In the present study, the molecular state of drug-rich amorphous nanodroplets was evaluated using NMR techniques to reveal the mechanism underlying the crystallization inhibition of drug-rich amorphous nanodroplets by a polymer. Ibuprofen (IBP) with a low glass transition temperature was used for direct characterization of drug-rich amorphous nanodroplets. Highly supersaturated IBP formed IBP-rich amorphous nanodroplets through phase separation from aqueous solution. Increasing the concentration of hypromellose (HPMC) in the aqueous solution contributed to the inhibition of IBP crystallization and maintenance of supersaturation at IBP amorphous solubility. Solution 1H NMR measurements of IBP supersaturated solution containing IBP-rich amorphous nanodroplets clearly showed two kinds of 1H peaks derived from the dissolved IBP in bulk water phase and phase-separated IBP in IBP-rich amorphous nanodroplets. NMR spectral analysis indicated that HPMC did not affect the chemical environment and mobility of the dissolved IBP. However, 1H spin–spin relaxation time measurements clarified that the dissolved IBP in the bulk water phase was exchanged with the IBP-rich amorphous nanodroplets with an exchange lifetime of more than 10 ms. Moreover, the 1H peaks of HPMC partially disappeared due to the formation of IBP-rich amorphous nanodroplets, suggesting that a part of HPMC distributed into the IBP-rich amorphous nanodroplets from the bulk water phase. The incorporation of HPMC significantly changed the chemical environment of the phase-separated IBP in the IBP-rich amorphous nanodroplets and strongly suppressed molecular mobility. The resulting molecular mobility suppression effectively inhibited IBP crystallization from the IBP-rich amorphous nanodroplets. Thus, direct investigation of drug-rich amorphous nanodroplets using NMR can be a promising approach for selecting appropriate pharmaceutical excipients to suppress drug crystallization in supersaturated drug solutions.

In this study, the molecular states of supersaturated drugs and crystallization inhibitors in aqueous solutions were characterized using NMR to elucidate the inhibition mechanism of drug crystal nucleation in drug-supersaturated solutions. Polyvinylpyrrolidone (PVP) K12, PVP K25, and 1-ethyl-2-pyrrolidone (NEP) were used as additives to evaluate their ability to inhibit chlorthalidone (CLT) crystal nucleation. Although an inhibitory effect of NEP on the crystal nucleation of CLT was not observed, PVP effectively inhibited CLT crystallization and maintained CLT in a supersaturated state in the long term. In the 1D 1 H NMR spectra, the aromatic proton peaks of CLT showed an upfield shift with increasing CLT concentration, reflected by the self-association of CLT in aqueous solution; the number of self-associates increased with increasing supersaturation level. The presence of the additives in the aqueous solution induced downfield shifts of the CLT peaks, which were the largest in the PVP K25 solution and the smallest in the NEP solution. Nuclear Overhauser effect spectroscopy (NOESY) measurements revealed that the PVP formed a hydrophobic interaction with CLT via the carbon chain of PVP. Furthermore, the spin−spin relaxation rate of the supersaturated CLT was significantly increased by the addition of PVP K25, indicating the mobility suppression of supersaturated CLT by PVP K25, which can be caused by the intermolecular interactions between CLT and PVP K25. Furthermore, CLT mobility suppression by PVP K25 became more significant with increasing CLT supersaturation levels. This implies that PVP K25 interacted particularly with CLT self-associates formed in the supersaturated solution and suppressed their molecular mobility, thereby inhibiting the reorientation from the CLT self-associates to the crystal nucleus. The present study highlights the importance of the molecular weight of crystallization inhibitors on the ability of mobility suppression of a self-associated drug due to the nucleation inhibition effect.