Tablets and compaction - Philadelphia University

Tablets and compaction - Philadelphia University

Tablets and compaction The oral route is the most common way of administering drugs, and among the oral dosage forms tablets of various different types are the most common. In 1843 the first patent for a hand-operated device

used to form a tablet was granted. A tablet consists of one or more drugs (active ingredients) as well as a series of other substances used in the formulation of a complete preparation. They are intended for oral administration. Some are swallowed whole,

some after being chewed, some are dissolved or dispersed in water before being administered some are retained in the mouth. Thus, a variety of tablets exists and the type of excipients vary between the different types.

Tablets are popular for several reasons: The oral route represents a convenient and safe way of drug administration. Compared to liquid dosage forms tablets have general advantages in terms of the chemical and physical stability of the dosage form. The preparation procedure enables accurate

dosing of the drug. Tablets are convenient to handle and can be prepared in a versatile way with respect to their use and to the delivery of the drug. Finally, tablets can be mass produced, with robust and quality-controlled production

procedures giving an elegant preparation of consistent quality and, in relative terms, low price. The main disadvantage of tablets as a dosage form: the bioavailability of poorly water-soluble or

poorly absorbable drugs. some drugs may cause local irritant effects or otherwise cause harm to the gastrointestinal mucosa. QUALITY ATTRIBUTES OF TABLETS tablets should fulfill a number of specifications

regarding their chemical, physical and biological properties. Tests and specifications for some of these properties are given in pharmacopoeias. The most important of these are: dose content

dose uniformity, the release of the drug in terms of tablet disintegration and drug dissolution, the microbial quality of the preparation. In addition, the authorities and manufacturers define a set of other specifications.

One such important property is the resistance of the tablet towards attrition and fracture. The quality attributes a tablet must fulfill can be summarized as follows: 1. The tablet should include the correct dose of the drug. 2. The appearance of the tablet should be elegant and its

weight, size and appearance should be consistent. 3. The drug should be released from the tablet in a controlled and reproducible way. 4. The tablet should be biocompatible, i.e. not include excipients, contaminants and microorganisms that could cause harm to patients.

5. The tablet should be of sufficient mechanical strength to withstand fracture and erosion during handling. 6. The tablet should be chemically, physically and microbiologically stable during the lifetime of the product. 7. The tablet should be formulated into a product acceptable by the patient. 8. The tablet should be packed in a safe manner.

TABLET MANUFACTURING Tablets are prepared by forcing particles into close proximity to each other by powder compression, which enables the particles to cohere into a porous, solid specimen of defined geometry. The compression takes place in a die by the action of two

punches, the lower and the upper, by which the compressive force is applied. Powder compression is defined as the reduction in volume of a powder owing to the application of a force. Because of the increased proximity of particle surfaces accomplished during compression,

bonds are formed between particles which provides coherency to the powder, i.e. a compact is formed. Compaction is defined as the formation of a porous specimen of defined geometry by powder compression.

The process of tabletting can be divided into three stages (sometimes known as the compaction cycle): 1. Die filling 2. Tablet formation 3. Tablet ejection

Die filling This is normally accomplished by gravitational flow of the powder from a hopper via the die table into the die (although presses based on centrifugal die filling are also used). The die is closed at its lower end by the lower punch.

Tablet formation The upper punch descends and enters the die and the powder is compressed until a tablet is formed. During the compression phase, the lower punch can be stationary or can move upwards in the die. After maximum applied force is reached, the upper

punch leaves the powder, i.e. the decompression phase. Tablet ejection During this phase the lower punch rises until its tip reaches the level of the top of the die. The tablet is subsequently removed from the die and die table by a pushing device.

Tablet presses There are two types of press in common use during tablet production: 1. the single-punch press 2. The rotary press. in research and development work hydraulic presses

are used as advanced equipment for the evaluation of the tabletting properties of powders and the prediction of scale-up on the properties of the formed tablets Single-punch press (eccentric press) possesses one die and one pair of punches. The powder is held in a hopper which is connected to a

hopper shoe located at the die table. The hopper shoe moves to and fro over the die, by either a rotational or a translational movement. When the hopper shoe is located over the die, the powder is fed into the die by gravity. The amount of powder filled into the die is controlled by

the position of the lower punch. When the hopper shoe is located beside the die, the upper punch descends and the powder is compressed. The lower punch is stationary during compression and the pressure is thus applied by the upper punch After ejection the tablet is pushed by the hopper shoe

Technical Problems during tabletting Tablet production via granulation Lactose Other sugars

Celluloses: MCC Dicalcium phosphate dihydrate Mechanism of action of disintegrants

Antiadherent The function of an antiadherent is to reduce adhesion between the powder and the punch faces and thus prevent particles sticking to the punches. Many powders are prone to adhere to the punches, a phenomenon (known in the industry as sticking or picking) which is affected by the moisture content of the powder.

