The authors profiled O6-methylguanine-DNA methyltransferase (MGMT) status across a diverse cohort of primary acute myeloid leukemia (AML) samples, including 23 de novo and 16 relapsed/refractory (R/R) cases. MGMT evaluation used an integrated approach of mRNA expression, promoter methylation, and protein expression. Approximately 25–30% of AMLs in this series exhibited MGMT silencing. In contrast, healthy CD34+ hematopoietic stem and progenitor cells showed consistent MGMT expression. These findings indicate that a clinically relevant subset of AMLs harbors epigenetic MGMT loss, supporting the consideration of MGMT as a biomarker in AML.
The cohort analysis also revealed frequent loss of mismatch repair (MMR) proteins among AML samples. MMR deficiency is an established mechanism of resistance to temozolomide (TMZ) in other cancers, notably glioblastoma. The investigators therefore examined whether MMR loss could explain the modest and variable responses to TMZ observed in prior clinical studies of AML. Their data identify MMR deficiency as an important genetic contributor to TMZ resistance in MGMT-silenced AML, potentially accounting for limited clinical activity of TMZ in this disease context.
To systematically evaluate genetic drivers of TMZ resistance, the study employed CRISPR knockout screening and genetically engineered AML models. These functional genomics approaches demonstrated that loss of MMR components induces both upfront (primary) and acquired resistance to TMZ in MGMT-silenced AML. The results extend observations from glioblastoma to AML and emphasize that MGMT silencing alone is insufficient to guarantee TMZ sensitivity when MMR is deficient.
Aiming to therapeutically exploit MGMT silencing while bypassing MMR-based resistance mechanisms, the authors evaluated a panel of MGMT-dependent DNA-alkylating agents. They identified N3-(2-fluoroethyl) imidazotetrazine (referred to as KL50), a fluoroethylating analog of TMZ, which showed pronounced and selective antileukemic activity in MGMT-silenced AML. Importantly, KL50 retained activity irrespective of MMR status, distinguishing it from TMZ in the presence of MMR deficiency. KL50 demonstrated efficacy in multiple preclinical settings, including human primary AML samples and in vivo models.
Mechanistically, KL50 produced DNA damage via the time-dependent formation of interstrand DNA crosslinks, a lesion type distinct from the canonical O6-methylguanine adducts typically reversed by MGMT. The crosslinking activity of KL50 bypassed reliance on functional MMR for cytotoxicity. KL50-induced lesions triggered replication stress and activated a DNA damage response dominated by ATR signaling, implicating ATR as a central mediator of the cellular response to KL50-induced replication stress.
Given the dominance of ATR signaling in the replication stress response to KL50, the investigators tested pharmacologic ATR inhibition in combination with KL50. ATR inhibition synergized with KL50 in preclinical models, producing marked antileukemic activity and significantly extending survival compared with single agents. The reported combination effects were achieved without compromising hematologic safety in the described experimental systems, supporting a therapeutic strategy that couples low-dose O6-fluoroethylating imidazotetrazines with ATR inhibitors to enhance efficacy while managing toxicity.
KL50 significantly prolonged survival in humanized MISTRG6 mice engrafted with primary AML patient-derived xenografts. The antileukemic effect of KL50 was observed across genetic backgrounds, including models with MMR deficiency. Combining KL50 with ATR inhibitors further extended survival across AML models. The authors report that these efficacy gains did not come at the cost of detectable hematologic safety signals within the reported experiments, though full safety characterization and dose optimization would require additional study and clinical investigation.
This body of work positions MGMT silencing as a prevalent and actionable biomarker in AML and identifies MMR status as a key modifier of TMZ responsiveness but not of KL50 sensitivity. The data provide a translational rationale for biomarker-directed strategies that pair MGMT-targeted O6-fluoroethylating agents with ATR inhibitors to treat both de novo and relapsed/refractory AML across diverse genetic backgrounds. The abstract notes that detailed dosing regimens, specific ATR inhibitor identities, and full experimental protocols are reported in the preprint; those implementation details are not included in the abstract and should be consulted in the full manuscript for clinical translation and trial design considerations.
Notes: This study is reported as a preprint and has not been peer reviewed. Competing interests and funding sources are declared by the authors in the preprint: several grants from NIH/NCI and foundations were listed, and licensed intellectual property related to the work was disclosed for some authors.