Validation of a Novel Rac1 Inhibitor As Potent Secdrug Against Activating Ras Mutant and Drug-Resistant Multiple Myeloma

Multiple myeloma is the second-most common hematopoietic malignancy in the United States. Although several drugs have so far been approved by the FDA, myeloma still remains an incurable disease with dose-limiting toxicities and resistance to primary drugs like proteasome inhibitors (PIs) and Immunomodulatory drugs (IMiDs). Furthermore, activating Ras gene (NRAS, KRAS, and HRAS) mutations are known to occur in >50% of myeloma patients, which are also associated with greater tumor burden, cancer aggressiveness, high recurrence rates, and poor survival. We have recently established a novel drug development/re-purposing pipeline called secDrug that incorporates a pharmacogenomics data-driven computational algorithm to introduce several new drug candidates (secDrugs), including EHT1864 (a Rac1 (Ras superfamily GTPase) inhibitor), CCT018159 (Hsp90 inhibitor), and CP466722 (a reversible ATM inhibitor) as synergistic partners of primary drugs against drug-resistant myeloma. The overall objective of this study is to validate EHT1864 using in vitro and ex vivo models of relapsed and refractory myeloma (RRMM) patients. First, we used single-cell RNA sequencing (scRNAseq) as a novel screening tool to demonstrate that EHT1864 is potentially effective against myeloma subclones based on the enrichment of target genes (Figure 1). For in vitro validation, we used a human myeloma cell line (HMCL) panel representing drug-sensitive (FLAM76), innate/refractory resistance (LP1), and acquired/relapsed resistance (parental and clonally derived PI-resistant and IMiD-resistant HMCL pairs U266 P/VR, RPMI P/VR, MM1S P/VR/LenR). FLAM76 KRas (FLAM76-K12) and FLAM76 Nras (FLAM76-N12) cell lines were generated using Adeno-associated viral (AAV) vector-mediated delivery of CRISPR-Cas9 for genome editing in humans. Ras mutations were confirmed using Sanger DNA sequencing method. Cell viability measured using CellTiter-Glo luminescent cell viability assay showed the potency of EHT1864 as a single-agent (IC₅₀ range 15-40µM) as well as in combination with PIs (represented by Ixazomib) and IMiDs (represented by Lenalidomide). Combination index (CI) values calculated using Chou-Talalay's CI theorem were consistently less than 0.9, while all the four synergy scores (HSA, Bliss, Loewe, and ZIP) were higher than 5, demonstrating high synergy. Cell death due to apoptosis was confirmed using flow cytometry (Annexin-V-Propidium Iodide staining), Caspase 3/7 activity assay, and immunoblotting. Interestingly, EHT1864 was most potent against drug-resistant and Ras-mutant HMCLs. Further, we showed that EHT1864 treatment also affects ROS generation and mitochondrial membrane potential, indicating that EHT1864 induces apoptosis via mitochondria-mediated pathway. Finally, we performed pre-vs-post-treatment genome-wide transcriptome analysis (tumor mRNAseq) and single-cell multi-ome analysis (single-cell RNA sequencing/scRNAseq + single-cell Assay of Transposase Accessible Chromatin sequencing/scATACseq) analysis to derive molecular signatures and pathways representing the basis of (on-target and off-target) efficacy of EHT1864 and drug synergy at the bulk tumor and subclonal levels. Finally, we will perform single-cell proteomics (high-dimensional immunophenotyping using mass cytometry/CyTOF/Cytometry time of flight) in CD138+ bone marrow cells derived from newly-diagnosed and relapsed myeloma patients (ex vivo) to demonstrate efficacy in RRMM. Thus, our work lays a framework to establish EHT1864 as a clinical trial-ready therapeutic option for the management of drug-resistant myeloma.

Kumar:AbbVie,: Membership on an entity's Board of Directors or advisory committees, Research Funding; Celgene: Membership on an entity's Board of Directors or advisory committees, Research Funding; Janssen,: Membership on an entity's Board of Directors or advisory committees, Research Funding; Takeda,: Membership on an entity's Board of Directors or advisory committees, Research Funding; Adaptive,: Membership on an entity's Board of Directors or advisory committees, Research Funding; KITE,: Research Funding; MedImmune/Astra Zeneca,: Membership on an entity's Board of Directors or advisory committees, Research Funding; Merck,: Research Funding; Novartis,: Research Funding; Roche: Research Funding; Sanofi: Research Funding; Oncopeptides: Other: Independent review committee. Baughn:Roche-Genentech: Consultancy. Van Ness:NIH: Research Funding. Mitra:NIH: Research Funding.