Jean-Marie Glantzmann of Novartis Pharma, in Basel, and Jean-Louis Raton of Sotax, in Allschwil, Switzerland, review dissolution testing along with the benefits of using the flow through method
The use of flow through cells for the dissolution testing of tablets and capsules is not a new idea. More than 40 years ago a flow through method was developed in the laboratories of the US Food and Drug Administration (FDA) and discussions with the dissolution experts from the US Pharmacopoeia (USP) were initiated. The flow through cell used today is based on subsequent development work by Dr F Langenbucher and Professor H Moeller and was incorporated as Apparatus 4 in the USP, the European Pharmacopoeia and the Japanese Pharmacopoeia during the 1990s.1-4
Today, the most commonly used apparatus (USP 1 and 2) is equipped with a paddle stirrer or a basket. The fixed solvent volume of maximal 2,000 ml is kept at a temperature of 37°C.
The flow through method uses an unlimited amount of solvent and offers a number of advantages compared with the apparatus 1 and 2. When using apparatus 4 for the dissolution testing of solid dosage forms, the drug product is placed in a cell through which the medium will flow. The sample solution is filtered before leaving the cell and is directly analysed or fractions are collected for subsequent analysis.
The flow through method is an excellent alternative to the stirrer methods, especially for extended release forms and low soluble active ingredients. Sink conditions (defined as the volume of medium at least three times greater than that required to form a saturated solution of a drug substance) can easily be reached with Apparatus 4. Also, pH changes during the test are easily performed.
The media change is performed by a valve switching from one medium to another. As the cell volume is only about 10ml and a typical flow rate is 16ml/min, it requires only about one minute for a complete pH change.
Sampling in stirrer methods often leads to problems. The introduction of the sampling probe can change the hydrodynamics and, therefore, the dissolution conditions. In addition, the sampling position must always be at the same point to guarantee reproducibility. In the flow through method there are no problems related to sampling. Neither manual nor automated manipulations are necessary. The sample solution is automatically filtered upon leaving the cell and can be analysed directly or after fractioning without interfering with the dissolution process.
When carrying out the flow through method, the drug product is placed in a small volume cell with precisely defined dimensions. The dimensions along with the test method are defined in the USP chapter 7241. A ruby ball is placed at the bottom of the cell to act as a check valve and prevent glass beads (when used) from blocking the cell channel.
Two cell types are specified in the USP, the 12mm and the 22.6mm tablet cells including tablet holder. In the EP 43 a suppository cell is also described.
Six different test cell types are available from Sotax. A piston pump is used for media delivery with usual flow rates from 1.5 to 50ml/min and a pulse rate of 120 ± 10. The pulse rate remains constant, independent of the selected flow rate; the latter is determined by the stroke length of the piston. The cell type and the flow rate are the two most important method parameters.
Typically, an unlimited volume of the medium is pumped through the cells and is filtered after leaving the cell via the filter head. The filter material can be selected from a wide range of available types. The release rate of the drug product depends on the flow rate of the medium and, therefore, a constant flow is essential.
The flow rate of the medium during the test must be constant even if filter resistance is increasing. The pump design must, therefore, be able to handle a range of filter types, maintain a sinusoidal flow profile and a constant flow rate. In addition, the material has to be inert to prevent absorption of the drug. Studies have also shown that a sinusoidal flow profile with a pulsation of 120 ± 10 pulses/min is optimal2 and it has become a USP requirement.1 The piston pump fulfils all these requirements and has proven to be both precise and reliable, while peristaltic pumps can be used in special cases, such as in testing of implants.
De-aeration of the test media is essential to ensure that no bubbles are formed. There are a number of test variations associated with air bubbles and these are given in table 1.
There are different methods of de-aeration. Some groups do it manually according to the USP method,1 which is time-consuming for the required larger volumes. Others work with a commercial medium preparation station. An on-line degasser can also be used, in which medium flows through gas permeable tubes under vacuum.
In an open system with a constant flow of new medium, the eluate is collected in the off-line mode in a fraction collector and then analysed afterwards in a spectrophotometer or by HPLC. The results obtained are the differential release rates and can easily be converted to cumulative values.
