Thiol and pH dual-responsive dynamic covalent shell cross-linked micelles for triggered release of chemotherapeutic drugs

We report on the fabrication of dynamic covalent shell cross-linked (SCL) micelles of amphiphilic diblock copolymers functionalized with aldehyde moieties in the hydrophilic block by utilizing difunctional crosslinkers cleavable in response to pH and thiols. Well-defined amphiphilic diblock copolymer, PCL-b-P(OEGMA-co-MAEBA), was synthesized via ring opening polymerization (ROP) of epsilon-caprolactone (CL) and atom transfer radical polymerization (ATRP) of oligo(ethylene glycol) monomethyl ether methacrylate (OEGMA) and p-(methacryloxyethoxy) benzaldehyde (MAEBA) comonomers. In aqueous solution, the diblock copolymer self-assembles into micelles consisting of hydrophobic PCL cores and hydrophilic P(OEGMA-co-MAEBA) coronas covalently anchored with aldehyde groups. The subsequent shell cross-linking reaction was conducted at pH 6.2 upon addition of difunctional dithiolbis(propanoic dihydrazide) (DTP). The formation of dynamic acylhydrazone cross-linking linkages was facilitated under the catalysis of aniline. The obtained SCL micelles can be de-crosslinked via two biologically relevant modes, namely, acidic pH-triggered cleavage of acylhydrazone bonds into aldehyde and hydrazide and thiol-triggered cleavage of disulfide linkages, which have been utilized for triggered release of physically encapsulated chemotherapeutic drugs. The dual-responsive dynamic covalent SCL micelles were examined by dynamic laser light scattering (LLS), H-1 NMR, Ellman's assay, and enzymatic degradation tests. In addition, camptothecin (CPT)-loaded SCL micelles were used to investigate thiol and pH-modulated CPT release profiles. Compared with CPT-loaded non-cross-linked (NCL) micelles, CPT-loaded SCL micelles can largely minimize drug leakage under physiological conditions, whilst exhibiting accelerated drug release under mildly acidic or thiol-rich microenvironments, which are relevant to those of acidic organelles (endosomes and lysosomes) or cytosol within tumor cells. Cell cytotoxicity studies revealed that drug-free SCL micelles are almost nontoxic, whereas CPT-loaded SCL micelles can efficiently deliver chemotherapeutic drug (CPT) into HepG2 cells, leading to considerable nucleic accumulation at extended incubation duration. The reported dynamic covalent shell cross-linking strategy can exert intricate control concerning the micellar stability and the release profile of encapsulated drugs in response to biological microenvironments, which augurs well for their potential use as novel smart nanocarriers for drug delivery in cancer chemotherapy.