Coupled equilibria of a self-associating drug loaded into polymeric nanoparticles

Doxorubicin (DOX) and other anti-cancer drugs are often formulated using nanoparticles for passive or active targeting and reducing detrimental side effects. Anionic polymers have been shown to effectively facilitate loading of cationic DOX hydrochloride into nanoparticles with high efficiency. One powerful method to study DOX loading into anionic polymeric nanoparticles has been isothermal titration calorimetry (ITC), but the curves are complex and were previously interpreted in a largely qualitative manner only. Here we present detailed quantitative modelling of such ITC data, corroborated by zeta potential measurements and dynamic light scattering. The model takes into account 3 coupled equilibria. First, DOX self-associates in solution to dimers and larger aggregates. This effect is modelled in terms of the stepwise aggregation model. Second, DOX binds with a 1:1 stoichiometry to the carboxylic acids in the polymer at low salt. At about 33% saturation, the nanoparticles collapse in size and the enthalpy of further binding becomes less exothermic. Third, free DOX also stacks onto polymer-bound DOX. This stacking effect is very weak and hardly detected by ITC. It is, however, revealed by a positive zeta potential. The present work demonstrates the power of combining ITC with light scattering and zeta potential measurements for studying the thermodynamics of drug loading into polymeric nanoparticles.