factors affecting bioavailibility and bioequivalence for mpharm and B.pharm students

1. Physicochemical Properties Affecting Bioequivalence
Studies
-AADITYA THOLE
M.PHARM PHARMACEUTICS
9420017639

2. What is Bioequivalence
• It is a relative term which denotes that the drug substance in two or more
identical dosage forms, reaches the systemic circulation at the same relative
rate and to the same relative extent i.e. their plasma concentration-time
profiles will be identical without significant statistical differences.
• When statistically significant differences are observed in the bioavailability
of two or more drug products, bio-inequivalence is indicated.

3. Factors affecting Bioequivalence
studies
Factors
affecting
Physical Chemical

4. Physical factors
• Crystal morphology
• Polymorphism
• Solvates
• Hydrates
• Complexation
• Surface activity
• Hgyroscopicity
• Particle size
• Solubility
• Dissolution

5. Crystal Morphology
• A crystalline species can be defined as the solid that is composed of the
atoms, ions or molecules arranged in a three dimensional pattern.
• Crystallization is invariably employed as the final step for the purification of
solid.
• Changes in crystal habit leads to significant variations in the raw material.
• Various dosage form parameters like flowability, particle orientation,
compaction, suspension stability and dissolution can be altered significantly
because of different crystal habits.

6. continued
• Changes in crystal leads to polymorphic transformation
morphology and
serious implications of physical
stability in dosage forms
Remedy- To minimize raw material characteristics
To ensure reproducibility and to judje poor performance of the
dosage form
Recognize the importance of changes in crystal surface appereance and
habit of pharmaceutical powders.

7. Polymorphism
• Both organic and inorganic pharmaceutical compounds can crystallize into
two or more solid forms that have the same chemical composition and is
called as polymorphism.
• Polymorphs have different relative intermolecular and/or interatomic
distances as well as unit cells, resulting in different physical and chemical
properties such as density, solubility, dissolution rate, bioavailability, etc.
• Polymorphs and pseudopolymorphs can be also classified as either
monotropes or enantiotropes, depending upon whether or not one form can
transform reversibly to another.

9. Effect of polymorphic forms

10. Continued
• It has been suggested that almost 40% of all organic compounds can exist in
various polymorphic forms; sometimes in as many as five different forms, as
in the case of cortisone acetate; almost 50% of all barbiturates and 70% of
steroids exhibit polymorphism.

11. Methods to determine
• Optical crystallography
• Hot stage microscopy
• x ray diffraction method
• Differential Thermal Analysis (DTA)
• Differential Scanning Calorimetric (DSC)
• Thermo Gravimetric Analysis (TGA)

12. Parameter to checked by preformulator
1.No of polymorphs
2. Relative degree of stability
3. Presence of glassy state
4. Stabilization of metastable form
5. Temperature stability range
6. Solubility of each polymorph
7. Method of preparation
8. Effect of micronization
9. Excipients incompatibility

13. Solvates
• In additions to polymorphs, solvates (inclusion of the solvent of
crystallization) are also often formed during the crystallization process.
• These forms are also called pseudopolymorphs.
• If the solvate contains an organic solvent, this would not be admitted
by regulatory authorities.

14. Types of solvents
• Class 1- Avoid these as per ICH eg-benzene,
carbon tetrachloride,1,2-dichloromethane
• Class 2- Should be limited and include non-genotoxic
animal carcinogens such as cyclohexane, acetonitrile
• Class 3- have low-toxicity potential including acetic acid,and
acetone and usually permissible

15. • The solvates themselves may exist in various polymorphic forms and are
referred to as pseudopolymorphs.
• Some examples of pseudopolymorphs include mercaptopurine,
fluprednisolone, and succinylsulfathiazole.
• The number of drug solvates is well over 100 and some of the most
common examples include steroids, antibiotics, sulfonamides,
barbiturates, xanthines, and cardiac glycosides..

16. Hydrates
• When solvate happens to be water, these are called hydrates
wherein water is entrapped through hydrogen bonding inside the
crystal and strengthens crystal structure and thereby invariably
reduces the dissolution rate.
• Solvate water
Hydrates Reduces Dissolution rate
Crystalline hydrates are classified into three classes as follows:

17. • Class I includes isolated lattice sites, represent the structures with
water molecules that are isolated and kept from contacting other
water molecules directly in the lattice structure.
• Therefore, water molecules exposed to the surface of crystals may be
easily lost.
• Class II includes hydrates that have water molecules in channels.
The water molecules in this class lie continuously next to the other
water molecules, forming channels through the crystal.
• Class III includes ion-associated hydrates. Hydrates contain metal-
ion coordinated water and the interaction between the metal ions and
water molecules is the major force in the structure of crystalline
hydrates.

