Current Pharmaceutical Design - Volume 17, Issue 6, 2011

P53 was discovered 30 years ago and research during the last three decades has produced over 55,000 publications leading to deeper understanding of this crucial protein [1]. Majority of cancers have dysfunctional p53, either through mutation of the p53 gene itself or alterations in factors that modulate p53's stability and activity. This has led to numerous strategies to combat cancer by re-activating p53 or related pathways in tumor cells [2]. Although much has been accomplished in understanding the regulation of its activity, there is still much to learn regarding the re-activation of p53 pathways. Gene therapy strategies such as the use of p53 adenovirus that selectively kill p53 deficient cells and pharmaceutical agents that induce conformational change in re-shaping the mutant p53 to enhance DNA binding are currently being pursued all around the globe [3]. However, in over half of cancers, wild type p53 is functional, but is tightly regulated by interacting with MDM2 (or the human homologue, HDM2) [4]. Genetic approaches to decrease the expression of MDM2 or development of synthetic peptides that target MDM2 are steadily developing [5]. From a pharmaceutical perspective, small molecular weight inhibitors have been demonstrated to disrupt the MDM2-p53 interaction without causing damage to the genome [6]. Such chemicals have been discovered by high-throughput screening methods and computational techniques based on crystal structure of MDM2-p53 interaction. A newer, more comprehensive approach is systems biology and network modeling which incorporates most of the published literature on the subject with results from cell-based assays to develop models which then can be verified and tested further in cell-based assays and in animals [7]. Pharmaceutical agents which promote wild type p53 re-activation are mainly small chemical molecules that inhibit the interaction between p53 and its regulator MDM2. These small molecules can potentially target p53 directly or target MDM2, either action resulting in the disruption of MDM2-p53 interaction and initiate the re-activation of p53 functional activity. More recently, interest has been generated in understanding the effect of these small molecules on other p53 family members, p63 and p73, which, in the absence of functional p53, may act through salvage pathways to effect a similar outcome; cell cycle arrest or cell death in tumor cells. Whether future pharmaceutical research will include the development of new agents to directly target p53-MDM2 interaction and agents that initiate the activation of pathways such as p73, independent of p53, will depend upon interaction of scientists with expert knowledge, sound data and future ideas expressed as a theme in this issue. This interaction is critical for the development of pharmaceutical agents to be successfully utilized as a single cure, in a preventative regimen or as adjuvants to reduce the necessary toxic chemotherapeutics presently used as anti-cancer agents. This theme issue is comprised of a collection of highly informative reviews from recognized experts in the area of p53 research. The list of thought provoking articles begins with a comprehensive update from Dr. Nouri Neamati's group on current status of small molecule inhibitors of HDM2 in their pre-clinical and clinical development. In the second article, Drs. Shen and Maki update the audience, through a very informative review on the current trends in p53 reactivating drugs, giving special emphasis to Nutlin-3. This is followed by an article from Dr. Zauli's group on therapeutic perspective on MDM2 inhibition by Nutlin-3 and their chemotherapy combination. In this article, readers can gain insight on the synergistic combinations of Nutlin-3 with novel drugs such as genotoxic platinum chemotherapeutics and TRAIL. In the next review, Drs. Bisso and Del Sal describe the significance of p73 as a critical therapeutic partner in the p53 pathway. Their article is an overview of the principal mechanism of p73 regulation, and describes several examples of pharmacological tools that can induce p73 accumulation and activate function by acting on upstream p73 modulators or displacing inhibitory p73 interactors. The fifth review, contributed by Dr. Slade, comprehensively highlights that p53 family member p73 is equally important and its potential cannot be ignored while designing pharmaceutical dugs. This contribution discusses how understanding the role of p73 has affected drug discovery and outlines current aspects of anticancer therapy. Then, Drs. Changyou Zhan and Wuyuan Lu elaborate on the emerging concept of peptide based MDM2 and MDMX inhibition for the treatment of cancer. In review number seven of this theme issue, Dr Park and group, describe a novel p53 inactivation mechanism by oncogenic K-Ras-Snail axis and a smart strategy to overcome this suppressive mechanism by chemical inhibition of snail-p53 interaction. Following this, Dr. Boyd and his team highlight the differential sensitivity of different tumors to p53 targeted regimens in review eight. Their article provides a clinical perspective and gives prominence to the understanding that not all tumors respond equally to p53 re-activating drugs. Dr. Athar, in his review, describes the potential of re-activating mut-p53 by natural agents, small molecule inhibitors and other strategies for cancer therapy. And finally, this special issue is concluded by an article from my group that describes the utility of systems biology and molecular network modeling in understanding the complex p53-HDM2 network. Our article builds a strong case for use of integrated technology in the design of optimal combination therapies involving HDM2 inhibitors and platinum based genotoxic agents. It is my opinion that this comprehensive collection of reviews from experts in the field encompassing wide ranging topics on pharmaceutical re-activation of p53 and related pathways is timely and will be definitely enhance the knowledge of the researchers actively engaged in the field. Therefore, I sincerely wish the audience enjoyable reading. ACKNOWLEDGEMENTS First and foremost, I would like to thank the Editor-in-Chief Dr. William A. Banks for giving me opportunity to organize this special theme issue. I would like to thank all of the contributing authors for their outstanding articles. I also would like to especially acknowledge all of the anonymous reviewers who kindly reviewed the articles. Dr. Irfana Muqbil's assistance in arranging peer review and proof reading the manuscripts is deeply acknowledged. I am indebted to the Editorial Director Mr. Mirza Kazim Ali Baig and the staff members of Bentham Science Publishers for their assistance in coordinating the publication of this special issue on Pharmaceutical Reactivation of p53 Pathways in Cancer in the journal Current Pharmaceutical Design.

