## 3 Results
### Controlled Release Study of \"Free\" RIS, Ca-RIS and Sr-RIS  
Three controlled delivery systems (CDSs) were evaluated in the form of tablets: (a) the \"free\" drug risedronate, (b) Ca-RIS, and (c) Sr-RIS. The active agents, the \"free\" RIS drug and its metal-RIS coordination compounds were mixed and ground with the appropriate recipients in the solid form, and these powders were pressed into tablets. These tablets were immersed into acidic solutions and aliquots were withdrawn at specific time intervals. The detailed protocols for tablet preparation, sampling and drug quantification are described in detail in Sections 2.4 and 2.5. Under the acidic conditions of the drug release experiments, hydrolysis of the metal-O bonds occurs that leads to the degradation of the crystal lattice and the release of RIS into the acidic medium. The drug release was quantified by \({}^{1}\)H NMR spectroscopy and the results were plotted in graphs as \"% drug\" vs. time (in hours). Figure 5: Various views of the crystal structure of Ca-RIS: (**a**) a single 1D chain along the a-axis, (**b**) the coordination sphere of the of Ca2+ center with Ca–O bond distances, and (**c**) the hydrogen bonding schemes (dashed green lines) between the 1D chains. Color codes: Ca, purple; P, orange; O, red; C, black; N, blue; H, white. With tablets containing the \"free\" RIS drug (with excipients), a fast release was observed during the first 2 h of the experiments, followed by a gradual drop in the quantity of RIS in solution (Figure 7). Crystal formation was noted. The experiment was stopped after 72 h; the clear colorless crystals were collected by filtration and were analyzed by powder X-ray diffraction. It was unequivocally proven that the precipitated solid was risedronic acid monohydrate, which is sparingly soluble in acidic solutions (inset of Figure 6) [19]. The gradual drop on measured \"free\" RIS is undoubtedly due to the low solubility of risedronic acid monohydrate (at the experimental pH). This is evidenced by the complicated H-bonding network of the phosphonate and pyridinium groups and the lattice water molecule, and by the strong \(\pi\)-\(\pi\) interactions between the pyridyl rings (centroid-to-centroid 3.485 A) of neighboring RIS molecules. The insolubility of the risedronic acid under these acidic conditions hampered the implementation of reliable release experiments and prohibited any meaningful comparisons with the release profiles of the Ca/Sr-RIS systems. Therefore, appropriate cationic additives were sought that could act as drug solubilizers. This strategy was based on the notion that the additive's cationic groups will form electrostatic interactions with the anionic phosphonate groups of the RIS drug, thus preventing its crystallization. Two cationic additives were tested, namely, an amine-terminated polyamidoamine dendrimer (PAMAM of generation 2 with 16 surface -NH\({}_{2}\) groups) and a linear polyethyleneimine polymer (LPEI, of MW 22,000 Daltons). Their schematic structures are shown in Figure 8. These additives were used in our group for the stabilization of anionic silicate in aqueous systems [20; 21; 22; 23; 24; 25]. Figure 6: Various views of the crystal structure of Sr–RIS: (**a**) view of 2D layers (hydrogen atoms are omitted for clarity), (**b**) \(\pi\)–\(\pi\)–\(\pi\)” stacking interactions between the pyridyl rings, and (**c**) the coordination sphere of the of Sr\({}^{2+}\) center with Sr–O bond distances. Color codes: Sr, green; P, orange; O, red; C, black; N, blue; H, white. LPEI was used in such a quantity that the RIS:amine molar ratio was 1:1. PAMAM-2 was added in excess, so the RIS:amine(surface) molar ratio was 1:4.5. Two different experiments were conducted. Initially, the PAMAM-2 and LPEI additives were tested as drug solubilizers for the \"free\" RIS system in a \"proof-of-concept\" experiment. The results are depicted in Figure 9 and demonstrate the efficacy of the additives to prevent the precipitation of RIS acid monohydrate.