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First Aid > Pharmacology - Pharmacokinetics and Pharmacodynamics > Flashcards

Pharmacology - Pharmacokinetics and Pharmacodynamics Flashcards

1
Q

Michaelis-Menten kinetics

A
  • Km = [S] at 1/2 (Vmax)
    • Km is inversely related to the affinity of the enzyme for its substrate.
    • [S] = concentration of substrate
    • V = velocity
    • Vmax is directly proportional to the enzyme concentration.
  • Most enzymatic reactions follow a hyperbolic curve (follow Michaelis-Menten kinetics)
    • However, enzymatic reactions that exhibit a sigmoid curve usually indicate cooperative kinetics (i.e., hemoglobin).
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2
Q

Lineweaver-Burk plot

A
  • Increase y-intercept –> decrease Vmax.
  • The further to the right the x-intercept (i.e., closer to zero), the greater the Km and the lower the affinity.
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3
Q 
Enzyme inhibition:
Competitive inhibitors (reversible)
  • Resemble substrate?
  • Overcome by increased [S}?
  • Bind active site?
  • Effect on Vmax
  • Effect on Km
  • Pharmacodynamics
  • Cross each other competitively (on graph)?
A
  • Resemble substrate?
    • Yes
  • Overcome by increased [S]?
    • Yes
  • Bind active site?
    • Yes
  • Effect on Vmax
    • Unchanged
  • Effect on Km
    • Increase
  • Pharmacodynamics
    • Decrease potency
  • Cross each other competitively (on graph)?
    • Yes
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4
Q 
Enzyme inhibition:
Competitive inhibitors (irreversible)
  • Resemble substrate?
  • Overcome by increased [S}?
  • Bind active site?
  • Effect on Vmax
  • Effect on Km
  • Pharmacodynamics
  • Cross each other competitively (on graph)?
A
  • Resemble substrate?
    • Yes
  • Overcome by increased [S]?
    • No
  • Bind active site?
    • Yes
  • Effect on Vmax
    • Decreased
  • Effect on Km
    • Unchanged
  • Pharmacodynamics
    • Decrease efficacy
  • Cross each other competitively (on graph)?
    • Yes
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5
Q

Enzyme inhibition:
Noncompetitive inhibitors

  • Resemble substrate?
  • Overcome by increased [S}?
  • Bind active site?
  • Effect on Vmax
  • Effect on Km
  • Pharmacodynamics
  • Cross each other competitively (on graph)?
A
  • Resemble substrate?
    • No
  • Overcome by increased [S]?
    • No
  • Bind active site?
    • No
  • Effect on Vmax
    • Decreased
  • Effect on Km
    • Unchanged
  • Pharmacodynamics
    • Decrease efficacy
  • Cross each other competitively (on graph)?
    • No
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6
Q

Pharmacokinetics vs. pharmacodynamics

  • Pharmacokinetics
  • Pharmacodynamics
A
  • Pharmacokinetics
    • The effects of the body on the drug.
    • ADME:
      • Absorption
      • ƒƒDistribution
      • ƒƒMetabolism
      • Excretion
  • Pharmacodynamics
    • The effects of the drug on the body.
    • Includes concepts of receptor binding, drug efficacy, drug potency, toxicity.
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7
Q

Bioavailability (F)

  • Definition
  • IV
  • Orally
A
  • Definition
    • Fraction of administered drug that reaches systemic circulation unchanged.
  • IV
    • For an IV dose, F = 100%.
  • Orally
    • F typically <100% due to incomplete absorption and first-pass metabolism.
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8
Q

Volume of distribution (Vd)

  • Definition
  • For each
    • Compartment
    • Drug types
  • Low
  • Medium
  • High
A
  • Definition
    • Theoretical volume occupied by the total absorbed drug amount at the plasma concentration.
    • Apparent Vd of plasma protein–bound drugs can be altered by liver and kidney disease (decrease protein binding, increase Vd).
    • Drugs may distribute in more than one compartment.
    • Vd = amount of drug in the body / plasma drug concentration
  • Low
    • Compartment: Blood (4–8 L)
    • Drug types: Large/charged molecules; plasma protein
      bound
  • Medium
    • Compartment: ECF
    • Drug types: Small hydrophilic molecules
  • High
    • Compartment: All tissues including fat
    • Drug types: Small lipophilic molecules, especially if bound to tissue protein
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9
Q

Half-life (t1/2)

A
  • The time required to change the amount of drug in the body by 1⁄2 during elimination (or constant infusion).
    • t1/2 = ( 0.693 × Vd ) / CL
      • CL = clearance
  • Property of first-order elimination.
  • A drug infused at a constant rate takes 4–5 half-lives to reach steady state.
  • It takes 3.3 half-lives to reach 90% of the steady-state level.
  • # of half-lives –> % remaining
    • 1 –> 50%
    • 2 –> 25%
    • 3 –> 12.5%
    • 4 –> 6.25%
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10
Q

Clearance (CL)

