What is the absorption?
Transfer of a drug from its site of administration to the
bloodstream
What is the distribution?
Movement of the drug from bloodstream to the interstitial and intracellular fluids throughout the body
What is the metabolism?
Biotransformation of the drug molecules by the liver or other tissues
What is the excretion?
Elimination of the drug and its metabolites from the body in urine, bile, or feces
Which routes of drug administration exist?
Eternal Application
Oral
Sublingual
Rectal
Paraternal Application (Injections)
Intravenous
Intramuscular
Subcutaneous
Others
Inhalation
Transdermal
Topical
What are pros and cons of Oral (PO)drug application?
Pros
- Most common, safest and the most convenient route.
- Self-medication is possible, economical.
- Low risk of systemic infections.
- In case of drug overdose or accidental poisoning, the drug can be
removed quickly by gastric lavage or induced vomiting.
Cons
- Onset of action is delayed.
- Changes in absorption rate or absorption extent due to food effects.
- Low pH of stomach may inactivate some drugs.
- Drugs are subjected to first-pass metabolism. (Enteric-coated
preparations).
- Not suitable for patients with persistent vomiting
What is the sublingual and buccal drug application and what are pros and cons?
Sublingual = Placement under the tongue
Buccal = Placement between cheek and gum
Rapid onset, convenient (suitable for emergency), Self-medication is possible, low incidence of infection, bypass of the harsh gastrointestinal environment, avoidance of first-pass metabolism.
- Limited to certain types of drugs (Not suitable for hydrophilic drugs, drugs with large molecular weight, irritant drugs or drugs with unpleasant taste).
- Not suitable for large doses of drugs.
- Not suitable for patients with dry mouth.
- Drugs have short duration of action. Frequent administration may be
necessary.
What are the pros and cons of rectal drug application?
Suitable for patients who are vomiting, unconscious, or unable to
swallow.
Partial avoidance of first-pass metabolism, bypasses destruction by
stomach acid.
Suitable for drugs that cause stomach irritation.
Can be used to produce local or systemic effect.
Inconvenient and uncomfortable for chronic use.
Not suitable for drugs that cause local irritation in the rectum.
Not suitable in case of persistent diarrhea or certain rectal diseases.
Rectal absorption is often erratic and incomplete (Not suitable for
drugs with low TI).
What are the general pros and cons of injections?
Most suitable for unconscious patients and emergency cases that
require a rapid onset of action.
Used for drugs that are poorly absorbed from the GI tract and for
agents that are unstable in the GI
Have the highest bioavailability, drugs are not subjected to first-pass
metabolism or harsh GI environments.
Provide the most controllable drug dose delivery to the body.
Irreversible, self-medication requires training.
May cause pain, fear, local tissue damage, and infections.
What are the pros and cons of intravenous (IV) injections?
Rapid effect: Drug is immediately introduced into the systemic
circulation (most suitable for emergency cases).
Maximum control: Allows for the highest degree of control over
drug levels in the blood, especially for drugs not absorbed orally.
Flexible dosing: Can be delivered as a bolus for immediate effect or
through an infusion to maintain drug levels over time.
Dilution of Irritating Drugs: Substances that may cause irritation when administered by other routes are diluted quickly in the
bloodstream.
Suitable for high-molecular-weight proteins and peptide drugs. Ideal
if dosed in large volumes.
Irreversibility: Once administered, the drug cannot be "recalled,"
unlike oral drugs.
Risk of Contamination: Bacteria or other infectious agents can be
introduced if sterile techniques are not followed.
Potential for Adverse Reactions: Rapid delivery of high drug
concentrations can cause complications such as hemolysis or toxic
effects of the drug.
Need for Monitoring: Patients must be closely observed for any
unfavorable drug reactions, and infusion rates must be carefully
managed. Self-administration is unsafe.
Unsuitable for oily substances and suspensions.
What are the pros and cons of Intramuscular (IM) injections?
Suitable for rapid absorption or sustained release: Aqueous solutions of drugs are absorbed quickly into the bloodstream while oily and specialized depot formulations allow for slow and extended release of drugs.
Slower absorption compared to IV: Drugs have slower onset of action compared to IV route, especially for depot preparations.
