FAST DISSOLVING TABLETS: A PROMISING APPROACH FOR DRUG DELIVERY

Aim of novel drug delivery system is to enhance safety and efficacy of drug molecules by formulating a convenient dosage form for administration and to achieve better patient compliance. One such approach led to development of fast dissolving tablets. Nowa-days fast disintegrating tablets (FDTs) gaining significance with wide variety of drugs serving many purposes. These are novel types of tablets that disintegrate/dissolve/disperse in saliva within few seconds (less than 60 seconds) without chewing and additional water. The basic approach used in development of FDTs is the use of superdisintegrants and the elimination of bitterness. FDTs reduces the disadvantages of conventional dosage form especially dysphasia (difficulty in swallowing) in pediatric and geriatric patients. These oral dosage forms have many benefits such as self medication, increased compliance, ease of manufacturing and noninvasive. This review presents description of fast dissolving tablets including need for development, challenges in formulation, suitability of drug candidates, composition, various technologies involved, advantages, disadvantages, and evaluation parameters.


INTRODUCTION
The conventional dosage forms (tablets and capsules) have wide range of acceptance up to 50-60 % of the total dosage forms 1 . Tablet is most popular dosage form of existing forms because of ease of self administration, compact in nature, easy to manufacture and it can be delivered in accurate dose 2 . There is one drawback associated with tablets and that is difficulty in swallowing and chewing in some patients particularly in geriatric and pediatric patients. This problem can be avoided by means of fast dissolving tablets. One such problem can be solved in the novel drug delivery system by formulating "mouth dissolving tablets" which disintegrates or dissolves rapidly without water within few seconds in the mouth due to the action of super disintegrant or maximizing pore structure in the formulation 3 . Fast dissolving tablets are defined as "a solid dosage form containing medicinal substance or active ingredient which disintegrates rapidly usually within a matter of seconds when placed upon the tongue". Fast dissolving tablets are especially designed for dysphagia, geriatric, pediatric, bed-ridden, travelling and psychotic patients who are unable to swallow or refuse to swallow conventional oral formulations. Fast dissolving tablets are also referred as quick disintegrating tablets, mouth dissolving tablets, oral rapid disintegrating tablets, rapid dissolving tablets, porous tablets and rapid melt tablets 4 . The basic approach used in development of FDT is the use of superdisintegrants like ac-di-sol, sodium starch glycol ate, kollidon, crospovidone, crosscarmellose sodium, l-hydroxypropyl cellulose, in a concentration of 1.5-7.5%. Superdisintegrants provide instantaneous disintegration of tablet, thereby releasing the drug in saliva and absorbed through oral mucosa thus drug enters directly into systemic circulation which in turn provides rapid onset of action. The fast dissolving drug delivery system are specially designed for the drugs which have extensive first pass metabolism and have low dose, for the enhancement of bioavailability. Elimination of bitter taste of drug is an important criterion in the development of mouth dissolving tablets 5

Mechanism of superdisintegrants
The four major mechanisms for tablet disintegration are as following-: 1. Swelling-When tablet comes in contact with water then swelling occurs and thus adhesiveness of other ingredients of the tablet is lost, causing the tablet disintegration 18 . 2. Porosity and Capillary Action (Wicking)-Due to the porous nature of the tablet, the liquid is drawn (wicking action) into the tablet through capillary action, thus the inter-particulate bonds get ruptured causing disintegration of tablet 19 . 3. Deformation-The superdisintegrants get deformed during tablet compression and upon contact with water they regain their normal structure which causes an increase in size of deformed particles resulting in the breaking of tablet. 4. Due to repulsive forces-Another mechanism of disintegration attempts the swelling of tablet made with"nonswellable" disintegrants. Nonswelling particle also cause disintegration of tablets. Generation of electric repulsive forces between particles promotes the disintegration of tablet and water is required for it.

Conventional techniques used in the preparation of fast dissolving tablets
The various techniques used for the preparation of fast dissolving tablets are-:

Direct Compression
It is considered as the best method to prepare orally disintegrating dosage forms 20 . The general scheme of direct compression is as follows: Milling→ Sieving → Mixing → Compression It is one of the most popular and convenient techniques for the preparation of FDTs. Here the FDTs are punched at significantly lower forces (4-10 kN) than the conventional tablets.

