Designing porous metal-organic frameworks to store and deliver large amounts of nitric oxide.

The overall aim of this work is to design and prepare new porous materials that can be used to store and deliver larger quantities of nitric oxide than was previously possible, and then to demonstrate the biological activity of the materials. To do this requires that the following milestones be reached

1. The synthesis of the metal-organic frameworks containing suitable functionality for nitric oxide storage
2. Quantification of the storage and release properties of the NO-loaded materials
3. Full characterisation of the materials
4. Determination of the relative potency and duration of action of different materials in human platelets, inflammatory cells and arteries
5. Determination of the therapeutic potential of lead compounds in suitable in vitro and in vivo models

Nitric oxide (NO) is an amazing Jekyll and Hyde compound. We go to great lengths to remove NO from the exhaust gases of automobiles as it is a harmful pollutant. However, NO is also extremely important in mammalian biology. It is implicated in many processes in the body including vasodilation, the prevention of platelet aggregation and thrombus formation, neurotransmission and wound repair. There are tremendous possibilities for the use of NO in prophylactic and therapeutic processes, including potential applications in anti-thrombogenic medical devices, improved dressings for wounds and ulcers and the treatment of fungal and bacterial infections (amongst many many others). It is predicted that the global market for treatments that involve NO will rise to ~US$100 bn by 2010.

Unfortunately, because of its gaseous nature, it is difficult to deliver NO in controlled amounts to areas of specific need. There is a great need to design materials that can store and release large amounts of NO, which can then be used in medical applications. In this project we will design new materials based on metal organic frameworks (MOF) with the right chemical characteristics to allow high storage capacity. We will prepare new materials that have two different sites for NO storage (and a mixture of both) and we will thoroughly characterise the materials to find their structure and their NO release properties. We will then test the materials to show their biological activity. The biological testing will involve three main strands; a study of the pharmacology and mechanism of action of the materials in human platelets and monocytes, an in vitro assessment of platelet and monocyte adhesion and an in vivo assessment of anti-clotting effects of the materials.

The work in the above grant, led by Professor Rossi, stems from a highly successful collaboration between Professor Ian Megson (then at the University of Edinburgh and now Professor of Diabetes and Cardiovascular Science at the University of the Highlands and Islands) and Professor Russell Morris (Professor of Chemistry from the University of St Andrews). The project funded Sarah Fox a first class honours Pharmacology student from the University of Edinburgh.



We discovered that transition metal-exchanged zeolite-A adsorbs and stores nitric oxide in relatively high capacity (up to 1 mmol of NO/g of zeolite). The stored NO is released on contact with an aqueous environment under biologically relevant conditions of temperature and pH. The release of the NO can be tuned by altering the chemical composition of the zeolite, by controlling the amount of water contacting the zeolite, and by blending the zeolite with different polymers. The high capacity of zeolite for NO makes it extremely attractive for use in biological and medical applications, and our experiments indicate that the NO released from Co-exchanged zeolite-A inhibits platelet aggregation and adhesion of human platelets in vitro. Furthermore, gas adsorption experiments were carried out on a copper benzene tricarboxylate metal-organic framework material, HKUST-1. Detailed chemical analysis was performed on HKUST-1 and chemiluminescence and platelet aggregometry experiments indicate that the amount of NO recovered on exposure of the resulting complex to water is enough to be biologically active, completely inhibiting platelet aggregation in platelet rich plasma.



Importantly, and as a direct consequence of the funding we showed that NO which is important for the regulation of a number of diverse biological processes, including vascular tone, neurotransmission, inflammatory cell responsiveness could have important implication for defence against invading pathogens and wound healing. Our work had shown that transition metal exchanged zeolites are nanoporous materials with high-capacity storage properties for gases such as NO. We showed that teh NO stores are liberated upon contact with aqueous environments, thereby making them ideal candidates for use in biological and clinical settings. We have demonstrated that the NO release capacity and powerful bactericidal properties of a novel NO-storing Zn(2+)-exchanged zeolite material at a 50 wt.% composition in a polytetrafluoroethylene polymer. Further to our published data showing the anti-thrombotic effects of a similar NO-loaded zeolite, we have demonstrated that the anti-bacterial properties of NO-releasing zeolites against clinically relevant strains of bacteria, namely Gram-negative Pseudomonas aeruginosa and Gram-positive methicillin-sensitive and methicillin-resistant Staphylococcus aureus and Clostridium difficile. Thus our findings highlight the potential of NO-loaded zeolites as biocompatible medical device coatings with anti-infective properties.



The above findings were highlighted in scientific and general press outlets (as described in the linked impact statement).