Drug adsorption onto activated charcoal as a means of formulation

The in vitro adsorption of metoprolol, pindolol, salbutamol, furosemide and clonidine onto activated charcoal was determined. The affinity of the drugs for charcoal decreased with increasing hydrophilicity. Also the rate of adsorption of clonidine onto four charcoal preparations having different particle sizes was studied. The equilibrium was reached rapidly with the charcoal having the smallest particle size, and the adsorption rate decreased as the particle size of the charcoal increased. The desorption of drugs from charcoal was investigated in the Sartorius dissolution apparatus at constant initial drug:charcoal ratio. The desorption had a two-step kinetics. The quantity of the initial rapid release, indicating the development of equilibrium, agreed with the adsorption data determined under the same conditions, except for salbutamol and furosemide at pH 7. The same was true for the Langmuir isotherms determined for adsorption and desorption. The following release step was slow and there were only minor differences between the release rates of the different drugs. The desorption rate (time to reach equilibrium) from the different charcoals was studied using a batch technique. An increase in the particle size of charcoal had, however, only limited sustaining effect on desorption. Although the affinity of the drugs for charcoal was in good agreement with their hydrophobicity, their desorption behavior was not necessarily proportional to their hydrophilicity. The in vitro release of the drugs from charcoal was retarded and was not significantly affected by the charcoal particle size.