In-Vitro Dissolution Tests for Solid Oral Dosage Forms

Standards of Quality for Solid Oral Dosage Forms

Although Pharmacists are able to formulate, granulate and compress small batches of tablets, but for economic reasons Pharmacists depend normally upon the products of a highly mechanized Pharmaceutical industry.
By whatever means tablets are made, however, they must conform to those standards of elegance and accuracy expected from medicines dispensed by pharmacists. Therefore tablets:

Should be free from adherent powder and not show any physical
imperfections
The surface of the tablets should be free from specks or blemishes
of any kind
If coloured, the colouring should be uniform and not mottled
Tablets should be made sufficiently hard to withstand reasonable
handling and yet should disintegrate within the required time.
Coated tablets should retain their high polish.

OFFICIAL STANDARDS

Many Pharmacopoeias (BP, USP, EP) define certain standards to which tablets should conform. These are:

Uniformity of weight
Permitted tolerances for Active Agent
The diameter to which the tablet must be compressed
The time allowed for Disintegration and
Satisfied Dissolution tests requirement

No Official requirements tests

Relative hardness and
Friability.

INTRODUCTION OF DISSOLUTION TEST

It has been emphasized that the accuracy of dose of drug in tablet form is meaningless unless it is made readily available for absorption by the system.
This means that where the drug is sparingly soluble in water, the tablet when swallowed must break down into numerous particles within a reasonable time.
This “within a reasonable time” is known as the Disintegration Time of the tablet and the time specified in most pharmacopoeias is 15mins for uncoated tablets.
However, Levy & Co-workers (1963) criticized the disintegration test: They indicated that although the test may be a useful routine check during manufacture of tablets, it has no value in assessing the availability of the drug for absorption by the patient.
Contrary to popular belief, they showed that when a tablet is swallowed, it does not necessarily disintegrate into numerous particles in the stomach. For example, they showed that when a Radio Opaque tablet of
Barium Sulphate, having a disintegration time of 3-5mins by the official test was swallowed and its behavior examined over a period of 1hour, by which time it had left the stomach and reached the intestine.
X-ray photographs, showed that the component particles of the disintegrated tablet remained as an aggregate in the stomach and were not freely dispersed until they reached the intestines.
Another criticism of thee disintegration test is that the rate of agitation of the tablets during the test is more vigorous than that in the gastric contents in the stomach.
Levy.et al proposed therefore, that the test for disintegration be replaced by one for Dissolution of the tablet. This is because the
dissolution test is a more reliable measure of the rate at which the active ingredient will be available for utilization by the system.
This led to the introduction of the dissolution test as a standard for many oral solid dosage forms (capsules & tablets in most pharmacopoeias in the early 70`s.
Examples of a few solid dosage forms that must undergo dissolution test.
Dissolution test is recommended for the following preparations using as the medium, 1000 ml of 0.1m Hcl and rotating the basket at 100 RPM.

Chloramphenicol capsules
Chlortetracycline capsules
Oxytetracycline capsules
Phenoxymethylpenecilin capsules
Tetracycline capsules
Chloroquine Phosphate/Sulphate tablets
Chlorpropamide tablets
Dapsone tablets
Digoxin Tablets
Metformin Tablets
Isoniazid Tablets

THE PURPOSE OR VALUES OF IN-VITRO DISSOLUTION TESTS

There are 3 main functions of the in-vitro dissolution tests for solid dosage forms

It can be used to study the effect of formulation and process changes on the bioavailability of a drug.
It can be employed to evaluate the reproducibility within a batch as well as between batch variations of solid dosage forms.
In an ideal situation, if the dissolution characteristics correlate with the absorption or clinical effectiveness of a solid dosage form, the in-vitro test can be used as a regular quality control requirement of the product.

(NOTE: This 3rd function is an ideal one and so far, it has not been achieved)

METHODS AVAILABLE FOR THE DETERMINATION OF DISSOLUTION RATES

It has been shown that the number of methods is proportional to the number of researchers/workers in the field.
The methods to be considered here are typical examples of the various methods that have been reported in the literature.

