Pre formulation Studies

 DEFINITION:-Investigation of physico-chemical properties of the new drug compound that could affect drug performance and development of an efficacious dosage form”.Preformulation commences when a newly synthesized drug shows a sufficient pharmacologic promise in animal model to warrant evaluation in man.

 INTRODUCTION

The preformulation is the first step in the rational development of a dosage form of a drug substance alone and when combined with excipients.

Objective :

To generate useful information (physico-chemical , kinetics, stability and compatibility with excipients) to the formulator to design an optimum drug delivery system.

Before embarking on a formal programme of preformulation, scientist must consider the following :

1. Available physicochemical data (including chemical structure, different salt available)

2. Anticipated dose

3. Supply situation and development schedule

4. Availability of stability – indicating assay

Organoleptic properties

 Color:

Color is generally a function of a drug’s inherent chemical structure relating to a certain level of unsaturation (double or triple bond).Color intensity relates to the extent of conjugated unsaturation as well as the presence of chromophores.Some compound may appear to have color although structurally saturated.

 Odor

·      The substance may exhibit an inherent odor characteristic of major functional groups present.

·      Odor greatly affects the flavor of a preparation or food stuff

Taste

·      If taste is considered as unpalatable, consideration is to be given to the use of a less soluble chemical form of the drug.

·      The odor and taste may be suppressed by using appropriate flavors and excipients or by coating the final product.

 PURITY

Designed to estimate the levels of all known & significant impurities & contaminates in the drug substance under evaluation. Study performed in an analytical research & development group.It is another parameter which allows for comparison with subsequent batches.

The techniques used for characterizing the purity of a drug are the same as those used for other purpose in a preformulation study.

Thin layer chromatography is a wide ranging applicability & is an excellent tool for characterizing the purity. HPLC, paper chromatography & gas chromatography are also useful. More quantitative information can be obtained by using quantitative differential scanning colorimetry.

 PARTICLE SIZE

Particle size is characterized using these terms :

Very coarse (#8)

Coarse (#20)

Moderately coarse (#40)

Fine (#60)

Very fine (#80)

Particle size can influence variety of important factors :

- Dissolution rate

- Suspendability

- Uniform distribution

- Penetrability

- Lack of grittiness

 SURFACE AREA

Particle size & surface area are inversely related to each other.Smaller the drug particle, greater the surface area.Specific surface is defined as the surface area per unit weight (Sw) or unit volume (Sv) of the material

HOWEVER SIZE REDUCTION IS NOT REQUIRED IN FOLLOWING CASES:

WHEN DRUG IS UNSTABLE.

DEGRADE IN SOLUTION FORM.

PRODUCE UNDESIRABLE EFFECTS.

WHEN SUSTAINED EFFECT IS DESIRED

 Particle Shape

Particle shape will influence the surface area, flow of particles, packing & compaction properties of the particles.

A sphere has minimum surface area per unit volume.

Therefore, these properties can be compared for spheres & asymmetric particles, in order to decide the shape.

 SOLUBILITY

The amount of substance that passes into solution in order to establish equilibrium at constant temperature and pressure to produce a saturated solution.

If solubility is <1mg/ml indicates need for salt formation to improve solubility.

If solubility is <1mg/ml in pH= 1 to 7, preformulation study should be initiated.

Solubility should ideally be measured at two temperatures: 4°C and 37°C.

·      4°C to ensure Physical stability.

·      37°C to support Biopharmaceutical evaluation.

DESCRIPTIVE SOLUBILITIES

Description	Parts of solvent
Very soluble	1
Freely soluble	1-10
soluble	10-30
Sparingly soluble	30-100
Slightly soluble	100-1000
Very slightly soluble	1000-10,000
Insoluble	> 10,000

 

 SOLUBILITY ANALYSIS

Analytic methods that are particularly useful for solubility measurement include HPLC, UV spectroscopy, Fluorescence spectroscopy and Gas chromatography. Reverse phase HPLC offer accurate and efficient mean of collecting solubility data of drug.

 Ionization constant (pKa)
Can be calculated by Henderson Hasselbach equation-For acidic drugs….pH= pKa+ log [ionized drug]/[unionized drug]

For basic drugs….pH= pKa+ log[unionized drug]/[ionized drug]

 pH Solubility Profile

The solubility of acidic or basic drug will show difference with changes in pH.So according to general role (salt formation), solubility of basic drugs will increased as the pH decreased, and the solubility of acidic drugs increased as the pH increased.pH solubility profile of a drug can be established by running the equilibrium solubility experiment within pH range of 3-4.

