Preclinical Evaluation of CBP/β-catenin Inhibition as a New Strategy for Drug Resistant Acute Lymphoblastic Leukemia.

Despite advances in chemotherapeutic treatment of acute lymphoblastic leukemia (ALL), 20% of children relapse with high death rates, so that new treatment modalities are needed. Recent studies have demonstrated that survivin, a member of the inhibitor of apoptosis (IAP) family proteins, is upregulated in ALL of relapsed patients but not in drug-sensitive ALL. The expression of survivin depends on the formation of a complex between β-catenin and its co-activator CBP. Selective suppression of CBP/β-catenin signaling using the novel small-molecule inhibitor ICG-001 offers the opportunity to sensitize leukemia cells to conventional treatment. We hypothesize that inhibition of CBP/β-catenin signaling by combining ICG-001 with conventional therapy represents a promising therapeutic principle to eradicate drug resistant ALL. To test this hypothesis, we used a NOD/SCID xenograft model engrafted with drug-resistant human pre-B ALL leukemia cells (1x10⁶ cells/mouse) to first model the outcome of the patient in vivo. When human CD45 engraftment of 1% was detected by flow cytometry on day 26 post-leukemia-injection, VDL (Vincristine, Dexamethasone, L-Asparaginase) (n=7) or with saline as control (n=7) was administered for 4 weeks intraperitoneally (i.p.). Without treatment, all mice died between days 31–38 post-treatment with a median survival time (MST) of 36 days. In contrast, one animal of the VDL group died at day 14 post-treatment, the remaining 6 mice between days 67–77 post-treatment (MST=70 days, p<0.05 compared to control group), demonstrating that our xenograft model can mirror the outcome of the patient. Next, we tested whether ICG-001 in combination with standard chemotherapy can improve survival of mice engrafted with the resistant human pre-B ALL cells (1.5x10⁶ cells/mouse). Leukemic animals were treated i.p. with a combination of VDL and ICG-001 (25mg/kg/d) (n=3) or with VDL only as a control (n=2). The animals in the control group died on day 18 and 62 post-treatment (MST=40). In marked contrast, the animals treated with a combination of VDL+ICG-001 died on day 71, 72, 77 post-treatment (MST =72 days, p<0.05 compared to VDL group). Blood count analysis did not show side effects of ICG-001 on hematopoietic cells. We next determined the effect of ICG-001 on the expression of survivin by real-time (RT) PCR in recipients of human relapse T-ALL. Survivin mRNA expression was found to be downregulated after VPL+ICG treatment compared to treatment with VPL only. A greater number of animals and a higher dose of ICG-001 with optimized delivery via osmotic pump are being evaluated. Although limited by the small numbers of mice studied, the sustained survival of the mice treated with combination of standard chemotherapy and ICG-001 is compatible with the hypothesis that ICG-001 can sensitize drug resistant leukemia cells to treatment with standard chemotherapy and may lead to novel therapeutic options to overcome drug resistance.