Significance of "extravascular" protein binding for antimicrobial pharmacodynamics in an in vitro capillary model of infection

Abstract

The effect of protein binding in an "extravascular" space on antimicrobial pharmacodynamics was studied in an in vitro capillary model of infection. Simulated 500-mg oral doses of dicloxacillin (-96% bound) or cephalexin (<5% bound) were administered every 6 h for four doses. A 10-fold-higher dose of dicloxacillin was also studied to determine the effect of drug concentration on Studies in animals (6,19,20), in vitro models (21), and humans (16,22,25) have shown that high serum protein binding significantly affects the kinetics of extravascular drug distribution. High binding to serum proteins (e.g., albumin) in the intravascular space tends to reduce the amount of drug available for diffusion into extravascular spaces (22,25). However, a considerable amount of serum protein also is distributed to the extravascular space; for example, approximately 60% of the total exchangeable albumin in the body is found in the extravascular space, but at lower concentrations than in the intravascular space (17). These proteins can bind a drug and result in higher concentrations of total (i.e., bound plus unbound) drug than those observed in protein-free spaces (2,9,11,19,21).There are few data on the effect of protein binding in an infected extravascular space on antibacterial pharmacodynamics following multiple simulated human doses. Therefore, the pharmacodynamics of simulated doses of a highly protein-bound drug (dicloxacillin) was compared with that of a drug with negligible protein binding (cephalexin) in an in vitro model of infection. The results show that during the early stages of treatment, high protein binding significantly retards the antimicrobial effects of dicloxacillin. MATERIALS AND METHODSDrugs. Cephalexin and dicloxacillin analytical-grade powders were supplied by Eli Lilly & Co., Indianapolis, Ind. Stock solutions were prepared in appropriate amounts of sterile, distilled, deionized water, which was further diluted in Mueller-Hinton broth (MHB) for dose administration. * Corresponding author.Bacteria. Staphylococcus aureus ATCC 25923 was used as the test strain. Susceptibility testing was done by the method of Stratton and Reller (24), with the exception that serumsupplemented medium (MHB-SER) was 75% MHB and 25% human serum. The MIC and MBC for dicloxacillin against this strain in MHB were 0.25 ,g/ml; in MHB-SER, they were 2 and 8 ,ug/ml, respectively. The MIC and MBC for cephalexin against this strain were 4 and 8 ,ug/ml, respectively, in both MHB and MHB-SER.Protein binding. The binding of dicloxacillin in MHB-SER was measured with an Amicon MPS-1 unit with YMT membranes (Amicon Corp., Danvers, Mass.). Dicloxacillin was prepared in MHB-SER, and approximately 1 ml was added to ultrafiltration units. Samples were centrifuged for 20 min at 1,000 x g (swinging bucket) at 37°C. The ultrafiltrate was collected and assayed for dicloxacillin content by agar well diffusion assay using Bacillus subtilis.In vitro capillary model of infection. A two-compartment in vitro model was modified to simulate first-order ...

The Impact of Protein Binding on Antibacterial Activities of Antibiotics is more than Predicted by Considering its Numerical Value Alone: Impact of Preparative and Incubation Methods on Different Pharmacodynamic Endpoints of ß-Lactams, Macrolides, or Fluoroquinolones Against Gram-positive and Gram-negative Bacteria-Part I

“…Kill rates of amoxicillin were much higher in active serum, too, despite its low protein binding. This disconnect between MICs and kill rates is very well in agreement with previously published data [8,20,[40][41][42][43][44][45][46][47]. Wouldn't it be more conclusive to derive PD measures not from static but from dynamic endpoints like kill-rates [65]?…”

“…The bacteriostatic activities of tetracyclines were converted into a rapid and pronounced bactericidal effect by fresh human serum [8]. Others described that bactericidal activities of highly protein bound dicloxacillin were reduced during the initial 6-hour phase of incubation only but not thereafter [40]. MICs of cefditoren, being bound to albumin to 88%, increased more than fourfold in the presence of albumin, but its bactericidal activity was reduced by albumin only and not by 90% human serum [41]; the authors concluded that the extrapolation of antibacterially active drug from the free fraction only results in an underestimation of antimicrobial activity [42,43].…”

Effect of 2',3'-didehydro-3'-deoxythymidine in an in vitro hollow-fiber pharmacodynamic model system correlates with results of dose-ranging clinical studies

“…Several types of in vitro systems have been developed in which cells or bacterial pathogens are grown in a suitable medium and exposed to fluctuating concentrations of drugs which are adjusted to mimic concentrations in serum obtained in humans following single or multiple doses. The results of studies with antibacterial compounds alone or in combination have been consistent with observations in established animal models of infection and have proven useful in the design of subsequent clinical investigations (3,6,7,9).…”

Preclinical Profile of BI 224436, a Novel HIV-1 Non-Catalytic-Site Integrase Inhibitor

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