INTRODUCTION:

Now a day with advancement in pharmaceutical sciences there is launching of number of new Active Pharmaceutical Ingredients having different physicochemical and biological properties. Thus it is biggest challenge for pharmaceutical scientist to formulate that Active Pharmaceutical Ingredients in a suitable dosage form to give proper mode of administration. Generally, most of the formulations contain higher proportion of excipients than Active Pharmaceutical Ingredients. Thus physicochemical properties of dosage form such as drug release, bio-availability and stability are mainly affected by pharmaceutical excipients. Due to this the Study of Excipients is become very essential for formulation scientist.

Excipients are the additives used to convert pharmacologically active compounds into pharmaceutical dosage forms suitable for the administration. The International Pharmaceutical Excipients Council (IPEC) defines excipient as “Substances, other than the API in finished dosage form, which have been appropriately evaluated for safety and are included in a drug delivery system to either aid the processing or to aid manufacture, protect, support, enhance stability, bioavailability or patient acceptability, assist in product identification, or enhance any other attributes of the overall safety and effectiveness of the drug delivery system during storage or use.”¹

Generally the most formulation contains excipients at higher concentration (~70-80%) than that of active Pharmaceutical ingredient. Consequently, the excipients contribute significantly to the formulation functionality and processability. In case of tablet manufacturing the most excipients used are solid in nature (except some solvents in granulation, flavors etc.). Solid substances are characterized by three levels of solid state: Molecular level, particle level and bulk level. The molecular level comprises the arrangement of individual molecules in the crystal lattice and includes phenomena such as polymorphism, pseudo-polymorphism, and the amorphous state. Particle level comprises individual particle properties such as shape, size, surface area, and porosity. The bulk level is composed of an ensemble of particles and properties such as flowability, compressibility, and dilution potential, which are critical factors in the performance of excipients. All these properties of solids (excipients) affect the pharmaceutical characteristics of tablet such as release performance, bioavailability and stability.²

Following are the some ideal characteristics of pharmaceutical excipients required for development of robust and successful formulation.³

Physiological inertness

Physical and chemical stability

Purity

Better tableting characteristics

Compatibility with drug and other excipients

No interference with drug bioavailability

Free from contamination

Cost effective

Excipients may be classified according to the role they play in the finished tablet. Those excipients that helps to impart satisfactory processing and compression characteristics to the formulation include fillers–diluents, binders, glidants, and lubricants. The second group of excipients that helps to impart additional desirable physical characteristics to the finished tablet it includes in disintegrating agents, coloring agents, wetting agents and surface-active agents. For chewable tablets, flavors, sweeteners, and taste-modifiers are employed. For controlled- or modified- release tablets, polymers or waxes or other solubility-retarding or modifying excipients are used. The choice of excipients in a tablet formulation depends on the properties of active Pharmaceutical ingredient, the type of tablet, the desired tablet characteristics, and the process used to manufacture the tablet.^4,5,6

Today it is reckoned that over one thousand different materials are used in the pharmaceutical industry to fulfill its various requirements such as diluents, bulking agents, Disintegrating Agents, lubricants, colouring agents, sweeteners, etc. They are chemically heterogeneous compounds that go from simple molecules (water) to complex mixtures of natural, semisynthetic or synthetic substances. According to their chemical nature and the role that can exert, excipients also classified into various classes. The classification of excipients is summarized in table No. 1.

Table 1: Classification of excipients (Based on Chemical Nature)⁷

Chemical Class	Role
Water, alcohols	Compliance
Esters, ethers, carboxylic acids	Dose precision and accuracy
Glycerides and waxes	Stability
Carbohydrates (mono, di, and polysaccharides)	Manufacturability
Hydrocarbons and halogen derivatives	Tolerability
Polymers (natural and synthetic)	Desaggregation
Minerals	Dissolution
Protein	Controlled release
Various: preservatives, dyes, sweeteners, surfactants.	Absorption

Source of excipients:

