Mixed Lineage Leukaemia (MLL) gene rearrangement is detected in 80% of acute lymphoblastic leukaemias (ALL) diagnosed in infancy (<12 months). A biologically distinct disease, infant MLL-rearranged leukaemia is highly aggressive and often refractory to treatment, with less than 50 % of patients surviving their disease. Hence there is an urgent need for targeted therapies to improve survival in MLL patients.
High Throughput Screening identified a novel small molecule inhibitor, OT-82, with selectivity against haematopoietic cancers. Cytotoxicity assays showed OT-82 to be particularly effective against cell lines harbouring MLL gene rearrangement, causing reduced cell viability at concentrations in the low nanomolar (nM) range (IC50 0.31-1.8 nM). Affinity chromatography identified nicotinamide adenine phosphoribosyltransferase (NAMPT) as the likely target of OT-82. This was further confirmed in cytotoxicity assays where nicotinic acid addition had a protective affect against OT-82 in haematopoietic cells.
A significant reduction in leukaemia burden was also observed in relevant preclinical animal models of MLL-rearranged ALL. Severe combined immunodeficient (SCID) mice subcutaneously xenografted with the leukaemia cell line MV4;11 (MLL-AF4) were treated with single agent OT-82 which resulted in an 80 day event free survival (EFS), representing a 60 day leukaemia growth delay (LGD) compared to mice receiving vehicle control. In systemic leukaemia models where primary infant MLL-ALL samples, derived from either MLL-AF4 or MLL-ENL patients, were engrafted in non-obese diabetic/SCID (NOD/SCID) mice, OT-82 significantly increased EFS and LGD for both xenografts (MLL-AF4: 45.6 day EFS, 41.4 day LGD; MLL-ENL: 67.8 day EFS, 57.6 day LGD, p < 0.0001 in each case).
In summary, we have identified a novel small molecule compound, OT-82, as a promising inhibitor of the NAD+ pathway that may ultimately be clinically useful for the treatment of infant leukaemia and other MLL-translocated cancers. Further preclinical studies are underway to determine the utility of OT-82 in combination with currently used standard and high-risk treatment regimens.