Such adherence is especially prone to happen if the tablet punches are engraved or embossed. Adherence can lead to a build-up of a thin layer of powder on the punches, which in turn will lead to an uneven and matt tablet surface with unclear engravings.

Many lubricants, such as magnesium stearate, have also antiadherent properties. However, other substances with limited ability to reduce friction can also act as antiadherents, such as talc and starch. Sorbent Sorbents are substances that are capable of sorbing

some quantities of fluids in an apparently dry state. Thus, oils or oil-drug solutions can be incorporated into a powder mixture which is granulated and compacted into tablets. Microcrystalline cellulose and silica are examples of sorbing substances used in tablets.

Flavour Flavouring agents are incorporated into a formulation to give the tablet a more pleasant taste or to mask an unpleasant one. The latter can be achieved also by coating the tablet or the drug particles. Flavouring agents are often thermolabile and so cannot be

added prior to an operation involving heat. They are often mixed with the granules as an alcohol solution. Colourant Colourants are added to tablets to aid identification and patient compliance. Colouring is often accomplished during coating , but a colourant

can also be included in the formulation prior to compaction. In the latter case the colourant can be added as an insoluble powder or dissolved in the granulation liquid. The latter procedure may lead to a colour variation in the tablet caused by migration of the soluble dye during the drying stage TABLET TYPES

Based on their drug-release characteristics, tablets can be classified into three types: 1. immediate release, 2. extended release 3. delayed release. immediate release tablets

the drug is intended to be released rapidly after administration, or the tablet is dissolved and administered as a solution. This is the most common type of tablet and includes disintegrating, chewable, effervescent, sublingual and buccal tablets.

Modified-release tablets should normally be swallowed intact. The formulation and thus also the type of excipients used in such tablets might be quite different from those of immediate-release tablets. The drug is released from an extended-release tablet slowly at a nearly constant rate.

If the rate of release is constant during a substantial period of time, a zero order type of release is obtained, i.e. M = kt (where M is the cumulative amount of drug released and t is the release time). However, for most type of extended-release tablets a perfect zeroorder release is not obtained. delayed-release tablets

the drug is liberated from the tablet some time after administration. After this period has elapsed, the release is normally rapid. The most common type of delayed-release tablet is an enteric tablet, for which the drug is released in the upper part of the small intestine after the preparation has passed the stomach.

However, a delayed-release can also be combined with a slow drug release, e.g. for local treatment in the lower part of the intestine or in the colon. Disintegrating tablets The most common type of tablet is intended to be swallowed and to release the drug in a relatively short time

thereafter by disintegration and dissolution, i.e. the goal of the formulation is fast and complete drug release in vivo. Such tablets are often referred to as conventional or plain tablets. A disintegrating tablet includes normally at least the following type of excipients: filler (if the dose of the drug is low), disintegrant, binder, glidant, lubricant and

antiadherent. the drug is released from a disintegrating tablet in a sequence of processes, including tablet disintegration, drug dissolution and drug absorption. All these processes will affect, and can be rate-limiting steps

for, the rate of drug bioavailability. The disintegration time of the tablet can be markedly affected by the choice of excipients, especially disintegrant . The type of filler and lubricant can also be of significant importance for tablet disintegration. The dissolution rate of

salicylic acid, as assessed by an in vitro dissolution method based on agitated baskets, from tablets formed from mixtures of salicylic acid (325 mg) and a series of different types of

starches as disintegrant. Tablet disintegration may also be affected by production conditions during manufacture. Important examples are: 1. the design of the granulation procedure (which will affect the physical properties of the granules),

