Hydrolytically Degradable Polymer Micelles for Drug\nDelivery: A SAXS/SANS Kinetic Study

Abstract

We report kinetic studies of therapeutically\nhighly potent polymer–drug conjugates consisting of amphiphilic <i>N</i>-(2-hydroxypropyl) methacrylamide (HPMA)-based copolymers\nbearing the anticancer drug doxorubicin (Dox). Highly hydrophobic\ncholesterol moieties as well as the drug were attached to the polymer\nbackbone by a pH-sensitive hydrazone bond. Moreover, the structure\nof the spacer between the polymer carrier and the cholesterol moiety\ndiffered in order to influence the release rate of the hydrophobic\nmoiety, and thus the disintegration of the high-molecular-weight micellar\nnanoparticle structure. We performed time-dependent SAXS/SANS measurements\nafter changing pH from a typical blood value (pH 7.2) to that of tumor\ncells (pH 5.0) to characterize the drug release and changes in particle\nsize and shape. Nanoparticles composed of the conjugates containing\nDox were generally larger than the drug-free ones. For most conjugates,\nnanoparticle growth or decay was observed in the time range of several\nhours. It was established that the growth/decay rate and the steady-state\nsize of nanoparticles depend on the spacer structure. From analytical\nfitting, we conclude that the most probable structure of the nanoparticles\nwas a core–shell or a core with attached Gaussian chains. We\nconcluded that the spacer structure determined the fate of a cholesterol\nderivative after the pH jump. Fitting results for 5α-cholestan-3-onecholestan-3-one\nand cholesteryl-4-oxopentanoate (Lev-chol) implied that cholesterol\nmoieties continuously escape from the core of the nanoparticle core\nand concentrate in the hydrophilic shell. In contrast, cholest-4-en-3-one\nspacer prevent cholesterol escaping. Dox moiety release was only observed\nafter a change in pH. Such findings justify the model proposed in\nour previous paper. Lastly, the cholesteryl 4-(2-oxopropyl)­benzoate\n(Opb-Chol) was a different case where after the release of hydrophobic\nOpb-Chol moieties, the core becomes more compact. The physicochemical\nmechanisms responsible for the scenarios of the different spacers\nare discussed.