Executive Summary
In healthy cells, p53, a tumor suppressor protein produced by the gene TP53, is a crucial defense against tumor formation, growth, and metastasis, earning its title as the ‘Guardian of the Genome’ [1]. P53 is either deleted or mutated in 50-60% of all cancers, and the key signaling pathways that it regulates are disrupted in the remaining 40-50% of all cancers [2]. Patients with Li-Fraumeni Syndrome, who are born with altered or absent p53, face a lifetime cancer risk of 70% for men and nearly 100% for women [3]. Given its vital role, p53 has long been a key target for cancer research.
RLIP76 is a critical protein that is involved in the detoxification of toxic metabolites and in the control of key cancer signaling pathways. Inhibition of RLIP76 has produced complete regression across multiple tumor types in preclinical models, and the absence of RLIP76 protects from the formation and growth of neoplasms [4,5,6]. Most critically, inhibition of RLIP76 circumvents abnormal p53 status to regulate and reactivate anti-cancer genetic pathways.
Signaling Pathways and p53
Signaling pathways are a series of chemical reactions in which a group of molecules in a cell work together to control a cell function. These pathways are critical to maintaining normalcy within cells, and when they are disrupted or activated abnormally, it can cause diseases such as cancer.
P53 regulates a vast signaling pathway network that includes some of the most important cancer targets: AKT, PI3K, PKCA MDM2, JNK1, CDKN1A, BRCA1, BRCA2, PTEN Chk1, Bcl-2, MAP and Cyclins among many others [7]. When p53 is normal, it regulates this entire network and acts as a tumor suppressor. When p53 is mutated or deleted, signal disruptions or abnormalities can cause tumors to form at many different parts of these pathways.
For decades, scientists have attempted to solve the problem of p53 mutation and deletion. Many attempts have been focused on reactivating p53 in some capacity [8]. This has included MDM2 and MDMX inhibitors, gene therapies, and targeting p53 mutants. None have succeeded at creating a solution that successfully reactivates p53’s tumor suppressive function as an effective cancer treatment.
Avesta Bio has generated a proprietary dataset that reveals that RLIP76 regulates p53 and its entire signaling network. In mice who are p53-/- and RLIP76+/-, this dataset has very clearly shown that RLIP76 depletion or inhibition reactivates the p53 signaling pathway network and restores the tumor suppressive functions.
Function of RLIP76
RLIP76 is a stress-protective protein that plays critical roles in the transport of glutathione-electrophile conjugates, clathrin-dependent endocytosis, and the mediation of oxidative stress. These roles translate to the following: cancer cells use RLIP76 to remove high levels of oxidants and prevent apoptosis (programmed cell death). This allows cancer cells to grow to 2-4 times the size of a non-cancerous cell, and permits tumors to form, grow, and spread [4].
RLIP76 also regulates p53’s signaling pathways. In the absence of p53, RLIP76 regulates tumor suppression, angiogenesis, cell cycle progression, cell cycle arrest, apoptosis, cell survival, mitochondrial respiration, glycolysis, autophagy, senescence, DNA repair, and several other key functions [9].
Together these point to RLIP76 being an essential protein in cancer formation, growth, survival, and spread, and thus a prime target for anti-cancer therapeutics.
RLIP76 Haploinsufficiency
Mice completely lacking in RLIP76 have been shown to be highly resistant to cancer formation, even when subjected to the most potent chemical carcinogens. This result prompted Avesta Bio’s scientists to examine the effect of combining RLIP76 and p53 deficiencies. These studies showed that even just a hemizygous deficiency of RLIP76 was enough to exert a strong dominant negative effect on the spontaneous carcinogenesis phenotype of homozygous p53-null mice. Additional studies have demonstrated that depleting RLIP76 by half is enough to double the median overall survival of animals with RLIP76+/+.[4]
Depletion of RLIP76 to ~50% after eight months of antisense treatment caused a dramatic normalization of the methylation status of the p53-/- mouse genome. These findings indicate that altered expression of carcinogenic proteins in p53-/- mice depends on changes in DNA methylation regulated by RLIP76 [9].
Overall, recent data indicates that RLIP76 deficiency switches off the cancer susceptibility of the p53-/- mouse phenotype through a haploinsufficiency mechanism that regulates stress-induced epigenetic remodeling of DNA through binding interactions between p53, HSF, and RLIP76 [9].
There are strong clinical ramifications of the ability to reactivate p53’s tumor suppressive effect through only a partial depletion of RLIP76. Chief among them are that patient dosing and potential toxicities are both reduced by not needing to completely reduce the amount of RLIP76 in the body. As RLIP76 serves a function to help protect the body from oxidative stress, retaining half of the RLIP76 levels allows normal cells to ward off the effects of stress on the body.
Broader Ramifications in Cancer
As previously mentioned, p53 dysfunction is ubiquitous across human cancers. Its nature as the key tumor suppressor is critical to nearly all types of cancer. RLIP76’s regulation of p53’s signaling pathways presents RLIP76 as essential to cancer formation, growth, and spread. This essential nature has borne out in in vivo studies in more than a dozen different types of cancer, where depletion or inhibition of RLIP76 has produced marked antitumor activity [4, 5, 10, 11, 12, 13].
Critically, these results have also come without overt negative toxicities. Systemic RLIP76-depleting treatments did not cause significant organ toxicity, and the animals gained weight normally after treatment, with no adverse effects [9]. Most incredibly, RLIP76 depletion saw p53-/- mice survive a full lifespan of 3 years, an unprecedented result when the median lifespan of a p53-/- mouse is 4½ months [5, 14].
The ramifications of these results are that a treatment focused on RLIP76 has the potential to treat a wide range of cancers, while showing a favorable safety profile in preclinical studies.
Novel Therapeutics That Target RLIP76
Several research groups have generated anti-RLIP76 monoclonal and polyclonal antibodies and short interfering RNA as chemical tools to parse the biology of RLIP76. However, there is, to date, no known RLIP76 drug discovery work being performed to generate lead clinical candidates. Teams that produce lead molecules and develop clinical candidates are likely to enjoy the benefits of first-mover advantage in the RLIP76 drug discovery space and targeting of RLIP76 has the potential to transform the cancer therapy landscape with universal clinical applications.
Avesta Bio has formed a team of highly experienced drug development experts to generate lead clinical candidates that will be first-in-class drugs targeting RLIP76. Avesta Bio’s lead compound, AV108, has shown promising results in vivo.
Given the potential of RLIP76 inhibition to restore the tumor suppressive function of p53 and regress tumors of more than a dozen cancer histologies, RLIP76-targeted therapies hold transformative potential for cancer treatment.
References
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