# Cloxacillin versus cefazolin for meticillin-susceptible Staphylococcus aureus bacteraemia (CloCeBa): a prospective, open-label, multicentre, non-inferiority, randomised clinical trial

Charles Burdet, Nadia Saidani, Céline Dupieux, Adrien Lemaignen, Etienne Canouï, Laure Surgers, Marc-Olivier Vareil, Agnès Lefort, Raphaël Lepeule, Nathan Peiffer-Smadja, et al.

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## ► To cite this version:

Charles Burdet, Nadia Saidani, Céline Dupieux, Adrien Lemaignen, Etienne Canouï, et al. Cloxacillin versus cefazolin for meticillin-susceptible Staphylococcus aureus bacteraemia (CloCeBa): a prospective, open-label, multicentre, non-inferiority, randomised clinical trial. The Lancet, 2025, 406 (10517), pp.2349-2359. 10.1016/S0140-6736(25)01624-1. hal-05380439

HAL Id: hal-05380439

https://hal.science/hal-05380439v1

Submitted on 3 Dec 2025

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# Cloxacillin versus cefazolin for meticillin-susceptible *Staphylococcus aureus* bacteraemia (CloCeBa): a prospective, open-label, multicentre, non-inferiority, randomised clinical trial

Charles Burdet, Nadia Saidani, Céline Dupieux, Adrien Lemaignen, Etienne Canoui, Laure Surgers, Marc Olivier Vareil, Agnès Lefort, Raphaël Lepeule, Nathan Peiffer-Smadja, Alexandre Charmillon, Vincent Le Moing, David Boutoille, Violaine Tolsma, Sophie Abgrall, Michel Wolff, Pierre Tattevin, Marina Esposito-Farèse, François Vandenesch, Xavier Duval*, Sarah Tubiana*, François-Xavier Lescure*, for the CloCeBa Study Group†

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## Summary

**Background** Although widely used, cefazolin efficacy for the treatment of meticillin-susceptible *Staphylococcus aureus* (MSSA) bacteraemia has not thus far been investigated in a clinical trial. In this study, we aimed to compare the efficacy and safety of cefazolin with that of cloxacillin in patients with MSSA bacteraemia.

**Methods** We conducted an open-label, non-inferiority, randomised clinical trial in 21 university and non-university hospitals in France in adults (aged ≥18 years) with MSSA bacteraemia, without intravascular implant or suspicion of CNS infection. Participants were randomly assigned (1:1) to receive intravenously cefazolin (25–50 mg/kg every 8 h) or cloxacillin (25–50 mg/kg every 4–6 h) for the first 7 days of therapy using computer-generated blocks of various sizes and stratification on vascular-access associated bacteraemia and centre. Subsequent treatment was left to the choice of the investigator (total duration ≥14 days). The primary endpoint was a composite of sterile blood cultures at day 3 (day 5 for endocarditis) without relapse of bacteraemia, survival, and clinical success at day 90, and was assessed in the intention-to-treat population. A non-inferiority margin of 12% was chosen. This trial is registered on ClinicalTrials.gov (NCT03248063) and is complete.

**Findings** Between Sept 5, 2018, and Nov 16, 2023, 315 participants were enrolled and assigned to cefazolin (n=158) or cloxacillin (n=157); 12 participants were excluded from analysis in the cefazolin group, and 11 in the cloxacillin group (final population of 146 in each group). Mean age was 62.7 years (SD 16.4), 215 (74%) participants were male, and race or ethnicity data were not collected. Median Pitt score was 0 (IQR 0–0). The primary endpoint was met in 109 (75%) of 146 participants in the cefazolin group versus 108 (74%) of 146 participants in the cloxacillin group (treatment difference -1%; 95% CI -11 to 9; p=0.012). At the end of study treatment, 22 (15%) of 146 participants assigned to cefazolin and 40 (27%) of 146 participants assigned to cloxacillin had had a serious adverse event (p=0.010). Acute kidney injury occurred more frequently in participants assigned to cloxacillin (15 [12%] of 128) than in those assigned to cefazolin (one [1%] of 134; p=0.0002).

**Interpretation** Cefazolin constitutes an alternative to cloxacillin for the treatment of MSSA bacteraemia, offering non-inferior clinical efficacy and potentially enhanced tolerability.

**Funding** French Ministry of Health.

**Copyright** © 2025 The Author(s). Published by Elsevier Ltd. This is an Open Access article under the CC BY-NC-ND 4.0 license.

## Introduction

*Staphylococcus aureus* bacteraemia is a leading cause of bloodstream infections both in the primary care and hospital setting. Its annual incidence has been estimated to be 20–30 cases per 100 000 people,¹,² and the clinical consequences of infection are often severe: common complications include deep-seated infections such as infective endocarditis or device-associated infection. The overall 3-month mortality rate is around 25%.³

In Europe, most *S. aureus* responsible for bacteraemia are susceptible to anti-staphylococcal penicillins, such as

cloxacillin, the historical reference treatment for meticillin-susceptible *S. aureus* (MSSA) infections. Combination antibiotic therapy of anti-staphylococcal penicillin with either fosfomycin, rifampicin, or daptomycin has not been proven to improve outcomes of *S. aureus* bacteraemia.⁴⁻⁶ Although anti-staphylococcal penicillins are still the reference treatment, concerns have emerged in recent years about toxicity, with reports of both hypersensitivity reactions and renal impairment in more than 10% of patients.⁷ Repeated antibiotics shortages have also occurred, leading clinicians to seek out effective alternatives.

Lancet 2025; 406: 2349-59

Published Online
October 17, 2025
https://doi.org/10.1016/
S0140-6736(25)01624-1

See Comment page 2300

*Contributed equally

†Members of the CloCeBa Study Group are listed in the appendix (pp 23–25)

Centre d'Investigation Clinique, INSERM CIC-1425 (C Burdet MD, Prof X Duval MD, S Tubiana PharmD), Service des Maladies Infectieuses et Tropicales (N Peiffer-Smadja MD, Prof F-X Lescure MD), and Département d'Épidémiologie, Biostatistique et Recherche Clinique (M Esposito-Farèse PhD), AP-HP, Hôpital Bichat-Claude Bernard, Paris, France; Université Paris Cité et Université Sorbonne Paris Nord, IAME, INSERM, Paris, France (C Burdet, Prof A Lefort MD, N Peiffer-Smadja, Prof X Duval, S Tubiana, Prof F-X Lescure); Centre Hospitalier de Corneouille, Service de Maladies Infectieuses et Tropicales, Quimper, France (N Saidani MD); Centre National de Référence des Staphylocoques, Institut des Agents Infectieux, Hospices Civils de Lyon, Lyon, France (C Dupieux PharmD PhD, Prof F Vandenesch MD); CHRU de Tours, Hôpital Bretonneau, Service de Maladies Infectieuses et Tropicales, Tours, France (Prof A Lemaignen MD); AP-HP, Hôpital Cochin, Equipe Mobile d'Infectiologie, Paris, France (E Canoui MD); GHU AP-HP, Sorbonne Université, Service des Maladies Infectieuses et Tropicales, Hôpital Saint-Antoine, Paris, France (L Surgers MD PhD); Sorbonne Université, INSERM, Institut Pierre Louis d'Épidémiologie et
de Santé Publique, Paris, France
(L. Surgers); Centre Hospitalier de
la Côte Basque, Service de
Maladies Infectieuses, Bayonne,
France (M O Vareil MD); AP-HP,
Hôpital Beaujon, Service de
Médecine Interne, Clichy, France
(Prof A Lefort); AP-HP, Hôpital
Henri Mondor, Unité
Transversale de Traitement des
Infections, Créteil, France
(R Lepeule MD); CHU de Nancy,
Service des Maladies
Infectieuses et Tropicales,
Vandoeuvre-lès-Nancy, France
(A Charmillon MD); CHU de
Montpellier, Service des
Maladies Infectieuses et
Tropicales, Montpellier, France
(Prof V Le Moing MD); CHU de
Nantes, Service des Maladies
Infectieuses et Tropicales, Centre
d'Investigation Clinique,
INSERM 1413, Nantes, France
(Prof D Boutouille MD); Centre
Hospitalier Annecy Genevois,
Service des Maladies
Infectieuses et Tropicales,
Annecy, France (V Tolsma MD);
APHP, Hôpital Béclère, Service
de Médecine Interne, Clamart,
France (Prof S Abgrall MD);
Université Paris-Saclay, Univ
Paris-Sud, UVSQ, CESP INSERM
U1018, Le Kremlin-Bicêtre,
France (Prof S Abgrall); GHU Paris
Psychiatrie-Neurosciences,
Service d'Anesthésie-
Réanimation, Paris, France
(Prof M Wolff MD); Hôpital
Pontchaillou, Service des
Maladies Infectieuses et
Emergentes, Rennes, France
(Prof P Tattevin MD)
Correspondence to:
Prof François-Xavier Lescuré,
Service de Maladies Infectieuses
et Tropicales, AP-HP, Hôpital
Bichat-Claude Bernard,
Paris 75018, France
xavier.lescuré@aphp.fr
See Online for appendix

