Crystallization rate of amorphous nifedipine analogues unrelated to the glass transition temperature.

To examine the relative contributions of molecular mobility and thermodynamic factor, the relationship between glass transition temperature (T(g)) and the crystallization rate was examined using amorphous dihydropyridines (nifedipine (NFD), m-nifedipine (m-NFD), nitrendipine (NTR) and nilvadipine (NLV)) with differing T(g) values. The time required for 10% crystallization, t(90), was calculated from the time course of decreases in the heat capacity change at T(g). The t(90) of NLV and NTR decreased with decreases in T(g) associated with water sorption. The t(90) versus T(g)/T plots almost overlapped for samples of differing water contents, indicating that the crystallization rate is determined by molecular mobility as indicated by T(g). In contrast, differences in the crystallization rate between these four drugs cannot be explained only by molecular mobility, since the t(90) values at a given T(g)/T were in the order: NLV>NTR>NFD approximately m-NFD. A lower rate was obtained for amorphous drugs with lower structural symmetry and more bulky functional groups, suggesting that these factors are also important. Furthermore, the crystallization rate of NTR in solid dispersions with poly(vinylpyrrolidone) (PVP) and hydroxypropyl methylcellulose (HPMC) decreased to a greater extent than expected from the increased T(g). This also suggests that factors other than molecular mobility affect the crystallization rate.