Coordination bonding based pH-responsive drug delivery systems

pH-responsive drug delivery systems are promising in their applicability to many areas, especially the anti-cancer therapy. Coordination complexes have the capability of storing and releasing drugs through the formation and cleavage of coordination bonds, respectively, in response to pH variations. This review outlines recent advances in coordination bonding based pH-responsive systems in both host carrier systems and coordination polymer systems. Various strategies have been developed for drug molecules with strong coordination binding sites, weak coordination binding sites and no coordination binding sites. The binding strengths of pH-responsive systems, using different types of metal sources, are discussed in this article. The construction of various pH-responsive systems using different categories of carriers, including mesoporous silica nanoparticles, mesoporous hybrid monoliths, mesoporous metal oxides and coordination polymer nanoparticles, has also been achieved. This review also provides a perspective on utilising this system in in vitro cell assays and in vivo tumour assays as well as further possibilities for clinical applications.