Targeting MYC Expression With CDK Inhibitors Shows Potency In Preclinical Models Of High-Risk Diffuse Large B-Cell Lymphom

Diffuse large B-cell lymphoma is the most common lymphoid malignancy and results in death in a third or more of patients. R-CHOP chemoimmunotherapy (rituximab, cyclophosphamide, doxorubicin, vincristine, and prednisone) is the front-line standard of care, but better treatments are needed for patients who are at high risk of failing this regimen. Recent studies found that co-expression of the oncogenic transcription factor MYC and the antiapoptotic protein BCL2, detected by immunohistochemistry, is a reliable predictor of risk: Less than 40 percent of R-CHOP-treated patients with co-expressing tumors will achieve long-term disease-free survival. Inhibition of cell-cycle promoting kinases is a rational approach in high-risk DLBCL because of frequent deregulation detected by high-resolution cytogenetics and other methods. We studied a panel of inhibitors against multiple members of the cyclin-dependent kinase (CDK) family, the aurora kinases, and other cell-cycle kinase targets in models of MYC-BCL2 co-expressing DLBCL. The multi-CDK inhibitor dinaciclib emerged with particularly high potency, with IC50 5 nM or less against multiple human cell lines. The drug also had strong activity against a genetically defined murine lymphoma model driven by MYC and BCL2, which accurately reflects the pathology and clinical behavior of high-risk DLBCL. Dinaciclib alone is sufficient to kill MYC-BCL2 DLBCL, as neither chemotherapy drugs nor other kinase inhibitors showed synergy. Mechanistically, the drug targets CDK1, 2, 5, and 9, and any or all of these could contribute to its potency against these tumors. CDK1 and 2 directly promote progression through the cell cycle by phosphorylating the retinoblastoma (Rb) protein. But we found loss of Rb phosphorylation not to be a reliable marker of potency. Previous studies of other multi-CDK inhibitors, meanwhile, have pointed to lost expression of the antiapoptotic factor MCL1, an effect of CDK9 inhibition, to be a key anti-tumor activity. We find high potency of dinaciclib, however, in multiple systems with constitutive expression of BCL2, which can replace MCL1 in protecting cells from apoptosis. Instead, most reliably, we found loss of c-MYC gene transcription to be a key effect of dinaciclib therapy, resulting in rapid loss of MYC protein and tracking with the drug’s potency. CDK1 regulation of SMAD3 and CDK9 regulation of RNA polymerase II are mechanistic possibilities we are exploring to explain the effect on c-MYC transcription. Either way our study shows the promise of CDK inhibition to improve prognosis for high-risk DLBCL and points to c-MYC regulation as a key mechanism. Dinaciclib is entering phase 3 clinical evaluation in other tumor types and may be appropriate for evaluation in DLBCL based on our work, although it remains to be determined if this drug’s pharmacologic properties would permit replication of the mechanism we see preclinically. By precisely defining the mechanism by which CDK inhibition targets MYC expression, however, our study promotes development of more precise therapies for high-risk DLBCL in the future.