Eprenetapopt in the Post-Transplant Setting: Mechanisms and Future Directions

Acute myeloid leukemia (AML) and myelodysplastic syndrome (MDS) with TP53 mutations represent a major unmet medical need. This is due to poor response to cytotoxic chemotherapy, DNA methyltransferase inhibitors alone or in combination with venetoclax, with high rates of persistent measurable residual disease, shorter durations of response, and a poor overall survival (OS) of 5-9 months.^1,2 Allogenic hematopoietic stem-cell transplantation (ASCT) offers the best chance at a cure by harnessing graft-versus-leukemia effect and can reduce the risk of death or relapse by up to 80% with a reported median OS of 8 months among transplanted patients with TP53-mutated AML.^3,4 Nonetheless, the 1-year OS for TP53-mutant AML/MDS remains poor at 5%-40% across various studies despite incorporation of modern therapies and ASCT.^5-8 Consequently, novel approaches are needed before and after ASCT to improve patient outcomes in TP53-mutant myeloid malignancies. In the companion to this article, Mishra et al⁹ evaluated the combination of eprenetapopt with azacitidine as maintenance therapy after ASCT for TP53-mutant AML/MDS.

Eprenetapopt is a prodrug and methylated derivative of PRIMA-1 (p53 reactivation and induction of massive apoptosis; Fig 1).¹⁰ The prodrug spontaneously converts to methylene quinuclidinone and other compounds including MIRA-1 and STIMA-1. These compounds form adducts via covalent binding to thiol groups in mutant p53, leading to changes in conformation of the core domain, which restores wild-type DNA-binding capacity.^10,11 Subsequent restoration of tumor suppressor function and upregulation of p53 target genes have been implicated as potential downstream mechanisms leading to cell death.¹²

FIG 1.

Eprenetapopt is postulated to work via binding to mutant p53, leading to restoration of wild-type conformation and DNA-binding capacity. This leads to activation of tumor suppressor function and p53 target genes, leading to cell cycle arrest and apoptosis. Eprenetapopt also works in a p53-independent manner. This includes depletion of GSH, triggering ferroptosis and increased ROS. In addition, inhibition of TrxR1 leads to impaired redox balance, ROS production, and decrease in dNTPs, contributing to cell cycle arrest and apoptosis. dNTP, deoxyribonucleotide; GSH, glutathione; ROS, reactive oxygen species; Trx, thioredoxin; TrxR1, thioredoxin reductase 1.

Given the proposed mechanism of action, the clinical development of eprenetapopt has focused on cancers with TP53 mutations. During its early clinical development, the drug showed a favorable safety profile and p53-dependent biologic effects on tumor cells.¹³ In AML, eprenetapopt has shown synergy with azacytidine and cytotoxic chemotherapy in primary AML samples, with the former process mediated by activation of the p53 pathway and downregulation of FLT3 signaling and MYC expression.^14,15 On the basis of these results, two clinical trials evaluated this combination of eprenetapopt with azacitidine in patients with TP53-mutated MDS or AML and have demonstrated safety and encouraging clinical activity.^16,17 Patients with biallelic TP53 defects or complex karyotype had significantly higher complete response (CR) rate compared with those who did not (49% v 8%, P = .01).¹⁸ Forty percent of all patients achieved clearance of TP53 mutation to a variant allele frequency of < 5%, a threshold that has been shown to be associated with improved OS.^16,17 Immunohistochemical quantification of p53 in bone marrow mononuclear cells was predictive of response. This was anticipated from preclinical studies showing a lower efficacy in cells with truncating mutations lacking detectable p53 protein, where mutant p53-dependent activity is lost with residual, and overall, lower activity of eprenetapopt occurring via p53 independent mechanisms.^14,18,19 Despite these encouraging results, the pivotal frontline phase III trial in TP53-mutant higher-risk MDS did not meet the prespecified primary end point despite an improved CR rate of 33% with eprenetapopt and azacytidine compared with a CR rate of 22% with azacytidine alone.²⁰

