1043. Activity of Mecillinam Against Enterobacterales Isolates Collected From Patients With Urinary Tract Infections (UTIs) in the USA During 2019

Abstract Background Mecillinam is a β-lactam antibiotic that exerts its antibacterial activity by binding to penicillin-binding protein 2. In the USA, intravenous (IV) mecillinam is in development for the treatment of complicated UTIs in the hospital setting and as step-down therapy transitioning from IV mecillinam to oral pivmecillinam so that patients can continue treatment at home. To support the clinical development of mecillinam in the USA for the treatment of both complicated and uncomplicated UTI, this observational study investigated the activity of mecillinam against Enterobacterales isolates from patients with UTI in the USA, collected during 2019. Methods This study evaluated the activity of mecillinam and other antimicrobial agents against 1075 selected Enterobacterales clinical isolates collected from patients with UTI in the USA during 2019. Antibiotic activity (minimum inhibitory concentration [MIC]) was determined by Clinical & Laboratory Standards Institute (CLSI) agar dilution methodology, and susceptibility was interpreted according to CLSI guidelines. Results Among the selected 1075 isolates, producers of extended-spectrum beta-lactamase (ESBL) represented 9.6% of Escherichia coli and 50% of Klebsiella pneumoniae. Ninety-five percent of the isolates tested were susceptible to mecillinam (Table 1). The MIC50 and MIC90 values for mecillinam were 0.25 and 2 µg/mL, respectively. Fosfomycin MIC50 and MIC90 were 1 and 32 µg/mL, respectively (97.6% of isolates were susceptible). Mecillinam showed the lowest MIC90 value of all single antibiotics tested. The highest MIC90 was 128 µg/mL for both nitrofurantoin and cefotaxime. The lowest percentage of resistance was obtained with fosfomycin (1.7%), followed by mecillinam (4%). Table 1: Summary MIC and susceptibility data for all isolates tested (n=1075) Conclusion Overall, mecillinam showed excellent activity and a comparable resistance profile to fosfomycin. Resistance rates to ceftazidime, cefotaxime, ciprofloxacin and trimethoprim/sulfamethoxazole of greater than 20% are concerning due to the frequent use of these antibiotics in clinical practice to treat UTIs. Taken together, these data demonstrate that mecillinam has promising activity, with low resistance observed in Enterobacterales species causing UTIs in the USA. Clinical development of mecillinam in the USA is ongoing. Disclosures Stephen Hawser, PhD, Utility Therapeutics (Grant/Research Support) Ian Morrissey, Utility Therapeutics (Grant/Research Support) Anne Santerre Henriksen, MS, Advanz (Consultant)Shionogi BV (Consultant)UTILITY Therapeutics (Consultant)

. S J Ryan Arends, PhD, AbbVie (formerly Allergan) (Research Grant or Support)GlaxoSmithKline, LLC (Research Grant or Support)Melinta Therapeutics, LLC (Research Grant or Support)Nabriva Therapeutics (Research Grant or Support)Spero Therapeutics (Research Grant or Support) Abby L. Klauer, n/a, Cidara Therapeutics, Inc. (Research Grant or Support)Spero Therapeutics (Research Grant or Support) Nicole Cotroneo, Spero Therapeutics (Employee, Shareholder) Ian A. Critchley, Ph.D., Spero Therapeutics (Employee, Shareholder) Rodrigo E. Mendes, PhD, AbbVie (Research Grant or Support)AbbVie (formerly Allergan) ( Background. Microbiota-based treatments have shown promise to reduce recurrence, morbidity, and mortality for recurrent Clostridioides difficile infections (rCDI), but consistent and reliable safety data are needed to support regulatory approvals and broaden patient access. Here we provide cumulative safety data from 5 prospective clinical studies evaluating RBX2660-a standardized, microbiota-based investigational live biotherapeutic-for reducing rCDI.
Methods. This analysis included three Phase 2 (PUNCH CD, PUNCH CD2, PUNCH CD Open Label) and two Phase 3 trials (PUNCH CD3, PUNCH CD3-OLS ad hoc analysis). Participants were ≥18 years old with documented rCDI who completed standard-of-care oral antibiotic therapy prior to treatment with RBX2660. PUNCH CD3-OLS allowed participants with comorbidities of irritable bowel syndrome (IBS) or inflammatory bowel disease (IBD). Depending on the trial, assigned study treatment was 1 or 2 doses of RBX2660 (or placebo), administered rectally. Participants whose CDI recurred within 8 weeks were eligible for additional RBX2660 treatment. Treatment-emergent adverse events (TEAEs) were recorded for at least 6 months following last study treatment; CD2 and CD Open Label recorded TEAEs for 24 months.
Conclusion. Across five clinical studies with consistent investigational product, RBX2660 was well-tolerated in rCDI participants. In aggregate, this data provides compelling and consistent safety data for RBX2660.

