preformaulation cource kau 2013-2014...12/23/2013 1 ahmed zidan, ph.d. bader aljaeid, ph.d....

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12/23/2013 1 Ahmed Zidan, Ph.D. Bader Aljaeid, Ph.D. 2013/2014 Definition: it is a group of studies that focus on the physicochemical properties of a new drug candidate that could affect the drug performance and the development of a dosage form. This could provide an important information for formulation design or support the need for molecular modification. The overall objective of preformulation testing is to generate information useful to the formulator in developing stable and bioavailable dosage forms which can be mass-produced. During the early development of a new drug substance, the synthetic chemist, alone or in cooperation with specialists in other disciplines (including preformulation), may record some data which can be appropriately considered as preformulation data. What should we know before starting the preformulation studies? 1.The properties of the drug. 2.Potency relative to the competitive products and the dosage form. 3.A literature search providing stability and decay data. 4.The proposed route of drug administration. 5.A literature search regarding the formulation approaches, bioavailability, pharmacokinetics of chemically related drugs. Preformulation Preliminary investigations and molecular optimization: Step I: the drug should be tested to determine the magnitude of each suspected problem area. Step II: if a deficiency is detected, a molecular modification should be done to overcome this deficiency. Molecular modification is done be salts, prodrugs, solvates, polymorphs or even new analogues. 1.Salt formation: The dissolution rate of a salt form of a drug is generally quite different from that of the parent compound. Sodium and potassium salts of weak organic acids and hydrochloride salts of weak organic bases dissolve much more readily than do the, respective free acids or bases. The result is a more rapid saturation of tile diffusion layer surrounding the dissolving particle and the consequent more rapid diffusion of the drug to the absorption sites. Preformulation Preliminary investigations and molecular optimization: 1.Salt formation: Numerous examples could be cited to demonstrate the increased rate of drug dissolution due to the use of the salt form of the drug rather than the free acid or base, but the following will suffice: the addition of the ethylenediamine moiety to theophylline increases the water solubility of theophylline 5-fold. The use of the ethylenediamine salt of theophylline has allowed the development of oral aqueous solutions of theophylline and diminished the need to use hydroalcoholic mixtures, e.g., elixirs. Ephedrine base is very poorly water soluble molecules that characterized by low solubility and dissolution rates. So, it is modified in the form of the salt “ephedrine HCL” that is ionized and offer higher water solubility and dissolution rate. Preformulation 2. Prodrug formation: it is the formation of a synthetic derivatives of the drug (e.g. esters or amides) that liberate the active drug in-vivo. Prodrug may or may not has a pharmacological activity. The active drug is released by acidic medium, enz action, … etc. Prodrug formation may increase the absorption rate due to its lipophilicity (passive) or its water solubility (active). Prodrug formation may increase the duration of action by 1) blockade of a key metabolic pathway or 2) change drug organs distribution. Prodrug formation may improve the drug stability, solubility, crystallinity, taste, odor and reduced pain on injection.

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Page 1: Preformaulation cource KAU 2013-2014...12/23/2013 1 Ahmed Zidan, Ph.D. Bader Aljaeid, Ph.D. 2013/2014 Definition: •it is a group of studies that focu s on the physicochemical properties

12/23/2013

1

Ahmed Zidan, Ph.D. Bader Aljaeid, Ph.D.

2013/2014

Definition:  

• it is a group of studies that focus on the physicochemical properties of a new drug candidate that could affect the drug performance and the development of a dosage form.

•This could provide an important information for formulation design or support the need for molecular modification.

•The overall objective of preformulation testing is to generate information useful to the formulator in developing stable and bioavailable dosage forms which can be mass-produced.

•During the early development of a new drug substance, the synthetic chemist, alone or in cooperation with specialists in other disciplines (including preformulation), may record some data which can be appropriately considered as preformulation data.

What should we know before starting the

preformulation studies?

1.The properties of the drug.

2.Potency relative to the competitive products and the dosage form.

3.A literature search providing stability and decay data.

4.The proposed route of drug administration.

5.A literature search regarding the formulation approaches, bioavailability, pharmacokinetics of chemically related drugs.

Preformulation

Preliminary investigations and molecular optimization:  

•Step I: the drug should be tested to determine the magnitude of each suspected problem area.

•Step II: if a deficiency is detected, a molecular modification should be done to overcome this deficiency.

•Molecular modification is done be salts, prodrugs, solvates, polymorphs or even new analogues.

1.Salt formation: The dissolution rate of a salt form of a drug is generally quite

different from that of the parent compound. Sodium and potassium salts of weak organic acids and hydrochloride salts of weak organic bases dissolve much more readily than do the, respective free acids or bases. The result is a more rapid saturation of tile diffusion layer surrounding the dissolving particle and the consequent more rapid diffusion of the drug to the absorption sites.

Preformulation

Preliminary investigations and molecular optimization:  

1.Salt formation: Numerous examples could be cited to demonstrate the increased rate

of drug dissolution due to the use of the salt form of the drug rather than the free acid or base, but the following will suffice: the addition of the ethylenediamine moiety to theophylline increases the water solubility of theophylline 5-fold. The use of the ethylenediamine salt of theophylline has allowed the development of oral aqueous solutions of theophylline and diminished the need to use hydroalcoholic mixtures, e.g., elixirs.

Ephedrine base is very poorly water soluble molecules that

characterized by low solubility and dissolution rates. So, it is modified in the form of the salt “ephedrine HCL” that is ionized and offer higher water solubility and dissolution rate.

Preformulation

2. Prodrug formation: it is the formation of a synthetic derivatives of the drug (e.g. esters or amides) that liberate the active drug in-vivo.

• Prodrug may or may not has a pharmacological activity.

• The active drug is released by acidic medium, enz action, … etc.

• Prodrug formation may increase the absorption rate due to its lipophilicity (passive) or its water solubility (active).

• Prodrug formation may increase the duration of action by 1) blockade of a key metabolic pathway or 2) change drug organs distribution.

• Prodrug formation may improve the drug stability, solubility, crystallinity, taste, odor and reduced pain on injection.

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Preformulation

2. Prodrug formation: it is the formation of a synthetic derivatives of the drug (e.g. esters or amides) that liberate the active drug in-vivo.

Example:

• Erythromycin base: has a bitter taste and is rapidly hydrolyzed in stomach to inactive products.

•Erythromycin estolate (prodrug of erythromycin):

• is inactive and tasteless. It has 4 times absorption rate.

•It is hydrolyzed by the acid in stomach to liberate the free base which is active.

•It is only 24% hydrolyzed to inactive products.

Steps in preformulation pharmaceutical research 

1. Stability i. Solubility a. Solid State (1) Water and Other Solvents (1) Temperature (2) pH-Solubility Profile (2) Light (3) Salt Forms (3) Humidity (4) Cosolvents b. Solution (5) Complexation (1) Solvent (6) Prodrug (2) pH j. Effect of pH on UV Spectra (3) Light k. Ionization Constant 2, Solid State Compatibility l. Volatility a. TLC Analysis m. Optical Activity b. DRS Analysis n. Polymorphism Potential 3. Physico-chemical Properties o. Solvate Formation a. Molecular Structure and Weight 4. Physico-mechanical Properties b. Color a. Bulk and Tapped Density c. Odor b. Compressibility d. Particle size, Shape, and Crystallinity c. Photomicrograph e. Melting Point 5. In Vitro Availability Properties f. Thermal Analysis Profile a. Dissolution of Drug Crystal Per se (1) DTA b. Dissolution of Pure Drug Pellet (2) DSC c. Dissolution Analysis of Pure Drug (3) TGA d. Rat Everted Gut Technique g. Hygroscopicity Potential 6. Other Studies h. Absorbance Spectra a. Plasma Protein Binding (1) UV b. Effect of Compatible Excipients (2) IR on Dissolution c. Kinetic Studies of Solution Degradation d. Use of Radio-labeled Drug

In this course, we will cover:  1.Organoleptic properties 2.Assay development (UV, TLC and HPLC). 3.Bulk characterization:

• Crystallinity and polymorphism. • Hygroscopicity. • Fine particle characterization. • Bulk density. • Powder flow properties. • Compression properties. • Physical description.

3. Solubility analysis: • Intrinsic solubility. • PKa determination. • Partition coefficient. • Dissolution. • Common ion effect.

