Quantification of Fosfomycin in Combination with Nine Antibiotics in Human Plasma and Cation-Adjusted Mueller-Hinton II Broth via LCMS

Fosfomycin-based combination therapy has emerged as an attractive option in our armamentarium due to its synergistic activity against carbapenem-resistant Gram-negative bacteria (CRGNB). The ability to simultaneously measure fosfomycin and other antibiotic drug levels will support in vitro and clinical investigations to develop rational antibiotic combination dosing regimens against CRGNB infections. We developed an analytical assay to measure fosfomycin with nine important antibiotics in human plasma and cation-adjusted Mueller–Hinton II broth (CAMHB). We employed a liquid-chromatography tandem mass spectrometry method and validated the method based on accuracy, precision, matrix effect, limit-of-detection, limit-of-quantification, specificity, carryover, and short-term and long-term stability on U.S. Food & Drug Administration (FDA) guidelines. Assay feasibility was assessed in a pilot clinical study in four patients on antibiotic combination therapy. Simultaneous quantification of fosfomycin, levofloxacin, meropenem, doripenem, aztreonam, piperacillin/tazobactam, ceftolozane/tazobactam, ceftazidime/avibactam, cefepime, and tigecycline in plasma and CAMHB were achieved within 4.5 min. Precision, accuracy, specificity, and carryover were within FDA guidelines. Fosfomycin combined with any of the nine antibiotics were stable in plasma and CAMHB up to 4 weeks at −80 °C. The assay identified and quantified the respective antibiotics administered in the four subjects. Our assay can be a valuable tool for in vitro and clinical applications.

Fosfomycin single- and multiple-dose pharmacokinetics in patients undergoing prolonged intermittent renal replacement therapy

BACKGROUND

Fosfomycin is used increasingly in the treatment of MDR bacteria. It is eliminated by renal excretion, but data regarding dosing recommendations for patients undergoing modern means of renal replacement therapies are scarce.

OBJECTIVES

Evaluation of the pharmacokinetics (PK) of fosfomycin in patients undergoing prolonged intermittent renal replacement therapy (PIRRT) to guide dosing recommendations.

METHODS

Fosfomycin was given in 11 (7 female) patients with severe infections undergoing PIRRT. Plasma levels were measured at several timepoints on the first day of fosfomycin therapy, as well as 5-6 days into therapy, before and after the dialyser, to calculate its clearance. Fosfomycin was measured in the collected spent dialysate.

RESULTS

The median (IQR) plasma dialyser clearance for fosfomycin was 183.4 (156.9-214.9) mL/min, eliminating a total amount of 8834 (4556-10 440) mg of fosfomycin, i.e. 73.9% (45.3%-93.5%) of the initial dose. During PIRRT, the fosfomycin half-life was 2.5 (2.2-3.4) h. Data from multiple-dose PK showed an increase in fosfomycin Cmax from 266.8 (166.3-438.1) to 926.1 (446.8-1168.0) mg/L and AUC0-14 from 2540.5 (1815.2-3644.3) to 6714 (4060.6-10612.6) mg·h/L. Dialysis intensity during the study was 1.5 L/h. T>MIC was 100% in all patients.

CONCLUSIONS

Patients undergoing PIRRT experience significant fosfomycin elimination, requiring a dose of 5 g/8 h to reach adequate plasma levels. However, drug accumulation may occur, depending on dialysis frequency and intensity.

Elimination of fosfomycin during dialysis with the Genius system in septic patients

To assess fosfomycin (FOS) elimination in patients with sepsis and acute kidney injury (AKI) undergoing slow-extended daily dialysis (SLEDD) with the Genius system in a prospective observational study. After ethics committee approval ten patients with sepsis and AKI stage 3 underwent daily SLEDD sessions of eight hours. FOS was applied i.v. at doses of 3 × 5 g per day. FOS serum levels were measured pre- and post hemofilter before, during, and after SLEDD sessions, and instantaneous clearance was calculated. In five of the patients, we analyzed FOS levels after the first dose, in the other five patients serum levels were measured during ongoing therapy. FOS was eliminated rapidly via the hemofilter. FOS clearance decreased from 152 ± 10 mL/min (start of SLEED session) to 43 ± 38 mL/min (end of SLEDD session). In 3/5 first-dose patients after 4–6 h of SLEDD the FOS serum level fell below the EUCAST breakpoint of 32 mg/L for Enterobacterales and Staphylococcus species. In all patients with ongoing fosfomycin therapy serum levels were high and above the breakpoint at all times. FOS toxicity or adverse effects were not observed. FOS serum concentrations exhibit wide variability in critically ill patients with sepsis and AKI. FOS is eliminated rapidly during SLEDD. A loading dose of 5 g is not sufficient to achieve serum levels above the EUCAST breakpoint for common bacteria in all patients, considering that T > MIC > 70% of the dosing interval indicates sufficient plasma levels. We thus recommend a loading dose of 8 g followed by a maintenance dose of 5 g after a SLEDD session in anuric patients. We strongly recommend therapeutic drug monitoring of FOS levels in critically ill patients with AKI and dialysis therapy.

