Acquisition of p53 mutations in response to the non-genotoxic p53 activator Nutlin-3

BH3-mimetics or DNA-damaging agents in combination with RG7388 overcome p53 mutation-induced resistance to MDM2 inhibition

The development of drug resistance reduces the efficacy of cancer therapy. Tumor cells can acquire resistance to MDM2 inhibitors, which are currently under clinical evaluation. We generated RG7388-resistant neuroblastoma cells, which became more proliferative and metabolically active and were less sensitive to DNA-damaging agents in vitro and in vivo, compared with wild-type cells. The resistance was associated with a mutation of the p53 protein (His193Arg). This mutation abated its transcriptional activity via destabilization of the tetrameric p53-DNA complex and was observed in many cancer types. Finally, we found that Cisplatin and various BH3-mimetics could enhance RG7388-mediated apoptosis in RG7388-resistant neuroblastoma cells, thereby partially overcoming resistance to MDM2 inhibition.

Targeting apoptotic pathways for cancer therapy

Apoptosis is a form of programmed cell death that is mediated by intrinsic and extrinsic pathways. Dysregulation of and resistance to cell death are hallmarks of cancer. For over three decades, the development of therapies to promote treatment of cancer by inducing various cell death modalities, including apoptosis, has been a main goal of clinical oncology. Apoptosis pathways also interact with other signaling mechanisms, such as the p53 signaling pathway and the integrated stress response (ISR) pathway. In addition to agents directly targeting the intrinsic and extrinsic pathway components, anticancer drugs that target the p53 and ISR signaling pathways are actively being developed. In this Review, we discuss selected and promising anticancer therapies in various stages of development, including drug targets, mechanisms, and resistance to related treatments, focusing especially on B cell lymphoma 2 (BCL-2) inhibitors, TRAIL analogues, DR5 antibodies, and strategies that target p53, mutant p53, and the ISR.

Translating p53-based therapies for cancer into the clinic

Inactivation of the most important tumour suppressor gene TP53 occurs in most, if not all, human cancers. Loss of functional wild-type p53 is achieved via two main mechanisms: mutation of the gene leading to an absence of tumour suppressor activity and, in some cases, gain-of-oncogenic function; or inhibition of the wild-type p53 protein mediated by overexpression of its negative regulators MDM2 and MDMX. Because of its high potency as a tumour suppressor and the dependence of at least some established tumours on its inactivation, p53 appears to be a highly attractive target for the development of new anticancer drugs. However, p53 is a transcription factor and therefore has long been considered undruggable. Nevertheless, several innovative strategies have been pursued for targeting dysfunctional p53 for cancer treatment. In mutant p53-expressing tumours, the predominant strategy is to restore tumour suppressor function with compounds acting either in a generic manner or otherwise selective for one or a few specific p53 mutations. In addition, approaches to deplete mutant p53 or to target vulnerabilities created by mutant p53 expression are currently under development. In wild-type p53 tumours, the major approach is to protect p53 from the actions of MDM2 and MDMX by targeting these negative regulators with inhibitors. Although the results of at least some clinical trials of MDM2 inhibitors and mutant p53-restoring compounds are promising, none of the agents has yet been approved by the FDA. Alternative strategies, based on a better understanding of p53 biology, the mechanisms of action of compounds and treatment regimens as well as the development of new technologies are gaining interest, such as proteolysis-targeting chimeras for MDM2 degradation. Other approaches are taking advantage of the progress made in immune-based therapies for cancer. In this Review, we present these ongoing clinical trials and emerging approaches to re-evaluate the current state of knowledge of p53-based therapies for cancer.

Enhancement of colorectal cancer therapy through interruption of the HSF1-HSP90 axis by p53 activation or cell cycle inhibition

The stress-associated molecular chaperone system is an actionable target in cancer therapies. It is ubiquitously upregulated in cancer tissues and enables tumorigenicity by stabilizing hundreds of oncoproteins and disturbing the stoichiometry of protein complexes. Most inhibitors target the key component heat-shock protein 90 (HSP90). However, although classical HSP90 inhibitors are highly tumor-selective, they fail in phase 3 clinical oncology trials. These failures are at least partly due to an interference with a negative feedback loop by HSP90 inhibition, known as heat-shock response (HSR): in response to HSP90 inhibition there is compensatory synthesis of stress-inducible chaperones, mediated by the transcription factor heat-shock factor 1 (HSF1). We recently identified that wildtype p53 (p53) actively reduces the HSR by repressing HSF1 via a p21-CDK4/6-MAPK-HSF1 axis. Here we test the hypothesis that in HSP90-based therapies simultaneous p53 activation or direct cell cycle inhibition interrupts the deleterious HSF1-HSR axis and improves the efficiency of HSP90 inhibitors. Indeed, we find that the clinically relevant p53 activator Idasanutlin suppresses the HSF1-HSR activity in HSP90 inhibitor-based therapies. This combination synergistically reduces cell viability and accelerates cell death in p53-proficient colorectal cancer (CRC) cells, murine tumor-derived organoids and patient-derived organoids (PDOs). Mechanistically, upon combination therapy human CRC cells strongly upregulate p53-associated pathways, apoptosis, and inflammatory immune pathways. Likewise, in the chemical AOM/DSS CRC model in mice, dual HSF1-HSP90 inhibition strongly represses tumor growth and remodels immune cell composition, yet displays only minor toxicities in mice and normal mucosa-derived organoids. Importantly, inhibition of the cyclin dependent kinases 4 and 6 (CDK4/6) under HSP90 inhibition phenocopies synergistic repression of the HSR in p53-proficient CRC cells. Even more important, in p53-deficient (mutp53-harboring) CRC cells, an HSP90 inhibition in combination with CDK4/6 inhibitors similarly suppresses the HSF1-HSR system and reduces cancer growth. Likewise, p53-mutated PDOs strongly respond to dual HSF1-HSP90 pathway inhibition and thus, providing a strategy to target CRC independent of the p53 status. In sum, activating p53 (in p53-proficient cancer cells) or inhibiting CDK4/6 (independent of the p53 status) provide new options to improve the clinical outcome of HSP90-based therapies and to enhance colorectal cancer therapy.

Advancements in MDM2 inhibition: Clinical and pre-clinical investigations of combination therapeutic regimens

Cancer cells often depend on multiple pathways for their growth and survival, resulting in therapeutic resistance and the limited effectiveness of treatments. Combination therapy has emerged as a favorable approach to enhance treatment efficacy and minimize acquired resistance and harmful side effects. The murine double minute 2 (MDM2) protein regulates cellular proliferation and promotes cancer-related activities by negatively regulating the tumor suppressor protein p53. MDM2 aberrations have been reported in a variety of human cancers, making it an appealing target for cancer therapy. As a result, several small-molecule MDM2 inhibitors have been developed and are currently being investigated in clinical studies. Nevertheless, it has been shown that the inhibition of MDM2 alone is inadequate to achieve long-term suppression of tumor growth, thus prompting the need for further investigation into combination therapeutic strategies. In this review, possible clinical and preclinical MDM2 combination inhibitor regimens are thoroughly analyzed and discussed. It provides a rationale for combining MDM2 inhibitors with other therapeutic approaches in the management of cancer, taking into consideration ongoing clinical trials that evaluate the combination of MDM2 inhibitors. The review explores the current status of MDM2 inhibitors in combination with chemotherapy or targeted therapy, as well as promising approach of combining MDM2 inhibitors with immunotherapy. In addition, it investigates the function of PROTACs as MDM2 degraders in cancer treatment. A comprehensive examination of these combination regimens highlights the potential for advancing MDM2-inhibitor therapy and improving clinical outcomes for cancer patients and establishes the foundation for future research and development in this promising area of study.

