* British firms to square off at oncology meeting
* Investors seek insights from Zejula, Lynparza trials
* Readouts could be indicative of size of future markets
By Ludwig Burger
Sept 26 (Reuters) - GlaxoSmithKline and AstraZeneca will each present trial data on a promising new class of drugs against ovarian cancer on Saturday as Britain's top drugmakers compete to burnish their oncology credentials.
They will unveil results of studies for rival drugs to block a cancer's ability to repair its genetic code, presenting their findings at the European Society For Medical Oncology's (ESMO) annual conference in Barcelona over the weekend.
Both companies have already flagged that their respective drug candidates, part of a class known as PARP inhibitors, did well in staving off a return of ovarian cancer in women that had responded to initial standard treatment.
GSK and Astra, which are also competing in respiratory drugs, have held back full trial details for the ESMO event.
Analysts and investors will be looking for clues as to how large a patient group will likely benefit in the future.
Many cancer cells have a limited ability to make DNA repairs during cell division, as healthy cells would. This feature, one of the hallmarks of cancer, makes tumours genetically volatile and helps them develop resistance to treatment over time.
Drugmakers try to use that to their advantage with PARP inhibitors that block what is left of the DNA repair mechanism so cancer cells fail to replicate and a tumour cannot sustain itself.
The drug class was initially targeted at patients that had a mutation in BRCA genes but the two British drugmakers are keen to add to evidence that wider patient groups can be helped with their respective drugs.
"The utility of the PARP inhibitors could be much broader than just patients that have a tumour with the BRCA mutation," said John Bowler, who manages more than $300 million in healthcare and biotech stocks at asset manager Schroders.
"That becomes relevant when you start thinking about the drugs' role in other tumour types like prostate cancer and breast cancer where the incidence of new patients each year is much greater than in ovarian cancer," he said.
GlaxoSmithKline cancer treatment Zejula slowed the progression of ovarian cancer in a late-stage study called PRIMA, the British drugmaker said in a release of headline data in July.
The drug originated in the labs of U.S. cancer specialist Tesaro, which GSK acquired for $5.1 billion in December. The deal, masterminded by Chief Scientific Officer Hal Barron, was seen as too pricey by many analysts.
"The outcome of this trial is the key test of Hal Barron saying this will justify the deal on its own if it comes good. This would be vindication for the first big acquisition that he has done for Glaxo," said Bowler of Schroders.
Astra and its U.S. development partner Merck & Co announced on Aug. 14 that their joint drug Lynparza was shown to slow the progression of ovarian cancer that had started to spread in the body, in the so-called PAOLA-1 trial.
An inclusion requirement for trial participants had been a successful initial treatment with chemotherapy and Roche's Avastin following a diagnosis of advanced ovarian cancer.
Analysts see Astra and Merck's Lynparza in the lead with an average annual sales estimate of $3.1 billion in 2023. GSK's Zejula is seen at about 870 million pounds ($1.1 billion) in revenues that year.
Two more PARP inhibitors, Pfizer's Talzenna and Clovis Oncology's Rubraca, are seen as further behind.
($1 = 0.8039 pounds) (Editing by Edmund Blair)