Increasing knowledge of the relationship between p53 and MDM2 has led to development of potential small molecule inhibitors useful for clinical studies. Herein, we discuss the patented (2006-2010) inhibitors of p53-MDM2 interaction. The anticancer agents discussed in this review belong to several different chemical classes including benzodiazepinediones, cis-imidazolines, oxindoles, spirooxindoles, and numerous miscellaneous groups. This review also provides comprehensive information on inhibitors of p53-MDM2 interaction that are currently being tested in clinical trials. It is important to note that many of the disclosed inhibitors need further validation to be considered as bona fide inhibitors of p53-MDM2 interaction and some will not be further considered for future studies. On the other hand, JNJ-26854165, a novel tryptamine derivative and RG7112, a cis-imidazoline representative have shown promising results in early phases of trials in cancer patients. AT-219, a spiroindolinone in late stage preclinical studies is a likely candidate to proceed into clinical trials. It remains to be seen how these inhibitors will perform in future clinical studies as single agents and in combination with the currently approved chemotherapeutic agents.

Restoring p53 activity by inhibiting the interaction between p53 and MDM2 represents an attractive approach for cancer therapy. To this end, a number of small-molecule p53-MDM2 binding inhibitors have been developed during the past several years. Nutlin-3 is a potent and selective small-molecule MDM2 antagonist that has shown considerable promise in pre-clinical studies. This review will highlight recent advances in the development of small-molecule MDM2 antagonists as potential cancer therapeutics, with special emphasis on Nutlin-3.

Nutlin-3 is a small molecule inhibitor of the MDM2/p53 interaction, which leads to the non-genotoxic p53 stabilization, activation of cell cycle arrest and apoptosis pathways. A series of recent studies have strengthened the concept that selective, non-genotoxic p53 activation by Nutlin-3 might represent an alternative to the current cytotoxic chemotherapy, in particular for pediatric tumors and for hematological malignancies, which retain a high percentage of p53wild-type status at diagnosis. Like most other drugs employed in cancer therapy, it will be unlikely that Nutlin-3 will be used as a monotherapy. In this respect, Nutlin-3 shows a synergistic cytotoxic effect when used in combination with innovative drugs, such as TRAIL or bortozemib. Although Nutlin-3 is currently in phase I clinical trial for the treatment of retinoblastoma, its effects on normal tissues and cell types remain largely to be determined and will require further investigation in the future years.