Pharmaceutical excipients are broadly categorized into two groups based on their source as Natural excipients and synthetic excipients. A large number of plant-based pharmaceutical excipients are available today. Ability to produce a wide range of material based on their properties and molecular weight, natural polymers became a thrust area in majority of investigations in drug.^8,9 In recent years, plant derived polymers have evoked tremendous interest due to their diverse pharmaceutical applications such as diluent, binder, disintegrating agents in tablets, thickeners in oral liquids, protective colloids in suspensions, gelling agents in gels and bases in suppository, sustaining agents in designing of modified release dosage forms, stabilizing and coating agents. They are also used in cosmetics, textiles, paints and paper-making.These polymers such as natural gums and mucilage are biocompatible, low cost and easily available, biocompatible, bio-acceptable, with a renewable source, environmental friendly processing, better patient tolerance, as well as public acceptability, fairly free from toxicity and can enhance overall safety, effectiveness and delivery of the drug during storage as well as use.¹⁰^{,11, 12} Today we have several pharmaceutical excipients of plant origin, like starch, agar, alginates, carrageenan, guar gum, xanthan gum, gelatin, pectin, acacia, tragacanth, and cellulose.¹³

Along with these natural excipients Synthetic excipients are also popular. These excipients are obtained by modification of natural excipients or by chemical synthesis. The various techniques used to obtained synthetic excipients includes Chemical and Physical modification, Grinding or sieving, Crystallization, Spray drying, Granulation/agglomeration and Dehydration.^2,14

Apart from natural and synthetic excipients, modification of physicochemical properties of the existing excipients has been the most successful strategy for the development of newer excipients in past three decades, a process that has been supported by the introduction of better performance grades of excipients. The modifications of existing excipients involve either particle engineering or co-processing.¹⁵

Particle Engineering is a young discipline that combines elements of microbiology, chemistry, formulation science, colloid and interface science, heat and mass transfer, solid state physics, aerosol and powder science, and nanotechnology. In this core of the particle is covered by one or more layers that are defined by having distinct composition or properties.¹⁶ Modification of excipient by particle engineering can be carried out using various techniques such as size reduction, spray drying, Nano particle technology and super critical fluid technology.¹⁷

Although particle engineering gives excipient with modified functionality, modification of single entity can provide only a limited quantum of functionality improvement. As a result, formulation scientists rely on excipients used in combination. Such combinations fall into two broad categories: physical mixtures and co-processed excipients. Co-processed excipients are combinations of two or more excipients that possess performance advantages that cannot be achieved using a physical admixture of the same combination of excipients. The use of different co-processed excipients has been investigated to overcome deficiencies arising from single component excipients and existing formulations.¹⁸ Co-processing is defined as combining two or more established excipients by an appropriate process. Co-processing is based on the novel concept of two or more excipients interacting at the sub particle level, the objective of which is to provide a synergy of functionality improvements as well as to mask the undesirable properties of individual excipients. Excipients are co-processed, to make use of the advantages of the individual excipient and to overcome the specific disadvantages if any. These binding and blending characters of co-processed excipients are much better than those of a physical mixture of the starting materials.¹⁹

Co-processing of excipients leads to the formation of excipient granulates with superior physicochemical properties such as superior flow, tremendous improvement in the pressure–hardness relationship, marked improvement in the compressibility profile, fewer fill-weight variation problems and low lubricant sensitivity.²⁰Various well known co-processed excipients includes Ludipress, Cellactose, Pearlitol SD, Dipac, Advantose FS-95, Prosolv, Avicel ce‐15, Finlac DC, Formaxx, Plasdone S-630, Microcelac, Pharmatose dcl 40 and LycatabC.^14,15,21,22

Selection criteria for excipient:

Dosage form and site of drug delivery:

Pharmaceutical dosage forms contain active pharmaceutical ingredient along with excipients added to aid the formulation and manufacture of the suitable dosage form. As most of the formulations contain higher proportion of excipients than active pharmaceutical ingredient, the properties of final dosage form such as bioavailability and stability are highly influenced by excipient characteristics, their concentration and compatibility with active pharmaceutical ingredient. In designing of sustained released or extended release dosage form polymeric excipients are required which retard the release of the drug from dosage form. Excipients showing varying characteristics at various physiological conditions are used in case of site specific drug delivery system. To achieve immediate release of drug from the dosage form disintegrating agents and super-disintegrating agents are used. Gas generating agents like citrate and carbonate are used for effervescent and floating drug delivery system. Bioadhesive polymers are useful in formulation of buccal and palatable dosage forms. In short in designing of various dosage forms and drug delivery systems, the physicochemical characters of excipients special consideration should be given.

