Here you can find a simple and short note on Pharmaceutical Preformulation studies. Reference book: The theory and practice of industrial pharmacy by Lachman and Lieberman.

1. DIPTO KUMER SARKER
1
PREFORMULATION
Bulk Characterization
Crystallinity and polymorphism:
Crystal habit and the internal structure .of a drug can affect bulk and physicochemical
properties, which range from flowability to chemical stability.
Habit is the description of the outer appearance of a crystal.
Internal structure is the molecular arrangement of the solid.
Changes with internal structure usually alter the crystal habit. Ex (chemical changes as
conversion of a sodium salt to its free acid form produce both a change in internal structure and
crystal habit).
Different shapes of crystals
Depending on internal structure compounds is classified as
1. Crystalline
2. Amorphous
Crystalline compounds are characterized by repetitious spacing of constituent atom or molecule
in three dimensional array.
In amorphous form atom or molecule are randomly placed.

2. DIPTO KUMER SARKER
2
Amorphous forms:
I. Prepared by rapid precipitation, lyophilization, rapid cooling of liquid metals.
II. Having higher thermodynamic energy than corresponding crystalizing form.
III. Solubilities and dissolution rate is also higher.
Disadvantages:
I. Upon storage, tend to revert to more stable form.
Crystalline forms:
1. Stoichiometric
2. Nonstoichiometric
 Nonstoichiometric adducts:
Ex: Inclusions or clathrates
Nonstoichiometric adducts involve entrapped solvent molecules within the crystal lattice.
Usually this adduct is undesirable, owing to its lack of reproducibility, and should be
avoided for development.
Depending on the shape they are of three types:-
i. Channel: - When the crystal contains continuous channels in which the solvent
molecule can be included. E.g. Urea forms channel.
ii. Layers: - Here solvent molecules are entrapped in between layers of crystals.
iii. Clathrates (Cage):- Solvent molecules are entrapped within the cavity of the crystal
from all sides
 Stoichiometric adducts:
It is a molecular complex that has incorporated the crystallizing solvent molecules into
specific sites within the crystal lattice.
When the incorporated solvent is water, the complex is called a hydrate.
1. Hemihydrate (molar equivalents of water corresponding to half)
2. Monohydrate (equivalents of water corresponding to one)
3. Dehydrate (equivalents of water corresponding to two)
A compound not containing any water within its crystal structure is termed anhydrous.
 Aqueous solubilities of hydrated compound can be significantly less than their
anhydrous forms.

3. DIPTO KUMER SARKER
3
 Conversion of an anhydrous compound to a hydrate within the dosage form may reduce
the dissolution rate and extent of drug absorption
Polymorphism:
Polymorphism is the ability of a compound to crystalize as more than one distinct species with
different internal lattices.
Polymorphs differ from each other with respect to their physiochemical property such as:
 Solubility
 Melting point
 Density
 Hardness
 Compression characteristic
 Optical properties
 Vapor pressure
Ex: Chloramphenicol palmitate exists in three crystalline polymorphic forms (A, B & C) and
amorphous form.
Analytical methods for the characterization of solid forms:
1. Microscopy
2. Fusion method
3. Different scanning calorimetry
4. Infrared spectroscopy
5. X- ray power diffraction
6. Scanning electron microscopy
7. Thermogravimetric analysis
8. Dissolution/solubility analysis
Microscopy:
Isotropic material have single refractive index and this substance do not transmit light with
crossed polarizing filter and appears black.

