Predictive Calculation of the Crystallization Tendency of Model Pharmaceuticals in the Supercooled State from Molecular Dynamics Simulations.

Abstract

Molecular dynamics (MD) simulations were used to perform a comparative study of the crystallization tendency from the melt of two model pharmaceutical compounds: felodipine and nifedipine. Two crystalline polymorphs of nifedipine (N(α), N(β)) and felodipine (FI, FII) have been studied. Calculations were performed on liquid and crystal systems separately in order to determine their main physical properties (diffusivity, density, and enthalpy). A fair agreement was found between the simulation and the known experimental data confirming the ability of the force field GAFF to reproduce accurately the experimental data for both compounds. Simulations of the crystal-liquid interface enabled the determination of the melting temperature and the interfacial free energy of the different polymorphs. Guided by the classical nucleation theory (CNT) predictions and different growth mechanism models (normal, two-dimensional, and screw dislocation), the nucleation and growth rates have been determined. The present investigation particularly raises the very important role of the solid-liquid interfacial free energy and its interplay with the driving force during the crystallization. The origin of the higher crystallization tendency of nifedipine with respect to felodipine is discussed from the present computed kinetic and thermodynamical factors.