## Research in context

### Evidence before this study

We searched MEDLINE using MESH terms from database inception to May 22, 2025, for randomised clinical trials with the terms "*Staphylococcus aureus*"[Mesh] AND "Cefazolin"[Mesh] AND ("Cloxacillin"[Mesh] OR "Oxacillin"[Mesh]) for completed trials comparing the efficacy of cefazolin and cloxacillin in patients with meticillin-susceptible *Staphylococcus aureus* bacteraemia, without any language restrictions. Our search retrieved only observational studies that suggested that cefazolin had similar efficacy to cloxacillin, with a possible lower risk of adverse effect. No randomised clinical trial was available.

### Added value of this study

CloCeBa is a multicentre, prospective, open-label, randomised, controlled non-inferiority trial evaluating the efficacy and safety

of cefazolin compared with cloxacillin for treatment of *S aureus* bacteraemia. Using a 12% non-inferiority margin, we observed that cefazolin has a non-inferior efficacy regarding mortality, microbiological or clinical endpoints. Cefazolin administration was associated with a lower rate of serious adverse events.

### Implications of all the available evidence

We provide evidence from the first randomised clinical trial that cefazolin can be used efficiently for treatment of *S aureus* bacteraemia. Cefazolin had a better safety profile and a better renal tolerance than cloxacillin. Results should be generalised with caution as some groups were under-represented in the trial.

To date, no randomised clinical trial has assessed the efficacy of cefazolin in MSSA bacteraemia. Along with extensive clinical experience, data from several observational studies have supported its use. No difference was observed between cefazolin and anti-staphylococcal penicillins in terms of mortality and treatment failure, although potential biases inherent to the studies' design cannot be ruled out.⁸

The safety profile of cefazolin has consistently been shown to be superior to that of anti-staphylococcal penicillins. However, some studies reported an inoculum effect—ie, an increase in cefazolin minimum inhibitory concentration in the case of bacterial strains carrying the *bla*Z gene (mostly type A) and high bacterial inoculum, raising the question of reduced overall efficacy.⁹,¹⁰

Here, we report the results of the CloCeBa trial, a prospective, randomised, non-inferiority clinical trial which aimed to compare the efficacy and safety of cefazolin with that of cloxacillin in patients with MSSA bacteraemia.

## Methods

### Study design

CloCeBa was a prospective, open-label, multicentre, randomised, controlled, non-inferiority trial conducted in 21 French university and non-university hospitals, which compared the efficacy of cefazolin versus cloxacillin for the treatment of MSSA bacteraemia.

The trial was approved by the Ethics Committee (Centre de Protection des Personnes Sud-Est I, #2017-87-PP), and the French National Agency for the Safety of Medicines and Health Products (#170661A-43), sponsored by Assistance Publique-Hôpitaux de Paris, and conducted in accordance with the Declaration of Helsinki. No data safety and monitoring board was set up for the trial. Results of the trial are reported according to the CONSORT statement for randomised trials

(appendix pp 20–22). The full protocol of the trial has previously been published.¹¹ All modifications made to the protocol are presented in the appendix (pp 11–14). This trial is registered on ClinicalTrials.gov (NCT03248063) and is complete.

## Participants

Participants were eligible to enter the trial if they were aged 18 years and older, hospitalised, and had a blood culture yielding MSSA, identified using standard microbiological techniques or GeneXpert PCR (Cepheid, Sunnyvale, CA, USA). Penicillin-susceptible *S aureus* strains were considered to be meticillin-susceptible strains. Participants with polymicrobial bacteraemia (excluding contaminants) were not eligible.

Patients were eligible if they had received antimicrobial therapy active against MSSA, provided it had been initiated within the 72 h before randomisation. Patients who had received such treatment for more than 72 h at the time of screening were not eligible for inclusion. Infective endocarditis was not an exclusion criterion, but participants with intravascular implant (vascular or valvular prothesis or cardiovascular implantable electronic device) or any material suspected to be infected, or with planned antibiotic treatment exceeding 70 days, were not eligible for enrolment, nor were those presenting clinical signs suggestive of a recent stroke (ie, within the past month), brain abscess, or meningitis. Participants were not eligible for enrolment if they had a stage 4 renal failure (estimated glomerular filtration rate <30 mL/min), a pre-established limitation of care (eg, no escalation to intensive or life-sustaining treatment), or a projected life expectancy less than 90 days. Participants with previous type 1 or Common Terminology Criteria for Adverse Events (CTCAE) grade 3 or grade 4 hypersensitivity reaction to β-lactams, with a pre-existing
coagulopathy and a prothrombin time of less than 50% not due to anticoagulant treatment, or treated with any another antibiotic therapy that could not be ceased or substituted by study treatment were not eligible; participants who were pregnant or breast-feeding were also excluded. Participants under guardianship or trusteeship, those without affiliation to the French Health insurance, or those already involved in another interventional clinical research evaluating a medicinal product were not eligible for enrolment. Participants who were initially enrolled in the trial but were subsequently diagnosed with meningitis, brain abscess, or a polymicrobial infection were later excluded and did not continue in the trial.

Sex was collected by the investigator following self-report by the participant. The provided options were male and female. Race and ethnicity data were not collected. Written informed consent was obtained from all included participants.

## Randomisation and masking

Participants were randomly assigned in a 1:1 ratio to receive either cefazolin or cloxacillin. The randomisation sequence was constituted by an independent statistician who had no further involvement in the trial. Randomisation was performed in the electronic Case Report Form to ensure appropriate allocation concealment and used computer-generated blocks of 4, 6, or 8. Participants were stratified based on vascular access-associated bacteraemia (including both peripheral intravenous catheters and central venous lines) and trial centre. Participants, study investigators, those assessing the outcomes, and those analysing the data were not masked to treatment assignment.

## Procedures

Participants assigned to cefazolin treatment received 25–50 mg/kg of cefazolin every 8 h (maximum 6 g/day), administered as a 60-min intravenous infusion. The dosing regimen was adapted in participants with chronic renal failure according to the Summary of Product Characteristics (SmPC).

