24 terms

Pharmacology: Drug Absorption and Distribution

Absorption describes the transfer of drug from the site of administration to the general circulation
The aim of drug therapy is to
rapidly deliver & maintain therapeutic, yet nontoxic levels of drug in the target tissues.
the speed of onset of drug action, the intensity of the drug´s effect & the duration of the drug action are controlled by 3 fundamental pathways of drug movement in the body:
1. drug absorption from the site of administration.
2. drug leaving the blood stream & its distribution into the interstitial & intracellular fluids (distribution)
3. hepatic metabolism & urinary excretion of the drug & its metabolites.
What route does not require the drug be transported from the site of administration into the systemic circulation?
IV route
The transport of drugs across membrane involves one or more of these four processes:
passive diffusion, active transport, endocytosis, exocytosis
Passive diffusion is
high to low concentration across a semi-permeable membrane
Active transport: (4 and example)
1. Specific carrier proteins
2. Energy-dependent
3. Capable of moving drugs against a 4. concentration gradient
4. Can be saturated
E.g. Methyldopa
Endocytosis: engulfment of a drug molecule by the cell membrane & transport into the cell by pinching off the drug-filled vesicle
Exocytosis and what molecules are transported this way
Exocytosis is the reverse of endocytosis
Many protein molecules are transported via this mechanism
IMPORTANT Factors that influence absorption of drugs includes (9)
1. Molecular size
2. Solubility of drug at site of administration
3. Concentration of drug
4. Route of administration
5. pH (acidic drus are unionized and absorbed in stomach. bases, e.g. atropine are largely ionized & are absorbed only when they reach the intestine)
6. Blood flow to site of administration
7. Total surface area for absorption
8. Contact time at the absorptive surface
9. Biopharmacutic factors e.g. tablet compression, coating, excipient etc.
Drugs in what sort of solution are more rapidly absorbed and why?
Drugs given in aqueous solution are more rapidly absorbed than those given in oily solution, suspension or solid form because they mix more readily with the aqueous phase at the absorptive site.
The extent or fraction of active drug that reaches the systemic circulation
What affects bioavailability?
destroyed in gut
not absorbed
destroyed by gut wall
destroyed by liver
->remainder to systemic circulation
What are the factors that affect bioavailability?
First-pass hepatic metabolism
Solubility of the drug
Chemical instability
Nature of the drug formulation
Drug Distribution
It means the reversible transfer of drug from one location to another within the body.
What sort of process is distribution and what is its driving force?
it is a passive force and is driven by concentration gradient between the blood and tissues
When does the process of distribution end?
when diffusion of the free drug reaches equilibrium
The pharmacological action of a drug depends upon its
concentration at the site of action
Distribution plays a significant role in
in the Onset, Intensity, and Duration of Action.
Why is distribution of a drug not uniform throughout the body?
different tissues receive the drug from plasma at different rates and to different extents.
IMPORTANT What are the drug reservoirs?
1. Plasma proteins - bind drugs=inactive
2. Cellular reservoirs - drugs accumulate in muscles/cells in high concentrations
3. Transcellular reservior - drugs cross epithelial cells and accumulate in transcellular fluid like in GIT
4. Fats - phenobarbitol
5. Bone - tetracycline, heavy metals; bone can incorporate them and slowly release toxins
Free drugs
only the free, unbound drug can act on target sites in the tissues and elicit a biological response
Bound drugs
By binding to plasma proteins, drug become "trapped" and in effect, inactive.
IMPORTANT- List the pharmacokinetics
1 Drug admin
2. Absorption & distribution
3. Binding to target site (neuron receptor) or storage depots (protein, fat, bone)
4. Inactivation in liver - metbolites excreted
5. Excretion