Condition to be treated:

Formulators must take all factors into consideration to design a holistic formulation, including physicochemical properties, stability and compatibility issues, pharmacokinetic attributes, permeation characteristics, segmental absorption behavior, drug delivery platforms, intellectual property issues, and marketing drive. Early characterization of these factors allows formulation scientists to determine the absorption challenges and desired delivery platform for the API. Furthermore, excipients are not totally inert to the human body, and they may contribute significantly to some pharmacological activities.^23,24,25 Thus while selecting excipient, it is desirable to consider the condition to be treated. For example in case of diabetics use of sugars like glucose as excipients should avoided.

Compatibility:

Pharmaceutical dosage form is a combination of active pharmaceutical ingredients (API) and excipients. Excipients are included in dosage forms to aid manufacture, administration or absorption. The ideal excipients must be able to fulfill the important functions i.e. dose, stability and release of API from the formulation. Although considered pharmacologically inert, excipients can initiate, propagate or participate in chemical or physical interactions with drug compounds, which may compromise the effectiveness of a medication.²⁶Drug-excipient interaction can either be beneficial or detrimental, which can be simply classified as Physical interactions, Chemical interactions and Biopharmaceutical interactions.

Physical interactions are quite common, but they are very difficult to detect. Physical interaction doesn’t involve any chemical changes. Physical interactions are frequently encountered during manufacturing of dosage forms to modify drug dissolution and bioavailability. Solid dispersion is a technique used to improve solubility of poorly soluble drug is an example for physical drug excipient interaction. But some time opposite effect also seen as in one study it was found that solid state interaction between povidone and stearic acid resulted in formation of solid dispersion at the storage temperature of 40°C/23% RH and above in capsule formulation containing both povidone and stearic acid, which resulted in slow dissolution of drug possibly because of such interaction. Degradation of active pharmaceutical ingredients may also be due to chemical interactions such as hydrolysis/dehydration, isomerization/ epimerization decarboxylation, rearrangements, and some kinds of polymerization reactions.²⁷

It is not uncommon that an API (active pharmaceutical ingredient) will be stable as a bulk drug but unstable when blended with the excipients required for formulation of dosage forms. Therefore, understanding the reactivity of the API in the solid state when mixed with excipients is critical to commercial formulation development.²⁷

Drug-excipient interactions/incompatibilities are major concerns in formulation development. Selection of proper excipient during pre-formulation studies is of prime importance. Acid-base interactions and Millard reactions are probably the most common API excipient interactions reported. The excipients lactose and magnesium stearate are the most widely used excipients in oral solid dosage forms. They are involved in higher number of incompatibilities and should always be used with caution. Use of lactose as diluents should be avoided for active pharmaceutical ingredients containing amines due to the possibility of a Millard reaction. Stearate salts should be avoided as tablet lubricants if the API is subject to hydrolytic ion-catalyzed degradation. Alkaline excipients such as Di Calcium Phosphate Dihydrate should not be used in the formulation of acidic drugs. The use of Eudragit RL should be avoided with drugs containing a carboxyl group (e.g. ibuprofen, lower pKa) since these exhibit strong electrostatic interaction with ammonium groups present in Eudragit polymer affecting the release profile of the active ingredient. Care should also be taken when formulating drugs containing hydroxyl groups.^[28]Such common solid state drug excipient interactions are summarized in table No. 2.

