Lecture 2: Drug Absorption and Distribution

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Bioavailability

% of dose that gets into body

Bioequivalence

Similarity between two formulations of same drug

The rate of drug absorption will be affected by:

1. Rate of release of drug from pharmaceutical preparation
2. Membrane permeability of drug
3. Surface area in contact with drug
4. Blood flow to site of absorption
5. Destruction of drug at or near site of absorption

Excipients

Additives

Additives that decrease rate of dissolution

-Binders
-Lubricants
-Coating agents

Additives that increase rate of dissolution

-Disintegrants

Additives that have variable effects on rates of dissolution

-Diluents
-Coloring agents
-Flavoring agents

Enteric coating

Dissolve in intestines (alkaline), not stomach (acidic)

Reservoir diffusion products

Drug diffuses from pill core through membrane shell

Matrix diffusion products

Drug diffuses through matrix in which it is embedded

Matrix dissolution products

Drug released as matrix dissolves

Osmotic tablets

Drug pumped out of tablet by osmotic forces

Ion-exchange products

Drug bound to resin exchanges with endogenous ions

Lipophilicity

Increases membrane permeability
a) Presence of aliphatic and aromatic structures
b) Absence of polar groups

Ionization

Decreases membrane permeability
a) Weak ACIDS in INTESTINES are mostly ionized
b) Weak BASES in STOMACH are mostly ionized

High surface area

Increased absorption
-Small intestines
-Lungs

High blood flow

Increased absorption
-Small intestines
-Lungs
-Muscles
-Buccal cavity
-Nasal cavity

First-pass effect

Hepatic enzymes metabolize before going into blood stream

Sublingual administration

Rapid absorption; bypasses liver

Rectal administration

Great for patient that is vomiting or cannot swallow medicine; 50% bypasses liver

Drug Distribution

1. Where drugs act
2. Where drugs are eliminated
3. How long drugs last in the body

Enterohepatic recirculation

Drug transported by the liver into the intestines where it is reabsorbed; increases the time it lasts in the body

Determinants of drug distribution

1. Organ blood flow
2. Barriers to drug diffusion
3. Adipose tissue
4. Tissue protein binding
5. Plasma protein binding
6. Drug transport
7. Ion trapping

Organs with high blood flow

Experience larger initial effects

Capillaries with tight junctions

Drug molecules must diffuse across (transcellular); must be lipophilic

Effects of adipose tissue

The distribution of lipophilic drugs will be different in thin vs. obese patients

Plasma protein binding

1. Acid drugs bind to albumin
2. Basic drugs bind to alpha-acid glycoprotein

Bilirubin encephalopathy

Displacement of unconjugated bilirubin from albumin by drugs in newborns

Ion trapping

Distribute drugs into the urinary compartment to increase the urinary excretion of poisons; only unionized forms can cross the membrane

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