The control and measurement of plasma pH in equilibrium dialysis; influence on drug plasma protein binding DMD # 36988 2 RUNNING TITLE: Plasma pH control in equilibrium dialysis

Abstract

Abstract Past publications have highlighted the influence of post-dialysis plasma pH on the measured fraction unbound in plasma (fup). There is disparity in the industry as to which of two main methods is more suitable for controlling post-dialysis plasma pH; the use of either a stronger buffer or a CO 2 atmosphere for the incubation. In the current study, it has been found that 10% CO 2 could be too high for the buffering capacities of both 100 mM sodium phosphate (pH 7.40 decreased to pH 6.90 after 6 h incubation) and plasma (decreased below pH 7.40 after 6 h incubation). In order to provide appropriate control over the post-dialysis plasma pH, for a range of species, it is proposed that a standard phosphate buffer strength (100 mM) and pH (7.40) in combination with a 5% CO 2 atmosphere is utilised for equilibrium dialysis. Furthermore, statistically significant differences in fup values obtained with a pH difference of less than 0.32 pH unit have been demonstrated. An acceptance range for post-dialysis plasma pH in routine in vitro fup screening assays of pH 7.40 ± 0.10 is recommended