CHICAGO ( TheStreet) -- Treatment with Roche's ( RHHBY) experimental GA101 -- the next-generation version of the company's blockbuster drug Rituxan -- helped elderly patients with chronic lymphocytic leukemia more than double the time before their disease worsened compared to chemotherapy. Initial data, not yet mature, also suggests GA101 may be superior to Rituxan. Results from a pivotal phase III study of GA101 were posted in a research abstract Wednesday night ahead of a planned presentation at the American Society of Clinical Oncology (ASCO) annual meeting. Last January, Roche announced that the GA101 study had met its primary endpoint but detailed data were held for presentation at the ASCO meeting. Roche has engineered GA101, also known as obinutuzumab, to be a more potent version of Rituxan, the company's top-selling hematological monoclonal antibody therapy with 2012 sales of approximately $7 billion. Rituxan sales, however, could start to fade in 2016 when a biosimilar version of the drug is expected to enter the market. For this reason, development and approval of GA101 has taken on some urgency for Roche in order to protect a key drug franchise. An FDA approval approval application for GA101 in CLL was submitted earlier this year and another phase III study of the drug in non-Hodgkin's lymphoma -- where Rituxan generates the bulk of its sales -- is also underway. In the phase III study, elderly CLL patients with other co-morbidities were randomized to treatment with GA101 plus chemotherapy, Rituxan plus chemotherapy or chemotherapy alone. In the first stage of the study, patients treated with GA101 plus chemotherapy had a median progression-free survival of 23 months (and growing because data are not yet mature) compared to 10.9 months for patients treated with chemotherapy alone. At the time of the data cutoff, GA101 reduced the risk of disease progression in these CLL patients by 86% relative to chemotherapy. The result was statistically significant. Seventy-five percent of CLL patients treated with GA101 reported tumor shrinkage, including 22 percent complete responses, compared to an overall response rate of 30% for chemotherapy and zero complete responses.