Sublimation/effervescent
Rapid disintegration of fast dissolving tablets is due to the presence of a porous structure in tablet matrix. Hence to generate porous matrix, volatile ingredients are used then removed by sublimation, leaving behind a porous matrix, were compressed along with other excipients into a tablet.

Mass extrusion
It involves softening of the active blend using the solvent mixture of water soluble polyethylene glycol and methanol and expulsion of softened mass through the extruder or syringe to get a cylindrical shaped extrude which are finally cut into even segments using heated blade to form tablets.

Tablet molding
Molding process is of two types i.e. solvent method and heat method. Solvent method involves moistening the powder blend with a hydro alcoholic solvent followed by compression at low pressures in molded plates to form a wetted mass. The solvent is then removed by air-drying.

Lyophillization/Freeze drying
This technique creates an amorphous porous structure that can dissolve rapidly. The active drug is dissolved or dispersed in an aqueous solution of a carrier/polymer. The mixture is poured in the walls of the preformed blister packs. The trays holding the blister packs are passed through liquid nitrogen freezing tunnel to freeze the drug solution or dispersion. Then the frozen blister packs are placed in refrigerated cabinets to continue the freeze-drying 21 .

Melt granulation
It is a process by which powders are efficiently agglomerated by a meltable binder which can be a molten liquid, a solid or a solid that melts during the process. It is used to enhance the dissolution rate of poorly water-soluble drugs, such as griseofulvin. This approach is used to prepare FDTs with sufficient mechanical integrity.

Nanonization
A Nanomelt technology involves size reduction of drug to nanosize by milling the drug using a proprietary wetmilling technique. The nanocrystals of the drug are stabilized against agglomeration by surface adsorption on selected stabilizers, which are then incorporated into FDTs. 8. Cotton candy process -This process is so named as it utilizes an inimitable spinning mechanism to produce floss like crystalline structure, which mimics cotton candy. In this process matrix of polysaccharides are formed by simultaneous action of flash melting and spinning. This candy floss matrix is then milled and blended with active ingredients and excipients after recrystallization and subsequently compressed to FDT 22 .

Three-dimensional printing (3DP) -
It is a rapid prototyping technology, which involves constructing specific layers that uses powder processing and liquid binding materials.

Superdisintegrants
To promote moisture penetration and disintegration/dispersion of the matrix of dosage form in dissolution fluids. These are generally used in a low concentration (1-10%) 14 .

2 .
Bulking materials Contributes functions of a diluent, filler and cost reducer. These agents improve the textural characteristics which further enhance the disintegration.

Lubricants
Remove grittiness and assist in the drug transport mechanism from the mouth down into the stomach 15 .

Flavours and Sweetners
Makes more palatable and pleasing products for patients, mask bitterness and disagreeable tastes of some active ingredients. Both natural and synthetic flavors are used.
Emulsifying agent They aid in rapid disintegration and drug release without chewing, swallowing or drinking water. They also stabilize the immiscible blends and enhance bioavailability 16 .

Important patented technologies for fast dissolving tablets 1. Zydis Technology
This is the patented technology of Catalent Pharma solutions. Zydis formulation is a unique freeze dried tablet in which drug is physically entrapped or dissolved within the matrix of fast dissolving carrier material. When zydis units are put into the mouth, the freeze-dried structure disintegrates instantaneously and does not require water to aid swallowing. To impart strength and resilience during handling, polymers such as gelatin, dextran or alginates are incorporated. These form a glossy amorphous structure, which imparts strength. Blister packs are used for Zydis products to protect the formulation from moisture present in the environment 23 .

Durasolv Technology
Durasolv is the patented technology of CIMA labs. Tablets produced by this technology have much higher mechanical strength and the production is a faster and effective. The tablets made by this technology consist of drug, filler and a lubricant. Tablets are prepared by using conventional tabletting equipment and have good rigidity. These can be packaged into conventional packaging system like blisters. Durasolv is an appropriate technology for product requiring low amount of active ingredients 24 .

Ziplets Technology
This technology is patented by Passano con Barnago, Italy. It can be used with water insoluble compounds as both bulk actives and as coated microparticles. These tablets are with improved mechanical strength and optimal disintegration time at low compression force. In fact, tablets composed primarily of water-soluble components often tend to dissolve rather than disintegrate, resulting in a much longer disintegration time 24 .

