The Anti-Malarial Mefloquine Demonstrates Preclinical Activity In Leukemia and Myeloma, and Is Dependent Upon Toll-Like Receptor Signaling for Its Cytotoxicity

Abstract 290

Known drugs with previously unrecognized anti-cancer activity can be rapidly repurposed for this new indication, given their prior safety and toxicity testing. To identify such compounds, we compiled and screened an in-house library of on-patent and off-patent drugs and screened them to identify agents cytotoxic to hematologic malignancies. From this screen, we identified mefloquine, a quinoline licensed for malaria treatment and prophylaxis. In secondary assays, leukemia and myeloma cell lines were treated with mefloquine for 72 hours and cell viability measured by MTS. Mefloquine decreased the viability of 10/10 human and murine leukemia (LD50 <8.0 μM) and 9/9 human myeloma (LD50 <5.0 μM) cell lines; cell death was confirmed by Annexin V staining. Mefloquine also reduced the viability of 6/6 primary AML samples with LD50 < 5 μ M. These concentrations of mefloquine appear pharmacologically achievable based on prior studies conducted in the context of malaria treatment. In contrast to the effects on malignant cells, mefloquine was significantly less cytotoxic to normal hematopoietic cells (LD50 31.83 ± 5.38 μM) and murine monocyte-derived dendritic cells (LD50 17.56 ± 2.69 μM), Given its in vitro activity, we evaluated the effects of oral mefloquine in mouse xenograft models of leukemia and myeloma. Sublethally irradiated SCID mice were injected subcutaneously with OCI-AML2 or K562 human leukemia cells, MDAY-D2 murine leukemia cells, or LP1 human myeloma cells, and treated with 50 mg/kg mefloquine, or vehicle alone, by gavage. Oral mefloquine delayed tumor growth by up to 60% in all 4 mouse models without toxicity at doses that appear pharmacologically relevant to humans based on scaling for body surface area. Mefloquine's mechanism of action as an anti-malarial agent is unknown. Therefore, to determine the mechanism by which mefloquine induced cell death in malignant cells, we performed gene expression oligonucleotide array analysis of mefloquine-treated OCI-AML2 cells. At times preceding cell death, mefloquine altered the expression of genes associated with Toll-like receptor (TLR) signaling. For example, we detected 4.5-fold up-regulation of STAT1 and >10-fold up-regulation of its downstream targets, including OAS1, IFIT3 and TRIM22, by 24 hr after treatment. Upregulation of additional TLR targets IRF1, IRF7 and IL-8 was also noted by 8 hours after treatment. Mefloquine also induced early activation of NF-κB with a 2.5± 0.2-fold increase noted after 1 hr, using an ELISA-based DNA binding assay. In contrast to TLR activation in malignant cells, changes in TLR targets were not detected in mefloquine-resistant normal dendritic cells, suggesting that mefloquine's effects on TLR signaling were specific to malignant cells. We next investigated whether TLR activation was functionally important for mefloquine's cytotoxicity in malignant cells. STAT1 activity was required for mefloquine-mediated cell death, as U4A bladder sarcoma cells lacking JAK1 were resistant to mefloquine (LD50 14.6± 4.9 μM), compared to the mefloquine sensitive parental line (LD50 2.3± 0.4 μM). TLR signaling requires the immediate downstream adapter proteins MyD88 and TRIF1. To assess the functional importance of TLR activation for mefloquine induced cell death, we knocked down MyD88 and TRIF1 with siRNA. Double knockdown of MyD88 and TRIF1 completely abrogated mefloquine-induced cell death in K562 leukemia cells at concentrations where control cells exhibited up to 80% loss of viability. TLR signaling and up-regulation of STAT1 can increase reactive oxygen species (ROS) generation. Therefore, we measured ROS generation in leukemia cells after mefloquine treatment. Mefloquine increased ROS production in leukemia cells in a dose-dependent manner within 24 hr. Co-treatment with the ROS scavenger N-Acetyl-L-Cysteine abrogated mefloquine-induced ROS production and cell death. Mefloquine-induced ROS production was also abrogated in MyD88 and TRIF1 double knockdown cells. Our data suggest that the known anti-malarial mefloquine displays preclinical activity in leukemia and myeloma through a mechanism related to TLR activation. Thus, these results highlight TLR activation as a novel therapeutic strategy for the treatment of leukemia and myeloma. Moreover, given its prior toxicology and pharmacology testing, mefloquine could be rapidly advanced into clinical trial for patients with leukemia and myeloma.