Adjunctive fosfomycin for treatment of Staphylococcus aureus bacteraemia: protocol for a pooled post-hoc analysis of two randomised clinical trials (BACSAFO)

Background . Improving outcomes in patients with methicillin-susceptible (MSSA) and methicillin-resistant (MRSA) Staphylococcus aureus bacteraemia (SAB) is a critical healthcare goal. Two recent randomised clinical trials (RCTs), the BACSARM trial and the SAFO trial, assessed the efficacy of fosfomycin as an adjunctive therapy for MRSA and MSSA SAB respectively. Although neither trial demonstrated statistically significant differences in their primary endpoints of treatment success and reduced mortality respectively, both studies observed lower rates of persistent bacteraemia in the fosfomycin groups. Methods. We will perform a post-hoc analysis of pooled individual patient data from the BACSARM and SAFO trials, which will be referred to as the BACSAFO study. The primary exposure of interest is fosfomycin adjunctive therapy, and the primary outcome will be treatment success at 8 weeks, defined as the patient being alive, without signs of relapse, and showing improvement in clinical signs and symptoms. We will use both Bayesian and frequentist frameworks: the Bayesian analysis will use a hierarchical Bayesian log-binomial model, while the frequentist analysis will apply a hierarchical log-binomial model. In addition, we will investigate whether adjunctive fosfomycin is particularly beneficial in specific patient subgroups (created according to age, methicillin resistance, place of acquisition, and complicated bacteraemia status). Discussion . The BACSAFO study aims to clarify the role of fosfomycin as an adjunctive therapy for improving outcomes in SAB patients. Although previous trials have not demonstrated significant differences in the primary endpoints, the significant reductions in rates of persistent bacteraemia observed suggest that fosfomycin might offer a clinical benefit in certain cases. By analysing pooled data and attempting to identify subgroups that might benefit most, this study has the potential to refine treatment strategies and inform trial design and planning for future RCTs investigating combination antibiotic therapies for SAB. Ethics and dissemination . Ethical approval has been obtained from the Ethics Committee of Bellvitge University Hospital (EOM033/24). The results will be presented at international meetings and will be made available to patients and funders. Trial registration number . ClinicalTrials.gov Identifier: NCT06695832.