Biophysical methods to study the binding of oritavancin, a lipoglycopeptide, to serum protein are confounded by nonspecific drug adsorption to labware surfaces. We assessed oritavancin binding to serum from mouse, rat, dog, and human by a microbiological growth-based method under conditions that allow near-quantitative drug recovery. Protein binding was similar across species, ranging from 81.9% in human serum to 87.1% in dog serum. These estimates support the translation of oritavancin exposure from nonclinical studies to humans.Estimates of serum protein binding are essential to translate drug exposure from nonclinical species to humans during assessments of toxicology, pharmacokinetics, and pharmacodynamics since the free fraction dictates drug activity (3,7,17,18). Recent evidence supports the concept of an "active fraction" that offers insight into the pharmacodynamic behavior of highly protein-bound drugs, such as daptomycin (20).Oritavancin is a late-stage investigational lipoglycopeptide under study for treatment of serious Gram-positive infections (6). Nonspecific binding of oritavancin to labware surfaces (1, 2) and to dialysis membranes has called into question the accuracy of previous oritavancin human serum binding estimates (85.7% to 89.9% [16]). We therefore used conditions that minimize nonspecific oritavancin binding (4, 5) to estimate its binding to serum by a single in vitro methodology for three nonclinical species (mouse, rat, and dog) and humans. Protein binding estimates were derived from serum-induced increases in oritavancin MICs (9, 10, 21). To control for any impact of serum components on bacterial growth and antibiotic activity, oritavancin activity in serum was compared to its activity in serum ultrafiltrate, which is devoid of albumin, the protein responsible for the majority of oritavancin serum binding (23). The method was benchmarked using daptomycin and ceftriaxone (8,18,22 Pooled serum from humans, mice, and rats was from Equitech-Bio (Kerrville, TX); pooled serum from beagle dogs was from Bioreclamation (Liverpool, NY). Serum ultrafiltrate was prepared using Centricon Plus-50 ultrafilters (Millipore, Billerica, MA), whose molecular mass cutoff (50 kDa) excludes albumin. MICs against Staphylococcus aureus ATCC 29213 were determined by broth microdilution (4) using arithmetic drug dilutions in 95% serum or 95% serum ultrafiltrate, each supplemented with 5% cation-adjusted Mueller-Hinton broth (CAMHB). Serum protein binding for each drug was calculated using the following formula: % bound ϭ (1 Ϫ [mean MIC in serum ultrafiltrate/mean MIC in serum]) ϫ 100.The MICs for each condition, serum source, and test agent were precise (Table 1), with a mean coefficient of variation of 17%. MICs as determined under CLSI M7-A8 conditions (Table 1) (4) were within the quality control ranges (5).Increases in the oritavancin MICs in serum compared to its MICs in serum ultrafiltrate, by species, were similar across species (5.5-to 7.8-fold) ( Table 2). Such shifts yielded similar mean values of or...