Open or closed
With the flow through method the system can be set in two ways: as an open or a closed system. The open system has a configuration where fresh medium is pumped through the cell and the fractions are collected. This set-up ensures that the dissolution test is always performed at the best possible sink conditions. Every 30 to 60 minutes fractions are collected and this results in rather high fraction volumes. A typical flow rate of 16ml/min with a 30 min fraction interval results in a fraction volume of 480ml. This is not very practicable for the laboratory and therefore a volume-splitting device (splitter) is used. The splitter manages a three-way valve, time-controlled from collection to waste. Typically only 10% of the eluate would be collected, which results in 48ml fractions.
In addition, pH changes are easily performed. To do this a medium selector is required that switches a seven-channel valve from medium one to medium two.
It is recommended that an open system configuration is used for poorly soluble drugs or when pH changes have to be performed. Also formulation variances of highly soluble drugs can be tested and differences at the beginning of the dissolution process are more easily detectable than with the USP 1 and 2 methods.
The closed system (figure 6) is a configuration in which medium is pumped in a circle and not replaced by fresh medium. The eluate is collected in a beaker and is stirred with a magnetic stirrer. Samples are taken from the beaker or readings taken on-line with a spectrophotometer to measure the cumulated concentration of drug, whereas in an open system the amount of drug released over time is measured.
The closed system has a configuration that is similar to the stirrer methods as the volume is limited and the released drug is accumulating over time. This configuration is used for drug products with very low dosage strength and, essentially, can be performed with very small volumes. Typical example applications include the testing of implants.
The question of an in vivo/in vitro correlation is extensively discussed but statements cannot easily be validated. However, the flow through method with constant feeding of fresh medium is sometimes better related to in vivo conditions than the stirrer method with fixed solvent volumes. In addition, the pH changes allow one to design an in vitro test under in vivo conditions. Questions pertaining to this problem are discussed in the specified literature. One publication discussed various conditions for a "Biorelevant Dissolution Test with the Flow-Through Apparatus".5
There are two different types of medium flow through the cells - laminar and turbulent flow. A laminar flow is characterised by fluid particles moving in parallel to each other in the flow direction.
A turbulent flow is a type of flow regime characterised by the rapid movement of fluid particles in all directions within the flow direction. Laminar flow is most commonly used, however, and turbulent flow is preferable for implant testing or products that require agitation.
To achieve a laminar flow, standardised 1mm glass beads are placed in the cell. The medium pumped through has a laminar flow pattern. Depending on the dosage form and cell types there are slight variations in the test conditions.
Dissolution tests with semi-solid dosage forms are often tedious. The implementation of an efficient in vitro method is important to assess the delivery rate through the skin. The Franz cell is widely used due to the lack of an alternative, but is not suitable for routine use and automatic sampling.
As an alternative, the USP 4 flow through system is a sophisticated piece of equipment used for various dosage forms with good reproducibility. A new design of trans-cap ointment capture cell was recently introduced for testing of ointments and creams6.
Flow through instruments have been available since the mid 1970s and have been redesigned over time. Today, many user-friendly instruments are also available, such as a fraction collector, pump, splitter and media selector. The dissolution software WinSOTAX is capable of controlling flow through instruments and has the calculation tools to calculate the percentage dissolved and to display differential and cumulative release profiles.
With the available components the system can be easily configured to meet individual automation needs. This can be an on-line system with immediate evaluation in the spectro-photometer or an off-line system with collection of eluate fractions. Programmable medium selection; the use of an in-line de-aeration device as well as the integrated test preparation; and cleaning functions are also key features of the flow through system.
Qualification
After the physical dimensions of the cells, the flow rate and the temperature are the most important test parameters that can influence the test. Checking the flow rate before the test is recommended if the system has not been used for some time and always after a test run to calculate the accurate release rate. The flow rate can be measured by metering the pump volume and measuring the volume vs time.
The cells are made of polycarbonate and there is very little potential of variability of the geometry. Currently no official USP calibrator tablets for the USP 4 method are available. In the USP project team 5, Dissolution Calibrators, discussions are ongoing for a calibrator tablet for USP 4. Some laboratories use in-house calibrator tablets.
The flow through method (USP 4) for the determination of the dissolution- and release-rate is a convincing alternative to the known stirrer methods. It is especially recommended for samples that introduce problems with sink conditions, for poorly soluble active substances and for methods that require media changes during the test.
The flow through method is not only successfully used for tablets and capsules but, with the aid of special flow through cells, also for suppositories, powders, granulates, ointments, creams and implants.
