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J Pharmacol Ther Res 2017 Volume 1 Issue 2
November 02-03, 2017 Chicago, USA
4 th International Congress on
International Conference and Exhibition on
Drug Discovery, Designing and Development
Biochemistry, Molecular Biology: R&D
&
The p53 transcription factor functions as one of cancer’s
most potent tumor suppressors and is the most
frequently mutated gene in human cancer. The majority
of p53 mutations (>70%) are missense that generate
a defective protein found at high levels in cells that is
targetable. Restoration of wild type structure and function of
mutant p53 with a small molecule (so-called reactivation) is
a highly sought-after goal in anti-cancer drug development.
The p53 protein requires the binding of a single zinc ion to
fold properly and mutations that impair the protein’s ability
to bind zinc (and cause it to misfold) are highly prevalent
in cancer. We recently discovered a new class of small
molecule zinc chelators named zinc metallochaperones
(ZMCs) that reactivate zinc deficient mutant p53 through
a novel mechanism involving both zinc ionophore activity
to raise intracellular zinc concentrations and donation
to restore zinc binding to mutant p53. This induces a wild
type conformation change and a p53 mediated apoptotic
program. The lead compound (ZMC1) displays a transient
pharmacodynamics (p21 levels) in vitro. We hypothesized
that the regulation of these pharmacodynamics is governed
by cellular zinc homeostatic mechanisms that function to
restore zinc to its physiologic picomolar levels. We examined
the entire suite of zinc homeostatic genes in response to
ZMC1 and manipulated several metallothionein genes by
knockout and knockdown. The net effect of this was to
increase the peak and duration of intracellular zinc levels
that lead to a more potent and sustained duration of p21
expression. This translated to increased sensitivity to ZMC1.
We further postulated that this pharmacodynamics would
allow the drug to function with very minimal exposure and
colony formation studies in vitro indicated that a two-hour
exposure was as effective as a 72-hour exposure. We then
sought to translate this mechanism in vivo using a genetically
engineered murine model of KPC pancreatic cancer (Pdx-
1Cre; KrasG12D) that expresses either the p53R172H
(zinc deficient) allele or p53R270H (non-zinc deficient).
Pharmacokinetic (PK) studies of the drug revealed a short
half-life (15 minutes) indicating a minimal exposure. Despite
this, daily, intermittent dosing at the maximum tolerated
dose resulted in a statistically significant increase in the
overall survival of the KPC-p53R172H mice while having no
such effect in the KPC-p53R270H. We sought to improve the
efficacy of ZMC1 by preloading it with zinc in a 2:1 molar
ratio based on the crystal structure. The drug-zinc complex
(Zn-1) increased the median survival of KPCp53-R172H
mice from 26 days to 35 days (ZMC1 monomer versus Zn-
1). These studies indicate that cellular zinc homeostatic
mechanisms function as an “off” switch for ZMC’s which
has important implications for the translation of ZMCs in
humans. Principally, this allows the drug to function with
minimal exposure which minimizes potential zinc toxicity.
ZMC1 as monotherapy improves survival in an allele-specific
mutant p53manner. Furthermore, ZMC1 can be optimized by
synthesizing it complexed with zinc. Overall, this “off” switch
is novel for a targeted molecular therapeutic and represents
a significant departure from the traditional paradigm where
the goal is to develop a compound that binds the target with
a PK profile that provides maximal exposure.
e:
Cellular zinc homeostatic mechanisms function as an off switch for zinc metallochaperone mediated
reactivation of mutant p53
Xin Yu1, 2, Samuel Kogan1, 2, 3, Ying Chen 2, Ashley T Tsang1, 2, 5, Tracy Withers2, Hongxia Lin2, Chunxia Chen2, Dirk F Moore2, Joseph Bertino2, Chang Chan2, 4
and Darren R Carpizo1, 2, 3
1Rutgers Robert Wood Johnson Medical School, USA
2Rutgers Cancer Institute of New Jersey, USA
3Rutgers University, USA
4Rutgers Robert Wood Johnson Medical School, USA
5Mount Sinai St. Luke’s and Mount Sinai West, USA