Precipitation Profiles of Dipyridamole in Biorelevant pH-Shift Dissolution Test with Clinically Relevant Dose-to-Fluid Volume Ratio Focusing on Liquid–Liquid Phase Separation

The purpose of this study was to evaluate the precipitation profiles of dipyridamole (DPM) in a biorelevant pH-shift dissolution test under a clinically relevant dose-to-fluid volume (Dose/FV) ratio, focusing on liquid–liquid phase separation (LLPS). In the pH-shift dissolution test, DPM was added to an hydrochloric acid solution (pH 2.0, 450 mL) as the solid drug substance or formulation (Dose/FV = 400 µg/mL, corresponding to a 100 mg dose in 250 mL). In the pH-shift precipitation test, DPM was fully dissolved before the pH shift. After 10 min, the pH was adjusted to 6.5 by adding a 10-fold concentrated bicarbonate buffer (BCB), yielding the conditions of FV = 500 mL and BCB = 10 mM. Under pH 2.0, approximately 90% of the dose dissolved within 10 min. Immediately after the pH shift, turbidity appeared within 1 s and the dissolved DPM concentration dropped to the LLPS concentration (97 µg/mL) (STEP 1), remained there for 3–5 min, and then declined toward the crystalline solubility (6 µg/mL) (STEP 2). In the pH-shift precipitation test examined under polarized light microscopy, noncrystalline droplets appeared instantaneously and persisted for approximately 2 min before crystallization. These results indicate that STEP 1 and STEP 2 corresponded to LLPS and crystallization, respectively. Although DPM is a free-base drug that does not generate LLPS on its own and has been categorized as a rapid crystallizer, this study experimentally demonstrated a two-step precipitation profile under clinically relevant pH-shift and Dose/FV conditions, in which LLPS preceded crystallization.