Deciphering pharmacokinetics and pharmacodynamics of fosfomycin

Fosfomycin, a low molecular weight and hydrophilic drug with negligible protein binding, is eliminated almost exclusively by glomerular filtration, whose clearance is subject to patient renal function. The volume of distribution approximates to the extracellular body water (about 0.3 L/Kg) in healthy volunteers, but it is increased in critically ill patients with bacterial infections. Fosfomycin presents a high ability to distribute into many tissues, including inflamed tissues and abscess fluids. Based on PK/PD analysis and Monte Carlo simulations, we have evaluated different fosfomycin dosing regimen to optimize the treatment of septic patients due to Enterobacterales and Pseudomonas aeruginosa. As PK/PD targets, we selected %T>MIC > 70% for all pathogens, and AUC24/MIC > 24 and AUC24/MIC > 15 for net stasis of Enterobacterales and P. aeruginosa, respectively. Pharmacokinetic parameters in critically ill patients were obtained from the literature. Several dosing regimens were studied in patients with normal renal function: fosfomycin 2-8 g given every 6-12 hours, infused over 30 minutes- 24 hours. At the susceptibility EUCAST breakpoint for Enterobacterales and Staphylococcus spp. (MIC ≤ 32 mg/L), fosfomycin 4 g/8h or higher infused over 30 minutes achieved a probability of target attainment (PTA) > 90%, based in both %T>MIC and AUC24/MIC. For MIC of 64 mg/L, fosfomycin 6 g/6h in 30-minute infusion and 8 g/ 8h in 30-minute and 6 hours infusions also achieved PTA values higher than 90%. No fosfomycin monotherapy regimen was able to achieve PK/PD targets related to antimicrobial efficacy for P. aeruginosa with MICs of 256-512 mg/L.

Intravenous fosfomycin for the treatment of multidrug-resistant pathogens: what is the evidence on dosing regimens?

INTRODUCTION

The intravenous (IV) formulation of fosfomycin has been re-introduced in clinical practice mainly to overcome treatment failures against multidrug-resistant (MDR) bacteria. Appropriate dosing schedules of the IV formulation have not yet been established. Areas covered: The mechanism of action and resistance development, commercial IV formulations, pharmacokinetic/pharmacodynamic (PK/PD) properties, IV dosing regimens for the treatment of MDR infections along with efficacy and safety issues were reviewed. Data regarding specific MDR pathogens, daily doses and patients' outcomes, gaps in the current literature, and in progress research agenda are presented. Expert opinion: The doses of fosfomycin IV range between 12 and 24 grams/day depending on the severity of infection. The efficacy and safety of the commonly administered doses have been shown mainly in observational non-comparative trials. The optimal dose ensuring maximal efficacy with minimal toxicity along with the most appropriate co-administered antibiotic(s) need further evaluation. The pharmacokinetic/pharmacodynamic parameter associated with maximum efficacy has not yet been established, although, the ratio of the area under the concentration-time curve (AUC) for the free unbound fraction of fosfomycin versus the MIC (fAUC/MIC) may be linked to optimal treatment. RCTs and other comparative studies are underway to address gaps of knowledge in adult patients and neonates.

Pharmacokinetic and Pharmacodynamic Considerations of Antibiotics of Last Resort in Treating Gram-Negative Infections in Adult Critically Ill Patients

PURPOSE OF REVIEW

We provide an overview of antimicrobials that are considered last resort for the treatment of resistant gram-negative infections in adult critically ill patients. The role in therapy, pharmacodynamic (PD) goals, and pharmacokinetic (PK) changes in critical illness for aminoglycosides, polymyxins, tigecycline, fosfomycin, and fluoroquinolones are summarized.

RECENT FINDINGS

Altered PK in septic patients in the intensive care unit (ICU) is observed with many of our agents of last resort. Based on the available literature, dosage adjustments may be required to optimize PK parameters and meet PD targets for most effective bacterial killing. Data is limited, studies are conducted in heterogeneous patient populations, and conclusions are frequently conflicting. Strategic dosing regimens such as high-dose extended interval dosing of aminoglycosides or loading doses with colistin and polymyxin B are examples of ways to optimize antibiotic PK in critically ill patients. Benefits of these strategies must be balanced with risks of increased toxicity. Patients with resistant gram-negative infections may present with septic shock in the ICU. Sepsis can significantly alter the PK of antibiotics and require dosage adjustments to attain optimal drug levels. An understanding of PK and PD properties of these agents of last resort will help to maximize therapeutic efficacy while minimizing toxic effects.