Flash Dose Technology
Flash dose technology has been patented by fuisz. Flash dose tablets consist of self-binding shear form matrix termed as "floss". Shear form matrices are prepared by flash heat processing. Nurofen meltlet, a new form of ibuprofen as melt in mouth tablets prepared using flash dose technology is the first commercial product launched by Biovail Corporation 25 .

Flash Tab Technology
Prographarm laboratories have patented the Flash tab technology. Active ingredient is used in the form of micro crystals. Drug micro granules may be prepared by using the conventional techniques like coacervation, micro encapsulation and extrusion spheronisation. All the processing utilized conventional tabletting technology 25 .

Novel Hole Technology
It is developed to minimize the disintegration time and maximize the patient compliance. Tablets formulated by this technique usually disintegrate in 10-20s. In this technology, highly volatile ingredients like ammonium bicarbonate, ammonium carbonate, benzoic acid, camphor, naphthalene, urea, urethane and phthalic anhydride may be compressed along with other excipients into a tablet. These volatile materials are then removed by sublimation creating a highly porous matrix. Absolute surface area of the tablet increases due to pore formation. The main mechanism involved in whole technology is sublimation.

Oraquick Technology
The Oraquick fastdissolving/disintegrating tablet formulation utilizes a patented taste masking technology. There is no utilization of solvents to mask taste, thus leads to faster and more efficient production.
Since there is lower heat of production than alternative fast dissolving/disintegrating technologies, so it is appropriate for heat sensitive drugs. Oraquick claims quick dissolution in a matter of seconds, with good taste masking 24 .

Quicksolv Technology
This technology is patented by Janssen Pharmaceutica, Beese, Belgium. It utilizes two solvents in formulating a matrix, which disintegrates instantly. Product formed by this technology has uniform porosity and adequate strength for handling. Methodology includes dissolving matrix components in water and the solution or dispersion is frozen. Then dry the matrix by removing water using an excess of alcohol (solvent extraction) 23 .

Lyoc Technology
Lyoc technology is patented by pharmalyoc. It utilizes a freeze drying process but it differs from Zydis in that the product is frozen on the freeze dryer shelves. Oil in water emulsion is prepared and placed directly into blister cavities followed by freeze-drying. Nonhomogeneity during freeze-drying is avoided by incorporating inert filler to increase the viscosity finally the sedimentation. High proportion of filler reduces porosity of tablets due to which disintegration is lowered.

Pharmabrust Technology
Pharmaburst technology is being patented by SPI pharma. Tablet prepared by this technique dissolve within 30-40 seconds. The tablet manufactured by this process involves a dry blend of a drug, flavors, and lubricant then followed by compression into tablets 26 .

Frosta Technology
Akina patents this technology. Plastic granules are prepared and compressed at low pressure to produce strong tablets with high porosity. Plastic granules composed of porous and plastic material, water penetration enhancer, and binder. The process involves mixing the porous plastic material with water penetration enhancer followed by granulating with binder. The tablets obtained have excellent hardness and rapid disintegration time ranging from 15 to 30 sec depending on size of tablet 27 .

Nanocrystal Technology
This is patented by Elan, King of Prussia. It includes lyophilization of colloidal dispersions of drug substance and water-soluble ingredients filled in to blister pockets. This method avoids manufacturing process such as granulation, blending, and tableting, which is more advantageous for highly potent and hazardous 26 .

Wowtab
Wowtab technology was developed by Yamanouchi Pharma Technologies. "Wow" means without water. The active ingredients may constitute up to 50% w/w of the tablet. Saccharides of both low and high moldability are used to prepare the granules. Moldability is the capacity of a compound to be compressed. Highly Moldable substance has high compressibility and thus slow dissolution. The combination of high and low moldability is used to produce tablets of adequate hardness and a rapidly melting strong tablet. Wowtab product dissolves quicklyin 15 seconds or less 28 .

