Clinical Lymphoma, Myeloma & Leukemia, Vol.21, Suppl.1 - September 2021

S148 E XABS-196-C LL New Targetable Pathways i n Chro n ic L y m phocytic L euke m ia (C LL ) Alexey V. D a n ilo v 1, * 1 City of Hope National Medical Center, Duarte, CA, USA *Correspo n di n g author: adanilov@coh.org Keywords CLL, MCL1, SYK, CDK9 Abstract In the past decade, inhibition of B-cell receptor-associated (BCR) Bruton tyrosine kinase (BTK), phosphoinotiside-3 kinase (PI3K), and the pro-survival protein BCL2 using small molecules has led to a paradigm shift in CLL therapy. 1,2 However, resistance to targeted agents is inevitable. In addition to acquiring target protein mutations, CLL cells may lose dependence on Bcl-2. 3–6 Emerging data suggest that such cells may be susceptible to inhibition of alternate BCL2 family proteins, as well as signaling pathways that are activated in this setting. 6 Several novel BH3-mimetics that target MCL1 have entered clinical development. AZD5991 is a highly selective BH3-mimetic that demonstrates high in vitro potency in MCL1-dependent cell lines with an IC50 <1 nM. 7 We have demonstrated that direct MCL1 inhibition with AZD5991 disrupts survival of neoplastic B-cells in lymph-node mimicking conditions, induces mitochondrial dysfunction and cooperates with BCL2/X inhibitors in vitro and in vivo . 8 A phase 1 clinical trial of AZD5991 alone or in combination with venetoclax in hematologic malignancies is ongoing (NCT03218683). AMG-1 7 6 is an alternate BH3- mimetic that inhibits MCL1 with high-affinity binding. AMG-1 7 6 demonstrated synergy with venetoclax in AML models; however, it also caused cytopenias. 9 AMG-1 7 6 was shown to induce apoptosis of CLL cells independent of prognostic markers and overcame the protective effect of stromal microenvironment. 10 However, suppressive effects on hematopoiesis will likely become the dose- limiting factor in clinical trials of MCL1-targeting agents. The effect of MCL1 inhibition on cord blood-derived CD34+ cells was studied using a small-molecule inhibitor, S63845, 11 resulting in almost complete depletion of human hematopoietic stem and progenitor cells, while mature blood cells survived normally. In this work, combined inhibition of MCL1 and BCLX resulted in “synthetic lethality” against the colony-forming hematopoietic stem cells. 11 MCL1 can also be targeted indirectly. We have shown that SYK inhibition with entospletinib leads to downmodulation of MCL1 protein in CLL both in vitro and in the clinic . 12,13 Treatment of CLL cells with entospletinib, but not other BCR-signaling inhibitors, led to a disruption of BAFF-BCR cross-talk and downmodulation of MCL1 mRNA and protein, thus implicating SYK in transduction of multiple pro-survival signals emanating from the tumor microenvironment. 13 Entospletinib has shown promising clinical activity in CLL. 12,14 It is currently being developed in myeloid malignancies. 15 A novel dual SYK/BTK inhibitor luxeptinib also was shown to downregulate MCL1 and other pro-survival BCL2 proteins in CLL. 16 Transcriptional cyclin-dependent kinases (CDK 7 /9) regulate activity of RNA polymerase, thereby controlling production of mRNA. Downmodulation of MCL1 has been considered a key mechanistic event accounting for the pro-apoptotic activity of CDK inhibitors in neoplastic B-cells. 1 7 ,18 Pan-CDK inhibitors (flavopiridol, dinaciclib) demonstrate clinical efficacy in lymphoid malignancies. We and others reported that preclinical efficacy of AZ55 7 6, a selective inhibitor of CDK9, and its clinical congener AZD45 7 3 in lymphoid tumors depended on rapid downmodulation of MCL1, BFL1, and MYC. 19–21 CDK9 inhibition can increase efficacy of BH3-mimetics, but safety of this combination will need to be explored. 22,23 CDK9 inhibitors in development, in addition to AZD45 7 3, include VIP152, CYC065, and voruciclib. BET inhibitors have also entered preclinical development in CLL. BCLX is also upregulated in NHL. Navitoclax is a small-molecule inhibitor of BCL2, BCLX, and BCL2L2; however, its use in CLL has been plagued by thrombocytopenia. 24 AZD4320 is an alternative agent that co-targets BCL2/X. It is being developed as an intravenous, rather than an oral formulation, with a hope to mitigate its effect on platelets while retaining the anti-tumor effect. 25 Preclinical studies to date demonstrate its anti-tumor effect across multiple lymphoid and myeloid cell lines, with increased sensitivity compared with venetoclax. ABBV-155, an antibody-drug conjugate targeting BCLX, where BCLX is being used as a payload to avoid “off-tumor” effects, demonstrated promise in solid tumor models. 26 Finally, as many microenvironment-driven pro-survival pathways converge on NF  B, leading to induction of chemokines, cell cycle regulators, and BCL2 family proteins themselves, this transcription factor continues to be an attractive target in CLL. We have successfully disrupted NF  B signaling in CLL by targeting the Nedd8-activating enzyme. 2 7