Effect of serum on the in vitro activities of 11 broad-spectrum antibiotics

Abstract

We evaluated the effect of serum on the in vitro activities of 11 antimicrobial agents against gram-negative isolates obtained from 100 patients with nosocomial bacteremia. The test organisms included 25 strains of Pseudomonas aeruginosa and 75 strains of the family Enterobacteriaceae. MICs were determined by broth microdilution with Mueller-Hinton broth alone or supplemented with 25 or 50% pooled, heat-inactivated human serum (25S or 50S, respectively). Among the antibiotics evaluated, the protein binding ranged from 9 to 95%. The antibiotics tested and their MICs for 90% of the strains tested in 50S included ciprofloxacin (0.12 ,Lg/mi), ceftazidime (1 ,ug/ml), imipenem (1 ,ug/ml), aztreonam (4 ,ug/ml), cefpirome (4 ,ug/ml), cefotaxime (16 ,ug/ml), cefoperazone (16 ,Lg/ml), desacetylcefotaxime plus cefotaxime (32 ,ug/ml), ceftriaxone (>32 tig/ml), ticarcillin (128 ,ug/ml), and desacetylcefotaxime (>128 ,ug/ml). MICs for 90% of the strains tested were calculated with 95% confidence intervals to show the precision of the MICs for these strains. With the exceptions of ceftriaxone (>95% protein bound) and cefoperazone (90% protein bound), serum had no significant effect on the in vitro activities of various agents. A fourfold-or-greater increase in the MIC of ceftriaxone was observed in 45 of 100 isolates with 50S and in 30 of 100 isolates with 25S. With cefoperazone, 17 of 100 isolates demonstrated more than 2 twofold dilution increases in 50S. Testing of antibiotics which were less protein bound illustrated minor effects primarily with members of the Enterobacteriaceae. The presence of serum did not adversely affect the in vitro activities of broad-spectrum agents against these nosocomial isolates.A reversible, noncovalent binding of antimicrobial agents to human proteins has been described for penicillins (4) and cephalosporins (4-6). For the former, increases in MICs were noted in the presence of serum or albumin when tested against both Staphylococcus and Streptococcus species (4). Similar studies using ceforanide (98% protein bound) showed a protein-binding effect compared with less proteinbound antibiotics (2). More recently, Jones and Barry demonstrated a protein-binding effect with a broad-spectrum cephalosporin, ceftriaxone, on the MICs for Staphylococcus aureus and selected gram-negative isolates, in which an increase in the MICs resulted in significant susceptibility category changes (5). With the addition of serum, antibiotic resistance increased in 53% of 34 gram-negative strains tested against ceftriaxone (5). In contrast, Leggett and Craig demonstrated that the MICs of highly protein-bound antibiotics, such as ceftriaxone (95% protein bound) and cefoperazone (90% protein bound), were enhanced in a linear fashion in the presence of increasing concentrations of serum when tested against Pseudomonas species (6). Minor effects with less protein-bound antibiotics, such as moxalactam (64% protein bound) and ceftizoxime (30% protein bound), were seen. Some newer cephalosporins with similar spectra and vari...

Potentiation of antibacterial activity of azithromycin and other macrolides by normal human serum

“…In contrast, roxithromycin is highly bound by whole serum, and the presence of serum in the MIC test increased the MIC against S. aureus fivefold at pH 7.2. Increased MICs in serum-supplemented broth have been reported previously (16a) for roxithromycin against grampositive organisms and for other highly protein bound antibiotics (5,16,24,32). However, roxithromycin decreases the MIC for E. coli by two-to fourfold in serum ( Table 1).…”

Importance of Relating Efficacy Measures to Unbound Drug Concentrations for Anti-Infective Agents

“…In vitro, the impact of protein binding on antimicrobial activity is often investigated through determination of the MIC, time-kill curves, and cell culture assays (13,16). Numerous in vitro studies have been published, which evaluated the impact of protein binding on antimicrobials (17)(18)(19)(20)(21)(22)(23)(24)(25)(26)(27)(28), antivirals (29,30), and antifungals (31,32) by using protein supplements and/or serum to mimic in vivo conditions. In the majority of studies, free drug concentrations are not measured directly in the experimental setting, and the extent of protein binding is accounted for by the using binding values reported in the literature.…”

Effect of Ertapenem Protein Binding on Killing of Bacteria

“…Some investigators have disputed the importance of protein binding except with drugs that exhibit very high binding levels (17). Others have asserted that only free-drug concentrations are important and that pharmacodynamic activity is determined solely by the free-drug concentration (3,5,6).…”