Shared Gene Targets of the ATF4 and p53 Transcriptional Networks

The master tumor suppressor p53 regulates multiple cell fate decisions, such as cell cycle arrest and apoptosis, via transcriptional control of a broad gene network. Dysfunction in the p53 network is common in cancer, often through mutations that inactivate p53 or other members of the pathway. Induction of tumor-specific cell death by restoration of p53 activity without off-target effects has gained significant interest in the field. In this study, we explore the gene regulatory mechanisms underlying a putative anticancer strategy involving stimulation of the p53-independent integrated stress response (ISR). Our data demonstrate the p53 and ISR pathways converge to independently regulate common metabolic and proapoptotic genes. We investigated the architecture of multiple gene regulatory elements bound by p53 and the ISR effector ATF4 controlling this shared regulation. We identified additional key transcription factors that control basal and stress-induced regulation of these shared p53 and ATF4 target genes. Thus, our results provide significant new molecular and genetic insight into gene regulatory networks and transcription factors that are the target of numerous antitumor therapies.

miR-221/222 induce instability of p53 By downregulating deubiquitinase YOD1 in acute myeloid leukemia

Acute myeloid leukemia (AML) is a hematological malignancy characterized by the impaired differentiation and uncontrolled proliferation of myeloid blasts. Tumor suppressor p53 is often downregulated in AML cells via ubiquitination-mediated degradation. While the role of E3 ligase MDM2 in p53 ubiquitination is well-accepted, little is known about the involvement of deubiquitinases (DUBs). Herein, we found that the expression of YOD1, among several DUBs, is substantially reduced in blood cells from AML patients. We identified that YOD1 deubiqutinated and stabilized p53 through interaction via N-terminus of p53 and OTU domain of YOD1. In addition, expression levels of YOD1 were suppressed by elevated miR-221/222 in AML cells through binding to the 3' untranslated region of YOD1, as verified by reporter gene assays. Treatment of cells with miR-221/222 mimics and inhibitors yielded the expected effects on YOD1 expressions, in agreement with the negative correlation observed between the expression levels of miR-221/222 and YOD1 in AML cells. Finally, overexpression of YOD1 stabilized p53, upregulated pro-apoptotic p53 downstream genes, and increased the sensitivity of AML cells to FLT3 inhibitors remarkably. Collectively, our study identified a pathway connecting miR-221/222, YOD1, and p53 in AML. Targeting miR-221/222 and stimulating YOD1 activity may improve the therapeutic effects of FLT3 inhibitors in patients with AML.

Shared gene targets of the ATF4 and p53 transcriptional networks

The master tumor suppressor p53 regulates multiple cell fate decisions, like cell cycle arrest and apoptosis, via transcriptional control of a broad gene network. Dysfunction in the p53 network is common in cancer, often through mutations that inactivate p53 or other members of the pathway. Induction of tumor-specific cell death by restoration of p53 activity without off-target effects has gained significant interest in the field. In this study, we explore the gene regulatory mechanisms underlying a putative anti-cancer strategy involving stimulation of the p53-independent Integrated Stress Response (ISR). Our data demonstrate the p53 and ISR pathways converge to independently regulate common metabolic and pro-apoptotic genes. We investigated the architecture of multiple gene regulatory elements bound by p53 and the ISR effector ATF4 controlling this shared regulation. We identified additional key transcription factors that control basal and stress-induced regulation of these shared p53 and ATF4 target genes. Thus, our results provide significant new molecular and genetic insight into gene regulatory networks and transcription factors that are the target of numerous antitumor therapies.

MDM2-PROTAC versus MDM2 Inhibitors: Beyond p53 Reactivation

SUMMARY

In this issue of Cancer Discovery, Adams and colleagues present the discovery of a potent PROTAC, MDM2 degrader, which activates wild-type p53 leading to cancer cell death. Importantly, in a number of in vitro and in vivo experiments, the authors show that the depletion of MDM2 by PROTAC kills p53-mutant or p53-null cancer cells. See related article by Adams et al., p. 1210 (5).

CopA3 peptide inhibits MDM2-p53 complex stability in colorectal cancers and activates p53 mediated cell death machinery

The diverse expression patterns of the tumor suppressor p53 in cancer cells reflect the regulatory efficiency of multiple cellular pathways. By contrast, many human tumors are reported to develop in the presence of wild-type p53. Recently, several oncogene inhibitors have been used clinically to suppress tumor development by functionally reactivating other oncoproteins. On the other hand, p53 reactivation therapies have not been well established, as few of the p53-MDM2 complex inhibitors such as Nutlin-3 induces mutation in p53 gene upon prolonged usage. Therefore, in this study CopA3, a 9-mer dimeric D-type peptide with anticancer activity against the human colorectal cancer cells, was used to explore the efficacy of p53 reactivation in-vitro and in-vivo. The anticancer activity of CopA3 was more selective towards the wild-type p53 expressing cells than the p53 deficient or mutant colorectal cancer cells. In response to this, this study investigated the signaling pathway in vitro and validated its anti-tumor activity in-vivo. The protein-peptide interaction and molecular docking efficiently provided insight into the specific binding affinity of CopA3 to the p53-binding pocket of the MDM2 protein, which efficiently blocked the p53 and MDM2 interaction. CopA3 plays a crucial role in the binding with MDM2 and enhanced the nuclear translocation of the p53 protein, which sequentially activated the downstream targets to trigger the autophagic mediated cell death machinery through the JNK/Beclin-1 mediated pathway. Collectively, CopA3 affected the MDM2-p53 interaction, which suppressed tumor development. This study may provide a novel inhibitor candidate for the MDM2-p53 complex, which could ultimately suppress the growth of colorectal cancer cells without being cytotoxic to the healthy neighboring cells present around the tumor microenvironment.

PPM1D suppresses p53-dependent transactivation and cell death by inhibiting the Integrated Stress Response

The p53 transcription factor is a master regulator of cellular stress responses inhibited by repressors such as MDM2 and the phosphatase PPM1D. Activation of p53 with pharmacological inhibitors of its repressors is being tested in clinical trials for cancer therapy, but efficacy has been limited by poor induction of tumor cell death. We demonstrate that dual inhibition of MDM2 and PPM1D induces apoptosis in multiple cancer cell types via amplification of the p53 transcriptional program through the eIF2α-ATF4 pathway. PPM1D inhibition induces phosphorylation of eIF2α, ATF4 accumulation, and ATF4-dependent enhancement of p53-dependent transactivation upon MDM2 inhibition. Dual inhibition of p53 repressors depletes heme and induces HRI-dependent eIF2α phosphorylation. Pharmacological induction of eIF2α phosphorylation synergizes with MDM2 inhibition to induce cell death and halt tumor growth in mice. These results demonstrate that PPM1D inhibits both the p53 network and the integrated stress response controlled by eIF2α-ATF4, with clear therapeutic implications.

Resistance mechanisms to inhibitors of p53-MDM2 interactions in cancer therapy: can we overcome them?

Since the discovery of the first MDM2 inhibitors, we have gained deeper insights into the cellular roles of MDM2 and p53. In this review, we focus on MDM2 inhibitors that bind to the p53-binding domain of MDM2 and aim to disrupt the binding of MDM2 to p53. We describe the basic mechanism of action of these MDM2 inhibitors, such as nutlin-3a, summarise the determinants of sensitivity to MDM2 inhibition from p53-dependent and p53-independent points of view and discuss the problems with innate and acquired resistance to MDM2 inhibition. Despite progress in MDM2 inhibitor design and ongoing clinical trials, their broad use in cancer treatment is not fulfilling expectations in heterogenous human cancers. We assess the MDM2 inhibitor types in clinical trials and provide an overview of possible sources of resistance to MDM2 inhibition, underlining the need for patient stratification based on these aspects to gain better clinical responses, including the use of combination therapies for personalised medicine.

Long transcripts minus touchdown qPCR (LTMT-qPCR): a simplified and convenient method for the screening and quantification of microRNA profiles

Due to the short length and differences in abundance of microRNAs, microRNA profile screening and quantification is challenging. In this study, we found that size selection magnetic beads could be employed to easily and efficiently remove long RNA transcripts. After removing the long transcripts, the remaining small RNAs could be concentrated and then reverse-transcribed using universal stem-loop primers (USLP), with six randomized nucleotides at the 3' end region. The efficiency of reverse transcription decreased when the number of randomized nucleotides was reduced. In addition, we found that touchdown qPCR improved microRNA profile detection, with lower C_T values and better detection efficiency than the regular qPCR protocol, especially for those low-abundance microRNAs. Finally, we incorporated these observations to create a new protocol we named long transcripts minus touchdown qPCR (LTMT-qPCR). We performed a side-by-side comparison of LTMT with USLP and traditional stem-loop primer (TSLP) protocols. We found that LTMT has higher detection efficiency than USLP, especially for the detection of low-abundance microRNAs. Although LTMT was equivalent to TSLP in terms of microRNA profile detection, LTMT is more convenient, user-friendly, and cost-effective. Taken together, the present data indicate that LTMT is a simple, rapid, and user-friendly approach that has higher precision, accuracy, and sensitivity than the previously described methods, making it more suitable for microRNA profile screening and quantification.

