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Stability

The ability of a drug to retain its potency and safety over time.

Factors Affecting Stability

1. Hydrolysis 2. Oxidation 3. Photolysis 4. Thermal Degradation 5. Humidity

Hydrolysis

A reaction with water that can affect esters, amides, and weak acids or strong bases.

Oxidation

An electron-transfer reaction that can affect amines, sulfides, and phenols.

Photolysis

Degradation caused by exposure to UV or fluorescent light, affecting compounds with certain functional groups.

Thermal Degradation

Acceleration of chemical reactions due to heat.

Humidity

Can cause hydrolysis or microbial growth; the distinction between free and bound water is important.

Stability Testing

Assessment of drug behavior under environmental factors to establish shelf life, storage conditions, and predict potential degradation products.

Stereochemistry

The study of the spatial arrangement of atoms or functional groups in molecules.

Types of Stereoisomers

1. Enantiomers 2. Diastereomers

Enantiomers

Non-superimposable mirror images that differ in spatial orientation.

Diastereomers

Stereoisomers that are not mirror images and have different configurations at stereocenters.

Formulation Considerations for Racemates and Enantiomers

Choosing between racemates and enantiomers depends on pharmacodynamic activity, toxicity, pharmacokinetics, and potential for in vivo chiral inversion.

Omeprazole vs Esomeprazole

Omeprazole is a racemate, while esomeprazole is an enantiomer; they show differences in clinical effect.

Chiral Excipients

Excipients that can interact with enantiomers to produce stereospecific release and absorption.

Geometrical Configuration

Enantiomers are labeled R or S based on chiral center orientation.

Biological Activity Differences

R and S forms of a drug can exhibit different biological activities. Examples: propranolol and ethambutol.

Optical Rotation

The direction a substance rotates polarized light.

(+) Dextrorotatory

Clockwise rotation of polarized light.

(−) Levorotatory

Counterclockwise rotation of polarized light.

d/l System

A system used in specific contexts to describe stereochemistry, but it can be confusing.

IUPAC Recommendation

IUPAC recommends using (+) and (−) designations to indicate optical rotation to avoid confusion with the d/l system.

Solubility

The ability of a substance (solute) to dissolve in a solvent, such as water, to form a saturated solution at a specific temperature and pressure.

Intrinsic Solubility

The concentration of the unionized molecule in a saturated aqueous solution.

Apparent Solubility

Metastable solubility that may be higher than the thermodynamically stable value.

Key Factors Influencing Solubility

1. pH of the solvent 2. Buffer composition 3. Common ion effect 4. Ionic strength 5. Additives 6. Temperature

pH of the Solvent

Affects ionization of weak acids and bases; solubility is typically highest when the API is in its ionized form.

Buffer Composition

Different buffers at the same pH can affect solubility because of interactions with the API.

Common Ion Effect

The presence of a common ion can decrease ionization and solubility.

Ionic Strength

Higher ionic strength can reduce solubility by affecting dissociation.

Additives that Enhance Solubility

Cosolvents such as DMSO and propylene glycol, and surfactants such as polysorbates, can enhance solubility.

Temperature and Solubility

Increased temperature generally enhances solubility for solids but reduces solubility for gases.

Permeability

The movement of molecules across lipid bilayers in biological membranes, influenced by molecular size, lipophilicity, polar surface area, and molecular flexibility.

Membrane Regions

1. Outermost Region 2. Second Region 3. Third Region 4. Fourth Region

Outermost Region of the Membrane

Has high water content.

Second Region of the Membrane

Has a high density of polar headgroups and presents the greatest barrier to diffusion.

Third Region of the Membrane

Has a high density of nonpolar tails and affects molecular size and shape limitations.

Fourth Region of the Membrane

Is the most hydrophobic region and serves as a hydrophobic barrier.

Factors Affecting Permeability

1. Molecular size 2. Lipophilicity 3. Polar surface area 4. Molecular flexibility

Molecular Size and Permeability

Larger molecules typically have lower permeability.

Lipophilicity and Permeability

Higher lipophilicity generally increases permeability.

Polar Surface Area and Permeability

Larger polar surface areas typically reduce permeability.

Molecular Flexibility and Permeability

Molecular flexibility affects the ability to form hydrogen bonds and traverse membranes.

Impurities

Substances that can affect the stability, appearance, toxicity, and effectiveness of APIs.

Common Impurities

1. Water 2. Small electrophiles such as aldehydes 3. Metals

Effects of Impurities

Common impurities can lead to hydrolysis, oxidation, and other degradation pathways.

Importance of Solubility and Permeability

Understanding factors affecting solubility and permeability is crucial for effective drug formulation and development.