18. Drug substance as hydrates

19. Complexation
• A molecular complex consists of constituents held together by
weak forces such as hydrogen bonds.
• Complexation will generally increase the total solubility of a
poorly water-soluble drug if the complex itself is soluble in
aqueous media.
• If the complexation process is reversible (Drug+Complexing
agent complex) then the absorption rates and the extent of
absorption will be increased for poorly soluble drugs.
• The most frequently observed complex formation is between
various drugs and macromolecules, such as gums, cellulose
derivatives, high-molecular weight polyols and nonionic
surfactants.

20. Examples of complexation type
reactions

21. Surface Activity
• The lowering of surface tension increases the dissolution rates by
increasing the solubility of drugs if the concentration of the surfactant
is above the critical micelle concentration.
• The lowering of surface tension also increases the diffusion of free
molecules in the medium, increasing the contact between free drug
and the absorption surface.
• A number of drugs have surface active properties themselves and form
their own micelles, thus facilitating the absorption.
• Examples of these drugs include potassium benzyl penicillin, mixtures
of penicillin and streptomycin salts, amphetamine sulfate,
cyclopentamine hydrochloride, ephedrine sulfate, propoxyphene
hydrochloride, ionic derivatives of phenothiazines, dyes, quarternary
ammonium salts of drugs, and liquorice.

22. Effect of surfactant on biovailibility

23. Hygroscopicity
• Water molecules have polar ends and readily form hydrogen
bonding. As a result, several compounds interact with water
molecules by surface adsorption, condensation in capillaries,
bulk retention, and chemical interaction and are called
hygroscopic.
• In tightly bound state, the water molecules are generally not
available to induce chemical reactions.
• Free water molecules can participate in the creation of a liquid
environment around crystal lattice where the pH may be altered
due to dissolution process.

24. Particle Size
• Increased absorption due to reduction of particle size is a result
of increased dissolution, which is in turn the result of a larger
specific surface area being exposed to the fluids in the
gastrointestinal tract or other sites of administration.
• Reduction in particle size can be achieved by several methods,
including milling, grinding, precipitating the drug on an
absorbent, and dispersing the drug in an inert watersoluble
carrier (referred to as a solid dispersion).
• The conventional methods of particle size reduction have long
been employed to improve the bioavailability of drugs,
• some of these examples include: vitamin A, medroxyprogesterone
acetate, 4-Acetamidophenyl 2,2,2-trichlorethyl carbonate,
nitrofurantoin, aspirin, phenobarbital, bishydroxycoumarin,
phenacetin

25. Not always desirable
• Nitrofurantoin, when administered in its fine particle size, causes
more gastrointestinal irritation than when administered in its coarser
size.
• This is due to the higher plasma and gastrointestinal concentrations
resulting from use of a fine particle size.
• The use of the coarser size is therefore preferred even though this
results in retarded absorption.
• Eg 2- When chemical instability is a problem the reduction of particle
size is also contraindicated, as with penicillin G and erythromycin,
which decompose in the gastrointestinal tract quickly.
• Even in the solid state a small particle size means a greater surface
area available for the absorption of moisture, which can result in an
increased rate of decomposition

26. SOLUBILITY
• The discussions of pKa, log P, log D above is relevant to
understanding the factors that affect the solubility of a drug at
the site of administration and thus determining the activity,
toxicity, stability, and dosage form and route of administration.
• High solubility is defined as the highest dose strength that is
soluble in 250 mL or less of aqueous media across the physiologic
pH range.
• Poorly soluble drugs can be defined as those with an aqueous our
solubility of less than 100 mg/mL.

27. Solubility criteria
V
F
S
S
S
V
P

28. Thumb rules
• Electrolytes dissolve in conducting solvents.
• Solutes having significant dipole moments dissolve in solvents
having significant dipole moments.
• Solutes with low or zero dipole moments dissolve in solvents
with low or zero dipole moments.
• Like dissolves like

29. Classes of solvents
• Protic solvents such as methanol and formamide which are
hydrogen bond donors,
• Dipolar aprotic solvents (e.g., acetonitrile nitrobenzene) with
dielectric constants greater than 15 but which cannot form
hydrogen bonds with the solute.
• Aprotic solvents in which the dielectric constant is weak and the
solvent is nonpolar, e.g.,pentane or benzene.