2. mixing conditions during the addition of lubricants and antiadherents, 3. the applied punch force during tabletting , increased compaction pressure can either increase ordecrease disintegration time For poorly water-soluble drugs the dissolution rate is often

the rate-limiting step for bioavailability. The dissolution rate is a function of the solubility and the surface area of the drug . dissolution rate will increase if the solubility of the drug is increased, e.g. by the use of a salt of the drug. It is also possible to speed up the dissolution process by incorporating into the formulation a substance that forms a

salt with the drug during dissolution. This has been a common means to increase the dissolution rate of aspirin by using magnesium oxide in the formulation. The drug dissolution rate will also increase with an increase in the surface area of the drug. control of drug particle size is important to control drug

dissolution. However, a reduced particle size will make a powder more cohesive. A reduction in drug particle size might thus give aggregates of particles which are difficult to break up, with the consequence that the drug dissolution rate from the tablet will be reduced. It is thus important to ensure that the tablet is formulated in such a way that it will disintegrate, and the aggregates thus

formed break up into small drug particles so that a large surface area of the drug is exposed to the dissolution medium. Single disintegrating tablets can also be prepared in the form of multilayers, i.e. the tablet consists of two or three layers cohered to each other (double and triple-layered tablets).

During the preparation of multilayer tablets the die is filled in two or three consecutive steps with different granulations from separate feed stations. Each layer is normally compressed after each fill. Multilayer tablets are made primarily to separate incompatible drugs from each other, i.e. incompatible drugs

can be incorporated into the same tablet. Although intimate contact exists at the surface between the layers, the reaction between the incompatible drugs is limited. The use of layered tablets where the layers are differently coloured represents an approach to preparing easily identifiable tablets.

Another variation of the disintegrating tablet is coated tablets which are intended to disintegrate and release the drug quickly (in contrast to coated tablets intended for modified release).

Chewable tablets Chewable tablets are chewed and thus mechanically disintegrated in the mouth. The drug is, however, normally not dissolved in the mouth but swallowed and dissolves in the stomach or intestine. Thus, chewable tablets are used primarily 1. to accomplish a quick and complete disintegration of the

tablet and hence obtain a rapid drug effect 2. or to facilitate the intake of the tablet. A common example of the former is antacid tablets. In the latter case, the elderly and children in particular have difficulty in swallowing tablets, and so chewable tablets are attractive forms of medication. Important

examples are vitamin tablets. Another general advantage of a chewable tablet is that this type of medication can be taken when water is not available. Chewable tablets are similar in composition to conventional tablets except that a disintegrant

is normally not included in the composition. Flavouring and colouring agents are common, and sorbitol and mannitol are common examples of fillers Effervescent tablets Effervescent tablets are dropped into a glass of water before

administration, during which carbon dioxide is liberated. This facilitates tablet disintegration and drug dissolution; the dissolution of the tablet should be complete within a few minutes. the effervescent carbon dioxide is created by a reaction in water between a carbonate or bicarbonate and a weak acid such as citric or tartaric.

Effervescent tablets are used: to obtain rapid drug action, for example for analgesic drugs or to facilitate the intake of the drug, for example for vitamins.

Concentration of salicylates in plasma after administration of acetylsalicylic acid tablets (1 g). Circles, effervescent tablet;

squares, conventional table The amount of sodium bicarbonate in an effervescent tablet is often quite high (about 1 g). After dissolution of such a tablet, a buffered water solution will be obtained which normally temporarily

increases the pH of the stomach. The result is a rapid emptying of the stomach and the residence time of the drug in the stomach will thus be short. effervescent tablets can thus 1. show a fast drug bioavailability, which can be

advantageous, for example, for analgesic drugs. 2. drug-induced gastric irritation can be avoided, e.g. for aspirin tablets, as the absorption of aspirin in the stomach can cause irritation

Effervescent tablets also often include a flavour and a colourant. A water-soluble lubricant is preferable in order to avoid a film of a hydrophobic lubricant on the surface of the water after tablet dissolution. A binder is normally not included in the composition. Effervescent tablets are prepared by both direct compaction and by compaction via granulation.

In the latter case, traditional wet granulation is seldom used; instead, granules are formed by the fusion of particles as a result of their partial dissolution during wet massing of a moistened powder. Effervescent tablets should be packaged in such a way that they are protected against moisture.

This is accomplished with waterproof containers, often including a dessicant, or with blister packs or aluminium foils. Lozenges Lozenges are tablets that dissolve slowly in the mouth and so release the drug dissolved in the saliva.