4. Stability analysis: • Solution stability • Solid state stability.

Preformulation Organoleptic properties 

1. Colour:

• Unappealing to the eye instrumental methods or variable from batch to batch

• Record of early batches establishing “specs” is very useful for later production

• Undesirable incorporation of a dye variable color in the body or coating

2. Odor and taste:

• Unpalatable use of less soluble chemical form or suppress it by flavors, excipients, coating

• Drug substances irritating to skin handling precautions

• Flavors, dyes, excipients used will affect stability and bioavailability

Preformulation Organoleptic properties 

• This table suggests terminology to describe organoleptic properties

of pharmaceutical powders

Preformulation Assay development 

•The first step in preformulation is to develop an analytical method for the quantitation of the drug in different media.

1.UV spectroscopy: • It is used for drugs that absorb energy at the ultraviolet region. • The drug must be in a solution form. The type of the chosen solvent

depends on drug solubility and functional groups it has. • Steps:

1. Establish the UV spectrum of the drug. 2. Choose a suitable wavelength for drug quantitation (Usually λmax). 3. Apply Beer-Lambert law: Absorbance (A) = log I°/I = eCL

SO, the amount of light absorbed is directly proportional with concentration and path length of the solution.

• Uses of UV spectroscopy: 1. Quantitative determination of drugs that absorb in UV region. 2. Quantitative determination of drug degradation products when it absorbs at

different wave length. 3. Determination of Pka of drugs.

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Preformulation Assay development 

2. Thin layer chromatography (TLC): • Steps:

1. Prepare the extracted sample that contain different components. 2. Prepare the mobile phase (eluting solution) with the suitable composition. 3. Choose the appropriate stationary phase (plate packing material). 4.Spot the sample mixture at the base of the plate and place it horizontally

in the elution tank saturated with the mobile phase vapors. 5. After sample development, determine the Rf value for qualitative analysis.

6.Scratch each spot after separation and extract its components for quantitative analysis by either UV or HPLC.

• Uses of TLC: 1. Qualitative determination of mixtures of components. 2. Estimating their concentration using reference standard.

Preformulation Assay development 

3. High performance liguid chromatography (HPLC): It is column chromatography with elution under high pressure

• Characteristics (compared to column chrom.):

1. Much smaller column. 2. Shorter retention times. 3. Higher sensitivity (less than 1 ng/ml). 4. Smaller sample volume (1-200 μl). 5. Higher separation selectivity especially for complex mixtures.

• HPLC modes according to type of stationary phase:

1. Normal phase HPLC. 2. Reversed phase HPLC.

Preformulation Assay development 

3. High performance liguid chromatography (HPLC):

Normal phase HPLC 1. The stationary phase is silica (hydrophilic) with a non polar mobile phase. 2. Mobile phase is usually hexane with the addition of chloroform, isopropyl 

alcohol, or methanol (the order of increasing the polarity). 3. Separation occur by a process of partitioning of the solute between the 

stationary and mobile phases (Adsorption and desorption). 4. Polar solutes will be retained and eluted faster as the polarity of the mobile 

phase increases and vice versa.

Reversed phase HPLC 1. The stationary phase is made lipophilic by coating the silica (derivatization). 

Examples: Octadecylsilane “ODS” (C18) and octylsilane (C8). 2. The mobile phase is hydrophilic such as methanol, acetonitrile, THF or mixtures 

to modify the polarity. 3. Polar solutes will be eluted quickly with a short retention time. Non polar 

solutes will be retained and eluted faster as the polarity of the mobile phase decreased and vice versa.  

Preformulation Bulk characterization 

Quest. Why do we need bulk characterization of a drug

candidate? To identify all the solid forms that may exist as a consequence of the synthetic stage such as the presence of polymorphs. 

Bulk properties such as particle size, bulk density, surface morphology may be changed during the development process. 

To avoid mislead predictions of solubility and stability which depends on a particular crystalline form. 

  

Preformulation Bulk characterization 

 

Bulk characterization testing includes: • Crystallinity and polymorphism. • Hygroscopicity. • Fine particle characterization. • Bulk density. • Powder flow properties. • Compression properties. • Physical Description

   

Preformulation 1.Crystallinity and polymorphism:

   

Chemical compound 

Internal structure 

Crystalline (regular arrangement of molecules)  

Single entity and polymorphs 

Molecular adducts 

Stoichiometric solvates or hydrates (Pseudopolymorph) 

Non‐ stiochiometric inclusion complexes 

Channel Channel 

Layer Layer 

Cage  

(Clathrates) 

Cage  

(Clathrates) 

Amorphous (random arrangement of molecules) 

Habit  (external shape) 

The structure of a solid compound can be classifies as shown beside:

• These structures disappear in the liquid and vapor states.

• Internal structures such as cubic, tetragonal, hexagonal, rhombic, etc.

• Solid habits such as platy, needle, tabular, prismatic, bladed, ….. etc.

• Changing the internal structures alter the crystal habits.

• Changing the chemical form (e.g. salt formation) alter both the internal structure and crystal habit.

• Different polymorphs are obtained by crystallization from different solvents and by solidification after melting.

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Preformulation 1.Crystallinity and polymorphism:

   

Chemical compound 

Internal structure 

Crystalline (regular arrangement of molecules)  

Single entity and polymorphs 

Molecular adducts 

Stoichiometric solvates or hydrates (Pseudopolymorph) 

Non‐ stiochiometric inclusion complexes 

Channel Channel 

Layer Layer 

Cage  

(Clathrates) 

Cage  

(Clathrates) 

Amorphous (random arrangement of molecules) 

Habit  (external shape) 

• Stiochiometric adduct (solvates): a molecular complex that has incorporated solvent molecules into a specific sites within the crystal.

• When the incorporated solvent is water, it is called hydrates.

• The compound not containing any water within its crystal structure is called as anhydrous.

• Amorphous forms are produced by rapid precipitation, lyophilization, rapid cooling of liquid melt.

• It has higher energy, faster dissolution, lower melting point than the crystalline form of the drug.

Preformulation 1.Crystallinity and polymorphism: How do the crystals form?

Under certain conditions, solute molecules join together, this process is called Nucleation. In this process the attraction forces between solute molecules exceed the forces in the solution which tend to disrupt these aggregates. Furthermore, these aggregates counter other solute molecules until they become insoluble and precipitate as crystalline particles.

Classification of solid material:

1) Crystalline: atoms in crystalline matter are arranged in regular and repeating patterns in three dimensions. e.g. metal and mineral.

2) Amorphous: atoms or molecules randomly placed “without a regular atomic arrangement”.

Crystal habit: it is the description of the outer appearance of crystal and includes, for example, acicular (needle-like), prismatic, pyramidal, tabular, equant, columnar and lamellar.

Internal structure: it is molecular arrangement within the solid.

Preformulation 1.Crystallinity and polymorphism: 

Classification of solid material:

Change with internal structure usually alters crystal habit.

Amorphous forms have higher solubility and dissolution rate than crystalline forms because they have higher thermodynamic energy. e.g. amorphous form of Novobiocin is well absorbed whereas crystalline form results in poor absorption.

Amorphous forms tend to revert to more stable forms upon storage, which means changing in physicochemical properties; therefore, this conversion makes them unstable during storage.

Amorphous forms have a characteristic temperature at which there is a major change in properties. This is called the glass transition temperature (Tg). Below Tg, the material will be brittle and is described as the glassy state; as the temperature is increased above Tg, the molecule becomes more mobile and the material is said to become rubbery. The transition temperature may be lowered by the addition of plasticizer. e.g. water.

Crystal habit and internal structure of a drug can affect physicochemical properties such as; bulk density, particle size, flowability and stability.

Preformulation 1.Crystallinity and polymorphism: 

Different shapes of crystals: Cubic or isometric - not always cube shaped. Also find as octahedrons (eight

faces) and dodecahedrons (12 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.

Preformulation 1.Crystallinity and polymorphism: 

Different shapes of crystals:

Preformulation 1.Crystallinity and polymorphism: 

Why do different crystals have different shapes and sizes? This depends on the following factors:

1. The relative growth rate along the various directions in the crystal. 2. The internal symmetry of the crystal.

During crystallization, drug often entrap solvent in the crystal. Based on the amount of crystallization solvent, a crystalline compound is classified to either a stoichiometric or non-stoichiometric.

Stoichiometric: it involves entrapped solvent molecules into specific site and the quantity is uniform. It is also called solvate.

Non-stoichiometric: the entrapment is not uniform and it is difficult to reproduce. It also should be avoided in development. • When the incorporated solvent is water, the complex is called hydrate. • A compound not containing water within its crystal structure is termed

anhydrous. • Terms hemihydrate, monohydrate and dihydrate describe hydrated forms with

molar equivalents of water corresponding to half, one and two. • Hydrated compounds have less aqueous solubilities than their anhydrous forms.