In the companion to this article, Mishra et al⁹ evaluated the combination of eprenetapopt with azacitidine as maintenance therapy after ASCT for TP53-mutant AML/MDS. This phase II trial enrolled 33 patients with TP53-mutant AML/MDS with a median age of 65 years who underwent ASCT in at least a morphologic marrow remission or better. This regimen showed acceptable safety with a median relapse-free survival of 14.5 months and an encouraging median OS of 20.6 months. The 1-year relapse-free survival of 60% compared favorably with contemporary historical expectations of approximately 30% for this population.⁶ However, of 84 patients screened, only 33 patients were enrolled. Consequently, the results warrant further validation in a prospective randomized trial. These results suggest a potential future pathway of development for eprenetapopt and add to the growing data, supporting the use of biomarker-driven application of tolerable post-ASCT maintenance to improve outcomes, especially in very high-risk populations with mutations in FLT3 or TP53.²¹

Newer modes of action of PRIMA-1 compounds have recently been identified. PRIMA-1/eprenetapopt is now recognized to induce p53-dependent and p53-independent cell death via different mechanisms. PRIMA-1 induced depletion of the antioxidant glutathione (GSH), which has been shown to be one mechanism leading to reactive oxygen species production and p53-independent cell death.²² Depletion of GSH induces production of lipid peroxides, which trigger ferroptosis.²³ Consequently, eprenetapopt was found to work synergistically with other approaches to trigger ferroptosis including glutathione peroxidase 4 inhibition. Depletion of GSH has been hypothesized as the mechanism of neurologic toxicity noted in clinical trials with eprenetapopt, and it has been postulated that ferroptosis inhibitors, for example, iron chelators or vitamin E, could potentially reverse or prevent such neurotoxicity.²³ Another potential mechanism of antitumor activity of eprenetapopt is via inactivation of thioredoxin reductase 1, which is an important regulator of redox balance in cells.²⁴ Inhibition of thioredoxin reductase 1 leads to cell death via a massive increase in reactive oxygen species and potentially via a decrease in synthesis of deoxyribonucleotides. Unfolded protein response activated by accumulation of misfolded protein in endoplasmic reticulum leading to endoplasmic reticulum stress has also been noted with this agent in myeloma cells.²⁵

An increasing understanding of different modes of action of eprenetapopt present exciting opportunities to combine it with other agents. Synergy with cytotoxic chemotherapy could potentially be leveraged in different treatment settings in multiple diseases including AML, other hematologic malignancies, and solid tumors. Similarly, combination with proteasome inhibitors, which can promote unfolded protein response, would be of significant interest, particularly in multiple myeloma. Combination with other inducers of ferroptosis including approved agents like sorafenib and ferumoxytol and investigational agents like glutathione peroxidase 4 inhibitor BBP-954 would be of interest in AML.^26-28 Ferroptosis, being a proinflammatory immunogenic mode of cell death along with promising results observed in this trial in the post-transplant setting raises intriguing questions about possible modulation of the immune system with eprenetapopt that deserves further investigation and the potential to combine eprenetapopt with emerging immunotherapeutic approaches in AML including antibodies targeting CD47/SIRPα, TIM-3, and adoptive cellular therapies. These approaches would be further bolstered by preclinical proof-of-concept studies before being translated to the clinic. While the results observed by Mishra et al⁹ warrant validation in a larger population, these encouraging findings rekindle interest for novel directions of investigation with eprenetapopt and similar agents for TP53-mutated AML.

AUTHOR CONTRIBUTIONS

Conception and design: All authors

Collection and assembly of data: All authors

Data analysis and interpretation: All authors

Manuscript writing: All authors

Final approval of manuscript: All authors

Accountable for all aspects of the work: All authors

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Eprenetapopt in the Post-Transplant Setting: Mechanisms and Future Directions

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