The Covalent CDK7 Inhibitor THZ1 Potently Induces Apoptosis in Multiple Myeloma Cells In Vitro and In Vivo

Purpose: The goal of this study was to characterize the activity of the covalent CDK7 Inhibitor THZ1 in multiple myeloma models.

Experimental design: Multiple myeloma lines were exposed to varying THZ1 concentrations alone or with carfilzomib or ABT-199, after which Apoptosis was monitored by flow cytometry, protein expression by Western blot analysis, mRNA by RT-PCR. Analogous studies were performed in cells ectopically expressing c-Myc, Mcl-1, or Bcl-xL, or CRISPER-Cas CDK7 sgRNA knockout. Primary multiple myeloma cells were exposed to THZ1 ± carfilzomib or ABT-199. In vivo effects of THZ1 were examined in a systemic U266 xenograft model.

Results: THZ1 markedly diminished multiple myeloma cell proliferation and survival despite bortezomib or stromal cell resistance in association with G₂-M arrest, inactivation of CTD RNA Pol II, dephosphorylation of CDKs 7 as well as 1, 2, and 9, and Mcl-1, Bcl-xL, and c-Myc mRNA or protein downregulation. Ectopic Mcl-1, c-Myc, or BCL-X_L expression significantly protected cells from THZ1 lethality. Both THZ1 and CRISPR-Cas CDK7 knockout sharply diminished multiple myeloma cell proliferation and significantly increased carfilzomib and ABT-199 lethality. Parallel effects and interactions were observed in primary CD138⁺ (N = 22) or primitive multiple myeloma cells (CD138^-/CD19⁺/CD20⁺/CD27⁺; N = 16). THZ1 administration [10 mg/kg i.p. twice daily (BID), 5 days/week] significantly improved survival in a systemic multiple myeloma xenograft model with minimal toxicity and induced similar events observed in vitro, for example, Mcl-1 and c-Myc downregulation.

Conclusions: THZ1 potently reduces multiple myeloma cell proliferation through transcriptional downregulation of Mcl-1, BCL-X_L, and c-Myc in vitro and in vivo. It warrants further attention as a therapeutic agent in multiple myeloma.