Lozenges are used for local medication in the mouth or throat, e.g. with local anaesthesia, antiseptic and antibiotic drugs. They can thus be described as slow release tablets for local drug treatment. Lozenges are normally prepared by compaction at high

applied pressures in order to obtain a tablet of high mechanical strength and low porosity which can dissolve slowly in the mouth. Disintegrants are not used in the formulation, but otherwise such tablets are similar in composition to conventional tablets. In addition, lozenges are often coloured and include a

flavour. The choice of filler and binder is of particular importance in the formulation of lozenges, as these excipients should contribute to a pleasant taste or feeling during tablet dissolution. The filler and binder should therefore be water soluble

and have a good taste. common examples of fillers are glucose, sorbitol and mannitol. A common binder in lozenges is gelatin Sublingual and buccal tablets Sublingual and buccal tablets are used for drug release in the mouth followed by systemic uptake ofthe drug.

A rapid systemic drug effect can thus be obtained without first-pass liver metabolism. Sublingual tablets are placed under the tongue and buccal tablets are placed in the side of the cheek. Sublingual and buccal tablets are often small and porous, the latter facilitating fast disintegration and drug release.

Extended-release tablets tablets which should be swallowed and thereafter slowly release the drug in the gastrointestinal tract. Such tablets are denominated in various ways, such as slow release, prolonged release, sustained release and extendedrelease. In the European Pharmacopoeia the term extended-release has been chosen as denominator for these types of tablets

Extended-release tablets are often referred to as controlledrelease preparations. The aim: 1. to increase the time period during which a therapeutic drug concentration level in the blood is maintained. 2. to increase the release time for drugs that can cause local irritation in the stomach or intestine if they are released

quickly. Examples are potassium chloride and iron salts. 3. drugs for local treatment of diseases in the large intestine are sometimes formulated as extended release tablets. An extended-release tablet contains one dose of the drug which is released for a period of about 12-24 hours. The release pattern can vary, from being nearly

continuous to two or more pulses. In the latter case the pulses can correspond to a rapid release of the drug, or can be a combination of a rapid release of one portion of drug followed by a slow release of a second portion. An extended-release preparation can also be

categorized as a single-unit or a multiple-unit dosage form. in the first case the drug dose is incorporated into a single-release unit, and in the latter is divided into a large number of small release units often considered to give a more reproducible drug action.

the drug must fulfill certain criteria in order to render itself suitable for sustained-release medication, These rationales and criteria, as well as the pharmacokinetic aspects of extended-release drug administration, will described elsewhere

Extended-release tablets are often classified according to the mechanism of drug release. The following are the most common means used to achieve a slow, controlled release of the drug from tablets: 1. Drug transport control by diffusion 2. Dissolution control

3. Erosion control 4. Drug transport control by convective flow (accomplished by, for example, osmotic pumping) 5. Ion-exchange control. Diffusion-controlled release systems Depending on the part of the release unit in which the dissolved

drug diffusion takes place, diffusion controlled release systems are divided into: 1. Matrix systems (also referred to as monolithic systems): diffusion occurs in pores located within the bulk of the release unit (normally polymer) 2. reservoir systems: diffusion takes place in a thin water-insoluble film or membrane, often about 5-20 um thick, which surrounds the

release unit. Diffusion through the membrane can occur in pores filled with fluid, or in the solid phase that forms the membrane. The release unit can be a tablet or a nearly spherical particle of about 1 mm in diameter (a granule or a millisphere). In both cases the release unit should stay

more or less intact during the course of the release process. Drug is released from a diffusion-controlled release unit in two steps: 1. The liquid that surrounds the dosage form penetrates the release unit and dissolves the drug. A concentration gradient of

dissolved drug is thus established between the interior and the exterior of the release unit. 2. The dissolved drug will diffuse in the pores of the release unit or the surrounding membrane and thus be released, or, alternatively, the dissolved drug will partition into the membrane surrounding the dose unit and diffuse in the membrane.

A dissolution step is thus normally involved in the release process, but the diffusion step is the rate-controlling step. 1. The rate at which diffusion will occur depends on four variables: 2+3. the concentration gradient over the diffusion distance, 4. the area and distance over which diffusion occurs; the diffusion coefficient of the drug in the diffusion medium.

Some of these variables are used to modulate the release rate in the formulation Reservoir systems In a reservoir system the diffusion occurs in a thin film surrounding the release unit . This film is normally formed from a high molecular weight polymer.

The diffusion distance will be constant during the course of the release and, as long as a constant drug concentration gradient is maintained, the release rate will be constant, i.e. a zero-order release (M = kt). One possible process for the release of the drug from a reservoir system involves partition of the drug dissolved inside the release unit to the solid membrane, followed by transport by diffusion of

the drug within the membrane. Finally, the drug will partition to the solution surrounding the release unit. Schematic illustration of the mechanism of drug release from a diffusion-based reservoir tablet (t = time).

