AZD4573 in Combination with CHOP Increases Combination Benefit in Preclinical Peripheral T-Cell Lymphomas Models

Peripheral T-cell Lymphoma (pTCL) make up about 10% of non-Hodgkin's lymphoma in the United States. Patients with relapsed/refractory pTCL have limited treatment options and poor outcomes to standard of care therapy. Current standard of care for pTCL revolves around the chemotherapy regimens CHOP/E and for CD30-postive pTCLs, brentuximab vedotin is approved.

CDK9 regulates transcription elongation 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 frequently overexpressed in haematologic tumours. It has been previously 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 and dependencies of the anti-apoptotic BCL-2 family members is highly heterogeneous in pTCL; however MCL-1 is the most universally expressed BCL-2 family member. Using the MCL-1 inhibitor AZD5991, we have shown statistically significant benefit in survival when combined with CHOP in 2 MCL-1 dependent preclinical pTCL PDX models (Koch et al. 2019). In addition to MCL-1, we have previously reported the importance of the anti-apoptotic protein BFL-1 in mediating cell survival in NHL, including a subset of pTCL (Boiko et al 2021).

Consistent with these findings, AZD4573 treatment exhibited a range of activity across the panel of 18 human TCL cell lines, in a 6-hour caspase-3/7 activation assay. 14 of the cell lines were sensitive, having EC ₅₀ values < 100 nM, and 13 reached a max caspase-3/7 activation of greater than 40%. pTCL cell lines that responded to AZD4573 treatment including ALCL, NK-TCL, and PTCL-NOS and CTCL subtypes. Acute AZD4573 treatment of pTCL cell lines resulted in decreased p-SerRNApoll II and reduced levels of c-MYC and MCL-1 protein levels. To determine if MCL-1 or c-MYC 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 after long term KO, only MCL-1 KO impaired AZD4573 treatment, suggesting that AZD4573 efficacy is mediated through MCL-1, in ALCL pTCL cell lines tested. To determine if combining AZD4573 with CHOP improved efficacy, we tested 5 pTCL cell lines in a 6x6 combination dosing matrix and assessed for viability after 24-72 hours using RealTime-Glo after 18hr exposure to CHOP and AZD4573 added for the last 6hrs. Adding AZD4573 treatment to CHOP schedule in vitro increased the combination benefit in pTCL ALCL cell lines that show MCL-1 dependency, but not in CTCL cell line which show a BCL-xL dependency and resistance to AZD4573. CHOP treatment in vitro resulted in a decrease in c-MYC levels at 24-48 hours, but not MCL-1 suggesting that combination benefit may be driven through decreased c-MYC levels, as KO of c-MYC resulted is loss of cell viability in these models. These data suggested that treatment with AD4573 either as a monotherapy or in combination with CHOP, would be an effective therapeutic strategy in pTCL. AZD4573 monotherapy is currently being evaluated in a phase 2 study (NCT05140382) to assess the efficacy, safety, and PK in patients with relapsed/refractory pTCL.