Inhibition Of Nampt As A Novel Therapeutic Strategy For Infant Leukemia

Novel targeted therapies are urgently needed for infant leukemia as this disease is highly aggressive and refractory to treatment resulting in poor survival rates. The NAD (Nicotinamide Adenine Dinucleotide) producing enzyme NAMPT (Nicotinamide Phosphoribosyltransferase) has been considered an attractive selective target for anti-cancer therapy due to the high dependency of tumor cells on NAD for energy metabolism and activity of NAD-dependent enzymes such as poly-ADP-ribose polymerases (PARPs) and sirtuins that play key roles in cancer cell survival. A newly developed NAMPT inhibitor, OT-82, was initially isolated for its high selective toxicity against a panel of adult leukemia cell lines. Here we investigated NAMPT inhibition as a therapeutic strategy for infant leukemias characterized by rearrangement of the MLL gene (MLL-r), by testing the potency of OT-82 in a panel of preclinical in vitro and in vivo models of MLL-r leukemia that are based on the use of patient-derived xenograft (PDX) cells. OT-82, as a single agent, dramatically reduced the viability of all tested MLL-r leukemia cell lines (n=9) and MLL-r leukemia PDX (n=6) with IC50s ranging from 0.15 to 3.82nM. While the IC50 for OT-82 correlated significantly with the IC50 of other NAMPT inhibitors STF-118804 and FK866, OT-82 was the most potent compound. When combining OT-82 with chemotherapeutic agents currently used to treat infants with leukemia, we observed significant synergy between OT-82 and cytarabine indicating the potential of OT-82 for chemosensitization. Consistent with NAMPT inhibition, OT-82 reduced cytosolic NAD+ levels in MLL-r leukemia cells and inhibited the activity of the NAD-requiring enzymes PARP-1 and SIRT-1, as exemplified by a decrease in PARylated PARP-1 levels and a p53-mediated increase in p21 levels, leading to apoptosis induction. Interestingly, despite the remarkable potency of OT-82 in killing MLL-r leukemia cells, a 25-fold difference in IC50 levels was noted across the cell line panel, with those lines harboring the MLL translocations most prevalent in infants, namely t(4;11) and t(11;19) translocations, being the most sensitive to the compound. A positive correlation was noted between baseline NAMPT mRNA levels and OT-82 IC50 (r=0.7712, P=0.015). Further in vivo testing of OT-82 showed impressive efficacy of the compound in MLL-r leukemia PDX-based animal models (n=6). OT-82 (p.o. 3x week, 40-50 mg/kg, for 3 or 6 weeks) was well tolerated and significantly delayed leukemia progression in 6/6 MLL-r leukemia xenografts with 5/6 achieving objective responses. OT-82 given as monotherapy was as effective as the routinely used triple combination treatment of vincristine, L-asparaginase and dexamethasone. Overall, these results demonstrate that NAMPT inhibition using OT-82 is highly effective against MLL-r leukemia and when combined with current chemotherapies may offer a more selective and potent therapeutic option for infants suffering from this disease. Citation Format: Klaartje Somers, Shiloh Middlemiss, Asel Biktasova, Mawar Karsa, Leanna Cheung, Angelika Kosciolek, Kathryn Evans, Chelsea Mayoh, Ursula R. Kees, Lioubov Korotchkina, Olga B. Chernova, Richard B. Lock, Andrei V. Gudkov, Michelle Haber, Murray D. Norris, Michelle J. Henderson. Inhibition of NAMPT as a novel therapeutic strategy for infant leukemia [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1175. doi:10.1158/1538-7445.AM2017-1175