Protein binding of xanthine derivatives to guinea pig serum albumin.

Binding of the bronchodilators N3-alkylxanthine and N3-alkyl-N1-methylxanthine derivatives to guinea pig serum albumin was investigated in vitro using the ultrafiltration method. A marked difference in the binding parameters of xanthine derivatives was observed, and binding was shown to be concentration dependent. Significant relations were observed among their binding parameter, dissociation constant (Kd), and hydrophobicity (log PC). The extent of binding of xanthine derivatives was increased both when a N3-methyl group was replaced by a longer alkyl chain and when a N3-alkylxanthine molecule was additionally replaced by a methyl group. Reversed-phase HPLC retention, as an index of hydrophobicity of xanthine derivatives, was also determined. Significant relationships were found between the adjusted retention time data for each xanthine derivative and their hydrophobicity or biological activities, such as their abilities to cause muscle relaxation and cyclic AMP phosphodiesterase (PDE) inhibition. These findings indicate that the difference in the extent of binding among xanthine derivatives is related to hydrophobicity, which is an important determinant of their biological activities.