127. Living on the Edge: The Impact of MIC Distributions on Empiric Antibiotic Selection

Background Due to variability in the precision of an MIC, concern may exist in optimizing PK/PD using standard doses when the MIC is at the susceptibility breakpoint (SBP). This is notable when treating infections in critically ill patients. Evaluating MIC distributions among commonly used antibiotics and accounting for isolates at the SBP represents an additional enhancement to inform empiric therapy. The aim of the study was to evaluate antibiotic susceptibility for commonly used β-lactams against Pseudomonas aeruginosa (PA) in a syndromic antibiogram, incorporating MIC distribution. Methods 20 US institutions submitted yearly up to 250 consecutive targeted Gram-negative pathogens from hospitalized patients as part of the Study for Monitoring Antimicrobial Resistance Trends (SMART) in 2016-2019. MICs were determined by broth microdilution and interpreted using 2021 CLSI breakpoints. The syndromic antibiogram included PA from a blood or respiratory source based on patient location. Based on CLSI guidance, an empiric antibiotic susceptibility threshold of ≥ 90% was deemed optimal. Results 2,500 PA blood (n=680) and respiratory (n=1,820) isolates were evaluated; piperacillin/tazobactam (P/T), cefepime (FEP), meropenem (MEM), and ceftolozane/tazobactam (C/T) susceptibilities were 69.6%, 74.2%, 75.3%, and 95%, respectively (Figure 1). Isolates with MICs at the SBP were observed in 12.1%, 18.7%, 7.5%, and 6.5% for P/T, FEP, MEM, and C/T, respectively. Susceptibilities were lower when stratified by ICU, 64.8%, 71.2%, 70.7%, and 93.7% for P/T, FEP, MEM, and C/T, respectively with a similar frequency of SBP isolates (Figure 2). Figure 1. Syndromic antibiogram evaluating P. aeruginosa blood and respiratory isolates. Figure 2. Syndromic antibiogram evaluating Pseudomonas aeruginosa blood and respiratory isolates stratified by ICU. *MIC breakpoints used to determine susceptibility included: P/T MIC ≤ 16/4 µg/ml, FEP ≤ 8 µg/ml, MEM ≤ 2 µg/ml, C/T ≤ 4 µg/ml Conclusion Our analysis demonstrated that first line antipseudomonal agents, P/T and FEP, have susceptibility rates lower than the CLSI recommended threshold. A significant portion of the MICs within the susceptible range are at the SBP. Due to the frequency of baseline resistance and challenge in achieving adequate PK/PD in critically ill patients, clinicians may be concerned with relying on certain antibiotics when the MIC is at the SBP. Antimicrobial stewardship programs should consider incorporating MIC distributions into syndromic antibiograms to better inform empiric therapy recommendations. Disclosures Karri A. Bauer, PharmD, Merck & Co., Inc. (Employee, Shareholder) Levita K. Hidayat, PharmD BCIDP, Merck & Co., Inc. (Employee, Shareholder) Kenneth Klinker, PharmD, Merck & Co., Inc. (Employee, Shareholder) Mary Motyl, PhD, Merck & Co., Inc. (Employee, Shareholder) C. Andrew DeRyke, PharmD, Merck & Co., Inc. (Employee, Shareholder)

1080. Relebactam Increases Imipenem Activity Against Imipenem-Nonsusceptible and -Susceptible Pseudomonas aeruginosa and Enterobacterales: Assessment of Isolates from RESTORE-IMI 2

