Abstract 4605: AZD4573 in combination with CHOP increases combination benefit in preclinical peripheral T-cell lymphoma models

Abstract Peripheral T-cell Lymphoma (pTCL) makes up about 10% of non-Hodgkin's lymphoma in the United States. Patients with relapsed/refractory pTCL have limited treatment options and poor prognosis. Current standard of care for pTCL revolves around the chemotherapy regimens CHOP/E. Here we examine the efficacy of CDK9 inhibition for the treatment of pTCL. CDK9 regulates transcription through phosphorylation of RNA polymerase II. AZD4573 is a highly potent and selective CDK9 inhibitor; acute inhibition with AZD4573 downregulates short-lived proteins such as MCL-1, BFL-1, and c-MYC, which are often overexpressed in hematologic tumors. It has been shown that the expression pattern of the anti-apoptotic BCL-2 family members in pTCL cell line models closely resembled the expression patterns in pTCL patient samples (Koch et al. 2019). Expression of BCL-2 family members is highly heterogeneous in pTCL, with MCL-1 being the most universally expressed. Using MCL-1 inhibitor AZD5991, we showed statistically significant benefit in survival when combined with CHOP in MCL-1 dependent preclinical pTCL PDX models (Koch et al. 2019). In addition, we have previously reported the importance of the anti-apoptotic protein BFL-1 in cell survival in NHL, including a subset of pTCL (Boiko et al 2021). AZD4573 treatment showed a range of activity across the panel of 18 human pTCL cell lines, in a 6-hour caspase-3/7 activation assay. 14 of the cell lines were sensitive, with EC50 values less than 100 nM, and 13 reached a max caspase activation of over 40%. pTCL subtypes that responded to AZD4573 treatment included ALCL, NK-TCL, and PTCL-NOS and CTCL. Acute AZD4573 treatment resulted in decreased pSer2-RNAP2 and reduced protein levels of c-MYC and MCL-1. To determine what was driving the apoptotic phenotype, we used ribonucleoprotein (RNP) mediated CRISPR knock out (KO) of MYC and MCL1 genes. While pTCL cell lines were dependent on c-MYC and MCL-1 for viability after KO, only MCL-1 KO impaired AZD4573 treatment, suggesting that efficacy is mediated through MCL-1 in ALCL pTCL cell lines assessed. To determine if combining AZD4573 with CHOP improved efficacy, we tested 5 pTCL cell lines in a 6x6 combination dosing matrix and measured viability after 24-72 hours using RealTime-Glo after 24hr exposure to CHOP with AZD4573 added for the last 6hrs. Adding AZD4573 to CHOP in vitro increased the efficacy in pTCL ALCL cell lines with MCL-1 dependency, but not in CTCL cell line with BCL-xL dependency and resistance to AZD4573. CHOP treatment in vitro resulted in a decrease in c-MYC levels but not MCL-1, suggesting that combination benefit may be driven through c-MYC. This data suggests that treatment with AZD4573 as a monotherapy or in combination with CHOP regimen would be an effective therapeutic strategy in pTCL. Citation Format: Danielle Sinead Potter, India Ott, Jamal Saeh, Richard Olsson, Stephen Fawell, Lisa Drew, Justine Roderick. AZD4573 in combination with CHOP increases combination benefit in preclinical peripheral T-cell lymphoma 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 (6Suppl): Abstract nr 4605.