SL-401, a Novel Targeted Therapy Directed to the Interleukin-3 Receptor (IL-3R), Blocks Plasmacytoid Dendritic Cell (pDC)-Triggered Myeloma Cell Growth and Prevents Osteoclastogenesis
Authors: Ray Arghya , Das Deepika Sharma , Ravillah Durgadevi , Song Yan , Macri Vincent , Brooks Christopher , Rowinsky Eric , Richardson Paul G. , Chauhan Dharminder , Anderson Kenneth C. 
Published: October 2019

Abstract
Introduction Despite the advent of novel therapies, relapse of multiple myeloma (MM) is common and the disease remains largely incurable. Our previous studies showed that bone marrow (BM) plasmacytoid dendritic cells (pDCs) play a central role in the immune deficiency characteristic of MM; as well as promote MM cell growth, survival, and drug resistance (Chauhan et al., Cancer Cell 2009, 16:309-323). These findings identify an integral role of pDCs in MM pathogenesis and provide the basis for targeting pDC-MM interactions as a novel therapeutic strategy in MM. In this context, we found that pDCs exhibit a high level of interleukin-3 receptor (IL-3Rα) expression, and pDC-MM interactions trigger secretion of interleukin-3 (IL-3), which in turn, induces MM cell growth and pDC survival, and promotes osteolytic bone disease in MM. These findings identified targeting IL-3R expressed on pDCs as a promising novel therapeutic strategy. Additionally, our preliminary data show that a significant number of clonogenic side population cells in MM (MM-SPs) with characteristic stem cell like features express IL-3R. Here we examined the effect of SL-401, a novel targeted therapy directed against IL-3R, on pDC-induced MM cell growth, osteoclast (OCL) formation, and MM-SPs.
Methods: Patient MM cells, pDCs, and MNCs were obtained from normal donors or MM patients. Cell growth/viability was analyzed using MTT/WST assays. OCL function and bone resorption were measured using the OsteoAssays and TRAP staining. RPMI-8226 cell line was used to isolate MM-SPs by flow-cytometry based Hoechst 33342 staining. SL-401 is a recombinant protein expressed in E. coli. The hybrid gene is comprised of human IL-3 fused to truncated diphtheria toxin (DT). The IL-3 domain of SL-401, which replaces the binding domain of DT, targets SL-401 to cells that overexpress IL-3R. SL-401 was obtained from Stemline Therapeutics, NY; bortezomib, lenalidomide, pomalidomide, and melphalan were purchased from Selleck chemicals, USA.
Results: SL-401 decreased the viability of pDCs even at low concentrations (IC50: 0.83 ng/ml; 14.6 pM, p < 0.001). SL-401 decreased the viability of MM cells at clinically achievable doses, without significantly affecting the viability of normal PBMCs. Co-culture of MM patient-derived pDCs induced proliferation of MM cell lines and patient MM cells; and importantly, SL-401 inhibited pDC-triggered MM cell growth (p < 0.005). Moreover, 3 of 5 samples were obtained from patients whose disease was progressing while on bortezomib, dexamethasone, and lenalidomide therapies. SL-401 blocked pDC-induced growth of dexamethasone-, doxorubicin- or melphalan-resistant MM cells. Combinations of SL-401 with melphalan, bortezomib, lenalidomide, or pomalidomide induced synergistic anti-MM activity (Combination index < 1.0). SL-401 blocked monocyte-derived osteoclast formation in a dose-dependent fashion, as well as restored MM patient BM-derived osteoblast formation. Finally, SL-401 also decreased the viability of IL-3R expressing MM-SPs.
Conclusions: Our preclinical study provides the basis for using SL-401 to directly target pDCs and inhibit the pDC-MM interactions, as well as target osteolytic bone disease and clonogenic side populations, in novel therapeutic strategies to enhance MM cytotoxicity, overcome drug-resistance, and improve patient outcome.

Disclosures
Macri: Stemline Therapeutics, Inc., New York, NY USA: Employment. Brooks:Stemline Therapeutics: Employment, Equity Ownership. Rowinsky:Stemline Therapeutics: Employment, Equity Ownership. Chauhan:Stemline Therapeutics: Consultancy. Anderson:BMS: Consultancy; Sanofi Aventis: Consultancy; Gilead: Consultancy; Onyx: Consultancy; Millenium: Consultancy; Celgene: Consultancy; Oncopep/Acetylon: Equity Ownership.