Molecular mobility of nifedipine−PVP and phenobarbital−PVP solid dispersions as measured by 13C-NMR spin-lattice relaxation time

Amorphous nifedipine−PVP and phenobarbital−PVP solid dispersions with various drug contents were prepared by melting and subsequent rapid cooling of mixtures of PVP and nifedipine, or phenobarbital. Chemical shifts and spin-lattice relaxation times (T1) of PVP, nifedipine, and phenobarbital carbons were determined by 13C-CP/MAS NMR to elucidate drug−PVP interactions and the localized molecular mobility of drug and PVP in the solid dispersions. The chemical shift of the PVP carbonyl carbon increased as the drug content increased, appearing to reach a plateau at a molar ratio of drug to PVP monomer unit of approximately 1:1, suggesting hydrogen bond interactions between the PVP carbonyl group and the drugs. T1 of the PVP carbonyl carbon in the solid dispersions increased as the drug content increased, indicating that the mobility of the PVP carbonyl carbon was decreased by hydrogen bond interactions. T1 of the drug carbons increased as the PVP content increased, and this increase in T1 became less obvious when the molar ratio of PVP monomer unit to drug exceeded approximately 1:1. These results suggest that the localized motion of the PVP pyrrolidone ring and the drug molecules is reduced by hydrogen bond interactions. Decreases in localized mobility appear to be one of the factors that stabilize the amorphous state of drugs. © 2005 Wiley-Liss, Inc. and the American Pharmacists Association