MI-773, a breaker of the MDM2/p53 axis, exhibits anticancer effects in neuroblastoma via downregulation of INSM1

Neuroblastoma (NB) is a common pediatric malignancy associated with poor outcomes. Recent studies have shown that murine double minute2 homolog (MDM2) protein inhibitors are promising anticancer agents. MI-773 is a novel and specific antagonist of MDM2, however, the molecular mechanism of its anti-NB activity remains unclear. NB cell viability was measured by Cell Counting Kit-8 assay following MI-773 treatment. Cell cycle progression was analyzed using PI staining and apoptosis was assessed using Annexin V/PI staining. The molecular mechanisms by which MI-773 exerted its effects were investigated using a microarray. The results showed that disturbance of the MDM2/p53 axis by MI-773 resulted in potent suppression of proliferation, induction of apoptosis and cell cycle arrest in NB cells. In addition, microarray analysis showed that MI-773 led to significant downregulation of genes involved in the G₂/M phase checkpoint and upregulation of hallmark gene associated with the p53 pathway. Meanwhile, knockdown of insulinoma-associated 1 decreased proliferation and increased apoptosis of NB cells. In conclusion, the present study demonstrated that MI-773 exhibited high selectivity and blockade affinity for the interaction between MDM2 and TP53 and may serve as a novel strategy for the treatment of NB.

Targeting the p53 Pathway in CLL: State of the Art and Future Perspectives

The p53 pathway is a desirable therapeutic target, owing to its critical role in the maintenance of genome integrity. This is exemplified in chronic lymphocytic leukemia (CLL), one of the most common adult hematologic malignancies, in which functional loss of p53 arising from genomic aberrations are frequently associated with clonal evolution, disease progression, and therapeutic resistance, even in the contemporary era of CLL targeted therapy and immunotherapy. Targeting the 'undruggable' p53 pathway therefore arguably represents the holy grail of cancer research. In recent years, several strategies have been proposed to exploit p53 pathway defects for cancer treatment. Such strategies include upregulating wild-type p53, restoring tumor suppressive function in mutant p53, inducing synthetic lethality by targeting collateral genome maintenance pathways, and harnessing the immunogenicity of p53 pathway aberrations. In this review, we will examine the biological and clinical implications of p53 pathway defects, as well as our progress towards development of therapeutic approaches targeting the p53 pathway, specifically within the context of CLL. We will appraise the opportunities and pitfalls associated with these therapeutic strategies, and evaluate their place amongst the array of new biological therapies for CLL.

Recent advances on the intervention sites targeting USP7-MDM2-p53 in cancer therapy

The ubiquitin-specific protease 7 (USP7)-murine double minute 2 (MDM2)-p53 network plays an important role in the regulation of p53, a tumor suppressor which plays critical roles in regulating cell growth, proliferation, cell cycle progression, apoptosis and immune response. The overexpression of USP7 and MDM2 in human cancers contributes to cancer initiation and progression, and their inhibition reactivates p53 signalings and causes cell cycle arrest and apoptosis. Herein, the current state of pharmacological characterization, potential applications in cancer treatment and mechanism of action of small molecules used to target and inhibit MDM2 and USP7 proteins are highlighted, along with the outcomes in clinical and preclinical settings. Moreover, challenges and advantages of these strategies, as well as perspectives in USP7-MDM2-p53 field are analyzed in detail. The investigation and application of MDM2 and USP7 inhibitors will deepen our understanding of the function of USP7-MDM2-p53 network, and feed in the development of effective and safe cancer therapies where USP7-MDM2-p53 network is implicated.

Pharmacokinetic-pharmacodynamic guided optimisation of dose and schedule of CGM097, an HDM2 inhibitor, in preclinical and clinical studies

BACKGROUND

CGM097 inhibits the p53-HDM2 interaction leading to downstream p53 activation. Preclinical in vivo studies support clinical exploration while providing preliminary evidence for dosing regimens. This first-in-human phase I study aimed at assessing the safety, MTD, PK/PD and preliminary antitumor activity of CGM097 in advanced solid tumour patients (NCT01760525).

METHODS

Fifty-one patients received oral treatment with CGM097 10-400 mg 3qw (n = 31) or 300-700 mg 3qw 2 weeks on/1 week off (n = 20). Choice of dose regimen was guided by PD biomarkers, and quantitative models describing the effect of CGM097 on circulating platelet and PD kinetics.

RESULTS

No dose-limiting toxicities were reported in any regimens. The most common treatment-related grade 3/4 AEs were haematologic events. PK/PD models well described the time course of platelet and serum GDF-15 changes, providing a tool to predict response to CGM097 for dose-limiting thrombocytopenia and GDF-15 biomarker. The disease control rate was 39%, including one partial response and 19 patients in stable disease. Twenty patients had a cumulative treatment duration of >16 weeks, with eight patients on treatment for >32 weeks. The MTD was not determined.

CONCLUSIONS

Despite delayed-onset thrombocytopenia frequently observed, the tolerability of CGM097 appears manageable. This study provided insights on dosing optimisation for next-generation HDM2 inhibitors.

TRANSLATIONAL RELEVANCE

Haematologic toxicity with delayed thrombocytopenia is a well-known on-target effect of HDM2 inhibitors. Here we have developed a PK/PD guided approach to optimise the dose and schedule of CGM097, a novel HDM2 inhibitor, using exposure, platelets and GDF-15, a known p53 downstream target to predict patients at higher risk to develop thrombocytopenia. While CGM097 had shown limited activity, with disease control rate of 39% and only one patient in partial response, the preliminary data from the first-in-human escalation study together with the PK/PD modeling provide important insights on how to optimize dosing of next generation HDM2 inhibitors to mitigate hematologic toxicity.

The role of SIRT5 and p53 proteins in the sensitivity of colon cancer cells to chemotherapeutic agent 5-Fluorouracil

BACKGROUND

In the treatment of colorectal cancer, it is important to develop drug combinations that will increase the effectiveness of chemotherapy and to determine the molecular targets of the drugs. Therefore, combined therapies that can increase the sensitivity of 5-Fluorouracil (5-FU) and the molecular pathways involved in this process are important in the treatment of the disease. Here we examined the SIRT5 (Resveratrol and Suramin) and p53 (Nutlin3a) modulators alone or in combination with 5-FU on the proliferation of colon cancer cells and effect of 5-FU on the SIRT5 and FOXO3a protein expressions whether p53 dependent or independent manner. METHODS AND RESULTS: According to our MTT assay results, Resveratrol (RSV), Nutlin3a and Suramin was found to be more effective in HCT-116 p53+/+ cells and these differences were evaluated together with the effect of 5-FU on the SIRT5, FOXO3a and Bim protein expressions in HCT-116 p53 +/+ and HCT-116 p53 -/- cells. SIRT5 is known to deacetylate FOXO3a which plays roles in the induction of apoptosis via Bim protein. Our western blot experiment results showed that while Suramin decreased SIRT5 and RSV decreased FOXO3a protein expressions significantly in HCT-116 p53 -/- cells, 5-FU decreased significantly SIRT5 and FOXO3a protein expressions in a p53 independent manner.

CONCLUSIONS

In this study, the effect of 5-FU on SIRT5 and FOXO 3a proteins was determined for the first time in HCT-116 p53 +/+ and HCT-116 p53 -/- cells. These results may help the discovery of new markers in colon cancer treatment.

Transient expansion of TP53 mutated clones in polycythemia vera patients treated with idasanutlin

Activation of the P53 pathway through inhibition of MDM2 using nutlins has shown clinical promise in the treatment of solid tumors and hematologic malignancies. There is concern, however, that nutlin therapy might stimulate the emergence or expansion of TP53-mutated subclones. We recently published the results of a phase 1 trial of idasanutlin in patients with polycythemia vera (PV) that revealed tolerability and clinical activity. Here, we present data indicating that idasanutlin therapy is associated with expansion of TP53 mutant subclones. End-of-study sequencing of patients found that 5 patients in this trial harbored 12 TP53 mutations; however, only 1 patient had been previously identified as having a TP53 mutation at baseline. To identify the origin of these mutations, further analysis of raw sequencing data of baseline samples was performed and revealed that a subset of these mutations was present at baseline and expanded during treatment with idasanutlin. Follow-up samples were obtained from 4 of 5 patients in this cohort, and we observed that after cessation of idasanutlin, the variant allele frequency (VAF) of 8 of 9 TP53 mutations decreased. Furthermore, disease progression to myelofibrosis or myeloproliferative neoplasm blast phase was not observed in any of these patients after 19- to 32-month observation. These data suggest that idasanutlin treatment may promote transient TP53 mutant clonal expansion. A larger study geared toward high-resolution detection of low VAF mutations is required to explore whether patients acquire de novo TP53 mutations after idasanutlin therapy.