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Preformulation 1.Crystallinity and polymorphism: Polymorphism It is the ability of the compound to crystallize as more than one distinct crystalline

species with different internal lattice. Polymorphism: different crystal forms (at different free energy states) of the same

compounds. They have different physicochemical properties (melting point, density, vapor pressure, X-ray, color, crystal shape, hardness, solubility, dissolution rate and bioavailability).

Type of polymorphs:

1) Enantiotropic: polymorphs can be interconverted below the melting point of either polymorph and the conversion is reversible at a define temperature. e.g. sulfur.

2) Monotropic: the transition takes place in one direction (irreversible). e.g. glyceryl stearate and diamond graphite.

• During preformulation, it is important to identify the polymorph that is stable at

room temperature.

Preformulation 1.Crystallinity and polymorphism: 

Type of polymorphs: For examples: Chloromphenicol exist in A,B & C forms, of these B form is more stable and most

preferable. Riboflavin has I,II & III forms, the III form shows 20 times more water solubility

than form I. Stable polymorph: it has low free energy, low solubility and high melting point. Metastable polymorph: it is less stable with higher solubility and bioavailability

and lower melting point.

Pharmaceutical applications of polymorphism The transformation between polymorphic forms can lead to formulation problems.

For example: • In suspension: phase transformation from unstable form to more stable polymorph can cause

changes in crystal size and caking. e.g. Oxyclozanide (anthlemintic) • In cream: crystal growth as a result of phase transformation can cause grittiness. • In suppositories: changes in polymorphic forms could cause product with different and

unacceptable melting characteristic ( failure to melt after administration or premature melting during storage). e.g. theobroma oil “suppositories base”.

Preformulation 1.Crystallinity and polymorphism: 

Type of polymorphs: For examples: Chloromphenicol exist in A,B & C forms, of these B form is more stable and most

preferable. Riboflavin has I,II & III forms, the III form shows 20 times more water solubility

than form I. Stable polymorph: it has low free energy, low solubility and high melting point. Metastable polymorph: it is less stable with higher solubility and bioavailability

and lower melting point.

Pharmaceutical applications of polymorphism The transformation between polymorphic forms can lead to formulation problems.

For example: • In suspension: phase transformation from unstable form to more stable polymorph can cause

changes in crystal size and caking. e.g. Oxyclozanide (anthlemintic) • In cream: crystal growth as a result of phase transformation can cause grittiness. • In suppositories: changes in polymorphic forms could cause product with different and

unacceptable melting characteristic ( failure to melt after administration or premature melting during storage). e.g. theobroma oil “suppositories base”.

Preformulation 1.Crystallinity and polymorphism: 

•Characterization of solids involves:

1. Verifying the solid is the expected chemical compound. 

2. Characterization the internal structure. 

3. Describing the habit of the crystal. 

4. Determine how many polymorph may exist for the compound, 

5. Determine how stable are the metastable forms. 

6. Screening for the presence of an amorphous form. 

7. Can the metastable form be stabilized? 

8. What is the stability of each form? 

9. Will the more soluble form survive during the processing and storage? 

 

•Methods of characterization:

1. Microscopy. 

2. Thermal analysis. 

3. X‐ray 

Preformulation 1.Crystallinity and polymorphism: 

•Microscopy: • Most rapid technique, but for quantitative size determination requires

counting large number of particles.

• For size ~ 1 mm upward (magnification x400).

• Suspending material in nondissolving fluid (water or mineral oil)

• Polarizing lens to observe birefringence change in amorphous state after grinding?

• Isotropic substance: it has one refractive index when examined cross polarized microscope. For example: amorphous substances and cubic crystals e.g. NaCl

• Anisotropic substance: it is transparent with more than one refractive index. It appear bright with brilliant colors. For example: different crystalline substances.

• Polarizing microscope fitted with hot stage is useful for investigating polymorphism, melting points, transition temperatures, polymorphic crystal habit.

Preformulation 1.Crystallinity and polymorphism: 

•Microscopy: • There are different types of microscopes could be used:

1. Optical microscopy: crystal habit and crystal size could be determined.

2. Polarized microscopy: by exposing the sample to polarized beam and rotating the slide, different and beautiful colors image from the sample.

3. Electron microscopy: it is powerful tool for studying the surface properties of crystal. Due to its high resolution, it could be used to study crystal surface during reaction.

4. Hot stage microscopy: the polarizing microscopy fitted with hot stage is useful for investigating polymorphism, melting point and transition temperature.

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Preformulation 1.Crystallinity and polymorphism: 

•X-ray diffraction: Mechanism: the orientation of crystal lattice can scatter X-rays to give a

pattern of peak intensities at distinct angels (2θ) relative to the incident beam.

When beam of non-homogenous X-ray is allowed to pass through the crystal, X-ray beam is diffracted and it is recorded by means of photographic plate 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 A crystalline material has its characteristic X-ray pattern. Amorphous substances has a hallow pattern with no peaks. It is used to investigate the interaction exist in combinations such as drug

plus excipients in a given dosage form.

Preformulation 1.Crystallinity and polymorphism: 

•Thermal analysis:

DSC and DTA measure the heat loss or gain resulting from physical or chemical changes within a sample as a function of temperature.

Examples of endothermic transition: fusion, boiling, sublimation, vaporization, desolvation and solid-solid transition.

Examples of exothermic transition: crystallization and degradation.

The heat of fusion ∆Hf can be obtained by calculating the area under the transition peak. It is a useful tool to calculate the heat of transition from polymorph to another.

A sharp endothermic melting peak is indicative of the relative purity of a material.

A broad asymmetric peak suggests the presence of impurities.

TGA (thermal gravimetric analysis): measures the change in sample weight as a function of time or temperature.

Preformulation 1.Crystallinity and polymorphism: 

•Thermal analysis: • Quantitative measurements of these processes have many applications in

preformulation studies including purity, polymorphism, solvation, degradation, and excipients compatibility.

• For characterizing crystal forms, the heat of fusion, ΔHs, can be obtained from the area-under-the-DSC-curve for the melting endotherm.

• 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.

• In following figure, the dihydrate form of an acetate salt loses two moles of water via all endothermic transition between 70° and 90°C. The second endothermic peak at 155°C corresponds to the melting process, with the accompanying weight loss due to vaporization of acetic acid as well as to decomposition.

• TGA and DSC analysis can also be used to quantitate the presence of a solvated species within a bulk drug sample. For the above example, 10% of the dihydrate form was easily detected by both methods.

Preformulation •Significances of identification of crystal shape and internal structure: it can influence the following parameters:

1.Solubility and stability. For example: Chloramphenicol palmitate exists in 3 crystalline polymorphic

forms (A, B and C) and an amorphous form (D). Increasing the concentration of the form B led to increase the serum level due to its higher water solubility.

2. Melting point. For example : Cacao butter as an oily base for suppositories exist in 4

polymorphic forms (α, β-prime, γ and β-stable). Only the β-stable form can be used as a suppository base due to its higher melting point.

3. Density (influence the flow properties of powders).

4. Crystal shape (influence the flow properties of powders).

5. Tablet hardness (influence the compression properties and grinding processes)

Preformulation 2. Hygroscopicity: 

Many drug substances exhibit a tendency to adsorb moisture. The amount of moisture adsorbed by a fixed weight of anhydrous sample in equilibrium with the moisture in the air at a given temperature is referred to as equilibrium moisture content.

The equilibrium moisture content may influence the flow and compression characteristics of powders and the hardness of final tablets and granulation.

The knowledge of the rate and extent of moisture pickup of new drug substances permits the formulator to take appropriate corrective steps where problems are anticipated.

In general, hygroscopic compounds should be stored in a well-closed container, preferably with a desiccant.

If a granulation step is needed in tablet operation these compounds, non-aqueous granulating solvents should be recommended.

Preformulation 2. Hygroscopicity: 

Deliquescent: a substance which absorb sufficient moisture from the atmosphere to dissolve itself (higher extreme). Efflorescent: a substance which loses water to form a lower hydrates or become anhydrous (lower level). Hygroscopic: a substance that exist in a dynamic equilibrium with water. This process depends on the relative humidity of the surroundings. Very hygroscopic materials (moisture sensitive) should stored at RH of 40%.

Methods of characterizations: A thin powder bed is placed in an open container to assure maximum atmospheric humidity exposure. They are then exposed to a range of controlled RH environment. Moisture uptake is monitored at a handling time points (0-24h) and at a storage time points (0-12 weeks). Moisture contents is analyzed by Karl fisher, gravimetric, TGA, or GC.

Significances: Changes in moisture content can affect stability, flowability, compatibility, … etc. Weight gain against time plots and weight gain against humidity plot are good indicatives for the proper storage condition for different materials.