Background Relebactam (REL) inhibits class A and C β-lactamases (BLs) and is approved in the US in the combination imipenem/cilastatin/REL (IMI/REL) for hospital acquired bacterial pneumonia (HABP) and ventilator associated bacterial pneumonia (VABP), and also in patients with limited treatment options for complicated urinary tract infections and complicated intraabdominal infections. The objective of this study was to evaluate the potentiation of imipenem (IMI) by REL in baseline respiratory isolates from the recently completed Phase 3 RESTORE-IMI 2 study that demonstrated efficacy and safety of IMI/REL in the treatment of patients with HABP/VABP. Methods Baseline lower respiratory tract (LRT) isolates were evaluated for IMI MICs in the presence and absence of REL using broth microdilution and CLSI interpretive criteria. All Pseudomonas aeruginosa and Enterobacterales for which IMI/REL is either indicated or the MIC90 is less than or equal to the susceptibility breakpoint were evaluated. Results Summary statistics and the MIC distribution for P. aeruginosa are shown in the figure. For P. aeruginosa, REL reduces the IMI mode MIC of IMI-nonsusceptible (IMI-NS) (MIC >2) isolates ≥8-fold (from 16-32 to 2 µg/mL) and that of IMI-susceptible (IMI-S) (MIC ≤2) isolates ≥2-fold (from 1 to ≤0.5 µg/mL). Among Enterobacterales, the IMI mode MIC of IMI-NS (MIC >1) isolates was reduced ≥4-fold (from 2 to ≤0.5 µg/mL). REL enhanced the activity of IMI among IMI-S isolates (MIC ≤1), most notably observed in Enterobacterales species that produce a chromosomal AmpC, increasing the proportion with MIC ≤0.5 µg/mL from 76% to 98%. MIC Distribution and Summary Statistics of RESTORE IMI-2 Isolates Conclusion Among baseline LRT isolates from RESTORE-IMI 2 the potentiation of IMI by REL results in the restoration of susceptibility among IMI-NS P. aeruginosa and Enterobacterales and enhanced IMI activity among IMI-S isolates. This enhanced activity among IMI-S Enterobacterales is most notable among species with reported chromosomal expression of AmpC. This lowering of IMI MICs upon addition of REL contributes to the high probability of target attainment (≥90%) observed following administration of IMI/REL 1.25g every 6 hours, further supporting the IMI/REL efficacy data observed in RESTORE-IMI 2. Disclosures David W. Hilbert, PhD, Merck (Employee) C. Andrew DeRyke, PharmD, Merck & Co., Inc. (Employee, Shareholder) Maria C. Losada, BA, Merck (Employee) Pamela Moise, PharmD, Merck (Employee) Luke F. Chen, MBBS MPH MBA FRACP FSHEA FIDSA, Merck (Employee) Katherine Young, MS, Merck (Employee)

1109. Pharmacokinetics and Exposure of Cefepime in Critically Ill Patients Receiving Extracorporeal Membrane Oxygenation (ECMO)

Background ECMO is a life-saving tool utilized in critically ill patients that require respiratory and/or cardiac support. ECMO may also affect the pharmacokinetics (PK) of certain medications, including some antibiotics. Cefepime is a widely used antibiotic in this population due to its broad spectrum activity but limited data are available to guide dosing in patients requiring ECMO. Methods This was a prospective, single-center study of 6 critically ill adult patients requiring ECMO and receiving cefepime 2g q8h as a 3h infusion. After obtaining informed consent, 4-6 blood samples within the dosing interval were collected to determine cefepime concentrations. Population PK was conducted in Pmetrics using R. Final MAP Bayesian parameter estimates were used to simulate free time above MIC (%ƒT >MIC) for various cefepime dosing regimens. The target pharmacodynamic exposure was 70% fT >MIC. Results Patients were between 31-62 years old; 4/6 (66.7%) were on veno-venous (VV) ECMO and 2 veno-arterial (VA) ECMO. Two patients required continuous venovenous hemodiafiltration (CVVHDF) while the other 4 had a CrCL between 92-199 ml/min. A two compartment model fitted the data better than a one compartment model. Median (range) final population PK parameters were: clearance (CL), 9.8 L/h (7.6-33.1); volume of central compartment (VC ), 6.9 L (4.7-49.8); and intercompartment transfer constants (k12), 2.04 h-1 (1.48-2.29); and k21, 1.49 h-1 (0.75-1.71). The 2g q8h (3h infusion) regimen resulted in target exposure in all patients up to an MIC of 8 mg/L (the susceptibility breakpoint for Pseudomonas), with 5/6 patients achieving this at 16 mg/L. A standard 2g q12h (0.5h infusion) regimen would have resulted in 5/6 patients achieving 70% ƒT >MIC at 8 mg/L and 1/6 at 16 mg/L. Conclusion These are the first data describing cefepime PK and exposure attainment in critically ill patients receiving ECMO. Cefepime 2g q8h (3h infusion) achieved target pharmacodynamic exposure up to the susceptibility breakpoint of 8 mg/L in all 6 patients, including 2 with concomitant CVVHDF. Additional studies are warranted to define cefepime PK in patients on ECMO across a robust range of CrCL to guide dosing. Disclosures David P. Nicolau, PharmD, Abbvie, Cepheid, Merck, Paratek, Pfizer, Wockhardt, Shionogi, Tetraphase (Other Financial or Material Support, I have been a consultant, speakers bureau member, or have received research funding from the above listed companies.) Joseph L. Kuti, PharmD, Allergan (Speaker’s Bureau)BioMérieux (Consultant, Research Grant or Support, Speaker’s Bureau)Contrafect (Scientific Research Study Investigator)GSK (Consultant)Merck (Research Grant or Support)Paratek (Speaker’s Bureau)Roche Diagnostics (Research Grant or Support)Shionogi (Research Grant or Support)Summit (Scientific Research Study Investigator)

