4: volume of distribution

Question 1

How do you calculate the absorbed dose?

Answer 1

F1S1Dose1 = absorbed drug

Question 2

How do you calculate administration rate?

Answer 2

Ra = DoseFS/tau
amount absorbed divided by time

tau = dosing interval (how often?)

ex: Ra = (2mg/kg)/4hrs

Question 3

Patient variability of high vs low bioavailability (which can be affected by lots of factors)?

Answer 3

High bioavailability usually doesnt have much patient variability

Low bioavailability usually can be a lot of patient variability (a lot of factors can contribute to low bioavailability

Question 4

What is salt factor? How do you calculate it?

Answer 4

Salt factor is the fraction/percentage of dosage form that is active drug.

S= (MW of the drug)/(MW of the drug salt)

Question 5

How do you calculate absorbed drug?

Answer 5

F (bioavailability) * S (salt factor) * Dose

!remember that absorbed dose DOESNT equal administered dose!

Question 6

define Ra?

Answer 6

average rate drug reaches systemic circulation (how fast does it take to reach system?)

Question 7

Drug A is dosed at 5mg/kg as a form of drug A.HCl (s=0.8) with F=0.5. If a patient needs to take drug A.phosphate (s=0.4) with F=0.8. What is the dose? What equation would you use?

Answer 7

F1S1dose1 = F2S2dose2
(.5)(.8)(5mg/kg) = (.8)(.4)(dose2)
dose2 = 6.25 mg/kg (dose given per weight of patient

Question 8

What is the total volume of water in the body?

Answer 8

60% of body weight

IF = 40% BW
EF = 20% BW

Question 9

How to solve for volume of distribution?

Answer 9

Vd = dose/Cp

Question 10

What is important to assume when calculating Vd or dose?

Answer 10

always assume that concentration everywhere is equal to Cp - plasma concentration

Question 11

What are factors that affect how fast the drug will distribute?

Answer 11

membrane permeability and blood perfusion (blood flow)

Question 12

What are factors that affect how much the drug will distribute in the body?

Answer 12

lipid solubility, pH-pKa (ionization), plasma protein binding (drugs will only stay in circulation -> low Vd), tissue protein binding (little Cp -> Vd is high)

Question 13

How do you calculate loading dose?

Answer 13

LD = Cp * Vd / (F*S)
(assume no excretion/clearance)

Question 14

If a patient has Cp at one level, but you want to increase the level, how will you determine incremental loading dose?

Answer 14

= V * (Cp desired - Cp current) / (S*F)