Participants assigned to cloxacillin treatment received 25–50 mg/kg of cloxacillin every 4 h or 6 h (between 8 g/day and a maximum of 12 g/day), administered as a 60-min intravenous infusion. The dosing regimen was adapted in participants with chronic renal failure or with impaired hepatic function, according to SmPC.

The suggested total duration of antibiotic treatment was based on a Delphi expert consensus conducted in France as part of the TEPSTAR study (appendix pp 15–18). A minimum of 14 days of antibiotic therapy was recommended. At the time of trial design, it was common practice to switch antibiotic treatment after 7 days. For feasibility reasons, investigators were thus allowed to switch treatment after 7 full days of the randomised therapy.

After enrolment (day 1), participants were assessed on day 3, day 5, and day 7, at the end of study treatment, at the end of the antibiotic treatment, and by phone call on day 90 (appendix p 10). Clinical data, concomitant medications, adverse events, blood cell counts, serum creatinine and liver aminotransferases were collected for efficacy and safety evaluations.

All participants had a transthoracic echocardiography within 7 days following random allocation. Any additional radiological exams were made per the investigator's discretion, based on clinical suspicion regarding the source of bacteraemia or the presence of a deep abscess.

Bacterial strains were characterised in the French National Reference Centre for staphylococci (Hospices Civils de Lyon, Lyon, France). Genomic DNA was extracted with the Maxwell RSC system (Promega, Madison, WI, USA) using the Maxwell RSC Blood DNA kit (Promega, Madison, WI, USA). The absence of the *mecA* and *mecC* genes was confirmed by multiplex PCR, along with *agr* typing, using primers previously published.<sup>12-14</sup>

Whole-genome sequencing was performed on all isolates. Libraries were prepared using the DNA Prep library kit (Illumina, San Diego, CA, US) and sequencing was performed on a NextSeq 550 sequencer (Illumina) to generate 2×150 bp paired-end reads. Raw reads were trimmed to remove adapters and indexes before assembly using SPAdes (version 3.14).<sup>15</sup> Multilocus sequence typing was carried out from the assemblies using *mlst* and the PubMLST database, with results reported as clonal complexes according to the classification of Monecke and colleagues.<sup>16</sup>

The presence of the *blaZ* gene, encoding the staphylococcal β-lactamase, was assessed using ABRicate and AMRFinderPlus databases.<sup>17</sup> When detected, *blaZ* sequences were analysed using PointFinder (2024/02 version), and *blaZ* type determined based on amino acid positions 128 and 216: type A (T128, S216), type B (K128, N216), type C (T128, N216), and type D (A128, S216).<sup>18,19</sup> Results of the bacterial strains characterisation were obtained after the end of the trial and were not available for participant management.

## Outcomes

The primary outcome measure was therapeutic success, defined as a composite endpoint of bacteriological success without relapse of bacteraemia before day 90, clinical success at day 90, and survival at day 90. Bacteriological success was defined as sterile (ie, bacteria negative) blood cultures at day 3 (or day 5 for patients with infective endocarditis). Relapse of bacteraemia was defined as new positive blood cultures yielding a MSSA strain with the same antibiotic susceptibility profile in a participant who had bacteriological success at day 3 or day 5. Clinical success at day 90 was defined as the resolution of symptoms and signs related to the infection. Clinical success was collected through a standardised evaluation form.

For more on *mlst* and the PubMLST database see https://github.com/tseemann/mlst and https://pubmlst.org

For more on ABRicate see https://github.com/tseemann/abricate

For the Summary of Product Characteristics for cefazolin see https://base-donnees-publique.medicaments.gouv.fr/medicament/61242331/extrait#tab-rcp and for cloxacillin see https://base-donnees-publique.medicaments.gouv.fr/medicament/67153708/extrait#tab-rcp
Secondary efficacy outcome measures included all separate outcomes constitutive of the primary outcome, clinical success at day 7, duration of the antibiotic treatment in accordance with the recommended duration adapted to the final diagnosis, and the desirability of outcome ranking.²⁰ The two last outcomes will be reported in future publications.

Safety outcomes were the cumulative incidence of any adverse event and of any grade 3 or grade 4 adverse event at day 7, at the end of the treatment allocated by randomisation (ie, end of study treatment), and at the end of any antibiotic treatment; the cumulative incidence of premature study treatment discontinuation due to an adverse event; and *Clostridioides difficile* infection, diagnosed according to local laboratory protocols, which were generally PCR-based. Adverse events were classified in grades from mild (grade 1) to death (grade 5) following the CTCAE (version 4.0) of the National Institutes of Health and National Cancer Institute.

We also added, as a post-hoc safety outcome, the cumulative incidence of any serious adverse event at day 7, at the end of study treatment, and at end of any antibiotic treatment. In accordance with the International Conference on Harmonisation of Technical Requirements (ICH) E2A guideline, a serious adverse event was

defined as any adverse event that resulted in death, threatened the life of the participant, required or prolonged hospitalisation, caused severe or long-term disability or impairment, led to a congenital abnormality or deformity, or was considered medically significant by the investigator. Serious adverse events were coded using the Medical Dictionary for Regulatory Activities (MedDRA) of the ICH and grouped using MedDRA system organ classes or preferred terms. Additional non-prespecified safety outcomes were the cumulative incidence of specific serious adverse events overall and at day 7, at end of study treatment, and at end of any antibiotic treatment; and the occurrence of acute kidney injury as defined by the Kidney Disease: Improving Global Outcomes classification, determined in participants with available data using the change of serum creatinine level from inclusion to day 7, and to end of study treatment.

## Statistical analysis

The trial was designed to determine whether cefazolin was non-inferior to cloxacillin for the treatment of MSSA bacteraemia. We anticipated that 85% of participants would have therapeutic success at day 90.²¹ Setting a non-inferiority margin of 12% and with balanced group sizes, the inclusion of 139 patients in each group would allow non-inferiority of cefazolin over cloxacillin to be shown with a power of 80% and a one-sided α risk of 0.025. As we anticipated that 5% of participants would present secondary exclusion criteria, we determined that 292 participants would need to be included. This number was increased to 315 participants to account for non-evaluable participants.

The intention-to-treat population included all participants who were randomly assigned, irrespective of whether they received the treatment assigned by randomisation or not. The modified intention-to-treat population included participants from the intention-to-treat population who received at least one dose of the treatment allocated by randomisation. The intention-to-treat and modified intention-to-treat populations ended up being identical. The per protocol population included all participants who received at least a 7-day course of intravenous cefazolin or cloxacillin and whose total antibiotic treatment lasted for at least 14 days.

