The Oral CDK9 Inhibitor, TP-1287, Is Active in Non-Clinical Models of Multiple Myeloma
Authors: Tyagi Ethika , Whatcott Clifford , Foulks Jason M , Siddiqui-Jain Adam , Bearss David J. , Warner Steven L. 
Published: October 2019

Abstract
Background
With 31,000 new cases expected in 2018 (US), and a 50% five-year overall survival rate, there is yet a significant unmet need in the treatment of patients with multiple myeloma (MM). The proteasomal inhibitor bortezomib is approved for the treatment of patients with multiple myeloma. Bortezomib inhibits the degradation of many proteins, including the pro-apoptotic protein NOXA. However, low basal levels of NOXA and/or high levels of the anti-apoptotic protein MCL-1 have been implicated in bortezomib resistance and negative patient outcomes. NOXA functions to sequester MCL-1 and prevent its interaction with the apoptosis inducing proteins, BAK or BAX. The BCL-2 inhibitor, venetoclax, has also been investigated in clinical trials for the treatment of multiple myeloma. Increased MCL-1 expression has been shown to be key in the resistance to venetoclax. Considering the central role of MCL-1 to survival and treatment efficacy in myeloma, we investigated the ability of an MCL-1-lowering agent, namely the CDK9 inhibitor, TP-1287, to suppress tumor growth in non-clinical models of multiple myeloma. TP-1287 is an oral form of the CDK9 inhibitor, alvocidib, and suppresses MCL-1 expression via CDK9-mediated regulation of RNA polymerase II. Alvocidib is currently under clinical investigation in patients with acute myeloid leukemia (AML), in both the frontline and relapse/refractory settings.
Hypothesis
We hypothesized that TP-1287 would suppress tumor growth in models of multiple myeloma and would be active in combinations with bortezomib or venetoclax.
Methods
Celltiter-Glo and Caspase-Glo assays were used to evaluate the in vitro anti-tumor activity of TP-1287, bortezomib, and venetoclax. We utilized real time PCR to measure gene expression changes in treated cells. We also measured protein expression changes following treatment, using standard gel electrophoresis and immunoblotting technique. In order to assess the anti-tumor activity of these compounds in vivo, we initiated xenograft studies in the RPMI-8226 model for multiple myeloma.
Results
In cell viability assays, we observed IC50s ranging from 0.1 nM to over 1000 nM with alvocidib or venetoclax treatment. The addition of up to 100 nM venetoclax resulted in a 2.8-fold reduction in the IC50 of alvocidib in the cultured OPM-2 cell line. Venetoclax activity was potentiated with the addition of alvocidib, resulting in a more than 500-fold decrease in IC50 in the relatively venetoclax-resistant OPM-2 cells. The cleaved form of TP-1287, or alvocidib, was able to reduce MCL-1 protein and mRNA expression in several multiple myeloma cell lines, in a time-dependent fashion. In the RPMI-8226 xenograft model for multiple myeloma, TP-1287 treatment frequency and dose level were explored, with administration of doses up to 15 mg/kg. As a single agent, TP-1287 achieved tumor growth inhibition (%TGI) of 56.0, 76.6, and 93.9% at doses of 2.5, 7.5, and 15 mg/kg, respectively. Additional studies are currently underway to investigate the efficacy of alvocidib and venetoclax in the context of bortezomib resistance where low NOXA may contribute to enhanced cell survival via MCL-1.
Conclusions
Taken together, our data suggest that the combination of alvocidib with venetoclax may constitute a novel therapeutic regimen in the treatment of MM. Further, it suggests that CDK9-mediated targeting of MCL-1 may offer a route to addressing intrinsic resistance in multiple myeloma patients.

Disclosures
Tyagi: Tolero Pharmaceuticals, Inc: Employment. Whatcott:Tolero Pharmaceuticals, Inc: Employment. Foulks:Tolero Pharmaceuticals, Inc: Employment. Siddiqui-Jain:Tolero Pharmaceuticals, Inc: Employment. Bearss:Tolero Pharmaceuticals, Inc: Employment. Warner:Tolero Pharmaceuticals: Employment.