MDM4 inhibition: a novel therapeutic strategy to reactivate p53 in hepatoblastoma

Hepatoblastoma (HB) is the most common pediatric liver malignancy. High-risk patients have poor survival, and current chemotherapies are associated with significant toxicities. Targeted therapies are needed to improve outcomes and patient quality of life. Most HB cases are TP53 wild-type; therefore, we hypothesized that targeting the p53 regulator Murine double minute 4 (MDM4) to reactivate p53 signaling may show efficacy. MDM4 expression was elevated in HB patient samples, and increased expression was strongly correlated with decreased expression of p53 target genes. Treatment with NSC207895 (XI-006), which inhibits MDM4 expression, or ATSP-7041, a stapled peptide dual inhibitor of MDM2 and MDM4, showed significant cytotoxic and antiproliferative effects in HB cells. Similar phenotypes were seen with short hairpin RNA (shRNA)-mediated inhibition of MDM4. Both NSC207895 and ATSP-7041 caused significant upregulation of p53 targets in HB cells. Knocking-down TP53 with shRNA or overexpressing MDM4 led to resistance to NSC207895-mediated cytotoxicity, suggesting that this phenotype is dependent on the MDM4-p53 axis. MDM4 inhibition also showed efficacy in a murine model of HB with significantly decreased tumor weight and increased apoptosis observed in the treatment group. This study demonstrates that inhibition of MDM4 is efficacious in HB by upregulating p53 tumor suppressor signaling.

Targeting p53-MDM2 Interaction Using Small Molecule Inhibitors and the Challenges Needed to be Addressed

MDM2 protein is the core negative regulator of p53 that maintains the cellular levels of p53 at a low level in normal cells. Mutation of the TP53 gene accounts for 50% of all human cancers. In the remaining malignancies with wild-type TP53, p53 function is inhibited through other mechanisms. Recently, synthetic small molecule inhibitors have been developed which target a small hydrophobic pocket on MDM2 to which p53 normally binds. Given that MDM2-p53 antagonists have been undergoing clinical trials for different types of cancer, this review illustrates different aspects of these new cancer targeted therapeutic agents with the focus on the major advances in the field. It emphasizes on the p53 function, regulation of p53, targeting of the p53-MDM2 interaction for cancer therapy, and p53-dependent and -independent effects of inhibition of p53-MDM2 interaction. Then, representatives of small molecule MDM2-p53 binding antagonists are introduced with a focus on those entered into clinical trials. Furthermore, the review discusses the gene signatures in order to predict sensitivity to MDM2 antagonists, potential side effects and the reasons for the observed hematotoxicity, mechanisms of resistance to these drugs, their evaluation as monotherapy or in combination with conventional chemotherapy or with other targeted therapeutic agents. Finally, it highlights the certainly intriguing questions and challenges which would be addressed in future studies.

Targeting p53 for the treatment of cancer

Dysfunction of the TP53 (p53) gene occurs in most if not all human malignancies. Two principal mechanisms are responsible for this dysfunction; mutation and downregulation of wild-type p53 mediated by MDM2/MDM4. Because of its almost universal inactivation in malignancy, p53 is a highly attractive target for the development of new anticancer drugs. Although multiple strategies have been investigated for targeting dysfunctional p53 for cancer treatment, only 2 of these have so far yielded compounds for testing in clinical trials. These strategies include the identification of compounds for reactivating the mutant form of p53 back to its wild-type form and compounds for inhibiting the interaction between wild-type p53 and MDM2/MDM4. Currently, multiple p53-MDM2/MDM4 antagonists are undergoing clinical trials, the most advanced being idasanutlin which is currently undergoing testing in a phase III clinical trial in patients with relapsed or refractory acute myeloid leukemia. Two mutant p53-reactivating compounds have progressed to clinical trials, i.e., APR-246 and COTI-2. Although promising data has emerged from the testing of both MDM2/MDM4 inhibitors and mutant p53 reactivating compounds in preclinical models, it is still unclear if these agents have clinical efficacy. However, should any of the compounds currently being evaluated in clinical trials be shown to have efficacy, it is likely to usher in a new era in cancer treatment, especially as p53 dysfunction is so prevalent in human cancers.

Small-molecule MDM2/X inhibitors and PROTAC degraders for cancer therapy: advances and perspectives

Blocking the MDM2/X–P53 protein–protein interaction has been widely recognized as an attractive therapeutic strategy for the treatment of cancers. Numerous small-molecule MDM2 inhibitors have been reported since the release of the structure of the MDM2–P53 interaction in 1996, SAR405838, NVP-CGM097, MK-8242, RG7112, RG7388, DS-3032b, and AMG232 currently undergo clinical evaluation for cancer therapy. This review is intended to provide a comprehensive and updated overview of MDM2 inhibitors and proteolysis targeting chimera (PROTAC) degraders with a particular focus on how these inhibitors or degraders are identified from starting points, strategies employed, structure–activity relationship (SAR) studies, binding modes or co-crystal structures, biochemical data, mechanistic studies, and preclinical/clinical studies. Moreover, we briefly discuss the challenges of designing MDM2/X inhibitors for cancer therapy such as dual MDM2/X inhibition, acquired resistance and toxicity of P53 activation as well as future directions.

Targeting apoptosis in cancer therapy

For over three decades, a mainstay and goal of clinical oncology has been the development of therapies promoting the effective elimination of cancer cells by apoptosis. This programmed cell death process is mediated by several signalling pathways (referred to as intrinsic and extrinsic) triggered by multiple factors, including cellular stress, DNA damage and immune surveillance. The interaction of apoptosis pathways with other signalling mechanisms can also affect cell death. The clinical translation of effective pro-apoptotic agents involves drug discovery studies (addressing the bioavailability, stability, tumour penetration, toxicity profile in non-malignant tissues, drug interactions and off-target effects) as well as an understanding of tumour biology (including heterogeneity and evolution of resistant clones). While tumour cell death can result in response to therapy, the selection, growth and dissemination of resistant cells can ultimately be fatal. In this Review, we present the main apoptosis pathways and other signalling pathways that interact with them, and discuss actionable molecular targets, therapeutic agents in clinical translation and known mechanisms of resistance to these agents.

Tumor suppressor OTUD3 induces growth inhibition and apoptosis by directly deubiquitinating and stabilizing p53 in invasive breast carcinoma cells

Background

P53 pathway inactivation plays an important role in the process of breast cancer tumorigenesis. Post-translational protein modification abnormalities have been confirmed to be an important mechanism underlying inactivation of p53. Numerous deubiquitinating enzymes are aberrantly expressed in breast cancer, and a few deubiquitination enzymes can deubiquitinate and stabilize p53. Here, we report that ovarian tumor (OTU) deubiquitinase 3 (OTUD3) is a deubiquitylase of p53 in breast carcinoma (BC).

Methods

Correlations between the mRNA expression levels of OTUD3, TP53 and PTEN and the prognosis of BC were assessed with the Kaplan-Meier Plotter tool. OTUD3 protein expression in 80 pairs of specimens in our cohort was examined by immunohistochemistry and western blotting. The relationship among OTUD3, p53, and p21 proteins was analyzed. Half-life analysis and ubiquitylation assay were performed to elucidate the molecular mechanism by which OTUD3 stabilizes p53. The interaction between OTUD3 and p53 in BC cells was verified by a co-immunoprecipitation assay and GST pulldown experiments. MTS assay for proliferation detection, detection of apoptosis induced by cisplatin and colony formation assay were employed to investigate the functional effects of OTUD3 on breast cancer cells.

