Feasibility of 19F-NMR for assessing the molecular mobility of flufenamic acid in solid dispersions.
The purpose of the present study was to clarify the feasibility of 19F-NMR for assessing the molecular mobility of flufenamic acid (FLF) in solid dispersions. Amorphous solid dispersions of FLF containing poly(vinylpyrrolidone) (PVP) or hydroxypropylmethylcellulose (HPMC) were prepared by melting and rapid cooling. Spin-lattice relaxation times (T1 and T1r) of FLF fluorine atoms in the solid dispersions were determined at various temperatures (-20 to 150oC). Correlation time (tc), which is a measure of rotational molecular mobility, was calculated from the observed T1 or T1r value and that of the T1 or T1r minimum, assuming that the relaxation mechanism of spin-lattice relaxation of FLF fluorine atoms does not change with temperature. The tc value for solid dispersions containing 20% PVP was 2-3 times longer than that for solid dispersions containing 20% HPMC at 50oC, indicating that the molecular mobility of FLF in solid dispersions containing 20% PVP was lower than that in solid dispersions containing 20% HPMC. The amount of amorphous FLF remaining in the solid dispersions stored at 60oC was successfully estimated by analyzing the solid echo signals of FLF fluorine atoms, and it was possible to follow the overall crystallization of amorphous FLF in the solid dispersions. The solid dispersion containing 20% PVP was more stable than that containing 20% HPMC. The difference in stability between solid dispersions containing PVP and HPMC is considered due to the difference in molecular mobility as determined by tc. The molecular mobility determined by 19F-NMR seems to be a useful measure for assessing the stability of drugs containing fluorine atoms in amorphous solid dispersions.