Direct evaluation of molecular States of piroxicam/poloxamer nanosuspension by suspended-state NMR and Raman spectroscopies

A nanosuspension of piroxicam (PXC) and poloxamer 407 (poloxamer) prepared by the wet milling method was directly evaluated at the molecular level from the viewpoint of both solution and solid phases. (13)C solution-state NMR measurements revealed a reduction in the concentration of dissolved poloxamer in the nanosuspension. Furthermore, the fraction of dissolved poly(ethylene oxide) (PEO) chain, which is the hydrophilic part of poloxamer, was higher than that of dissolved poly(propylene oxide) (PPO) chain, the hydrophobic part. (13)C suspended-state NMR and Raman spectroscopies detected both solid-state PXC and poloxamer involved in the nanoparticles. Interestingly, the coexistence of crystalline and amorphous PXC in the nanoparticle was demonstrated. The yellow color of the nanosuspension strongly supported the existence of amorphous PXC. Changes in the peak intensity depending on the contact time in the suspended-state NMR spectrum revealed that the PEO chain of poloxamer in the nanoparticle had higher mobility compared with the PPO chain. The PEO chain should project into the water phase and form the outer layer of the nanoparticles, whereas the PPO chain should face the inner side of the nanoparticles. Amorphous PXC could be stabilized by intermolecular interaction with the PPO chain near the surface of the nanoparticles, whereas crystalline PXC could form the inner core.