SL-401, a Novel IL-3Rα/CD123-Directed Agent Targets Stem-like Cells in Multiple Myeloma
Authors: Ray Arghya , Song Yan , Das Deepika Sharma , Macri Vincent , Chen Janice , Paba-Praba Claudia , Richardson Paul G. , Brooks Christopher L. , Chauhan Dharminder , Anderson Kenneth C. 
Published: October 2019

Abstract
Introduction Multiple myeloma (MM) remains incurable despite novel therapies, highlighting the need for further identification of factors mediating disease progression and resistance. One such factor is the development of a drug-resistant stem cell-like subpopulation. Previous studies have shown that MM side population cells (MM-SPs) exhibit stem-cell like features and contribute to relapse of MM. Recent research efforts, which have focused the biology of MM stem-like cells in order to derive specific therapy, have shown that stem-cell transcription factor Oct-4 is linked to stemness and chemoresistance. Here we examined the effect of enforced expression of Oct4 in MM cells and MM-SPs on the development of stem cell-like characteristics and drug-resistance in MM. These studies allow us to establish stable MM cell lines with characteristic stem-cell like features, which in turn facilitate screening of novel agents that effectively target this cell population in MM.
Methods MM-SPs were isolated from RPMI-8226 cells by flow-cytometry based Hoechst 33342 staining. RPMI-8226 and RPMI-8226-SP cells were transfected with a phOct4-GFP construct (Gerrard et al., Stem Cell 2005, 23:124-133), and selected with G418 (0.5 mg/ml) to derive stable RPMI-8226-Oct4 and RPMI-8226-SP-Oct4 cell lines. Oct-4 expression was confirmed using FACS. Cell viability was analyzed by WST assays. SL-401 is a targeted therapy directed to IL-3Rα/CD123, comprised of recombinant human IL-3 fused to truncated diphtheria toxin. Drug and reagent source: SL-401 was obtained from Stemline Therapeutics; Bortezomib and flow antibodies were purchased from Selleck Chemicals and BD Biosciences, respectively. Statistical significance was derived using GraphPad Prism.
Results 1) RPMI-8226 and RPMI-8226-Oct4 cells were analyzed for the expression of surface markers associated with stem cells (CD123/IL-3Rα, CD133 and CD27) by multicolor flow analysis. Oct-4 transfection does not affect the overall CD123 expression in RPMI-8226 cells, as the % MFI-CD123hi in RPMI-8226 versus RPMI-8226-Oct4 remains unchanged. However, the stable selection makes RPMI-8226-Oct4 more clonal in nature (%CD123hi : RPMI8226; 14.9% vs RPMI-8226-Oct4; 60%). 2) A significant increase in the frequency of CD133+ve cells was observed in RPMI-8226-SP-Oct4-tranfected cells versus either RPMI-8226-SP cells or RPMI-8226-Oct-4 cells [RPMI-8226-SP: 6.3%; RPMI-8226-Oct4: 27.8%; RPMI-8226-SP-Oct4: 40%; p< 0.05]. 3) Analysis of CD27 surface marker showed highest expression in RPMI-8226-SP-Oct4 cells compared to RPMI-8226-Oct4, RPMI-8226-SP, or RPMI-8226 cells (% MFI: RPMI-8226-SP-Oct4 > RPMI-8226-Oct4 > RPMI-8226-SP > RPMI-8226 cells). 4) Treatment of RPMI-8226 and RPMI-8226-Oct4 cells with proteasome inhibitor bortezomib decreased the viability of RPMI-8226 cells; in contrast, bortezomib did not significantly alter the viablity of RPMI-8226-Oct4 cells [% Viability after bortezomib: RPMI-8226; <50% versus RPMI-8226-Oct4; 95%]. Finally, 5) SL-401 significantly decreased the viability of RPMI-8226-Oct4 cells [IC50: RPMI-8226-Oct4 cells: 75 pM; RPMI-8226-SP cells: 350 pM; RPMI-8226 cells: 1367 pM]. We have previously shown anti-MM activity of SL-401 by an additional mechanism of targeting IL-3Rα-expressing plasmacytoid dendritic cells (pDCs) localized in the tumor microenvironment and blocking pDC-induced MM cell growth.
Conclusions Our data show that stem-like cells in MM are relatively resistant to proteasome inhibitor therapy. Importantly, a novel agent SL-401 effectively targets these cells. Oct4-driven stable RPMI-8226 MM cell line serves as a novel tool to screen and develop newer agents targeting stem-like cells in MM. Overall, we show the ability of SL-401 to target a drug-resistant stem-like cell population in MM, and provide an additional rationale for clinical evaluation of SL-401 to improve patient outcome. A clinical trial of SL-401 in MM is currently ongoing (NCT02661022).

Disclosures
Macri: Stemline Therapeutics, Inc.: Employment. Chen:Stemline Therapeutics, Inc.: Employment, Equity Ownership. Richardson:Jazz Pharmaceuticals: Consultancy, Membership on an entity's Board of Directors or advisory committees. Brooks:Stemline Therapeutics, Inc.: Employment, Equity Ownership, Patents & Royalties. Chauhan:Oncopeptide AB: Consultancy; Epicent Rx: Consultancy; C4 Therapeutics: Equity Ownership; Stemline Therapeutics, Inc.: Consultancy. Anderson:Celgene: Membership on an entity's Board of Directors or advisory committees; Sonofi Aventis: Membership on an entity's Board of Directors or advisory committees; Acetylon: Other: Scientific Founder; Oncopep: Other: Scientific Founder; Gilead: Membership on an entity's Board of Directors or advisory committees; Onyx: Membership on an entity's Board of Directors or advisory committees.