Perspectives in solubility measurement and interpretation

Area of Science:

• Pharmaceutical Science
• Physical Chemistry
• Drug Discovery

Background:

• Accurate solubility measurement is crucial for drug development and formulation.
• Traditional models like the Henderson-Hasselbalch equation may not fully capture complex solubility behaviors of ionizable molecules.
• Understanding solution equilibria and solid-state properties is key to predicting and controlling drug solubility.

Purpose of the Study:

• To summarize key aspects of solubility measurement and interpretation, focusing on pH-dependent solubility.
• To illustrate deviations from predicted solubility-pH curves and explore their underlying causes.
• To enhance understanding of solution chemistry relevant to drug suspensions and improve assay design.

Main Methods:

• Review and case study analysis of published solubility determinations as a function of pH.
• Discussion of limitations in kinetic solubility measurements.
• Exploration of advanced concepts such as supersaturation, pre-nucleation clusters, and hydrotropic solubilization.

Main Results:

• Ionizable molecules can exhibit solubility-pH curves that deviate from predictions based on the Henderson-Hasselbalch equation.
• Factors like drug-buffer complexation, hydrotropy, and cocrystal formation influence solubility.
• Supersaturation phenomena are linked to pre-nucleation clusters and sub-micellar aggregates.

Conclusions:

• A deeper understanding of solution equilibria and factors influencing solubility is essential for robust drug development.
• Improved assay design and data quality assessment are needed for reliable solubility measurements.
• Recognizing deviations from simple models and considering complex solution phenomena can lead to better drug formulations.