Protein binding: do we ever learn?

Abstract

Although 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.

Fig. 1.

In vitro mean protein binding of moxifloxacin in Mueller-Hinton broth (MHB), pure serum, and MHB containing various amounts of albumin or serum. The broken line indicates protein binding (PB) achieved in pure serum. PB was corrected for nonspecific membrane binding during ultrafiltration. Data are presented as means plus standard deviations (SD) (error bars) (n = 6). The albumin was fatty acid free. (Reprinted from reference with permission of the publisher.)

Fig. 2.

In vitro mean protein binding on different media. In vitro mean protein binding of ceftriaxone (CRO) and ertapenem (ERT) in Todd-Hewitt broth (THB), THB with bovine serum albumin (BSA) at a protein concentration of 40 g/liter, THB with human serum albumin (HSA) at a protein concentration of 40 g/liter, pooled adult bovine serum (ABS), and pooled human plasma (HP). The HSA was fatty acid free. (Reprinted from reference with permission.)