* Patients negative to Myriad HRD diagnostic test also benefit
* Their median progression-free survival benefit 3.1 months
* Lead researcher sees no need for Myriad test
* Growing interest in PARP inhibitor cancer drug class (Adds further comments from study leader and Myriad)
By Ben Hirschler
COPENHAGEN, Oct 8 (Reuters) - Tesaro's experimental drug niraparib improved outcomes for all women with recurrent ovarian cancer in a clinical study, boosting prospects for the product, part of a closely watched class of new medicines called PARP inhibitors.
The finding suggests that a Myriad Genetics companion diagnostic test, designed to select suitable patients, may not be necessary.
U.S. biotech company Tesaro already cheered investors in June by saying that the study had met the main goal of prolonging survival without disease worsening, but full results were only reported at the annual European Society for Medical Oncology congress in Copenhagen.
The treatment helped patients live longer without their disease progressing. Even those least likely to be helped by the drug because of their genetic profile saw a benefit of 3.1 months, researchers told the meeting on Saturday.
Niraparib and other PARP inhibitors block enzymes involved in repairing damaged DNA, thereby helping to kill cancer cells.
AstraZeneca's Lynparza became first of the new class of drug to reach the market when it won U.S. approval at the end of 2014, but it was only cleared for patients whose cancer tested positive for defective BRCA genes.
Tesaro hopes to show its drug can work across the entire population of women needing maintenance therapy for ovarian cancer after being given platinum-based chemotherapy.
Study leader Mansoor Raza Mirza of Copenhagen University believes that case has now been made.
"Our conclusion is that all patients have a benefit and all patients must be treated," he told reporters at the meeting.
"This is a breakthrough for patients with ovarian cancer. We have never seen such large benefits in progression-free survival (PFS) in recurrent ovarian cancer."
The latest findings showed that various patient populations all responded, with the biggest median PFS of 15.5 months seen in the BRCA mutation group.
However, even those with no BRCA mutation and who tested negative to another test called HRD, made by Myriad, still saw a benefit of 3.1 months.
There is now likely to be debate among doctors and investors as to whether this is enough for Tesaro to win very broad approval for its drug or whether it should be reserved for a narrower group of patients based on genetic testing.
Mirza is convinced it should be used widely and dismissed the need for the HRD test. "You don't need it because all patients benefit," he said.
He estimates niraparib could benefit 70 percent of all ovarian cancer patients, against 15 to 20 percent who are currently deemed suited for AstraZeneca's Lynparza.
Myriad's chief medical officer, Johnathan Lancaster, contested Mirza's view, arguing that a treatment benefit of just over three months was modest and patient selection still made sense.
Results of the study were also published online in the New England Journal of Medicine.
Interest in PARP inhibitors has grown apace over the last year as drug developers try to target DNA repair mechanisms inside cells as a way to fight cancer.
Other companies with PARP inhibitors in development include Clovis, AbbVie and Medivation, which was recently bought by Pfizer for $14 billion. (Editing by Alexander Smith and John Stonestreet)