A CK1δ/CK1ε-to-Wnt/β-Catenin Circuit Is a Therapeutic Vulnerability in Primary and Drug Resistant Multiple Myeloma

Aberrant canonical Wnt/β-catenin signaling has long been known to play a role in cancer development and progression, where Wnt binding provokes nuclear localization of β-catenin, which functions as a coactivator for the TCF/LEF family of transcription factors that induce an oncogenic transcriptional program. Indeed, hallmarks of several tumor types are gain-of-function somatic mutations in β-catenin, and loss-of-function mutations in components of the β-catenin destruction complex, including the scaffold proteins APC and Axin1 and the serine/threonine kinase casein kinase1-α (CK1α) that phosphorylates β-catenin, priming it for destruction by the βTrCP E3 ubiquitin ligase. Interestingly, the CK1α-related kinase CK1δ antagonizes CK1α in Wnt signaling, where CK1δ triggers disassembly of the β-catenin destruction complex by phosphorylation of disheveled-1 (Dvl1). Notably, we have shown that CK1δ is amplified, and is necessary and sufficient, for β-catenin activation and Wnt signaling in some tumor types. Interestingly, lenalidomide resistance in multiple myeloma (MM) cell lines and patients has been shown to correlate with increased expression and activity of Wnt/β-catenin signaling.

Here, we report that roughly 40% of MM patients have mutations in the Wnt/β-catenin pathway and that this pathway is active in a large panel of human MM cell lines. Notably, a highly potent and selective in-house CK1δ/CK1ε kinase inhibitor coined SR-3029 rapidly compromises the growth and survival of MM cell lines. Importantly, the anti-MM activity of SR-3029 is augmented in MM cell lines with selected resistance to bortezomib and lenalidomide relative to paired naïve myeloma cells. In concordance with the anti-MM activity of SR-3029, treatment of MM cells with this kinase inhibitor leads to marked reductions in the levels of β-catenin target genes (e.g., MYC, CCND1 and WNT3) and also with the suppression of CK1δ and CK1ε. Further, using an ex vivo platform that accurately quantifies the sensitivity of primary MM samples to agents in a reconstructed tumor microenvironment, we examined CD138-selected patient specimens from 29 patients against a panel of 31 drugs simultaneously. Notably, MM patient samples, including those that are quad-resistant, were highly sensitive to SR-3029 with a mean LD₅₀ of 300nM and a range between 30nM and 990nM. Indeed, SR-3029 was by far the most potent kinase inhibitor assessed in this platform, where its mean LD₅₀ is an order of magnitude more potent than all other kinase inhibitors included in the screen. Finally, using the well-established 5TGM1/Kal-Ridge (C57B6/KaLwRijHsd) syngeneic mouse model of multiple myeloma, we show that tumors derived from 5TGM1 cells, which are highly sensitive to SR-3029 ex vivo, are also sensitive to CK1δ/CK1ε inhibition in vivo, where the SR-3029 treated cohort of animals demonstrated decreased tumor burden as assessed by IgG2b levels and imaging, and by significantly improved survival relative tovehicle treated recipients. Collectively, these findings demonstrate that SR-3029 has potent activity in both naïve and therapy resistant multiple myeloma and establish CK1δ and/or CK1ε as attractive targets for anti-MM therapy.