Breaking patterns of press-coated tablets during the diametral compression test: Influence of the product, geometry and process parameters

Press-coated tablets are a high-interest technology in chronopharmaceutics, for modified release applications. As for any kind of tablet, the test of the mechanical resistance is of primary importance at the industrial level during both the development and production steps. For this purpose, the diametral compression test is commonly used in the industry for press-coated tablets. Nevertheless, the result of this test can be much more complex compared to the case of single layer tablets. This work aims to study the applicability of this test to press-coated tablets. Diametral compression tests were performed on press-coated tablets obtained with different products (shell/core), shell sizes and compaction pressures. Four types of breaking profiles were found: total diametral, shell diametral, around the core and laminated depending on the process parameters/products used to obtain the tablet. Digital image correlation was used in order to understand the breaking patterns especially in terms of failure initiation and propagation. The kind of breaking pattern obtained is dependent on the final structure of the tablet in terms of density distribution and thus of elastic properties. To confirm the findings, numerical simulations by the finite element method was used to visualize the stress distribution inside the tablet and confirm the influence of the process parameters. The multiple failure profiles obtained imply that the output value of the diametral compression test applied to press-coated tablets should be taken with caution.

Erodible coatings based on HPMC and cellulase for oral time-controlled release of drugs

Oral drug delivery systems for time-controlled release, intended for chronotherapy or colon targeting, are often in the form of coated dosage forms provided with swellable/soluble hydrophilic polymer coatings. These are responsible for programmable lag phases prior to release, due to their progressive hydration in the biological fluids. When based on high-viscosity polymers and/or manufactured by press-coating, the performance of functional hydroxypropyl methylcellulose (HPMC) layers was not fully satisfactory. Particularly, it encompassed an initial phase of slow release because of outward diffusion of the drug through a persistent gel barrier surrounding the core. To promote erosion of such a barrier, the use of a cellulolytic product (Sternzym® C13030) was here explored. For this purpose, the mass loss behavior of tableted matrices based on various HPMC grades, containing increasing percentages of Sternzym® C13030, was preliminarily studied, highlighting a clear and concentration-dependent effect of the enzyme especially with high-viscosity polymers. Subsequently, Sternzym® C13030-containing systems, wherein the cellulolytic product was either incorporated into a high-viscosity HPMC coating or formed a separate underlying layer, were manufactured. Evaluated for release, such systems gave rise to more reproducible profiles, with shortened lag phases and reduced diffusional release, as compared to the reference formulation devoid of enzyme.

The influence of core tablets rheology on the mechanical properties of press-coated tablets

Press-coating (also called compression coating or dry coating) consists of a second compression of an outer layer of material over a preformed tablet core. Despite being old, this technology has returned to popularity due to its widespread use in preparation of chronotherapeutic dosage forms. The literature available on press-coated tablets has mainly investigated drug release kinetics, while there is a lack of information about their mechanical properties. Here we study, for the first time, the effect of material properties and manufacturing parameters on the mechanical characteristics of press-coated tablets. Firstly, we show that the stiffness of the bare core tablets depends on the material type and, in case of viscoelastic materials, also depends on the compression pressure. We then demonstrate that less stiff (i.e. more viscoelastic) core tablets deform to a greater extent upon the second compression and thus allow the formation of less porous, harder coats and with a more homogenous density distribution. Finally, we find that changes in the mechanical properties of press-coated tablets over one month storage are almost negligible. Our data suggest that viscoelastic rather than stiff cores should be used in dry coating, as they promote the formation of more homogenous coats and with better mechanical properties.