Abstract 4836: Targeting transcriptional regulation in multiple myeloma with a covalent CDK7 inhibitor THZ1
Authors: Zhou Liang , Zhang Yu , Sharma Kanika , McCarter Jacquelyn , Craun William , Grant Steven 
Published: August 2018

Abstract
Previous studies have shown that certain tumors such as MYCN-driven neuroblastoma and T-cell ALL may be sensitive to CDK7 inhibition. However, the impact of CDK7 disruption on the proliferation and survival of multiple myeloma (MM) cells, which are regulated, at least in part, by c-Myc, has not as yet been examined. To address this issue, the effects of THZ1, a selective covalent inhibitor of cyclin-dependent kinase CDK7, a component of the general transcription factor TFIIH, was examined in MM cells. Virtually all MM cell lines tested were sensitive to the growth-inhibitory and pro-apoptotic effects of very low (e.g., nM) THZ1 concentrations administered for 24 hr, including H929, OPM2, RPMI8226, and U266 cells, as well as bortezomib- or revlimid- resistant cells. THZ1 induced G2/M arrest and inhibited CDK7-mediated phosphorylation of CDK1/2 at T161 and T160 sites. Notably, THZ1 significantly inhibited general transcription by blocking the phosphorylation of the CTD (carboxy-terminal domain) of RNA Pol II at Ser5, Ser7, and Ser2 sites. Washout studies, in which cells were washed free of THZ1 and resuspended in drug-free medium, demonstrated thatTHZ1 acted irreversibly in blocking CTD phosphorylation. CDK7 inhibition was accompanied by down-regulation of MCL-1, c-Myc (L-myc in U266) and BCL-XL gene expression, all of which exert pro-survival effects. Concordant results were obtained when protein levels were monitored. Moreover, ectopic expression of c-Myc, MCL-1, or BCL-XL in MM cells markedly diminished THZ1 lethality, documenting the functional significance of down-regulation of these proteins in THZ1 activity. CRISPR/Cas9 editing of the CDK7 gene was them utilized to confirm the on-target impact of CDK7 disruption. Cells treated with constructs encoding two independent small guiding RNAs targeting CDK7 (sgCDK7) displayed markedly reduced CDK7 protein levels and sharply diminished H929 and OPM2 cell growth. Furthermore, co-culture with HS-5 cells failed to protect MM cells from THZ1, indicating that THZ1 can circumvent micro-environment forms of drug- resistance. Notably, THZ1 alone showed pronounced antitumor effects (e.g., inhibition of viability), and significantly enhanced proteasome inhibitor (e.g., bortezomib or carfilzomib) lethality in human primary CD138+ MM cells and primitive MM cells (CD138-/19+/20+/27+), but not normal hematopoietic cells (e.g., CD34+ cord blood cells). Finally, administration of THZ1 reduced tumor burden and significantly enhanced survival (P &lt; 0.001) in flank and orthotopic xenograft mouse MM models with minimal toxicity. Collectively, these findings indicate that CDK7 is a potential therapeutic target in MM, and suggest that abrogating CDK7 activity by small molecules like THZ1 warrants further consideration as a therapeutic strategy in MM.
Citation Format: Liang Zhou, Yu Zhang, Kanika Sharma, Jacquelyn McCarter, William Craun, Steven Grant. Targeting transcriptional regulation in multiple myeloma with a covalent CDK7 inhibitor THZ1 [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 4836.