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29 terms

Lecture 2: Drug Absorption and Distribution

% of dose that gets into body
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
Additives that decrease rate of dissolution
-Coating agents
Additives that increase rate of dissolution
Additives that have variable effects on rates of dissolution
-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
Increases membrane permeability
a) Presence of aliphatic and aromatic structures
b) Absence of polar groups
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
High blood flow
Increased absorption
-Small intestines
-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