In vitro efficacy of CDK9 inhibitor tambiciclib (SLS009) in ASXL1 mutated colorectal cancer cell lines.

Abstract

3121 Background: ASXL1 (Additional sex combs-like 1) gene encodes ASXL1 protein thought to disrupt chromatin, enhancing transcription of certain genes while repressing the transcription of others. ASXL1 mutations occur in ~20% of acute myeloid leukemia (AML) patients and ~50% of ASXL1 AML mutations are frameshift or nonsense mutations generating a truncated ASXL1 form with oncogenic gain of function in AML. Recently ASXL1 mutated AML patients were treated with a CDK9 inhibitor tambiciclib (SLS009) with promising results. ASXL1 mutations were also reported in 55% of Colorectal Carcinoma with High Microsatelite Instability (CRC MSI-H) cell lines, but it is not known whether those mutations are similar to ASXL1 mutations observed in AML and whether CDK9 inhibitors have enhanced cytotoxic effect in CRC MSI-H cells with those mutations. Methods: Twelve CRC MSI-H cell lines were treated with SLS009 at various concentrations. Staurosporin was used as positive control. Cytotoxicity analysis was performed by CellTiter-Glo 2.0 assay. Data analyses were performed using GraphPad Prism 9. NGS was used to determine mutations in studied cell lines. The experiment was designed to compare ASXL1 mutations in CRC MSI-H cell lines to those observed in AML and determine efficacy of SLS009 cytotoxicity in CRC MSI-H cells with and without ASXL1 mutations. Highly effective concentrations in this experiment were considered those with IC50 values below 100 nM. Results: Among the 12 tested cell lines, 8 (67%) had non-synonymous ASXL1 mutations of any kind, similar to the literature reported 55% ASXL1 mutations rate in CRC MSI-H. Among cell lines with ASXL1 mutations, 4 (50%) had high impact frameshift mutations, similar to estimated rate of high impact frameshift mutations in AML (~50%). Among cell lines with high impact frameshift mutations, all had mutations in protein position regions 581-582 and 642-643. Three out four had in addition high impact frameshift mutations in the region of protein position 637-638. Protein positions of these frameshift ASXL1 mutations were similar to those observed in AML (591 – 592 and 635 – 646). Among the cell lines with any ASXL1 mutation, 4/8 (50%) had IC50 values for SLS009 below 100 nM (highly efficacious) vs 0/4 (0%) among cell lines without ASXL1 mutations. Among cell lines with ASXL1 frameshift mutations, SLS009 was highly efficacious in 3/4 (75%) cell lines vs 1/8 (12.5%) in cell lines without ASXL1 frameshift mutations. High efficacy was observed in all cell lines (3/3, 100%) with frameshift mutations in the protein position region 637-638. Presence of high impact TP53 mutations did not appear to significantly affect SLS009 efficacy. Conclusions: Results indicate that ASXL1 mutations may be oncogenic drivers in some solid tumors, like CRC MSI-H, similar to those in AML and that efficacy of CDK9 inhibition with SLS009 may be similar in some solid tumors to the efficacy observed in AML.