Insights into interspecies protein binding variability using clindamycin as an example.

Background: In the preclinical development of new drugs, animal models are often employed to predict their efficacy in humans, relying on translational pharmacokinetic/pharmacodynamic (PK/PD) studies. We performed in vitro experiments focusing on the comparison of plasma protein binding (PPB) and bacterial growth dynamics of clindamycin, a commonly used antimicrobial agent, across a range of drug concentrations and plasma environments.

Methods: Human, bovine and rat plasma were used for determining PPB of clindamycin at various antibiotic concentrations in buffer and media containing 20% to 70% plasma or pure plasma using ultrafiltration (UF) and equilibrium dialysis (ED). Also bacterial growth and time-kill assays were performed in Mueller-Hinton broth (MHB) containing various percentages of plasma.

Results: Protein binding of clindamycin correlated well between UF and ED. Notably, clindamycin exhibited substantially lower protein binding to rat plasma compared with human and bovine plasma. Staphylococcus aureus growth was significantly reduced in 70% human, bovine, and rat plasma after 4, 8 and 24 h compared with standard MHB. Time-kill data demonstrated that bacterial counts at both 20% and 70% plasmas were less when compared with MHB at drug concentrations lower than MIC after 4 and 8 h of incubation. For rat plasma, the difference was maintained over 24 h of incubation. Furthermore, a complete bacterial killing at 16 mg/L was observed after 24 h in 20% and 70% human and bovine plasma, but not for rat plasma.

Conclusions: Recognizing interspecies differences in PB might be essential for optimizing the translational relevance of preclinical studies.