The influence of poly(allylamine hydrochloride) hydrogel crosslinking density on its thermal and phosphate binding properties.

Sevelamer hydrochloride (SH) or Renagel® is an effective phosphate binder prescribed to prevent the absorption of phosphate in end stage renal disease (ESRD) patients. The relationship between SH structure and binding capacity and affinity is very important and can be used in characterising the sensitivity of the hydrogel to binding conditions. Thus, a series of hydrogels were prepared by varying the amount of crosslinker, whilst the other hydrogel components were kept constant. Variation of this parameter influenced the hydrogel structure as shown by swelling data, differential scanning calorimetry and solid state nuclear magnetic resonance spectroscopy. The hydrogels' physical characteristics were found to correlate with the number of phosphate binding sites and affinity obtained from the Langmuir-Freundlich Isotherm (L-FI) and affinity distribution spectra (AD). The hydrogels formed using lower amounts of crosslinker showed a slight increase in binding capacity but with lower affinity. However, the influence of the pH of the binding media on the binding parameters of sevelamer hydrochloride was significant. This is the first report on the use of AD spectra generated from L-FI binding parameters in hydrogels, which demonstrates the sensitivity of the affinity and binding site numbers to changes in hydrogel physical properties and the pH of the binding media.

Kinetic and thermodynamic evaluation of phosphate ions binding onto sevelamer hydrochloride.

Sevelamer hydrochloride is the first non-aluminium, non-calcium-based phosphate binder developed for the management of hyperphosphatemia in end stage renal diseases. It is a synthetic ion-exchange polymer which binds and removes phosphate ions due to the high content of cationic charge associated with protonated amine groups on the polymer matrix. This is the first in-depth study investigating phosphate removal in vitro from aqueous solutions using commercially available sevelamer hydrochloride at physiological conditions of phosphate level, pH and temperature. The kinetic and thermodynamic parameters of phosphate binding onto the sevelamer hydrochloride particles were evaluated in order to define the binding process. A series of kinetic studies were carried out in order to delineate the effect of initial phosphate concentration, absorbent dose and temperature on the rate of binding. The results were analysed using three kinetic models with the best-fit of the experimental data obtained using a pseudo-second order model. Thermodynamic parameters provide in-depth information on inherent energetic changes that are associated with binding. Free energy ΔG°, enthalpy ΔH°, and entropy ΔS° changes were calculated in this study in order to assess the relationship of these parameters to polymer morphology. The binding reaction was found to be a spontaneous endothermic process with increasing entropy at the solid-liquid interface.