Dissolution testing of solid dosage forms

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AIMST,Dissolution test

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K.JAYA RAJ KUMAR AMIST UNIVERSITY MALAYSIA Dissolution testing of solid dosage forms

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The rate of solution of a solid drug substance from a granule or a tablet is dependent to a large extent on its solubility in the solvent phase and its concentration in that phase. INTRODUCTION

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The physicochemical factors which point to the need for dissolution testing include: Low aqueous drug solubility. Poor product dissolution – evidence from the literature that the dissolution of one or more marketed products is poor. Drug particle size – evidence that particle size may affect bioavailability. The physical form of drug – when polymorphs , solvates and complexes have poor dissolution characteristics. Presence of specific excipients which may alter dissolution or absorption . Tablet or capsule coating which may interfere with the disintegration or dissolution of the formulation.

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In vitro tests provide the opportunity to make precise and reproducible release measurements to distinguish between different formulations of the same drug or the same formulation after ageing or processing changes or during production, i.e. batch-to-batch variation . They do not replace the need for clinical work, but an in vitro test can pinpoint formulation factors during development which are of importance in determining drug release . Physiological verisimilitude is not essential for validity in quality control, where reproducibility of a product is in itself a goal . KEY POINTS

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In vitro methods may be divided into two types : 1. Natural convection in which, for example, a pellet of material is suspended from a balance arm in the dissolution medium. Because there is no agitation, the conditions are not representative of in vivo conditions . 2. Forced convection in which a degree of agitation is introduced, so making this method more representative of in vivo conditions . Most practical methods fall into this category. There are two types of forced convection methods: those that employ non-sink conditions and those that achieve sink conditions in the dissolution medium.shows examples of simple types of forced convection methods employing non-sink conditions .

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With drugs of very low solubility it is sometimes necessary to consider the use of in vitro tests which allow sink conditions to be maintained. This generally involves the use of a lipid phase into which the drug can partition : alternatively it may involve dialysis or physical replacement of the solvent phase . Mixed-solvent systems such as ethanol–water or surfactant systems may have to be used to enhance the solubility of sparingly soluble drugs, but some prefer the use of flow through systems in these cases. KEY POINTS

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Fig-Simple types of forced convection methods of dissolution testing

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If released drug is not removed from the dissolution medium during dissolution testing, i.e. if testing is performed under non-sink conditions, the drug concentration in this medium may, in some cases, approach saturation level and if so the rate of release of the drug will be significantly reduced (see Noyes–Whitney equation). Sink conditions normally occur when the volume of the dissolution medium is at least 5–10 times the saturation volume. Tips

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The British Pharmacopoeia method involves a rotating wire mesh basket in which tablets or capsules are placed (Figure). The mesh is small enough to retain broken pieces of tablet but large enough to allow entry of solvent without wetting problems. The basket may be rotated at any suitable speed but most United States Pharmacopeia monographs specify 50, 100 or 1500 rpm. In all methods the appropriate pH for the dissolution medium must be chosen and there should be a reasonable degree of agitation. Rotating basket method

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In vitro evaluation of non-oral systems Suppositories Suppositories are difficult to study in vitro, because it is not easy to simulate the conditions in the rectum. One system employs a suppository placed in a pH 7.8 buffer in a dialysis bag which is then placed in a second dialysis bag filled with octanol and the whole is suspended in a flow system at 37°C. The amount of drug released into the outer liquid is monitored.

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In vitro testing of the lot-to-lot uniformity of semisolid dosage forms of creams, ointments and lotions is important in quality control. Ointments and transdermal systems encounter little water in use but useful data can be obtained by measuring release into aqueous media, which can sometimes be predictive of in vivo performance. Alternatively, a liquid biophase can be simulated using isopropyl myristate . A rotating bottle apparatus has been used to measure the release of nitroglycerin from Deponit transdermal patches. The British Pharmacopoeia specifies a distribution (release) test for transdermal patches based on the paddle apparatus for tablet and capsules.

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The cascade impactor is probably the most widely used instrument in categorizing airborne particles. In this instrument: Large particles leave the airstream and impinge on baffles or on glass microscope slides (Figure). The airstream is then accelerated at a nozzle, providing a second range of smaller-sized particles on the next baffle and so on. Progressively finer particles are collected at the successive stages of impingement owing to jet velocity and decreasing jet dimension

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Apparatus A employs the principle of liquid impingement and has a solvent in both chambers to collect the aerosol. Air is drawn through the system at 60 l min–1 and the inhaler is fired several times into the device Apparatus A (glass) There are several impaction surfaces at the back of the glass throat about 10 cm away from the activator (similar to human dimensions). The upper impinger (stage 1) has a cut-off at a particle size of ~6.4 μm . The last impact surface is in the lower impinger (stage 2) and is considered to be the respirable fraction

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We have seen a selection of tests which can be conducted to measure the key parameters of a variety of formulations. These tests are not necessarily predictive of performance in vivo, but can be used in a comparative sense, testing one product against others or different batches of a product to ensure batch-to-batch consistency. Release tests can be applied to rectal and transdermal products by adapting The method used for oral products, altering the receptor phase to mimic the medium in which the formulation resides in vivo. Key parameters are different for different routes of delivery and different formulations: particle size is a key factor in inhalation products and in topical preparations where the drug is dispersed rather than dissolved in the vehicle. Key word

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