Targeted protein degraders from an oncologist point of view: The Holy Grail of cancer therapy?

Confounding Factors in Targeted Degradation of Short-Lived Proteins

Targeted protein degradation has recently emerged as a novel option in drug discovery. Natural protein half-life is expected to affect the efficacy of degrading agents, but to what extent it influences target protein degradation has not been systematically explored. Using simple mathematical modeling of protein degradation, we find that the natural half-life of a target protein has a dramatic effect on the level of protein degradation induced by a degrader agent which can pose significant hurdles to screening efforts. Moreover, we show that upon screening for degraders of short-lived proteins, agents that stall protein synthesis, such as GSPT1 degraders and generally cytotoxic compounds, deceptively appear as protein-degrading agents. This is exemplified by the disappearance of short-lived proteins such as MCL1 and MDM2 upon GSPT1 degradation and upon treatment with cytotoxic agents such as doxorubicin. These findings have implications for target selection as well as for the type of control experiments required to conclude that a novel agent works as a bona fide targeted protein degrader.

PROTACs: A novel strategy for cancer drug discovery and development

Proteolysis targeting chimera (PROTAC) technology has become a powerful strategy in drug discovery, especially for undruggable targets/proteins. A typical PROTAC degrader consists of three components: a small molecule that binds to a target protein, an E3 ligase ligand (consisting of an E3 ligase and its small molecule recruiter), and a chemical linker that hooks first two components together. In the past 20 years, we have witnessed advancement of multiple PROTAC degraders into the clinical trials for anticancer therapies. However, one of the major challenges of PROTAC technology is that only very limited number of E3 ligase recruiters are currently available as E3 ligand for targeted protein degradation (TPD), although human genome encodes more than 600 E3 ligases. Thus, there is an urgent need to identify additional effective E3 ligase recruiters for TPD applications. In this review, we summarized the existing RING-type E3 ubiquitin ligase and their small molecule recruiters that act as effective E3 ligands of PROTAC degraders and their application in anticancer drug discovery. We believe that this review could serve as a reference in future development of efficient E3 ligands of PROTAC technology for cancer drug discovery and development.

An MDM2 degrader for treatment of acute leukemias

In acute myeloid leukemia (AML), p53 tumor suppressor activity can be reduced due to enhanced expression of MDM2 which promotes the degradation of p53. In TP53 wild-type malignancies, therapy with small molecule antagonists of MDM2 results in antileukemic activity. Current treatment strategies, however, have been limited by poor tolerability and incomplete clinical activity. We have developed a proteolysis-targeting chimera (PROTAC) MS3227 that targets MDM2 by recruiting the E3 ligase Von Hippel-Lindau, resulting in proteasome-dependent degradation of MDM2. In WT TP53 leukemia cell lines, MS3227 led to activation of p53 targets p21, PUMA, and MDM2 and resulted in cell-cycle arrest, apoptosis, and decreased viability. The catalytic PROTAC MS3227 led to more potent activation when compared to a stoichiometric inhibitor, in part by dampening the negative feedback mechanism in the p53 - MDM2 circuit. The effectiveness of MS3227 was also observed in primary patient specimens with selectivity towards leukemic blasts. The addition of MS3227 enhanced the activity of other anti-leukemic agents including azacytidine, cytarabine, and venetoclax. In particular, MS3227 treatment was shown to downregulate MCL-1, a known mediator of resistance to venetoclax. A PROTAC-based approach may provide a means of improving MDM2 inhibition to gain greater therapeutic potential in AML.

PROTAC: targeted drug strategy. Principles and limitations

The PROTAC (PROteolysis TArgeting Chimera) technology is a method of targeting intracellular proteins previously considered undruggable. This technology utilizes the ubiquitin-proteasome system in cells to specifically degrade target proteins, thereby offering significant advantages over conventional small-molecule inhibitors of the enzymatic function. Preclinical and preliminary clinical trials of PROTAC-based compounds (degraders) are presented. The review considers the general principles of the design of degraders. Advances and challenges of the PROTAC technology are discussed.