In dissolution test, there are different ways to maintain sink condition. Outline some methods.
1. Adjust the concentration of the API. Decide on volume (600 or 900 mL).
2. Use simulated gastric or intestinal fluid initially, being mindful of the effects of pH on the API and sample matrix.
3. Decide on paddle or basket.
4. Decide on speed of rotation (50 or 100 rpm).
Remove 5 ml of media at each sampling point and add 5mL of media about 37+/--2 C at the same sampling points.
First of all, to maintain the sink condition you need to
1. Fix the dissolution volume. The minimum dissolution volume should be more than 3 times the volume required to dissolve the dose equivalent API in the selected dissolution medium.
2. Selection of dissolution medium depends on the type of formulation you have to analyze. If you are testing DR tablets or enteric coated tablet then you should make sure the dissolution medium after initial 1.5 h should contain enzymes so as to mimic actual body condition.
3. Then comes selection of dissolution apparatus. It again depends on type of formulation.
4. The selection of paddle speed depends on the viscosity and stress applied on formulation by body when administered. If you are testing FDA approved formulation then you can get the RPM in USP. If you are trying something new then select speed based on type of formulation. Like if you are going to test MDT, then RPM cannot be more than 50 and media cannot be more than 150mL. If you are testing enteric coated, it will go inside intestine and colonic area where stress is more along with viscosity, then RPM should be more than 70. For floating tablets, RPM can be anything between 50 to 70. The selection of RPM is important for maintaining sink condition.
5. Lastly the sampling volume. Sampling volume depends on the dissolution medium volume. For less than 50 mL of dissolution volume, you can withdraw NMT 2mL and replenish 2mL with fresh media. For 50-150mL, 3 mL should be withdrawn. For 200-500 or 900 minimum 5mL should be withdrawn.
6. The sampling area makes a huge different in result obtained. Sampling should always be done from 2cm away from paddle rod and above 4cm from paddle blades.
Keep the concentration of the substance less than 10% of its solubility in the medium. This can be maintained by replacing the medium with portions of fresh solution. Depending on the expected rate of dissolution or diffusion, this can be adjusted. For example: at each sampling point, X amount of solution is withdrawn for analysis, and fresh medium solution is added to top up thetotal volume of the media
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