65. In Vivo Efficacy of Human Simulated Minocycline (MIN) against Stenotrophomonas maltophilia (STM)

Background The current susceptibility breakpoint for MIN against STM is 4mg/L, yielding >99% of isolates susceptible. Unfortunately, there are limited pre-clinical and clinical data to support this breakpoint for STM. The purpose of this study was to evaluate the efficacy of a MIN human simulated regimen (HSR) against STM across a wide range of MICs in the murine neutropenic thigh model. Methods Clinical STM with modal MIN MICS of 0.25-8mg/L were included. Confirmatory pharmacokinetic (PK) studies were performed in infected neutropenic mice to develop a MIN HSR providing an area under the curve (AUC) and maximum concentration (Cmax) exposure similar to MIN 100mg intravenous (IV) q12h at steady-state based on PK parameters from critically ill adult patients. The murine neutropenic thigh infection model was utilized to examine the antibacterial effects of the confirmed MIN HSR against 17 STM. Both thighs of neutropenic ICR mice were inoculated with bacterial suspensions of 107 colony forming units (CFU)/mL. Two hours after inoculation, the MIN HSR was administered subcutaneously (SC) over 24h. Control mice received normal saline. Efficacy was measured as the change in log10CFU/thigh at 24h compared with 0h controls. Results MIN 22, 10, 14, and 10mg/kg dosed SC at 0, 6, 12, and 18h best recapitulated the human Cmax and AUC profile. Mean ± standard deviation bacterial burden at 0h across all isolates was 6.03±0.32 log10CFU/thigh. Bacterial growth was 1.35±0.68 log10CFU/thigh in 24h controls. Six of 7 isolates (86%) with MIC ≤ 0.5mg/L achieved 1-log kill with MIN HSR (-1.44±1.37 log10CFU/thigh). All STM with MIC ≥ 1mg/L experienced bacterial growth (1.18±0.79 log10CFU/thigh) (Figure). Figure. Efficacy of a minocycline human simulated exposure of 100mg intravenous Q12h in the murine neutropenic thigh model against 17 clinical Stenotrophomonas maltophilia isolates Conclusion These data describe the in vivo efficacy of a MIN HSR with exposures similar to MIN 100mg IV q12h in critically ill adults. Lack of antibacterial activity against STM with MICs ≥ 1mg/L justifies a reassessment of the current susceptibility breakpoint. The study was funded under FDA Contract 75F40120C00171. Disclosures David P. Nicolau, PharmD, Abbvie, Cepheid, Merck, Paratek, Pfizer, Wockhardt, Shionogi, Tetraphase (Other Financial or Material Support, I have been a consultant, speakers bureau member, or have received research funding from the above listed companies.) Joseph L. Kuti, PharmD, Allergan (Speaker’s Bureau)BioMérieux (Consultant, Research Grant or Support, Speaker’s Bureau)Contrafect (Scientific Research Study Investigator)GSK (Consultant)Merck (Research Grant or Support)Paratek (Speaker’s Bureau)Roche Diagnostics (Research Grant or Support)Shionogi (Research Grant or Support)Summit (Scientific Research Study Investigator)

Levofloxacin pharmacodynamics against Stenotrophomonas maltophilia in a neutropenic murine thigh infection model: implications for susceptibility breakpoint revision

