Abstract
The lower critical solution temperature (LCST) of the thermo-responsive engineered elastin-like polypeptide (ELP) biopolymer is being exploited for the thermal targeted delivery of doxorubicin (Dox) to solid tumors. We examine the impact of Dox labeling on the thermodynamic and hydrodynamic behavior of an ELP drug carrier and how Dox influences the liquid-liquid phase separation (LLPS). Turbidity, dynamic light scattering (DLS), and differential scanning calorimetry measurements show that ELP undergoes a cooperative liquid-liquid phase separation from a soluble to insoluble coacervated state that is enhanced by Dox labeling. Circular dichroism measurements show that below the LCST ELP consists of both random coils and temperature-dependent β-turn structures. Labeling with Dox further enhances β-turn formation. DLS measurements reveal a significant increase in the hydrodynamic radius of ELP below the LCST consistent with weak self-association. Dox-labeled SynB1-ELP1 (Dox-ELP) has a significant increase in the hydrodynamic radius by DLS measurements that is consistent with stable oligomers and, at high Dox-ELP concentrations, micelle structures. Enhanced association by Dox-ELP is confirmed by sedimentation velocity analytical ultracentrifugation measurements. Both ELP self-association and the ELP inverse phase transition are entropically driven with positive changes in enthalpy and entropy. We show by turbidity and DLS that the ELP phase transition is monophasic, whereas mixtures of ELP and Dox-ELP are biphasic, with Dox-labeled ELP phase changing first and unlabeled ELP partitioning into the coacervate as the temperature is raised. DLS reveals a complex growth in droplet sizes consistent with coalescence and fusion of liquid droplets. Differential scanning calorimetry measurements show a -11 kcal/mol change in enthalpy for Dox-ELP coacervation relative to the unlabeled ELP, consistent with droplet formation being stabilized by favorable enthalpic interactions. We propose that the ELP phase change is initiated by ELP self-association, enhanced by increased Dox-ELP oligomer and micelle formation and stabilized by favorable enthalpic interactions in the liquid droplets.