Abstract 1691: SL-401 activity against AML blasts correlates with CD123 levels and does not down-regulate CD123 expression

Abstract

Abstract Despite the progress, the current therapies for acute myeloid leukemia (AML) are not curative partly due to failure to eradicate the leukemic stem cells (LSCs) that repopulate the leukemia. CD123 is the alpha-chain of interleukin-3 receptor (IL-3R) that is expressed on AML and AML LSCs, while it is undetectable in normal hematopoietic stem cells. We report here time and dose dependent cytotoxic activity of SL-401(Stemline Therapeutics, Inc), a diphtheria toxin interleukin-3 fusion protein which is designed to selectively bind, internalize and induce apoptosis of CD123+ primary AML cells isolated from patients. AML blasts were obtained from leukapheresis of patients and cultured in RPMI1640 with 20% fetal bovine serum and antibiotics and supplemented with growth factors GM-CSF and SCF (10ng/ml) overnight before treatment with increasing concentrations of SL-401. CD123 expression of AML blasts was analyzed using multicolor flow cytometry before and 24 hours after treatment with SL-401. The viability of the cells before and after treatment with increasing concentrations of SL-401 was measured by annexinV-FITC and propidium iodide staining. SL-401 induced dose-dependent (SL-401: 10, 100, 1000ng/ml) cytotoxicity in primary AML in nanomolar range (trend p = 0.0002 at 48hr), which correlated with surface expression levels of CD123. AML CD34+CD33+ blasts showed high CD123 expression levels and were susceptible to cytotoxicity with SL-401 (P<0.0001,1000ng/ml at 48hr). CD34−CD33+ committed leukemic cells were also CD123+ and responded to SL-401. To determine if SL-401 modulated CD123 levels after exposure, AML blasts pre-treated with SL-401 for 24hr were analyzed by flow cytometry for receptor expression. Importantly, SL-401 treatment after 24 hours did not down-regulate CD123 expression on primary AML blasts indicating a lack of an escape mechanism for malignant cells. These results provide evidence that time- and dose-dependent activity of SL-401 against CD123+ AML blasts correlates with CD123 expression. Moreover, treatment does not down-regulate IL-3R expression. Clinical studies of SL-401 in patients with relapsed/refractory AML with evaluation of CD123 expression and blastic plasmacytoid dendritic cell neoplasm (BPDCN) are ongoing. Citation Format: Rajeswaran Mani, Bhavani Gopalakrishnan, Xiaokui Mo, Chris Brooks, William Blum, Gerard Lozanski, Sumithira Vasu, Natarajan Muthusamy. SL-401 activity against AML blasts correlates with CD123 levels and does not down-regulate CD123 expression. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 1691. doi:10.1158/1538-7445.AM2015-1691