4. DIPTO KUMER SARKER
4
Material with more than one refractive index are anisotropic & appear bright with brilliant
colors against black polarized background.
The color intensity depends upon crystal thickness.
Advantages:
By this method, we can study crystal morphology & difference between polymorphic forms.
Disadvantages:
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 temperature.
Disadvantage:
 In this technique, the molecules can degrade during the melting process.
 Recrystallization of substance after melting.
Thermal analysis:
Differential scanning calorimetry (DSC) and differential thermal analysis (DTA) measure the
heat loss or gain resulting from physical or chemical changes within a sample as a function of
temperature.
Examples- of endothermic (heat-absorbing) processes are fusion, boiling, sublimation,
vaporization. desolvation, solid-solid transitions and chemical degradation.
Crystallization and degradation are usually exothermic processes.
Applications in preformulation studies:
1. Purity
2. Polymorphism
3. Salvation
4. Degradation
5. Excipient compatibility

5. DIPTO KUMER SARKER
5
Application of DTA and DSC in preformulation studies:
 To determine the purity of a sample
 To determine the number of polymorphs and to
 To determine the ratio of each polymorph.
 To determine the heat of solvation
 To determine the thermal degradation of a drug or excipients.
 To determine the glass-transition temperature of a polymer
For characterizing crystal forms, the heat of fusion, ∆𝐻f can be obtained from the area under
the DSC-curve for the melting endotherm.
Similarly, the heat of transition from one polymorph to another may be calculated.
 A sharp symmetric melting endotherm can indicate relative purity,
 Whereas broad, asymmetric curves suggest impurities or more than one thermal
process.
 DTA
In DTA instrument a record is produced where temperature difference (∆T) (between the
sample and reference material) is plotted against temperature (T) when two specimens are
subjected to an identically controlled temperature regime.
The reference material is alumina, keiselguhr
 DSC METHOD
In DSC method the difference in energy inputs (∆H) into a sample and reference material is
measured as a function of temperature as the specimens are subjected to an identically
controlled temperature programme.

6. DIPTO KUMER SARKER
6
 TGA ( Thermogravimetric analysis):
Thermogravimetric analysis (TGA) measures changes in sample weight as a function of time
(isothermal) or temperature. Desolvation and decomposition processes are frequently
monitored by TGA. Comparing TGA and DSC recorded under identical conditions can greatly
aid in the interpretation of thermal processes.
The dihydrate form of an acetate salt loses two moles of water via an endothermic transition
between 70° and 90°C. The second endotherm at 155°C corresponds to the melting process,
with the accompanying weight loss due to vaporization of acetic acid as well as to
decomposition.
X-ray diffraction
It is an important technique for establishing the batch-to-batch reproducibility of a crystalline
form is x-ray powder diffraction.
Random orientation of a crystal lattice in a powder sample causes the x-rays to scatter in a
reproducible pattern of peak intensities at distinct angles (𝜃) relative to the incident beam.
Each diffraction pattern is characteristic of a specific crystalline lattice for a given compound.
An amorphous form does not produce a pattern. Mixtures of different crystalline forms can be
analyzed using normalized intensities at specific angles, which are unique for each crystalline
form.
When beam of nonhomogeneous X-ray is allow to pass through the crystal, X-ray beam is
diffracted & it is recorded by means of photographic plate.

7. DIPTO KUMER SARKER
7
Diffraction is due to crystal which acts as 3 dimensional diffraction grating toward X-ray.
Single – Crystal x-ray provide the most complete information about the solid state.
Hygroscoopicity:
Many drug substances, particularly water –soluble salt forms, have a tendency to adsorb
atmospheric moisture( (especially water soluble salt forms). They are called hygroscopic
materials and this phenomenon is known as hygroscopicity.
Adsorption and moisture content depend upon
 The atmospheric humidity,
 Temperature,
 Surface area,
 Exposure and
 The mechanism of moisture uptake=0950
The degree of Hygroscopicity is classified into four classes:
 Slightly hygroscopic: increase in weight is ≥ 0.2% w/w and < 2% w/w
 Hygroscopic : increase in weight is ≥ 0.2 % w/w and < 15 % w/w
 Very hygroscopic : increase in weight is ≥ 15% w/w
 Deliquescent : sufficient water is adsorbed to form a solution
Deliquescent materials: They absorb sufficient amount of moisture and dissolve completely in
it. (E.g. anhydrous calcium chloride).
Tests of hygroscopicity:
Bulk drug samples are placed in open containers with thin powder bed to assure maximum
atmospheric exposure. These samples are then exposed to a range of controlled relative
humidity (RH) environments prepared with saturated aqueous salt solutions.
The amount of moisture adsorbed can be determined by the following methods:
o Gravimetry
o Thermogravimetric analysis (TGA)
o Karl-Fischer titration (KF-titration)