The driving force for the release is the concentration gradient of dissolved drug over the membrane. The release rate can be described by the following equation C is the solubility of the drug in the liquid, A and h are the area and thickness of the membrane, D is the diffusion coefficient of the drug in the

membrane K the partition coefficient for the drug between the membrane and the liquid at equilibrium For oral preparations the film surrounding the release units is normally based on high molecular weight, water-insoluble polymers, such as certain

cellulose derivatives (e.g. ethyl cellulose) and acrylates. The film often also includes a plasticizer. In the case of drug release through liquid-filled pores a small amount of a water-soluble compound is also added, the membrane surrounding the release unit often includes a watersoluble component.

This can be small particles of a soluble substance, such as sucrose, or a water-soluble polymer, such as a water soluble cellulose derivative (e.g. hydroxypropyl methylcellulose). In the latter case the polymer is used together with a water-insoluble polymer as the filmforming materials that constitute the coating. In such a membrane the water-soluble component will dissolve and form pores filled with liquid in which the drug can thereafter diffuse.

The area and length of these pores will thus constitute the diffusion area and distance. These factors can be estimated from: 1. the porosity of the membrane (E) and 2. the tortuosity () of the pores (the tortuosity refers to the ratio between the actual transport distance in the pores between two positions and

the transport distance in a solution). The release rate can thus be described in a simplified way as follows: Reservoir systems today are normally designed as multiple-unit systems rather than

single units. Matrix systems In a matrix system the drug is dispersed as solid particles within a porous matrix formed of a water-insoluble polymer, such as polyvinyl chloride Initially, drug particles located at the surface of the release unit will

be dissolved and the drug released rapidly. Thereafter, drug particles at successively increasing distances from the surface of the release unit will be dissolved and released by diffusion in the pores to the exterior of the release unit. Thus, the diffusion distance of dissolved drug will increase as the release process proceeds.

Schematic illustration of the mechanism of drug release from a diffusionbased matrix tablet (t = time). The drug release, in terms of the cumulative amount of drug (M) released from a matrix in which drug particles are suspended is proportional to the square root of time i.e. M = kt1/2.

The main formulation factors by which the release rate from a matrix system can be controlled are: 1. 2. 3. 4.

the amount of drug in the matrix, the porosity of the release unit, the length of the pores in the release unit (dependent on the size of the release unit and the pore tortuosity) 5. the solubility of the drug (which regulates the

concentration gradient). Matrix systems are traditionally designed as single-unit systems, normally tablets, prepared by tabletting. However, alternative preparation procedures are also used, especially for release units that

are smaller than tablets. Examples of such techniques are extrusion, spray-congealing and casting. Dissolution-controlled release systems the rate of dissolution in the gastrointestinal juices of the drug or another ingredient is the

release controlling process. It is obvious that a sparingly water-soluble drug can form a preparation of a dissolutioncontrolled extended-release type. Approaches 1. A reduced drug solubility can be accomplished by preparing poorly soluble salts or derivatives of the drug. In practice, this

approach is a less common way of formulating an extendedrelease preparation. 2. incorporate the drug in a slowly dissolving carrier. 3. covering drug particles with a slowly dissolving coating. The release of the drug from such units occurs in two steps: The liquid that surrounds the release unit dissolves the coating (rate-limiting dissolution step). The solid drug is exposed to the liquid and subsequently

dissolves In order to obtain an extended release based on dissolution of a coating, the tablet is designed to release the drug in a series of pulses. Although this type of release is not continuous it is normally referred to as extended release, as a similar bioavailability as with

continuous-release systems can often be achieved. A pulsatile drug release can be accomplished by dividing the drug dose into a number of smaller release units, which are coated in such a way that the dissolution time of the coatings will vary . Schematic representation of the cumulative amount of drug released from a dissolution-based (due to differences in coating thickness)

pulsatile-release preparation. The release unit is often a nearly spherical granule about 1 mm in diameter. A variation in dissolution time of the coating can be accomplished by varying its thickness or its solubility. Release units with different release times will be mixed and

formed into tablets. After disintegration of the tablet, the release units will deliver the drug in a sequence of pulses. The procedure described here is also the most common means to prepare a delayed-release system, such as enteric-coated dosage forms.

