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Pharmacology > lecture 4 (pharmacokinetics) > Flashcards

lecture 4 (pharmacokinetics) Flashcards

1
Q

Describe the relationship between pKa and pH

A
  • if pKa is GREATER then the pH, then the drug will exist predominantly in the pronated form
  • If pKa is LESS than the pH, then the drug will exist predominantly in the unpronated form
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2
Q

describe the ionized/non-ionized forms

A
  • Non-ionized forms more readily cross membranes

–> pronated form of WEAK ACID is non-ionized

–> unprotonated form of a WEAK BASE is non-ionized

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3
Q

describe the factors that affect distribution of a drug in the body

A
  • Proportion of cardiac output (greater the CO the greater the distribution)
  • regional blood flow (more blood flowing to the tissue then the greater the distribution of the drug to the tissue)
  • Capillary permeability
  • binding to plasma proteins (the more drug that sticks to proteins, the LESS of it is distributed to tissues)
  • lipid solubility (lipid soluble drugs can move across lipid membranes and therefore can access targets more readily)
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4
Q

what are the sites of biotransformation

A
  • GI, lungs, skin, kidney, brain and LIVER
  • liver is main metabolizing organ due to vascular architecture
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5
Q

describe the phase 1 of biotransforamtion

A
  • generate a more polar molecule by EXPOSING a functional group on parent compound

–> rate limiting step is mediated by cytochrome P450s (CYP) enzymes in the NEDOPLASMI RETICULUM of cells

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6
Q

describe the phase 2 of biotransformation

A
  • Yeilds a more WATER SOLUBLE conjugated product

–> transported to bile cuniculi –> bile duct –> excreted with feces

–> transported back into the blood to be excreted by kidneys

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7
Q

describe the first-pass effect

A
  • Large percentage of oral drug absorbed by the stomach or intestine will be initially carried to the LIVER via portal vein

–> most of the orally administered drug will be exposed to metabolizing enzymes in the liver before distributing to the entire body.

–> results in LARGE amount of drug inactivation prior to reaching the inteded target

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8
Q
A
  • C = concentration of drug in blood
  • V = volume of distribution
  • The larger the volume of distribution, the GREATER the extent to which the drug distributes to extravascular tissue (e.g. FAT)
  • V is an apparent volume and can exceed actual body volume
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9
Q
A
  • Clearance = rate of elimination in relation to drug concentration
  • Clearance is additive
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10
Q

define zero-order elimination

A
  • Specific amount of drug is eliminated over a period of time
  • Elimination process is AT CAPACITY or SATURATED. Independent of drug conc.
  • 100mg –2hr–> 80mg –2hr–> 60mg –2hr–> 40mg etc (amount of drug is constant)
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11
Q

define first-order elimination

A
  • Clearance is constant over the range of drug concentration in the body
  • The FRACTION of drug eliminated is constant and not dependent upon the drug conc
  • 65mg –2hr–> 32.5mg –2hr–> 16.25mg etc
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12
Q

Define Capacity-limited elimination

A
  • Concentration of a drug has SATURATED the elimination capacity or capabilities of our system
  • -> MIMICS ZERO-ORDER KINETICS
  • -> If dosing rate is greater than the elimination rate, the drug concentration will CONTINUE TO INCREASE in the blood (lead to TOXIC EFFECTS)
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13
Q

Define Flow-dependent elimination

A
  • Organ has HIGH capacity or is NOT SATURATED by the drug concentration
  • -> Kinetic would mimic FIRST-ORDER
  • -> Limiting factor for elimination of a drug is the blood flow to the metabolizing organ (Greater the blood flow the greater the extend of elimination)
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14
Q
A

–> Half-life is dependent upon the volume of distribution and clearance when considering a drug eliminated by first-order kinetics

–> Half-life is variable if the drug is eliminated by zero-order kinetics

  • As volume increases, so does the half life
  • As CL increases, the half-life decreases

–> At about 4-5 half-life, we consider 1 dose of the drug to be gone

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15
Q
A
  • Accumulation of drugs in the body will be detectable if the dosing interval is shorter than four half-lives
  • Accumulation is inversely proportional to the fraction of drug lost with each dosing interval
  • -> If a drug is given once every half life: AF = 1/0.5 = 2 (drug will accumulate to a concentration twice as much as was seen following the first dose)
  • -> If a drug is given once eveyr 3 half lifes: AF = 1/.875 = 1.143
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16
Q

describe bioavailability

A
  • Bioavailability = amount of drug that reaches the systemic circulation

–> what happens if hepatic blood flow decreases?

–> what if absorption of gut increaes?

17
Q

define therapeutic window

A

–> The range between concentration of drug that produces desired response and concentration that will produce adverse effects = THERAPEUTIC WINDOW

–> MEC = minimum concentration for the toxic or unwanted drug effect

18
Q
A
  • Steady state concentration (Css) = the point during a dosing regimenw hen elimination of a drug is equal to the bioavailability of the drug

–> Only obtained after 4-5 half-lives (dependent upon the half-life)

  • If CL of a drug decreases then the Css will increase; CL is constant and the dosing rate increases then the Css will increase
19
Q

describe the maintenance dose

A
  • Maintenance dose = dose needed to maintain steady state conc.
    1) Dosing Ratess = CL x TC
    2) Dosing Rateoral = (Dosing Ratess)/(Foral)
    3) Maintenance Dose = dosing rate x Dosing interval
  • Increase dosing interval (decreasing frequency) –> decreases the amount of drug –> greater variation from peak to lowest conc
  • decreasing doseing interval (increasing frequency) –> less variation in the peaks and lowest conc.
20
Q

Describing loading dose

A
  • Loading dose = initial dose that can be given in order to achieve the target concentration rapidly
21
Q

describe the advantage/disadvantage of loading dose

A

Advantage:

  • -> Useful when time to steady state is long but need to rapidly reach TC
  • -> This can occur with drugs that have a long half-life

Disadvantage

  • -> Abrupt high concentrations may be toxic
  • -> Calculation takes into account final volume of distribution but drug is initially restricted to central compartment (blood)
  • -> Due to disadvantages, LD is typically divided into multiple smaller dose or infused over a period

Pharmacology (19 decks)

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