Amphiphilic core cross-linked star polymers as water-soluble, biocompatible and biodegradable unimolecular carriers for hydrophobic drugs

Unimolecular polymeric architectures are ideal candidates for drug encapsulation. In this study we report the facile yet well-controlled formation of a series of biocompatible and biodegradable core cross-linked star (CCS) polymers via the easily scalable, metal free, one- or two- step ring-opening polymerization (ROP) of e-caprolactone, using poly(ethylene glycol) (PEG) as initiator and [4,4′-bioxepane]-7,7′-dione (BOD) as cross-linker. The resulting CCS polymers, which exhibit hydrophilic PEG blocks in their outer shell and hydrophobic poly(e-caprolactone) (PCL) and BOD segments in their inner core, are water-soluble and amphiphilic and exist in a unimolecular state, both in organic solvents and in water. These properties provide the opportunity to easily stabilise water-insoluble, hydrophobic drugs in aqueous environments without the need for conjugation of the drug to the carrier and/or complex encapsulation techniques. The impact of hydrophilic/hydrophobic block length and core size on polymer properties was investigated via gel permeation chromatography (GPC) and dynamic light scattering (DLS). In addition, the change in drug encapsulation properties with varying hydrophilic/hydrophobic balance was studied using pirarubicin – a potent anthracycline – as a model hydrophobic drug. Formation of a drug–CCS polymer conjugate purely based on hydrophobic–hydrophobic interaction of the drug and the hydrophobic component of the CCS was verified by 1H NMR and UV-Vis measurements, and the size change confirmed by DLS and transmission electron microscopy (TEM). The in vitro study of drug–CCS conjugate demonstrated significantly faster release of anthracycline from the CCS polymer under acidic conditions (pH = 5.5) compared with normal physiological pH level (7.4). Furthermore, cytotoxicity and cellular uptake tests performed using Hela cells, demonstrated extremely low toxicity of the macroinitiators and CCS polymers even at high concentrations, while anthracycline-loaded CCS polymers exhibited similar IC50 values to the free drug. Confocal laser scanning microscopy and flow cytometry confirmed high uptake efficiency and intracellular localisation of the CCS polymers upon uptake, respectively.