Drug Crystal Precipitation in Biorelevant Bicarbonate Buffer: A Well-Controlled Comparative Study with Phosphate...

Area of Science:

• Pharmaceutical Sciences
• Physical Chemistry
• Drug Development

Background:

• Understanding drug precipitation is crucial for formulation development.
• Buffer selection can influence drug solubility and precipitation kinetics.
• Physiological relevance of buffers is key for in vitro studies.

Purpose of the Study:

• To compare drug precipitation profiles in bicarbonate buffer (BCB) versus phosphate buffer (PPB).
• To investigate the impact of buffer type on precipitation kinetics of diverse drug classes.
• To assess the relevance of BCB for simulating intestinal drug precipitation.

Main Methods:

• Comparative study using pH-shift and solvent-shift precipitation tests.
• Evaluated seven weak acids, six weak bases, one unionizable, and one zwitterionic drug.
• Controlled pH, buffer capacity, and ionic strength to mimic small intestine conditions.
• Identified precipitate solid forms using powder X-ray diffraction and differential scanning calorimetry.

Main Results:

• Precipitation of weak acids (pKa ≤ 5.1) and weak bases (pKa ≥ 7.3) was slower in BCB compared to PPB.
• Similar precipitation profiles were observed for less ionizable or nonionizable drugs in both buffers at pH 6.5.
• All precipitates were in their free forms, with final bulk pH maintained at 6.5 ± 0.1.

Conclusions:

• Bicarbonate buffer (BCB) exhibits slower precipitation kinetics for ionizable weak acids and bases compared to phosphate buffer (PPB).
• Differences may arise from the CO2 hydration process in BCB, affecting particle surface pH.
• BCB is a physiologically relevant buffer and a viable option for studying ionizable weak acid and base precipitation.