Solubility
Contents
• Drug classification
• Physio chemical properties influencing biological action
• Solubility
• Solubility prediction
Intended
learning outcomes
At the end of this
lecture, student will be able to:
• Explain the importance of solubility of drug molecules on
biological action.
Drug
Classification
Classified according
to their origin:
Pure organic compounds are the chief source of agents for
the cure, mitigation or the prevention of disease. These remedial agents could
be classified according to their origin:
• Natural compounds:
materials obtained from both plant and animal, e.g. vitamins, hormones, amino
acids, antibiotics, alkaloids, glycosides…. etc.).
• Synthesis
compounds: either pure synthesis or synthesis naturally occurring compounds
(e.g. morphine, atropine, steroids and cocaine) to reduce their cost.
• Semi-synthesis
compounds: Some compounds either cannot be purely synthesized or cannot be
isolated from natural sources in low cost. Therefore, the natural intermediate
of such drugs could be used for the synthesis of a desired product (e.g. semi
synthetic penicillins).
Classified according
to their Use:
Drugs can be classified according to their medicinal uses
into two main classes:
I- Pharmacodynamic
agents: Drugs that act on the various physiological functions of the body
(e.g. general anaesthetic, hypnotic and sedatives, analgesic etc.).
II- Chemotherapeutic
agents: Those drugs which are used to fight pathogenic (e.g. sulphonamides,
antibiotics, antimalarial agents, antiviral, anticancer etc.).
Drugs can treat
different types of diseases:
1- Infectious
diseases: Born (transmitted) from person to person by outside agents,
bacteria (pneumonia, salmonella), viruses (common cold, AIDS), fungi (thrush,
athletes foot), parasites (malaria)
2- Non-infectious
diseases: disorders of the human body caused by genetic malfunction,
environmental factors, stress, old age etc. (e.g. diabetes, heart disease,
cancer. Haemophilia, asthma, mental illness, stomach ulcers, arthritis).
3- Non-diseases:
alleviation of pain (analgesic), prevention of pregnancy (contraception),
anesthesia.
Physico-chemical
properties in relation to biological action
• Drug action
results from the interaction of drug molecules with either normal or abnormal
physiological processes. Drugs normally interact with targets (which they are
proteins, enzymes, cell lipids, or pieces of DNA or RNA).
• The ability of
a chemical compound to elicit a pharmacologic /therapeutic effect is related to
the influence of its various physical and chemical (physicochemical) properties
• The most
pharmacologically influential physicochemical properties of organic medicinal
agents (OMAs) are:
1. Solubility
2. Acidity and basicity
3. Reactivity
SOLUBILITY
• Drugs must be
in solution to interact with receptors.
• Drugs have some
degree of solubility in both aqueous and lipid compartments (PC).
• Solubility is a
function of:
1- Ionization
2- Molecular structure
3- Molecular weight
4- Stereochemistry
5- Electronic structure
Importance of solubility:
(1) Formulation of the drug in an appropriate dosage form
and
(2) Bio-disposition: Disposition of OMAs in the living
system after administration (absorption, distribution, metabolism, and
excretion).
• The solubility
expression: in terms of its affinity/philicity or repulsion/phobicity for
either an aqueous (hydro) or lipid (lipo) solvent.
hydrophilic………………..water loving
lipophobic…………………lipid hating
lipophilic…………………..lipid loving
hydrophobic………………water hating
• Drug
administered orally as a solid or in suspension have to dissolve in the aqueous
gastric fluid (dissolution)Before they can be absorbed and transported via
systemic circulation to their site of action
• The rate &
extent of dissolution of a drug is a major factor in controlling the absorption
of that drug.
• This is because
the concentration of the drug in fluid in gut lumen is one of the main factors
governing the transfer of the drug through the membranes
SOLUBILITY
OF ORGANIC MEDICINAL AGENTS
• Majority of
OMAs possess balanced solubility (have some degree of solubility in both
aqueous and lipid media).
