Effects of Different Centrifugation Parameters on Equilibrium Solubility Measurements

Area of Science:

• Pharmaceutical Sciences
• Drug Development
• Physical Chemistry

Background:

• Drug bioavailability is intrinsically linked to solubility, necessitating early and accurate determination during drug development.
• The saturation shake-flask (SSF) method is the standard for solubility measurement, with phase separation being a critical step.
• The accuracy of solubility measurements can be significantly impacted by the phase separation technique, particularly centrifugation.

Purpose of the Study:

• To investigate the influence of centrifugation parameters (time and speed) on drug solubility measurements using the SSF method.
• To compare the impact of different sample preparation protocols (continuous stirring vs. stirring followed by sedimentation) on solubility results.
• To identify optimal centrifugation conditions for reliable and accurate solubility determination.

Main Methods:

• Four model compounds were tested at three pH values using Britton-Robinson buffers.
• Solubility was measured using the saturation shake-flask (SSF) method.
• Centrifugation was performed at varying speeds (5000 or 10,000 rpm) and durations (5, 10, or 20 min).
• Two sample preparation methods were compared: 24-hour continuous stirring versus 6-hour stirring followed by 18-hour sedimentation before centrifugation.

Main Results:

• Pre-sedimentation of samples before centrifugation resulted in solubility values closer to sedimentation-only references.
• Continuous stirring protocols often led to overestimated solubility values, especially at higher speeds and longer centrifugation times.
• Papaverine hydrochloride showed 60-70% higher solubility values after prolonged, high-speed centrifugation without pre-sedimentation.
• Shorter, slower centrifugation (5 min at 5000 rpm) exhibited lower standard deviations and results closest to reference values.

Conclusions:

• Centrifugation parameters and sample preparation significantly affect drug solubility measurements.
• Pre-sedimentation is recommended to improve the accuracy of solubility data obtained via centrifugation.
• Optimized, gentler centrifugation conditions (shorter time, lower speed) are crucial for reliable drug solubility determination in early drug development.