For the primary outcome, analyses were performed both on the intention-to-treat and per protocol populations. According to recommendations from the European Medicines Agency, we originally selected the per protocol population for the primary analysis, and the intention-to-treat population for the coprprimary analysis. However, more recent guidance recommends that the intention-to-treat population is used as the primary analysis set.²² Therefore, we have focused the presentation of the results on the intention-to-treat population. Other efficacy analyses were performed only on the intention-to-treat population. Safety analyses were performed on the

<figure><img src="image_5.png" /><figcaption>Figure 1: Trial profile</figcaption></figure>
<table><thead><tr><th></th><th>Cefazolin (n=146)</th><th>Cloxacillin (n=146)</th></tr></thead><tbody><tr><td>Age, years (mean)</td><td>62.7 (15.3)</td><td>62.8 (17.4)</td></tr><tr><td>Age, years (median)</td><td>66 (54-74)</td><td>67 (52-76)</td></tr><tr><td>Sex</td><td></td><td></td></tr><tr><td>Male</td><td>106/146 (73%)</td><td>107/146 (73%)</td></tr><tr><td>Female</td><td>40/146 (27%)</td><td>39/146 (27%)</td></tr><tr><td>Charlson comorbidity score*</td><td>4.4 (3.2)</td><td>4.3 (3.0)</td></tr><tr><td>At least one comorbid condition</td><td>100/146 (68%)</td><td>105/145 (72%)</td></tr><tr><td>Myocardial infarction</td><td>6/146 (4%)</td><td>13/145 (9%)</td></tr><tr><td>Congestive heart failure</td><td>6/146 (4%)</td><td>4/145 (3%)</td></tr><tr><td>Peripheral vascular disease</td><td>24/146 (16%)</td><td>19/145 (13%)</td></tr><tr><td>Cerebrovascular disease</td><td>11/146 (8%)</td><td>12/145 (8%)</td></tr><tr><td>Chronic pulmonary disease</td><td>12/146 (8%)</td><td>12/145 (8%)</td></tr><tr><td>Connective tissue disease</td><td>10/146 (7%)</td><td>10/145 (7%)</td></tr><tr><td>Liver disease</td><td>17/146 (12%)</td><td>13/145 (9%)</td></tr><tr><td>Diabetes requiring medication</td><td>28/146 (19%)</td><td>19/145 (13%)</td></tr><tr><td>Diabetes with end-organ damage</td><td>14/146 (10%)</td><td>12/145 (8%)</td></tr><tr><td>Moderate or severe renal disease</td><td>21/146 (14%)</td><td>21/145 (14%)</td></tr><tr><td>Solid tumour</td><td></td><td></td></tr><tr><td>Non-metastatic</td><td>20/146 (14%)</td><td>23/145 (16%)</td></tr><tr><td>Metastatic</td><td>15/146 (10%)</td><td>14/145 (10%)</td></tr><tr><td>History of intravenous drug use</td><td>5/146 (3%)</td><td>3/146 (2%)</td></tr><tr><td>Immunosuppression</td><td>45/146 (31%)</td><td>38/146 (26%)</td></tr><tr><td>Treatment with corticosteroids (&gt;5 mg/day for &gt;1 week)</td><td>12/146 (8%)</td><td>8/146 (5%)</td></tr><tr><td>Treatment with cancer chemotherapy</td><td>24/146 (16%)</td><td>25/146 (17%)</td></tr><tr><td>Treatment with other immunosuppressive therapy</td><td>12/146 (8%)</td><td>8/146 (5%)</td></tr><tr><td>Other†</td><td>9/146 (6%)</td><td>3/146 (2%)</td></tr><tr><td>Presence of a venous access at time of bacteraemia diagnosis</td><td>38/146 (26%)</td><td>43/146 (29%)</td></tr><tr><td>Prosthetic joint</td><td>6/146 (4%)</td><td>18/146 (12%)</td></tr><tr><td>Other medical device</td><td>10/146 (7%)</td><td>17/146 (12%)</td></tr><tr><td>Cardiovascular devices (ie, cardiac stents, femoral artery stent, implantable chamber)</td><td>2/146 (1%)</td><td>8/146 (5%)</td></tr><tr><td>Digestive device (biliary prosthesis)</td><td>0</td><td>1/146 (1%)</td></tr><tr><td>Subcutaneous glucose monitor</td><td>0</td><td>1/146 (1%)</td></tr><tr><td>Ventriculo-atrial shunt</td><td>1/146 (1%)</td><td>0</td></tr><tr><td>Ocular prosthesis</td><td>1/146 (1%)</td><td>0</td></tr><tr><td>Bone fixation material</td><td>3/146 (2%)</td><td>4/146 (3%)</td></tr><tr><td>Pleural catheter</td><td>1/146 (1%)</td><td>0</td></tr><tr><td>Urological devices (ie, urethral prosthesis, ureterostomy tube, indwelling bladder catheter)</td><td>1/146 (1%)</td><td>3/146 (2%)</td></tr><tr><td>Peripheral catheter</td><td>1/146 (1%)</td><td>0</td></tr></tbody><tfoot><tr><td colspan="3">(Table 1 continues in next column)</td></tr></tfoot></table>

modified intention-to-treat group (ie, in this case the intention-to-treat group).

For the primary outcome, missing data were imputed using multiple imputation by the chain equations method. Predictors for the incomplete variables were selected from baseline variables. Full information for the

<table><caption>Table 1: Baseline characteristics in the intention-to-treat population</caption><thead><tr><th></th><th>Cefazolin (n=146)</th><th>Cloxacillin (n=146)</th></tr></thead><tbody><tr><td>(Continued from previous column)</td><td></td><td></td></tr><tr><td>Hospital stay in the year before randomisation</td><td>75/146 (51%)</td><td>79/146 (54%)</td></tr><tr><td>Surgery in the year before randomisation</td><td>34/146 (23%)</td><td>40/146 (27%)</td></tr><tr><td>Pitt bacteraemia score at inclusion‡</td><td>0 (0-0)</td><td>0 (0-0)</td></tr><tr><td>Intensive care unit admission at inclusion</td><td>5/146 (3%)</td><td>9/146 (6%)</td></tr><tr><td>Laboratory data at inclusion§</td><td></td><td></td></tr><tr><td>Leukocytes, g/L</td><td>10.5 (6.3)</td><td>9.9 (6.4)</td></tr><tr><td>Creatinine, µmol/L</td><td>81.8 (35.0)</td><td>79.3 (30.8)</td></tr><tr><td>Estimated glomerular filtration rate, mL/min</td><td>88.6 (30.3)</td><td>89.7 (31.5)</td></tr><tr><td>C-reactive protein, mg/L</td><td>156.6 (105.0)</td><td>147.9 (106.1)</td></tr><tr><td>Aspartate aminotransferase, UI/L</td><td>46 (41-1)</td><td>48.1 (57-1)</td></tr><tr><td>Alanine aminotransferase, UI/L</td><td>45.6 (50.2)</td><td>47.3 (61-9)</td></tr><tr><td>Prothrombin time, % of control</td><td>81.4 (16.5)</td><td>79.7 (16.8)</td></tr><tr><td>Infection focus at day 7¶</td><td></td><td></td></tr><tr><td>Bacteraemia of unknown origin</td><td>43/142 (30%)</td><td>42/142 (30%)</td></tr><tr><td>Catheter-related infection</td><td>52/142 (37%)</td><td>51/142 (36%)</td></tr><tr><td>Skin and soft-tissue infection</td><td>26/142 (18%)</td><td>36/142 (25%)</td></tr><tr><td>Arthritis or osteoarthritis</td><td>30/142 (21%)</td><td>40/142 (28%)</td></tr><tr><td>Urinary infection</td><td>10/142 (7%)</td><td>7/142 (5%)</td></tr><tr><td>Respiratory infection</td><td>5/142 (4%)</td><td>7/142 (5%)</td></tr><tr><td>Infective endocarditis</td><td>8/142 (6%)</td><td>6/143 (4%)</td></tr><tr><td>Deep abscess</td><td>9/142 (6%)</td><td>15/142 (11%)</td></tr></tbody><tfoot><tr><td colspan="3">Data are mean (SD), median (IQR), or n/N (%). *Charlson comorbidity score was missing for one participant in the cloxacillin group. †Other immunosuppression includes solid organ transplantation, haematopoietic stem-cell transplantation, and neutropenia. ‡Pitt bacteraemia score was missing for one participant in the cefazolin group and two participants in the cloxacillin group. §The following numbers of participants had missing data for these variables: leukocytes (cefazolin n=2, cloxacillin n=6); creatinine (cloxacillin n=2); estimated glomerular filtration rate (cefazolin n=1, cloxacillin n=2); C-reactive protein (cefazolin n=12, cloxacillin n=10); aspartate aminotransferase (cefazolin n=17, cloxacillin n=17); alanine aminotransferase (cefazolin n=17, cloxacillin n=17); prothrombin time (cefazolin n=16, cloxacillin n=20). ¶Data were missing for four participants in each treatment group.</td></tr></tfoot></table>

imputation methods is provided in the appendix (p 19). For other secondary efficacy endpoints, missing data were not replaced.

