(M0930-05-33) Development of a Water-Soluble Mini-Tablet Formulation for Flow Cytometry Applications

Purpose: The objective of this study was to develop a water-soluble tablet formulation capable of effectively carrying micron-scale polystyrene beads as a flow cytometry counting standard. The goal was to ensure complete water solubility of the tablet formulation, allowing for accurate counting and analysis of small particles and cells while minimizing potential interference from residual particulate matter. The tablet formulations were designed for achieving sufficient hardness for shipping durability and rapid dissolution in small volumes of aqueous samples, aiming to enhance the reliability and reproducibility of flow cytometry measurements. The final product consisted of 3.21 mg mini-tabs prepared using both single-tip and multi-tip 2mm SRC tooling, each carrying approximately 50,000 polystyrene beads.

Methods: The final product was produced via a modified wet granulation procedure. A slurry comprising of a diluted portion of the bead suspension was mixed with mannitol in a high shear mixer (Freund-Vector GMX-Lab Micro) at 30rpm. The slurry was convection dried at 60°C overnight and tested for %LOD (Sartorius Moister Analyzer) to ensure proper drying. The dried cake was then milled utilizing either a Gerteis Small Scale Mill (Gerteis Maschinen, Stampfstrasse, Switzerland) or a rotary conical mill (Quadro Comil U5) running at 1000-3000rpm with the 1607 Impeller, and multiple screen types (018R, 024R, 032R, and 156Q). PEG6000 was micronized using Fluid-Energy Modell 00 jet mill. The resulting granule were subsequently blended with extra-granular components in amber jars at 23 rpm. Final blends were tableted using a single station press (Natoli RD10A) with a 2-mm SRC tooling targeting a 3.21 mg tablet weight to achieve the appropriate number of beads in each tablet.

Results: SEM imaging revealed minimal deformation of polystyrene micro-spheres during milling and compaction, addressing initial concerns about breakage or deformation (Fig. 1). The formulation approach employed sugar alcohols, with mannitol serving as the primary bead carrier, maltitol as the main brittle filler, sorbitol as the primary plastic filler, and micronized PEG 6000 as a water-soluble tablet lubricant. Additional components, such as Kollidon 30 and Polaxomer 407, were included for bead binding and separation, respectively. Adequate tablet strength and processability were achieved by incorporating equal amounts of maltitol and sorbitol (Fig. 2). The final blend exhibited minimal defects during tableting. However, punch sticking was observed under high humidity conditions due to the hygroscopic nature of the excipients. The resulting tablets demonstrated sufficient hardness for shipping and rapid dissolution in test samples. Initial prototypes exhibited a reduced concentration of detected beads, along with a smaller proportion of single beads and debris (Fig. 3).

Conclusion: This study demonstrated the proof-of-concept feasibility of a water-soluble tablet formulation as a carrier for insoluble polystyrene micro-spheres for use in applications such as diagnostics and flow cytometry. Optimization specific to the application was crucial for achieving accuracy of the bead content uniformity. Although the formulation fell short of the stringent project requirements for bead count due to the low number of beads and resulting low weight percentage, the framework established in this study holds promise for further exploration of fully water-soluble mini-tablet formulations in pharmaceutical and other life science applications

Figure 1. SEM image of micro-spheres suspended in dried Mannitol.

Figure 2. Prototype water-soluble tablet formulation.

Figure 3. Flow cytometry data displaying final concentrations and % singlets. Target concentration is 100 beads/uL and >95% singlets.