Background Levofloxacin displays in vitro activity against Stenotrophomonas maltophilia (STM); however, current susceptibility breakpoints are supported by limited data. We employed the murine neutropenic thigh infection model to assess levofloxacin pharmacodynamics against STM. Methods Twenty-six clinical STM were studied using the neutropenic murine thigh infection model. Human simulated regimens (HSR) of levofloxacin 750 mg q24h were administered over 24 h. Efficacy was measured as the change in log10 cfu/thigh at 24 h compared with 0 h. Composite cfu data were fitted to an Emax model to determine the fAUC/MIC needed for stasis and 1 log10 reduction at 24 h. Monte Carlo simulation was performed to determine PTA. Results Levofloxacin MICs ranged from 0.5–8 mg/L. Mean bacterial burden at 0 h was 6.21 ± 0.20 log10 cfu/thigh. In the 24 h controls, bacterial growth was 1.64 ± 0.66 log10 cfu/thigh. In isolates with levofloxacin MICs ≤1, 2 and ≥4 mg/L, changes in bacterial density following levofloxacin HSR were −1.66 ± 0.89, 0.13 ± 0.97 and 1.54 ± 0.43 log10 cfu/thigh, respectively. The Emax model demonstrated strong agreement between fAUC/MIC and change in bacterial density (R2 = 0.82). The fAUC/MIC exposure needed for stasis and 1 log10 reduction was 39.9 and 54.9, respectively. PTAs for the 1 log10 reduction threshold were 95.8, 72.2, and 26.6% at MICs of 0.5, 1 and 2 mg/L, respectively. Conclusions These are the first data to describe fAUC/MIC thresholds predictive of cfu reductions for levofloxacin against STM. Due to poor in vivo efficacy and PTA at MICs ≥2 mg/L, reassessment of the current susceptibility breakpoint is warranted.

Determination of Susceptibility Breakpoint for Cefquinome against Streptococcus suis in Pigs

Streptococcus suis (S. suis), a zoonotic pathogen, causes severe diseases in both pigs and human beings. Cefquinome can display excellent antibacterial activity against gram-negative and gram-positive bacteria. The aim of this study was to derive an optimal dosage of cefquinome against S. suis with a pharmacokinetic/pharmacodynamic (PK/PD) integration model in the target infection site and to investigate the cutoffs monitoring the changes of resistance. The minimum inhibitory concentration (MIC) distribution of cefquinome against 342 S. suis strains was determined. MIC50 and MIC90 were 0.06 and 0.25 μg/mL, respectively. The wild-type cutoff was calculated as 1 μg/mL. A two-compartmental model was applied to calculate the main pharmacokinetic parameters after 2 mg/kg cefquinome administered intramuscularly. An optimized dosage regimen of 3.08 mg/kg for 2-log10 CFU reduction was proposed by ex vivo PK/PD model of infected swine. The pharmacokinetic-pharmacodynamic cutoff was calculated as 0.06 μg/mL based on PK/PD targets. Based on the clinical effectiveness study of pathogenic MIC isolates, the clinical cutoff was calculated as 0.5 μg/mL. A clinical breakpoint was proposed as 1 μg/mL. In conclusion, the results offer a reference for determining susceptibility breakpoint of cefquinome against S. suis and avoiding resistance emergence by following the optimal dosage regimen.

A novel dosing strategy of ceftolozane/tazobactam in a patient receiving intermittent hemodialysis

We describe a case of a 54 years old male receiving intermittent hemodialysis (iHD) who was found to have P. aeruginosa bacteremia secondary to osteomyelitis of the calcaneus bone. The patient was clinically cured without recurrence using a ceftolozane/tazobactam (CTZ) dosing strategy of 100/50mg every 8 hours (standard dosing) and 1000/500mg thrice weekly following iHD. Utilizing a susceptibility breakpoint of ≤ 4 µg/ml for P. aeruginosa, the T>MIC for standard dosing and the 1000/500mg thrice weekly following iHD regimen were calculated to be 92.7% and 94.1%, respectively. Ceftolozane total body clearance for the standard q 8 h dosing and the 1000/500mg thrice weekly following iHD regimen were calculated to be 0.196 L/h and 0.199 L/h, respectively. To our knowledge, this is the first report to illustrate the administration of CTZ at a dose of 1000/500mg thrice weekly following iHD.

