Ultra-Microporous Fe-MOF with Prolonged NO Delivery in Biological Media for Therapeutic Application

Abstract

Nitric oxide (NO), a key element in the regulation of essential biological mechanisms, presents huge potential as therapeutic agent in the treatment and prevention of chronic diseases. Metal-organic frameworks (MOFs) with open metal sites are promising carriers for NO therapies but delivering it over an extended period in biological media remains a great challenge due to i) a fast degradation of the material in body fluids and/or ii) a rapid replacement of NO by water molecules onto the Lewis acid sites. Here, a new ultra-narrow pores Fe bisphosphonate MOF, denoted MIP-210(Fe) or Fe(H₂O)(Hmbpa) (H₄mbpa = p-xylenediphosphonic acid) is described that adsorbs NO due to an unprecedented sorption mechanism: coordination of NO through the Fe(III) sites is unusually preferred, replacing bound water, and creating a stable interaction with the free H₂O and P-OH groups delimiting the ultra-narrow pores. This, associated with the high chemical stability of the MOF in body fluids, enables an unprecedented slow replacement of NO by water molecules in biological media, achieving an extraordinarily extended NO delivery time over at least 70 h, exceeding by far the NO kinetics release reported with others porous materials, paving the way for the development of safe and successful gas therapies.

Keywords: biological stability; iron(III)‐MOF; nitric oxide donors; open metal sites; phosphonate.

Figure 1

Examples of different NO‐releasing porous materials and respective limitations that inspired the development of MIP‐210(Fe). The combination of unique properties leads to a more controlled and extended NO delivery from MIP‐210(Fe) compared to the existent NO‐releasing porous solids expands its use to several therapeutic applications.

Figure 2

Crystal structure of MIP‐210(Fe). A) General view of MIP‐210(Fe) at the a axis, highlighting the linker (shown in its fully deprotonation form although in the structure one of the phosphonate groups is only partially deprotonated (R‐HPO₃ ⁻) coordinated to two adjacent Fe(III) cations) and FeO₅(H₂O) polyhedra (H₂O is omitted for clarity). The bonded water can either be removed and create an OMS or can be replaced by NO. B) Representation of one channel along the [1 0 0] emphasizing its highly confined environment. C) Double FeO₅(H₂O) chains in MIP‐210(Fe) viewed at the [0 0 1] (H₂O is omitted for clarity). Color code: FeO₅(H₂O), dark yellow polyhedra; O, red; C, grey; P, pink. Hydrogen atoms on the MOF framework (and on the linker representation) and non‐coordinated guest molecules have been omitted for clarity.

Figure 3

Experimental NO adsorption studies at 25 °C on MIP‐210(Fe) and computational understanding of the NO adsorption mechanism. A) Gravimetric NO adsorption profile for 80 kPa NO; B) NO desorption profile under high vacuum and C) IR spectra for the sample previously activated at 120 °C for 12 h under vacuum, followed by 24 h of NO exposure at room temperature (1.3 kPa) and its evacuation (30 min of vacuum). D) periodic DFT‐calculated potential energy profile for the adsorption of NO in MIP‐210(Fe) with Fe initially coordinated by water molecule using climbing image nudged elastic band (CI‐NEB)^{[ 30 ]} method. Color codes: carbon (dark brown), hydrogen (white), oxygen (red), phosphorus (purple), nitrogen (blue), and iron (light brown). The distances and energies are in Å and kJ mol⁻¹, respectively. For the sake of clarity, only one inorganic node of the periodic structure is shown.

Figure 4

NO release profile from MIP‐210 in oxyhemoglobin solution at 25 °C using the oxyhemoglobin quantification assay. Inset depicts the changes over 70 h in the main peak of the oxyhemoglobin‐containing solution spectrum, triggered by NO‐mediated conversion to methemoglobin. The concentration of meta(Hb) quantified is considered stoichiometric to the concentration of NO released.^{[ 36 ]}

Figure 5

Effects of NO released by MIP‐210(Fe) on HUVEC cells migration and on Geltrex tube‐forming assay. A) Representative images of the Oris cell migration assay in the pre‐migration stage and after allowing cells to migrate in the presence (or not) of the MIP‐210(Fe) (unloaded and loaded with NO) at the concentration of 11.75 µg mL⁻¹. Colored areas represent the migration zone that is still not occupied by cells. B) Example images of HUVEC tube formation after being treated with (or without) NO‐releasing MIP‐210(Fe) (11.75 µg mL⁻¹) after 18 h. C) HUVEC cells migration data after 24 and 48 h. D) Quantification of tube formation (measured by the number of branches) after 18 h. Graph values are expressed as mean ± SD (n ≥ 3) and statistical differences were performed using unpaired t‐test student (^* P < 0.05).