CLASSIFICATION OF METHODS

Hersey (1969) classified the methods for the in-vitro test procedures according to the type and degree of agitations employed
during the test.
Hersey classified them into:

Natural or Free Convection Methods
(This is streamline flow in which there is no physical agitation)
Forced Convection Methods (This is turbulent flow in which there is agitation).

These methods can be further divided into:
(a.) Non-sink methods and
(b.) Sink methods
(a.) For Non-Sink Conditions: There is a gradual increase in the concentration of drug in the dissolution medium.
(b.) For Sink Condition: There must be a constant or low level concentration of drug in the dissolution medium.

IN-VITRO DISSOLUTION METHODS

A. Natural or Free Convection (Non-sink) Methods.
B. FORCED CONVECTION (NON-SINK METHODS)
C. FORCED CONVECTION (SINK METHODS)

A. Natural or Free Convection (Non-sink) Methods

The various techniques that are employed in this section, use nondisintegrating dosage forms and it is assumed that the surrounding dissolution medium is continuously replaced by fresh medium as a result of density differences.
In addition, the dissolution rates are studied without the need to assay for the drug in solution.

1. SOLVOMETER METHOD

In this method, the non-disintegrating solid dosage form is placed in a boat completely immersed in the dissolution medium through a float.
The float is attached to a scale. As the material dissolves, the float becomes lighter and the loss in weight of the tablet, makes the pointer to move up the scale.
SOLVOMETER METHOD
From the loss in weight of the tablet with time intervals the dissolution rate can then be calculated.

Disadvantage of the method Solvometer Method

The primary disadvantage of the method is that it is very crude and the concentration of the drug builds up in the dissolution medium.

2. HANGING PELLET METHOD (Nelson, 1958)

In this method, circular, non-disintegrating discs of the drug under study are moulded on a wax-coated aluminum strip so that only ONE circular face is exposed.
The strip on which the pellet is mounted is suspended from one arm of a balance so that the strip and pellet are completely immersed in the dissolution medium.
The dissolution process is followed by recording the weight loss of the disc/tablet over a period of time. Readings are discontinued after 4- 5mg of the drug has dissolved.

3. STATIC DISC METHOD (Levy, 1963)

This technique is similar to the previous two in that a nondisintegrating disc or tablet is mounted in holder so that only one surface
is exposed to the dissolution medium.

Exposed surface of the Disc/Pellet with a constant surface area.
The disc holder is inserted through a rubber stopper that is used to close a vial containing 25ml dissolution fluid at 370C. The position of the holder and the volume of the fluid used are such that the exposed surface of the disc or tablet is completely immersed in the dissolution medium.
The holder is removed at appropriate time intervals and placed in a second vial with dissolution fluid. The contents of the first vial are then filtered and assayed for drug content. By repeating this procedure the
necessary number of times, the dissolution rate may be obtained.

INTRINSIC DISSOLUTION RATE AND TOTAL /APPARENT DISSOLUTION RATE

In the 3 methods just considered the drug under investigation is in the form of a highly compressed non-disintegrating pellet/disc/tablet:

Therefore, the surface area of drug exposed to the dissolution medium remains constant during the test. Under such conditions, one measures the intrinsic Dissolution Rate of the drug. This is expressed mathematically in terms of
[(Wt. of drug dissolved) / (Unit time x Unit surface area.)]
In the case of total or Apparent Dissolution Rate, the surface
area of the drug changes appreciably during the test run due to the presence of excipients in the tablet formulation. The Total or apparent dissolution Rate is therefore expressed mathematically as (Wt. of drug dissolved / Unit time.)
Thus, with a particular method it is possible to obtain either the intrinsic or Apparent Dissolution Rate depending on whether a non-disintegrating or disintegrating form of the drug is being employed in the
test.

B. FORCED CONVECTION (NON-SINK METHODS)

These techniques incorporate stirring, rotation or oscillation to induce relative motion between the dissolving particulate system and the dissolution medium. In most cases, disintegrating dosage forms are
employed.

1. WRUBLE METHOD (WRUBLE, 1930)

This method uses a series of test tubes, each containing a tablet in the dissolution medium and clamped to a revolving disc rotated at a speed of 6-12rpm in a water bath at 37ºC.

