Inhibition of the solid state transformation of carbamazepine in aqueous solution: impact of polymeric properties.

The effects of polymers on the anhydrate-to-hydrate transformation of carbamazepine (CBZ) was investigated. The three types of polymers studied were polyvinylpyrrolidone (PVP), polyvinyl alcohol (PVA) and substituted celluloses which included hydroxypropyl methylcellulose (HPMC) and methylcellulose (MC). Anhydrous CBZ was added to dilute aqueous polymer solutions and Raman spectroscopy measurements were collected to monitor the kinetics of the solution-mediated transformation to CBZ dihydrate. Polymers exhibiting the greatest inhibition were able to reduce the growth phase of the solution-mediated transformation and change the habit of the hydrate crystal indicating polymer adsorption to the hydrate crystal surface as the mechanism of inhibition. The results of the various polymers showed that short chain substituted celluloses (HPMC and MC) inhibited the CBZ transformation to a much greater extent than longer chains. The same trend was observed for PVP and PVA, but to a lesser extent. These chain length effects were attributed to changes in polymer confirmation when adsorbed on the crystal surface. Additionally, decreasing the percentage of hydroxyl groups on the PVA polymer backbone reduced the ability of the polymer to inhibit the transformation and changing the degree of substitutions of methyl and hydroxypropyl groups on the cellulosic polymer backbone had no effect on the transformation.