Partition Coefficient
It is the ratio of unionized drug distributed between organic and aqueous phase at equilibrium. It’s a measurement of a drug's lipophilicity and its ability to cross the cell membrane.

P o/w = ( C oil / C water )equilibrium

 

 Effect Of Temperature

The heat of solution ∆Hs, represents the heat released or absorbed when a mole of solute is dissolved in large quantity of solvent.

most commonly the solution process is Endothermic reaction (+) , thus by increasing the temp. the solubility will be increased.

Exothermic reaction (-) for such solutes as lithium chloride and other HCL salts that are ionized when dissolved ,thus increasing the temp. will decrease the solubility.

Determination of solubility
The following points should be considered:

-The solvent & solute must be pure.

-A saturated solution must be obtained before any solution is removed for analysis.

-The method of separating a sample of saturated solution from undissolved solute must be satisfactory.

-The method of analyzing solution must be reliableTemperature must be adequately controlled .

 Solubility Determination Method
-Solubility is normally depends on temperature, so temperature is recorded in each solubility measurement.

- Plot of solubility against temperature is commonly used for solubility determination.

- Two methods are available for determination are as follow.

1.   Synthetic method

2.   Analytical method: Temp. of equilibrium is fixed and conc. Of the solute in the saturated sol. Is determined at equilibrium by a suitable analytical procedure.

General Methods of Increasing the Solubility (Solubilization)
Addition of co-solventSalt formationReduction of particle sizeThe choice of proper polymorphProdrug approachAddition of SurfactantpH-change of solutionTemp.effectComplexation

 Process Of Solubilization
The process of solubilization involves the breaking of inter-ionic or intermolecular bonds in the solute, the separation of the molecules of the solvent to provide space in the solvent for the solute, interaction between the solvent and the solute molecule or ion.

Step 1: Holes opens in the solvent DEPT. OF PHARMACEUTICS

Step2: Molecules of the solid breaks away from the bulk

Step 3: The free solid molecule is intergraded into the holes in the solvent

 SOLUBILIZATION

When surfactants are added to the liquid at low concentration they tend to orient at the air-liquid interface .On further addition of surfactant the interface becomes completely occupied and excess molecules are forced into the bulk of liquid.At very high concentration surfactant molecules in the bulk of liquid begin to form micelles and this concentration is know as CRITICAL MICELLECONCENTRATION{CMC}DEPT. OF PHARMACEUTICS

Solubilization is thought to occur by virtue of the solute dissolving in or being adsorbed onto the micelle.Thus the ability of surfactant solution to dissolved or solubilize water insoluble materials starts at the CMC and increase with increase in the concentration of micelles.Solubilization of any material in any solvent depends on proper selection of solubilising agents.

Addition Of Co-Solvent
Weak Electrolyte :- PhenobarbitoneNon polar :- Nitro CelluloseThese are poorly soluble in given solvent.For such poorly soluble materials, to enhance their solubility, the water miscible solvents are used in which the drug has good solubility.This process of improving solubility is known as co-solvency and the solvent used is known as co-solvents.

e.g. Phenobarbitone is insoluble in water. A clear solution is obtained by dissolving it in a mixture of Alcohol, Glycerin, Propylene glycol.e.g. Of Cosolvents:-PG, glycerin, sorbitol, PEG, Glyceryl formal, glycofurol, ethyl carbamate, ethyl lactate and dimethyl acetamide.

 pH change Method Weak base:- Alkaloids, Local Anaesthesia

Weak acid:- Sulphonamides, BarbituratesIn aqueous medium they dissociate poorly and undissociated portion is insoluble.e.g. Benzoic acid, PhenobarbitoneSo, solubility of the undissociated portion is improved by pH control.For weak acidic drug:- increase pH, solubility is increase.For weak base drug:- decrease pH, increase solubility.

 

Reduction Of Particle size
Reduction in Particle size improve solubility of drug.Basically reduction in particle size increase contactsurface area of the particle, there by ultimately it increase rate of solubility of drug.

 Applications of solubilization
Drugs with limited aqueous solubility can be solubilized. These include oil-soluble vitamins, steroid hormones and antimicrobial agents etc.

 Solubilization of orally administered drugs results in an improved appearance and improves unpleasant taste.Both oil-soluble and water-soluble compounds can be combined in a single phase system as in case of multivitamin preparations.

Solubilization may lead to enhanced absorption and increased biological activity.Improves the intestinal absorption of vitamin A.

Drug absorption from ointment bases and suppositories also increased.Liquid preparations with small quantity of preservative can be prepared by solubilization.

Aqueous concentrates of volatile oils can be prepared by solubilization.