Results

OTUD3 downregulation is correlated with a poor prognosis in BC patients. OTUD3 expression is decreased in breast cancer tissues and not associated with the histological grade. OTUD3 also inhibits cell proliferation and clone formation and increases the sensitivity of BC cells to apoptosis induced by chemotherapy drugs. Reduced OTUD3 expression accompanied by decreased p53 abundance is correlated with human breast cancer progression. Ectopic expression of wild-type OTUD3, but not its catalytically inactive mutant, stabilizes and activates p53. Mechanistically, OTUD3 interacts directly with p53 through the amino-terminal OTU region. Finally, OTUD3 protects p53 from murine double minute 2 (Mdm2)-mediated ubiquitination and degradation, enabling the deubiquitination of p53 in BC cells.

Conclusions

In summary, we found that OTUD3 may be a potential therapeutic target for restoring p53 function in breast cancer cells and suggest that the OTUD3-p53 signaling axis may play a critical role in tumor suppression.

Systematic target actionability reviews of preclinical proof-of-concept papers to match targeted drugs to paediatric cancers

Background

Children with cancer are in urgent need of new therapies, as approximately 25% of patients experience a relapse and 20% succumb to their disease. Moreover, the majority of survivors suffer from clinically relevant health problems. Repurposing of targeted agents developed for adult indications could provide novel therapeutic options for paediatric cancer patients. To prioritise targeted drugs for paediatric clinical development, we applied a systematic review methodology to develop a Target Actionability Review (TAR) strategy. These TARs assess the strength and completeness of published preclinical proof-of-concept (PoC) data by structured critical appraisal of and summarising the available scientific literature for a specific target (pathway) and the associated drugs in paediatric tumours.

Methods

A sensitive literature search in PubMed was performed and relevant papers were identified. For each paper, the individual experimental findings were extracted, marked for paediatric tumour type and categorised into nine separate PoC data modules. Each experimental finding was scored for experimental outcome and quality independently by two reviewers; discrepancies were assessed by a third reviewer and resolved by adjudication. Scores corresponding to one PoC module were merged for each tumour type and visualised in a heat map matrix in the publicly available R2 data portal [r2.amc.nl].

Results and conclusions

To test our TAR methodology, we conducted a pilot study on MDM2 and TP53. The heat map generated from analysis of 161 publications provides a rationale to support drug development in specific paediatric solid and brain tumour types. Furthermore, our review highlights tumour types where preclinical data are incomplete or lacking and for which additional preclinical testing is advisable.

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A new strategy to review literature on targeted compounds in paediatric cancer.

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Results help to guide and prioritise clinical development of novel targeted agents.

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Outcomes are visualised in a publicly available, interactive heat map.

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We applied this unique methodology to MDM2 and TP53 and MDM2 inhibitors.

Long-term cultivation using ineffective MDM2 inhibitor concentrations alters the drug sensitivity profiles of PL21 leukaemia cells

Acquired MDM2 inhibitor resistance is commonly caused by loss-of-function TP53 mutations. In addition to the selection of TP53-mutant cells by MDM2 inhibitors, MDM2 inhibitor-induced DNA damage may promote the formation of TP53 mutations. Here, we cultivated 12 sublines of the intrinsically MDM2 inhibitor-resistant TP53 wild-type acute myeloid leukaemia cell line PL21 for 52 passages in the presence of ineffective concentrations of the MDM2 inhibitor nutlin-3 but did not observe loss-of-function TP53 mutations. This suggests that MDM2 inhibitors select TP53-mutant cells after mutations have occurred, but do not directly promote TP53 mutations. Unexpectedly, many sublines displayed increased sensitivity to the anti-cancer drugs cytarabine, doxorubicin, or gemcitabine. Consequently, therapies can affect the outcome of next-line treatments, even in the absence of a therapy response. This finding is conceptually novel. A better understanding of such processes will inform the design of improved therapy protocols in the future.

Targeting MDM2 for novel molecular therapy: Beyond oncology

The murine double minute 2 (MDM2) oncogene exerts major oncogenic activities in human cancers; it is not only the best-documented negative regulator of the p53 tumor suppressor, but also exerts p53-independent activities. There is an increasing interest in developing MDM2-based targeted therapies. Several classes of MDM2 inhibitors have been evaluated in preclinical models, with a few entering clinical trials, mainly for cancer therapy. However, noncarcinogenic roles for MDM2 have also been identified, demonstrating that MDM2 is involved in many chronic diseases and conditions such as inflammation and autoimmune diseases, dementia and neurodegenerative diseases, heart failure and cardiovascular diseases, nephropathy, diabetes, obesity, and sterility. MDM2 inhibitors have been shown to have promising therapeutic efficacy for treating inflammation and other nonmalignant diseases in preclinical evaluations. Therefore, targeting MDM2 may represent a promising approach for treating and preventing these nonmalignant diseases. In addition, a better understanding of how MDM2 works in nonmalignant diseases may provide new biomarkers for their diagnosis, prognostic prediction, and monitoring of therapeutic outcome. In this review article, we pay special attention to the recent findings related to the roles of MDM2 in the pathogenesis of several nonmalignant diseases, the therapeutic potential of its downregulation or inhibition, and its use as a biomarker.

Intact-Cell MALDI-ToF Mass Spectrometry for the Authentication of Drug-Adapted Cancer Cell Lines

The use of cell lines in research can be affected by cell line misidentification. Short tandem repeat (STR) analysis is an effective method, and the gold standard, for the identification of the genetic origin of a cell line, but methods that allow the discrimination between cell lines of the same genetic origin are lacking. Here, we use intact cell MALDI-ToF mass spectrometry analysis, routinely used for the identification of bacteria in clinical diagnostic procedures, for the authentication of a set of cell lines consisting of three parental neuroblastoma cell lines (IMR-5, IMR-32 and UKF-NB-3) and eleven drug-adapted sublines. Principal component analysis (PCA) of intact-cell MALDI-ToF mass spectrometry data revealed clear differences between most, but not all, of the investigated cell lines. Mass spectrometry whole-cell fingerprints enabled the separation of IMR-32 and its clonal subline IMR-5. Sublines that had been adapted to closely related drugs, for example, the cisplatin- and oxaliplatin-resistant UKF-NB-3 sublines and the vincristine- and vinblastine-adapted IMR-5 sublines, also displayed clearly distinctive patterns. In conclusion, intact whole-cell MALDI-ToF mass spectrometry has the potential to be further developed into an authentication method for mammalian cells of a common genetic origin.

Drug-adapted cancer cell lines as preclinical models of acquired resistance

Acquired resistance formation limits the efficacy of anti-cancer therapies. Acquired and intrinsic resistance differ conceptually. Acquired resistance is the consequence of directed evolution, whereas intrinsic resistance depends on the (stochastic) presence of pre-existing resistance mechanisms. Preclinical model systems are needed to study acquired drug resistance because they enable: (1) in depth functional studies; (2) the investigation of non-standard treatments for a certain disease condition (which is necessary to identify small groups of responders); and (3) the comparison of multiple therapies in the same system. Hence, they complement data derived from clinical trials and clinical specimens, including liquid biopsies. Many groups have successfully used drug-adapted cancer cell lines to identify and elucidate clinically relevant resistance mechanisms to targeted and cytotoxic anti-cancer drugs. Hence, we argue that drug-adapted cancer cell lines represent a preclinical model system in their own right that is complementary to other preclinical model systems and clinical data.

Inhibition of p53 inhibitors: progress, challenges and perspectives

p53 is the major tumor suppressor and the most frequently inactivated gene in cancer. p53 could be disabled either by mutations or by upstream negative regulators, including, but not limited to MDM2 and MDMX. p53 activity is required for the prevention as well as for the eradication of cancers. Restoration of p53 activity in mouse models leads to the suppression of established tumors of different origin. These findings provide a strong support to the anti-cancer strategy aimed for p53 reactivation. In this review, we summarize recent progress in the development of small molecules, which restore the tumor suppressor function of wild-type p53 and discuss their clinical advance. We discuss different aspects of p53-mediated response, which contribute to suppression of tumors, including non-canonical p53 activities, such as regulation of immune response. While targeting p53 inhibitors is a very promising approach, there are certain limitations and concerns that the intensive research and clinical evaluation of compounds will hopefully help to overcome.

