Multiple myeloma (MM) remains incurable despite advances in available therapy. Mutations within the RAS-MAPK pathway are frequently associated with relapsed/refractory disease, with KRAS being particularly prevalent. Increased efforts to target this pathway with the MEK inhibitor, trametinib (Tr) have been limited by toxicities and the development of resistance due to adaptive signalling networks. Dexamethasone (Dex) is a corticosteroid commonly used in clinical practice, to synergistically enhance efficacy of anti-myeloma therapy. Therefore, we hypothesised that the combination of Tr and Dex would yield synergistic activity in KRAS-mutant MM.
Sensitivity to Tr and Dex was determined via CellTiter-Blue (CTB) assay in the KRAS-mutant multiple myeloma cell lines (MMCLs): MM1R (Dex-refractory) & MM1S (Dex-sensitive). Apoptosis and cell cycle were evaluated by flow cytometry. Reverse phase protein array (RPPA) was employed for quantitative analysis of 60 proteins and validated by Western blotting.
CTB assay demonstrated a dose-dependent reduction in cell proliferation with Tr and Dex individually in MM1S, while MM1R was resistant to both treatments. TrDex demonstrated synergistic cytotoxicity in MM1S using CTB and annexin V staining. An accumulation of sub-G1 cells was observed during cell cycle analysis in MM1S, confirming increased cell death. These effects were accompanied by activation of pro-apoptotic proteins, such as cleaved PARP and increased BIM. RPPA revealed the following phospho-proteins were downregulated with TrDex in MM1S compared to MM1R: FAK, PYK2, FLT3, NDRG1 and 4EBP1. Changes in phospho-NDRG1 were statistically significant (P < 0.001; 2-way ANOVA). This was confirmed by Western blotting, where expression levels were downregulated by TrDex in MM1S but unaffected in the resistant cell line MM1R.
TrDex demonstrates synergistic activity in KRAS-mutant MMCLs by suppression of pro-survival signalling and engagement of apoptotic pathways. Our data support further investigation of this combination in KRAS-mutant MM.
Clinical trial identification
Legal entity responsible for the study
Institute of Cancer Research.
Cancer Research UK.
P. Sriskandarajah: CRUK funding grant awarded in 2016. All other authors have declared no conflicts of interest.