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Preformulation 3. Fine particle characterization: • Certain physical and chemical properties of drug substances are affected by the

particle size distribution, including drug dissolution rate, bioavailability, content uniformity, taste, texture color, and stability.

• In addition, properties such as flow characteristics and sedimentation rates, among others, are also important factors related to particle size.

• It is essential to establish as early as possible how the particle size of the drug substance may affect formulation and product efficacy.

• Of special interest is the effect of particle size on the drug's absorption. Particle size has been shown to significantly influence the oral absorption profiles of certain drugs as griseofulvin, nitrofurantoin, spironolactone, and procaine penicillin.

• There are several methods available to evaluate particle size and distribution including sieving or screening, microscopy, sedimentation, and stream scanning.

• For powders in the range of approximately 44 microns and greater, sieving or screening is the most widely used method of size analysis.

• The difficulty with using sieve analysis early in the pre-formulation program is the requirement of a relatively large sample size. The main advantage of the sieve method is simplicity, both in technique and equipment requirements.

Preformulation 3. Fine particle characterization: • Certain physical and chemical properties of drug substances are affected by the

particle size distribution, including drug dissolution rate, bioavailability, content uniformity, taste, texture color, and stability.

• In addition, properties such as flow characteristics and sedimentation rates, among others, are also important factors related to particle size.

• It is essential to establish as early as possible how the particle size of the drug substance may affect formulation and product efficacy.

• Of special interest is the effect of particle size on the drug's absorption. Particle size has been shown to significantly influence the oral absorption profiles of certain drugs as griseofulvin, nitrofurantoin, spironolactone, and procaine penicillin.

• There are several methods available to evaluate particle size and distribution including sieving or screening, microscopy, sedimentation, and stream scanning.

• For powders in the range of approximately 44 microns and greater, sieving or screening is the most widely used method of size analysis.

• The difficulty with using sieve analysis early in the pre-formulation program is the requirement of a relatively large sample size. The main advantage of the sieve method is simplicity, both in technique and equipment requirements.

Preformulation 3. Fine particle characterization methods: 1. Light microscope with a calibrated grid: • Optical microscopy is frequently the first step in the determination of particle

size and shape for the new drug substance. This is usually a qualitative assessment since quantitation by the microscope technique is tedious and time consuming. A key element in utilizing the microscope for particle size determination is preparation of the slide.

• It must be representative of the bulk of the material and be properly suspended and thoroughly dispersed in a suitable liquid phase. In order to do a quantitative particle size evaluation a minimum of 1000 of the particles should be counted.

2. Sedimentation techniques utilize the relationship between rate of fall of particles and their size. Techniques utilizing devices that continuously collect a settling suspension are used. These methods share the disadvantage of the microscope technique in that it is tedious to obtain the data. Also, proper dispersion, consistent sampling, temperature control, and other experimental variables must be carefully controlled in order to obtain consistent and reliable results.

Preformulation 3. Fine particle characterization: 3. Stream scanning is a valuable method for determining particle size distribution of

powdered drug substances.

• This technique utilizes a fluid suspension of particles which pass the sensing zone where individual particles are sized, counted, and tabulated.

• Sensing units may be based on light scattering or transmission as well as conductance.

• Two popular units in the pharmaceutical industry for this purpose are the Coulter Counter and Hiac Counter. Both units electronically size, count, and tabulate the individual particles that pass through the sensing zone data be generated in a relatively short time with reasonable accuracy.

• Thousands of particles can be counted in seconds and used to determine the size distribution curve.

• All stream scanning units convert the particles to effective diameter, and therefore have the shortcoming of not providing information relative to particle shape.

• Nevertheless, stream scanning methods are powerful tools and can be used for evaluation of such parameters as crystal growth in suspension formulation.

Preformulation 3. Fine particle characterization: Coulter counter: it characterizes the size distribution of a compound.

Mechanism: A dispersed sample in a conducting material (saline with surfactant) is drown into a tube with small orifice (0.4 : 800 μm) by applying an electric current. For each particle passes through the hole, it is counted and sized according to the resistance difference generated.

Counting particles and biological cells is not much of a problem with the coulter counter. The sample concentrations are low enough to ensure that the particles travel through the orifice only one at a time.

Pulse generated when a particle passes through the orifice is directly proportional to particle volume.

In Coulter counter, there is an isotonic solution namely 0.9% Na Cl in which the drug particles are suspended.

Preformulation 3. Fine particle characterization: 

Surface area determination by BET nitrogen adsorption method: • When a drug particle is reduced to a larger number of smaller particles, the total

surface area created is increased. For drug substances that are poorly or slowly soluble, this generally results in an increase in the rate of dissolution. The actual solubility of a pure drug remains the same.

• Increased therapeutic response to orally administered drugs due to smaller particle size has been reported for a number of drugs, among them theophylline, a xanthine derivative used to treat bronchial asthma; griseofulvin, an antibiotic with antifungal activity; sulfisoxazole, an anti-infective sulfonamide and nitrofurantoin, a urinary anti-infective drug.

• To achieve increased surface area, pharmaceutical manufacturers frequently use micronized powders in their solid dosage form products. Micronized powders consist of drug particles reduced in size to about 5 microns and smaller. The use of micronized drugs is not confined to oral preparations.

• For example, ophthalmic ointments and topical ointments utilize micronized drugs for their preferred release characteristics and nonirritating quality after application.

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Preformulation 3. Fine particle characterization: 

Surface area determination by BET nitrogen adsorption method: • Mechanism: The determination of the surface area is based on the Brumauer,

Emmett, Teller (BET) theory of adsorption. The theory states that most substances will adsorb a monomolecular layer of a gas under certain condition of partial pressure (of the gas) and temperature.

• Knowing the monolayer capacity of an adsorbent (i.e., the quantity of adsorbate which can be accommodated as a monolayer on the surface of a solid (adsorbent) and the area of the adsorbate molecule, the surface area can be calculated.

• Nitrogen is used as the adsorbate at a specific partial pressure established by mixing it with an inert gas (helium) in concentration of 30%, and at liquid nitrogen temperature (- 195°C), a monolayer is adsorbed onto most solids.

• SSA is the specific surface area; (m) is the specific weight of adsorbate in a monolayer. N is the Avogadro number, (= 6.0233x1023 atoms / mole), AN2, the area of the adsorbate molecule (generally taken to be, for nitrogen, 16.2 X 10-20 m2 per molecule), and MN2 the molecular weight of the adsorbate.

Preformulation 4. Bulk density: 

Knowledge of the true and bulk densities of the drug substance is very useful in forming some idea as to the size of the final dosage form. Obviously, this parameter is very critical for drugs of low potency, which may constitute the bulk of the final granulation or table. The density of solids also affects their flow properties. In the case of a physical mixture of powders, significant difference in the absolute densities of the components could lead to segregation.

Bulk density, true density and bulkiness Bulk density of a compound varies substantially with the method of crystallization, milling or formulation once a density problem is identified it is often easily corrected by milling slugging or formulation. Usually bulk density is of great importance when one considers the size of a high dose capsule product or the homogeneity of low dose formulation in which there are large differences in drug and excipients densities .

Preformulation 4. Bulk density: 

Bulk density, true density and bulkiness Apparent bulk density (g/ml) is determined by pouring presieved (40-mesh) bulk drug into a graduated cylinder via a large funnel and measuring the volume and weight "as is“. Tapped density is determined by placing a graduated cylinder containing a known mass of drug or formulation on a mechanical tapper apparatus which is operated for a fixed number of tabs(~1000) until the powder bed volume has reached a minimum using the weight of drug in the cylinder and this minimum volume the tapped density may be computed knowing the anticipated dose and tapped formulation density. One may use the following figure to determine the appropriate size for a capsule formulation:

Preformulation 4. Bulk density: 

Bulk density, true density and bulkiness True density: in addition to bulk density, it is frequently desirable to know the true density of a powder for computation of void volume or porosity of packed powder beds. Experimentally, the true density is determined by suspending drug particles in solvents of various densities and in which the compound is insoluble. Wetting and pore penetration may be enhanced by the addition of a small quantity of surfactant to the solvent mixtures. After vigorous agitation, the samples are centrifuged briefly and then left to stand undisturbed until floatation or settling has reached equilibrium. The sample that remains suspended corresponds to the true density of the material. Density of the test solution corresponding to the drug's true density should be checked with a calibrated pycnometer preferably after the test to include any density changes due to dissolved material.

Significances: It can affect powder flow properties. It affects the size of high dose capsule product or the homogenity of a low dose formulation in which there are large differences in drug and excipients densities.

