Routes of Drug Administration

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Routes of Drug Administration

• Robert L. Copeland, Ph.D.
• Department of Pharmacology
• www.med.howard.edu/pharmacology
• 202.806.6311

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Routes of Drug Administration

• The route of administration (ROA) that is chosen
  may have a profound effect upon the speed and
  efficiency with which the drug acts

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• The possible routes of drug entry into the body
  may be divided into two classes
• Enteral
• Parenteral

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Enteral Routes

• Enteral - drug placed directly in the GI tract
• sublingual - placed under the tongue
• oral - swallowing (p.o., per os)
• rectum - Absorption through the rectum

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Sublingual/Buccal

• Some drugs are taken as smaller tablets which are
  held in the mouth or under the tongue.
• Advantages
• rapid absorption
• drug stability
• avoid first-pass effect

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Sublingual/Buccal

• Disadvantages
• inconvenient
• small doses
• unpleasant taste of some drugs

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Oral

• Advantages
• Convenient - can be self- administered, pain
  free, easy to take
• Absorption - takes place along the whole length
  of the GI tract
• Cheap - compared to most other parenteral routes

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Oral

• Disadvantages
• Sometimes inefficient - only part of the drug may
  be absorbed
• First-pass effect - drugs absorbed orally are
  initially transported to the liver via the portal
  vein
• irritation to gastric mucosa - nausea and vomiting

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Oral

• Disadvantages cont.
• destruction of drugs by gastric acid and
  digestive juices
• effect too slow for emergencies
• unpleasant taste of some drugs
• unable to use in unconscious patient

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First-pass Effect

• The first-pass effect is the term used for the
  hepatic metabolism of a pharmacological agent
  when it is absorbed from the gut and delivered to
  the liver via the portal circulation. The
  greater the first-pass effect, the less the agent
  will reach the systemic circulation when the
  agent is administered orally

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First-pass Effect cont.
Magnitude of first pass hepatic effect
Extraction ratio (ER) ER CL liver / Q where Q
is hepatic blood flow (usually about 90 L per
hour. Systemic drug bioavailability (F) may be
determined from the extent of absorption (f) and
the extraction ratio (ER)
F f x (1 -ER)
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First-pass Effect
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Rectal
1. unconscious patients and children 2. if
patient is nauseous or vomiting 3. easy to
terminate exposure 4. absorption may be variable
5. good for drugs affecting the bowel such
as laxatives 6. irritating drugs contraindicated
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Parenteral Routes

• Intravascular (IV, IA)- placing a drug directly
  into the blood stream
• Intramuscular (IM) - drug injected into skeletal
  muscle
• Subcutaneous - Absorption of drugs from the
  subcutaneous tissues
• Inhalation - Absorption through the lungs

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Intravascular
Absorption phase is bypassed (100
bioavailability) 1.precise, accurate and almost
immediate onset of action, 2. large quantities
can be given, fairly pain free 3. greater risk
of adverse effects a. high concentration
attained rapidly b. risk of embolism
c. OOPS factor or !_at_
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Intramuscular
1. very rapid absorption of drugs in aqueous
solution 2.repository and slow release
preparations 3.pain at injection sites for
certain drugs
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Subcutaneous
1. slow and constant absorption 2. absorption is
limited by blood flow, affected if
circulatory problems exist 3. concurrent
administration of vasoconstrictor
will slow absorption
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Inhalation
1.gaseous and volatile agents and aerosols
2.rapid onset of action due to rapid access to
circulation a.large surface area
b.thin membranes separates alveoli from
circulation c.high blood
flow Particles larger than 20 micron and the
particles impact in the mouth and throat. Smaller
than 0.5 micron and they aren't retained.
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Inhalation cont.

• Respiratory system. Except for IN, risk hypoxia.
• Intranasal (snorting) Snuff, cocaine may be
  partly oral via post-nasal dripping. Fairly fast
  to brain, local damage to septum. Some of the
  volatile gases also appear to cross nasal
  membranes.
• Smoke (Solids in air suspension, vapors) absorbed
  across lung alveoli Nicotine, opium, THC,
  freebase and crack cocaine, crystal
  meth.Particles or vapors dissolve in lung fluids,
  then diffuse. Longer action than volatile gases.
  Tissue damage from particles, tars, CO.
• Volatile gases Some anaesthetics (nitrous oxide,
  ether) precise control, petroleum distillates.
  Diffusion and exhalation (alcohol).
• Lung-based transfer may get drug to brain in as
  little as five seconds.

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Topical

• Mucosal membranes (eye drops, antiseptic,
  sunscreen, callous removal, nasal, etc.)
• Skin
• a. Dermal - rubbing in of oil or ointment
  (local action)
• b. Transdermal - absorption of drug through
  skin (systemic action)
• i. stable blood levels
• ii. no first pass metabolism
• iii. drug must be potent or patch
  becomes to large

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Route for administration -Time until effect-

• intravenous 30-60 seconds
• intraosseous 30-60 seconds
• endotracheal 2-3 minutes
• inhalation 2-3 minutes
• sublingual 3-5 minutes
• intramuscular 10-20 minutes
• subcutaneous 15-30 minutes
• rectal 5-30 minutes
• ingestion 30-90 minutes
• transdermal (topical) variable (minutes to hours)

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Time-release preparations

• Oral - controlled-release, timed-release,
  sustained-release
• designed to produce slow,uniform absorption for 8
  hours or longer
• better compliance, maintain effect over night,
  eliminate extreme peaks and troughs

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Time-release preparations

• Depot or reservoir preparations - parental
  administration (except IV), may be prolonged by
  using insoluble salts or suspensions in
  non-aqueous vehicles.

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The ROA is determined by the physical
characteristics of the drug, the speed which the
drug is absorbed and/ or released, as well as
the need to bypass hepatic metabolism and achieve
high conc. at particular sites
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Very Important Info!
No single method of drug administration is ideal
for all drugs in all circumstances
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THATS IT!!
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