Abstract 5720: Targeting CDK9 in cutaneous T-cell lymphoma using patient-derived xenograft models

Abstract Cutaneous T-cell lymphoma (CTCL) is a rare form of non-Hodgkin lymphoma. Mycosis fungoides and Sézary syndrome (MF/SS) are the most common subtypes in CTCL. Early-stage MF/SS has a favorable prognosis. The advanced-stage CTCL has limited treatment options and a median survival of <5 years, highlighting the urgent need to develop targeted therapies in CTCL. Preclinical studies of CDK9 inhibitors have shown anti-tumor activity in various subtypes of hematologic malignancies. Several CDK9 inhibitors have entered clinical trials for lymphomas and leukemias. However, whether targeting CDK9 in CTCL is a therapeutic option is little known and has resulted in CTCL not being included in ongoing clinical investigations of CDK9 inhibitors. AZD4573 is a highly selective and potent CDK9 inhibitor currently under clinical investigation for Hodgkin and non-Hodgkin lymphomas. It was shown that AZD4573 induces tumor cell death through depletion of MCL-1. Overexpression of MCL-1 has been shown in CTCL cell lines and skin biopsy specimens of CTCL patients. MCL-1 expression appears to be particularly elevated in patients with advanced stage disease. Here, we investigated the ex vivo anti-tumor activity of AZD4573 using patient-derived CTCL xenograft models (PDXs). Primary tumor cells harvested from CTCL PDX mice were activated with CD2/CD3/CD28 beads and incubated in hIL-2 and hIL-7 at an optimal cell density. Subsequently, cells were exposed to AZD4573 at various concentrations for 8 hr, then treated cells were harvested, washed, and placed in fresh media for another 48 hr. Growth inhibition was measured by the Real-Time Glo cell viability assay, and apoptotic activity was assessed by flow cytometry analysis of Caspase 3/7 activity and Annexin V + propidium iodide (PI) double staining. Additionally, we conducted RNA-seq analysis for MCL-1 expression in CTCL PDX primary tumor cells. AZD4573 inhibited cell proliferation in a dose-dependent manner in primary tumor cells from three CTCL PDX mice (PRS-1, PRS-2 PRS-3), with GI50 of 43.5 nM, 175.4 nM, and 113.5 nM, respectively. Furthermore, AZD4573 significantly increased caspase 3/7 activity and apoptosis in PRS-1 and PRS-3, but not PRS-2 primary tumor cells. In addition, our RNA-seq analysis showed that compared with normal controls (normal PBMC from healthy donors), the MCL-1 gene expression is increased 10-fold (P-value <0.0001) in PRS1 cells. The MCL-1 gene expression in PRS2 and PRS3 cells is the same as normal control. Furthermore, PRS-1, which has the highest MCL-1 expression level, is most sensitive to AZD4573. In conclusion, AZD4573 demonstrated dose-dependent growth inhibition and apoptosis induction in primary CTCL tumor cells. In addition, the expression level of MCL-1 appeared to be correlated with the ex vivo anti-tumor activity of AZD4573. Our findings provide a rationale for clinical investigation of targeting CDK9 in CTCL. Citation Format: Yan-Jin Liu, Chi-Heng Wu, Laura Pincus, Weiyun Z. Ai. Targeting CDK9 in cutaneous T-cell lymphoma using patient-derived xenograft models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 5720.