More Is Less: Curcumin and Paclitaxel Formulations Using Poly(2-Oxazoline) and Poly(2-Oxazine) Based Amphiphiles Bearing Linear and Branched C9 Side Chains

A known limitation of polymer micelles for the formulation of hydrophobic drugs is their low loading capacity, which rarely exceeds 20 wt.%. One general strategy to overcome this limitation is to increase the amphiphilic contrast, i.e. to make the hydrophobic core of the micelles more hydrophobic. However, we reported earlier that for poly(2-oxazoline) based amphiphilic triblock copolymers, a minimal amphiphilic contrast is beneficial, which was tentatively attributed to possible side chain crystallization. Here, we revisit this subject in more detail using more hydrophobic side chains that are either linear (nonyl) or branched (3-ethylheptyl), the latter of which should not crystallize. Moreover, we investigate two different backbones within the hydrophobic block, in particular poly(2-oxazoline) and poly(2-oxazine), for the solubilization and co-solubilization of the two highly water insoluble compounds curcumin and paclitaxel. Even though high loading capacities could be achieved for curcumin within poly(2-oxazine) based triblock copolymers, the solubilization capacity of all investigated polymers with longer side chains was significantly lower compared to poly(2-oxazoline)s and poly(2-oxazine)s with shorter side chains. Although the even lower loading capacity for paclitaxel could be somehow attenuated by co-formulating curcumin, this study corroborates that in the case of poly(2-oxazoline) and poly(2-oxazine) based polymer micelles, an increased amphiphilic contrast leads to less drug solubilization.