Understanding the structure and stability of paclitaxel nanocrystals

Previously, PTX/Pluronic F127 nanocrystals were prepared in our laboratory using the stabilization of nanocrystals (SNC) method. For PTX nanocrystals, dosages could be increased to yield improved antitumor activity over Taxol® without incidence of acute toxicity. The objectives of this current study are to further understand the structure and stability of PTX nanocrystals. More Pluronic F127 surfactant was added in the formulation to attempt to further stabilize the nanocrystals against thermal induced aggregation. However, this resulted in formation of micelles that worsened the stability of nanocrystals. The F127 desorption experiment suggested different surfactant adsorption affinity to nanocrystal surface below and above the CMC. Below the CMC monomers bound to nanocrystal surface with high affinity, but above the CMC low affinity surfactant aggregates readily left the surface upon dilution. At higher temperature the tendency of F127 micellization is enhanced due to drastically lower CMC. Consequently, at 37 °C there was F127 desorption even for nanocrystals prepared with low amounts of F127 (1:5 (w/w) PTX/F127). To improve the stability of nanocrystals, re-nanonization by incubation–sonication procedure was used to disrupt the preferred crystal growth pattern of PTX. Furthermore, we have demonstrated that a higher heating temperature (45 °C vs. 37 °C) used in the incubation–sonication procedure was able to provide even better nanocrystal stability for long periods of incubation time.