SM09419, a Novel, Small-Molecule CDC-like Kinase (CLK) Inhibitor, Demonstrates Strong Inhibition of the Wnt Signaling Pathway and Antitumor Effects in FMS-like Tyrosine Kinase 3 (FLT3)-Mutant Acute Myeloid Leukemia Models

Acute myeloid leukemia (AML) with the FLT3 internal tandem duplication (FLT3-ITD) mutation accounts for ~25% of all AMLs, carries a poor prognosis, and is prone to relapse despite targeted therapy. FLT3 mutations are associated with aberrant activation of the Wnt signaling pathway, which itself is implicated in AML initiation/progression and is required for the self-renewal and survival of leukemic stem cells. CLKs regulate the activity of serine/arginine-rich splicing factors (SRSFs) that modulate spliceosome assembly, mRNA splicing, and gene expression. SM09419 is a novel, oral, small-molecule pan-CLK inhibitor that potently inhibits the Wnt pathway. These studies examined the antitumor activity of SM09419 as a single agent and in combination with targeted and standard therapies in preclinical models of FLT3-ITD AML.

In MV-4-11 and MOLM-13 AML cells carrying the FLT3-ITD mutation, SM09419 dose-dependently inhibited SRSF6 phosphorylation and potently suppressed expression of Wnt pathway-related genes (CCND1, MYC, TCF7, DVL2). The effect on cell proliferation was tested in 8 AML cell lines with varying mutation profiles as well as 26 different leukapheresis-derived primary human AML cells. Proliferation was strongly impaired by SM09419 across all tested cell lines (average EC₅₀=0.2 + 0.048 µM]); MV-4-11 and MOLM-13 cells had EC₅₀ of 0.049 and 0.144 µM, respectively. SM09419 also potently inhibited proliferation in all primary AML cells (average EC₅₀=0.048 + 0.0097 µM) regardless of FLT3 mutation status, cytogenetics, or AML diagnosis (de novo or relapsed/refractory). SM09419 also induced apoptosis in MV-4-11 and MOLM-13 cells, increasing caspase 3/7 activation and PARP cleavage while reducing survivin and MCL-1 expression relative to vehicle.

In vivo antitumor effects and tolerability of oral SM09419 (QD) alone or combined with either midostaurin (FLT3 inhibitor) or venetoclax (BCL2 inhibitor) and/or azacitidine were assessed in FLT3-ITD xenograft models (n=5-6/group). In MOLM-13 xenografts, SM09419 (12.5 and 25 mg/kg) induced strong tumor growth inhibition (TGI) vs. vehicle at Day 14 (TGI 52% [p<0.05] and 74% [p<0.001], respectively). Midostaurin (50 mg/kg) induced significant TGI vs. vehicle (50%, p<0.05), which was increased when administered in combination with 12.5 mg/kg SM09419 (81%, p<0.001). In MV-4-11 xenografts, single-agent SM09419 (6.25, 12.5, and 25 mg/kg) induced significant TGI vs. vehicle (56% [p<0.05], 94%, and 95% [p<0.001], respectively) at Day 26 with tumor regression in all mice dosed at 12.5 mg/kg and 25 mg/kg. In a subsequent experiment, midostaurin (50 mg/kg) alone and combined with 6.25 mg/kg SM09419 for 23 days induced tumor regression in MV-4-11 xenografts (100% TGI vs. vehicle, p<0.0001). After treatment discontinuation, tumor regression was maintained in all mice (6/6) treated with the combination for 26 days, whereas tumor regrowth was immediately observed in midostaurin-treated mice. In another MV-4-11 xenograft study, the combination of 6.25mg/kg SM09419 with azacitidine (0.8 mg/kg QD) and/or venetoclax (25 mg/kg QD) induced significant TGI (95-98% vs. vehicle, p<0.001) with tumor regression at Day 26. Azacitidine + venetoclax induced 79% TGI (p<0.001), but no tumor regression was observed. The triple combination induced tumor regression in all mice and complete regressions in 4/6 mice (67%); it had a greater effect on slowing tumor regrowth after treatment discontinuation vs. a single agent or doublet. SM09419 alone or in combination was well tolerated in these xenograft models based on body weight measurements.

In summary, SM09419 potently inhibited SRSF6 phosphorylation and Wnt signaling pathway activity and induced apoptosis in FLT3-ITD cell lines. It also inhibited proliferation in cell lines and primary AML cells regardless of FLT3 status. The strong in vivo antitumor effects observed as combination treatment suggest that SM09419 combined with standard therapies may provide a clinical benefit by slowing or preventing relapse in AML with a marker of poor prognosis such as FLT3-ITD. A Phase 1 study assessing safety, tolerability, and pharmacokinetics of SM09419 in subjects with advanced hematologic malignancies is being initiated.