Anticancer Activities and Molecular Mechanisms of 2-Methoxy-1,4-Naphthoquinone: A Systematic Review of Preclinical Evidence

Background/Objectives: 2-Methoxy-1,4-naphthoquinone (MNQ) is a naturally occurring, redox-active compound with reported anticancer activity, but its preclinical evidence has not been systematically reviewed. This review evaluated the anticancer-related activities and associated molecular mechanisms of MNQ across preclinical cancer models. Methods: PubMed, Scopus, Cochrane Library, Embase, and Web of Science were systematically searched, with the final search performed on 26 October 2026. Original English-language studies evaluating the anticancer effects or cancer-related biological activities of MNQ were included. Findings were narratively synthesised and mapped to the Hallmarks of Cancer framework. Risk of bias was assessed using adapted OHAT and SYRCLE tools for in vitro and in vivo evidence, respectively. Results: Fourteen studies were included, all involving in vitro experiments, with one also reporting an in vivo hepatoma xenograft study. MNQ showed cytotoxic and antiproliferative activities, alongside apoptosis, necrosis, cell-cycle alterations, oxidative stress, DNA damage, mitochondrial dysfunction, and suppression of glycolysis, migration, and invasion. Alterations in autophagy-related markers, inflammatory and cellular stress responses, inhibition of EBV early antigen activation, and alterations in MAPK, PI3K/Akt, NF-κB, and PKC-related signalling were also reported. The in vivo study reported reduced tumour growth. Risk-of-bias assessments identified methodological reporting limitations in in vitro studies and predominantly unclear risk-of-bias judgements for the in vivo study. Conclusions: Current preclinical evidence suggests that MNQ affects biological processes associated with six Hallmarks of Cancer, highlighting its potential for further anticancer investigation. Nevertheless, the evidence remains predominantly in vitro, while experimental heterogeneity and methodological limitations emphasise the need for further mechanistic and in vivo validation.