Summary of "Routes of Drug Administration (Part I)"
Main ideas and lessons (Routes of drug administration — Part I)
1) What “route of administration” means
- Route of drug administration is defined as: the way (portal of entry) by which a drug is introduced into the body, and thus where/when/how it reaches its site of action.
- Key point: the route determines how quickly and how efficiently the drug can enter circulation and/or act locally.
2) Basic drug definition
A drug (as used here) is a chemical substance used for:
- Prophylaxis (prevention),
- Diagnosis,
- Cure, or
- Relief of disease symptoms.
3) Main factors used to select a route (the “four important factors”)
The lecture groups route-selection factors around:
- Drug properties (chemical/physical/organoleptic)
- Dose requirements
- Desired drug action (local vs systemic)
- Onset and duration of action (linked to pharmacokinetics/pharmacodynamics)
4) Drug properties and how they influence route
A) Chemical properties (structure-related limitations)
- A drug’s chemical structure affects:
- Stability in the body,
- Whether it can survive GI conditions,
- Ability to remain intact long enough to be absorbed.
- Example: insulin
- Insulin is a peptide/protein-like molecule.
- If taken orally, it is degraded by:
- Low stomach pH (HCl) and
- Enzymes (peptidases).
- Result: insulin becomes inactive amino acids, so oral insulin is ineffective.
- Therefore, insulin is typically given by injection (e.g., subcutaneous).
B) Physical properties: solubility and partitioning
Two described requirements:
- Water solubility: needed to dissolve in GI fluids for oral absorption.
- Lipid solubility: needed to cross cell membranes (phospholipid membranes) via diffusion. - The partition coefficient is mentioned conceptually as the balance of water vs lipid affinity.
C) Solid-state form: crystalline vs amorphous
Drugs can exist in different physical arrangements while keeping the same chemical identity:
- Amorphous
- No fixed molecular arrangement
- Weaker interactions
- More water penetration
- Faster dissolution rate → faster onset expected
- Crystalline
- Fixed structure with stronger bonds
- Slower dissolution rate
- Practical formulation link:
- If fast onset is desired → prefer amorphous.
- If fast onset is not critical → crystalline may be acceptable.
D) Polymorphism (different crystal forms)
- Polymorphs are different crystal forms of the same drug with different dissolution rates/stabilities.
- Some forms may be:
- Unstable/metastable → dissolve faster
- Stable → dissolve slower
- Challenge: selecting a fast-dissolving polymorph isn’t enough—the form must be preserved during shelf life, or it may convert over time.
E) Organoleptic properties (taste/odor) and formulation impact
- Many drugs have unpleasant taste/odor.
- Dosage form selection depends on this:
- If bitter → syrup/suspension less likely; may prefer tablet/capsule
- If using liquid forms → may require taste-masking additives
5) Dose-related factors
- Dose amount can limit feasible routes/dosage forms.
- Example pattern:
- Low-dose drugs may sometimes be formulated as sublingual tablets.
- High-dose drugs (e.g., ~1000 mg class) are “highly unlikely” for sublingual use due to practicality.
6) Desired action: local vs systemic
Local effect (near the administration site)
- Drug acts mainly at/near where it’s given.
- Examples:
- Skin: ointments with antihistamines/corticosteroids
- GI for diarrhea: oral drugs acting in the GI tract (not necessarily absorbed systemically)
- Eyes (allergy): histamine eyedrops
Systemic effect (away from administration site)
- Drug is absorbed into general circulation, reaching distant organs.
- Example:
- An oral pain/headache drug → absorbed → reaches brain/CNS.
“Local but oral” example
- Oral delivery doesn’t always mean systemic action.
- Example:
- An antacid swallowed for effect in the stomach (local action despite oral delivery)
7) Onset and duration of action (speed controlled by route)
Routes differ in:
- How fast the drug starts working
- How long it persists
Examples aligned with speed:
- Intravenous injection → onset in seconds
- Sublingual and intramuscular injection → onset in minutes
- Tablets/patches → onset and duration can extend for hours
8) Pharmacokinetics framework (linked to onset/duration)
The lecture uses an ADME-style (Absorption, Distribution, Metabolism, Elimination) process and introduces pharmacokinetic parameters that shape the time course.
Oral dosing process (example: tablet)
- Liberation (drug release)
- Drug leaves the dosage form into the GI environment.
- Failure mode: drug may remain “trapped” in the form.
