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

Make sure you know the learning objectives. 1. Describe 4 processes by which drugs penetrate biological barriers. 2. Explain how changes in pH can affect absorption and excretion of drugs. 3. Describe 4 common routes of drug administration, their advantages …

Passive Diffusion High conc. -> low majority of drugs work this way does not involve carrier, is not saturable, and shows a low structural specificity Facilitated Diffusion involves carrier proteins High conc -> low no energy required, an be saturated, …

Define Half-Life t½ means half life – The time it takes for blood levels of drug to decrease to half of what it was at equilibrium. If maximum level is 16 mg and the half life is 2 hours, after …

Make sure you know the learning objectives. Describe the pharmacokinetic parameters: clearance, apparent volume of distribution, bioavailability, and half-life. Explain how plasma drug concentration is dependent on the route and schedule of drug administration. Explain how plasma drug concentration in …

3 phases a PO drug goes through – pharmaceutic (dissolution) – pharmacokinetic – pharmacodynamic pharmaceutic phase The drug becomes a solution so it may cross the biologic membrane. WE WILL WRITE A CUSTOM ESSAY SAMPLE ON ANY TOPIC SPECIFICALLY FOR …

Pharmaceutic Phase (First Phase) The drug becomes a solution so that it can cross the biologic membrane **When drug is given subQ, IM, or IV routes there is NO pharmaceutic phase. Pharmacokinetic Phase (Second Phase) Is composed of 4 processes: …

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