For primary and secondary efficacy outcomes, statistical inference for non-inferiority of cefazolin over cloxacillin was based on the lower limit of the 95% CI of the difference of proportions of participants who met each outcome. The analyses accounted for the stratification by presence of vascular-access associated bacteraemia, but not for stratification by the centre, as the distribution of participants between centres was extremely unbalanced. Stratified analyses used the Miettinen–Nurminen method to estimate CIs for risk differences between groups, with Cochran–Mantel–Haenszel weighting used to combine strata.²³ We also performed non-prespecified non-inferiority subgroup analyses of efficacy outcomes according to the presence of
<table><thead><tr><th></th><th>Cefazolin<br>(n=146)</th><th>Cloxacillin<br>(n=146)</th></tr></thead><tbody><tr><td colspan="3"><strong>Treatments received before randomisation</strong></td></tr><tr><td>Antibiotic treatment before randomisation</td><td>139 (95%)</td><td>132 (90%)</td></tr><tr><td>Duration of antibiotic treatment before randomisation, days</td><td>1.6 (0.8)</td><td>1.7 (0.8)</td></tr><tr><td colspan="3"><strong>Treatments received after randomisation</strong></td></tr><tr><td>Duration of the allocated treatment, days</td><td>12.9 (8.2)</td><td>11.6 (6.5)</td></tr><tr><td>Total duration of antibiotic treatment, days</td><td>27.4 (19.6)</td><td>27.0 (19.5)</td></tr><tr><td colspan="3"><strong>Treatments received in the 7 days after randomisation</strong></td></tr><tr><td>Additional anti-staphylococcal antibiotic between day 2 and day 7</td><td>12 (8%)</td><td>31 (21%)</td></tr><tr><td>Cloxacillin</td><td>1 (1%)</td><td>..</td></tr><tr><td>Cefazolin</td><td>..</td><td>8 (5%)</td></tr><tr><td>Fluoroquinolones</td><td>4 (3%)</td><td>8 (5%)</td></tr><tr><td>Lincosamides</td><td>3 (2%)</td><td>8 (5%)</td></tr><tr><td>Aminoglycosides</td><td>1 (1%)</td><td>6 (4%)</td></tr><tr><td>Rifampicin (alone or in anti-tuberculous combination)</td><td>2 (1%)</td><td>4 (3%)</td></tr><tr><td>Third-generation cephalosporins</td><td>0</td><td>3 (2%)</td></tr><tr><td>Sulfamethoxazole plus trimethoprim</td><td>1 (1%)</td><td>2 (1%)</td></tr><tr><td>Daptomycin</td><td>1 (1%)</td><td>1 (1%)</td></tr><tr><td>Glycopeptides</td><td>0</td><td>1 (1%)</td></tr><tr><td>Macrolides</td><td>0</td><td>1 (1%)</td></tr><tr><td>Penicillin plus β-lactamase inhibitor</td><td>0</td><td>1 (1%)</td></tr><tr><td>Trimethoprim</td><td>0</td><td>1 (1%)</td></tr><tr><td colspan="3"><strong>Treatments received at day 8</strong></td></tr><tr><td>Allocated treatment exclusively</td><td>99 (68%)</td><td>84 (57%)</td></tr><tr><td>Allocated treatment and additional anti-staphylococcal antibiotic</td><td>40 (27%)</td><td>50 (34%)</td></tr><tr><td>Cloxacillin</td><td>1 (1%)</td><td>..</td></tr><tr><td>Cefazolin</td><td>..</td><td>7</td></tr><tr><td>Fluoroquinolones</td><td>17</td><td>10</td></tr><tr><td>Lincosamides</td><td>9</td><td>13</td></tr><tr><td>Sulfamethoxazole plus trimethoprim</td><td>5 (3%)</td><td>5 (3%)</td></tr><tr><td>Rifampicin (alone or in anti-tuberculous combination)</td><td>7 (5%)</td><td>2 (1%)</td></tr><tr><td>Linezolid</td><td>0</td><td>3 (2%)</td></tr><tr><td>Oxacillin</td><td>1 (1%)</td><td>2 (1%)</td></tr><tr><td>Third-generation cephalosporins</td><td>0</td><td>2 (1%)</td></tr><tr><td>Fusidic acid</td><td>0</td><td>1 (1%)</td></tr><tr><td>Aminoglycosides</td><td>0</td><td>1 (1%)</td></tr></tbody><tfoot><tr><td colspan="3">(Table 2 continues in next column)</td></tr></tfoot></table>

<table><thead><tr><th></th><th>Cefazolin<br>(n=146)</th><th>Cloxacillin<br>(n=146)</th></tr></thead><tbody><tr><td colspan="3">(Continued from previous column)</td></tr><tr><td>Other anti-staphylococcal antibiotic exclusively</td><td>7 (5%)</td><td>12 (8%)</td></tr><tr><td>Cloxacillin</td><td>1 (1%)</td><td>..</td></tr><tr><td>Cefazolin</td><td>..</td><td>8 (5%)</td></tr><tr><td>Fluoroquinolones</td><td>5 (3%)</td><td>6 (4%)</td></tr><tr><td>Lincosamides</td><td>1 (1%)</td><td>9 (6%)</td></tr><tr><td>Rifampicin</td><td>2 (1%)</td><td>3 (2%)</td></tr><tr><td>Sulfamethoxazole plus trimethoprim</td><td>2 (1%)</td><td>2 (1%)</td></tr><tr><td>Daptomycin</td><td>2 (1%)</td><td>1 (1%)</td></tr><tr><td>Penicillin plus β-lactamase inhibitor</td><td>1 (1%)</td><td>1 (1%)</td></tr><tr><td>Trimethoprim</td><td>1 (1%)</td><td>1 (1%)</td></tr><tr><td>Linezolid</td><td>1 (1%)</td><td>1 (1%)</td></tr><tr><td>Aminoglycosides</td><td>1 (1%)</td><td>0</td></tr><tr><td>First-generation cephalosporins</td><td>0</td><td>1 (1%)</td></tr><tr><td colspan="3">Data are n (%) or mean (SD).</td></tr><tr><td colspan="3">Table 2: Antibiotic treatments received by participants in the intention-to-treat population</td></tr></tbody></table>

## Role of the funding source

The funder of the study had no role in study design, data collection, data analysis, data interpretation, or writing of the report.

## Results

Between Sept 5, 2018, and Nov 16, 2023, 315 participants were randomised to one of the two treatment groups across 21 sites. 23 participants were excluded from the analysis and 292 (146 in the cefazolin group and 146 in the cloxacillin group) were evaluable for analysis (ie, the intention-to-treat population; figure 1). 69 participants were not included in the per protocol analysis; therefore the per protocol population comprised 223 participants (115 in the cefazolin group and 108 in the cloxacillin group).

Baseline characteristics of participants included in the intention-to-treat population are presented in table 1 (characteristics of the per protocol population are presented in the appendix pp 2–3). The mean age was 62.7 years (SD 16.4) and 213 (73%) of 292 participants were male. Overall, 205 (70%) of 291 participants had at least one underlying condition (data were missing for one participant). The mean Charlson score was 4.4 (SD 3.1). The most frequent underlying conditions were diabetes (73 [25%] of 291), solid tumour (72 [25%]), and peripheral vascular disease (43 [15%]). 83 (28%) of 292 participants were immunosuppressed and 81 (28%) had venous access in place at the onset of bacteraemia.

