Adsorstion of Selected Biochemicals onto coated and Uncoated Charcoal

Creatinine, glutethimide and pentobarbital isotherms and preliminary kinetic data have been obtained in vitro on coated and uncoated charcoal from two commercially available hemoperfusion devices (B-D Hemodetoxifier and Gambro-Adsorba 300C). The uptake characteristics of these devices were examined in both buffer and plasma environments at physiological conditions (pH:7.4; temperature:37^oC), using radiolabelled solutes. Equilibrium adsorption data were analyzed by both Freundlich and Langmuir isotherm equations and isotherm constants determined. For both drugs, buffer and plasma data were well correlated by a Langmuir isotherm (r>0.99) while creatinine data were fitted better by the Freundlich equation (r>0.99). For all solutes, the amount adsorbed was lower in plasma (after correction for protein-binding) due to reversible competitive inhibition. However, charcoal capacity remained high enough for both coated and uncoated charcoals to enable satisfactory clinical hemoperfusion to be performed. Kinetic data were reduced assuming a homogeneous model of intraparticle diffusion. Calculated charcoal diffusivities were 0.3–3.8×10^--⁸ cm²/sec, with plasma values four to six times lower than corresponding values for buffer. Charcoal diffusivities were about two orders of magnitude lower than corresponding effective diffusivities through Cuprophan, indicating that the mass transfer resistance of the charcoal coating is inconsequential under conditions of intraparticle diffusion control. It was also deduced that the commonly observed decrease in clearance as a function of time during clinical charcoal hemoperfusion is mainly a consequence of diffusive rather than site-availability limitations.