The decision to withdraw the study was made due to delayed logistics of the supply chain of ceftolozane-tazobactam along with the immense complexities of conducting clinical research felt because of the COVID-19 pandemic.

The purpose of this study is to determine whether ceftolozane-tazobactam is as effective as meropenem with respect to 30 day mortality in the treatment of bloodstream infection due to third-generation cephalosporin non-susceptible Enterobacterales or a known chromosomal AmpC-producing Enterobacterales (Enterobacter spp., Citrobacter freundii, Morganella morganii, Providencia spp. or Serratia marcescens).

Enterobacterales are common causes of bacteraemia, and may produce extended-spectrum beta-lactamases (ESBLs) or AmpC beta-lactamases. ESBL or AmpC producers are typically resistant to third generation cephalosporins such as ceftriaxone, but susceptible to carbapenems. In no study has the outcome of treatment for serious infections for ESBL producers been significantly surpassed by carbapenems. Despite the potential advantages of carbapenems for treatment of ceftriaxone non-susceptible organisms, widespread use of carbapenems may cause selection pressure leading to carbapenem-resistant organisms. This is a significant issue since carbapenem-resistant organisms are treated with last-line antibiotics such as colistin.

Ceftolozane-tazobactam is a combination of a new beta-lactam antibiotic with an existing beta-lactamase inhibitor, tazobactam, and is active against ESBL and most AmpC producing organisms. In a large sample of ESBL- and AmpC-producing Enterobacterales isolates from urinary tract and intra-abdominal specimens, ceftolozane-tazobactam was susceptible in over 80%. It has been FDA approved for complicated urinary tract infections (cUTI) and complicated intra-abdominal infections (cIAI), and more recently for hospital-acquired and ventilator-associated pneumonia (HAP/VAP). In addition, a pooled analysis of phase 3 clinical trials has shown favourable clinical cure rates with ceftolozane-tazobactam for cUTI and cIAI caused by ESBL-producing Enterobacterales. Given the issues of carbapenem resistant organisms, there is a need for establishing the efficacy of an alternative to carbapenems for serious infections.

Inclusion Criteria:

• Bloodstream infection defined as presence in at least one peripheral blood culture draw demonstrating Enterobacterales with proven non-susceptibility to third generation cephalosporins or cephalosporin susceptible species known to harbour chromosomal AmpC-beta-lactamases (Enterobacter spp., Klebsiella aerogenes, Citrobacter freundii, Morganella morganii, Providencia spp. or Serratia marcescens) during hospitalisation
• Patient is aged 18 years and over (21 and over in Singapore)
• The patient or approved proxy is able to provide informed consent
• ≤72 hours has elapsed since the first positive qualifying (index) blood culture collection
• Expected to receive IV therapy for ≥5 days

Exclusion Criteria:

• Known hypersensitivity to a cephalosporin or a carbapenem, or anaphylaxis to beta-lactam antibiotics
• Participant with significant polymicrobial bloodstream infection (i.e. not a contaminant)
• Treatment is not with the intent to cure the infection (i.e. palliative intent) or the expected survival is ≤4 days
• Participant is pregnant or breast-feeding (tested for in women of child-bearing age only)
• Use of concomitant antimicrobials with known activity against Gram-negative bacilli (except trimethoprim/sulfamethoxazole for Pneumocystis prophylaxis and when adding metronidazole for suspected IAI) in the first 5 days post-randomisation
• Participant with CrCl <15 mL/minute or on renal replacement therapy (in addition, participants will be withdrawn from the study if CrCl reaches this level)
• Previously randomised in the MERINO-3 trial or concurrently enrolled in another therapeutic antibiotic clinical trial
• Blood culture isolate with in-vitro resistance to either meropenem or ceftolozane-tazobactam (known either at time of enrolment or during the course of study treatment, in which case the participant will be withdrawn)