Prediction of Liquid-Liquid Phase Separation at the Dissolving Drug Salt Particle Surface.

During the dissolution of drug salt particles, liquid-liquid phase separation (LLPS) of a free form can occur within the unstirred water layer (UWL) of the particles (UWL-LLPS). Theoretically, UWL-LLPS occurs when the free form concentration at the salt particle surface ( C 0 ) exceeds the intrinsic LLPS concentration ( S 0 LLPS ) of the free form. In the present study, we attempted to predict UWL-LLPS based on the intrinsic physicochemical properties of drugs. Cyproheptadine hydrochloride (CPH-HCl), diclofenac sodium (DCF-Na), papaverine hydrochloride (PAP-HCl), and propafenone hydrochloride (PRF-HCl) were selected as model drug salts. The pH0 and C 0 values at pHs 4.0-9.5 (citric acid, phosphoric acid, and boric acid, buffer capacity = ca. 4 mM/ΔpH) were calculated using the pK a , solubility product ( K sp ), and diffusion coefficient ( D ) of a drug. S 0 LLPS was measured using the pH-shift method. UWL-LLPS was predicted to occur when C 0 ≥ S 0 LLPS . The prediction result was then compared with UWL-LLPS observed at each pH by polarized light microscopy (PLM). The pH-LLPS concentration ( S pH LLPS ) profile of each drug was also measured. UWL-LLPS was approximately correctly predicted for CPH-HCl, DCF-Na, and PRF-HCl. However, UWL-LLPS was not observable when C 0 was close to S 0 LLPS . Furthermore, UWL-LLPS was not accurately predicted in the case of PAP-HCl. The pH- S pH LLPS profile of PAP did not follow the Henderson-Hasselbalch equation, probably because of the formation of cationic aggregates. In conclusion, UWL-LLPS was approximately predictable for drug salts using their intrinsic physicochemical properties ( K sp , pK a , D , and S 0 LLPS ), except for PAP-HCl.