pharmacokinetics

Question 1

what is Vd?

Answer 1

apparent Volume of distribution in L

• volume of fluid a drug would need to be dissolved in to account for the dose to produce a measured plasma conc of the drug

Question 2

what is Co?

Answer 2

initial concentration of drug in plasma at time = 0 seconds

• corresponds to peak plasma conc

Question 3

what is t1/2?

Answer 3

half life in minutes/hours/days

• length of time is takes for the plasma concentration of a drug to half

Question 4

what is Cl?

Answer 4

clearance of plasma

• the volume of plasma that has been cleared of drug (removed from) per unit time

Question 5

what is D?

Answer 5

loading dose

• initial dose of a drug

Question 6

define dosing rate

Answer 6

the rate at which the dose of drug must be administered in order to acheive and maintain the desired plasma concentration of the drug

Question 7

define volume distribution

Answer 7

the distribution of a drug between the plasma and the rest of the body after dosing

Question 8

what is volume distribution dependent on?

Answer 8

drug permeability across membranes

binding within compartments (plasma protein binding, accumulation in tissues)

pH partition

Question 9

formula involving Vd, Co, and D

Answer 9

Vd = D / Co

Question 10

what determines the rate of reduction of drug plasma concentration?

Answer 10

metabolic reactions and clearance of the drug from plasma

Question 11

explain first order kinetics

Answer 11

greater dose/amount of drug present, the more metabolising enzymes recruited and the greater the rate at which the drug is removed

• curve becomes steeper with higher

Question 12

explain zero order pharmacokinetics

Answer 12

when all metabolishing enzymes become saturated, so rate of decrease will be linear

Question 13

define steady state

Answer 13

the equilibrium point where the amount of drug administered exactly replaces the amount of drug excreted

Question 14

how do you calculate steady state?

Answer 14

Question 15

what is a loading dose?

Answer 15

higher intial dose to rapidly achieve therapeutic range to inducd drug effects as quickly as possible

• increased risk of overshooting and therefore of drug risk effects

Question 16

formula to calc loading dose

Answer 16

loading dose = Vd x desired Css (steady state plasma conc)

Question 17

define maintenance dose

Answer 17

susequent doses needed to replace amount of drug lost through metabolism and excretion

Question 18

formula to calc maintenance dose

Answer 18

maintenance dose = clearance x desired Css

Question 19

explain the pharmacokinetics of repeated dosing, including time to ‘steady state’

Answer 19

• if a drug is administered via IV or continuous infusion then a steady state is readily achieved where plasma conc of drug can be effectively maintained within desired therapeutic range
• infrequent, high dosing can achieve desired plasma conc, but there are periods of inital overshoots that can lead to drug adverse effects and periods of undershooting that mean the drug is not producing its desired effect