Nano-scale and molecular-level understanding of wet-milled indomethacin/poloxamer 407 nanosuspension with TEM, suspended-state NMR, and Raman measurements
We investigated the formation and stabilization mechanisms of indomethacin (IMC)/poloxamer 407 nanosuspensions. Stable nanosuspensions were prepared via 24 h wet-milling of three IMC forms (γ form, α form, and amorphous) with poloxamer 407. Cryogenic-transmission electron microscopy images of nanoparticles obtained using γ-form IMC indicated a rhombic-plate shape. In contrast, needle-like nanoparticles were observed in the nanosuspensions of α-form and amorphous IMC. Suspended-state cross polarization 13C NMR and Raman measurements directly detected the molecular states of IMC in nanosuspensions. IMC existed in its initial crystal form when γ-form and α-form IMC were used; amorphous IMC transformed into crystalline α-form IMC. Suspended-state 13C pulse saturation transfer NMR measurements revealed the molecular state of poloxamer 407 on the surface of IMC crystals. The polypropylene oxide group adsorbed to the IMC crystal surface via hydrophobic interactions, while the polyethylene oxide group on the surface was as flexible as that in polymeric micelles. The equilibrium of poloxamer 407 between micelle and nanocrystal surfaces was slower than the NMR time scale, which could stabilize the dispersion of the nanoparticles in water. The time interval evaluation during the wet-milling process revealed that α-form IMC nanocrystals could be efficiently prepared via wet-milling using amorphous IMC as the starting material.