Calcium Medronate-Based Metal–Organic Frameworks as Multifunctional Biomaterials

Metal-organic frameworks (MOFs) can be prepared from bioactive molecules, which are released during the degradation of the material in the body. Particularly, MOFs have recently emerged as bisphosphonate (BP) drug delivery systems. In this work, two novel MOFs based on the smallest bisphosphonate medronic acid (MA) and calcium with formulas [Ca-(CH₄O₆P₂)·H₂O] (GR-MOF-23) and [Ca-(CH₄O₆P₂)·CH₃OH] (GR-MOF-24) in aqueous and/or methanolic solutions at room temperature were synthesized and fully characterized. The stability test performed in simulated physiological conditions (a phosphate buffer saline (PBS, pH = 7.4, 10 mM) solution at 37 °C) showed a progressive Ca²⁺ leaching from both GR-MOF-23 and GR-MOF-24, achieving 38.0 ± 2.8 and 35.8 ± 3.9% release of calcium after 1 week of suspension. Interestingly, the recovered solid residues from the stability tests were identified as apatite and calcium phosphate phases, which might facilitate the formation of bone apatite and collagen. The antibacterial activity of GR-MOF-23 and GR-MOF-24 was investigated against Escherichia coli and Staphylococcus aureus, among the most relevant human pathogens, causing a wide variety of infections in bone fracture in osteoporosis and prosthesis. While both materials exhibited bacteria growth inhibition, GR-MOF-24 also showed a bactericide action, likely due to a more progressive release of Ca²⁺, which is the ion related to the improved stability of the biofilm. These innovative materials present exciting opportunities for developing antibacterial surfaces in prosthetics and the treatment of bone fracture infections.