(W1130-10-68) Development of Stable Flocculated Suspension Formulation for Preclinical Studies

Purpose: Due to the ease of preparation covering a wide range of dosing concentrations, suspension formulations are commonly used in drug discovery for preclinical animal PK, PD, and toxicity studies. Additionally, a suspension formulation serves as a surrogate for a disintegrated immediate release dosage form (capsule or tablet) that allows screening and selection of compounds with better developability attributes. The dose accuracy of the suspension depends on its homogeneity. The suspension should be flocculated and easily resuspended upon settling to provide better dosing accuracy. Additionally, when formulating for any animal study, the tolerability for each excipient at the proposed concentration needs to be considered for each specific species. This study describes the development of a flocculated suspension for a preclinical study. PTC compound is a weak base with a molecular weight just under 500 g/mol that had a very high tendency to agglomerate (Compound A). When compound A was initially formulated at 2 mg/mL and 30 mg/mL in aqueous 0.5% w/w hydroxypropyl methyl cellulose (HPMC) and 0.1% w/w polysorbate 80, the resulting suspension was not flocculated and settled to form a gum at the bottom of the container. Efforts to develop a physically stable, well flocculated suspension formulation for this compound are described below.

Methods: To develop a well-flocculated and physically stable suspension, three different approaches were explored: 1. Higher surfactant concentrations, 2. Use of different polymers, and 3. Modulation of pH. For the first approach, the concentration of polysorbate 80 was increased from 0.1% w/w to up to 2% w/w for the suspension formulation. This approach was used to facilitate wetting and to minimize agglomeration. The surface charge of weakly basic compounds can be altered by reducing the pH. With this mechanism in mind, the suspensions were formulated at 2 mg/mL and 30 mg/mL by reducing the pH using acid (pH between 3-5.5). Finally, two alternate polymers were tested with the intention of coating the particles to create steric hinderance for prevention of aggregation. The prepared suspensions were evaluated for physical and chemical stability for a period of 7 days.

Results: In the first approach, caking or aggregation was observed with polysorbate 80 levels below 2% w/w (Table 1); however, 2% w/w polysorbate 80 was not well tolerated in rats and dogs. Therefore, this approach is not appropriate. For the second approach, hydroxypropyl methyl cellulose acetate succinate (HPMC-AS) and sodium carboxymethyl cellulose (CMC) were evaluated as alternatives to HPMC with concentrations ranging from 0.5% to 1% w/w. HPMC-AS produced no improvement and the suspension incorporating sodium CMC (0.5% w/w) and polysorbate 80 (0.1% w/w) initially produced a flocculated suspension; however, caking was observed after 24 hours. Therefore, this approach was also not appropriate. In the final approach, the suspension pH was reduced to fully protonate the test article (pKa ~ 8) and, thereby, prevent aggregation. Phosphoric and hydrochloric acid both were used to reduce the pH of the suspensions. All pH adjusted formulations showed good stability at room temperature for up to 21 days with ease of re-dispersion on mixing.

Conclusion: Neither increasing the amount of surfactant nor switching the polymers improved the physical stability of the aqueous suspension of Compound A. This study demonstrated that adjusting the pH to ~5 (so that it is well tolerated in the animals) produced a flocculated, resuspendable, and chemically stable formulation suitable for animal dosing.