Equations

Question 1

How does the elimination rate (k) relate to both clearance and drug concentration?

Answer 1

K = Cl * C

Question 2

How does clearance relate to volume of distribution and elimination rate?

Answer 2

Cl = V * k

Question 3

How the concentration of unbound drug relate to the free fraction and the overall drug concentration?

Answer 3

C(unbound) = free fraction (alpha) * C (overall drug concentration)

Question 4

How does AUC(free) relate to the free fraction of drug and AUC?

Answer 4

AUC(free) = free fraction (alpha) * AUC

Question 5

How is the steady state concentration (Css) related to the fraction absorbed (F), the total dose (D), the clearance of the drug (Cl) and the interval of administration (tau)?

Answer 5

Css = (FD)/(Cltau)
The steady state concentration is equivalent to the fraction absorbed x total drug dose divided by the clearance x the interval of administration.

Question 6

How is AUC related to the fraction of drug absorbed (F), the total drug dose (D), the elimination rate constant (k) and the volume of distribution (V)?

Answer 6

AUC = (F * D)/ (K * V)

Or simplified further:
AUC = (F * D)/ Cl

As Cl = K * V

Question 7

How can the half-life of a drug be worked out if the only information we have is the elimination rate constant (K)?

Answer 7

T1/2 = 0.693/k

Question 8

What is the ‘rate in’?

Answer 8

Rate in = (F * D)/Tau.

Rate out = Cl * Css

Question 9

What is the ‘rate out’?

Answer 9

Rate out = Cl * Css

Rate in = (F * D)/ Tau

Question 10

How is V calculated if the only other variables we know are the amount of the drug in the body and the blood plasma concentrations of the drug?

Answer 10

V = (amount of drug in body)/C plasma.

Question 11

How is the C(free fraction) worked out if we know the value of the free fraction (alpha) and the total concentration (Ctotal)?

Answer 11

Cfree = alpha * Ctotal.

Question 12

How is the F(absolute) calculated?

Answer 12

F(absolute) = [AUC(test)/D(test)] / [AUC(reference) / D(reference)]

Question 13

How is the F(relative) calculated?

Answer 13

F(relative) = [AUC(test)/D(test)] / [AUC(reference)/D(reference)]

Question 14

How is the rate of presentation of a drug to the organ of excretion calculated?

Answer 14

Rate of presentation = QH * Concentration of drug entering organ.

Question 15

How does the rate of elimination relate to:
QH (bloodflow)
C(entering organ)
C(leaving organ)

Answer 15

Rate of Elimination = QH * [C(entering) - C(leaving)]

Question 16

How does the Cl (hepatic) relate to the elimination rate and the C(entering)?

Answer 16

Cl(hepatic) = Rate of elimination/C(entering)

Question 17

How can the extraction ratio (EH) of a drug be calculated if we know:
The free fraction in blood (alpha blood)
The intrinsic clearance (Cl intrinsic)
The organ blood flow (QH)

Answer 17

EH = alpha blood * Cl (intrinsic) / QH + (alpha blood * Cl (intrinsic)

Question 18

How does the maximum BA relate to the EH of drug?

Answer 18

BA = 1 - EH.

Question 19

How is EH calculated?

Answer 19

EH = alpha blood * Cl (intrinsic) / QH + (alpha blood * Cl (intrinsic)

Question 20

How does Cl relate to AUC and D?

Answer 20

Cl = D/AUC

Question 21

Css

Answer 21

Css = DF/Clt

Question 22

Cl

Answer 22

Cl = D/AUC
Cl = K * V

Question 23

What is the equation to work out the terminal half-life?

Answer 23

-k = ln(C2) - ln(C1) / t2 - t1

then using t1/2 = 0.693/k