Helping the Released Guardian: Drug Combinations for Supporting the Anticancer Activity of HDM2 (MDM2) Antagonists

The protein p53, known as the "Guardian of the Genome", plays an important role in maintaining DNA integrity, providing protection against cancer-promoting mutations. Dysfunction of p53 is observed in almost every cancer, with 50% of cases bearing loss-of-function mutations/deletions in the TP53 gene. In the remaining 50% of cases the overexpression of HDM2 (mouse double minute 2, human homolog) protein, which is a natural inhibitor of p53, is the most common way of keeping p53 inactive. Disruption of HDM2-p53 interaction with the use of HDM2 antagonists leads to the release of p53 and expression of its target genes, engaged in the induction of cell cycle arrest, DNA repair, senescence, and apoptosis. The induction of apoptosis, however, is restricted to only a handful of p53wt cells, and, generally, cancer cells treated with HDM2 antagonists are not efficiently eliminated. For this reason, HDM2 antagonists were tested in combinations with multiple other therapeutics in a search for synergy that would enhance the cancer eradication. This manuscript aims at reviewing the recent progress in developing strategies of combined cancer treatment with the use of HDM2 antagonists.

Anti-tumor activity of the MDM2-TP53 inhibitor BI-907828 in dedifferentiated liposarcoma patient-derived xenograft models harboring MDM2 amplification

PURPOSE

Dedifferentiated liposarcoma (DDLPS) is a soft tissue malignancy characterized by amplification of the mouse double minute 2 homolog (MDM2) gene. MDM2 is a negative regulator of tumor protein 53 (TP53). We tested the in vivo efficacy of BI-907828, a small molecule inhibitor of the MDM2-TP53 interaction, in two DDLPS patient-derived xenografts (PDX).

METHODS

Partially immunodeficient mice were bilaterally engrafted with UZLX-STS3 (n = 24) and UZLX-STS5 (n = 24) human DDLPS tissue harboring MDM2 amplifications. Mice were grouped as follows: (a) vehicle (0.5% hydroxyethylcellullose) 10 ml/kg daily per os (p.o.); (b) doxorubicin 5 mg/kg weekly intraperitoneally (i.p.); (c) BI-907828 2.5 mg/kg daily p.o. and (d) BI-907828 10 mg/kg daily p.o. The treatment lasted for 15 days, all mice treated with BI-907828 were followed for 37 days post-treatment. Efficacy was assessed by tumor volume and histopathological evaluation.

RESULTS

The 15-day treatment with 2.5 mg/kg and 10 mg/kg BI-907828 significantly inhibited tumor growth in UZLX-STS5 and -STS3 (p < 0.0001 compared to control for both models). All UZLX-STS5 and -STS3 tumors treated with BI-907828 decreased in size during treatment, and BI-907828-treated UZLX-STS5 tumors even disappeared completely. During the follow-up period, no tumor regrowth was observed in the UZLX-STS5 model and both doses of BI-907828 led to a pathological complete response, whereas a dose-dependent regrowth was seen in the UZLX-STS3 model.

CONCLUSION

BI-907828 showed significant anti-tumor activity in DDLPS PDX harboring MDM2 amplifications, providing a strong rationale for early clinical testing of BI-907828 in a DDLPS patient population.

Surface plasmon resonance and cytotoxicity assays of drug efficacies predicted computationally to inhibit p53/MDM2 interaction

Docking on the p53-binding site of murine double minute 2 (MDM2) by small molecules restores p53's tumor-suppressor function. We previously assessed 3244 FDA-approved drugs via "computational conformer selection" for inhibiting MDM2 and p53 interaction. Here, we developed a surface plasmon resonance method to experimentally confirm the inhibitory effects of the known MDM2 inhibitor, nutlin-3a, and two drug candidates predicted by our computational method. This p53/MDM2 interaction displayed a dosage-dependent weakening when MDM2 is pre-mixed with drug candidates. The inhibition efficiency order is nutlin-3a (IC₅₀ = 97 nM) > bepridil (206 nM) > azelastine (307 nM). Furthermore, we verified their anti-proliferation effects on SJSA-1 (wild-type p53 and overexpressed MDM2), SW480 (mutated p53), and SaOs-2 (deleted p53) cancer cell lines. The inhibitory order towards SJSA-1 cell line is nutlin-3a (IC₅₀ = 0.8 μM) > bepridil (23 μM) > azelastine (25 μM). Our experimental results are in line with the computational prediction, and the higher IC₅₀ values from the cell-based assays are due to the requirement of higher drug concentrations to penetrate cell membranes. The anti-proliferation effects of bepridil and azelastine on the cell lines with mutated and deleted p53 implied some p53-independent anti-proliferation effects.

TP53 mutant cell lines selected for resistance to MDM2 inhibitors retain growth inhibition by MAPK pathway inhibitors but a reduced apoptotic response

Background

Emergence of resistance to molecular targeted therapy constitutes a limitation to clinical benefits in cancer treatment. Cross-resistance commonly happens with chemotherapeutic agents but might not with targeted agents.

Methods

In the current study, TP53 wild-type cell lines with druggable MAPK pathway mutations [BRAF ^V600E (WM35) or NRAS ^Q61K (SJSA-1)] were compared with their TP53 mutant sublines (WM35-R, SN40R2) derived by selection for resistance to MDM2/p53 binding antagonists.

Results

The continued presence of the druggable MAPK pathway targets in the TP53 mutant (TP53 ^MUT) WM35-R and SN40R2 cells was confirmed. Trametinib and vemurafenib were tested on the paired WM35/WM35-R and SJSA-1/SN40R2 cells and similar growth inhibitory effects on the paired cell lines was observed. However, apoptotic responses to trametinib and vemurafenib were greater in WM35 than WM35-R, evidenced by FACS analysis and caspase 3/7 activity, indicating that these MAPK inhibitors acted on the cells partially through p53-regulated pathways. SiRNA mediated p53 knockdown in WM35 replicated the same pattern of response to trametinib and vemurafenib as seen in WM35-R, confirming that p53 plays a role in trametinib and vemurafenib induced apoptosis. In contrast, these differences in apoptotic response between WM35 and WM35-R were not seen with the SJSA-1/SN40R2 cell line pair. This is likely due to p53 suppression by overexpressed MDM2 in SJSA-1.

Conclusion

The TP53^MUT cells selected by resistance to MDM2 inhibitors nevertheless retained growth inhibitory but not apoptotic response to MAPK pathway inhibitors.

TP53 and therapy-related myeloid neoplasms

Therapy-related myeloid neoplasms (t-MNs) are the most serious late complications in patients treated with traditional cytotoxic chemotherapy and/or radiation. T-MNs are aggressive and chemorefractory hematologic malignancies, with a median survival of less than 6 months. TP53 mutations are highly enriched in t-MN patients, though the mechanism for this selective enrichment has only come to light over the past several years. In this review, we discuss the history and function of p53, and the role of TP53 mutations in the origin and progression of t-MNs. Emerging data has begun to elucidate who may be at highest risk of developing t-MNs, which ideally will enable us to develop preventative strategies for this devastating disease. As t-MNs may not be avoidable, novel therapies are urgently needed for this patient group and are underway as exemplified by recent investigation in restoring wild-type p53 function as well as directly targeting TP53 mutant variants. With better prevention and treatment, outcomes will hopefully begin to improve in the near future.

PRIMA-1MET-induced neuroblastoma cell death is modulated by p53 and mycn through glutathione level

Background

Neuroblastoma is the most common extracranial solid tumor in children. This cancer has a low frequency of TP53 mutations and its downstream pathway is usually intact. This study assessed the efficacy of the p53 activator, PRIMA-1^MET, in inducing neuroblastoma cell death.

Methods

CellTiter 2.0 was used to study susceptibility and specificity of NB cell lines to PRIMA-1^MET. Real-time PCR and western blot were used to assess the most common p53 transactivation targets. Induction of p53 and Noxa, and inhibition of Cas3/7, were used to assess impact on cell death after PRIMA-1^MET treatment. Flow cytometry was used to analyze cell cycle phase and induction of apoptosis, reactive oxygen species, and the collapse of mitochondrial membrane potential.

Results

Neuroblastoma cell lines were at least four times more susceptible to PRIMA-1^MET than were primary fibroblasts and keratinocyte cell lines. PRIMA-1^MET induced cell death rapidly and in all cell cycle phases. Although PRIMA-1^MET activated p53 transactivation activity, p53’s role is likely limited because its main targets remained unaffected, whereas pan-caspase inhibitor demonstrated no ability to prevent cell death. PRIMA-1^MET induced oxidative stress and modulated the methionine/cysteine/glutathione axis. Variations of MYCN and p53 modulated intracellular levels of GSH and resulted in increased/decreased sensitivity of PRIMA-1^MET. PRIMA-1^MET inhibited thioredoxin reductase, but the effect of PRIMA-1^MET was not altered by thioredoxin inhibition.

