Urinary tract infections (UTIs) are among the most common bacterial infections in children. Up to 50% of UTI's are caused by multi-drug resistant ESBL-producing gram negative bacteria that do not respond to treatment with oral penicillin's or cephalosporins. Instead, children often require hospital admission to receive broad-spectrum intravenous antibiotics when they may otherwise be safely managed at home; resulting in prolonged hospital stays and an increased use of health resources. Fosfomycin is a broad-spectrum antibiotic discovered in 1969 that remains susceptible to a large number of organisms due to its low international use. Fosfomycin can be prepared as an oral solution with an orange/tangerine flavour and is currently approved for use in females >12 years old. Despite extensive evidence of its efficacy in adults and safety in neonates, the use of fosfomycin in children remains limited and fosfomycin is not currently licensed for use in children <12 years old in Australia.

The aim of this clinical trial is to compare the use of oral fosfomycin against standard of care antibiotics for the treatment of antibiotic resistant urinary tract infections in children. The main questions the trial aims to answer are:

1. Is oral fosfomycin non-inferior in efficacy to the current standard of care for the treatment of antibiotic-resistant urinary tract infections in children?
2. Is oral fosfomycin a safe and well-tolerated antibiotic in children?
3. What is the best dosing regimen of oral fosfomycin for the treatment of antibiotic-resistant UTIs in children?

This is a Phase 3, non-inferior, pragmatic, multi-centre, adaptive, randomised clinical trial.

Up to 300 participants aged ≥6 months to <18 years of age with a clinical and microbiological diagnosis of an antibiotic-resistant urinary tract infection will be recruited for the trial. Children will be randomised 1:1 to receive either oral fosfomycin (the intervention) or standard of care. The duration of antibiotic treatment will be dependent on whether the child's UTI is complicated or uncomplicated. Children with at least two of the following signs and symptoms will meet criteria of having a complicated UTI: fever, nausea or vomiting, other signs or symptoms of systemic illness: including chilld or rigors, significant fatigue, irritability or malaise above baseline, flank or back pain, ultrasound findings suggestive of an upper renal tract infection, a known history of structural and/or mechanical urological abnormalities, patients with a stent in situ or known renal tract calculi, patients who are immunocompromised. Children who do not meet the criteria for a 'complicated' UTI will meet criteria for an 'uncomplicated' UTI.

For children allocated to the standard of care (S)C) arm: the choice of antibiotic will be as per physician preference based on institutional prescribing practices, local antibiograms and medication availability. Children with an uncomplicated UTI will be prescribed a 3-day course of SOC antibiotics. Then, on Day 3 either; (a) Stop if the child has clinically improved, or (b) Give an additional 48 hours of SOC antibiotics if the child has ongoing fever or clinical symptoms consistent with an uncomplicated UTI, or (c) Give an additional 7 days of SOC antibiotics (to make up a total 10-day duration) if the child has evidence of a complicated UTI. Children with a complicated UTI diagnosis at the time of enrolment will be administered a total 10-day course of SOC antibiotics.

For children allocated to the intervention arm: Children with an uncomplicated UTI will be administered a single dose of oral fosfomycin trometamol. Then, on Day 2 either; (a) Stop if the child has clinically improved, or (b) Give an additional dose of oral fosfomycin trometamol if the child has ongoing fever or clinical symptoms consistent with an uncomplicated UTI, or (c) Give an additional 8 days of oral fosfomycin trometamol with repeat doses every 48 hours (to make up a total 10-day treatment duration) if the child has evidence of a complicated UTI. Children with a complicated UTI diagnosis at the time of enrolment will be administered a total 10-day course of oral fosfomycin trometamol with repeat doses administered every 48 hours.

Children enrolled into the oral fosfomycin group as well as children enrolled in the standard of care group who receive an oral antibiotic will be asked to complete a tolerability questionnaire following the first dose of the oral antibiotic. The child's parent and a member of their clinical team will also complete this questionnaire. All examinations, investigations and clinical interventions during the study period will be tailored to the child's needs as per routine standard of care. Clinical observations and results will be recorded on a case report form and entered into a secure electronic database. At Day 3, 10 and 28 after enrolment, parents will receive an SMS questionnaire to monitor their ongoing health, capture adverse events and/or capture their medication adherence.

50 children aged ≥6 months to <12 who have been randomised to the fosfomycin arm will be co-enrolled in a pharmacokinetic sub-study. For these children, two blood samples will be taken within 24 hours after the first dose of fosfomycin to determine the plasma fosfomycin concentrations. Co-enrolment into the pharmacokinetic sub-study is optional and will only be performed at a subset of study sites.

Inclusion Criteria:

Children aged ≥6 months to <18 years with:

1. Symptoms consistent with a clinical diagnosis of a UTI (as per the treating clinician); AND

2. Microbiological confirmation: Defined as a urine culture revealing a predominant growth of a bacterial uropathogen [≥10^6 CFU/L, or ≥10^3 CFU/mL] together with ≥10x10^6 white blood cells on microscopy; AND

3. The bacterial uropathogen is a non-pseudomonal gram-negative organism likely to cause urinary tract infections in children; being one of either: Escherichia coli, Proteus spp., Klebsiella spp., Enterobacter spp., Serratia spp., or Citrobacter spp., AND

4. The uropathogen has in vitro evidence of resistance to all oral penicillins and oral first- and second- generation cephalosporins (or is presumed to be resistant based on the pattern of phenotypic testing); AND

5. The patient has not yet received >48 hours of antibiotics with in vitro activity against the urinary pathogen prior to enrolment.

   Exclusion Criteria:

6. Evidence of bacteraemia due to the same uropathogen within the same clinical illness; OR

7. Evidence of infection at a secondary site (such as meningitis or endocarditis); OR

8. Children with features suggestive of sepsis (defined as requiring inotropic support, or >20ml/kg fluid bolus); OR

9. Children who are unable to tolerate or absorb oral antibiotics; OR

10. Children with severe renal unsifficiency (creatinine clearance <10ml/minute/1.73m^2); OR

11. Known allergy to fosfomycin; OR

12. A decision by the primary treating physician that enrolment in the trial is not in the child's best interest.

Efficacy and AEs/SAEs will be summarised and pooled. Deidentified individual data will be shared with the DSMB and ethics for SAEs only.

Investigators have the protocols and the ICFs. SAP will be available for all investigators once authored. Reports and analytical code will be shared when approved by trial statisticians.

Open-label study. Statisticians will be blind to groups when analysing results