*4.2. Endogenous Protein-Bound Solute Dialysis*

Protein-bound uremic toxins (PBUTs) such as p-cresyl sulphate (PCS) and indoxyl sulphate (IS), which are products of bacterial intestinal amino acid metabolism, accumulate in end-stage renal disease. Their accumulation is important because, although PBUTs are not the cause of initial renal failure, PBUT concentrations are associated with increased mortality and morbidity in long-term haemodialysis patients. Increased PBUT concentrations are also associated with higher rates of vascular complications and more rapid progression of renal failure. Their clearance by traditional HD and PD is poor due to their high protein binding in plasma [17]. In order for PBUT clearance by dialysis to be increased by a nanoparticle, PBUTs must first dissociate from their binding protein into the free fraction in plasma, diffuse from the bloodstream into the dialysate and subsequently be bound in the dialysate to maintain a concentration gradient for the toxin to continue leaving the bloodstream. This process must also occur within the span of a capillary (for PD) or dialysis cartridge (for HD) transit time [18]. Preliminary studies imply that adding albumin or soy phospholipid-based liposomes as binders to dialysate could improve PBUT removal compared with conventional glucose-based peritoneal dialysis [18]. The addition of soy phospholipid-based liposomes to the dialysate in haemodialysis for uraemic rats also increased PBUT clearance [17].

Strategies to improve the extraction capacity of liposomes in dialysate for PBUTs have been evaluated. Liposomes loaded with molecules that compete with PBUTs for albumin binding sites, thus displacing PBUTS and increasing the free fraction concentration in the blood available for dialysis, have been investigated. Linoleic acid-modified liposomes (LA-liposomes) were used in the dialysate and assessed for their capacity to increase PBUT clearance [19]. As previously discussed, toxins not in the free fraction are inaccessible to traditional dialysis. It was shown that using linoleic acid as a "displacer" to compete with PBUTs for albumin binding sites significantly increased the clearance of PBUTs in in vitro dialysis models. The same effect was observed for LA liposomes for liver failure-related cholestatic solutes.

PCS and IS are molecules based on an aromatic ring with a negatively charged sulphate group attached. Liposomes bind PBUTs via electrostatic and lipophilic interactions with the liposome membrane. A positively charged amphiphile on the liposome phospholipid was trialed to increase liposome affinity for PBUTs, with increased extraction of IS and p-Cresol using an in vitro dialysis model [20].

Preclinical work has identified investigational targets other than ammonia for liposomes to augment dialysis in the management of liver failure. Both unconjugated bilirubin and bile salts were found to have significantly higher reduction ratios with liposome dialysis when compared to conventional dialysate using in vivo haemodialysis models [21].