8. DIPTO KUMER SARKER
8
o Gas chromatography (GC)
Time of monitoring depends on the purpose:
 For the purpose of ‘handling’ data points from 0 to 24 hours are taken
 For the purpose of ‘storage’ data points from 0 to 12 weeks are taken.
 Moisture level in a powder sample may affect the flowability and compactibility which,
are important factors during tableting and capsule filling.
 After adsorption of moisture, if hydrates are formed then solubility of that powder may
change affecting the dissolution characteristics of the material.
 Moisture may degrade some materials. So humidity of a material must be controlled.
Fine particle characterization:
Parameters those are measured:
 Particle size and size-distribution
 Shape of the particle
 Surface morphology of the particles
Samples are exposed
to the moisture
exposed to controlled
relative humidity
environments
moisture
uptake is
monitored at
different time
points

9. DIPTO KUMER SARKER
9
Particle size is characterized using these terms:
 Very coarse
 Coarse
 Moderately coarse
 Fine
 Very fine
Methods to determine particle size:
Microscopy: Range: 0.2 – 100 μm
Particle size can be determined by the use of calibrated grid background.
Most direct method. Slow & tedious method.
In conjunction with light microscopy, stream counting devices, such as the Coulter counter
and HIAC counter often provide a convenient method of for characterizing the size
distribution of a compound.
Samples are prepared by dispersing the material in a conducting medium such as isotonic saline
with the aid of ultrasound and a few drops of surfactant.
A known volume (0.5 to 2 ml) of this suspension is then drawn into a tube through a small
aperture (0.4 to 800 microns in diameter), across which a voltage is applied.
As each particle passes through the hole, it is counted and sized according to the resistance
generated by displacing that particle's volume of conducting medium.
Given that the instrument has been calibrated with standard spheres, the counter provides a
histogram output (frequency versus size) within the limits of that particular aperture tube.
Several different sizes of aperture tubes should be used to assure accurate count-ing of single
particles.
Limitations of coulter-counter method:
1. Each re-sistance arises from a spherical particle; thus, nonspheres are sized
inaccurately.

10. DIPTO KUMER SARKER
10
2. The tendency of needle-shaped crystals to block the aperture hole, the dissolution of
compound in the aqueous conducting medium, and stratification of particles within the
suspension.
Sieving method: Range: 50 – 150 μm
 Simple, inexpensive
 If powder is not dry, the apertures get clogged.
Measurement of the surface area:
Nitrogen absorption method(brunauer, emmett and teller method) (BET method):
1. In this method ,a layer of the nitrogen molecules is absorbed to the sample surface at -
196℃
2. Once surface absorption is reached in equilibrium, the sample is heated to room
temperature, the nitrogen gas is desorbed, and its volume is measured and converted to
the number of adsorbed molecules via the ideal gas law.
3. Since each nitrogen molecule (N2) occupies an area of 16A2
, one may readily compute
the surface area per gram for each preweighed sample.
4. By determining the surface area at several partial pressures of nitrogen (5% to 35% N2
in He), extrapolation to zero nitrogen partial pressure yields the true monolayer surface
area.
Light energy diffraction: Range: 0.5 – 500 μm
Particle size is determined by the reduction in light reaching the sensor as the particle, dispersed
in a liquid or gas, passes through the sensing zone.
Quick & fast.
Laser holography: Range: 1.4 – 100 μm
A pulsed laser is fired through an aerosolized particle spray & photographed in three
dimensional with holographic camera, allowing the particles to be individually imaged & sized.

