Predicting Antimicrobial Activity at the Target Site: Pharmacokinetic/Pharmacodynamic Indices versus Time-Kill Approaches

Antibiotic dosing strategies are generally based on systemic drug concentrations. However, drug concentrations at the infection site drive antimicrobial effect, and efficacy predictions and dosing strategies should be based on these concentrations. We set out to review different translational pharmacokinetic-pharmacodynamic (PK/PD) approaches from a target site perspective. The most common approach involves calculating the probability of attaining animal-derived PK/PD index targets, which link PK parameters to antimicrobial susceptibility measures. This approach is time efficient but ignores some aspects of the shape of the PK profile and inter-species differences in drug clearance and distribution, and provides no information on the PD time-course. Time–kill curves, in contrast, depict bacterial response over time. In vitro dynamic time–kill setups allow for the evaluation of bacterial response to clinical PK profiles, but are not representative of the infection site environment. The translational value of in vivo time–kill experiments, conversely, is limited from a PK perspective. Computational PK/PD models, especially when developed using both in vitro and in vivo data and coupled to target site PK models, can bridge translational gaps in both PK and PD. Ultimately, clinical PK and experimental and computational tools should be combined to tailor antibiotic treatment strategies to the site of infection.

Pharmacokinetics/Pharmacodynamics Evaluation of Flomoxef against Extended-Spectrum Beta-Lactamase-Producing Escherichia coli In Vitro and In Vivo in a Murine Thigh Infection Model

PURPOSE

Although flomoxef (FMOX) has attracted substantial attention as an antibiotic against extended-spectrum beta-lactamase-producing Escherichia coli (ESBL-producing E. coli), the pharmacokinetics/pharmacodynamics (PK/PD) characteristics of FMOX against ESBL-producing E. coli is unclear. The aim of this study was to determine the PK/PD index of FMOX against ESBL-producing E. coli.

METHODS

In vitro time-kill curve studies and in vivo PK/PD experiments were carried out.

RESULTS

Time-kill curves exhibited a unique bactericidal activity: time-dependent activity at low concentrations and concentration-dependent activity at high concentrations. In neutropenic murine thigh infection experiments, the antibacterial activity of FMOX correlated with the time that the free drug concentration remaining above the minimum inhibitory concentration (MIC) (fT>MIC) and the ratio of the area under the free drug concentration-time curve for a 24 h period to the MIC (fAUC₂₄/MIC). However, the burden of ESBL producing E. coli significantly reduced when the time intervals for administration were shorter among three dosage regimens with same magnitude of fAUC₂₄/MIC, indicating that fT>MIC is significant PK/PD index. The target value of fT>MIC for 1 log₁₀ kill reduction was 35.1%.

CONCLUSIONS

fT>MIC is the most significant PK/PD index of FMOX against ESBL-producing E. coli and its target value is ≥ 40%.

Antibacterial Activity of Arbutus pavarii Pamp against Methicillin-Resistant Staphylococcus aureus (MRSA) and UHPLC-MS/MS Profile of the Bioactive Fraction

Arbutus pavarii Pamp is a medicinal plant commonly used by local tribes in East Libya for the treatment of many diseases, such as gastritis, renal infections, cancer and kidney diseases. In this study, the antibacterial activity of the leaf and stem bark extracts of the plant against methicillin-resistant Staphylococcus aureus (MRSA), as well as the metabolite profiles of the bioactive fractions, was investigated. The antibacterial activity was determined by disc diffusion method, minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC), while the microbial reduction by the bioactive fraction was evaluated using time-kill test. The bioactive fraction was further subjected to ultrahigh-performance liquid chromatography-mass spectrometry (UHPLC-ESI-MS/MS) analysis to putatively identify the chemical constituents contained therein. All the extracts and fractions showed different levels of antibacterial activity on the tested MRSA strains. The highest total antibacterial activity, i.e., 4007.6 mL/g, was exhibited by the crude leaf methanolic extract. However, the ethyl acetate fraction of the leaf showed moderate to significant antibacterial activity against MRSA at low MIC (0.08-1.25 mg/mL). Metabolite profiling of this fraction using UHPLC-ESI-MS/MS resulted in the putative identification of 28 compounds, which included phenolic acids, flavan-3-ols and flavonols. The results of this study showed that the ethyl acetate fraction of Arbutus pavarii leaf possessed potential antibacterial activity against MRSA and hence can be further explored for pharmaceutical applications as a natural antibacterial agent.