 1998. It has become standard treatment for a certain type of breast cancer.
But the new drugs are likely to be very expensive - up to £6,360 a month - and so far have not proved to be cures. Doctors hope they might be when given to women with early-stage cancers when cure is possible, rather than the very advanced cases treated in these studies.