Abstract

Restoration of wild-type structure and function to mutant p53 with a small molecule (hereafter referred to as “reactivating” mutant p53) is one of the holy grails in cancer therapeutics. The majority of TP53 mutations are missense which generate a defective protein that is targetable. We are currently developing a new class of mutant p53 reactivators called zinc metallochaperones (ZMCs) and, here, we review our current understanding of them. The p53 protein requires the binding of a single zinc ion, coordinated by four amino acids in the DNA binding domain, for proper structure and function. Loss of the wild-type structure by impairing zinc binding is a common mechanism of inactivating p53. ZMCs reactivate mutant p53 using a novel two-part mechanism that involves restoring the wild-type structure by reestablishing zinc binding and activating p53 through post-translational modifications induced by cellular reactive oxygen species (ROS). The former causes a wild-type conformation change, the later induces a p53-mediated apoptotic program to kill the cancer cell. ZMCs are small molecule metal ion chelators that bind zinc and other divalent metal ions strong enough to remove zinc from serum albumin, but weak enough to donate it to mutant p53. Recently we have extended our understanding of the mechanism of ZMCs to the role of cells’ response to this zinc surge. We found that cellular zinc homeostatic mechanisms, which normally function to maintain free intracellular zinc levels in the picomolar range, are induced by ZMCs. By normalizing zinc levels, they function as an OFF switch to ZMCs because zinc levels are no longer sufficiently high to maintain a wild-type structure. This on/off switch leads to a transient nature to the mechanism of ZMCs in which mutant p53 activity comes on in a few hours and then is turned off. This finding has important implications for the translation of ZMCs to the clinic because it indicates that ZMC concentrations need not be maintained at high levels for their activity. Indeed, we found that short exposures (as little as 15 min) were adequate to observe the mutant p53 reactivating activity. This switch mechanism imparts an advantage over other targeted therapeutics in that efficacy can be accomplished with minimal exposure which minimizes toxicity and maximizes the therapeutic window. This on/off switch mechanism is unique in targeted cancer therapeutics and will impact the design of human clinical trials.

Highlights

  • This on/off switch mechanism is unique in cancer therapeutics and the in vivo translation suggests that zinc metallochaperones (ZMCs) might not need maximal or continuous dosing for optimal efficacy

  • Zn-1 treatment increased the survival of the KPC-p53R172H mice both compared to vehicle control (p = 0.00065) and the ZMC1 monomer (p = 0.023)

  • Significant progress has been made in the understanding of the mechanism of ZMCs indicating some very unique properties in the field of cancer therapeutics

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Summary

The Multi-Functional Role of p53

The TP53 gene is the most frequently mutated gene in all of human cancer, with at least 50%. Accumulating evidence suggests that the wide range of p53 mutations have varied consequences with respect to loss of wild-type activity, ability to inhibit remaining wild-type protein function, and acquisition of GOF [11] Another GOF property of mutant p53 is the upregulation of chromatin regulatory genes, leading to genome-wide increases in methylation and acetylation [14]. Patient-derived tumors expressing GOF p53 mutants show upregulation of MLL1, MLL2, and MOZ, whereas wild-type or p53 null tumors did not These genetic changes contribute to cancer cell proliferation and can be attenuated by knockdown of MML1 or pharmacologic inhibition of the MLL1 methyltransferase complex [14]. Numerous approaches have been described, many of which have been discovered in the last ten years and are summarized in several reviews [2,21,22]

Classes of p53 Mutations
Discovery of Thiosemicarbazones as Mutant p53 Reactivators
Elucidation of a Novel Mechanism of Action in Cancer Therapeutics
Pre-Clinical Translation of ZMCs
ZMCs Synthesized in Complex with Zinc: A Novel Drug Formulation
Pharmacokinetic Parameters That Distinguish ZMCs
1.10. Design Principles for Future Clinical Trials of a ZMC
1.10.1. Patient Selection
1.10.2. Proof-of-Concept Clinical Trial
Findings
Conclusions
Full Text
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