Understanding the Host-Guest Chemistry of CuMOF-Gliotoxin Complexes by X-Ray Absorption and X-Ray Photoelectron Spectroscopy

<b>Introduction:</b> Gliotoxin, a secondary metabolite isolated from the fungi, <i>Aspergillus fumigatus</i> has shown cytotoxic activity against neuroblastoma (1), a type of cancer accounting for 15% of all childhood cancer-related deaths (2). But as with all other chemotherapeutic agents, this compound is toxic to both tumour cells and healthy cells. To minimize toxicity towards the healthy cells, a biocompatible nanoparticulate system is needed. For this reason, our study has explored the use of nanosized metal organic frameworks (MOFs) – a class of organic-inorganic hybrid materials made up of metal nodes held together by organic bi or tridentate linkers, as drug delivery systems with the goal of achieving controlled delivery of gliotoxin to neuroblastoma cells. Amongst the MOF systems we have investigated, Cu-MOFs with RAWYAS and GIFKUC phases showed interesting results and are the focus of this contribution. By means of microwave-assisted solvothermal method, we synthesized some Cu-based MOFs at 1 (MOF1Cu), 5 (MOF5Cu), and 30 (MOF30Cu) minutes, using copper (ii) nitrate trihydrate as the metal ion and 5-aminoisophthalic acid as the ligand, and studied their structural characteristics by means of scanning electron microscopy (SEM) and X-ray diffraction (XRD). We also explored their adsorption capacities using high performance liquid chromatography (HPLC) and investigated their cytotoxic effect against the SHSY-5Y neuroblastoma cell line by means of 3-(4,5-dimethylthiazol-2-yl)-2,5 diphenyltetrazolium bromide (MTT) assay. To rationalize our findings from the MTT assay, we sought to understand the nature of the intra-and inter-molecular interactions between gliotoxin and the synthesized Cu-MOFS by measuring the K-edges of Cu, C, N, O, and S of the MOF-gliotoxin complexes by means of soft and hard x-ray absorption spectroscopy (XAS) and x-ray photoelectron spectroscopy (XPS) in an ex-situ fashion.<br/><b>Results:</b> The MTT assay showed that upon immobilization of gliotoxin within the synthesized MOFs, its cytotoxic effect was increased. This enhanced cytotoxicity suggests that not only do the MOFs act as carrier systems, but they also provide a synergistic effect. The changes in the near edge x-ray absorption fine structure (NEXAFs) spectra on immobilization of gliotoxin within the synthesized Cu-MOFs showed that there is an interaction between the drug and the MOF systems. This is particularly evident in the N K edge spectrum of the MOF sample after immobilization of gliotoxin 1:5 ratio with a shift of the 1s→p* transition of the amino group of MOFs to higher energy. Moreover, the ratio between the 1s→p* and 1s→s* in the OK edge changes significantly upon immobilization of gliotoxin, indicating a significant structural modification of the MOFs. Also, marked changes can be seen in the S2p XPS spectra upon gliotoxin immobilization, which suggests a direct co-ordination between the sulphur of gliotoxin and the Cu metal sites of the MOFs. Based on these results, we posit that the interaction of the gliotoxin with the MOFs involves mainly the N site of the ligand. More investigations on the S L edge XAS will clarify any modification on the structure of the gliotoxin.<br/><b>Conclusion:</b> Taken together, our findings highlight an approach to enhancing the pharmacological activity of gliotoxin. As an outlook, we would further explore the nature of the host-guest chemistry between gliotoxin and the synthesized Cu-based MOFs from a computational viewpoint using molecular dynamic studies.<br/><br/><b>References</b><br/>1. Axelsson V, Holback S, Sjögren M, Gustafsson H, Forsby A. Gliotoxin induces caspase-dependent neurite degeneration and calpain-mediated general cytotoxicity in differentiated human neuroblastoma SH-SY5Y cells. Biochem Biophys Res Commun. 2006 Jul 7;345(3):1068–74.<br/>2. Maris JM, Hogarty MD, Bagatell R, Cohn SL. Neuroblastoma. Vol. 369, Lancet. Elsevier; 2007. p. 2106–20.