Preformulation 5. Powder flow properties: 

The flow properties of powders are critical for an efficient tablet operation. A good flow of the powders or granulation to be compressed is necessary to assure efficient mixing and acceptable weight uniformity for the compressed tablets. Powders considered "poorly flowable” may have to be pre-compressed or pre-granulated. During the pre-formulation evaluation of the drug substance, therefore, its flowability characteristic should be studied, especially when the anticipated dose of the drug is large.

Preformulation 5. Powder flow properties: 

Powders may be free flowing or cohesive (non free flowing). Flow properties are affected by changes in particle size, density, shape, electrostatic charges, and adsorbed moisture.

Methods of characterization: Carr’s index and Hausner ratio:

Carr’s index

(Consolidation index)=

Ratios less than 1.25 indicate good flow Ratios greater than 1.25 indicates poor flow. Ratios between 1.25 and 1.5, added glidant normally improves flow.

Carr’s index  Flowability 

5‐15  Excellent 

12‐16  Good 

18‐21  Fair to possible* 

23‐35  Poor* 

33‐38  Very poor 

>40  Extremely poor 

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Preformulation 5. Powder flow properties: 

Methods of characterization: Angle of repose:

It is the maximum angle that can be obtained between the free standing surface of the powder heap and the horizontal plane. Significances: It is important when attempting to develop a commercial solid dosage form containing a large percentage of cohesive materials. It can give direction (formulation recommendation such as granulation) for the development project team about the pharmaceutical consequence of each process improvement. Poor flowability is due a number of factors including shape and size of particles where spherical particles flow better than needles, however very fine particles do not flow as freely as large particles.

Angle of repose  Flowability 

<25  Excellent 

25‐30  Good 

30‐40  Possible 

>40  Very poor* 

Preformulation 6. Compression properties: 

The compression properties (elasticity, plasticity, fragmentability, and punch filming propensity) for a small quantities of a new drug candidate can be established by the sequence outlined below:

500 mg drug + 1% Mg stearate 

Sample code  A  B  C 

1. Blend in tumbler mixer for:  5 min  5 min  30 min 

2. Compress 13mm compacts in a hydrophilic press at compression force of: 

75 Mpa  75 Mpa  75 Mpa 

3. Dwell time of :  2 sec  30 sec  2 sec 

4. Store tablets in a sealed container at Tr to allow equilibration for: 

24 hours  24 hours  24 hours 

5. The crushing strength of tablets:  A  N  B  N  C  N 

Preformulation 6. Compression properties: 

1.Plastic materials: when the ductile material deform by changing shape. The bonding of these materials is time dependant, So, increasing the dwell

time increases the bonding strength (sample B). During compression, more intimate mix of the lubricant (Mg stearate) leads

to poor bonding (sample C). These tablets can be arranged according to the crushing strength as B>A>C

2. Fragmenting materials: it is the materials that lubricant mixing time (sample C) nor dwell time (sample B) do not affect the bonding strength. Usually these materials showed low crushing strength and high friability.

3. Elastic materials: they are materials that showed very little permanent change upon compression e.g. paracetamol. The material rebound (elastically recover) when the compression load released. These materials usually showed capping and lamination after compression

(samples A and C).

4. Punch filming (sticking): the adherence of the powder to the surfaces of the punches and the die. Generally, the lubricant is very effective antiadherent. Efficient mixing of the lubricant should prevent sticking (sample C).

Significances: Proper selection of the formulation ingredients

Preformulation 7. Physical Description: • It is important to have an understanding of the physical description of a drug

substance prior to dosage form development. • Liquid drugs are used to a much lesser extent than solid drugs, gases even less

frequently. • Among the few liquid medicinal agents in use today are the following:

1. Amyl nitrite, vasodilator by inhalation. 2. Castor oil, Mineral oil cathartic and Glycerin, cathartic in suppository form. 3. Clofibrate, antihyperlipidemic. 4. Dimercaprol, antidote for arsenic, gold, and mercury poisoning. 5. Dimethylsulfoxide, analgesic in interstitial cystitis. 6. Ethchlorvynol, hypnotic. 7. Nitroglycerin (as tablets), anti-angina 8. Paraldehyde, sedative-hypnotic 9. Paramethadione, anticonvulsant 10.Prochlorperazine, tranquilizer and antiemetic. 11.Propylhexedrine, vasoconstrictor by nasal inhalation, 12.Undecylenic acid, fungistatic agent.

Preformulation 7. Physical Description: Problems of liquid drugs 1. Liquid drugs pose an interesting problem in the design of dosage forms or drug

delivery systems. Many of the liquids are volatile substances and as such must be physically sealed from the atmosphere to prevent their loss.

• Amyl nitrite for example, is a clear yellowish liquid that is volatile even at low temperatures and is also highly flammable. It is maintained for medicinal purposes in small sealed glass cylinders wrapped with gauze or another suitable material. When amyl nitrite is administered, the glass is broken between the fingertips and the liquid wets the gauze covering, producing vapors that are inhaled by the patient requiring vasodilatation.

• Propylhexedrine provides another example of a volatile liquid drug that must be contained in a closed system to maintain its presence. This drug is used as a nasal inhalant for its vasoconstrictor action. A cylindrical roll, of fibrous material is impregnated with propylhexedrine, and the saturated cylinder is placed in a suitable, generally plastic, sealed nasal inhaler. The inhaler's cap must be securely tightened each time it is used. Even then, the inhaler maintains its effectiveness for only a limited period of time due to the volatilization of the drug.

Preformulation 7. Physical Description: Problems of liquid drugs 2. Another problem associated with liquid drugs is that those intended for oral

administration cannot generally be formulated into tablet form, the most popular form of oral medication, without undertaking chemical modification of the drug.

• An exception to this is the liquid drug nitroglycerin, which is formulated into sublingual tablets that disintegrate within seconds after placement under the tongue. However, because the drug is volatile, it has a tendency to escape from the tablets during storage and it is critical that the tablets be stored in tightly sealed glass containers.

• For the most part, when a liquid drug is to be administered orally and a solid dosage form is desired, two approaches are used.

1. First, the liquid substance may be sealed in a soft gelatin capsule. Paramethadione. (Paradione) and ethchlorvynol (Placidyl) are examples of liquid drugs commercially available in capsule form.

2. Secondly, the liquid drug may be developed into a solid ester or salt form that will be suitable for tableting or drug encapsulating. For instance, scopolamine hydrobromide is a solid salt of the liquid drug scopolamine and is easily produced into tablets.

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Preformulation 7. Physical Description: Advantages of liquid drugs 1. For certain liquid drugs, especially those employed orally in large doses or applied

topically, their liquid nature may be of some advantage in therapy. For example, 15-mL doses of mineral oil may be administered conveniently as such.

2. Also, the liquid nature of undecylenic acid certainly does not hinder but rather enhances its use topically in the treatment of fungus infections of the skin.

Preformulation 7. Physical Description: Evaluation tests of solid drug substances 1- Purity

• Materials with unnecessary impurities should be rejected. • Testing purity is another control parameter for comparison with subsequent

batches. • Impurity can affect:

o Stability: metal contamination in ppm o Appearance: off-colour recrystallized white o Toxic: aromatic amine carcinogenic

• Techniques used for characterizing purity are: o Thin Layer Chromatography (TLC) o High-Pressure Liquid Chromatography (HPLC) o Gas Chromatography (GC) o Melting Point Depression

o Impurity index (II) defined as the ratio of all responses (peak areas) due to components other than the main one to the total area response.

o Homogeneity index (HI) defined as the ratio of the response (peak area) due to the main component to the total response.

Preformulation 7. Physical Description: Evaluation tests of solid drug substances 1- Purity

• Example: o Main component retention time: 4.39 min with area response: 4620 o 7 Impurities existed with minor peaks have total area response : 251

o Impurity index = 251/(4620 + 251) = 0.0515 o Homogeneity index = 1 - 0.0515 = 0.9485

o USP Impurity Index is defined as a ratio of responses due to impurities to that response due to a defined concentration of a standard of the main component. General limit 2 % impurities

o All II, HI, USP II are not absolute measures of impurity since the specific response (molecular absorbance or extinction coefficient) due to each impurity is assumed to be the same as that of the main component.

o More accurate analysis - identification of each individual impurity followed by preparation of standards for each one of them.

Preformulation 7. Physical Description: Evaluation tests of solid drug substances 1- Purity

• One parameter in determining chemical purity is melting point depression.

• The melting point, or freezing point, of a pure crystalline solid is defined as that temperature where the pure liquid and solid exist in equilibrium.

• This characteristic can be used as an indicator of purity of chemical substances (a pure substance would ordinarily be characterized by a very sharp melting peak).

