Dual Targeting of Cell-Cycle Checkpoint Kinase and Unfolded Protein Response As a Novel Therapeutic Approach for High-Risk Multiple Myeloma with TP53 Deletion or Mutations

Multiple myeloma (MM) is a hematological malignancy characterized by abnormal clonal proliferation of malignant plasma cells. Despite the introduction of novel agents such as proteasome inhibitors, immunomodulatory drugs, and antibodies that have significantly improved clinical outcomes of the patients of MM, most patients eventually relapse and develop drug resistance. In particular, the prognosis of patients harboring either chromosome TP53 deletion or mutations remains very poor, suggesting the prevalence of TP53 abnormalities increases with disease progression. Therefore, novel therapeutic strategies to overcome this unfavorable feature are urgently needed in clinical settings. WEE1 is a cell-cycle checkpoint kinase and a key regulator of DNA damage surveillance pathways. In response to extrinsically induced DNA damage, WEE1 kinase induces cell cycle arrest, allowing damaged DNA to be repaired before the cell undergoes DNA replication in S phase, and preventing cells harboring unrepaired, damaged DNA from mitotic lethality. Furthermore, WEE1 overexpression has been observed in many types of cancers. In addition, our previous studies revealed that monotherapy with AZD1775, a potent and highly selective inhibitor of WEE1, inhibited the proliferation of various MM cell lines irrespective of TP53 status. (Blood 2016; 128: 3256). On the other hand, one of the defining features of MM cells is the production of large amounts of protein, such as immunoglobulin, that must be processed within the endoplasmic reticulum (ER). Due to the accumulation of abundant immunoglobulin in ER, MM cells are constitutively under conditions of ER stress. The unfolded protein response (UPR)-signaling pathway is a cytoprotective mechanism against ER stress, and is therefore activated in MM cells to survive these conditions. Activation of the UPR has been observed in many types of cancers, and loss of TP53 has shown to enhance the UPR. Protein kinase RNA-like endoplasmic reticulum kinase (PERK) is one of three ER transmembrane protein kinases implicated as primary effectors of the UPR. Recent studies have suggested that PERK inhibition resulted in dose-dependent inhibition of tumor growth both in vitro and in vivo. In addition, more recent studies have proposed that PERK induces resistance to cell death elicited by chemotherapy. The combination of WEE1 and PERK inhibitors might thus offer an attractive therapeutic option against this incurable hematological malignancy. Here, we investigated the therapeutic utility of AZD1775 and GSK2606414, a highly selective inhibitor of PERK kinase, alone and in combination in various MM cells including TP53 wild-type (MM1.S) as well as TP53-deficient (KMS-11) and TP53-mutated (U266, RPMI8226, OPM-2) cell lines. AZD1775 and GSK2606414 alone induced dose-dependent cell growth inhibition in all investigated MM cells irrespective of TP53 status. Interestingly, GSK2606414 in combination with AZD1775 inhibited proliferation of all MM cells more effectively than either single agent. Assays for apoptotic cell death demonstrated that AZD1775 in combination with GSK2606414 induced significant and marked apoptotic cell death in MM cells used in this study compared to monotherapy alone. Next, western blotting analysis was performed to address the mechanisms of apoptotic cell death by the treatment of WEE1 and PERK inhibitors in MM cells. GSK2606414 inhibited PERK activation and decreased its downstream substrates (phospho-eIF2a, ATF4, and CHOP). Combination treatment with WEE1 inhibitor and various doses of PERK inhibitor significantly increased PARP and caspase 3 cleavage, CDK1 phosphorylation, and histone H2AX expression. Taken together, these data suggest that combining AZD1775 and GSK2606414 synergistically induced DNA damage and promoted premature mitotic entry, resulting in apoptotic cell death of TP53-deleted or -mutated MM cells. In conclusion, dual targeting of WEE1 and PERK might be a promising therapeutic approach for MM irrespective of TP53 status.