Session: P-59. New Drug Development
Background. Mecillinam is a β-lactam antibiotic that exerts its antibacterial activity by binding to penicillin-binding protein 2. In the USA, intravenous (IV) mecillinam is in development for the treatment of complicated UTIs in the hospital setting and as step-down therapy transitioning from IV mecillinam to oral pivmecillinam so that patients can continue treatment at home. To support the clinical development of mecillinam in the USA for the treatment of both complicated and uncomplicated UTI, this observational study investigated the activity of mecillinam against Enterobacterales isolates from patients with UTI in the USA, collected during 2019.
Methods. This study evaluated the activity of mecillinam and other antimicrobial agents against 1075 selected Enterobacterales clinical isolates collected from patients with UTI in the USA during 2019. Antibiotic activity (minimum inhibitory concentration [MIC]) was determined by Clinical & Laboratory Standards Institute (CLSI) agar dilution methodology, and susceptibility was interpreted according to CLSI guidelines.
Results. Among the selected 1075 isolates, producers of extended-spectrum beta-lactamase (ESBL) represented 9.6% of Escherichia coli and 50% of Klebsiella pneumoniae. Ninety-five percent of the isolates tested were susceptible to mecillinam ( Table  1). The MIC 50 and MIC 90 values for mecillinam were 0.25 and 2 µg/mL, respectively. Fosfomycin MIC 50 and MIC 90 were 1 and 32 µg/mL, respectively (97.6% of isolates were susceptible). Mecillinam showed the lowest MIC 90 value of all single antibiotics tested. The highest MIC 90 was 128 µg/mL for both nitrofurantoin and cefotaxime. The lowest percentage of resistance was obtained with fosfomycin (1.7%), followed by mecillinam (4%).

Conclusion.
Overall, mecillinam showed excellent activity and a comparable resistance profile to fosfomycin. Resistance rates to ceftazidime, cefotaxime, ciprofloxacin and trimethoprim/sulfamethoxazole of greater than 20% are concerning due to the frequent use of these antibiotics in clinical practice to treat UTIs. Taken together, these data demonstrate that mecillinam has promising activity, with low resistance observed in Enterobacterales species causing UTIs in the USA. Clinical development of mecillinam in the USA is ongoing.
Disclosures Methods. A total of 10 isolates recovered from patients with infections in 2018 were tested for antimicrobial susceptibility to TBP and ETP in the absence or presence of 1%, 5%, or 10% BPS (Infasurf; ONY Biotech). These isolates included the following species: C. freundii, E. cloacae, E. coli, H. influenzae, H. parainfluenzae, K. pneumoniae, methicillin-susceptible S. aureus, M. catarrhalis, S. pneumoniae, and S. pyogenes. Isolates were tested with the appropriate broth microdilution method for each organism as specified by CLSI. For most organisms, MICs were determined in cation-adjusted Mueller-Hinton broth (CAMHB). CAMHB was supplemented with 2.5-5% lysed horse blood for streptococci and Haemophilus Test Medium broth for Haemophilus spp. Daptomycin (DAP) was tested against S. aureus ATCC 29213 as a positive control.
Results. All isolates displayed TBP MIC values ranging from ≤0.004 to 0.06 mg/L in media without BPS. There were no observed MIC increases >2-fold in the presence of BPS. 4 of the 10 isolates displayed slightly higher (≥4-fold) ETP than TBP MIC values. The ETP MIC values ranged from 0.015-0.25 mg/L in media without BPS. Similarly, there were no observed instances of a >2-fold shift toward lower potency in the presence of BPS. For both TBP and ETP, MIC endpoint values were easily determined, except for in the case of the 2 Haemophilus strains growing in the presence of 5% or 10% BPS. For these conditions, resazurin was added to establish an MIC value.