• The latent heat of fusion is the quantity of heat absorbed when 1 g of a solid melts; the molar heat of fusion (ΔHf) is the quantity of heat absorbed when 1 mole of a solid melts. High-melting-point substances have high heats of fusion and low- melting-point substances have low heats of fusion. These characteristics are related to the types of bonding in the specific substance.

Preformulation 7. Physical Description: Evaluation tests of solid drug substances 1- Purity

• For example, ionic materials have high heats of fusion (NaCl melts at 801°C with a heat of fusion of 124 cal /G) and those with weaker van der Waals forces have low heats of fusion (paraffin melts at 52°C with a heat of fusion of 35.J cal/G). Ice, with weaker hydrogen bonding, has a melting point of 0°C and a heat of fusion of 80 cal /G.

• Additions of second ingredient to a pure compound (A), resulting in a mixture with a melting point that is lower than that of the pure compound. The degree to which the melting point is lowered is proportional to the mole fraction (NA) of the second component that is added. This can be expressed as:

• Where: ΔHf is the molar heat of fusion; T is the absolute equilibrium temperature, To is the melting point of pure A, and R is the gas constant.

Preformulation 7. Physical Description: Evaluation tests of solid drug substances 1- Purity

• Two things are noteworthy in contributing to the extent of melting-point lowering.

1. Evident from this relationship is the inverse proportion between the lowering of melting point (∆T) and the heat of fusion. When a second ingredient is added to a compound with a low molar heat of fusion a large lowering of the melting point is observed; substances with a high molar heat of fusion will show little change in melting point with the addition of a second component.

2. The extent of lowering of the melting point is also related to the melting point itself. Compounds with low melting points are affected to a greater extent than compounds with high melting points upon the addition of a second component (i.e., low-melting-point, compounds will result in a greater lowering of the melting point than those with high melting points.

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Preformulation Solubility analysis 

It focus on drug‐solvent  interactions that could occur during the delivery of a drug candidate.  For example, orally administered drug should be examined for solubility in simulated gastric media.  

Quest. Why do we need to perform solubility analysis of a new drug?

To provide a basis for later formulation work. It can affect drug performance. Drugs with an aqueous solubility less than 

1% (10 mg/ml) will suffer from bioabsorption problems.  

Solubility analysis include: • Solubility determination. • pKa determination. • Partition coefficient. • Dissolution behavior. • Common ion effect. • Membrane permeability.

Preformulation 1.Solubility One important goal of the pre‐formulation effort is to devise a method for making solutions of the drug. A drug must possess some aqueous solubility for therapeutic efficacy. In order for a drug to enter the systemic circulation to exert a therapeutic effect, it must first be in solution. Relatively insoluble compounds often exhibit incomplete absorption. 

When a solute dissolves, the substance's inter molecular forces of attraction must be overcome by forces of attraction between solute and solvent molecules. This involves breaking the solute‐solute forces and the solvent‐solvent forces to achieve the solute‐solvent attraction. 

 

Factors affecting the solubility of a drug 

1‐ Effect of temperature 

Temperature is an important factor in determining the solubility of a drug and in preparing its solution. Most commonly, the solution process is endothermic, and thus increasing the solution temperature increases the drug solubility. For such solutes as lithium chloride and other hydrochloride salts that are ionized when dissolved, the process is exothermic such that higher temperature suppresses the solubility. 

Preformulation 1.Solubility 

Factors affecting the solubility of a drug 

1‐ Effect of temperature 

2‐ Chemical and physical properties of both the solute and the solvent. 

3‐ Pressure. 

4‐ The acidity or basicity of the solution. 

5‐ The state of subdivision of the solute and 

6‐ The physical agitation applied to the solution during the dissolving process. 

 

The solubility of a pure chemical substance at a given temperature and pressure is constant; however, its rate of solution, that is, the speed at which it dissolves, depends upon the particle size of the substance and the extent of agitation.  

The more fine the powder, the greater the surface area that comes in contact with the solvent, and the more rapid the dissolving process.  

Also; the greater the agitation, the more unsaturated solvent passes over the drug and the faster the formation of the solution. 

 

Preformulation 1.Solubility 

Methods to improve drug solubility: 

The methods used will depend on the chemical nature of the drug and the type of drug product under consideration.  

1‐ The chemical modification of the drug into salt or ester forms is a technique frequently used to obtain more soluble compounds.  

2‐ Through selection of a different solubilizing agent. 

3‐ A pharmacist can, in certain instances, dissolve greater quantities of a solute than would otherwise be possible. For example, iodine granules are soluble in water only to the extent of 1 g in about 3000 ml of water. Using only these two agents the maximum concentration possible would be approximately 0.03% of iodine in aqueous solution. However, through the use of an aqueous solution of potassium or sodium iodide as the solvent, much larger amounts of iodine may be dissolved as the result of the formation of a water‐soluble complex with the iodide salt. This reaction is taken advantage of, for example, in Iodine Topical Solution, USP, prepared to contain about 2% of iodine and 2.4% of sodium iodide. 

 

 

Preformulation 1.Solubility 

Methods to improve drug solubility: 

The methods used will depend on the chemical nature of the drug and the type of drug product under consideration.  

4‐ In many cases, it is desirable to utilize co‐solvents or other techniques such as micronization, or solid dispersion to improve aqueous solubility.  

5‐ Another technique, if the drug is to be formulated into a liquid product, involves the adjustment of the pH of the solvent in which the drug is to be dissolved to enhance solubility. However, there are many drug substances for which pH adjustment is not an effective means of improving solubility. Weak acidic or basic drugs may require extremes in pH that are outside accepted physiologic limits or may cause stability problems with formulation ingredients. Adjustment of pH usually has little effect on the solubility of non‐electrolytes. 

 

 

 

 

 

Preformulation 1.Solubility 

Methods to improve drug solubility: 

The methods used will depend on the chemical nature of the drug and the type of drug product under consideration.  

• A great many of the important organic medicinal agents are either weak acids or weak bases, and their solubility is dependent to a large measure upon the pH of the solvent.  

• These drugs react either with strong acids or strong bases to form water‐soluble salts.  

• For instance, the weak bases, including many of the alkaloids (atropine, codeine, and morphine), antihistamines (di‐phenyhydramineand tripelennamine), (cocaine, procaine, and tetracaine), and other important drugs are not very water‐soluble, but they are soluble in dilute solutions of acids.  

• Pharmaceutical manufacturers have prepared many acid salts of these organic bases to enable the preparation of aqueous solutions. 

 

 

 

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Preformulation 1.Solubility 

Methods to improve drug solubility: 

The methods used will depend on the chemical nature of the drug and the type of drug product under consideration.  

• It must be recognized, however, that if the pH of the aqueous solutions of these salts is changed by the addition of alkali, the free base may separate from solution unless it has adequate solubility in water.  

• Organic medicinals that are weak acids include the, barbiturate drugs (as phenobarbital and pentobarbital) and the sulfonamides (as sulfadiazine and sulfacetamide). These and other weak acids form water‐soluble salts in basic solution and may separate from solution by a lowering of the pH.  

• Although there are no exact rules for predicting the solubility of a chemical agent in a particular liquid, experienced pharmaceutical chemists can estimate the general solubility of a chemical compound based on its molecular structure and functional groups. Perhaps the most written guideline for the prediction of solubility is that "like dissolves like,” meaning that a solvent having a chemical structure most similar to that of the intended solute will be most likely to dissolve it. Thus, organic compounds are more soluble in organic solvents than in water. 

Preformulation 1.Solubility 

Methods to improve drug solubility: 

The methods used will depend on the chemical nature of the drug and the type of drug product under consideration.  

• Organic compounds may, however, be somewhat water‐soluble if they contain polar groups capable of forming hydrogen bonds with water. In fact, the greater the number of polar groups present, the greater will likely be the organic compound's solubility in water. Polar groups include OH, CHO, COH, CHOH, CH2OH, COOH, NO2, CO, NH2 and SO3H.  

• The introduction of halogen atoms into a molecule tends to decrease water‐solubility because of an increase in the molecular weight of the compound without a proportionate increase in polarity. An increase in the molecular weight of an organic compound without a change in polarity generally results in decreased solubility in water. 

 

Preformulation 1.Solubility 

Solubility determination  First, all factors that affect the solubility and dissolution should be defined.  Steps:  

1. An excess amount of the drug is dispersed in the medium and agitated at constant temperature. 

2. Samples of the slurry are withdrawn as a function of time. 3. Samples are clarified by filtration or centrifugation. 4. The clear samples then assayed for its drug content to establish a plateau 

concentration. Analysis can be done using UV, HPLC, GC, …. etc.  