About half of all human tumors contain an inactivating mutation of p53, while in the remaining tumors, the p53 pathway is frequently abrogated by alterations of other components of its signaling pathway. In humans, the p53 tumor suppressor is part of a small gene family that includes two other members, p73 and p63, structurally and functionally related to p53. Accumulating evidences indicate that all p53-family proteins function as molecular hubs of a highly interconnected signaling network that coordinates cell proliferation, differentiation and death in response to physiological inputs and oncogenic stress. Therefore, not only the p53-pathway but the entire “p53-family pathway” is a primary target for cancer drug development. In particular, the p53-related protein p73 has a crucial role in determining cellular responses to chemotherapy, and can vicariate p53 functions in triggering cell death after DNA damage in multiple experimental models. The biology and regulation of p73 is complex, since the TP73 gene incorporates both tumor-suppressive and protooncogenic functions. However, the p73 gene is rarely mutated in tumors, so appropriate pharmacological manipulation of the p73 pathway is a very promising approach for cancer therapy. Here we provide an overview of the principal mechanism of p73 regulation, and describe several examples of pharmacological tools that can induce p73 accumulation and function by acting on upstream p73 modulators or displacing inhibitory p73 interactors. A better understanding of how the p73 pathway works is mandatory to discover additional players intervening in this pathway and has important implications for the improvement of cancer treatment with the development of new molecules or with the reposition of currently available drugs.

The p53 family consists of three members: p53, p63 and p73. All three of them have a role in cell cycle arrest and induction of apoptosis. However, despite structural and partly functional similarity, there are striking differences in their functions and each of them has its own and unique identity. All three genes encode multiple variants with opposing functions in cancer development - full length transactivation forms with proapoptotic and antiproliferative functions, and dominant-negative transactivation-deficient forms with antiapoptotic (oncogenic) functions. The functional interactions between family members are crucial to gain insight and understand their role in cancer biology. The discovery of p53/p73 network could have a major clinical impact in prognostic use and targeted drug design. In the current review we present the recent achievements in p73 research including very complex and sophisticated p73 regulation and response to DNA damage, and functional interactions among family members. We discuss how p73 has affected drug discovery. According to the p73 tumor suppressor function, we outline current aspects of anticancer therapy.

The oncoproteins MDM2 and MDMX negatively regulate the activity and stability of the p53 tumor suppressor, directly contributing to the development and progression of many tumors harboring wild type p53. Antagonizing MDM2 and MDMX to activate the p53 pathway has thus become an attractive new strategy for anticancer drug design. Several different classes of MDM2 and MDMX antagonists have been reported, including low molecular weight compounds, small peptides, miniature proteins, and peptidomimetics. This review aims to summarize the latest progress in the design of peptide activators of the p53 tumor suppressor.

Since p53 is the strongest tumor suppressor gene, which can regulate apoptosis, cell cycle arrest and senescence, re-activation of p53 and its pathway seem to be very plausible target for cancer therapy. However, in 50% of human cancers, p53 itself is mutated. In addition, in remaining half of cancers, it is inactivated by distortion of signaling pathways. Moreover, differentially from typical tumor suppressor genes such as Rb, p53 mutations in its DNA binding domain show the dominant negative effect on p53 function. Here, we describe the novel p53 inactivation mechanism by oncogenic K-Ras-Snail axis and smart strategy to reactivation of p53 suppressed by oncogenic K-Ras-Snail through small chemicals (GN25, 29). Since K-Ras mutation is frequently occurred in human pancreatic, colon, and lung cancer, we discuss the clinical implication of new small Snail-p53 inhibitor on these cancers. In addition, the possibility of reactivation of wild type p53, governed by mutant p53, is suggested using our chemicals. Through this, we will provide the new strategy to handling the K-Ras mutated human cancers including pancreatic, lung and colon cancers.

