In vitro and ex vivo activity of SDX-105 (bendamustine) in drug-resistant lymphoma cells

Abstract

1215 SDX-105 (bendamustine), currently in Phase II clinical trials in the US for the treatment of rituximab relapsed/resistant Non-Hodgkin’s Lymphoma (NHL), has been shown to have clinical activity in patients refractory to conventional alkylator chemotherapy. To better define the activity and mechanism of action of SDX-105, we generated resistant lymphoma cells and assayed ex vivo NHL specimens from patients previously shown to be resistant to cyclophosphamide, doxorubicin, and etoposide. Two SDX-105 resistant cell lines were generated by continuous exposure to increasing concentrations of the drug for over a year. SU-DHL-1 cells, an anaplastic large cell lymphoma cell line, demonstrate an IC50 of approximately 20 μM. SDX-105-resistant SU-DHL-1 cells displays an IC50 of approximately 150 μM. SU-DHL-9 cells, EBV-negative diffuse large B-cell lymphoma, have an IC50 of approximately 20 μM while the SDX-105-resistant SU-DHL-9 cells have an IC50 of approximately 80 μM. Initial experiments show no cross-resistance of both of these resistant cell sublines to chlorambucil, phosphoramide mustard (a cyclophosphamide metabolite), fludarabine, cisplatinum, and doxorubicin. An initial analysis of the effect of SDX-105 on the SU-DHL-1 gene expression profile using the Affymetrix genechip has been performed. A comparison between the parental and SDX-105-resistant cells will allow the identification of genes potentially important for the resistant phenotype. Activity of SDX-105 was also evaluated using the DiSC® assay, a clinically validated assay with the potential to predict patient treatment response to traditional chemotherapeutic agents. Twenty primary NHL specimens obtained from patients who failed prior chemotherapy were tested with SDX-105 and an active metabolite of cyclophosphamide (4-HC). At equipotent concentrations, SDX-105 was cytotoxic to NHL specimens defined as 4-HC resistant (P = 0.0004). In addition, SDX-105 showed a statistically significant increase in cytotoxic activity in the samples that were previously analyzed and defined as doxorubicin-resistant (P = 0.000098) and etoposide-resistant (P = 0.00038). These data suggest that SDX-105 is a novel chemotherapeutic agent with a mechanism of action and drug resistance potentially distinct from other DNA-damaging agents. In addition, the activity in multidrug resistant NHL tumors suggest that SDX-105 is a promising agent for the treatment of patients refractory to conventional cytotoxic therapy.