Acute lymphoblastic leukemia or acute lymphoid leukemia (ALL) is an aggressive type of blood cancer, characterized by overproduction and accumulation of cancerous, immature white blood cells, known as leukemic blasts  (Figure 1). These leukemic blasts are overproduced in the bone marrow affecting the synthesis of normal blood cells, causing a decrease in red blood cells, platelets and normal white blood cells. ALL is most common in childhood with around 60% of patients diagnosed younger than 20 years of age [1, 2]. Every year, approximately 6,000 new cases of ALL are reported in the United States .
Figure 1: Schematic of normal blood cells and ALL development steps
Standard treatment for ALL consists of three phases: (1) induction of remission, (2) consolidation of therapy, and (3) remission maintenance . During induction of remission, the patient is treated with intense chemotherapy to destroy the leukemic cells. However, small number of leukemic cells, undetectable in blood or bone marrow tests, may survive and leukemia is likely to come back. Consolidation therapy aims to stop this with more chemotherapy and, if the patient is suitable, with a bone marrow or stem cells transplant. Maintenance therapy aims to keep the leukemia in remission with low dose chemotherapy and short courses of steroids.
Approximately 90% of children with ALL will achieve remission after initial therapy and, among these, between 15 and 20% will experience relapse . While most adults experience remission after initial therapy, most patients also relapse, resulting in poor prognosis [2, 5, 6]. Treatment options for relapse or refractory ALL include different chemotherapy regimens, but these rarely result in long-term survival .
The lack of effective treatment options for patients that relapse after remission shows the urgent need for new therapies in ALL.