Pivmecillinam as Oral Step-Down Treatment for Escherichia Coli Febrile Urinary Tract Infection Versus Standard of Care

The goal of this clinical trial is to learn if oral treatment with pivmecillinam is effective to treat febrile urinary tract infections in adult patients. Hospitalized patients who have received 2-4 days of intravenous antibiotic therapy for febrile urinary tract infections, and have responded to treatment, will be randomized to either pivmecillinam or standard treatment (other oral or intravenous antibiotics). The main question the study aims to answer is if oral follow-up with pivmecillinam is as effective as standard treatment. Patients will be evaluated for clinical response (resolution of fever and urinary tract symptoms) and microbiological response (no growth of bacteria in urine) 7 and 28 days after the end of treatment. Patients will also be asked about side effects. Some of the participants will also be examined for changes in the gut microbiome and drug exposures in blood and urine. Participants will: 1. Keep a patient diary to record antibiotic intake, body temperature, urinary tract symptoms, and suspected side effects until 7 days after end of treatment. 2. Participate in phone interviews 7 and 28 days after end of treatment to assess clinical response. 3. Provide urine samples 7 and 28 days after end of treatment to evaluate microbiological response. 4. A subgroup (60 patients) will provide fecal samples at five time-points over three months to assess antibiotic-induced changes in the gut microbiome. 5. A subgroup (30 patients) treated with to pivmecillinam will provide blood and urine samples to determine the pharmacokinetics of mecillinam during one dosing interval.