Conclusions

PRIMA-1^MET could be a promising new agent to treat neuroblastoma because it demonstrated good anti-tumor action. Although p53 is involved in PRIMA-1^MET-mediated cell death, our results suggest that direct interaction with p53 has a limited role in neuroblastoma but rather acts through modulation of GSH levels.

Electronic supplementary material

The online version of this article (10.1186/s13046-019-1066-6) contains supplementary material, which is available to authorized users.

Prolonged Idasanutlin (RG7388) Treatment Leads to the Generation of p53-Mutated Cells

The protein p53 protects the organism against carcinogenic events by the induction of cell cycle arrest and DNA repair program upon DNA damage. Virtually all cancers inactivate p53 either by mutations/deletions of the TP53 gene or by boosting negative regulation of p53 activity. The overexpression of MDM2 protein is one of the most common mechanisms utilized by p53^wt cancers to keep p53 inactive. Inhibition of MDM2 action by its antagonists has proved its anticancer potential in vitro and is now tested in clinical trials. However, the prolonged treatment of p53^wt cells with MDM2 antagonists leads to the development of secondary resistance, as shown first for Nutlin-3a, and later for three other small molecules. In the present study, we show that secondary resistance occurs also after treatment of p53^wt cells with idasanutlin (RG7388, RO5503781), which is the only MDM2 antagonist that has passed phase II and entered phase III clinical trials, so far. Idasanutlin strongly activates p53, as evidenced by the induction of p21 expression and potent cell cycle arrest in all the three cell lines tested, i.e., MCF-7, U-2 OS, and SJSA-1. Notably, apoptosis was induced only in SJSA-1 cells, while MCF-7 and U-2 OS cells were able to restore the proliferation upon the removal of idasanutlin. Moreover, idasanutlin-treated U-2 OS cells could be cultured for long time periods in the presence of the drug. This prolonged treatment led to the generation of p53-mutated resistant cell populations. This resistance was generated de novo, as evidenced by the utilization of monoclonal U-2 OS subpopulations. Thus, although idasanutlin presents much improved activities compared to its precursor, it displays the similar weaknesses, which are limited elimination of cancer cells and the generation of p53-mutated drug-resistant subpopulations.

Alpha ketoglutarate levels, regulated by p53 and OGDH, determine autophagy and cell fate/apoptosis in response to Nutlin-3a

Activated p53 can promote apoptosis or cell cycle arrest. Differences in energy metabolism can influence cell fate in response to activated p53. Nutlin-3a is a preclinical drug and small molecule activator of p53. Alpha-ketoglutarate (αKG) levels were reduced in cells sensitive to Nutlin-3a-induced apoptosis and increased in cells resistant to this apoptosis. Add-back of a cell-permeable αKG analog (DMKG) rescued cells from apoptosis in response to Nutlin-3a. OGDH is a component of the αKGDH complex that converts αKG to succinate. OGDH knockdown increased endogenous αKG levels and also rescued cells from Nutlin-3a-induced apoptosis. We previously showed reduced autophagy and ATG gene expression contributes to Nutlin-3a-induced apoptosis. DMKG and OGDH knockdown restored autophagy and ATG gene expression in Nutlin-3a-treated cells. These studies indicate αKG levels, regulated by p53 and OGDH, determine autophagy and apoptosis in response to Nutlin-3a.

Role of p53 circuitry in tumorigenesis: A brief review

Maintenance of genome integrity under the stressed condition is paramount for normal functioning of cells in the multicellular organisms. Cells are programmed to protect their genome through specialized adaptive mechanisms which will help decide their fate under stressed conditions. These mechanisms are the outcome of activation of the intricate circuitries that are regulated by the p53 master protein. In this paper, we provided a comprehensive review on p53, p53 homologues and their isoforms, including a description about the ubiquitin-proteasome system emphasizing its role in p53 regulation. p53 induced E3(Ub)-ligases are an integral part of the ubiquitin-proteasome system. This review outlines the roles of important E3(Ub)-ligases and their splice variants in maintaining cellular p53 protein homeostasis. It also covers up-to-date and relevant information on small molecule Mdm2 inhibitors originated from different organizations. The review ends with a discussion on future prospects and investigation directives for the development of next-generation modulators as p53 therapeutics.

The role of p53 in cancer drug resistance and targeted chemotherapy

Cancer has long been a grievous disease complicated by innumerable players aggravating its cure. Many clinical studies demonstrated the prognostic relevance of the tumor suppressor protein p53 for many human tumor types. Overexpression of mutated p53 with reduced or abolished function is often connected to resistance to standard medications, including cisplatin, alkylating agents (temozolomide), anthracyclines, (doxorubicin), antimetabolites (gemcitabine), antiestrogenes (tamoxifen) and EGFR-inhibitors (cetuximab). Such mutations in the TP53 gene are often accompanied by changes in the conformation of the p53 protein. Small molecules that restore the wild-type conformation of p53 and, consequently, rebuild its proper function have been identified. These promising agents include PRIMA-1, MIRA-1, and several derivatives of the thiosemicarbazone family. In addition to mutations in p53 itself, p53 activity may be also be impaired due to alterations in p53s regulating proteins such as MDM2. MDM2 functions as primary cellular p53 inhibitor and deregulation of the MDM2/p53-balance has serious consequences. MDM2 alterations often result in its overexpression and therefore promote inhibition of p53 activity. To deal with this problem, a judicious approach is to employ MDM2 inhibitors. Several promising MDM2 inhibitors have been described such as nutlins, benzodiazepinediones or spiro-oxindoles as well as novel compound classes such as xanthone derivatives and trisubstituted aminothiophenes. Furthermore, even naturally derived inhibitor compounds such as a-mangostin, gambogic acid and siladenoserinols have been discovered. In this review, we discuss in detail such small molecules that play a pertinent role in affecting the p53-MDM2 signaling axis and analyze their potential as cancer chemotherapeutics.

The pathophysiological significance of PPM1D and therapeutic targeting of PPM1D-mediated signaling by GSK2830371 in mantle cell lymphoma

PPM1D is a serine/threonine phosphatase that negatively regulates key DNA damage response proteins, such as p53, p38 MAPK, histone H2A.X, and ATM. We investigated the pathophysiological significance of PPM1D and its therapeutic targeting by the novel PPM1D inhibitor GSK2830371 in mantle cell lymphoma (MCL). Oncomine-based analyses indicated increased PPM1D mRNA levels in MCL cells compared with their normal counterpart cells. Higher PPM1D expression was associated with higher expression of the proliferation gene signature and poorer prognosis in patients. Eight MCL (three p53 wild-type and five mutant) cell lines were exposed to GSK2830371. GSK2830371 inhibited the cell growth, being prominent in p53 wild-type cells. GSK2830371 induced apoptosis in sensitive cells, as evidenced by induction of phosphatidylserine externalization and loss of mitochondrial membrane potential. p53 knockdown de-sensitized cell sensitivity. GSK2830371 increased the levels of total and Ser15-phosphorylated p53, and p53 targets p21 and PUMA. GSK2830371 and the MDM2 inhibitor Nutlin-3a acted synergistically in p53 wild-type cells. Interestingly, GSK2830371 sensitized MCL cells to bortezomib and doxorubicin in p53 wild-type and mutant cells; p38 signaling appeared to be involved in the GSK2830371/bortezomib lethality. PPM1D inhibition may represent a novel therapeutic strategy for MCL, which can be exploited in combination therapeutic strategies for MCL.