A
  • The volume of plasma cleared of drug per unit time.
  • Clearance may be impaired with defects in cardiac, hepatic, or renal function.
  • CL = rate of elimination of drug / plasma drug concentration = Vd × Ke
    • Ke = elimination constant
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11
Q

Dosage calculations

  • Loading dose
  • Maintenance dose
  • In renal or liver disease…
  • Time to steady state depends primarily on…
A
  • Loading dose = ( Cp × Vd) / F
    • Cp = target plasma concentration at steady state
  • Maintenance dose = ( Cp × CL × t ) / F
    • t = dosage interval (time between doses), if not administered continuously
  • In renal or liver disease, maintenance dose decreases and loading dose is usually unchanged.
  • Time to steady state depends primarily on t1/2 and is independent of dose and dosing frequency.
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12
Q

Zero-order elimination of drugs

A
  • Rate of elimination is constant regardless of Cp (i.e., constant amount of drug eliminated per unit time).
    • Cp = target plasma concentration at steady state
  • Cp decreases linearly with time.
  • Examples of drugs—Phenytoin, Ethanol, and Aspirin (at high or toxic concentrations).
    • A PEA is round, shaped like the “0” in “zero-order.”
  • Capacity-limited elimination.
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13
Q

First-order elimination of drugs

A
  • Rate of elimination is directly proportional to the drug concentration (i.e., constant fraction of drug eliminated per unit time).
  • Cp decreases exponentially with time.
  • Flow-dependent elimination.
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14
Q

Urine pH and drug elimination

  • Ionized vs. neutral species
  • Weak acids
    • Examples
    • Trapped in/
    • Treat/
    • Equation
  • Weak bases
    • Example
    • Trapped in/
    • Treat/
    • Equation
A
  • Ionized vs. neutral species
    • Ionized species are trapped in urine and cleared quickly.
    • Neutral forms can be reabsorbed.
  • Weak acids
    • Examples: phenobarbital, methotrexate, aspirin.
    • Trapped in basic environments.
    • Treat overdose with bicarbonate.
    • RCOOH (lipid soluble) <–> RCOO– + H+ (trapped)
  • Weak bases
    • Example: amphetamines.
    • Trapped in acidic environments.
    • Treat overdose with ammonium chloride.
    • RNH3+ (trapped) <–> RNH2 + H+ (lipid soluble)
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15
Q

Drug metabolism

  • Phase I
  • Phase II
A
  • Phase I
    • Reduction, oxidation, hydrolysis with cytochrome P-450 usually yield slightly polar, water-soluble metabolites (often still active).
    • Geriatric patients lose phase I first.
  • Phase II
    • Conjugation (Glucuronidation, Acetylation, Sulfation) usually yields very polar, inactive metabolites (renally excreted).
      • Geriatric patients have GAS (phase II).
    • Patients who are slow acetylators have greater side effects from certain drugs because of decreased rate of metabolism.
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16
Q

Efficacy

A
  • Maximal effect a drug can produce.
  • High-efficacy drug classes are analgesic (pain) medications, antibiotics, antihistamines, and decongestants.
  • Partial agonists have less efficacy than full agonists.
17
Q

Potency

A
  • Amount of drug needed for a given effect.
  • Increased potency –> increased affinity for receptor.
  • Highly potent drug classes include chemotherapeutic (cancer) drugs, antihypertensive (blood pressure) drugs, and lipid-lowering (cholesterol) drugs.
18
Q

Receptor binding (242)

  • For each
    • Effect
    • Example
  • Competitive antagonist
  • Noncompetitive antagonist
  • Irreversible competitive antagonist
  • Partial agonist
A
  • Competitive antagonist [1]
    • Effect:
      • Shifts curve to right (decreased potency)
      • No change in efficacy.
      • Can be overcome by increasing the concentration of agonist substrate.
    • Ex: Diazepam + flumazenil (competitive antagonist) on GABA receptor.
  • Noncompetitive antagonist [2]
    • Effect:
      • Shifts curve down (decreased efficacy).
      • Cannot be overcome by increasing agonist substrate concentration.
    • Ex: Glutamate + ketamine (noncompetitive antagonist) on NMDA receptors.
  • Irreversible competitive antagonist [2]
    • Effect:
      • Shifts curve down (decreased efficacy).
      • Cannot be overcome by increasing agonist substrate concentration.
    • Ex: Norepinephrine + phenoxybenzamine (irreversible competitive antagonist) on α-receptors.
  • Partial agonist [3]
    • Effect:
      • Acts at same site as full agonist, but with lower maximal effect (decreased efficacy).
      • Potency is an independent variable.
    • Ex: Morphine vs. buprenorphine (partial agonist) at opioid μ-receptors.
19
Q

Therapeutic index

A
  • Measurement of drug safety.
  • TD50 / ED50 = median toxic dose / median effective dose
    • TITE: Therapeutic Index = TD50 / ED50.
  • Therapeutic window—measure of clinical drug effectiveness for a patient.
  • Safer drugs have higher TI values.
  • Examples of drugs with low TI values include digoxin, lithium, theophylline, and warfarin.
  • LD50 (lethal median dose) often replaces TD50 in animal studies.

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