The high drug-loading and excellent biocompatibilities of metal-organic frameworks (MOFs) have led to their application as drug-delivery systems (DDSs). Nanoparticle surface chemistry dominates both biostability and dispersion of DDSs while governing their interactions with biological systems, cellular and/or tissue targeting, and cellular internalization, leading to a requirement for versatile and reproducible surface functionalization protocols. Herein, we explore not only the effect of introducing different surface functionalities to the biocompatible Zr-MOF UiO-66 but also the efficacy of three surface modification protocols: (i) direct attachment of biomolecules [folic acid (FA) and biotin (Biot)] introduced as modulators for UiO-66 synthesis, (ii) our previously reported "click-modulation" approach to covalently attach polymers [poly(ethylene glycol) (PEG), poly-l-lactide, and poly-N-isopropylacrylamide] to the surface of UiO-66 through click chemistry, and (iii) surface ligand exchange to postsynthetically coordinate FA, Biot, and heparin to UiO-66. The innovative use of a small molecule with metabolic anticancer activity, dichloroacetate (DCA), as a modulator during synthesis is described, and it is found to be compatible with all three protocols, yielding surface-coated, DCA-loaded (10-20 w/w %) nano-MOFs (70-170 nm). External surface modification generally enhances the stability and colloidal dispersion of UiO-66. Cellular internalization routes and efficiencies of UiO-66 by HeLa cervical cancer cells can be tuned by surface chemistry, and anticancer cytotoxicity of DCA-loaded MOFs correlates with the endocytosis efficiency and mechanisms. The MOFs with the most promising coatings (FA, PEG, poly-l-lactide, and poly-N-isopropylacrylamide) were extensively tested for selectivity of anticancer cytotoxicity against MCF-7 breast cancer cells and HEK293 healthy kidney cells as well as for cell proliferation and reactive oxygen species production against J774 macrophages and peripheral blood lymphocytes isolated from the blood of human donors. DCA-loaded, FA-modified UiO-66 selectively kills cancer cells without harming healthy ones or provoking immune system response in vitro, suggesting a significant targeting effect and great potential in anticancer drug delivery. The results provide mechanistic insight into the design and functionalization of MOFs for drug delivery and underline the availability of various in vitro techniques to potentially minimize early-stage in vivo animal studies following the three Rs: reduction, refinement, and replacement.
Keywords: coordination modulation; drug delivery; endocytosis pathways; immune system response; metal−organic frameworks; surface modification.