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Editorials
homolog MDMX, which together form an E3 ubiquitin ligase complex that targets p53, resulting in nuclear export and subsequent degradation. Small molecule antagonists of MDM2 such as nutlins and related com- pounds, are under clinical evaluation in a variety of can- cers expressing wildtype p53.6 However, these drugs are ineffective in TP53-mutated or -deleted tumors. To target mutant p53, small molecules have been developed that are predicted to correct p53 protein folding, and thereby restore activity, which would re-enable TP53-mutated tumor cells to undergo apoptosis in response to (chemo)therapy. For instance, cell-based screening of compound libraries has led to the identification of the alkylating compound “p53 reactivation and induction of massive apoptosis” (PRIMA-1) and its methylated deriva- tive PRIMA-1Met, also known as APR-246. When taken up by cells, APR-246 is converted to the reactive electrophile methylene quinuclidinone (MQ), which is thought to reactivate conformationally unstable p53 mutants by binding to critical cysteine residues in the p53 DNA bind- ing domain.7 Consistent with such a mechanism of action, expression of p53 target genes is restored by these drugs while strongly promoting the induction of apopto- sis. However, since the first descriptions of the proposed mechanism of action of this drug,7,8 alternative mecha- nisms of action have been put forward. For instance, it was shown that APR-246 induces apoptosis by increasing intracellular levels of reactive oxygen species (ROS), even in fully p53-deficient cells, a mechanism involving glu- tathione depletion and MQ-mediated inactivation of the enzyme thioredoxin reductase.9 Tumor cells expressing only wildtype p53 would be less responsive to these effects, due to the protective role of p53 against oxidative stress.10 Therefore, irrespective of its precise mechanism of action, APR-246 appears to selectively target cancers with deficient p53 and for this reason it is under clinical evaluation in a number of TP53-mutated cancers, includ- ing myeloid malignancies such as myelodysplastic syn- dromes and acute myeloid leukemia.
The paper by Demir and colleagues in this issue of the journal describes the potential clinical use of APR-246 in pediatric TP53-mutated ALL.1 Using both established ALL cell line models and patient-derived xenografts, the authors compared the effects of this drug in wildtype vs. TP53- mutated cells. They observed that the presence of TP53 mutations selectively enhanced resistance to the DNA damaging drug doxorubicin, while increasing sensitivity to APR-246. The induction of apoptosis, seen in TP53-mutat- ed ALL in response to APR-246, was directly correlated with the upregulation of p53 target genes such as p21, PUMA and NOXA, which is at least consistent with pP53 reactivation contributing to these effects. Moreover, when mutant TP53 expression was reduced by shRNA-mediated knockdown, the apoptosis-inducing effects of APR-246 were no longer seen, indicating that in these leukemia models, the response to this drug requires the presence of mutant p53. Given the uncertainties pertaining to the pre- cise mechanism of action of APR-246, the authors exam- ined to what extent intracellular ROS levels may con- tribute to APR-246-induced apoptosis. Indeed, neutraliza-
tion of ROS using MnTBAP, a synthetic metalloporphyrin with antioxidant activity, was able to partially inhibit the apoptosis-inducing activity of APR-246, suggesting a more complex mode of action for this compound than simply restoring p53 target gene expression.
Since doxorubicin is part of the armamentarium used to treat (relapsed) ALL, in vivo synergy between doxorubicin and APR-246 was examined using a TP53-mutated ALL xenograft model. Whereas doxorubicin was ineffective as a single agent in prolonging survival, a clear therapeutic benefit was seen from the treatment with APR-246 alone. In addition, and consistent with the observed ex-vivo syn- ergy between doxorubicin and APR-246, this drug combi- nation further extended leukemia-free survival in the mice studied. Taken together, the work by Demir and col- leagues suggests that ALL patients suffering from relapsed TP53-mutated ALL could benefit from the use of APR-246 or similar p53-refolding agents, when used in addition to the current chemotherapy protocol. However, clinical implementation will require a more rigorous characteri- zation of resistance profiles and therapy responses in TP53-mutated ALL, such as the inclusion of more patient- derived xenografts and examining potential interactions between APR-246 and other components of the treat- ment regimen for relapsed ALL. With respect to potential toxicities related to the use of APR-246, it will be impor- tant to follow the results of ongoing (phase II/III) trials in acute myeloid leukemia/myelodysplastic syndrome with this drug, either as a single agent or in combination with azacitidine.11
References
1. DemirS,BoldrinE,SunQ,etal.Therapeutictargetingofmutantp53 in pediatric acute lymphoblastic leukemia. Haematologica 2020; 105(1):170-181.
2. Kruiswijk F, Labuschagne CF, Vousden KH. p53 in survival, death and metabolic health: a lifeguard with a licence to kill. Nat Rev Mol Cell Biol. 2015;16(7):393-405.
3. Stengel A, Kern W, Haferlach T, Meggendorfer M, Fasan A, Haferlach C. The impact of TP53 mutations and TP53 deletions on survival varies between AML, ALL, MDS and CLL: an analysis of 3307 cases. Leukemia. 2017;31(3):705-711.
4. Comeaux EQ, Mullighan CG. TP53 mutations in hypodiploid acute lymphoblastic leukemia. Cold Spring Harb Perspect Med. 2017;7(3). 5. Hof J, Krentz S, van Schewick C, et al. Mutations and deletions of the TP53 gene predict nonresponse to treatment and poor outcome in first relapse of childhood acute lymphoblastic leukemia. J Clin
Oncol. 2011;29(23):3185-3193.
6. Burgess A, Chia KM, Haupt S, Thomas D, Haupt Y, Lim E. Clinical
overview of MDM2/X-targeted therapies. Front Oncol. 2016;6:7.
7. Lambert JM, Gorzov P, Veprintsev DB, et al. PRIMA-1 reactivates mutant p53 by covalent binding to the core domain. Cancer Cell.
2009;15(5):376-388.
8. Bykov VJ, Issaeva N, Shilov A, et al. Restoration of the tumor sup-
pressor function to mutant p53 by a low-molecular-weight com-
pound. Nat Med. 2002;8(3):282-288.
9. Peng X, Zhang MQ, Conserva F, et al. APR-246/PRIMA-1(MET)
inhibits thioredoxin reductase 1 and converts the enzyme to a dedi-
cated NADPH oxidase. Cell Death Dis. 2017;8(4):e2751.
10. Sablina AA, Budanov AV, Ilyinskaya GV, Agapova LS, Kravchenko JE, Chumakov PM. The antioxidant function of the p53 tumor sup-
pressor. Nat Med. 2005;11(12):1306-1313.
11. ClinicalTrials.gov [Internet]. Bethesda (MD): National Library of
Medicine (US). Available from: http://clinicaltrials.gov/ct/show/ NCT00287391/NCT03931291/ NCT03745716
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