Graduation Date

Fall 12-15-2017

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Programs

Cancer Research

First Advisor

Michael G. Brattain

Second Advisor

Amarnath Natarajan

Abstract

Colorectal cancer (CRC) remains one of the leading causes of cancer related deaths in the United States. Currently, there are limited therapeutic options for CRC patients, none of which focus on the cell signaling mechanisms controlled by members of the cyclin dependent kinase (CDK) family. CDK5 has been implicated in a variety of cancers, and most recently as a tumor promoter in CRC.

As such, we evaluated a compound developed by Pfizer, CP-668863 (a.k.a. 20-223), that inhibits CDK5 in neurodegenerative disorders. In our CRC xenograft model, 20-223 reduced tumor growth and tumor weight, indicating its value as a potential anti-CRC agent. We subjected 20-223 to a series of cell-free and cell-based studies to understand the mechanism of its anti-tumor effects. Profiling the CDK family revealed that 20-223 was most potent against CDK2 and CDK5 in cell-free and cell-based systems. The clinically used CDK inhibitor AT7519 and 20-223 share the aminopyrazole core. 20-223 was comparable, or in some cases better, than clinically used AT7519, proving it to be a suitable lead compound.

Next we utilized the new PRoteolysis TArgeting Chimera (PROTAC) strategy to develop CDK5 degraders. Synthesis and evaluation revealed that the heterobifunctional aminopyrazole-based PROTAC capable of cereblon-mediated proteasomal degradation targeted CDK9 while sparing CDK2 and CDK5. While the degrader (3) did in fact bind to CDK5 and inhibit its kinase activity, it was unable to trigger its degradation, likely due to differentially exposed lysine residues. This is the first report of a PROTAC capable of degrading a member of the oncogenic CDK family.

Overall, these studies demonstrate that inhibition of CDK5 is a promising therapeutic strategy and warrants further evaluation. 20-223 is a favorable lead compound for CRC therapy as it exhibits anti-cancer activity both in vitro and in vivo. Additionally, the PROTAC strategy can be applied to develop selective CDK degraders.

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