A treatment to starve brain tumours of blood has shown positive results in clinical trials.
Glioblastomas can be difficult to treat
However, US doctors say it is not yet clear whether the drug, AZD2171, will extend the lives of patients with some of the deadliest cancers.
The trials, published in Cancer Cell, were carried out on 16 patients at Massachusetts General Hospital with glioblastoma brain tumours.
In eight of them, the cancer shrank by half or more after treatment.
The drug works by interfering with the tumour's efforts to supply itself with enough blood as it grows by creating new blood vessels around it.
Without enough blood, and the oxygen it carries, parts of the tumour cannot grow any bigger, and may start to die away.
The patients involved in the trial all had glioblastomas which had come back despite standard treatments such as surgery, radiotherapy and chemotherapy.
The outlook for these patients is normally extremely poor, with survival times of less than six months for most.
Dr Tracy Batchelor, who led the trial, said: "Patients with recurrent glioblastomas desperately need new, effective treatment alternatives.
"While these are preliminary results of an initial trial, it's looking like these agents may play an increasingly important role in the treatment of patients whose tumours have recurred - and perhaps in newly diagnosed patients as well."
Within a month of the first dose of the drug, cancers had begun to shrink in most of the patients.
Overall, a three-quarters of the patients saw their tumour reduce in size by 25% or more, and half the patients saw at least 50% reduction.
Blood supply to the tumour appeared to be becoming more 'normal', and this opens the possibility that a conventional chemotherapy given following the new drug might be more effective.
The researchers did make clear that these were preliminary results, and published before the effect of the drug on overall survival times could be assessed.
Julie Sharp, from Cancer Research UK, said that the relatively small size of the trial meant that more evidence of the treatment's power would be needed.
"A number of research teams around the world are investigating drugs that target a tumour's blood supply for their potential to treat brain cancer.
"This early study shows some positive effects in a small group of patients.
"But, more long-term research in a larger number of people will be needed before we can tell if this drug has real potential for treating this form of brain cancer."