The genetic background of 253 MSSA strains isolated from 252 participants (129 [88%] of 146 from the cefazolin group and 123 [84%] of 146 from the cloxacillin group) were analysed (appendix p 4). Two strains were isolated from one participant from the cloxacillin group. Most

the *blaZ* A allele in the *S aureus* strain isolated in participants. Secondary safety outcomes were analysed under a superiority assumption. Non-inferiority tests were one-sided with a type-I error of 0.025 and superiority tests were two-sided with a type-I error of 0.05.

We present the quantitative variables as means with SDs, and categorical variables as counts and their proportions. All analyses were performed using R software (version 4.4).
strains (178 [70%] of 253) carried the *blaZ* gene, with a similar distribution between A, B, and C types. The *blaZ* type A was present in 58 (33%) of 178 strains (appendix p 4).

In the 72 h preceding randomisation, 271 (93%) of 292 participants had received an anti-staphylococcal antibiotic (table 2): 139 (95%) of 146 participants in the cefazolin group, and 132 (90%) of 146 in the cloxacillin group. The blood culture sampled just before the initiation of the antibiotic assigned by randomisation was still positive in 65 (44%) of 146 participants in the cefazolin group and 73 (50%) of 146 participants in the cloxacillin group.

Participants in the intention-to-treat population were treated with the study-assigned antibiotic for a mean duration of 12·9 days (SD 8·2) in the cefazolin group and 11·6 days (6·5) in the cloxacillin group. Total daily doses of study-assigned antibiotic according to baseline creatinine clearance are presented in the appendix (p 5). Between day 2 and day 7, 31 (21%) of 146 participants in the cloxacillin group received an additional

anti-staphylococcal antibiotic compared with 12 (8%) of 146 in the cefazolin group. At day 8, the distribution between participants receiving the allocated antibiotic treatment alone, in combination with another antibiotic, or another antibiotic alone was not significantly different between treatment groups (p=0·16; table 2). The mean total duration of antibiotic treatment after randomisation was 27·4 days (SD 19·6) in the cefazolin group, and 27·0 days (19·5) in the cloxacillin group.

Cefazolin was found to be non-inferior to cloxacillin in both the intention-to-treat and per protocol populations for the composite primary outcome (figure 2). In the intention-to-treat population, the primary outcome was met in 109 (75%) of 146 participants in the cefazolin group, and in 108 (74%) of 146 participants in the cloxacillin group. After multiple imputations of missing data and stratification on the vascular-access associated bacteraemia, the resultant treatment difference was -1% (95% CI -11 to 9; p=0·012). In the per protocol population, the primary outcome was met in 83 (72%) of 115 participants in the cefazolin group, and in 75 (69%) of

<table><caption>**Figure 2:** Subgroup analyses of the efficacy outcomes in the intention-to-treat population</caption><thead><tr><th></th><th>Cefazolin</th><th>Cloxacillin</th><th></th><th>Treatment difference (95% CI)</th><th>p value</th></tr></thead><tbody><tr><td colspan="6"><strong>Primary outcome (therapeutic success)</strong></td></tr><tr><td>Intention-to-treat population</td><td>109/146</td><td>108/146</td><td></td><td>-1% (-11 to 9)</td><td>0.012</td></tr><tr><td>Per protocol population</td><td>83/115</td><td>75/108</td><td></td><td>2% (-10 to 14)</td><td>0.011</td></tr><tr><td colspan="6"><strong>Secondary efficacy outcomes (intention-to-treat population)</strong></td></tr><tr><td>Survival at day 90</td><td>134/146</td><td>134/146</td><td></td><td>0% (-7 to 7)</td><td>0.0005</td></tr><tr><td>Early bacteriological success</td><td>136/146</td><td>135/146</td><td></td><td>1% (-6 to 7)</td><td>0.0002</td></tr><tr><td>Absence of relapse of bacteraemia by day 90</td><td>145/146</td><td>144/146</td><td></td><td>1% (-3 to 4)</td><td>&lt;0.0001</td></tr><tr><td>Clinical success at day 90</td><td>116/144</td><td>111/139</td><td></td><td>1% (-9 to 10)</td><td>0.0041</td></tr><tr><td>Clinical success at day 7</td><td>105/142</td><td>98/140</td><td></td><td>4% (-6 to 14)</td><td>0.0013</td></tr><tr><td colspan="6"><strong>Subgroup analyses (intention-to-treat population)</strong></td></tr><tr><td colspan="6"><strong>Primary outcome</strong></td></tr><tr><td>blaZ type A allele</td><td>19/29</td><td>21/27</td><td></td><td>-12% (-34 to 12)</td><td>0.52</td></tr><tr><td>No blaZ type A allele</td><td>76/98</td><td>68/90</td><td></td><td>2% (-10 to 14)</td><td>0.012</td></tr><tr><td colspan="6"><strong>Secondary efficacy outcomes</strong></td></tr><tr><td colspan="6"><strong>Survival at day 90</strong></td></tr><tr><td>blaZ type A allele</td><td>24/30</td><td>26/28</td><td></td><td>-12% (-30 to 6)</td><td>0.51</td></tr><tr><td>No blaZ type A allele</td><td>93/99</td><td>89/95</td><td></td><td>0% (-7 to 8)</td><td>0.0016</td></tr><tr><td colspan="6"><strong>Early bacteriological success</strong></td></tr><tr><td>blaZ type A allele</td><td>27/30</td><td>25/28</td><td></td><td>0% (-17 to 19)</td><td>0.074</td></tr><tr><td>No blaZ type A allele</td><td>95/99</td><td>90/95</td><td></td><td>1% (-5 to 8)</td><td>0.0004</td></tr><tr><td colspan="6"><strong>Absence of relapse of bacteraemia by day 90</strong></td></tr><tr><td>blaZ type A allele</td><td>30/30</td><td>28/28</td><td></td><td>0% (-12 to 12)</td><td>0.024</td></tr><tr><td>No blaZ type A allele</td><td>98/99</td><td>93/95</td><td></td><td>1% (-4 to 6)</td><td>0.0001</td></tr><tr><td colspan="6"><strong>Clinical success at day 90</strong></td></tr><tr><td>blaZ type A allele</td><td>20/29</td><td>24/27</td><td></td><td>-20% (-40 to 2)</td><td>0.76</td></tr><tr><td>No blaZ type A allele</td><td>81/98</td><td>71/90</td><td></td><td>2% (-11 to 15)</td><td>0.0034</td></tr><tr><td colspan="6"><strong>Clinical success at day 7</strong></td></tr><tr><td>blaZ type A allele</td><td>20/28</td><td>19/27</td><td></td><td>1% (-23 to 25)</td><td>0.15</td></tr><tr><td>No blaZ type A allele</td><td>70/97</td><td>65/92</td><td></td><td>4% (-8 to 15)</td><td>0.017</td></tr></tbody></table>

Bacterial strains of 40 participants were not available for genetic analysis (17 from the cefazolin group; 23 from the cloxacillin group). The dotted line represents the non-inferiority margin.
108 participants in the cloxacillin group. After multiple imputations of missing data and stratification on the vascular-access associated bacteraemia, the resultant treatment difference was 2% (-10 to 14; p=0.011).

Cefazolin was also found to be non-inferior to cloxacillin for all secondary efficacy outcomes (table 3). Specifically, the survival rate at day 90 was 134 (92%) of 146 participants in the cefazolin group and 134 (92%) of 146 participants in the cloxacillin group (treatment difference 0; 95% CI -7 to 7; p=0.0005).