Intrinsic solubility determination 1. Solubility analysis for a new drug candidate is  performed in different media such as 

0.9% NaCl, 0.01M HCl, 0.1M HCl, 0.1M NaOH and buffer pH 7.4. 2. An increased solubility in acidic media indicating basic drug. 3. An increased solubility in alkaline media indicating acidic drug. 4. An increased solubility in both media indicating an amphoteric (Zwitter ion) drug. 5. No change in solubility in both media suggesting non‐ionizable, neutral molecule. 6. Intrinsic solubility for acids is measured in alkaline medium and vice versa. 7. Intrinsic solubility is performed at 5°C (to ensure good stability) and 37°C (to support 

biopharmaceutical evaluation). 8. Phase solubility diagrams is made to test effect of impurities on drug solubility. 

Preformulation 2. pKa determination: The interrelationship of the dissociation constant, lipid solubility, and pH at the absorption site and absorption characteristics of various drugs are the basis of the pH‐partition theory.  

Dissociation constant or pKa is usually determined by potentiometric titration.  

The majority of drugs today are weak organic acids or bases. Knowledge of their individual ionization or dissociation characteristics is important, because their absorption is governed to a large extent by their degrees of ionization as they are presented to the membrane barriers. 

Cell membranes are more permeable to the unionized forms of drugs than to their ionized forms, mainly because of the greater lipid solubility of the unionized forms and to the highly charged nature of the cell membrane which results in the binding or repelling of the ionized drug and thereby decreases cell penetration. 

Also, ions become hydrated through association with water molecules, resulting in larger particles than the undissociated molecule and again decreased penetrating capability.  

 

 

Preformulation 2. pKa determination: Also the extent of drug ionization has an important effect on the formulation and pharmacokinetic parameters of the drug.  The extent of dissociation/ionization is, in many cases, highly dependent on the pH of the medium containing the drug. In formulation, often the vehicle is adjusted to a certain pH in order to obtain a certain level of ionization of the drug for solubility, and stability purposes. In the pharmacokinetic area, the extent of ionization of a drug is an important factor of its extent of absorption, distribution, and elimination. For the practicing pharmacist, it is important in predicting precipitation in admixtures and in the calculating of the solubility of drugs at certain pH values. 

The degree of a drug's ionization depends both on the pH of the solution in which it is presented to the biologic membrane and on the pKa, or dissociation constant, of the drug (whether an acid or base).  

The concept of pKa is derived from the Henderson‐Hasselbalch equation: 

For acidic compounds   pH=pKa + log (ionized drug/ unionized drug) For basic compounds     pH= Pkw ‐ pKb + log (unionized drug/ionized drug) 

 

 

 

 

Preformulation 2. pKa determination:  

Significances: 1.Provided that the intrinsic solubility and pKa are known, the solubility at any pH can be predicted. 

2.Henderson equations can facilitate the selection of suitable salt forming compounds and predict salts’ solubility. 

3.Determination of the ratio of the ionized to the unionized form of a drug molecules. This useful to predict which form will predominate at different physiologic pHs. Mostly, the unionized form of the drug is the one absorbed. Consequently, acidic drugs will be absorbed in the acidic media of the stomach and vice versa. 

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Preformulation 3. Partition coefficient: The oil/water partition coefficient is a measure of a molecule's lipophilic character; that is, its preference for the hydrophilic or lipophilic phase. The partition coefficient should be considered in developing a drug substance into a dosage form.   If a solute is added to a mixture of two immiscible liquids, it will distribute between the two phases and reach equilibrium at a constant temperature. The distribution of the solute (un‐aggregated & un‐dissociated) between the two immiscible layers can be described as follows:   it is the ratio of the unionized drug distributed between the organic (upper phase) and aqueous (lower) phases at equilibrium.    For an ionizable drug, the following equation is applicable:     

Where α equals the degree of ionization.    The determination of the degree of ionization or, pKa value of the drug substance is an important physical‐chemical characteristic relative to evaluation of possible effects on absorption from various sites of administration. 

Preformulation 3. Partition coefficient:  

The most commonly used solvent for partitioning is n‐octanol because its polarity lies in the midway in the range for the majority of drugs.   The type of solvent used depends on the solute characteristics and the ability to measure drug concentration in both solvents. 

According to the discriminating power of the partitioning solvent, they are classified as: 1. Hyperdiscriminating: they are solvents less polar than octanol. It reflects more 

closely the transport across blood‐brain barrier.  2. Hypodiscriminating: they are solvents more polar than octanol. E.g., butanol. It 

gives values consistent with buccal absorption.   

Determination of partition coefficient: 1. Shake flask method: the drug dissolved in one solvent is shaken with the other 

partitioning solvent for 30 min. The mixture allowed to stand for 5 min. The aqueous solution is centrifuged and then assayed for drug content  (Cwater).  

Preformulation 3. Partition coefficient:

  Significances: 

It has a number of applications related to preformulation such as: 

1. Solubility: both in aqueous and mixed solvents. 

2. Drug absorption in‐vivo: It is applied to a homologous drug series for structure activity relationships. 

3. Partition chromatography: it can be helpful for column and stationary phase selection (HPLC), choice of plates for TLC and choice of mobile phases (eluents) 

 

This information can be effectively used in the:                                           

1. Extraction of crude drugs,                                                                         

2. Recovery of antibiotics from fermentation broths,                                    

3. Recovery of biotechnology‐derived drugs from bacterial cultures,          

4. Extraction of drugs from biologic fluids for therapeutic drug monitoring, 

5. Absorption of drugs from dosage forms (ointments, supp, TDDS), and 

6. Study of the distribution of flavoring oil between oil and water phases of emulsions.  

Preformulation 3. Partition coefficient:

Extraction principle: 

The basic relationship given before can be used to calculate the quantity of drug extracted from, or remaining behind in, a given layer and to calculate the number of extractions required to remove a drug from mixture. 

The concentration of drug found in the upper layer (U) of two immiscible layers is given by:          U = Kr/(Kr+1)  

Where K is the distribution or partition constant, and r is Vu / V1, or the ratio of the volume of upper and lower phases.  

The concentration of drug remaining in the lower layer (L) is given by:  

L = 1 / (Kr+1) 

 

If the lower phase is successively re‐extracted with n equal volumes of the upper layer, each upper (Un) contains the following fraction of the drug:  

Un = Kr/ (Kr + 1)n  

Where Un is the fraction contained in the nth extraction, and n is the nth successive volume. 

The fraction of solute remaining in the lower layer (Ln) is given by: Ln = l / (kr + 1)n 

Preformulation 3. Partition coefficient:

 

Extraction principle: 

More efficient extractions are obtained using successive small volumes of the extraction solvent (as compared to single larger volumes).  

This can be calculated as follows when the same volume of extracting solvent is used, but in divided portions. For example, the fraction Ln remaining after the nth extraction is given by: 

 

 

 

 

 

 

Preformulation 3. Partition coefficient: EXAMPLE 1 

 At 25°C and pH 6.8, the K for a second generation cephalosporin is 0.7 between 

equal volumes of butanol and the fermentation broth. Calculate the U, L, and Ln (using the same volume divided into fourths).  

The fraction of drug extracted into the upper layer   U = 0.7/(0.7 + 1 ) = 0.41  

The fraction of drug remaining in the lower layer   L = 1/(0.7 + 1) = 0.59  

The total of the fractions in the U and L = 0.41 + 0.59 = 1  

If the fermentation broth is extracted with four successive extractions accomplished by dividing the quantity of butanol used fourths, the quantity of drug remaining after the fourth extraction is  

 

 

From this, the quantity remaining after a single volume, single extraction is 0.59, but when the single volume is divided into fourths and four successive extractions are done, the quantity remaining is 0.525; therefore, more was extracted using divided portions of the extracting solvent. Inherent in this procedure is the selection of appropriate solvents, drug stability, use of salting‐out additives, and environmental concerns. 

525.0)1

4(

1

44

KrthL

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Preformulation 4. Dissolution studies: The speed or rate at which drug substance dissolves in a medium is called 

dissolution rate.   

Dissolution rate data when considered along with data on a drug’s solubility, dissociation constant and partion coefficient can provide an indication of the drug’s absorption potential following administration. 

 

Dissolution principle: 

As a drug particle undergoes dissolution, the drug molecules on the surface are the first to enter into solution creating a saturated layer of drug‐solution which envelops the surface of the solid drug particle.  This layer of solution is referred to as the diffusion layer.  Form this diffusion layer, the drug molecules pass throughout the dissolving fluid and make contact with the biologic membranes and absorption ensues.   

As the molecules of drug continue to leave the diffusion layer, the layer is replenished with dissolved drug from the surface of the drug particle and the process of absorption continues. 

