Influence Mechanism of Polymeric Excipients on Drug Crystallization: Experimental Investigation and Chemical Potential Gradient Model Analysis and Prediction

In this work, the influence of temperature, stirring speed, and polymeric excipients on crystal growth kinetics of active pharmaceutical ingredients (APIs) mesalazine and allopurinol was systematically investigated through experimental measurement and chemical potential gradient model. The results indicated that the two-step chemical potential gradient model showed good performance in modeling API crystal growth kinetics within the average relative deviations (ARDs) of 5%. Excipients (poly(ethylene glycol) (PEG), hydroxypropyl methyl cellulose (HPMC), and poly(vinyl pyrrolidone) (PVP)) played a crucial role in inhibiting the API crystal growth in all of the investigated systems. On one hand, excipients improved the API solubility, and on the other hand, the crystal growth rate of APIs was reduced remarkably by excipients. By comparing the diffusion rate constant (kd) and the surface reaction rate constant (ks) within the two-step chemical potential gradient model, it was found that the controlling step of mesalazine and allopurinol crystal growth was the surface reaction. Because of the presence of excipients, the diffusion rate constant was markedly decreased and the surface reaction order increased. Excipients hinder the surface reaction and decrease the crystal growth rate. Meanwhile, the crystal growth kinetics of mesalazine and allopurinol were predicted successfully with the ARDs of 2.5 and 6.5% under a certain temperature and stirring speed by the two-step chemical potential gradient model. This work provided a mechanistic understanding of how polymers inhibit API crystal growth.