• Because there
is a need for OMAs to move through both aqueous (plasma, extracellular fluid,
cytoplasm, etc.) and lipid media (biologic membranes) in the biological system.
• Solubility of
OMAs should be viewed as being on a continuum between high lipophilicity on one
end of the spectrum and high hydrophilicity on the other.
• In order for a
chemical compound to dissolve in a particular solvent/medium the compound must
establish attractive forces between itself and molecules of the solvent.
• In order for a
chemical compound to dissolve in a particular solvent/medium the compound must
establish attractive forces between itself and molecules of the solvent.
• It is possible
to estimate the solubility properties of an OMA (hydrophilic vs. lipophilic) by
examining the structure of the OMA and noting whether its structural features
promote affinity for aqueous or lipid media.
• The most
important intermolecular attractive forces (bonds) that are involved in the
solubilization process are:
1. Van der Waals
Attraction
• Weakest
intermolecular force (0.5-1.0 kcal/mole)
• Electrostatic
• Occurs between
nonpolar groups (e.g. hydrocarbons)
• Highly distance
and temperature dependent
2. Dipole-Dipole
Bonding
• Stronger (1.0
to 10 kcal/mole)
• Occurs
electrostatically between electron deficient and electron excessive /rich atoms
(dipoles)
• Hydrogen
bonding is a specific example of this bonding and serves as a prime contributor
to hydrophilicity
3. Ionic Bonding
• Electrostatic
attraction between cations and anions
• Common in
inorganic compounds and salts of organic molecules
• Relatively
strong (5
4. Ion-Dipole Bonding
• Electrostatic
between a cation/anion and a dipole
• Relatively
strong (1-5 kcal/mole)
• Low temperature
and distance dependence
• Important
attraction between OMAs and H2O
Solubility
Prediction
• The relative
solubility of an OMA is a function of the presence of both lipophilic and hydrophilic
features within its structure, which serve to determine the extent of
interaction of the OMA with lipid and/or aqueous phases.
• The relative
solubility of an OMA can be determined in the laboratory, i.e. the partition
coefficient [P; the ratio of the solubility of the compound in an organic
solvent to the solubility of the same compound in an aqueous environment (i.e.,
P=[Drug]lipid/[Drug]aqueous). P is
often expressed as a log value.
• A mathematical
procedures also have been developed to estimate the relative solubility of an
organic molecule based upon differential contributions of various structural
features to overall solubility.
• For example,
the relative solubility of an OMA is the sum of the contributions of each group
and substituent to overall solubility.
Example:
Examination of the structure of chloramphenicol (indicates the
presence of both lipophilic (nonpolar) and hydrophilic (polar) groups and
substituents.
The presence of oxygen and nitrogen containing functional groups usually
enhances water solubility. While lipid solubility is enhanced by non ionizable
hydrocarbon chains and ring systems.
1. Laboratory
Estimation of Relative Solubility
The relative solubility of an organic compound is measured
by determining the extent of its distribution into an aqueous solvent (usually
pH 7.4 buffer) and a lipid solvent (usually n-octanol). These experiments
generate a value, P, the partition coefficient for that particular compound.
2- Mathematical
Estimation of Relative Solubility
Solubility contributions (groups and substituents) are
expressed as hydrophilic (negative value) or lipophilic (positive value)
fragment constants.
Log Pcalc = ∑
π
Where; Log P calc = log of partition cofficient and ∑ π =
sum of hydrophilic-lipophilic constants.
Hydrophilic-Lipophilic constants
SUMMARY
• Drug can be
classified into Pharmacodynamic agents and chemotherapeutic agents
• The most
pharmacologically influential physicochemical properties of organic medicinal
agents (OMAs) are: solubility, acidity and basicity
• The relative
solubility of an organic compound is measured by determining the extent of its
distribution into an aqueous solvent (usually pH 7.4 buffer) and a lipid
solvent (usually n-octanol).