30. DISSOLUTION
• Dissolution is the conversion of solid state (highly aggregated
state) to a solution state (highly dispersed state).
• The dissolution models are derived from the known principles of
physics and chemistry such as Fick’s laws of diffusion,
concentration, or chemical potential gradients and the
hydrodynamic principles.

31. Types of models
• Diffusion Model
• Convection Model
• Surface Reaction Model
• Cube-Root Model
• Tablet Dissolution Model
• Noyes–Whitney Model

32. 2. Chemical factors
• Ionization
• Partitioning
• Distribution coefficient
• Chemical Structure and Form
• Salt Forms

33. Ionization
• Chemical moieties are known to attract to each other and under
appropriate conditions ,disassociate; when this process is driven
by the electrical charges on the components of the moiety, this
phenomenon, known as ionization.
• Acids give rise to excess of H+ in aqueous solution whereas a
base gives rise to excess of OH- in solution (Brønsted–Lowry
theory).
• A more general theory of acids and bases is the Lewis theory
wherein when an H+ ion combines with an OH- ion to form
water

34. Henderson–Hasselbach equation
• The Henderson–Hasselbach equation defines the relationship
between ionization and pH
• where [A–] is the concentration of the dissociated species and
[HA] is the concentration of the undissociated species.

36. pH partition hypothesis
• The theory states that for drugs of compounds having molecular
weight greater than 100 which are primarily transported across
the membrane by diffusion, the process of absorption is governed
by
• 1.The dissociation constant of the drug (Pka)
• 2.The lipid solubility on unionized drug
• 3.The pH of absorption site
Assumptions of hypothesis:
1. The GIT is a simple lipoidal barrier to the transportation of the drug
2. Larger the fraction of unionized drug faster is the absorption
3. Greater the lipophilicity of unionized drug better is absorption.

37. Optimum HLB balance

38. Log p and absorption

39. Partitioning
• The partition coefficient is a measure of the extent a substance
partitions between two phases, generally an oil phase and an
aqueous phase. This ratio is often expressed as log P (logarithm
of partition ratio).
• Both pKa and log P measurements are useful parameters in
understanding the dissolution and absorption behavior of drug
molecules.
• The pKa will determine the species of molecules, which is likely
to be present at the site of absorption and how quickly or
completely the species would cross a large number of transport
barriers in the body, regardless of the route of administration.

40. Partitioning
It is worth noting that this is a logarithmic scale, therefore, a log P=0 means that the
compound is equally soluble in water and in the partitioning solvent.
If the compound has a log P=5, then the compound is 100,000 times more soluble in the
partitioning solvent.
A log P=(-2) means that the compound is 100 times more soluble in water, i.e., it is quite
hydrophilic.

41. Partitioning
• Generally, compounds with log P values between 1 and 3 show
good absorption, whereas those with log P values greater than 6
or less than 3 often have poor transport characteristics.
• Highly nonpolar molecules have a preference to reside in the
lipophilic regions of membranes, and very polar compounds show
poor bioavailability because of their inability to penetrate
membrane barriers.

42. Distribution Coefficient
• If the drug has more than one ionization center, the distribution
of species present will depend on the pH.
• The concentration of the ionized drug in the aqueous phase will
therefore have an effect on the overall observed partition
coefficient.
• This leads to the definition of the distribution coefficient (log D)
of a compound, which takes into account the dissociation of weak
acids and bases.

43. Chemical Structure and Form
• Many important drugs are weak acids or bases.
• Salts of acidic or basic drugs have different solubility characteristics
and show different bioavailability.
• Sodium or potassium salts of weak acids dissolve much more rapidly
than the corresponding free acids, regardless of the pH of the
dissolution medium.
• The same is usually true of the hydrochloride salts or other strong acid
salts of weak bases, such as tetracycline hydrochloride, or atropine
sulfate.
• The salt form of the drug is generally more soluble in an aqueous
medium. However, the solubility of the salt depends on the strength
and quantity of the counterions; the smaller the counterion the more
soluble is the salt.
• For example, p-Amino salicylic acid (PAS) exists in various salt forms
and their solubility is given in following table

45. Not always desirable
• However, the use of salt forms is not always desirable such as demonstrated
for several drugs as listed in the table

46. Thank you

47. Reference
• 1.Handbook of bioequivalence testing
Sarfaraz .K Niazi
Pg no-163 -193
2. Biopharmaceutics and pharmacokinetics
a treatise,
D.M brahmankar and sunil Jaiswal
pg no-30-39