The IL-3Rα-Targeted Drug SL-401 Selectively Kills Blastic Plasmacytoid Dendritic Cell Neoplasm Cells
Authors: Angelot-Delettre Fanny , Frankel Arthur E. , Liu Jen Sing , Seilles Estelle , Biichle Sabeha , Drenou Bernard , Jacobson Mark L. , Cirrito Thomas P. , Yazid Arkam , Deconinck Eric , Brooks Christopher L. , Saas Philippe , Ottou Francine Garnache 
Published: October 2019

Abstract
Abstract 2588
Blastic plasmacytoid dendritic cell neoplasm (BPDCN) is a rare and aggressive neoplasm derived from plasmacytoid dendritic cell precursors that involves the skin and invariably leads to aggressive leukemic dissemination (Garnache Ottou et al., 2007). The incidence is more frequent in men and elderly subjects, but BPDCN may occur in young adults and even in children. Current treatment options are not codified, and although conventional chemotherapy often induces an initial response, relapse is frequent and rapid. The prognosis is poor, with a median overall survival of 9–13 months whatever the initial presentation was. Allogeneic hematopoietic transplantation is currently the only potentially curative treatment, but this is not an option for most elderly patients. Therefore, there is an urgent need to develop novel therapies that can specifically target this tumor type. Since BPDCN cells express high levels of the interleukin-3 receptor (IL-3R) α-chain (CD123), we tested SL-401, a novel biologic conjugate that targets IL-3Rα (Konopleva et al. 2010 and Frolova et al. 2010), against 2 BPDCN cell lines (GEN2.2 and CAL-1) and 6 primary BPDCN cells isolated from 5 patients (P#1-5). Cells from P#2 were tested at diagnosis (#2d) and at relapse (#2r). A CD123+ cell line (TF-1/H-ras) sensitive to SL-401 and a CD123− cell line (Daudi) were used as controls. Cytotoxicity was assessed by MTT assay as well as flow cytometry (FC) after Annexin-V (AV) and 7-AAD staining. Primary BPDCN cells were cultured with IL-3 alone (to maintain survival) or IL-3 in combination with SL-401 for 24 h (FC) or 48 h (MTT) (Figure 1). All BPDCN cell lines and primary cells were found to be sensitive to SL-401 by MTT assay (Figure 1A). These results were confirmed by FC with a dose-dependent decrease in cell viability observed for the GEN2.2 and CAL-1 lines, (47 ± 10 to 2 ± 2%, n = 4, and 90 ± 7 to 11 ± 5%, n = 4, respectively) when treated with SL-401 compared to untreated cells (48 ± 9% and 88 ± 5%, for GEN2.2 and CAL-1 respectively). Cell viability decreased for all the 6 primary cells tested by FC (Figure 1B). The Daudi negative control cell line was resistant to SL-401 (Figure 1A), which confirmed SL-401 specificity.Figure 1.In vitro sensitivity of BPDCN cell lines (CAL-1 and GEN2.2) and primary cells from 5 BPDCN patients to SL-401 assessed by FC and MTT assays.Figure 1. In vitro sensitivity of BPDCN cell lines (CAL-1 and GEN2.2) and primary cells from 5 BPDCN patients to SL-401 assessed by FC and MTT assays.
This is the first study evaluating the in vitro sensitivity of BPDCN using the IL-3R targeted drug candidate, SL-401, which is currently being evaluated in clinical trials of patients with acute myeloid leukemia (AML), myelodysplastic syndrome, and chronic myeloid leukemia. Importantly, the highest concentration of SL-401 evaluated in these in vitro studies was ten times lower than the peak plasma concentration achieved in AML patients (Frankel et al, 2008). Since all BPDCN patients evaluated to date express high levels of CD123 (Garnache Ottou et al, 2009), these results suggest that BPDCN patients may clinically benefit from SL-401 therapy. This new strategy should be evaluated in a clinical trial.
A. Percentage of viable cells from BPDCN patients (#1, #4, and #5, n = 1) or of viable CAL-1 and GEN 2.2 cells (n = 3) assessed by MTT assay after incubation with different concentrations of SL-401 or without drug (cells only). Untreated cells were considered as 100% viable cells. The Daudi cell line (CD123−) was used as a negative control (n = 3). B. Percentage of viable cells (AV and 7-AAD negative, as measured by FC) after incubation with different concentrations of SL-401 or without drug (cells only) for 24 h. The histogram represents a mean of 3 (P#1), 4 (P#2d), and 6 (P#2r) independent experiments. Samples (P#3- 4, and- 5) were each tested once.

Disclosures:
Frankel: Stemline Therapeutics: Patents & Royalties, Research Funding. Jacobson:Stemline Therapeutics, Inc: Employment, stock Options. Cirrito:Stemline Therapeutics, Inc: Employment, Equity Ownership, Patents & Royalties. Brooks:Stemline Therapeutics, Inc: Employment, equity options.