A Calorimetric Investigation of Doxorubicin-polymer Bead Interactions. 1 2

11 Isothermal titration calorimetry (ITC) was utilised to investigate suitability of the technique to determine 12 the stoichiometry and thermodynamics of the interactions that occur between a commonly used chemotherapeutic 13 drug, namely doxorubicin, and a polymer bead-based drug delivery embolisation system (DC BeadTM). Six 14 temperatures were selected for drug-polymer titrations (293 to 313 K) and in all cases an initially exothermic signal 15 reverted to an endothermic response upon the saturation of the beads with drug. From these experiments, and 16 subsequent calculations, the molar ratio of drug to SO3 (polymer) was found to be 0.4:1 at all temperatures studied. 17 Enthalpic data was calculated from the raw ITC data with an average enthalpy of drug-polymer binding of – 14.8 18 kJmol at 293 K through to – 19.4 kJmol at 313 K implying the process is enthalpically-driven yet only affected by 19 an increase in experimental temperature to a limited extent whereby an increase in experimental temperature results 20 in a small increase in the negativity in change in enthalpy recorded. The application of ITC in this study (with its 21 unique ability to monitor real-time interactions and facilitate stoichiometric calculations) resolves the lack of 22 knowledge regarding the thermodynamics of this specific drug-polymer interaction. This study confirms that ITC is 23 not only useful for this specific system, but also highlights the potential use of ITC for more general studies in this 24 area. 25