PREFORMULATION STUDIES OF BLENDS 1. Bulk density
Bulk density can be determined by pouring blend into a graduated measuring cylinder using a funnel and weigh 29 . The bulk density can be calculated using the formula-:

Tapped density
Same measuring cylinder should be set for the determination of tapped density that was used for the determination of bulk volume. Set measuring cylinder to 300 taps per minute and operate for 500 taps 29 . The tapped density is calculated by the following formula-:

Angle of repose (θ)
It is an indication of the flow properties of the powder. It is defined as maximum angle possible between the surface of the pile of powder and the horizontal plane. The powder mixture is allowed to flow through the funnel fixed to a stand at definite height (h). The angle of repose is then calculated by measuring the height and radius of the heap of powder formed 29 . It is calculated by the following formula-Where, θ = angle of repose, h =height in cm, r = radius in cm.

Carr's Index
It is a simplest way of measurement of the free flow of powder. Carr"s index measures the propensity of powder to be compressed and the flow ability of powder 30 . Carr"s index can be calculated from the bulk and tapped density by using following formula-

Hausner's ratio
Hausner"s ratio also measure the propensity and the flow ability of powder 30 . Hausner"s ratio can be calculated from the bulk and tapped density.Hausner ratio given by the equation-:

. Thickness
Tablet thickness is an important characteristic and is expressed in mm. The thickness and diameter of the tablets was determined using a micrometer screw gauge 31 .

Weight variation test
For this test 20 tablets are generally selected randomly from the lot and weighted individually for checking weight variation 32 .

Hardness
Force required to break a tablet in a diametric compression test is called hardness (crushing strength). Hardness is measured by using a Monsanto Hardness Tester 32 .

Friability
Roche friabilator is used for the measurement of friability using 20 tablets. Twenty tablets are weighed and rotated at 25 rpm for 4 minutes (100 revolutions 32 . The tablets were then reweighed after removal of fines and the percentage of weight loss was calculated.

Measurement of Tablet Porosity
Porosity of tablet can be determined by using mercury penetration porosimeter 33 . The tablet porosity (ε) can be calculated by using following equation, Where, ρt= true density, m and V= weight and volume of the tablet, respectively.

Water absorption ratio
A tablet is placed on the paper and the time required for complete wetting is determined by using following formula 33 Where, Wa = weight of tablet after absorption, Wb = weight of tablet before absorption.

In-vivo disintegration time
This test is performed on 6 tablets, by placing tablet into each tube (3 inches long and have 10 mesh screen) of apparatus using the distilled water (used as disintegration medium) at a frequency of 28-32 cycle/minute and 37±2 0 C and the time in second was noted when no lumps remaining in the apparatus 34 .

In-vivo dissolution study
Dissolution study is carried out by using USP type-II apparatus. The dissolution test is performed using 900 ml of the dissolution medium at 50 rpm and 37±0.5°C. 10ml of aliquots were periodically withdrawn and the sample volume was replaced with an equal volume of fresh dissolution medium to maintain sink condition 34 . The samples are analyzed spectrophotometrically at the specific wavelength. 9. Stability studies Stability testing of tablets is done to check whether it is a stable product or not and to check integrity of formulations during its shelf life. The formulation prepared should be packed in a special way, firstly the formulation is wrapped in a butter paper then aluminium foil is wrapped over it, then this is packed in an aluminium pouch and heat sealed. Storage conditions of formulation should be 45 o C/75% RH. Formulations should be stored for 3 months. During the course of stability study triplicate samples should be taken at three sampling intervals i.e. 0, 1 and 3 month, and tablets should be evaluated for physical changes and drug content 34 . Table 3: Weight variation and accepted % deviation CONCLUSION FDTs are innovative dosage forms specially designed to get disintegrated in saliva without the need of water, due to the porous structure of the tablet matrix or on the addition of superdisintegrants and/or effervescent excipients. Fast dissolving tablets have better patient compliance especially geriatric and paediatric populations for and offer improved biopharmaceutical properties and efficacy and better safety when compared with conventional. The development of a fast-dissolving tablet also provides an opportunity for extension in the market place. Pharmaceutical marketing is another reason for the development of fast dissolving products. Hence, patient demand and the availability of various technologies have increased the acceptance of Fast disintegrating tablets, which in turn prolongs the patent life of a drug. There is need for improved manufacturing processes for fast dissolving tablets to make them mechanically strong, allowing ease of handling and packaging and with production costs similar to that of conventional tablet.