Phenytoin

Question 1

Phenytoin use

MOA

Answer 1

prevention of tonic-clonic seizures, focal seizures, NOT adsence seizures
MOA: slows impulses in the brain that cause seizures

Question 2

pheny dosage forms

Answer 2

phenytoin sodium
phenytoin acid
fosphenytoin

Question 3

phenytoin sodium

1. difference from regular phenytoin
2. solubility in water
3. ___ and ___ are added to increase dissolution
4. S =
5. contained in _____

Answer 3

1. 92% phenytoin by weight
2. insoluble
3. ethanol and propylene glycol
4. S = 0.92 bc its specific activity is 92% of regular phenytoin
5. capsules!

Question 4

max infusion rate of pheny sodium

recommended concentration in NS

Answer 4

50mg/min
usual is 1/2-1/4 of the recommended max

10mg/mL

Question 5

fosphenytoin info
max infusion rate
half life
reaches peak [  ] in \_\_\_ hr
prescribed in \_\_\_\_\_\_\_\_\_\_
storage
can be prepared in \_\_\_\_\_\_\_
price?
S =

Answer 5

water-sol, phosphate ester prodrug of phenytoin
150mg/min
t 1/2= 15 min 
peak [  ] at 1 hour
prescribed in phenytoin sodium equivalents (PE)
refrigerate
dextrose or NS
expensive

Question 6

Dilantin ER capsules of 400, 800, and 1600 mg reach peak concentrations at ____

Answer 6

8, 13 and 30 hours

increase dose, increase time to ss

Question 7

can phenytoin dosage forms be given IM

Answer 7

NO, they will precipitate in muscle

Question 8

valproic acid concentrations exceeding _______ displaces phenytoin from albumin

Answer 8

30-40mg/L

Question 9

phenytoin follows _____ pk

ss drug [ ] ________ in a proportionate/disproportionate? manner after a dosage increase

Answer 9

Michaelis-Menten or saturable
(non linear pk where the # of drug molecules saturates the enzyme’s ability to metabolize the drug)

ss drug [ ] INCREASE in a DIS-proportionate manner after a dosage increase

Question 10

what is the expected max decline in pheny concentrations for the average patient with phenytoin concentrations >15mg/L

Answer 10

~10mg/L/day

Question 11

phenytoin therapeutic range

Answer 11

10-20mg/L total (unbound + bound)

fup = 0.1 so unbound concentration is 1-2mg/L

Question 12

serum phenytoin cencentrations are ______ during pregnancy due to _______

Answer 12

lower

due to enhanced hepatic metabolism

Question 13

uremic patients and liver disease patients have _______ and _______

Answer 13

reduced binding to serum proteins and impaired metabolism accordingly

Question 14

phenytoin Vd

Answer 14

0.65L/kg

Question 15

If a patient’s Albumin is lower than ____ then we need to adjust the measured Cp for altered plasma protein binding

Answer 15

4.4

Question 16

if given two Cp values from 2 regimens in the same patient, which dose and Cp do we use when calculating Vmax

Answer 16

the higher dose and its Cp