The Novel Multitarget Small-Molecule Inhibitor SRX3177 Overcomes Ibrutinib Resistance in Mantle Cell Lymphoma

Mantle cell lymphoma (MCL) is an aggressive, rare, and difficult to treat subtype of non-Hodgkin's Lymphoma (NHL) that accounts for about 6% of all cases. Although there is no defined standard of care for MCL treatment, some combination of chemo-immunotherapy and rituximab maintenance with or without autologous stem cell transplantation is generally employed depending on the age and fitness of the patient. Despite recent development of novel therapeutics, there is inevitable disease relapse with progressively declining efficacy and increasing frequency of resistance with single agent targeted therapy. Here, we describe the novel multitarget inhibitor SRX3177 which simultaneously hits three oncogenic targets: phosphatidylinositol-3 kinase (PI3K), cyclin-dependent kinases 4 and 6 (CDK4/6), and the epigenetic reader protein BRD4. This in silico designed, thieno-pyranone (TP) scaffold-based small molecule inhibitor orthogonally disrupt three targets within the cancer cell with one agent.

Targeting the cell cycle with small molecule inhibitors represents a reasonable attempt to treat MCL, as cell cycle-associated genes like ATM, TP53, CDKN2A, CCND1 and CDK4/6 are most frequently mutated in patients. Palbociclib is a well-known single agent CDK4/6 inhibitor that has been employed in both solid and hematological malignancies. Due to its cytostatic nature, treatment with single agent palbociclib often results in the emergence of treatment-resistant clones. Therefore, a combination strategy would theoretically be more effective and can overcome the development of resistance. Moreover, prolonged G1 arrest by CDK4/6 inhibition sensitizes lymphoma cells to PI3K inhibition, suggesting a synthetic lethality relationship between these two agents. Inhibiting the chromatin reader protein BRD4 causes downregulation of target genes c-MYC and BCL2, further increasing cytotoxic capabilities. Hence, we developed SRX3177 as a potent CDK4/6/PI3K/BRD4 triple inhibitor to synergistically inhibit cell cycle progression and induce cancer cell apoptosis.

SRX3177 is an ATP competitive CDK4/6 inhibitor (IC ₅₀: CDK4 = 2.54 nM, CDK6 = 3.26 nM), PI3K inhibitor (IC ₅₀: PI3Kα = 79.3 nM, PI3Kδ = 83.4 nM), and BRD4 inhibitor (IC ₅₀: BD1 = 32.9 nM, BD2 = 88.8 nM). We have tested the efficacy of SRX3177 against a panel of MCL cell lines and report that SRX3177 induces a strong antiproliferative activity with maximal IC ₅₀ 0f 340 nM in JeKo-1, 29 nM in Mino cells, and 630 nM for Rec-1 cells while IC ₅₀ values for cell lines Granta and JVM-2 were 1.3 µM and 1.5 µM, respectively. Further, we show that SRX3177 is more potent to tumor cells than the individual PI3K (BKM120), BTK (Ibrutinib), BRD4 (JQ1), and CDK4/6 (palbociclib) inhibitors, and dual PI3K/BRD4 inhibitor SF2523 (backbone for SRX3177) in JeKo-1 cells. Next, we examine the cytotoxic effect of SRX3177 in ibrutinib/palbociclib resistant primary MCL cells. Our results show that SRX3177 triggers cytotoxic response at 500 nM and 1000 nM as compared to the lack of cytotoxicity of combination Ibrutinib and palbociclib at 150 nM and 1000 nM (Fig 1). SRX3177 induces a strong apoptotic response and cell cycle arrest in JeKo-1 and Mino cells at 24hrs. Annexin V/7AAD apoptosis staining confirmed the induction of PCD by SRX3177with increase in c-PARP. Western blot analysis shows SRX3177 treatment blocks both PI3K/AKT signaling and Rb phosphorylation. Moreover, analysis by chromatin immunoprecipitation revealed that SRX3177 effectively blocked BRD4 binding to both the promoter and enhancer of c-MYC (p≤0.01 and p≤0.001) and BCL2 (p≤0.05). SRX3177 also suppresses the c-MYC and BCL2 transcriptional program in both a time- and dose-dependent manner. Our findings also demonstrate a SRX3177-dependent reduction in c-MYC half-life via induction of proteasomal-mediated degradation. This degradation is associated with decreased phosphorylation of c-MYC at Ser62 and increased phosphorylation of c-MYC at Thr58 - indicative of differential regulation of c-MYC stability. Finally, we show that SRX3177 overcomes chronic ibrutinib resistance in Jeko-1 cells with a maximal IC ₅₀ of 150 nM as compared to 64 µM with ibrutinib. Hence, the triple inhibitor SRX3177 has superior potency to ibrutinib in MCL cell lines and succeeds in overcoming ibrutinib-resistance at nanomolar doses. Taken together, our data supports the development of SRX3177 as a novel therapeutic agent for treatment of MCL.

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