Olaparib, an experimental drug from AstraZeneca found to have shrank or stabilized tumors in around half of ovarian cancer patients with an inherited gene mutation in early-stage clinical tests.

Stan Kaye, head of the Section of Medicine at the Institute of Cancer Research (ICR), said the results were very encouraging.

He said that the findings raise the possibility that carefully selected patients in future may well be offered olaparib as an alternative to chemotherapy.

Out of around 50 women with confirmed or suspected BRCA1 or BRCA2 gene mutations given olaparib in an expansion of a Phase I study, 20 responded with their tumors shrinking or with significant falls in a key cancer marker. The drug also stabilized disease in a further three patients.

It was effective for an average of seven months, although several patients were still taking olaparib nearly two years later, researchers from the ICR, the Royal Marsden Hospital and AstraZeneca reported in the Journal of Clinical Oncology.

Side-effects were mild, especially when compared to current chemotherapy treatments.

Platinum-based chemotherapy is one of the main treatments used for ovarian cancer. However, it can make patients very sick and typically loses effectiveness over time.

Olaparib efficacy was also lower in platinum-resistant patients but its clinical benefit rate was still 46 percent in this group, the study found.

Up to 15 percent of ovarian cancers have known BRCA1 or BRCA2 mutations. Experts believe olaparib may also be effective in cancers linked to other DNA repair defects, so the drug could have a role in about half of cases.

Randomized trials of olaparib in ovarian cancer are now underway and results will be available later this year.

Promising results with olaparib and another PARP drug from Sanofi-Aventis in women advanced breast cancer were one of the highlights of last year's meeting of the American Society of Clinical Oncology.

PARP is short for poly (ADP-ribose) polymerase, which is used by cancer cells to repair DNA damage. By blocking the enzyme, the drug is designed to undermine the ability of cancer cells to heal themselves.