p53, the “guardian of the genome” and the most mutated gene in cancer presents a considerable therapeutic opportunity as well as a challenge. In the past decade, several therapeutic strategies have been developed that aim to take advantage of a wealth of knowledge about p53, including insights into the biology and patho-biology of p53. Nevertheless, considerable challenges remain, not least as a result of tissue- and cancer-specific differences in p53 regulation and/or function. p53 does not act in the same manner in all tissues or in the cancers arising from them. Nor is p53 regulated in the same way in the wide variety of tissues from which cancers develop. Therefore, potential strategies for therapeutic targeting need to be tailored to each tumour/tissue type. This review summarises some of these tissue- and cancer-specific issues to suggest how different strategies are required for cancers arising from different tissues and to illustrate the complexities of therapeutic targeting of p53.

Tumor suppressor p53 is a transcription factor that regulates a large number of genes and guards against genomic instability. Under multiple cellular stress conditions, p53 functions to block cell cycle progression transiently unless proper DNA repair occurs. Failure of DNA repair mechanisms leads to p53-mediated induction of cell death programs. p53 also induces permanent cell cycle arrest known as cellular senescence. During neoplastic progression, p53 is often mutated and fails to efficiently perform these functions. It has been observed that cancers carrying a wild-type p53 may also have interrupted downstream p53 regulatory signaling leading to disruption in p53 functions. Therefore, strategies to reactivate p53 provide an attractive approach for blocking tumor pathogenesis and its progression. p53 activation may also lead to regression of existing early neoplastic lesions and therefore may be important in developing cancer chemoprevention protocols. A large number of small molecules capable of reactivating p53 have been developed and some are progressing through clinical trials for prospective human applications. However, several questions remain to be answered at this stage. For example, it is not certain if pharmacological activation of p53 will restore all of its multifaceted biological responses, assuming that the targeted cell is not killed following p53 activation. It remains to be demonstrated whether the distinct biological effects regulated by specific post-translationally modified p53 can effectively be restored by refolding mutant p53. Mutant p53 can be classified as a loss-of-function or gain-of-function protein depending on the type of mutation. It is also unclear whether reactivation of mutant p53 has similar consequences in cells carrying gain-of-function and loss-of-function p53 mutants. This review provides a description of various pharmacological approaches tested to activate p53 (both wild-type and mutant) and to assess the effects of activated p53 on neoplastic progression.

The discovery of small molecule inhibitors of HDM2-p53 interaction is considered one of the most significant therapeutic developments in the area p53 research. Intensive work on different classes of HDM2 inhibitors has proven their therapeutic utility as activators of p53 in multiple tumor models. Many laboratories have shown that HDM2 inhibitors can synergize with chemotherapeutic agents resulting in enhanced efficacy through both p53-dependent and independent mechanisms. In our hands HDM2 inhibitor and platinum drug combination showed remarkable antitumor activity that led tumor free survival in one of the most resistant and complex pancreatic xenograft models. Although antitumor efficacy of such combinations has been studied in detail, not much is known on the molecular mechanisms governing this synergy. This is partly due to complexity of multiple pathways modulated by p53 and HDM2. We are of the view that in order to decode this complexity, an integrated approach is needed that considers both HDM2 and p53 as components of a network and not in isolation. This review highlights recent advancements in our understanding of HDM2 inhibitor combination therapy based on network modeling and systems biology driven science. Our recent findings support such a network view as integrated gene expression profiling and pathway network modeling on MI-219-oxaliplatin treated cells revealed activation of multiple and closely knit biological networks. We anticipate that in the near future such network-centric approaches will benefit clinical development of HDM2 inhibitors for genetically predefined subsets of cancer patients and this will be a step towards personalized medicine.