Fast Tablet Coating with Xylitol

INTRODUCTION

The traditional drageification procedure dates back to the middle of the 18^th century. And it is still used without any significant improvements of recipes or technical procedures. It is a time-consuming process, requiring specific equipment and a specialized workforce. This hard coating process is therefore not the first formulator’s choice, despite its great organoleptic and galenic potential. A well-done hard coating offers plenty of additional formulation options, such as a new taste sensation, reduced friability, an important moisture seal and additional protection against fake products.

Sugar and polyol coating are based on crystallization while the drageification process does not conform to basic physical conditions of crystallization. For example, the high drying temperature in the coating pan hinders the aimed crystallization, favoring the re-dissolution of already formed crystals.

OBJECTIVES

Trials were done to understand why the traditional hard coating process is such a long and non-automated process. Improvements of the process conditions shorten the process itself, transforming it to a quasi-continuous crystallization cycle.

A new pharmaceutical-minded coating process with xylitol is developed, overcoming the traditional drawbacks of the old process.

MATERIALS AND METHODS

Materials

XYLISORB® 300 xylitol from Roquette Frères (Lestrem, France)

Titanium dioxide (grade 1171) from Kronos Titan GmbH (Leverkusen, Germany)

Modified starch from Roquette Frères (Lestrem, France)

Methods

Drageification process:

All hard coating trials were done in standard film coating equipment, using different batch sizes (from 1 kg scale to big quantities). For this example, we used:

RAMA COTA, with a 19” perforated pan

Spray gun: Schlick 970/7-1 S75 (nozzle diameter 0.8 mm)

Pump: Watson Marlow model 323, Head: 313 DW

The coating syrup was prepared out of a dry powder blend (see table 1). It is used as a 60% DS syrup, dissolved in water at room temperature.

Table 1. Composition of the coating syrup (in dry substance)

The hard coating runs via spraying the 60% DS syrup on the cores (quantities e.g., 5 kg), targeting a coating level between 5% and 30% weight gain. The basic process conditions are:

• Pan speed: 8 RPM
• Inlet air temperature: 45°C
• Tablet bed temperature: 30-35°C
• Spray rate: 3.5 to 8.5 g/min/kg

The spray rate is usually low in the first process phase, to trigger the continuous xylitol crystallization process. After the process stabilization (typically after 5% weight gain), spray rate increases up to 8.5 g/min/kg. In some rare cases, a reduced coating rate during the end phase of the process contributes to better surface smoothing and finishing.

RESULTS

The selected process conditions leverage some traditional obstacles to obtain a fast and complete crystallization.

Lowering the bed temperature limits the xylitol solubility and the viscosity of its saturated solutions (e.g., on the tablet surface during the coating process). This is particularly important for xylitol, having a significantly higher solubility/temperature dependence than saccharose (see figure 1).  

Figure 1. Solubility of xylitol and sugar at different temperatures

Targeting a lower but constant core bed temperature permits faster crystallization, due to a higher mobility of the xylitol molecules. The xylitol crystallization enthalpy assists the water evaporation and permits a rapid weight gain without excessive heating. In consequence, it is possible to switch to a continuous crystallization state, permitting very fast and homogenous weight gain. Anyhow, the tablet surface remains dry during the whole xylitol coating process. Tables 2 and 3 summarize the differences between the traditional hard coating process and the rationalized process.

Table 2. Traditional hard coating process. More than 50 separate production cycles are needed to obtain a satisfying result. 

Table 3. Rationalized hard coating process, example xylitol 

Applying coating levels up to 20% did not bridge tablet logos (see figure 3).

It is therefore possible to standardize/automatize the coating process, meeting pharmaceutical production standards.

Adding a polymer gives the coating layer more mechanical resistance (e.g., Arabic gum or pregel starch), preventing edge chipping.

The presence of titanium dioxide provides high opacifying properties (see figure 2). Typically, the coating level of about 15% could completely cover colored and/or inhomogeneous tablets (see figure 2). The pigment quantity can be adapted to the formulation needs. Alternatively, it is possible to design white, but titanium dioxide free, recipes and colored versions.

Figure 2: xylitol coated, initially red colored tablets.  

Figure 3. Xylitol coated tablet. Coating level 20% mass gain.

CONCLUSION

A rational approach of the hard coating process permitted a new, quasi-continuous coating process. It requires only one coating cycle and is therefore much faster than the traditional hard coating. In addition, it runs in traditional film coating equipment and does not request supplementary equipment. It helps obtain white or colored hard coatings, while also preserving tablet logos (without bridging) up to a coating level of about 20%.

REFERENCES

WO2018234248A1, Sugar coating method and sugar coated solid forms with irregular shapes, December 2018

https://worldwide.espacenet.com/patent/search/family/059521110/publication/US2020214315A1?q=US20200214315A1

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