At specified time intervals, a test tube is removed, sample of dissolution medium taken from it, filtered and assayed for drug content.

2. BEAKER METHOD OF LEVY & HAYES (1960)

The assembly consists of a 400ml Pyrex Griffin beaker containing 250ml of dissolution fluid which is agitated by a 3-blade, 5cm diameter, polyethylene stirrer attached to an automatically controlled stirring motor.
Stirring rate of 30-60r.p.m are usually used (this is sufficient speed to obtain a homogenous solution for sampling purposes, yet low enough, not to break down the “microenvironment” of the dosage form being tested).
In using the apparatus, the stirrer is immersed in the dissolution medium to a depth of 27mm and is accurately centered by means of a guide. The beaker is immersed in a constant temperature bath maintained at 37ºC ± 0.1ºC.
Samples are taken by means of a filtered glass immersion tube of medium porosity and 30mm diameter. The solution sample is then assayed for drug content.

DISADVANTAGE OF THE METHOD

The only disadvantage of the method is that there is a spread of powder/granules at the bottom of the beaker during the test.

ADVANTAGES OF THE METHOD

The capacity of the container and volume of the dissolution medium
can be modified.
The tablet is usually allowed to disintegrate and the total dissolution
rate determined.
By suitably mounting a hard pellet of the drug on a glass slide so that only one surface is exposed, the method can also be used for intrinsic dissolution rate measurement.
Most importantly, the publication of the method was a significant advance in dissolution rate methodology. It clearly indicated that for tablets and other solid oral dosage forms Low intensities of agitation are highly desirable, and more likely to allow distinguishing formulations and products and, correlating results with in-vivo data.
The method has since been modified in most of the pharmacopoeias (known as the Rotating Basket or the Rotating Paddle methods) and specified for the testing of the dissolution rates/profiles of tablets
and capsules.

MODIFICATIONS MADE TO THE LEVY & HAYES BEAKER METHOD

BY VARIOUS PHARMACOPOEIAS

The shape and dimension of the container and type of stirrer (e.g. whether a paddle or basket).
The composition of the dissolution medium.
The rate of rotation of the paddle/basket and their position in the container during the test.
The minimum amount of drug which must be in solution at specified time intervals.

The rotating basket and the rotating paddle methods are the most important stirred vessel methods. Their details can be found in any of the official monographs (BP or USP or EP).
Both use the same type of vessel which is filled with dissolution medium of controlled volume and temperature.
In the rotating paddle method, the tablet is placed in the vessel and the dissolution medium is agitated by a rotating paddle. In the rotating basket method, the tablet is placed in a small basket formed from a
screen. This is then immersed in the dissolution medium and rotated at a
given speed. In both cases, samples of the dissolution medium are removed at specified times, filtered and assayed.

3. OSCILLATION TUBE METHOD

This method is really an extension of the disintegration test. At
specified time intervals, samples are removed from the oscillating tube
containing the tablet and the amount of drug dissolved in the dissolution medium assayed.

DISADVANTAGES

There are two main disadvantages of this method. They are:

Agitation is serve and varies with volume of the dissolution medium employed. For example, it has been shown that the removal of samples for assay (i.e. reduction of the volume of the dissolution medium) results in an increase in the rate of agitation and therefore of dissolution rates.
In some cases it is impossible to distinguish between two formulations by this method.

4. ROTATION DISC METHOD

The method was originally proposed by Levy & Sahli (1962). The method requires a ½ inch plain-faced tablet to be mounted on an
acrylic holder with the aid of paraffin wax, so that one surface of the tablet is exposed to the dissolution medium. Levy & Sahli used a
round-bottom flask containing 200ml of dissolution medium. The tablet and its holder is placed/immersed to a depth of 1 inch of the dissolution medium and rotated as 555 rpm.

5. SOUDER & ELLENBOGEN METHOD (1958)

The method is similar to the Wruble method. It consists of a series of 90ml bottles, each containing 60ml of dissolution medium maintained at 370C in a water bath. A unit dose is placed in the 90ml bottles and
rotated at 40 rpm.

