The impact of serum protein binding on bacterial killing of minocycline

Objectives: Minocycline is increasingly used clinically for treating infections due to multidrug resistant bacteria. We previously reported that the serum protein binding of minocycline atypically correlated with total concentration using microdialysis, but the therapeutic implications of this finding remained unclear. The objective of this study was to ascertain the functional impact of serum protein binding on bacterial killing. Methods: Time-kill experiments using 4 strains of Acinetobacter baumannii were conducted comparing the activity of minocycline in mouse serum (50 mg/L) and 50% cation-adjusted Mueller-Hinton broth (CA-MHB) (4 mg/L). As a control, similar experiments were also conducted for a clinically achievable levofloxacin concentration (4 mg/L) in serum and 50% CA-MHB (2 mg/L). Serial samples were collected in duplicate over 6 hours, and bacterial burden was determined by quantitative culture. Results: Minocycline exhibited concentration-dependent bactericidal activity against the reference strain in mouse and human serum. Despite using approximately 10× the peak concentration associated with clinical dosing, only moderate bacterial killing was observed. All the minocycline killing profiles in serum were inferior to those observed in CA-MHB. In contrast, the reduction in bactericidal activity seen with levofloxacin was less dramatic. Conclusion: Antimicrobial activity of minocycline was dramatically reduced in the presence of serum, which corroborated with our atypical serum protein binding findings. If validated, these results implied dose escalation might not the best approach to improve the clinical efficacy of minocycline for bacteremia. Future investigations will focus on the specificity and mechanism(s) of minocycline protein binding.