Diffusion from Pill to Target Receptor

Diffusion

Spontaneous movement from higher to lower chemical potential; in solution, this usually means moving down a concentration gradient.

Why is diffusion important in pharmacy?

It helps drive drug liberation, absorption, distribution, controlled release, and elimination.

What is the usual journey from dosage form to target?

Drug is liberated from the dosage form, absorbed across barriers, distributed to its target, then eventually eliminated.

Concentration gradient

The difference in concentration across a distance; it drives passive diffusion.

Fick's First Law

At steady state, flux is proportional to the concentration gradient: J = −D(dC/dx).

What is flux (J)?

The amount of material passing through a unit area per unit time.

What does the minus sign in Fick's First Law mean?

Net flux is in the direction of decreasing concentration.

Steady-state diffusion

Concentration at each location stays constant over time, so the gradient does not change with time.

Fick's Second Law

Describes how concentration at a location changes over time: ∂C/∂t = D(∂²C/∂x²).

Fick's First vs Second Law

First law describes flux at steady state; second law describes changing concentration over time.

Diffusion coefficient (D)

A measure of how readily a substance diffuses through a particular medium under defined conditions.

What does a larger D mean?

Greater diffusivity: the substance moves more readily through that medium.

What affects D in the Stokes-Einstein relationship?

Higher temperature increases D; greater medium viscosity or particle radius decreases D.

How does temperature affect diffusion?

Higher temperature generally increases molecular motion and speeds diffusion.

How does molecular size affect diffusion?

Larger or heavier molecules generally diffuse more slowly.

How does the solvent affect diffusion?

Diffusion depends on the drug's solubility and the solvent's properties, including polarity, composition, and viscosity.

What is viscosity?

A measure of a fluid's resistance to flow; higher viscosity generally slows diffusion.

How does a barrier affect diffusion?

Barrier structure and permeability affect how readily a drug can cross it.

What is TEER?

Transepithelial electrical resistance: an electrical measure of resistance across epithelial or endothelial barriers.

What are the main factors that influence diffusion?

Concentration gradient, temperature, molecular size, drug and solvent properties, and barrier properties.

What driving forces can facilitate pharmaceutical processes?

Concentration, pressure, temperature, and electrical potential.

Examples of non-concentration driving forces

Pressure: osmotic release or jet injection; temperature: lyophilisation; electrical potential: iontophoresis or electrophoresis.

Why control drug release by diffusion?

Some treatments need rapid release; others need slow, sustained release to maintain drug delivery over time.

Where is diffusion-controlled release used?

Examples include tablets, creams, patches, depot injections, implants, and osmotic devices.

How can drugs cross a barrier to reach their site of action?

After dissolving, they diffuse across epithelial or endothelial barriers.