Drug Action Across the Lifespan
Ann MacLeod, RN, BScN, MPH
Agenda
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Identify 5 basic principles of drug action
 (absorption, distribution, metabolism, excretion, half-life)
Describe the meaning of disired action, side effects, adverse effects, allergic
reaction and ideosyncratic reactions
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List factors that cause variation in absorption, distribution, metabolism and
excretion of drugs across the lifespan
Drug Action
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Drug fits into a “recepeptor” or a key hole
May be several receptors on a cell
May be a better or worse “fit”
May stimulate a response - “ agonist”
 Morphine
May stop a response - “antagonists”
 Narcaine
Pharmacokinetics
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The study of drug absorption, distribution, metabolism and excretion of
individual medicines
Drug Absorption
 How the drug gets to its receptor site
 Enteral - by mouth (PO) or tubes GI stomach intestinal absorption into blood
stream (least direct)
 Parenteral - bypass GI system,
 SC Subcutaneous, just under skin
 IM Intramuscularly, into muscle
 IV Intraveneously (most direct)
 Percutaneous - inhalation (fast), sublingual (under tongue), topical (cream on
skin) slow
Absorption cont’d
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Enteral  on empty stomach.. But may irritate stomach
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Percutaneous  with higher concentration of drug, longer contact, more
vascular, thin skin
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Parenteral  more directly into blood SC, IM, then IV, more vascular, less scar
tissue
Distribution
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The way the drugs are transported to the receptors
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Lipid soluble drugs, sometimes bind and stored in adipose tissue
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“biotransformation” how the liver or other enzymes inactivate and “digest” the
drug
if attached to large proteins, lower concentration of “free” unattached
molecules to get to the receptor sites
May not cross blood- brain barrier or placenta
Metabolism
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Preparation for excretion
Excretion
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How the body rids itself of the metabolized or “digested” drug
 kidneys
 evaporation
 exhalation
 secretion into saliva or mother’s milk
Half- Life
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The ammount of time required for half of the drug to be eliminated from the
body
 e.g. half life of 100 mg is 12 hrs, 50 mg is left 12 hours.
Divide the dose by 2 ( 25 mg) and
Add half life time (12) to that time (12)… 24 hours
Longer the half life digoxin, the less frequent the drug needs to be given
Drug Actions
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Desired action - what you hope for
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Sever side effects - toxicity
Side or adverse effects - in addition to the desired action may have another
result - drowsiness
Idio syncratic response - unexpected response
Allergic reactions -- immune responses
Allergic vs
Analphylactic
 Urticaria - (hives)
 Rashes
 Edema - (local swelling)
 second reaction may be more serious
 Severe urticaria
 Edema around throat and face
 Bronchoconstriction and difficulty breathing ( dyspnea )
 tachycardia( high heart rate )
 hypotension ( low blood pressure)
Variables affecting drug action
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Body weight
 mg/kg, larger more drug required
Metabolic rate
 higher the metabolic rate the faster drug works and metabolizes
Illness such as renal or liver disease
 impair metabolism may have build up of drug
Psychological aspects
 placebo effect, works just because you think so
Variables affecting drug action cont’d
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Tolerance : liver ‘learns’ how to metabolise the drug more efficiently therefore
more of the drug is needed to reach therapeutic effects - morphine
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Dependence: physical or psychological so that if drug is discontinued
withdrawl effects happen - usually the opposite of drug action - sedatives
discontinued  agitated , tremors
Other drugs affecting drug action - Drug interactions
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Additive effect: sum of two drugs’ effects
Synergistic: augmented effect
Antagonistic: counteract each other
Displacement: one drug displaces the other at the receptor site
Interference: inhibits excretion or metabolism
Drug absorption & Age
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 age
  predictable topical absorption
 difficulty swallowing pills
 lower pH (more acidic) due to ulcer’s etc.
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Newborns and premies
 high pH
 may not have all the absorptive enzymes
Drug Distribution and Age
  age
 less percentage of water  lower concentrations of drug needed
 greater fat %  fat soluble drugs longer time to act, prolonged action
greater doses
 cirrhosis, liver failure  little protein to bind smaller mg/kg ofprotein
binding drugs
 Newborns and premies
 more water, higher concentrations per pound
 less %fat  smaller mg/kg of fat soluble drugs
 little protein to bind smaller mg/kg ofprotein binding drugs
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Drug Metabolism and Age
  age
 improperly functioning enzyme pathways due to disease  slower
metabolism, lower doses
 Premies and newborns
 immature enzymes  slower metabolism, lower doses
 frequent therapeutic drug blood levels to adjust dose
Drug Excretion and Age
  age
 potential kidney disease detected by a raised creatinine level mean a lower
dose/kg needs to be prescribed because the drug is not being excreted
 Premies and newborns
 immature kidneys mean a lower dose/kg needs to be prescribed because
the drug is not being excreted
Carcinogen vs Teratogen
 Mutates cell and can become cancerous
 A drug that can induce birth defects