Possible local side effects: Irritation or discomfort at the injection
site may occur, especially with certain drug formulations.
Not suitable for anticoagulants: Can lead to bleeding at the injection
site.
What are the pros and cons of Subcutaneous (SC) injections?
Constant and slow absorption: Allow for slower absorption
compared to IV injections, leading to more sustained effects.
Minimized Risk of Hemolysis/Thrombosis: Less risk of complications
like hemolysis and thrombosis compared to IV injections.
Self-administration is possible with lower risk (e.g, Insulin pen).
- Long-Term Drug Delivery: Certain SC devices, like implanted rods or
pumps, can provide long-term effects, such as contraceptive
implants or insulin pumps.
Limited to Non-Irritant Drugs: SC is appropriate for drugs that do not cause tissue damage at the injection site (tissue irritation can lead to severe pain or necrosis at the site of injection).
What is the Inhalation (Pulmonary route) drug application?
Involves administration of drug through the nose or mouth and into the lungs during respiratory inspiration.
Especially useful for the local administration of drugs (gases, aerosols) into the respiratory tract (systemic side effects are minimized).
Provides rapid delivery of drugs (Very rapid drug onset of action). Dose can be titrated.
What is the Transdermal drug application?
Suitable for achieving systemic effects by application of drugs to the skin, usually via a transdermal patch.
Rate of absorption can be variable and onset can be delayed, depending on the physical characteristics of the skin at the site of application and lipid solubility of the drug.
Convenient and painless, suitable for sustained delivery of drugs.
What is the Topical drug application?
(Application on skin and mucous membranes)
When a local effect of the drug is desired (in certain conditions used for systemic effect)
What are the four mechanisms of drug absorption?
Passive Diffusion
Facilitated Diffusion
Active Transport
Endocytosis
What is passive Diffusion?
Driving Force: Concentration gradient (high to low)
Characteristics: No carrier involved, not saturable, low structual specificity
Most drugs gain access to the body by this mechanism:
Water-soluble: Move through aqueous channels or pores. Lipid-soluble: Easily cross lipid bilayers
What is faciliated diffusion?
Uses specialized transmembrane carrier proteins: Undergo conformational changes, allowing the passage of drugs into the interior of cells.
Characteristics: No energy required, can be saturated, inhibited by competing compounds.
Function: Facilitates entry of larger molecules from high to low concentration
What is active transport?
Mechanism: Involves specific carrier proteins.
Characteristics: Energy-dependent (via ATP hydrolysis), can move drugs against concentration gradients (low to high), shows saturation kinetics.
Selectivity: Can be competitively inhibited by other substances.
Drugs that closely resemble the structure of naturally occurring metabolites are absorbed via this mechanism. e.g, levodopa (for Parkinson’s disease).
What is endocytosis?
Endocytosis: Engulfs large drug molecules and transports them into the cell via vesicle formation. Examples: Insulin, Peptide antibiotics, vaccine antigens / mRNA, Vit B12.
Exocytosis: Reverse process; used to secrete substances out of cells. Example: Release of neurotransmitters like norepinephrine.
What are factors, that affect the drug absorption?
pH at the absorption site:
Weak acids: HA <-> H+ + A-
Weak bases: BH+ <-> H+ + B
Uncharged forms of drugs diffuse through membranes more easily (faster absorption
rates)
The ratio between the charged and uncharged forms of a drug is determined by the
pH at the site of absorption and by the strength of the weak acid or base, which is represented by the ionization constant, pKa
Expression of P-glycoprotein:
A family of transmembrane transporter proteins (belong to ABC transporters) that pump drugs out of cells, reducing absorption
P-glycoproteins are associated with reduced drug absorption and resistance to multiple drugs
Blood flow at the absorption site:
Intestinal blood flow is higher than stomach blood flow, favoring absorption in the intestine.
In case of shock (marked reduction in blood flow) drug absorption from subcutaneous injections is markedly decreased
Surface area available for absorption:
Intestinal surface area (due to microvilli) is 1000-fold higher than that of the stomach, enhancing drug absorption in the intestine
Contact time at the absorption surface:
Short contact time (e.g., diarrhea) reduces absorption.
Presence of food in the stomach both dilutes the drug and slows gastric emptying (Drugs taken with meal are generally
absorbed more slowly)
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