Abstract
Compatibility between risperidone and soft drinks was examined by means of isothermal titration microcalorimetry, and the interaction of risperidone with tea tannin- (-)-epigallocatechin, (-)-epicatechin, theaflavin and their gallates-was investigated thermodynamically. Risperidone exothermically bound to tea tannin monomers with a binding affinity of 103-104 M-1 in aqueous solution to form an insoluble complex at a 1:1 molar ratio. The binding reaction was characterized by small enthalpy and entropy changes, reflecting hydrophobic and van der Waal's interactions.
After mixing Risperdal® liquid (1 mg/mL risperidone oral solution) with soft drinks containing tannin-such as green tea, black tea and oolong tea, there was an immediate decrease in the risperidone content and the mixtures became cloudy. The heat effect of risperidone titrated into these soft drinks was exothermic and proportional to the quantity of the complex formed in the mixture. However, no significant heat effect was noted when risperidone was titrated into drinks in PET bottles that do not contain tea tannin, such as mineral water and an infusion of parched barley. Isothermal titration microcalorimetry is thus a rapid and effective method for evaluating the compatibility and physico-chemical interactions of drugs.
