Protein Binding: Do We Ever Learn?

Abstract

6Although the influence of protein binding (PB) on antibacterial activity has been reported for many antibiotics and over many years, there is currently no standardization for pharmacodynamic models that account for the impact of protein binding of antimicrobial agents in vitro. This might explain the somewhat contradictory results obtained from different studies. Simple in vitro models which compare the MIC obtained in protein-free standard medium versus a protein-rich medium are prone to methodological pitfalls and may lead to flawed conclusions. Within in vitro test systems, a range of test conditions, including source of protein, concentration of the tested antibiotic, temperature, pH, electrolytes, and supplements may influence the impact of protein binding. As new antibiotics with a high degree of protein binding are in clinical development, attention and action directed toward the optimization and standardization of testing the impact of protein binding on the activity of antibiotics in vitro become even more urgent. In addition, the quantitative relationship between the effects of protein binding in vitro and in vivo needs to be established, since the physiological conditions differ. General recommendations for testing the impact of protein binding in vitro are suggested.Binding to plasma proteins plays a major role in drug therapy as this binding provides a depot for many compounds, affects pharmacokinetics (PK) and pharmacodynamics (PD) of drugs, and may influence the metabolic modification of ligands (34,104). Protein binding (PB) of antibiotics may affect the efficacy of antimicrobial therapy in two ways. First, only the non-protein-bound fraction of a drug in plasma can penetrate into and equilibrate with the extravascular space (13). Penetration into the extravascular space is highly important for antimicrobial therapy, as the majority of bacterial and fungal infections occur in the interstitial fluid of tissues or in other body fluids than blood (113). PB also affects drug clearance from the body. For antibiotics eliminated by tubular secretion or hepatic metabolism, high PB is associated with lowered drug elimination. Additionally, PB negatively correlates with glomerular filtration, since only the free drug is filtered (63). On the other hand, small fluctuations in protein binding usually have no effect on average unbound concentrations, and this is especially the case for drugs with a low extraction ratio (11). Changes in protein binding can also alter the volume of distribution and, hence, the half-life, which may, in turn, modify the time above MIC. Extensive reviews on the impact of PB on pharmacokinetics of drugs are available (12,13,27,29,82,86).The second effect of PB is demonstrated by the impact on the antibacterial activity. Over many decades, various methods were used to show that only the non-protein-bound fraction of an antibiotic is microbiologically active (10,28,70,111). For important antibiotic classes, such as penicillins and quinolones, clear relationships between the percenta...

Mathematical modeling of intraperitoneal drug delivery: simulation of drug distribution in a single tumor nodule

“…Note that the IC 50 values are taken from in vitro analysis and protein-binding in an in vivo setting might influence these values (Zeitlinger et al, 2011). Therefore, we found that when considering different drugs to use for IP chemotherapy, not only transport parameters, but also biological parameters like IC 50 values and protein binding should be taken into account.…”

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

“…Such differences really matter, as the free fraction used as basis for PK/PD calculations could be assumed to be as low as 30% and 48% or as high as 60% and 75%, respectively. These differences are very likely due to methodological differences as reviewed previously [3,59], but the question is which of the published data should be selected for PK/PD calculations and based on which criteria?…”

“…Therapeutic in vivo efficacy of antibacterials, however, is the complex result of a number of factors. On the one hand, protein binding has a strong impact on minimal inhibitory concentrations (MICs) of antibacterials for the pathogen to be treated as well as on pharmacokinetics (PK) and pharmacodynamics (PD) of the agent as the free fraction only crosses pro-as well as eukaryotic membranes thus reaching intracellular targets or distributing between the intra-and extravascular spaces; furthermore, protein binding can alter renal excretion, half-life and volume of distribution thus affecting PK/PD-surrogates [1][2][3][4][5]. On the other hand, serum proteins protect the host from serious sequelae of an infection and/or may increase antibacterial activities.…”

A Simple, Fast, Low Cost, HPLC/UV Validated Method for Determination of Flutamide: Application to Protein Binding Studies

“…However this increase is in opposite direction which is normally described in the literature. 4,18 The presence of a competitive protein binder at lower concentrations, or multiple binding sites with different affinities could explain the observed trend.…”

“…On the other hand analyzing the free fraction of the drug in study samples would be the main issue demanding the sensitive analytical methods. 4 The gas chromatographic determination of flutamide in blood was reported in 1988 by Schulz et al for a limited range of concentration (0.02-0.250 µg/ml). 1 In another study by Manjunath et al, HPLC system was equipped with a radioactive detector and a gradient solvent system was used for radioactive flutamide determination with 18.3 minutes retention time.…”