In this case dissolution is inhibited until the preparation reaches the higher pH of the small intestine, where the drug is released in a relatively short time. Erosion-controlled release systems The rate of drug release is controlled by the erosion of a matrix in

which the drug is dispersed. The matrix is normally a tablet, i.e. the matrix is formed by a tabletting operation, and the system can thus be described as a single-unit system. The erosion in its simplest form can be described as a continuous liberation of matrix material (both drug and excipient) from the surface of the tablet, i.e. a surface erosion.

The consequence will be a continuous reduction in tablet weight during the course of the release process. Schematic illustration of the mechanism of drug release from an erosion tablet. Drug release from an erosion system can thus be described in two steps:

1. Matrix material, in which the drug is dissolved or dispersed, is liberated from the surface of the tablet. 2. The drug is subsequently exposed to the gastrointestinal fluids and mixed with (if the drug is dissolved in the matrix) or dissolved in (if the drug is suspended in the matrix) the fluid.

the drug may be released both by erosion and by diffusion within the matrix. Thus, a mathematical description of drug release from an erosion system is complex. However, drug release can often approximate zero-order for a significant part of the total release time.

The eroding matrix can be formed from different substances: 1. lipids or waxes, in which the drug is dispersed. 2. polymers that gel in contact with water (e.g. hydroxyethyl cellulose). The gel will subsequently erode and release the drug dissolved or dispersed in the gel.

Diffusion of the drug in the gel may occur in parallel. Osmosis-controlled release systems The flow of liquid into the release unit, driven by a difference in osmotic pressure between the inside and the outside of the release unit, is used as the release controlling process.

Osmosis can be defined as the flow of a solvent from a compartment with a low concentration of solute to a compartment with a high concentration. The two compartments are separated by a semipermeable membrane, which allows flow of solvent but not of the solute.

In the most simple type of osmosis-controlled drug release the following sequence of steps is involved in the release process: 1. Osmotic transport of liquid into the release unit; 2. Dissolution of drug within the release unit; 3. Convective transport of a saturated drug solution by pumping of the solution through asingle orifice or

through pores in the semipermeable membrane. The pumping of the drug solution can be accomplished in different ways: 1. a tablet includes an expansion layer, i.e. a layer of a substance that swells in contact with water, the expansion of which will press out the drug solution

from the release unit. 2. the increased volume of fluid inside the release unit will increase the internal pressure, and the drug solution will thus be pumped out. The flow rate of incoming liquid under steadystate conditions is a zero-order process, and the release rate of the drug will therefore also

be a zero-order process. Osmosis-controlled release systems can be designed as single-unit or multiple-unit tablets. In the first case the drug solution can be forced out from the tablet through a single orifice formed in the

membrane by boring with a laser beam. the drug solution can flow through a number of pores formed during the uptake of water. Such pores can be formed by the dissolution of water-soluble substances in the membrane, or by straining of the membrane owing to the increased internal pressure in the release unit. In the case of multiple-unit release tablets the transport

occurs in formed pores. Schematic illustration of the mechanism of drug release from an osmosis-controlled release system designed as a single-unit tablet with a single release orifice. TABLET TESTING Uniformity of content of active ingredient

1. uniformity of weight 2. uniformity of active ingredient. Disintegration Dissolution Mechanical strength Uniformity of content of active

ingredient A fundamental quality attribute for all pharmaceutical preparations is the requirement for a constant dose of drug between individual tablets. In practice, small variations between individual preparations are accepted and the limits for this variation are defined as standards in pharmacopoeias. For tablets, uniformity of dose or dose variation is tested in two separate

tests: 1. uniformity of weight 2. uniformity of active ingredient. These either reflect indirectly or measure directly the amount of drug substance in the tablet. The test for uniformity of weight is carried out by

collecting a sample of tablets, normally 20, from a batch and determining their individual weights. The average weight of the tablets is then calculated. The sample complies with the standard if the individual weights do not deviate from the mean more than is permitted in terms of percentage.