Non-inferiority of cefazolin over cloxacillin was met for all outcomes in the subgroup of participants whose *S aureus* strain did not carry the *blaZ* A allele (figure 2). For participants whose *S aureus* strain carried the *blaZ* A allele, non-inferiority was not shown for any outcome, except for the absence of bacteraemia relapse by day 90. However, the proportion of participants with microbiological failure at day 3 (or day 5) did not differ significantly between participants infected with a *S aureus* strain carrying the *blaZ* type A (six [10%] of 58) and those whose bacterial strain did not carry this *blaZ* allele (nine [5%] of 194; p=0.12; data not shown).

Safety outcomes are presented in table 4 (additional information on safety outcomes are available in the

appendix pp 6–8). Treatment was discontinued because of the occurrence of an adverse event in five (3%) of 146 participants in the cefazolin group and 13 (9%) of 146 participants in the cloxacillin group (treatment difference -5%; 95% CI -12 to 0; p=0.052). *C difficile* infection occurred in three (2%) of 146 participants from the cefazolin group and three (2%) of 146 participants from the cloxacillin group (treatment difference 0%; -4 to 4; p>0.99; table 4). No significant difference was observed in the occurrence of adverse event between randomisation groups at any timepoints (appendix p 6).

Focusing on serious adverse events, we observed that 51 (35%) of 146 participants from the cefazolin group and 64 (44%) of 146 participants from the cloxacillin group had a serious adverse event over the study course (treatment difference -9%; 95% CI -20 to 2; p=0.12). At the end of study treatment, 22 (15%) of 146 participants from the cefazolin group and 40 (27%) of 146 participants from the cloxacillin group had a serious adverse event (treatment difference -12%; -22 to -3; p=0.01; table 4). These differences were not significant when restricting the analysis to participants who only received the allocated treatment between day 2 and day 7 (appendix p 9).

When investigating differences in specific serious adverse events between groups, we observed that participants from the cloxacillin group had serious adverse events belonging to the “renal and urinary disorders” system organ class and coded as “acute kidney injury” significantly more frequently than participants from the cefazolin group (appendix pp 7–8). Using the creatinine data collected at day 7 during the trial, we observed that acute kidney injury occurred in one (1%) of 134 participants in the cefazolin group and in 15 (12%) of 128 participants in the cloxacillin group (treatment difference -11%; -18 to -6; p=0.0002). In the cloxacillin group, the mean increase of serum creatinine was 8.9 µmol/L (SD 48.2) whereas a mean decrease of 13.0 µmol/L (SD 19.6) was observed in the cefazolin group.

<table><thead><tr><th></th><th>Cefazolin<br>(n=146)</th><th>Cloxacillin<br>(n=146)</th><th>Treatment difference<br>(95% CI)</th><th>p value</th></tr></thead><tbody><tr><td>Survival at day 90</td><td>134/146 (92%)</td><td>134/146 (92%)</td><td>0% (-7 to 7)</td><td>0.0005</td></tr><tr><td>Microbiological success at day 3<br>(or day 5 in participants with<br>endocarditis)</td><td>136/146 (93%)</td><td>135/146 (92%)</td><td>1% (-6 to 7)</td><td>0.0002</td></tr><tr><td>Absence of relapse of bacteraemia<br>at day 90</td><td>145/146 (99%)</td><td>144/146 (99%)</td><td>1% (-3 to 4)</td><td>&lt;0.0001</td></tr><tr><td>Clinical success at day 90</td><td>116/144 (81%)</td><td>111/139 (80%)</td><td>1% (-9 to 10)</td><td>0.0041</td></tr><tr><td>Clinical success at day 7</td><td>105/142 (74%)</td><td>98/140 (70%)</td><td>4% (-6 to 14)</td><td>0.0013</td></tr></tbody></table>

Data are n/N (%), unless otherwise indicated. p values refer to the test for non-inferiority of cefazolin over cloxacillin.

<table><caption>**Table 3: Primary and secondary endpoints in the intention-to-treat population**</caption><thead><tr><th></th><th>Cefazolin<br>(n=146)</th><th>Cloxacillin<br>(n=146)</th><th>Treatment difference<br>(95% CI)</th><th>p value</th></tr></thead><tbody><tr><td>Any serious adverse event over the<br>study course</td><td>51/146 (35%)</td><td>64/146 (44%)</td><td>-9% (-20 to 2)</td><td>0.12</td></tr><tr><td>Any serious adverse event by day 7</td><td>14/146 (10%)</td><td>30/146 (20%)</td><td>-11% (-19 to -3)</td><td>0.0090</td></tr><tr><td>Any serious adverse event by end of<br>study treatment</td><td>22/146 (15%)</td><td>40/146 (27%)</td><td>-12% (-22 to -3)</td><td>0.010</td></tr><tr><td>Any serious adverse event by end of<br>antimicrobial therapy</td><td>32/146 (22%)</td><td>48/146 (33%)</td><td>-11% (-21 to -1)</td><td>0.036</td></tr><tr><td>Premature discontinuation of study<br>treatment due to an adverse event</td><td>5/146 (3%)</td><td>13/146 (9%)</td><td>-5% (-12 to 1)</td><td>0.052</td></tr><tr><td>Clostridioides difficile infection</td><td>3/146 (2%)</td><td>3/146 (2%)</td><td>0 (-4 to 4)</td><td>&gt;0.99</td></tr><tr><td>Acute kidney injury by day 7</td><td>1/134 (1%)</td><td>15/128 (12%)</td><td>-11% (-18 to -6)</td><td>0.0002</td></tr><tr><td>Acute kidney injury by end of<br>study treatment</td><td>4/111 (4%)</td><td>17/99 (17%)</td><td>-13% (-23 to -6)</td><td>0.0008</td></tr></tbody></table>

Data are n/N (%), unless otherwise indicated. p values refer to the test of superiority. Some participants had more than a single serious adverse event.

<caption>**Table 4: Severe adverse events and safety outcomes in the intention-to-treat population**</caption>

## Discussion

In this prospective, open-label, multicentre, randomised, controlled, non-inferiority trial in patients with MSSA bacteraemia, the efficacy of cefazolin was non-inferior to that of cloxacillin. No significant differences were observed between cefazolin and cloxacillin in the rates of microbiological and clinical success, nor regarding mortality. Fewer serious adverse events, notably fewer renal disorders, were reported in participants allocated to cefazolin than those allocated to cloxacillin.

Although the use of cefazolin for treatment of MSSA bacteraemia has increased in the past decade, very few studies support its use. In particular, no randomised controlled trials have previously compared these two therapeutic agents. Non-interventional, observational clinical studies reported non-significant differences in mortality between cefazolin and anti-staphylococcal penicillins.⁸,²⁴ These studies suggested that the use of
cefazolin might be associated with lower mortality rates.³,²⁴⁻²⁷ However, the observational design of these studies requires cautious interpretation of results, due to potential biases that could not be accounted for. In the CloCeBa trial reported here, cefazolin was found to be non-inferior to cloxacillin in terms of survival.

During the trial, we were able to retrieve approximately 90% of the bacterial strains for genetic characterisation. The genetic background of the strains reflected the European setting of the trial. Some clonal complexes that are prevalent in the USA, such as the CC30, were also represented in the collection (42 [17%] of 253 strains). We observed similar proportions in each treatment group in terms of genetic background and regarding the presence and type of *blaZ* gene. Overall, 128 (70%) of the 253 retrieved MSSA strains carried the *blaZ* gene, whose expression product can hydrolyse β-lactams, including cefazolin. The *blaZ* type A allele had been associated with increased cefazolin minimum inhibitory concentration and reduced cefazolin efficacy in case of high inoculum.⁸⁻¹⁰ In the subgroup of participants whose strain carried this allele, non-inferiority of cefazolin over cloxacillin was not met.