- Dissolution
- Drug must dissolve in GI fluids to be absorbed.
- Absorption
- Crosses GI lining into blood vessels.
- Distribution
- Travels via blood to tissues.
- Interaction with plasma proteins (e.g., albumin):
- Bound drug → cannot readily act at receptors; can extend half-life but isn’t immediately active
- Free fraction → active; can distribute and interact with receptors
- Binding described as: strongly / intermediately / loosely / not bound.
- Metabolism
- Usually in the liver.
- Often inactivates/detoxifies, but sometimes active metabolites form → prolonged effect.
- Elimination
- Excreted, commonly via kidneys/urine.
Prodrugs concept (deliberate inactive → activated)
- Prodrug strategy: administer an inactive form that becomes active after metabolism (commonly in the liver).
9) Bioavailability (oral success metric)
Definition
- Bioavailability = the fraction/percentage of the administered dose that reaches systemic circulation intact (not degraded).
First-pass effect
- Oral drugs often pass through the liver after GI absorption, where extensive metabolism can reduce intact drug reaching blood.
Aminoglycosides example (local GI use vs systemic need)
- Kanamycin/gentamicin-type drugs are described as:
- Ionized in the GI tract
- Poor at crossing lipid membranes
- Poorly absorbed systemically
- Therefore:
- Oral use can be effective for local GI effects
- Systemic effects require injection
Sublingual example to bypass first-pass
- Nitroglycerine for angina:
- Given as sublingual tablets
- Avoids first-pass liver metabolism and reaches circulation faster
- Compared with oral forms that suffer first-pass degradation
10) Factors affecting GI absorption (detailed list)
A) Drug-specific physicochemical factors
- Amorphous vs crystalline form
- Polymorphism (stable vs unstable crystal forms)
- Partition coefficient (water vs lipid affinity)
- Particle size / surface area
- Smaller particles → larger surface area → faster dissolution → improved absorption
- Larger particles → slower dissolution
B) Stability challenges in the GI tract
- GI challenges include:
- Stomach acidity (low pH)
- Enzymes that degrade drugs
- Chemical and enzymatic stability determine whether the drug survives long enough.
C) Dosage form / excipients (formulation engineering)
- Coatings can protect against:
- Hostile stomach environment
- Enzymatic degradation
- Improve release timing (conceptually: enteric-like protection)
- Excipients:
- Included for manufacturability and drug release
- Lubricants assist manufacturing/powder ejection
- Manufacturing effects:
- Tablet compression can affect disintegration and water penetration
- Storage conditions:
- Temperature, humidity, and light can affect stability and later dissolution/release
D) GI physiology and patient factors
- Gastric emptying time
- Intestinal motility
- Fed vs fasted states (food can influence absorption)
- Diarrhea/vomiting: reduce residence time and absorption opportunities
- Drug-food interactions
- Example: tetracyclines complexing with calcium → antagonizes absorption
- Bile secretion / fatty meals
- Can affect solubilization/absorption for certain drugs
E) Transport/enzymatic transporters (absorption mechanism)
- Absorption depends on:
- Carriers/transport mechanisms
- Enzymes in the gut that may metabolize/modify drugs
- Emphasis: transporters and enzymes act selectively—route- and drug-specific.
11) Concentration–time curve and therapeutic window
Key concepts
- Concentration vs time curve
- Absorption phase: concentration rises
- Then metabolism/elimination: concentration declines
- Cmax
- Maximum concentration in blood
- Occurs at Tmax
- Therapeutic window / therapeutic range
- Drug works only when concentration remains within a beneficial range
- Below range: ineffective
- Above range: toxicity/overdose risk
12) Final recap: what determines route selection?
Route selection is determined by a combination of:
-
Drug-related factors
- Chemical/physical properties
- Organoleptic properties (taste/odor)
- Desired effect type (local vs systemic)
- Onset/duration needs
-
Patient-related and practical factors
- Patient condition and ability to tolerate a route
- Convenience and feasibility
Speakers / sources featured
- Speaker: lecture instructor (no name given in the subtitles)
- External sources: none clearly identified beyond general teaching-style examples.
- Examples of drugs mentioned: insulin, aspirin, nitroglycerine, tetracyclines, doxorubicin, aminoglycosides (e.g., kanamycin/gentamicin-like), digoxin/digoxin-like examples, progesterone/estrogen/testosterone (tablets), antacids, histamine/corticosteroids, and others.
Category
Educational
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