FOSTER CITY, Calif. ( TheStreet) -- Gilead Sciences' ( GILD) experimental cancer drug idelalisab demonstrated long-lasting tumor shrinkage in more than half of patients with advanced chronic lymphocytic leukemia (CLL). Responses were durable, with disease progression stalled for an average of 17 months -- noteworthy because CLL patients enrolled in the phase I study had already undergone an average of five prior therapies. Results from the idelalisab study in CLL were announced Wednesday night in advance of a presentation at next month's American Society of Clinical Oncology (ASCO) annual meeting. Gilead is known best these days for its dominant HIV drug business and soon-to-be-approved crop of hepatitis C therapies. But Gilead is also making a less-publicized push into cancer drug development. Idelalisab is the company's lead cancer asset, picked up through the acquisition of Calistoga Pharmaceuticals in 2011. In CLL and other blood-related cancers, idelalisab, a pill, is competing primarily against ibrutinib, a similar pill owned by Pharmacyclics ( PCYC) and Johnson & Johnson ( JNJ). In the Gilead phase I study, 54 patients with refractory or relapsed CLL received idelalisib. The rate of tumor shrinkage reported was 56%, including two patients with a complete response and 28 partial responses. Eighty-one percent of patients had a lymph node response. The median progression-free survival was 17 months -- better than typically expected for patients on their sixth-line therapy. The most common serious side effects associated with idelalisab were fatigue (2%), diarrhea (6%) and pneumonia (19%.) Fifteen percent of patients discontinued the study due to side effects, 7% of which were deemed to be potentially treatment related. "We are reaching a point in CLL where we have multiple agents in development that are very effective. Drugs like idelalisib are probably going to change the landscape of the disease in the next few years," said study investigator Dr. Jennifer Brown of the Dana-Farber Cancer Institute. "While this research is still early and ongoing, we hope this drug, along with others like it, will lead to prolonged survival and eventually help turn CLL into a condition that is treated like high blood pressure, where a patient can take a couple of pills every day. In the shorter term, these drugs may also provide an alternative to chemotherapy in elderly patients who tend not to tolerate chemotherapy well."