PT - JOURNAL ARTICLE AU - Andy Pike AU - Barry Jones AU - Roshini Markandu AU - Daniel O'Neill TI - <strong>Impact of inter-individual differences in plasma fraction unbound on the pharmacokinetics of a novel Syk kinase inhibitor in Beagle dogs </strong> AID - 10.1124/dmd.121.000409 DP - 2021 Jan 01 TA - Drug Metabolism and Disposition PG - DMD-AR-2021-000409 4099 - http://dmd.aspetjournals.org/content/early/2021/06/16/dmd.121.000409.short 4100 - http://dmd.aspetjournals.org/content/early/2021/06/16/dmd.121.000409.full AB - Inconsistencies in pharmacokinetic parameters between individual animals in preclinical studies are a common occurrence. Often such differences between animals are simply accepted as experimental variability, rather than indications of specific differences in animal phenotype which could lead to a different interpretation of the data. The fraction unbound in plasma is one factor influencing pharmacokinetic parameters and is typically determined using pooled plasma from multiple animals making the assumption that there is limited population variance. However, this assumption is not often tested and may not hold true if there are polymorphisms affecting binding or variation in the concentrations of individual plasma proteins which could give rise to different fraction unbound phenotypes in individual animals. During profiling of a novel Syk inhibitor, AZ8399, striking inter-individual differences in total plasma clearance and volume of distribution were observed between dogs consistent with differences in fraction unbound between animals. Determination of the fraction unbound showed an ~5 fold difference in fraction unbound between the animals in the study. Broader analysis of individual dogs across a colony demonstrated a correlation between individual animal fraction unbound with total plasma clearance and volume of distribution. The concentration of the common drug binding proteins albumin and a1-acid glycoprotein in plasma were determined and a1-acid glycoprotein levels found to correlate with fraction unbound. Finally, single nucleotide polymorphisms were identified at c502 and c522 of exon 5 of the dog a1-acid glycoprotein gene that may be correlated to the a1-acid glycoprotein concentration phenotype observed. Significance Statement The current work demonstrates the potential for significant inter-individual differences in plasma fraction unbound in beagle dogs and goes on to examine the underlying cause for the compound described. The findings suggest that the application of a population mean value of fraction unbound generated from a pooled sample may not always be appropriate and could introduce significant errors in scaling of in vitro clearance values, PBPK understanding and interpretation of PKPD or toxicokinetic data in the context of unbound concentrations.