Table 2: List of common solid-state incompatibilities^28,29

Functional Group (Example of API)	Incompatible with	Type of Reaction
Primary amine (e.g. Acyclovir)	Mono and disaccharides (e.g. lactose)	Maillard reaction (Amine-aldehyde and amine-acetal)
Esters (e.g. Moexipril)	Basic components (e.g. magnesium hydroxide)	Ester hydrolysis (Ring opening ester-base hydrolysis)
Lactone (e.g. Irinotecan HCl)	Basic components (e.g. magnesium hydroxide)	Ring opening (hydrolysis)
Carbonyl, hydroxyl	Silanol	Hydrogen bonding
Aldehyde	Amine carbohydrates	Aldehyde-amine Schiff base or glycosylamine
Alcohol (e.g. Morphine)	Oxygen	Oxidation to aldehydes and ketones
Sulfhydryl (e.g. Captopril)	Oxygen	Dimerization
Phenol	Metals, polyplasdone	Complexation
Gelatin	Cationic surfactants	Denaturation

Drug excipient compatibility investigation is the first step in development of pharmaceutical formulation as the interaction between drugs and excipients may alter both the stability and the bioavailability of the drug in the formulation. Consequently, a thorough drug excipient compatibility study is very important part of the development of a stable pharmaceutical dosage form. There are two main approaches to drug excipient compatibility screening: Thermal studies and non thermal studies. Thermal studies include Differential Scanning Calorimetry (DSC) and Differential Thermal Analysis (DTA) while non thermal studies include FT-IR Spectroscopy, TLC-Thin Layer Chromatography, HPLC-High Pressure Liquid Chromatography.^27,30,31

For this, protocol was suggested by researchers for the drug excipient compatibility as,²⁹

Run the New Chemical Entity (NCE) and excipients individually.

Run mixtures of the NCE and excipients immediately after mixing.

Run the NCE and excipients individually after 3 weeks at 55°C.

Run the NCE-excipient mix after 3 weeks at 55°C.

Run the single components and mixtures after 3 weeks at 55°C only if the curves of the mixtures before and after storage at this temperature differ from each other.

Purity and quality of excipients:

The huge number of starting materials and reagents are used in the synthesis of active pharmaceutical ingredients and excipients. Thus it is critical to understand and assess the purity of active pharmaceutical ingredients and excipients. From a scientific point of view, the suitability of a specification for excipients must be assessed on the basis of the following criteria for meaningful quality standards: ³²

Reliable identification

Detection and limitation of critical impurities from the toxicological point of view

Detection and limitation of impurities that can have an adverse effect on stability and efficacy of drugs owing to interaction with the active ingredient or other excipients

Differentiation of grades for pharmaceutical and technical application

Confirmation of the quantitative composition (in case of mixed substances)

Characterization and specification of physical properties with technological relevance, to ensure a frictionless, economic manufacture of dosage forms and to guarantee their constant physical properties, in vitro dissolution rate, and bioavailability

Pharmaceutical characteristics:

Many investigations have demonstrated the importance and impact of the physical and chemical properties of materials on powder processing. Physical properties such as particle size and shape clearly influence powder flow. However, compact mechanical properties (i.e., those properties of a material under the influence of an applied stress) are also of great importance for solid dosage form development and manufacturing particularly for tablet formulation. Mechanical properties are those properties of a material under an applied load which includes elasticity, plasticity, viscoelasticity, bonding, and brittleness. The mechanical properties of a material play an important role in powder flow and compaction. These properties are critical properties that influence the true areas of contact between particles. Therefore, it is essential to characterize the properties. Reliable mechanical property information can be useful in helping to choose a processing method such as granulation or direct compression, selecting excipients with properties that will mask the poor properties of the drug or helping to document what went wrong.³³

Particle characterization is an important component of data driven formulation development on a material-sparing scale. Particle size, size distribution, shape, and texture can have an impact on pharmaceutical processing and performance, hence due consideration must be given to the impact of these parameters on the robustness of processing.³³

For a plastically deforming material like Micro Crystalline Cellulose (MCC), powder particle size impacts compact strength of formulations added with lubricant (coarse MCC particles are more sensitive to lubricant). For a viscoelastic material like starch, powder particle size impacts compact strength of formulations added with and without lubricant (fine starch particles lead to stronger compacts and are less sensitive to lubricant). For a brittle material like dibasic calcium phosphate dihydrate, powder particle size has no significant impact on compact strength of formulations added with and without lubricant.³⁴ At low compression pressures larger initial particles are more compressible than smaller particles. The differences in compressibility between particle-size fractions decreased with increasing compaction pressure. At the same compaction pressure, smaller particles formed tablets of greater mechanical strength than larger particles. For materials with a high tendency to fragment, the original particle size is less important than for plastic materials, and so volume reduction of the powder bed and tablet strength are generally independent of particle size.³⁵

