Guide to Determine Sink Conditions in Dissolution Testing

Sink conditions refer to a state in which the dissolution medium has sufficient volume to ensure the drug dissolves rapidly without reaching saturation throughout the duration of the test. 

Sink conditions = V/Vsat ≥ 3

Where, 

• V = Volume of dissolution medium in one vessel
• Vsat = Volume required to form saturated solution of the drug that is in your dosage form 

Volume of Dissolution Medium

The volume of the dissolution medium should be at least 3-10 times the saturation volume (amount required to saturate the medium). This ensures that the drug concentration remains low throughout the test, maintaining sink conditions. This is typically why dissolution is performed in larger volumes such as 900 mL or 1litre. 

Examples of Sink Conditions

• If the drug product has a label claim of 250 mg and saturation solubility of the drug is 1 mg/mL, a dissolution medium volume of 900 mL would be appropriate to achieve sink conditions. 
• For a drug product with the label claim 2.5 mg and a saturation solubility of 0.1 mg/mL, a dissolution medium volume of 200 - 250 mL would be sufficient to maintain sink conditions. 

Understanding sink conditions is essential for accurate and reliable dissolution testing, which assesses the rate at which a drug substance dissolves in a dissolution medium.

Maintaining sink conditions in dissolution allows the drug substance to dissolve freely, mimicking the conditions in the gastrointestinal tract during drug absorption. The dissolution rate of the drug is thus not limited by its solubility, enabling accurate evaluation of drug release characteristics, bioavailability, and formulation performance.

Having said that, it is not mandatory to meet sink conditions. The Dissolution test conditions must be selected based on discriminatory ability with respect to the critical quality attributes of the product being evaluated.

In certain cases, dissolution test may be more discriminative if sink conditions are not met!

Read also: Non-Sink Conditions in Dissolution Testing

Resource Person: Pearl Pereira Nambiar