Example: soaps used for solubilising phenolic compounds for use as disinfectants- Lysol, Roxenol etc.

Barbiturates, anticoagulant, alkloidal drugs are dissolved with polysorbate by solubilization.

 Crystallinity

Crystal habit & internal structure of drug can affect bulk & physicochemical properties of molecule.

Bulk prop. Like hygroscopocity, flowabilty and density.Crystal habit is description of outer appearance of crystal.

Internal structure is molecular arrangement within the solid.

 Crystallinity and Polymorphism
There are two main classes of the solids: crystalline and amorphous.

what distinguishes them is the nature of their atomic scale structure.

Atomic positions in crystal exhibit a property called long-range order positions repeated in space in regular array.

Amorphous material is a random

Arrangement of the molecules .

 

 

 Different shapes of crystals

Cubic or isometric - not always cube shaped. Also find as octahedrons (eight faces) and dodecahedrons (10 faces).

Tetragonal- similar to cubic crystals, but longer along one axis than the other, forming double pyramids and prisms.

Orthorhombic - like tetragonal crystals except not square in cross section (when viewing the crystal on end), forming rhombic prisms or dipyramids (two pyramids stuck together).

Hexagonal - six-sided prisms. When you look at the crystal on-end, the cross section is a hexagon.

Trigonal - possess a single 3-fold axis of rotation instead of the 6-fold axis of the hexagonal division.

Triclinic - usually not symmetrical from one side to the other, which can lead to some fairly strange shapes.

Monoclinic - like skewed tetragonal crystals, often forming prisms and double pyramids.

Solubility & dissolution rate are greater for amorphous form then crystalline, as amorphous form has higher thermodynamic energy. Also amorphous solubility is higher because of its larger surface area, but at the same time stability is low and its more affected by PH and its factor. Amorphous powder with storage convert from unstable to stable form.

Amorphous form typically prepared by :

Lyophilization, rapid precipitation, and rapid cooling of the liquid melts.

When water incorporated in the structure , the complex called hydrate.
If it contains one molar equivalent of H₂O , called monohydrate.Contains two molar of H₂O , called dihydrate.Contains no water, called anhydrous.Hydrate compounds are important since their aqueous solubility are significantly less than that of anhydrous form. Conversion of anhydrous to hydrate within the dosage form reduce the solubility and dissolution rate of the drug like anhydrous and trihydrate forms of Ampicillin .

 Polymorphism

It is the ability of the compound to crystallize as more than one distinct crystalline species of the same pure substance with different internal lattice.Different crystalline forms are called polymorphs.

Physical properties differ among various polymorphs:

Molar vol. and density

Refractive index

Melting and sublimation temp.

Enthalpy (heat content)

Solubility

Dissolution rate

Stability

Hardness

Compatibilty , handling, flow and blending.

 

During preformulation it is important to identify the polymorph that is stable at room temp.Chloramphenicol exist in A,B & C forms, of these B form is more soluble & most preferable as the serum level of it is higher but at the same time it is less stable and may converted to stable form upon storage.The least stable polymorph has more solubility and vice versa.

 

 Techniques for studies of crystals
Microscopy

Hot stage microscopy

Thermal analysis

X-ray diffraction

 Microscopy

Material with more than one refractive index appears bright with brilliant colors against black polarized background.The color intensity depends upon crystal thickness.materials havesingle refractive indexand this substance do nottransmit light with crossedpolarizing filter and appearsblack.

Advantage :
By this method, we can study crystal morphology & difference between polymorphic form.

Disadvantage :

This require a well trained optical crystallographer, as there are many possible crystal habit & their appearance at different orientation.

 Hot stage microscopy

The polarizing microscope fitted with hot stage is useful for investigating polymorphism, melting point & transition temp.

Disadvantage :In this technique, the molecules can degrade during the melting process.

 DSC And Thermal analysis
Differential scanning calorimetry (DSC) & Differential thermal analysis are (DTA) are particularly useful in the investigation of polymorphism.

·      It measures the heat loss or gain resulting from physical or chemical changes within a sample as a function of temp.

·      Disadvantage : Degradation during thermal analysis mayprovide misleading results.

X-ray diffraction

Working :
When beam of nonhomogenous X-ray is allow to pass through the crystal, X-ray beam is diffracted & it is recorded by means of photographic plate.Diffraction is due to crystal which acts as 3 dimensional diffraction grating toward X-ray. Random orientation of crystal lattice in the powder causes the X-ray to scatter in a reproducible pattern of peak intensities.The diffraction pattern is characteristic of a specific crystalline lattice for a given compound. An amorphous form does not produce a pattern mixture of different crystalline forms.