Mechanical strength of coarse powders such as Starch 1500 and sodium chloride increased with an increase in irregularity of particles. The increase in mechanical strength was due to the increased area of contact between particles as they deformed.³⁶

The bulk density of excipient powder used for tablet preparation affects both DTM (disintegration time in the mouth) and STP (stationary time of upper punch displacement). As the value of bulk density increased, STP became longer and DTM shorter. For a DTM less than 60 s, a formulation with a bulk density greater 0.5 g/mL should be chosen with a compression force of 5 kN. The hardness of tablets could be greater than 3 kg if at least one high-compressibility excipient was used in the formulation. Rapidly disintegrating tablets can be obtained using powder with high bulk density.³⁷

Various terminologies are used to explain excipient characteristics such as dilution potential, elasticity, bonding index, brittle fracture index, compactibility and compressibility.

Dilution potential is the minimum amount of excipient needed in the blend with an active ingredient to form tablets of adequate compactibility and friability (<1%). The selection of appropriate excipients with a low dilution potential has generated great interest among formulation scientists.³⁸

Elasticity is another important property to test an excipient and it is assessed by measuring the increase in tablet thickness due to elastic recovery during ejection. This axial elastic recovery (ER) assesses the elasticity of the compact particles and a high value is associated with a decrease in tablet strength due to the reduction in bonding surface area. The elastic recovery has also been associated with a tendency of tablets to undergo capping and lamination. Compacts produced from materials with weak inter-particle attraction experience more relaxation than tablets made of materials with strong particle attraction. Thus, the ability of the tablet to withstand elastic recovery due to the release of stored elastic energy is a significant factor in determining the success of a compaction process.³⁸

Bonding index is a measure of the ability of a material, on decompression, to maintain a high fraction of the bond that was created during compression. A high bonding index indicates that, a larger portion of the strength remained intact after decompression. A low bonding index indicates that less of the strength remains. The term bonding index, then, is a good description since it, in effect, characterizes the tendency of the material to remain intact after it has been compressed. Tablets made of materials with poor bonding characteristics may be quite friable. Compacts made of materials with good bonding indices may, conversely, make strong tablets. A bonding index in excess of 0.01 (range 0.001 to 0.06) is typically desired.³³

Brittle fracture index is the ability of a compact to relieve stress around compact defects by plastic deformation. The brittle fracture index (BFI) is determined by comparing the tensile strength of a compact, with that of a compact with a small hole (stress concentrator) in it, using the tensile test. The tensile strength of a tablet with a hole in it will be about one-third that of a “defect free” tablet.³³

Compactibility is the ability of a powder to be transformed into tablets with a resulting strength. If one can achieve an acceptable tensile strength at an acceptable solid fraction with the application of pressure, a satisfactory tablet can be produced.³³

Tabletability is the capacity of a powder to be transformed into a tablet of specified strength under the effect of compaction pressure. Tabletability describes the effectiveness of the applied pressure in increasing the tensile strength of the tablet, and demonstrates the relationship between the cause (the compaction pressure), and the effect (the strength of the compact). Normally, a higher compaction pressure makes a stronger tablet.³³

Compressability is the ability of a material to undergo a reduction in volume as a result of an applied pressure. It is a measure of the ease with which a powder bed undergoes volume reduction under compaction pressure.³³

Flowability is also one of the important characters of excipient. Excipient should be free flowing. Flowability is required in case of high-speed rotary tablet machines, in order to ensure homogenous and rapid flow of powder for uniform die filling.

