Background/Objectives: Prolonged use of denosumab in patients with metastatic breast cancer has raised concerns about the development of medication-related osteonecrosis of the jaw (MRONJ). However, the threshold at which the risk increases remains unclear. Methods: This retrospective cohort study analyzed patients with breast cancer and bone metastases who received denosumab between May 2012 and August 2024. Associations between cumulative denosumab administration and MRONJ were evaluated using univariate and multivariate logistic regression analyses. A receiver operating characteristic (ROC) analysis was used to determine the optimal cutoff for cumulative doses. Results: MRONJ developed in 101 patients (31.2%). Multivariate analysis identified cumulative denosumab administration (odds ratio [OR]: 1.05, 95% confidence interval [CI]: 1.03–1.06; p < 0.001) and a history of tooth extraction (OR: 4.40, 95% CI: 2.23–8.71; p < 0.001) as independent risk factors for MRONJ. ROC analysis determined an optimal cutoff of 32 cumulative doses, with an area under the curve of 0.83 (95% CI: 0.78–0.88; p < 0.0001). Conclusions: Cumulative denosumab administration and history of tooth extraction were independent risk factors for MRONJ in patients with breast cancer and bone metastases. The risk of MRONJ increased after 32 cumulative doses, providing a clinically actionable threshold for risk assessment and patient monitoring.

Genetic manipulation tools are essential for elucidating the pathogenic mechanisms of microorganisms. Several species of Actinomyces, including A. israelii, are present in the oral cavity and they are the causative agents of actinomycosis. However, efficient gene-editing tools for these species have not yet been developed. In this study, the aim was to evaluate the introduction of foreign genes into Actinomyces using plasmids derived from Mycobacterium, which belong to the same class as Actinomycetes. A truncated derivative of pYT923, pYT923S, which contains the replication origin of the M. scrofulaceum plasmid pMSC262 was constructed and introduced into A. israelii by electrotransformation. pYT923S was successfully introduced into A. israelii. The transformation efficiency of A. israelii was approximately 7-66 CFU/μg of DNA, and all transformed colonies harbored pYT923S. The plasmid recovered from A. israelii replicated in Escherichia coli. pYT923S was introduced into and maintained within A. israelii. Therefore, the pYT923S vector represents a useful genetic tool for Actinomyces and it is expected to facilitate future studies on the biology and pathogenicity of Actinomyces.