Efficacy of temocillin against MDR Enterobacterales: a retrospective cohort study

Objectives EUCAST recently advised against temocillin use, except for non-serious urinary tract infections (UTI) caused by Escherichia coli, Klebsiella spp. (except Klebsiella aerogenes) and Proteus mirabilis (EKP) treated with a dose of 2 g q8h. We aimed to analyse our practice in the context of a larger temocillin use in France. Patients and methods All ≥3 day temocillin prescriptions from 2016 to 2019 were reviewed, with reference to French recommendations and a susceptibility breakpoint of 8 mg/L. The primary outcome was early clinical failure (antibiotic switch, relapse or death within 10 days after the completion of antibiotic treatment). Results Overall, 153 cases were analysed: 123 cases of UTI (80.4%) and 133 cases of monomicrobial infection with Enterobacterales (86.9%). A total of 160 Enterobacterales were isolated, comprising 108 (67.5%) ESBL producers and 30 (20.7%) non-EKP species. The rate of early clinical failure was 9.2% and was significantly lower for UTI compared with non-UTI (4.9% versus 26.7%, P = 0.001) and for sepsis compared with severe sepsis or septic shock (6.2% versus 25%, P = 0.011). It was not different between 2 g q12h and 2 g q8h doses (10% versus 7.4%, P = 0.81) and between EKP and other Enterobacterales (8.7% versus 14.3%, P = 0.41). Conclusions EUCAST recommendations on urinary isolates seem to be too restrictive. Our data support the efficacy of temocillin at a dose of 2 g q12h to treat patients with non-severe complicated UTI caused by MDR Enterobacterales with an MIC of ≤8 mg/L, whatever the species.

1599. Clinical Outcomes for Patients Treated with Fluoroquinolones for Bacteremia Caused by Enterobacteriaceae Reclassified as Not Susceptible by Updated CLSI Breakpoints

Background Antibiotic resistance remains a pressing public health challenge. Antibiotic susceptibility testing is crucial to identify resistance and predict which antibiotics are most likely to be effective. In vitro minimum inhibitory concentrations (MICs) are interpreted using MIC breakpoints set for the United States by The Clinical and Laboratory Standards Institute (CLSI). In 2019 CLSI updated fluroquinolone (FQ) breakpoints for Enterobacteriaceae. Previously any isolate with an MIC ≤ 1 µg/mL of ciprofloxacin would be considered susceptible but based largely on pharmacokinetic/pharmacodynamic simulations the susceptibility breakpoint was revised to ≤ 0.25 µg/mL. However, the clinical relevance of this decision remains unclear. Methods All cases of Enterobacteriaceae bacteremia with isolates previously considered susceptible but reclassified as resistant (MIC = 1 µg/mL) in adults treated with FQs between 08/01/2018 and 07/31/2019 were identified. Demographics, clinical characteristics and outcomes were compared with an equal number of randomly selected isolates with an automated MIC reported as ≤ 0.5 µg/mL. Available stored isolates with a reported MIC of ≤ 0.5 µg/mL had manual E-testing performed to identify a more precise MIC. Results 29 cases with an MIC = 1 μg/mL were compared with 29 controls with a MIC of ≤ 0.5. Only 3 cases and 1 control received FQs as empiric therapy, the remaining patients in each group were transitioned to FQ after a median of 4 days of other antibiotics. No significant difference was found for predetermined outcomes including 30 day mortality, escalation after starting FQ, length of hospital stay, and readmission in 30 days (see Table). No primary outcome was thought to be related to antibiotic failure. E-testing found no isolates with an MIC = 0.5 μg/mL. Table 1 Conclusion Patients with Enterobacteriaceae bacteremia treated with FQs for isolates reclassified as resistant had similar outcomes to those with lower MICs. While FQs are generally not recommended as first line empiric antibiotics, FQs may still be safe to use as stepdown therapy for isolates with a ciprofloxacin MIC = 1 μg/mL, particularly if the only alternative may be IV antibiotics. A larger study is needed to confirm this. Disclosures Gregory Weston, MD MSCR, Allergan (Grant/Research Support)