Disintegrating agent and their effect on tablet characteristics:

The release of active ingredient from solid dosage form follows a typical path as first disintegration, de-aggregation followed by dissolution. Hence disintegration has major role for facilitating the release of active ingredients from solid dosage forms.^39,40 Disintegration is a process in which the solid dosage form is breaks down into small particles. Disintegrating agents are substances or mixture of substances added to the drug formulation that facilitates the breakup or disintegration of tablet or capsule content into smaller particles for quick dissolution when it comes in contact with water. In the recent years, several newer agents have been developed known as Super-disintegrating agents. Super-disintegrating agents are used to improve the efficacy of solid dosage forms. This is achieved by decreasing the disintegration time which in turn enhances drug dissolution rate. Super-disintegrating agents are those substances, which facilitate the faster disintegration with smaller quantity, typically 1-10 % w/w in contrast to disintegrating agents. The disintegration of dosage forms are depends upon various physical factors of Disintegrating agents/ Super-disintegrating agents such as, percentage of Disintegrating agents present in the formulation, proportion of Disintegrating agents used, compatibility with other excipients, presence of surfactants, hardness of the tablets, nature of drug substances, mixing and types of addition.^39,41,42Apart from this various ingredients in formulation also affect disintegration such as fillers, binders and lubricants.The solubility and compression characteristics of fillers affect both rate and mechanism of disintegration of tablet. If soluble fillers are used then it may cause increase in viscosity of the penetrating fluid which tends to reduce effectiveness of strongly swelling disintegrating agents and as they are water soluble, they are likely to dissolve rather than disintegrate. Insoluble diluents produce rapid disintegration with adequate amount of disintegrating agent. Lubricating agents generally added to tablet formulation are hydrophobic in nature. They are usually used in smaller quantity than any other ingredient. But when the lubricating agents are mixed with other ingredients, the particles may adhere to the surface of other ingredient and produce hydrophobic coating over it. This reduces the water uptake by tablet essential for swelling of disintegrating agent and thus consequently affects disintegration of tablet. Surfactants are the agents which decrease surface tension and increase solubility. Surfactants are effective within certain ranges of concentration. The speed of water penetration in solid dosage form can increase by the addition of a surfactant. Surfactants are recommended to decrease the hydrophobicity of the drugs because the more hydrophobic the tablet the greater the disintegration time. Along with this, binding capacity of the binders will also affect the disintegration. As binding capacity of the binder increases, disintegration of tablet decreases. Even the concentration of the binder can also affect the disintegration time of a tablet.⁴³Various mechanisms responsible for tablet disintegration summarized in table No. 3.

Disintegrating agents can be classified into two categories as Natural disintegrating agents and Synthetic disintegrating agents on the basis of their source.^[3] Natural disintegrating agent means natural in origin like gums and mucilage have been extensively used in the field of drug delivery for their easy availability, cost effectiveness, non-irritating and nontoxic nature, capable of multitude of chemical modifications, potentially biodegradable and compatible. Apart from these natural disintegrating agents some synthetic disintegrating agents are also used to formulate drug delivery systems. Synthetic disintegrating agents are advantageous over natural as they are effective in lower concentrations, have less effect on compressibility and flow ability and are more effective intra-granularly. Some common used disintegrating agents summarized in the table No. 4.

Lubricating agents and their effect on tablet characteristics:

Lubricants are commonly used in tablet formulations to reduce die wall friction during tablet compression and ejection. Among the different lubricants that are utilized in the pharmaceutical industry, magnesium stearate is the most commonly used. It is generally accepted that magnesium stearate forms an adsorbed lubricant film around host particles during mixing. This results in a decrease in solid–solid contact, including contact between tablet and die wall, hence, reduction in die wall friction.^[34]Magnesium stearate is the most widely used lubricant. It eases the compaction process by reducing wall friction during tablet ejection, improves flowability, bulk and tap densities, compressibility and reduces the adhesion of the powder to metal surfaces. Since it is hydrophobic, the formation of an external film on the blended particles could reduce surface wettability, decreasing dissolution rates and prolonging disintegration times. Magnesium stearate could also weaken the bonding of the powder mixture by creating an interface on the surface which could reduce particle binding. These effects could be aggravated by increasing the amount of magnesium stearate and mixing time and by using mainly plastic-deforming materials. Therefore, it is essential to check excipients for the sensitivity of magnesium stearate. Fragmenting materials could be less sensitive to magnesium stearate because of the creation of lubricant-free surfaces during compression. In contrast, plastically deforming materials suffer from a high lubricant sensitivity, since the lubricant film is not destroyed during consolidation. Lubricants, especially magnesium stearate, increased the excipient bulk density. This improvement in bulk density due to the lubricants is an indication of good flowability, a small contribution to particle rearrangement and less friction during powder consolidation. A decrease in particle roughness due to lubrication could be associated with an increase in powder flowability. Materials with higher porosity (lower bulk density) have more void space to accommodate lubricant, resulting in a low lubricant sensitivity.³⁸