If the drug substance forms the greater part of the tablet mass, any weight variation obviously reflects variations in the content of active ingredient. Compliance with the standard thus helps to ensure that uniformity of dosage is achieved. In the case of potent drugs which are administered in low doses, the excipients form the greater part of the

tablet weight and the correlation between tablet weight and amount of active ingredient can be poor Thus, the test for weight variation must be combined with a test for variation in content of the drug substance. Nevertheless, the test for uniformity of weight

is a simple way to assess variation in drug dose, which makes the test useful as a quality control procedure during tablet production. Correlation between amount of active ingredient and tablet weight for: (a) a low dose (drug content 23% of tablet weight)

(b) a high dose (drug content 90% of tablet weight) The test for uniformity of drug content is carried out by collecting a sample of tablets, normally 10, followed by a determination of the amount of drug in each. The average drug content is calculated the content of the individual tablets should fall within

specified limits in terms of percentage deviationfrom the mean. Disintegration the drug release process from immediaterelease tablets often includes a step at which the tablet disintegrates into smaller fragments. In order to assess this, disintegration test

methods have been developed and examples are described as official standards in pharmacopoeias. The test is carried out by agitating a given number of tablets in an aqueous medium at a defined temperature, and the time to reach

the end-point of the test is recorded. The preparation complies with the test if the time to reach this end-point is below a given limit. The end-point of the test is the point at which all visible parts of the tablets have been eliminated from a set of

tubes in which the tablets have been held during agitation. The tubes are closed at the lower end by a screen and the tablet fragments formed during the disintegration are eliminated from the tubes by passing the screen openings, i.e. disintegration is considered to be achieved when no tablet fragments remain on the screen (fragments of coating may remain).

A disintegration apparatus consists normally of six chambers, i.e. tubes open at the upper end and closed by a screen at the lower. Before disintegration testing, one tablet is placed in each tube and normally a plastic disc is placed upon it. The tubes are placed in a water bath and raised and lowered

at a constant frequency in the water in such a way that at the highest position of the tubes, the screen remains below the surface of the water. Tests for disintegration do not normally seek to establish a correlation with in vivo behaviour. Thus, compliance with the specification is no

guarantee of an acceptable release and uptake of the drug in vivo and hence an acceptable clinical effect. However, it is reasonable that a preparation that fails to comply with the test is unlikely to be efficacious. Disintegration tests are, however, useful as: 1. a means to assess the potential importance of

formulation and process variables on the biopharmaceutical properties of the tablet, 2. as a control procedure to evaluate the quality reproducibility of the tablet during production. Dissolution Dissolution testing is the most important way to study,

under in vitro conditions, the release of a drug from a solid dosage form, and thus represents an important tool to assess factors that affect the bioavailability of a drug from a solid preparation. During a dissolution test the cumulative amount of drug that passes into solution is studied as a function of time. The test thus describes the overall rate of all the processes

involved in the release of the drug into a bioavailable form. Dissolution studies are carried out for several reasons: 1. To evaluate the potential effect of formulation and process variables on the bioavailability of a drug;

2. To ensure that preparations comply with product specifications; 3. To indicate the performance of the preparation under in vivo conditions. This last point requires that in vitro dissolution data correlate with the in vivo performance of the dosage form, which

must be experimentally verified. The term in vitro/in vivo correlation in this context is related to the correlation between in vitro dissolution and the release or uptake of the drug in vivo. The establishment of such a correlation is one of the most important aspects of a dissolution test for a preparation under formulation development

Dissolution is accomplished by locating the tablet in a chamber containing a flowing dissolution medium. So that the method is reproducible, all factors that can affect the dissolution process must be standardized. This includes factors that affect the solubility of the substance (i.e. the composition and temperature of the dissolution

medium) and others that affect the dissolution process (such as the concentration of dissolved substance in, and the flow conditions of, the fluid in the dissolution chamber). Normally, the concentration of the drug substance in the bulk of the dissolution medium shall not exceed 10% of the solubility of the drug, i.e. sink conditions.

Under sink conditions, the concentration gradient between the diffusion layer surrounding the solid phase and the concentration in the bulk of the dissolution medium is often assumed to be constant. A number of official and unofficial methods exist for dissolution testing, which can be applied to both

drug substances and formulated preparations. With respect to preparations, the main test methods are based on forced convection of the dissolution medium and can be classified into two groups: 1. stirred-vessel methods 2. continuous-flow methods.

