Phenytoin ADME

Question 1

Phenytoin is assumed to have what bioavailability?

Answer 1

100% (F=1)

Question 2

After PO administration of phenytoin, what happens to the absorption and peak concentrations?

Answer 2

Peak concentrations are delayed and there’s slow absorption

Question 3

The time to achieve peak concentration is dependent on what?

Answer 3

The product and dose

Question 4

What formulation of phenytoin isn’t recommended due to patient discomfort?

Answer 4

IM injection

Question 5

What formulations absorb faster than phenytoin capsules

Answer 5

Suspension and chewable tablets

Question 6

Obese patients have a ? Vd while taking phenytoin.

Answer 6

Larger

Question 7

When bound concentrations of phenytoin decrease, what happens to the unbound concentration?

Answer 7

Remains relatively unchanged

Question 8

Is there a change in loading dose needed with altered plasma binding?

Answer 8

Nope

Question 9

What is phenytoin primarily bound to?

Answer 9

Albumin in the plasma

Question 10

Disease states that are commonly associated with altered phenytoin plasma protein binding

Answer 10

Hypoalbuminemia, ESRD

Question 11

Phenytoin is primarily cleared by what form of metabolism?

Answer 11

Hepatic metabolism

Question 12

Enzyme that metabolizes ~90% of phenytoin

Answer 12

CYP2C9

Question 13

Enzyme that metabolizes ~10% of phenytoin

Answer 13

CYP2C19

Question 14

Phenytoin follows what form of PK?

Answer 14

Michaelis-Menten/saturable PK

Question 15

Steady state serum concentrations of phenytoin do what after a dosage increase?

Answer 15

Increase in a disproportionate manner

Question 16

Is Cssav of phenytoin proportional to the maintenance dose?

Answer 16

No

Question 17

Is the time to Css 3-5 half-lives for phenytoin?

Answer 17

No

Question 18

Elimination of phenytoin is limited to what?

Answer 18

Capacity limited

Question 19

Phenytoin CL decreases with what?

Answer 19

Increasing concentrations

Question 20

When the Css of phenytoin becomes greater than Km, what happens?

Answer 20

MD approaches Vmax and a small change in either the maintenance dose or Vmax can result in disproportionate changes in the new steady state concentration

Question 21

Phenytoin in pregnancy

Answer 21

Concentrations tend to fall

Question 22

Reduced GI motility in pregnancy does what to phenytoin absorption?

Answer 22

Enhance it

Question 23

Phenytoin in liver and renal disease

Answer 23

Binding of phenytoin to serum proteins is reduced

Question 24

What happens to the time to steady-state with an increase in the dose?

Answer 24

The time to Css increases

Question 25

True or false: Css increases more than proportionally with an increase in phenytoin dose

Answer 25

True

Question 26

True or false: different Vmax and Km values can provide the same steady state concentrations of phenytoin

Answer 26

True