We observed a safety profile that favoured cefazolin, although participants from the cloxacillin group received additional antibiotics more frequently, which might have a role in the occurrence of adverse events. Cefazolin was associated with a significantly lower rate of serious adverse events than cloxacillin during the course of antibiotic therapy (p=0.036). A safer profile of cefazolin was particularly observed for nephrotoxicity. This warning about the potential toxicity of cloxacillin was previously reported in observational studies and might be related to ageing and accumulating comorbidities.⁷,²¹,²⁸ More participants from the cloxacillin group had an acute kidney injury at the end of the study treatment than the cefazolin group. However, a limited increase in creatinine levels was observed, raising the question of their clinical significance, particularly in patients with typical baseline creatinine levels. As the day 90 assessment did not include further sample collection for control of serum creatinine levels, we were not able to evaluate the persistence of this elevation.

Besides its lower toxicity, cefazolin has some obvious advantages compared with cloxacillin, such as more convenient dosing and possible use in patients with allergy to penicillin. Furthermore, the presence of an effective and safe alternative treatment option represents an asset in the context of increasing risks of global drug shortages.

However, the results of the CloCeBa trial should be generalised with caution. The outcome of participants was generally good, with a global mortality rate of 8%, which is below the commonly reported rates in observational cohorts.¹ This finding might be explained in part by the exclusion of patients whose life expectancy was short, whose death might be attributable to the

underlying pathology rather than to the unfavourable evolution of the bacteraemia. We also excluded patients under guardianship or trusteeship, including patients who were sedated and in the intensive care unit. These patients, however, represent a non-negligible proportion of the patient population with *S aureus* bacteraemia in clinical practice. Enrolled participants had a mean Charlson comorbidity index of 4.4 (SD 3.1), with a Pitt severity score of 0. Approximately 30% of participants were immunosuppressed. Some patient categories were under-represented in the trial, such as patients with infective endocarditis or deep-seated infection, in comparison with a large cohort of *S aureus* bacteraemia studied in 2015.²⁹ We excluded patients with CNS infection. At the time of the protocol development, the diffusion of cefazolin in nervous tissue was uncertain. A 2020 study suggested that cefazolin is a reasonable option for treatment in this setting.³⁰ In the trial, we also excluded participants with indwelling medical devices suspected to be infected. The management of these patients is usually both medical and surgical, as patient outcome depends on removal of the implanted material. Moreover, the infected implant can be a source of relapse of bacteraemia. These patient populations should be further investigated, although results from the CloCeBa trial are reassuring and consistent with the existing literature. Platform trials such as the *S aureus* Network Adaptive Platform Trial (NCT05137119) might help to answer remaining questions.

The trial has several limitations. First, it was open label. This choice was driven by the dosing schedule of administered treatments: every 8 h for cefazolin, and every 4 h to 6 h for cloxacillin. Having a placebo-controlled trial would have lowered the feasibility of the trial. In addition, to control potential evaluation bias, we used a composite endpoint that included robust objective criteria, namely mortality, and results of laboratory analyses. Clinical success was evaluated using standardised evaluation forms to mitigate the risk of bias, although some degree of subjectivity might still remain.

Second, participants were allowed to receive antibiotic treatment before randomisation, and approximately half of the participants from each group had a negative blood culture at the time of the assigned antibiotic initiation. Obtaining the blood culture result was required for inclusion in the trial. Allowing antibiotic initiation before inclusion was necessary as it would not have been ethical to wait for this result before starting antibiotic treatment in patients with sepsis. We reduced this pre-randomisation treatment to its minimum duration, but the proportion of participants treated with antibiotics before inclusion was slightly higher in the cefazolin group than in the cloxacillin group. This might bias the results toward non-inferiority of cefazolin. Furthermore, the initial assumption that 85% of participants would have therapeutic success by day 90, which was based on
previous data, might have been overly optimistic, as the final observed rate of success was 75%. As a result, the underestimation of the sample size and the optimistic assumption about the primary outcome could have affected the study's ability to show non-inferiority. We did not collect data on race or ethnicity as part of our study, as these are not expected to influence the effectiveness of antibiotic treatment in *S aureus* bacteraemia; this might, however, represent an additional study limitation. Lastly, some categories of patients were not eligible for enrolment. Our results might form a foundation for further evaluation in other clinical settings.

In summary, findings from this randomised controlled trial support the non-inferiority of cefazolin over cloxacillin in the treatment of non-severe *S aureus* bacteraemia. The safety profile of cefazolin is more favourable than that of cloxacillin, especially regarding renal tolerance. Further studies could focus on more difficult-to-treat infections, such as infective endocarditis.

### Contributors

ME-F, ST, CB, XD, and F-XL were involved in the design, establishment, and day-to-day management and implementation of the trial. CB, F-XL, and XD obtained funding for the trial. NS, AdL, AgL, EC, LS, MOV, VLM, DB, VT, SA, and F-XL enrolled participants in the trial. FV and CD were responsible for the assessment of *S aureus* strains at the French reference laboratory. ME-F and ST were in charge of data curation and accessed and verified the data. ME-F, ST, CB, XD, and F-XL were involved in the statistical analyses. CB wrote the original draft of the manuscript, which was reviewed and edited by ST, XD, and F-XL. All authors contributed to refinement of and approved this manuscript. ME-F and ST accessed and verified the data. All authors had full access to all the data and accept the responsibility to submit for publication.

### Declaration of interests

VLM received consulting fees from Pfizer and Advanz Pharma; payment or honoraria for lectures from Advanz Pharma; and support for attending meetings or travel from Pfizer. F-XL is an advisor within the French Committee for the Surveillance and Anticipation of Health Risks (COVARS); Director of the French Mission of Operation Coordination for Epidemic and Biological Risk and of the Emerging Infections Subcommittee of European Society of Clinical Microbiology and Infectious Diseases. FV received a grant from the French National Research Agency for microbial identification and antibiotic resistance determination from positive blood cultures by mass spectrometry; holds a patent for a method for determining methicillin resistance in strains of *Staphylococcus aureus* (FR2200268); and is co-funder of Weezion, a patented technology for in-vitro diagnosis of sepsis bacteraemia or fungaemia. All other authors declare no competing interests.

### Data sharing

Data collected for the study, including individual participant data and a data dictionary defining each field in the set, will be made available on request after publication of the secondary analyses.

### Acknowledgments

We thank all patients and their relatives for participating in this study. We thank the staff (physicians, nurses, medical microbiologists, secretaries, study coordinators, and assistants) at all participating sites for their help in conducting the trial. We acknowledge members of the CloCeBa Steering Committee: Pierre Tattevin (CHU de Rennes, Rennes), Michel Wolff (GHU Paris, Paris), Charles Burdet (Hôpital Bichat, Paris), Xavier Duval (Hôpital Bichat, Paris), Tristan Ferry (Hospices Civils de Lyon, Lyon), Nathalie Grall (Hôpital Bichat, Paris), François-Xavier Lescure (Hôpital Bichat, Paris), and François Vandenesch (Hospices Civils de Lyon, Lyon). We thank the REseau NAational de Recherche Clinique en Infectiologie (RENARCI) for

support in the conduct of the trial. This study was supported by a grant from the French Ministry of Health (PHRC-N-2016-0291) and sponsored by the Assistance Publique - Hôpitaux de Paris (P160943)].

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