  

Preformulation 4. Dissolution studies: Dissolution principle: 

If the process of dissolution for a given drug particle is rapid, or if the drug is administered as a solution and remains present in the body as such, the rate at which the drug becomes absorbed would be primarily dependent upon its ability to traverse the membrane barrier.   

However, if the rate of dissolution for a drug particle is slow as may be due to the physicochemical characteristics of the drug substance or the dosage form, the dissolution process itself would be a rate‐limiting step in the absorption process. 

Slowly soluble drugs such as digoxin, may not only be absorbed at a slow rate. They may be incompletely absorbed, or in some cases largely unabsorbed following oral administration, due to the natural limitation of time that they may remain within the stomach or the intestinal tract.  Thus, poorly soluble drugs or poorly formulated drug products may result in a drug’s incomplete absorption and its passage, unchanged, out of the system via the feces. 

Under normal circumstances a drug may be expected to remain in the stomach for 2 to 4 hours (gastric emptying time) and in the small intestines for 4 to 10 hours, although there is substantial variation between people and even in the same person on different occasions.  

Preformulation 4. Dissolution studies: Dissolution principle: 

The gastric emptying time for a drug is most rapid with a fasting stomach, becoming slower as the food content is increased.  Changes in emptying time and/or intestinal motility can affect drug transit time and thus the opportunity for drug dissolution and absorption. 

 

Factors affecting dissolution rate: 

The dissolution rate of the drug in which the surface area is constant during dissolution is described by Noyes Whitney equation 

Preformulation 4. Dissolution studies: Factors affecting dissolution rate: 

The rate of dissolution is governed by the rate of diffusion of solute molecules through the diffusion layer into the body of the solution.   

The equation reveals that the dissolution rate of a drug may be increased by: 

1. Increasing the surface area (reducing the particle size) of the drug, 

2‐ Increasing the solubility of the drug in the diffusion layer. 

3‐ Factors embodied in the dissolution rate constant, k, including: 

a‐ The diffusion coefficient of the dissolving drug, for a given drug, the diffusion coefficient and usually the concentration of the drug in the diffusion layer will increase with increasing temperature. 

b‐ Also, increasing the rate of agitation of the dissolving medium will increase the rate of dissolution.  

c‐ A reduction in the viscosity of the solvent employed is another means which may be used to enhance the dissolution rate of a drug.  

d‐ Changes in the pH or the nature of the solvent which influence the solubility of the drug may be used to advantage in increasing dissolution rate.  

  

 

Preformulation 4. Dissolution studies:

Measurement of dissolution rate: The dissolution rates of chemical compounds are generally determined by two 

methods: the constant surface method which provides the intrinsic dissolution rate of the agent, and particulate dissolution method in which a suspension of the agent is added to a fixed amount of solvent without exact control of surface area.  

Compress the powdered drug into solid compacts. Dissolution study should made using apparatus I (basket)  or appratus II (paddle) USP dissolution apparatus. The amount of the drug released should be monitored with time. The dissolution rate is obtained by dividing the slope of the plot by the exposed surface area.   

This method eliminates surface area and surface electrical charges as dissolution variables.  

The dissolution rate obtained by this method is termed the intrinsic dissolution rate and is characteristic of each solid compound and a given solvent under the fixed experimental conditions.  

The value is generally expressed as milligrams dissolved per centimeter squared per minute (mg/cm2/min). 

 

Preformulation 4. Dissolution studies:

Significances: Taking into consideration the intrinsic solubility data, dissolution studies can 

identify potential bioavailability problems. For example: dissolution of solvates and polymorphs can have an impact on the bioavailability and drug delivery. 

 

It is useful in predicting probable absorption problems due to dissolution rate. In particulate dissolution, a weighed, amount of powdered sample is added to the dissolution medium in a constant agitation system.  

This method is frequently used to study the influence of particle size, surface area, and excipients upon the active agent.  

Occasionally, an inverse relationship of particle size to dissolution is noted due to the surface properties of the drug.  

 

Early formulation studies should include the effects of pharmaceutical ingredients on the dissolution characteristics of the drug substance. 

 

 

 

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Preformulation 5. Common ion effect: The addition of a common ion reduces the solubility of the slightly soluble 

electrolyte. This salting out (drug pptation) results from the removal of solvent  molecules from 

the surface of the electrolyte by the hydration of the common ion. Salting in: larger anions (hydrotropes) e.g. benzoates, salicylates can open the 

water molecules  allowing an increase in solubility of poorly water soluble drugs. Example: hydrochloride salts often exhibit lower solubility in gastric juice due to the

abundance of the chloride ions.  

Method of characterization: To explore a common ion interaction, the dissolution rate of a hydrochloride salt 

should be compared in different media: 1.Water and 1.2% w/v NaCl. 2.0.05 M HCl and 0.9% w/v NaCl in 0.05 M HCl. If a decrease in the dissolution rate was observed, other salts forms such as sulphates, tosylates, mesylates, …etc should be examined. 

 Significances: 

The choice of a suitable salt form for the proper dissolution and accordingly enhanced absorption. 

Preformulation 6. Membrane permeability: Modern pre‐formulation studies include an early assessment of passage of drug molecules across biological membranes.  

Data obtained from the basic physical‐chemical studies, specifically, pKa, solubility, and dissolution rate provide an indication of absorption expectations.  

To enhance these data, a technique utilizing the "everted intestinal sac" may be used in evaluating absorption characteristics of drug substances. In this method, a piece of intestine is removed from an intact animal, everted filled with a solution of the drug substance, and the degree and rate of passage of the drug through the membrane sac is determined. Through this method, both passive and active transport can be evaluated. 

In the latter stages of pre‐formulation testing or early formulation studies, animals and man must be studied to assess the absorption efficiency, pharmacokinetic parameters and to establish possible in vitro/in vivo correlation for dissolution and bioavailability. 

  

Preformulation Stability analysis 

Commercial pharmaceutical products should have a shelf life of about 5 years during which the drug content should not go below 90% and the product should look and perform as it did when first manufactured.  

Drug degradation mechanisms: 

1.Hydrolysis (Major): it occur by water for compounds having the following bonds: lactam, lactone, ester, amide and imide. Conditions that catalyze this process are: OH¯, H+, polyvalent ions, heat, light, polarity, … etc. 

2.Oxidation (Major): it occur by oxygen for compounds containing unsaturated bonds. Conditions that catalyze this process are: O2, oxidizing agents, polyvalent ions, heat, light, polarity, … etc. 

3.Photolysis (minor). 4.Trace metal catalysis (minor).  

Methods of characterization: • Decay experiments are done at extreme pHs and temperatures. • The degraded samples are used to confirm assay specificity as well as to provide estimates for maximum rates of degradations.   

Solution stability

Preformulation

Factors affecting degradation rates: 1.Temperature: Drug samples are stored at different temperatures and in each case the degradation constant is calculated. 

 Arrhenius plot: a plot of Log K values at different temperatures against 1/T . From the plot, K at room temperature can be obtained by extrapolation and the shelf life accordingly. Moreover, the activation energy (Ea) can be obtained from the slope. 

 2. Effect of pH: Drug samples prepared under the same condition at different pHs are stored at the same temperature. Degradation rates are then calculated and plotted against the pH values. From this profile, the pHs of the minimum and maximum degradation can be determined. 

 3. Other factors: Such as ionic strength, co‐solvent, presence and absence of O2, presence of antioxidants, presence of chelating agent.   

Solution stability (cont.)

Preformulation

Significances: 

The  primary goal of this approach is to identify the storage conditions and additives to form a stable solution preparation. For example: 

1. Antioxidants (Sod. sulphite, Sod. Thiosulphate, ascorbic acid, BHA and BHT) 

2. Chelating agents (EDTA) 

3. Replacement of O2 by CO2 or N2. 

4. Use of co‐solvents (propylene glycol, ethanol) to replace part of the aqueous vehicle for enhancing solubility and stability. 

5. Storage at law temperatures.     

Solution stability (cont.)

Preformulation

In contrast to solution stability, solid state may be severely affected by changes in purity and crystallinity which often results from process improvements. 

Due to the reduced molecular contacts between the drug and excipients, solid state reactions are slower than solution stability.  

Characterization methods: Solid samples are stored at different conditions such as Temperatures, humidities, 

presence or absence of O2, light,…… etc. the following tests are conducted:   1.Analysis of the appearance of decay products: Analytical data from TLC, fluorescence, UV, HPLC or GC may be required to precisely determine the kinetics of decay products and to establish a room temperature shelf life for drug candidate. 

 

2. Polymorphic changes: It can be detected by DSC.  

3. Excipients compatibility: To detect any interaction that may exist between the drug and the investigated excipients. This is helpful for the proper selection of the dosage form components. 

Solid state stability

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