* GA101 meets primary endpoint in first late-stage study
* Roche developing GA101 as follow-on to MabThera/Rituxan
* Data on direct comparison with MabThera due later in 2013 (Adds details)
ZURICH, Jan 31 (Reuters) - Roche's GA101 drug significantly improved progression-free survival in people with chronic lymphocytic leukaemia (CLL), giving the Swiss drugmaker the first signal it may have a viable successor to its current best-selling medicine.
Roche is banking on the success of GA101 to fend off the threat of so-called biosimilar copies of cancer treatment MabThera, its top seller in 2012 with sales of 6.7 billion Swiss francs ($7.3 billion).
Roche said on Thursday data from the first late-stage study on GA101 found taking the drug together with chlorambucil, a chemotherapy, significantly reduced the risk of the disease worsening or death compared with chlorambucil alone.
A second set of data specifically comparing GA101 with MabThera, also known as Rituxan, is due later this year.
Daniel O'Day, head of Roche pharmaceuticals had told analysts at a results presentation on Wednesday that Roche was awaiting the results of several studies "to get confidence that this has the potential to give us a significant advantage on MabThera".
Roche hopes the data will help prove GA101's superiority to MabThera and help it convince investors and doctors it has a viable product to treat non-Hogkin's Lymphoma (NHL) and CLL. It plans to test GA101 in NHL if it proves successful in CLL.
Its strategy of developing "biobetter" drugs to replace older products has already been tested in its breast cancer portfolio. It won U.S. approval last June for Perjeta, a treatment for women with an aggressive form of breast cancer, which is a follow-on to its second-biggest seller Herceptin.
Although MabThera loses patent protection in Europe at the end of 2013, Roche does not expect to face competition from a biosimilar copy until 2016 as rivals still have to prove the efficacy and safety of their drugs in trials.
Several companies, including local rival Novartis and Celltrion, are racing to produce biosimilar copies of injectable biotechnology drugs like MabThera. (Reporting by Caroline Copley)