Impact of solid state properties of sodium naproxen hydrates on their technological performance

Sodium naproxen, a member of the class of non-steroidal anti-inflammatory drugs (NSAIDs), exists in one anhydrous form and four hydrated ones: one monohydrate, two dihydrate and one tetrahydrate. Sodium naproxen (SN) forms can be summarised as follows: • the anhydrous sodium naproxen (ASN) is the commercialised form; • the monohydrated sodium naproxen (MSN), is obtained by dehydration of the dihydrated sodium naproxen (DSN), according to Kim and Rousseau (2004); • the dihydrated sodium naproxen (DSN) is obtained by exposing the ASN to 55% RH according to Di Martino et al. (2001); • the dihydrated sodium naproxen (CSN) is obtained by crystallizing sodium naproxen from water, according to Di Martino et al. (2001) and Kim and Rousseau (2004); • the tetrahydrated form (TSN) is obtained by exposing the ASN at 75% RH according to Di Martino et al. (2007). The hydration state of SN may strongly influence its physico-chemical and technological properties and consequently its bioavailability. Water exposure during storage or pharmaceutical processing can cause changes in the crystal lattice of the starting material. Therefore, a profound understanding and characterisation of SN solid state and phase transitions throughout storage or processing are important in predicting and defining its technological performance. In fact, it was observed that following wet granulation process by high-shear mixergranulator, the drug hydrated to the tetrahydrated form. Performing two different drying procedures, granules of different water content and crystallographic characteristics were obtained. This means that differences in drying procedures could lead to products of different crystallographic properties. The behaviour under compression revealed that one of the batches offered the best tabletability and compressibility. These results make it possible to state that differences in the crystallographic properties and water content of sodium naproxen are such that different hydration/drying processes can alter the drug crystal form and thus the tabletability of the resulting granules. Next, the water uptake of ASN during storage was evaluated. A correlation between water uptake by ASN at two different relative humidities and modifications in tableting and densification behaviour under hydration exists. Models for the hydration kinetics of ASN at 55% and 86%, corresponding to the formation of the dihydrated and tetrahydrated forms respectively, were evaluated assuming Eyring's dependence on temperature. Tabletability, compressibility, compactibility and densification behaviour were determined using an instrumented single punch tablet machine. Kinetic data is consistent with a model where water molecules enter the crystal preferentially along hydrophilic tunnels existing in the crystal structure and corresponding to the propionate side chain. Water inclusion perturbs the crystallographic structure, causing slight structural changes according to the amount and associated to an increase in entropy. The interposition of water molecules between SN molecules weakens intermolecular bonds, and these sites can behave like sliding planes under compression. Such structural changes may explain the improved compression behaviour and modified densification propensity mechanism. Kinetic data describing the water hydration mechanism of ASN explains in an original way the improved tableting and densification properties under hydration. Because different hydration/dehydration processes can alter the drug crystal form, the isothermal dehydration of some of SN hydrates was observed by thermogravimetry at several temperatures. The rate of water removal from the crystal was used to determine the mechanism of dehydration in the solid state, by fitting results with selected expressions corresponding to the most common solid-state processes. The water loss was then evaluated according to Eyring's equation, and both changes in activation enthalpy ( a'„H*) and activation entropy ( a'„S*) were estimated from rate constant values. Experiments made it possible to distinguish different dehydration mechanisms for these hydrate forms, and in particular, to discern the dehydration behaviour of CSN and DSN dihydrate forms. These results add new evidence supporting the X-ray powder diffraction study and showing different patterns for these two forms. X-ray powder diffractometry evaluation of the phase transitions occurring during dehydration of these two dihydrate forms showed that they vary according to dehydration temperature. To finalize our study, the technological and mechanical properties of several solid forms of SN were investigated. Particular attention has been made in order to reduce differences, among the samples, in crystal habit, particle size and distribution, amount of absorbed water, so that only the hydration degree and the crystalline structure might affect the technological behaviour of powders. Thus, the compression behaviours were determined by using an instrumented single punch tablet machine and evaluated through the tabletability, compressibility and compactibility analysis. The results showed that the compression ability was influenced by the hydration degree and the crystalline form. In general, the tabletability was mainly due to the ability of particles to close up by establishing numerous bonds.