The cytotoxic mechanisms of thymidylate synthase inhibitors, such as the multitarget antifolate pemetrexed, are not yet fully understood. Emerging evidence indicates that combining pemetrexed with histone deacetylase inhibitors (HDACi) may enhance therapeutic efficacy in non-small cell lung cancer (NSCLC). To explore this further, A549 NSCLC cells were treated with various combinations of pemetrexed and the HDACi MS275 (Entinostat), and subsequently assessed for cell viability, cell cycle changes, and genotoxic markers. Proteomic alterations were analyzed using label-free shotgun and targeted LC-MS/MS. MS275 enhanced the sensitivity of A549 cells to pemetrexed, but only when administered following prior treatment with pemetrexed. Both HeLa (p53 negative) and A549 (p53 positive) showed robust activation of γH2AX upon treatment with this combination. Importantly, CRISPR/Cas9 knockout of the uracil-DNA glycosylase UNG did not affect γH2AX activation or sensitivity to pemetrexed. Proteomic analysis revealed that MS275 altered the expression of known pemetrexed targets, as well as several proteins involved in pyrimidine metabolism and DNA repair, which could potentiate pemetrexed cytotoxicity. Contrary to the conventional model of antifolate toxicity, which implicates futile cycles of uracil incorporation and excision in DNA, we propose that ribonucleotide incorporation in nuclear and mitochondrial DNA significantly contributes to the cytotoxicity of antifolates like pemetrexed, and likely also of fluorinated pyrimidine analogs. HDAC inhibition apparently exacerbates cytotoxicity of these agents by inhibiting error-free repair of misincorporated ribonucleotides in DNA. The potential of HDACis to modulate pyrimidine metabolism and DNA damage responses offers novel strategies for improving NSCLC outcomes.
MS275 enhanced the sensitivity of A549 cells to pemetrexed, but only when administered following prior treatment with pemetrexed.
schedule comparisons support the direction in A549 cells, with independent viability replication unclear
CRISPR/Cas9 knockout of the uracil-DNA glycosylase UNG did not affect γH2AX activation or sensitivity to pemetrexed.
validated independent knockout clones support the observed null response in the tested models, with assay replication incompletely specified
we propose that ribonucleotide incorporation in nuclear and mitochondrial DNA significantly contributes to the cytotoxicity of antifolates like pemetrexed
ribonucleotide incorporation was neither directly measured nor causally tested
MS275 altered the expression of known pemetrexed targets, as well as several proteins involved in pyrimidine metabolism and DNA repair
LFQ and selected PRM measurements support expression changes, although differential-expression selection thresholds are ambiguously reported
HDAC inhibition apparently exacerbates cytotoxicity of these agents by inhibiting error-free repair of misincorporated ribonucleotides in DNA.
repair inhibition is inferred from protein abundance rather than demonstrated by functional or causal testing
Derived from the full evaluation — not a separate score.
Strengths
MS275 before pemetrexed reduces sensitivity, whereas the reversed sequence moderately increases sensitivity in A549 cells. This distinguishes schedules that would otherwise be grouped as the same drug combination.
↳ Results, Figs 1–2
Two independent clones per genotype, vector and intronic-guide controls, and targeted protein validation provide a controlled test of UNG involvement in the examined models.
↳ Fig. 3; Methods, CRISPR/Cas9 Clones
Biological-replicate LFQ analysis is complemented by targeted PRM measurements of selected proteins. This supports the measured expression observations without establishing their proposed causal consequences.
↳ Methods, proteomics sections; Figs 4–6
Limitations
Genomic ribonucleotides were not measured, and no causal RER perturbation or rescue establishes the proposed pathway. Expression changes and γH2AX therefore support a hypothesis rather than identify its mechanism.
↳ Figs 5–7; Abstract; Discussion
The central dose-response figures report three parallel wells without stating the number of independent experiments. Proteomic biological replication does not resolve uncertainty about these separate assays.
↳ Figs 1B, 2B, 3C
The Discussion lists FPGS among downregulated proteins, while the targeted-proteomics Results report it as markedly upregulated. This changes that component of the mechanistic explanation, although the measured schedule response and TYMS/DHFR findings remain intact.
↳ Results, Targeted Mass Spectrometry Corroborates the Results From LFQ Analysis; Discussion, final summary
Figures 1–3 establish bounded experimental observations about treatment order and UNG knockout, while the LFQ and PRM analyses provide a resource for mechanistic follow-up. The central limitation is the move from altered protein abundance to a ribonucleotide-driven explanation without direct lesion measurement or causal repair testing. Unclear independent replication in the viability assays further limits experimental certainty, and the Discussion does not explicitly acknowledge the decisive measurement gap. Its final summary also reverses the reported FPGS direction, although correcting that sentence does not overturn the observed treatment-order response.
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Confidence mediumThe score reflects bounded advances in treatment scheduling, validated UNG knockout observations, and a proteomic resource. The proposed ribonucleotide explanation remains an indirect model rather than a demonstrated mechanism.
Independent knockout clones, controls, biological-replicate proteomics, and targeted corroboration support the experimental work. Unclear independent replication in viability assays and the absence of direct mechanistic testing limit what the design establishes.
The experimental sequence is followable, but the Abstract and final Discussion assign a major cytotoxic role to a mechanism inferred from expression changes. The score includes a 0.4 deduction for that central wording–evidence mismatch.
The Introduction and Discussion engage the antifolate, UNG, HDAC-inhibitor, and DNA-repair literature. The omission of an explicit acknowledgment that genomic ribonucleotides were not measured materially limits interpretation of the central model.
Caveats4 of 4 checks
No reporting-and-transparency finding meeting the specified format is retained. Missing access to supplements and disclosure statements does not establish a conduct problem.
86 references in manuscript 86 of 86 checkable references found in an index 1 reference confirmed by manual review
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No retraction or expression of concern found in Crossref (includes Retraction Watch).
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Where this paper’s evidence sits on the path from initial observation to real-world use.
The evidence remains at the cultured-cell and molecular-assay stage. Neither therapeutic benefit beyond cell culture nor the proposed ribonucleotide mechanism is validated.
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