Leukotoxin (Leukothera™), a Targeted Therapy for Hematological Malignancies

Abstract 3284

Leukotoxin (Leukothera™) is a bacterial protein toxin that naturally targets and kills disease associated white blood cells (WBCs) expressing the activated form of leukocyte function antigen-1 (LFA-1). While leukotoxin has minimal effects on resting and healthy cells, it causes significant death of malignant WBCs associated with leukemias and lymphomas. Leukotoxin is a unique biologic in that it already provides both toxicity and specificity without requiring fusion of the protein to other molecules such as antibody fragments or cytokines. In vivo efficacy was demonstrated in an HL-60 SCID mouse leukemia model previously. In the present work, we compared leukotoxin to the standard chemotherapeutic agents, doxorubicin and cytarabine in a SCID mouse model using THP-1 leukemia cells. Mice (n=14 per group) were injected with THP-1 cells intravenously (i.v.) and then administered either leukotoxin or standard agents i.v. Mice received three doses, once daily, of leukotoxin (1.5 mg/kg) or five doses of doxorubicin (0.5 mg/kg) or cytarabine (10 mg/kg) once daily. Leukemic mice that were treated with leukotoxin maintained and increased body weight more effectively than those which received vehicle alone or the standard agents. The leukotoxin-treated mice showed significantly (p<0.001) higher mean survival than mice treated with vehicle or cytarabine over the 60-day observation period. Necropsy and histopathology of animals after the 60-day period revealed that test animals treated with vehicle or standard agents developed internal tumors on organs such as the thymus and lymph nodes. In contrast, none of the leukotoxin-treated mice developed internal tumors. To determine if leukotoxin could block migration of malignant cells, and thus formation of internal tumors, we performed a cellular migration assay using activated monocytes and human brain endothelial cells. It was found that even a low dose of leukotoxin (10 ng/ml) caused significant suppression (>80%) of monocyte migration across an endothelial barrier. Hence, leukotoxin has the potential to not only deplete malignant WBCs, but may also prevent their spread to other tissues. To test the general safety of leukotoxin, mice were injected with 1 mg/kg for three weeks. None of the mice showed signs of illness or changes in normal behavior and all continued to gain weight throughout the study period. In addition, to determine if a neutralizing antibody response to leukotoxin was generated, we challenged mice with high doses of leukotoxin and assayed monoclonal antibody from ten independent hybridomas. Results showed that antibody was able to bind to and recognize leukotoxin, but did not cause significant neutralization in a bioassay. In conclusion, leukotoxin may represent a highly effective and safe option for patients with hematologic malignancies, especially those with relapsed and refractory disease.