However, afore mentioned lubricant film also interferes with the bonding properties of the host particles by acting as a physical barrier. This causes a decrease in compact strength, especially with excessive lubricant amounts and/or prolonged mixing times. The effect of lubricant on compact strength depends on a number of factors such as its nature, concentration, mixing time, and specific surface area. There are other properties of the excipients themselves that influence their sensitivity to lubricant, the most important of which is the mechanism by which the material undergoes deformation. Lubricated tablets were shown to have larger relaxation than those compressed with no added lubricant, indicating a reduction of inter-particle bonding as a result of the added lubricant.³⁴

Apart from Magnesium Stearate other compounds such as sodium laurel sulfate, stearic acid can be used as lubricating agents. Research on effect of sodium laurel sulfate proved that there was no impact on the insertion point of sodium laurel sulfate into the process on API dissolution, and that the presence of sodium laurel sulfate improved dissolution by 5% compared to the control tablets. Adding sodium laurel sulfate just prior to tableting can improve tablet hardness and yield similar dissolution performance relative to sodium laurel sulfate addition prior to the initial blending step.⁴⁹

Research work on effect of lubricant on tablet dosage form showed that higher hardness and higher percentage of drug dissolved (p ≤ 0.05) was shown by formulations prepared with stearic acid compared to formulations prepared with magnesium stearate. This lower hardness of tablets prepared with magnesium stearate, as lubricant, could be explained by the formation of a hydrophobic film around host particles giving a molecular coverage which makes inter-particle bond formation more difficult. Colloidal silicon dioxide is widely used as a glidant in the manufacture of powders, capsules and tablets. Its use has been reported by some researchers as a factor that decreases the negative effect of lubricants on the hardness and drug release from tablets.⁵⁰

For pharmaceutical operations such as blending, roller compaction, tablet manufacturing, and capsule-filling, lubrication is essential in order to reduce the friction between the surfaces of manufacturing equipment and that of organic solids as well as to ensure the continuation of an operation. The main activities attributed to the lubricants are: (a) Prevention of sticking of granules to the tooling (anti adherent), (b) Improve of flowability of granules, (c) easily eject from the die cavity.^{51,52, 53}

The concentration of the lubricant in the formulations and the lubrication time should be balanced in terms of the adverse effects of the used lubricant. A very small quantity of lubricant is needed (usually 0.25%–5.0%, w/w) to improve the powder processing properties of formulations.⁵¹Generally, these agents are added to the premixed powder mixture as a final step and mixed for only a few minutes. Even small changes in mixing time and lubricant amount can significantly affect the product performance and quality properties. As a result of over-lubrication, the mechanical strength of the tablets can decrease and the dissolution of the drug may worsen.⁵⁴

Generally boundary lubricants are the most commonly used lubricants in the pharmaceutical processes; Boundary lubricants are the agents which forms layer between surfaces or at interfaces to reduce friction. These are long chain molecules with active end-groups such as stearic acid and its metallic salts like magnesium stearate. However, there are other lubricating agents, like fatty acid esters, inorganic materials, and polymers, which can also be used in the cases when both magnesium stearate and stearic acid fails meet their expected performance.It is expected that the lubrication efficiency of magnesium stearate improves with increasing its surface area or decreasing its particle size since the increase of surface area can provide more surface coverage. Consequently, with more coverage of particle surfaces by magnesium stearate, the particle-particle bonding is weakened, resulting in weak tablets. In addition, because the surface of API particles is covered with the lubricant which is hydrophobic, it causes slow-down of dissolution. The optimal size range for the lubricant was found to be from 350–500 μm.The mechanical properties of any compacts/tablets manufactured are lubricant dependent. Magnesium stearate can modified the compaction dynamics by influencing compression pressure as well as the nip angle of the compression process. The reduction of the maximum compression pressure and the nip angle increased with increasing lubricant concentration. In terms of the method of lubrication, it was found that mixing the lubricant with powders (bulk-lubrication) was much more effective than just spraying the lubricant. ⁵¹

