(A) Fulvestrant and (B) PPD were selected for their intrinsic chemotherapeutic efficacy and aggregation properties. Formulation of (C) fulvestrant and (D) PPD colloids in water or PBS in the presence of the following polymeric excipients: UP80, PLAC–PEG, Brij 58, Pluronic F127, VitE-PEG, Pluronic F68, and Brij L23. Initial diameters of the formulations are shown. Incubation of (E) fulvestrant and (F) PPD formulations at 37 °C, showing size changes over 48 h. UP80 was the optimal polymer to maintain the size of fulvestrant over time in buffered salt solution (PBS) compared to other polymers. Stability of PPD with F68 could not be assessed due to precipitation. PLAC–PEG was the optimal polymer to maintain the size of PPD, with the smallest nanoparticle size over the incubation period. (n = 3, mean + SD, *** p < 0.001.)