* Study meets main goal of improvement in progression free survival
* GSK acquired Zejula from Tesaro
* Treatment competes with AstraZeneca's Lynparza (Rewrites, adds analyst comment, details on competitors)
By Noor Zainab Hussain and Pushkala Aripaka
July 15 (Reuters) - GlaxoSmithKline Plc's cancer treatment Zejula met the main goal of helping patients with ovarian cancer live longer without their disease worsening in a late-stage study, the company said on Monday.
The company bought the drug when it acquired U.S. cancer specialist Tesaro for $5.1 billion in December and Zejula is already approved for certain ovarian cancer patients.
The study, called PRIMA, tested the treatment as maintenance therapy in women who have undergone platinum-based chemotherapy and met the main goal for women regardless of their biomarker status, GSK said.
A biomarker status helps identify if patients have a particular genetic mutation which determines how a drug can be used and how its effectiveness is tested and measured.
For GSK, the success of Zejula would help it access a wider population group and give it an edge over rival PARP inhibitors such as AstraZeneca and Merck & Co's Lynparza and Clovis Oncology's Rubraca.
"Zejula's potential to expand PARP use beyond BRCAm (BRCA mutation) patients, was a key justification for its Tesaro acquisition and this required a positive outcome for Zejula in the PRIMA study," Jefferies analysts said.
Mutations in the BRCA genes impair the ability to repair DNA damage, which can drive cancer growth.
PARP inhibitors are a class of drugs which work by blocking enzymes involved in repairing damaged DNA, thereby helping to kill cancer cells, and are a growing focus for drug research, with potential for use in breast, lung and prostate cancers.
Jefferies analysts estimate potential peak sales of $550 million for Zejula in first-line ovarian cancer maintenance.
About 300,000 women globally are diagnosed with ovarian cancer each year, and only about 15% of patients are currently eligible to receive PARP inhibitors as their initial therapy, GSK said in a statement.
Ovarian cancer is the fifth most frequent cause of cancer death among women, it added.
Currently, AstraZeneca's Lynparza, which sells nearly three times more than Zejula, is approved as a first-line maintenance treatment for advanced BRCA-mutated ovarian cancer.
GSK has been focusing on bolstering its cancer drug pipeline, and earlier this year agreed to pay up to 3.7 billion euros ($4.17 billion) to Germany's Merck KGaA for the rights to a next-generation immunotherapy.
($1 = 0.8871 euros) (Reporting by Noor Zainab Hussain and Pushkala Aripala in Bengaluru; Editing by Arun Koyyur and Edmund Blair)