11. DIPTO KUMER SARKER
11
Bulk density:
Apparent bulk density:
Bulk drug powder is sieved through 40 mesh screen. Weight is taken and poured into a
graduated cylinder via a large funnel. The volume is called bulk volume.
Significance:
 Bulk density is required during the selection of capsule size for a high dose drug.
 In case of low dose drug mixing with excipients is a problem if the bulk densities of the
drug and excipients have large difference.
Bulk density can varies due to:
Method of crystallization, milling, formulation.
Density problem is identified by milling, slugging or formulation.
Tapped density:
Bulk powder is sieved through 40 mesh screen. Weight is taken and poured into a graduated
cylinder.
The cylinder is tapped 1000 times on a mechanical tapper apparatus. The volume reached a
minimum that is called tapped volume.
Significance: Knowing the dose and tapped density of the formulation, the capsule size can be
determined.
Powder flow properties:
Pharmaceutical powders may be free flowing or cohesive. Powder flow properties can be
affected by change in particle size, shape & density.
The flow properties depends upon following-
1. Force of friction.
2. Cohesion between one particles to another.
Fine particle possess poor flow by filling void spaces between larger particles causing packing
& densification of particles.
By using glident we can alter the flow properties. E.g. Talc

12. DIPTO KUMER SARKER
12
Measurement of the free flowing powder:
1. Compressibility
% Compressibility=(
𝜌 𝑡−𝜌0
𝜌 𝑡
) × 100
Where 𝜌𝑡 is the tapped bulk density and 𝜌0 is the initial bulk density.
It is also known as Carr's index.
It is simple, fast & popular method of predicting powder flow characteristics.
Carr’s Index Type of flow
5-15 Excellent
12-16 Good
18-21 Fair To Passable
23-35 Poor
33-38 Very Poor
>40 Extremely Poor
2. Determination of angle of repose
A greater angle of repose indicate poor flow. It should be less than 30°. & can be determined
by following equation.
tan θ = h/r.
where, θ = angle of repose.
h=height of pile.
r= radius.

13. DIPTO KUMER SARKER
13
Angle of
Repose
( In degree)
Type of Flow
<25 Excellent
25-30 Good
30-40 Passable
>40 Very poor
Disadvantages:
Lack of precision, observation of powder flow from a glass funnel and then a grounded metal
funnel provides in-sight into the drug's flow properties, electrostatic properties, and tendency
to bridge in a cone shaped hopper.
Solubility analysis:
Preformulation solubility studies focus on drug-solvent system that could occur during delivery
of drug candidate.
Method:
The drug is dispersed in a solvent. The suspension is agitated at a constant temperature.
Samples of the suspension are withdrawn as a function of time, clarified by centrifugation, and
assayed to establish a plateau concentration.
Solubility depends on
 pH
 Temperature
 Ionic strength
 Buffer concentration
Solubility analysis include determination of pKa, temperature dependence, solubilization
product, pH solubility profile etc.

14. DIPTO KUMER SARKER
14
Solubility values that are useful in a candidates early development are those in distilled.
Solvents taken are
 0.9% NaCl at room temperature
 M HCl at RT
 M HCl at RT
 M NaOH at room temperature
 At pH 7.4 buffer at 37℃
Drug concentration is determined by the following analytical methods
 HPLC
 UV –Spectroscopy
 Fluorescence Spectroscopy
 Gas Chromatography
Significance:
 A drug for oral administrative should be examined for solubility in an isotonic saline
solution and acidic pH..
 This solubility data may provide the dissolution profile invivo.
 Solubility in various mediums is useful in developing suspension or solution
toxicologic and pharmacologic studies.
 Solubility studies identify those drugs with a potential for bioavailability problems.
 E.g. Drug having limited solubility (7 %) in the fluids of GIT often exhibit poor
or erratic absorption unless dosage forms are tailored for the drug.