TP53 mutant MDM2-amplified cell lines selected for resistance to MDM2-p53 binding antagonists retain sensitivity to ionizing radiation

Non-genotoxic reactivation of the p53 pathway by MDM2-p53 binding antagonists is an attractive treatment strategy for wild-type TP53 cancers. To determine how resistance to MDM2/p53 binding antagonists might develop, SJSA-1 and NGP cells were exposed to growth inhibitory concentrations of chemically distinct MDM2 inhibitors, Nutlin-3 and MI-63, and clonal resistant cell lines generated. The p53 mediated responses of parental and resistant cell lines were compared. In contrast to the parental cell lines, p53 activation by Nutlin-3, MI-63 or ionizing radiation was not observed in either the SJSA-1 or the NGP derived cell lines. An identical TP53 mutation was subsequently identified in both of the SJSA-1 resistant lines, whilst one out of three identified mutations was common to both NGP derived lines. Mutation specific PCR revealed these mutations were present in parental SJSA-1 and NGP cell populations at a low frequency. Despite cross-resistance to a broad panel of MDM2/p53 binding antagonists, these MDM2-amplified and TP53 mutant cell lines remained sensitive to ionizing radiation (IR). These results indicate that MDM2/p53 binding antagonists will select for p53 mutations present in tumours at a low frequency at diagnosis, leading to resistance, but such tumours may nevertheless remain responsive to alternative therapies, including IR.

Pre-clinical efficacy and synergistic potential of the MDM2-p53 antagonists, Nutlin-3 and RG7388, as single agents and in combined treatment with cisplatin in ovarian cancer

Ovarian cancer is the fifth leading cause of cancer-related female deaths. Due to serious side effects, relapse and resistance to standard chemotherapy, better and more targeted approaches are required. Mutation of the TP53 gene accounts for 50% of all human cancers. In the remaining malignancies, non-genotoxic activation of wild-type p53 by small molecule inhibition of the MDM2-p53 binding interaction is a promising therapeutic strategy. Proof of concept was established with the cis-imidazoline Nutlin-3, leading to the development of RG7388 and other compounds currently in early phase clinical trials. This preclinical study evaluated the effect of Nutlin-3 and RG7388 as single agents and in combination with cisplatin in a panel of ovarian cancer cell lines. Median-drug-effect analysis showed Nutlin-3 or RG7388 combination with cisplatin was additive to, or synergistic in a p53-dependent manner, resulting in increased p53 activation, cell cycle arrest and apoptosis, associated with increased p21^WAF1 protein and/or caspase-3/7 activity compared to cisplatin alone. Although MDM2 inhibition activated the expression of p53-dependent DNA repair genes, the growth inhibitory and pro-apoptotic effects of p53 dominated the response. These data indicate that combination treatment with MDM2 inhibitors and cisplatin has synergistic potential for the treatment of ovarian cancer, dependent on cell genotype.

Combination treatment with rucaparib (Rubraca) and MDM2 inhibitors, Nutlin-3 and RG7388, has synergistic and dose reduction potential in ovarian cancer

Ovarian cancer is the seventh most common cancer worldwide for females and the most lethal of all gynecological malignancies. The treatment of ovarian cancer remains a challenge in spite of advances in debulking surgery and changes in both chemotherapy schedules and routes of administration. Cancer treatment has recently been improving with the introduction of targeted therapies to achieve greater specificity and less cytotoxicity. Both PARP inhibitors and MDM2-p53 binding antagonists are targeted therapeutic agents entered into clinical trials. This preclinical study evaluated the effect of Nutlin-3/RG7388 and rucaparib as single agents and in combination together in a panel of ovarian cancer cell lines. Median-drug-effect analysis showed Nutlin-3/RG7388 combination with rucaparib was additive to, or synergistic in a cell type-dependent manner. Mechanism studies showed rucaparib alone had no effect on p53 stabilization or activity. Although treatment with Nutlin-3 or RG7388 induced stabilization of p53 and upregulation of p21WAF1 and MDM2, the addition of rucaparib did not enhance the p53 activation seen with the MDM2 inhibitors alone. These results demonstrate that the synergistic effect on growth inhibition observed in the combination between rucaparib and Nutlin-3/RG7388 is not the result of increased p53 molecular pathway activation. Nevertheless, combined treatment of Nutlin-3/RG7388 with rucaparib increased cell cycle arrest and apoptosis, which was marked for A2780 and IGROV-1. These data indicate that combination treatment with MDM2 inhibitors and rucaparib has synergistic and dose reduction potential for the treatment of ovarian cancer, dependent on cell type.

Dysfunctional diversity of p53 proteins in adult acute myeloid leukemia: projections on diagnostic workup and therapy

The heterogeneous nature of acute myeloid leukemia (AML) and its poor prognosis necessitate therapeutic improvement. Current advances in AML research yield important insights regarding AML genetic, epigenetic, evolutional, and clinical diversity, all in which dysfunctional p53 plays a key role. As p53 is central to hematopoietic stem cell functions, its aberrations affect AML evolution, biology, and therapy response and usually predict poor prognosis. While in human solid tumors TP53 is mutated in more than half of cases, TP53 mutations occur in less than one tenth of de novo AML cases. Nevertheless, wild-type (wt) p53 dysfunction due to nonmutational p53 abnormalities appears to be rather frequent in various AML entities, bearing, presumably, a greater impact than is currently appreciated. Hereby, we advocate assessment of adult AML with respect to coexisting p53 alterations. Accordingly, we focus not only on the effects of mutant p53 oncogenic gain of function but also on the mechanisms underlying nonmutational wtp53 inactivation, which might be of therapeutic relevance. Patient-specific TP53 genotyping with functional evaluation of p53 protein may contribute significantly to the precise assessment of p53 status in AML, thus leading to the tailoring of a rationalized and precision p53-based therapy. The resolution of the mechanisms underlying p53 dysfunction will better address the p53-targeted therapies that are currently considered for AML. Additionally, a suggested novel algorithm for p53-based diagnostic workup in AML is presented, aiming at facilitating the p53-based therapeutic choices.

Synergistic activity and heterogeneous acquired resistance of combined MDM2 and MEK inhibition in KRAS mutant cancers

There are currently no effective targeted therapies for KRAS mutant cancers. Therapeutic strategies that combine MEK inhibitors with agents that target apoptotic pathways may be a promising therapeutic approach. We investigated combining MEK and MDM2 inhibitors as a potential treatment strategy for KRAS mutant non-small cell lung cancers and colorectal carcinomas that harbor wild-type TP53. The combination of pimasertib (MEK inhibitor) + SAR405838 (MDM2 inhibitor) was synergistic and induced the expression of PUMA and BIM, led to apoptosis and growth inhibition in vitro, and tumor regression in vivo. Acquired resistance to the combination commonly resulted from the acquisition of TP53 mutations, conferring complete resistance to MDM2 inhibition. In contrast, resistant clones exhibited marked variability in sensitivity to MEK inhibition, which significantly impacted sensitivity to subsequent treatment with alternative MEK inhibitor-based combination therapies. These results highlight both the potential promise and limitations of combining MEK and MDM2 inhibitors for treatment of KRAS mutant NSCLC and CRC.

MDM2/X inhibitors under clinical evaluation: perspectives for the management of hematological malignancies and pediatric cancer

The two murine double minute (MDM) family members MDM2 and MDMX are at the center of an intense clinical assessment as molecular target for the management of cancer. Indeed, the two proteins act as regulators of P53, a well-known key controller of the cell cycle regulation and cell proliferation that, when altered, plays a direct role on cancer development and progression. Several evidence demonstrated that functional aberrations of P53 in tumors are in most cases the consequence of alterations on the MDM2 and MDMX regulatory proteins, in particular in patients with hematological malignancies where TP53 shows a relatively low frequency of mutation while MDM2 and MDMX are frequently found amplified/overexpressed. The pharmacological targeting of these two P53-regulators in order to restore or increase P53 expression and activity represents therefore a strategy for cancer therapy. From the discovery of the Nutlins in 2004, several compounds have been developed and reported with the ability of targeting the P53-MDM2/X axis by inhibiting MDM2 and/or MDMX. From natural compounds up to small molecules and stapled peptides, these MDM2/X pharmacological inhibitors have been extensively studied, revealing different biological features and different rate of efficacy when tested in in vitro and in vivo experimental tumor models. The data/evidence coming from the preclinical experimentation have allowed the identification of the most promising molecules and the setting of clinical studies for their evaluation as monotherapy or in therapeutic combination with conventional chemotherapy or with innovative therapeutic protocols in different tumor settings. Preliminary results have been recently published reporting data about safety, tolerability, potential side effects, and efficacy of such therapeutic approaches. In this light, the aim of this review is to give an updated overview about the state of the art of the clinical evaluation of MDM2/X inhibitor compounds with a special attention to hematological malignancies and to the potential for the management of pediatric cancers.