Tableting of fluidized bed granules containing living microorganisms

Tablets are the favored dosage form for numerous active pharmaceutical ingredients, among others because they are easy to take, ensure safe dosing and allow cost-effective production on a large scale. This dosage form is also frequently chosen for the administration of viable probiotic microorganisms. Saccharomyces cerevisiae cells granulated in a fluidized bed process, with dicalcium phosphate (DCP), lactose (LAC) and microcrystalline cellulose (MCC) as carrier materials, were tableted using a compaction simulator, varying the compression stress. The tablets were analyzed regarding physical properties, e.g., porosity and tensile strength, as well as microbial survival. Carrier material and compression stress showed a significant influence on survival rate and physical tablet properties. The dependencies were related to material specific deformation characteristics and linked to mechanistic approaches to explain the different sensitivities.

Introduction

Probiotic microorganisms confer health benefits for the patient, when administered in adequate amounts and viable form [1]. As their viability is essential, sufficient storage stability and special precautions during the processing are necessary to ensure effective dosage forms.

Drying is the most suitable method for long term conservation of microbial viability. Different drying techniques could be used, for example lyophilization, spray drying or fluidized bed drying. In a previous work, fluidized bed granulation was successfully used to dry microorganisms preserving their viability to a certain degree and dependency of survival rate on different formulation and process parameters was discussed [2]. The tableting of these granules was also briefly presented, but so far only a consideration of the microbiological properties of the tablets has been made and a mechanistic understanding of the stress on the cells is still lacking.

This research gap is addressed in the present study, in which the focus lays on the tablet preparation as the subsequent step of the process chain to tablets as final dosage forms. Tableting of viable, mostly probiotic microorganisms or spores is not completely new. However, in most cases lyophilized microorganisms were used [[3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19]]. Before tableting, lyophilizates have to be ground to small particles to enable reproducible die filling. It was shown that this step might be critical with respect to the microbial viability [19]. It was assumed that during comminution, cracks do not only pass through the surrounding cryoprotectant matrix, but also run through the cells and disintegrate the cell walls, especially when the surrounding matrix is mechanically stronger than the cells and the binding strength between cell surface and protectants [19].

Sometimes (hot) air dried (oven, laminar flow workbench, purged desiccator) [[20], [21], [22], [23], [24], [25], [26]] or spray dried [27] microorganisms were used, but especially for tableting of fluidized bed granulated microorganisms, literature is rare. However, in principle, it can be deduced from the aforementioned publications that tableting is a critical process step for the survival of the microorganisms contained. Even if the published survival rates are of different orders of magnitude, it is generally true that the higher the compaction pressure, the lower the survival [[4], [5], [9], [10], [11], [19], [20], [21], [22], [23], [24], [27], [28], [29]]. In some cases, it has also been reported that no further reduction in viability was observed when a certain compression stress was exceeded [[10], [27]], or that low compaction pressures were tolerated without loss of viability [[5], [21]]. Sometimes, however, no concrete statements on survival during tableting can be taken from the published results, since the determination of viable microorganisms is accompanied by testing for enteric resistance. Lower survival rates for tablets produced at higher compression stresses cannot always be observed here because of improved survival during tablet disintegration due to lower tablet porosity and by this slower disintegration in simulated gastric juice [8]. Some publications focus on the formulation and do not consider the influence of the compression stress at all.

At least for wet granulated and subsequently, e.g., oven-dried microorganisms, a few publications exist [[20], [21], [22], [23], [24], [25], [26]], in some of which the spatial distribution is also discussed [[20], [23]]. For example, the survival of Bacillus megaterium spores during compaction of granules prepared with the spore suspension as the binder, of granules prepared with sterile water and dry added spores as well as non-granulated excipients dry mixed with spores was investigated [20]. For brittle, fragmenting materials (LAC and Emdex) no influence of the procedure on the survival was found but for a plastic material (KCl), survival was reduced in case of granulation with spore suspension compared to dry addition of spores [20]. This was attributed to higher shear stresses when the spores were located in the inner of the plastically deforming granules [20]. However, it must be stated, that in general the survival was higher with the plastic KCl compared to LAC and nearly the same compared to Emdex. In addition, a potentially harmful KCl concentration must be taken into account, especially in the case of tablets made from KCl granules with the spore suspension as the granulation liquid, since in this case the spores are exposed to high concentrations for the longest time. In another study, Staphylococcus aureus or Enterobacter cloacae were wet mixed with LAC, maize starch and MCC and the survival during tableting of the dried powders was analyzed [23].

When more plastic materials (MCC and starch) were used, survival rates were lower compared to more brittle particles (LAC) and this was explained with the greater interparticulate contact enhancing mechanical disruption of cells in case of ductile deformation [23]. In this study, the aim is to consider the tableting of fluidized bed granulated microorganisms. For detailed information regarding fluidized bed granulation of the same strain, please refer to [2]. The yeast Saccharomyces cerevisiae was used due to its easy accessibility. However, the probiotic yeast Saccharomyces cerevisiae var. boulardii belongs to this specie [30]. Therefore, same dependencies of survival on process and formulation parameters were expected especially for the probiotic yeast but is also assumed for probiotic bacteria even if the concrete values of survival will not necessarily be the same due to strain specific tolerances. Three different tableting excipients were chosen as carrier materials for the granulation process (dicalcium phosphate (DCP), lactose (LAC), and microcrystalline cellulose (MCC)). The granules were compressed and tablets were analyzed regarding physical properties, e.g., tablet porosity, tensile strength as well as the survival of the yeast cells. Additionally, deformation characteristics were assessed in-die during compression. Based on these findings, the survival rates of the yeast cells throughout tableting are interpreted and approaches to explain relationships are derived.

Following excipients are mentioned in the study besides other: Emdex

Karl Vorländer, Lukas Bahlmann, Arno Kwade, Jan Henrik Finke, Ingo Kampen, Tableting of fluidized bed granules containing living microorganisms, European Journal of Pharmaceutics and Biopharmaceutics, Volume 187, 2023, Pages 57-67, ISSN 0939-6411, https://doi.org/10.1016/j.ejpb.2023.03.011.

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