Although weakly basic drugs with specific properties (e.g., dose number >1-10, pKa 5-9) are known to form supersaturated solutions and exhibit complex phase behavior in the gastrointestinal tract, key aspects of this process (namely, the physical stability of nanodroplets and the charge site) remain poorly characterized. This work employs the dual-pKa anticancer drug crizotinib (CZT; pKa 5.6, 9.4) as a model drug to establish a systematic methodological framework for investigating the complex behaviors of weakly basic drugs induced by pH shift. The results revealed that upon pH shift from 1.0 to above 5.4, a 1 mg/mL CZT solution underwent liquid-liquid phase separation with nanodroplets forming immediately. 1H nuclear magnetic resonance (NMR), 13C solid-state NMR, and synchrotron single-crystal X-ray diffraction (SCXRD) data together confirmed that this phase transition is attributed to the deprotonation from a dicationic species (with both piperidinium and pyridinium protonated) to a monocationic species (with only the piperidinium protonated) in both the drug-rich and drug-lean phases. Ultraviolet-visible spectroscopy, dynamic light scattering, confocal laser scanning microscopy, and polarized optical microscopy together revealed that the resulting additive-free CZT droplets are unstable at pH 6.5 with a low zeta potential of 5.1 mV, and sedimented into a bulk gel within minutes. This sedimentation significantly reduces the surface area of the colloidal particles (drug reservoir) and, consequently, slows the dissolution of CZT from the drug-rich phase─but interestingly, it has little effect on transmembrane flux through a cellulose membrane, suggesting that for CZT, transport across the membrane, not interphase diffusion, is the rate-limiting step. By establishing the methodology, which integrates NMR spectroscopy and X-ray crystallography to precisely locate the charge site of drug in both drug-rich phase (droplets or gel-like precipitate) and drug-lean phase (bulk solution), this work bridges the critical gap between macroscopic phase behavior and molecular-level understanding. These insights provide a mechanistic foundation for the rational design of oral formulations for weakly basic drugs.
Chen et al. (Thu,) studied this question.