Changes in the degree of substitution of HES in vivo and their influence on methods for determining HES concentrations in plasma

Abstract

Objective The degree of substitution (DS) of HES describes the average proportion of substituted glucose molecules in a starch molecule. Although no quantitative studies of the in vivo behavior of the DS have been conducted so far, most pharmacokinetic studies to date have measured HES concentrations using the enzymatic method. This method assumes that at any point in time after an infusion, the DS in a serum remains constant and is identical to the DS in the infused solution. In the present study, we examined the changes in the DS of HES 130/0.42 in vivo in an animal model and compared two methods of measuring HES concentrations in plasma (the enzymatic and the o-Toluidine method). Methodology We randomized 22 pigs into 2 groups. After induction of anesthesia, the pigs received 500 ml or 1000 ml of HES 130/0.42 (Tetraspan®). The DS was measured directly after the infusion, then after 30, 60, 120 and 240 minutes. In determining the DS, the hydroxyethyl starch was extracted from the plasma and hydrolyzed with hydrochloric acid to form non-substituted glucose and hydroxyethyl glucose. Subsequently, the concentration of free unsubstituted glucose was determined enzymatically and the total concentration of all (i.e., substituted and unsubstituted) glucose molecules was determined using the o-Toluidine method. From this, the concentration of the substituted glucose (hydroxyethyl glucose) and the DS could be calculated. In addition, the HES concentration was measured first in vitro and then in vivo at any point after the infusion by both the enzymatic method and the o-Toluidine method. Results The DS increased significantly directly after the infusion from 0.42 to 0.53 (for 500ml) or to 0.50 (for 1000ml); 4 hours later this had further increased to 0.55 and 0.54, respectively (p <0.0001). The HES concentration in vitro showed no significant difference (p = 0.17) when determined with the enzymatic and the o-Toluidine method. In contrast, the serum concentrations in vivo displayed significant differences (p<0.0001) between the two measurement methods. Immediately after the infusion of 500ml HES, the concentration measured with the o-Toluidine method was 31% higher than the one measured with the enzymatic method; 4 hours later, this discrepancy was still at 25%. For 1000 ml HES, the differences amounted to 16% and 25%, respectively. Conclusion The DS of HES in vivo increases significantly over time. As a result, an HES concentration measured with the enzymatic method in vivo will be significantly lower than the same concentration determined with the o-Toluidine method. In future pharmacokinetic studies, HES concentrations should be measured using a method that takes into account changes in the DS in vivo.