Lubricant(s) used for tablets are often incorporated into premixed powder mixture of the formulation as the last step prior to compression. This type of procedure is often called internal lubrication. As opposed to this internal lubrication, external lubrication often refers to a lubrication process in which only the lower punch and die, not the final blend material, are lubricated. This type of lubrication procedure may be used when tablet properties are very sensitive to lubricants. External lubrication gave 40% higher tablet crushing strength without prolonging tablet disintegration time. The tablets also showed lower compression energy, higher hardness, less total pore volume, faster dissolution.⁵⁵

CONCLUSION:

Most of the formulations comprises of excipients in higher concentration than the active pharmaceutical ingredients. The presence of different grades of excipients in varying concentrations decides the functionality and efficacy of the dosage form. The physicochemical properties of the dosage form such as drug release, bioavailability and stability can thus be improved by selecting proper grades and right concentration of excipients.

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Received on 12.05.2016 Accepted on 04.06.2016

Asian J. Pharm. Res. 2016; 6(3): 193-202.

DOI: 10.5958/2231-5691.2016.00028.9

Sr. No.	Action	Mechanism
1.	Swelling	Disintegrating agents impart the disintegrating effect by swelling when in contact with water. Eg. Sodium starch glycolate
2.	Wicking	In this Disintegrating agents do not swell but facilitate disintegration by enhancing porosity by capillary action and provide pathway for penetration of fluid into a tablet. Eg. Croscarmalose, crospovidone.
3.	Deformation	Disintegrating agent particles (elastic) get deformed (plastic) due to high compression pressure and these deformed particles get into their normal structure when they come in contact with aqueous media or water. Eg. Potato starch, maize starch.
4.	Particle repulsive forces	In case of Non-swellable Disintegranting agents, the electric repulsive forces between particles are responsible for disintegration of tablet.
5.	Enzymatic reactions	Some enzymes presents in the body act as Disintegranting agents. These enzymes destroy the binding action of binder and helps in disintegration. Eg. Pectin degradation by pectinase.
6.	Heat of wetting	When a Disintegranting agent with exothermic properties gets wetted, localized stress is generated due to capillary air expansion, which result in disintegration of tablet.
7.	Gas release	Carbon dioxide released within tablets on wetting due to interaction between bicarbonate and carbonate with citric acid or tartaric acid. The tablet disintegrates due to generation of pressure within the tablet. This effervescent mixture is used when pharmacist needs to formulate very rapidly dissolving tablets or fast disintegrating tablets.

Name of excipients	Type	Effect. conc.	Properties
CrosPovidone (Polyplasdone XL, Kollidon CL)	Polyvinyl-pyrrolidone	1-3%	Insoluble in water, Greatest rate of swelling compared to other disintegrants. Swelling index- 58±1.5% v/v.
Cros carmellose Sodium (Ac-di-sol, Primellose, Solutab.)	Cellulose, carboxy-methyl ether, sodium salt crosslinked	2-5%	Insoluble in water, swells to 4-8times its original volume on contact with water. Specific surface area- 0.81-0.83 m2/g. Swelling index- 65±1.7% v/v.
Sodium starch Glycolate (Primogel, Explotab, Glycolys.)	Sodium carboxymethyl starch	4-6%.	Absorbs water rapidly, swelling up to 6%.High concentration causes gelling and loss of disintegration. Swelling index-52±1.2% v/v.
Microcrystalline Cellulose (Avicel)	Cellulose	5-15%	Several different grades of microcrystalline cellulose are commercially available. The larger-particle-size grades generally provide better flow properties in pharmaceutical machinery. Low moisture grades are used with moisture-sensitive materials. Higher-density grades have improved flowability.
Polacrilin Potassium (Indion 294)	Crosslinked polymethacrylic		No lump formation after disintegration. High compatibility with excipients and common therapeutic