In simple language, Dissolution is a process in which solid substances dissolve in the solvent. Here we can more clarify it by saying "Dissolution is a process in which a solid substance solubilizes in a given solvent".
Here consider two phases one is solid and the other is liquid. Solid is a drug substance from dosage form and liquid is the fluid present in the body. In the dissolution mass of the solid surface of the drug, the substance is transferred to the liquid phase. Means dissolution is the physicochemical process by which a solid substance enters the solvent phase to yield a solution.
2. Why we are doing dissolution in pharma?
Dissolution and drug release tests are in-vitro tests that measure the "rate and extent of dissolution or release of the drug substance from a drug product", usually in an aqueous medium under specified conditions.
There are many reasons to perform a dissolution test. I am discussing 4 main important reasons behind the dissolution study.
a) Predicting in- vivo performance
Scientific evidence has shown that in-vitro dissolution testing provides the means to evaluate critical parameters such as adequate bioavailability and provides information necessary to the formulator in the development of more efficacious and therapeutic optimal dosage forms.
This dissolution test has high relevance in Quality Control and Quality Assurance. The data will have great importance when assessing changes in the manufacturing process or formulation and assist in decisions concerning the need for the bio-availability study.
b) Scale-up - Evaluation of bio-equivalence data
Drug release should same for every batch whether it is small in size or large. The batch size should not affect the dissolution of the dosage form. The drug release of every batch should be the same. To identify this we perform a Dissolution test. This is called bioequivalence; where the NDA sample and ANDA sample show the same drug release and bioavailability criteria.
c) Stability
When pharma companies manufacture one dosage form it is very important that it should be stable for its shelf life. This means at a particular temperature and Humidity condition it should not affect the drug release profile for that dosage form. To check the effect of all these parameters we perform a dissolution test.
d) Batch to Batch Drug Uniformity
When Pharma Company manufactures one dosage form they manufacture it by batch by batch. But for every batch drug release profile should be the same. To check the uniformity of the drug we perform a dissolution test.
3) Theories of Dissolution
a) Diffusion layer model/ film theory-
Thin-film layer forms on the solid and liquid interface called stagnant film or diffusion layer. Then it saturates with the drug. This soluble solute diffuses from this stagnant layer to the bulk of the solution.
b) Danckwart’s Model (penetration or surface renewal theory)-
Danckwart says that turbulence in dissolution medium exists at the solid-liquid interface which forms packets of solvent. These packets reach to solid-liquid interface and absorb solute by diffusion and carry it to the bulk of the solution. Such solute-containing packets are continuously replaced with new packets of fresh solvent.
c) Interfacial Barrier Model (double barrier or limited solvation theory)
According to this theory, an intermediate concentration can exist at the interface as a result of the solvation mechanism and is a function of solubility rather than the diffusion of a crystal. Each face of the crystal will have a different interfacial barrier.
4) Practical touch of Dissolution
How do we perform dissolution tests in the lab?
There are different types of apparatus available that are designed according to guidelines provided by regulatory authorities like USP, IP BP. According to them, some types of dissolution test apparatus are given below.
According to IP
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According to BP
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According to EP
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Type I- Paddle Apparatus
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Type I- Basket Apparatus
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Type I- Basket Apparatus
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Type I- Paddle Apparatus
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Type II- Basket Apparatus
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Type II- Paddle Apparatus
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Type II- Paddle Apparatus
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Type II- Basket Apparatus
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Type III- Reciprocating cylinder
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Type III- Flow through cell
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Type III- Flow through cell
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Type IV- Flow through cell
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Type V- Paddle over Disk
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Type VI- Cylinder
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Type VII- Reciprocating holder
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Dissolution Apparatus
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Mostly used apparatus are Paddle apparatus and Basket apparatus. There are baskets in the basket apparatus and paddles in the paddle apparatus. This is the only main difference in both apparatus otherwise remaining all functions are the same. Other functions are mentioned in other videos and PowerPoint presentations. Basically how we perform the dissolution test that we are going to discuss here.
The requirement for performing a dissolution test is dosage form, dissolution apparatus, and dissolution medium. Dissolution parameters for a particular drug dosage form are set by the research and development department.
Some important Parameters are
1) Name and volume of dissolution medium
2) time of test (start time and end time)
3) RPM
4) Temperature of dissolution medium
There are some parameters to check before starting the test
1) Status of apparatus
2) Cleanliness of apparatus
3) Water level of bath
4) Fitment of basket or paddle
5) Degassed dissolution medium
6) Vessels required are clear or amber?
7) Cleanliness of vessels
8) Temperature of dissolution medium of individual vessels
9) Dropped location of dosage form
10) Program set in apparatus
After performing the dissolution test we have to collect the sample manually or the apparatus collects it by autosampler. This will show the Drug release pattern of the dosage form in percentage by using UV Visible spectrophotometer or by HPLC System.
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If you want to know how to calculate percent drug release from dosage form write in the comment section below. I will write another post about it.
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