Bottles are removed and samples are withdrawn at specified time intervals, and the solution assayed. The method was originally used to measure the release rate of drugs from delayed-action pellets.

C. FORCED CONVECTION (SINK METHODS)

Sink conditions are designed to reproduce what happens in the GIT because if dissolution is the rate limiting step in drug absorption, then there will be no buildup of drug in the gastro-intestinal fluid and they will function as a perfect sink.
The drugs that give the greatest dissolution problems are those which have the least solubility and therefore give the greatest difficulty with respect to the maintenance 0f perfect sink conditions in a dissolution
test. Various techniques have been used to achieve sink conditions.

1. Adsorption Method

This is essentially the Levy & Hayes Beaker dissolution Method but this consists of an adsorbent (e.g. Charcoal or Bentonite) which is added to the dissolution medium to adsorb the dissolved solute molecules.
However, with Bentonite, a problem arises because it also alters the viscosity of the dissolution medium. This is important
because the dissolution rate of a diffusion controlled process is governed by the viscosity of the medium.

2. Partition Method

The method consists of a beaker, a 3-bladed stirrer and a magnetic stirrer. For the test, an organic solvent/phase is chosen into which it is known that the drug will partition readily. The stirrer is placed in the top
half of the beaker containing the organic phase and the tablet is placed, in the bottom half of the beaker containing the aqueous dissolution medium which is stirred by means of the magnetic stirrer.

As Dissolution of the tablet takes place in section B, it partitions between sections A and B, thereby creating sink condition for the drug in section B. Removal of sample for analysis is from a fixed position in
section B.

3. Dialysis Method

This is the most suitable and widely used method for achieving sink conditions.
The methods use dialysis membranes for obtaining the solute concentration without disturbing the dissolution process or the dissolution medium. The membrane selected usually has a short equilibrium time and adequate physical strength and ability to retain solid particles.
The rate at which material appears on the distal surface of the dialysis membrane should be a function of the dissolution rate and not of the dialysis rate (i.e. the dissolution must be the rate-limiting step).
Many variation of the methods are available.

(a.) Dialysis using a Rotating Cell

In this method a tablet is placed in the dissolution medium on one side of the dialysis membrane and the cell is rotated in a water bath at 15 rpm. Samples are removed from the distal chamber at
appropriate time intervals.

(b.) The other variation of the above method available is the Dialysis using oscillating cell.

COLUMN METHOD
In 1969, a number of workers including Marshall & Brook; Baun and Walker and Langenbucher, all described sample apparatus for measuring dissolution rates under perfect sink conditions.
In all these studies, a solid dosage form was exposed to a stream of liquid passed up a tube. Agitation rates were therefore controlled by regulating the flow of the liquid or the diameter of the flow cell.
Langenbucher carried out a complete analysis of the experimental conditions and claimed that this type of apparatus has many advantages over the various beaker methods with respect to simplicity and
reproducibility.

4. Column Method: (Langenbucher Apparatus)

This is a continuous flow method. In this method the solid dosage form (i.e. Tablet) is held (in a vertical column) or within a flow ceT16.

The amount of drug dissolved is usually analyzed more or less continuous, as the concentration in the vessel, at a series of consecutive times.
In some cases, a single measurement can be performed if this is required in the pharmacopoeia or required by the product specification (i.e. the amount of drug dissolved within a certain time period is
determined).
The composition of the dissolution medium may vary between different test solutions. Pure water may be used but in many cases
simulated gastric or intestinal fluids are used. These media show a closer resemblance to some physiological fluid. In these media, the pH and ionic strength are controlled and surface active agent are added in order to affect the surface tension of the fluid and the solubility of the drugs.
Advantages of the continuous flow method over stirred-vessel methods are that:

It maintains perfect sink conditions throughout the experiment.
It avoids floating of the solid dosage form.

Finally, Wagner (1970) considers that a beaker method using a very slow rate of stirring, or the column method of Langenbucher are the in-vitro methods most likely to give reasonable correlation with in-vivo testshthrough which the dissolution medium is pumped at a controlled rate from
a large reservoir.
The liquid which has passed the flow cell is collected for analysis of drug content.