Chronic lymphocytic leukaemia (CLL) is the most common form of leukaemia, a cancer of the white blood cells.
"The treatment today for CLL can be worse than the disease, leading to a great deal of side effects and death," said the lead investigator, Dr Richard R Furman from the Weill Cornell Medical College.
"This study, and others we have conducted on idelalisib, demonstrates that we may no longer need to use chemotherapy in CLL," Furman said.
"Even if this cancer remains incurable, it now can be treated as if it was a chronic disease with a pill, in the same way that high blood pressure is treated," Furman said.
CLL is a cancer of B cells, which normally produce antibodies to fight infections. In CLL, B cells grow out of control, accumulating in all of a patient's organs.
Patients are typically treated with a combination of chemotherapeutic drugs, to which they commonly respond. Unfortunately, patients ultimately relapse and require repeated cycles of chemotherapy.
With each relapse, the remissions become shorter until the patient either no longer responds, or is forced to stop taking the drugs because of their side effects, which are a result of the medications' inability to differentiate between healthy cells and cancer cells.
In this randomised, double-blinded study, researchers from 19 medical centres in five countries tested a combination of two targeted drugs - medications that attack cancer without damaging healthy cells.
They compared rituximab and idelalisib against rituximab and a placebo pill in 220 CLL patients who could not receive chemotherapy.
They found that those who received the combination of idelalsib and rituximab went longer without their disease worsening than those who received only rituximab, which has been the standard of care.
Six months into the study, cancers in 93 per cent of participants in the combination therapy group had not worsened, compared to 46 per cent of those in the rituximab plus placebo group.
Just 13 per cent of patients treated with rituximab alone responded to the therapy, compared to 81 per cent of the participants in the idelalisib treatment group.
A higher percentage of patients who received both drugs some 92 per cent were still alive a year after the study began, compared to 80 per cent of those who only received rituximab. About the same percentage of patients in each group suffered side effects from the treatments.
The contrast was so significant that an independent data-monitoring committee halted the study early, in October 2013, so that all of the study participants could receive idelalisib.