Stirred-vessel methods The most important stirred-vessel methods are 1. The paddle method 2. the rotating-basket method Details of these can be found in official monographs in the European or US

Pharmacopoeias. Both use the same type of vessel, which is filled with a dissolution medium of controlled volume and temperature. In the 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. Diagram of a dissolution instrument based on the rotating

paddle method for the testing of tablet dissolution rate Diagram of a dissolution

instrument based on the rotating-basket method for the testing of tablet dissolution rate Continuous-flow methods

In the continuous-flow method the preparation is held within a flow cell, through 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. The continuous- flow cell method may have advantages over stirred-vessel methods,

1. it maintains sink conditions throughout the experiment 2. and avoids floating of the preparation. The amount of drug dissolved is normally analysed more or less continuously as the concentration in the vessel at a series of consecutive times. However, sometimes a single measurement can be

performed if required in the Pharmacopoeia or product specification, i.e. the amount of drug dissolved within a certain time period is determined. The composition of the dissolution medium might vary between different test situations.

Pure water may be used, but in many cases a medium that shows a closer resemblance to some physiological fluid is used. In such media the pH and ionic strength can be controlled, and surface-active agents might be added to affect the surface tension of the liquid and the solubility

of the drug. Such fluids are often referred to as simulated gastric or intestinal fluids. Also, other dissolution media might be used, such as solvent mixtures, if the solubility of the drug is very low. Mechanical strength

The mechanical strength of a tablet is associated with the resistance of the solid specimen towards fracturing and attrition. An acceptable tablet must remain intact during handling between production and administration. Thus, an integrated part of the formulation and production of tablets is the determination of their

mechanical strength. Such testing is carried out for several reasons, such as: 1. To assess the importance of formulation and production variables for the resistance of a tablet towards fracturing and attrition during formulation work, process design and scaling up;

2. To control the quality of tablets during production (in-process and batch control); 3. To characterize the fundamental mechanical properties of materials used in tablet formulation. The most commonly used methods for strength testing can be subcategorized into

two main groups: 1. attrition-resistance methods 2. fracture-resistance methods. Attrition-resistance methods The idea behind attrition resistance methods is to mimic the kind of forces to which a tablet is subjected during

handling between its production and its administration. These are also referred to as friability tests: a friable tablet is one that is prone to erode mechanically during handling. During handling, tablets are subjected to stresses from collisions and tablets sliding towards one another and other solid surfaces, which can result in the removal of small fragments and particles from the tablet surface.

The result will be a progressive reduction in tablet weight and a change in its appearance. Such attrition can occur even though the stresses are not high enough to break or fracture the tablet into smaller pieces

application of a friability method : ability of the tablet to resist attrition so as to ensure that the correct amount of drug is administered and that the appearance of the tablet does not change during handling. to detect incipient capping, as tablets with no visible defects can cap or laminate when stressed

by an attrition method, e.g. a rotating cylinder. The most common experimental procedure to determine attrition resistance involves the rotation of tablets in a cylinder followed by the determination of weight loss after a given number of rotations. Another approach is to shake tablets intensively in a jar of similar dimensions to a pack-jar.

Normally, weight loss of less than 1 % during a friability test is required. In addition, the tablets should not show capping or cracking during such testing. Fracture-resistance methods Analysis of the fracture resistance of tablets involves

the application of a load on the tablet and the determination of the force needed to fracture or break the specimen along its diameter. In order to obtain a controlled loading, care must be taken to ensure that the load is applied under denned and reproducible conditions in terms of the type of load applied (compression, pulling, twisting etc.) and

the loading rate. diametric compression test For compressive loading of tablets, the test is simple and reproducible under controlled conditions, and the diametric compression test has therefore a broad use during formulation development and tablet production.

In such compression testing the tablet is placed against a platen and the load is applied along its diameter by a movable platen. The tablet fails ideally along its diameter, i.e. parallel to the compression load, in a single fracture into two pieces of similar size and the fracture force is recorded.

This mode of failure is actually a tensile failure even though it is accomplished here by compressive loading. The force needed to fracture the tablet by diametral compression is often somewhat unfortunately referred to as the crushing or breaking strength of the tablet. The term hardness is also used in the literature to denote the failure force, which is in this context incorrect

as hardness is a deformation property of a solid. Illustration of the tensile failure of a tablet during diametral compression. axial tensile test An alternative procedure to measure the tensile strength of

a tablet is to directly pull the tablet apart by the application of stresses along its main axes until fracture occurs, i.e. a direct axial tensile test. The use of this method is primarily to detect weaknesses in the compact in the axial direction, which is an indication of capping or lamination tendencies in the tablet. Thus, the strength value obtained by this procedure

indicates weak zones in the tablet rather than the mean strength of the whole tablet. Illustration of tablet defects referred to as capping and lamination.

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