Pharmacodynamics of venetoclax plus cirtuvivint a first-in-class splice variant regulator.

e15093 Background: NSC835563 (SM08502, cirtuvivint) is a first-in-class pan CDC-like kinase (CLK) and dual specificity tyrosine kinase (DYRK) inhibitor that targets alternative splicing of genes in multiple pathways and modifies their protein expression. Preclinical screening of hematologic malignancy models has shown sensitivity to cirtuvivint, leading to proposed clinical trials in AML in combinations with venetoclax. We performed pharmacodynamic studies to identify probable mechanisms of combination activity in AML models. Methods: In vivo tolerability, efficacy and pharmacodynamic (PD) studies were performed in two AML xenograft models, MV411 and KG1a in NSG mice, at 6.25-25 mg/kg PO cirtuvivint and 25 mg/kg PO venetoclax. Body weights (BW) and tumor volumes were recorded periodically to assess toxicity and anti-tumor efficacy. Tumor tissue samples were collected at 6 and 24 h post venetoclax to assess PD response. Multiplexed analysis of apoptosis pathway proteins/protein-protein complexes was performed by clinically validated assays (PMID 26446940). Results: Cirtuvivint alone at high doses (25 mg/kg) caused tumor regressions in MV411 but only a modest tumor growth inhibition (TGI) in KG1a, a resistant AM model. Importantly, the lowest dose of 6.25 mg/kg cirtuvivint combined with 25 mg/kg venetoclax led to tumor regression in the MV411 model and a moderate TGI in KG1a. Cirtuvivint alone caused a transient 25% reduction (p20-fold increased mitochondrial Bax/Bak heterodimer and >10-fold increased cytosolic cleaved caspase-3. In KG1a, combination treatment resulted in only modest (<20%) changes in biomarkers. Overall, biomarkers provided evidence of the combination MoA. Conclusions: Venetoclax and cirtuvivint in combination effectively induced tumor regressions at low tolerable doses in an AML model. Mitochondrial Bax/Bak heterodimer provided a sensitive biomarker of combination activity. Cirtuvivint contributed its combinatorial effect primarily through disruption of Bim complexes and inhibition of Mcl-1. PD biomarkers identified in this study can confirm MoA of drug combination in ongoing clinical trials.