Charge-reversal ZnO-based nanospheres for stimuli-responsive release of multiple agents towards synergistic cancer therapy

2,3-Dimethylmaleic anhydride (DMMA)-decorated zinc oxide (ZnO) nanoparticles with doxorubicin (DOX) and phenylsulfonyl furoxan (PSF) loaded were designed and prepared through a step-by-step strategy. Amino-terminated ZnO nanoparticles were synthesized, in which DOX was encapsulated by forming coordination bonding between DOX and Zn2+ ions. PSF as a NO donor was conjugated to the nanoparticles via amide bonding. Surface decoration of DMMA endowed the resultant (DOX,PSF)@ZnO-DMMA nanospheres with charge-reversal ability, as characterized by zeta potential measurements. The (DOX,PSF)@ZnO-DMMA nanospheres (an average size of ~7.9 nm) had a mildly negative surface charge and thus had a long blood circulation. The intratumoral microenvironment could reverse the surface charge of the nanospheres, which favored the cellular uptake of the nanospheres. Decomposition of ZnO occurred under acidic conditions characteristic of intracellular endosomal and lysosomal systems, leading to release of both Zn2+ ions and DOX. In the meantime, NO release from the PSF was triggered by the high concentration of GSH in the cancerous cells. The nanospheres were able to inhibit the proliferation of cancerous cells efficiently, as evidenced by in vitro cell assay and in vivo small animal experiments. The NO greatly reduced the multi-drug resistance (MDR) and increased the intracellular concentration of DOX, which killed cancerous cells in combination with the Zn2+ ions. This work has highlighted the delivery of multiple therapeutic agents in response to intracellular stimuli, which offered a new approach for rational design of nanomaterials towards synergistic cancer therapy.