Neutral styrene divinylbenzene copolymers for adsorption of toxins in liver failure

In artificial extracorporeal liver support systems, albumin-bound toxins such as bilirubin, bile acids, or aromatic amino acids are removed by adsorption to polymer beads. To overcome the potential weaknesses of anion exchange polymers currently used in liver support, namely, binding of heparin and activation of coagulation, we prepared two series of neutral polystyrene divinylbenzene resins with average pore sizes of 5-6 and 8-9 nm, respectively. In in vitro experiments using human plasma spiked with bilirubin, cholic acid, tryptophan, and phenol, we found that only pores larger than 5-6 nm were accessible to strongly albumin-bound substances, such as bilirubin. On the other hand, less strongly albumin-bound substances, such as bile acids, were efficiently bound by polymers of the small pore size range due to a higher accessible surface area. None of the neutral resins bound significant amounts of heparin. To assess the influence of the polymers on activation of coagulation, generation of thrombin-antithrombin complexes (TAT) was measured at different citrate concentrations. While none of the neutral polymers induced TAT generation, TAT levels were significantly elevated after incubation of plasma with an anion exchange polymer that is in clinical use for extracorporeal liver support. Binding characteristics of the neutral resins for the natural anticoagulants protein C and antithrombin showed remarkable differences, with weak binding of antithrombin but strong removal of protein C, not only for the anion exchanger, but also for neutral polymers of the large pore size range. In conclusion, neutral polystyrene divinylbenzene polymers with a pore size larger than 5-6 nm are efficient adsorbents for albumin-bound toxins that do not induce generation of thrombin-antithrombin complexes.