Pharmacokinetics Questions
1. A drug achieves a plasma concentration of 16 mg/L immediately





after the administration of the first dose. If the half life and the dosing
interval are both 6 hrs, what is the approximate plasma level shortly
before the administration of the 5th dose?
15 mg/L
24 mg /L
28 mg /L
30 mg /L
31 mg/L
Pharmacokinetics Questions 1) Answer





After first dose ---- plasma conc is 16mg/L, it will reduce to half of it
in 6 hrs --- 8mg/L
After second dose --- plasma conc is 8+16 = 24 mg/L, it will reduce
to half in 6 hrs –12 mg/L,
After 3rd dose --- plasma conc is 12+16 = 28mg/L, it will reduce to
half in 6 hrs -14mg/L
After 4th dose – plasma conc is 14 +16 = 30mg/L, it will reduce to
half in 6hrs – 15 mg/L
Therefore before administration of fifth dose, plasma conc is around
15 mg/L
Pharmacokinetics Questions 1) Answer




The drug concentration before the administration of 5th
dose is same as the concentration after 4 half lives.
The drug reaches 94% of the steady state after 4 half lives.
The drug achieves 50% of the steady state after 1 half life
~ is 8 mg/L.
The drug concentration after 4 half lives – 94% =
= 94 * 8 / 50 = 15.04 mg/L
Pharmacokinetics Questions
2. A patient was given a 160 mg dose of the drug IV, and 80 mg
was eliminated during the first 120 minutes. If the drug
follows the first order kinetics, how much of the drug will
remain 6 hrs after its administration?
 5 mg
 10 mg
 20 mg
 40 mg
 60 mg
Pharmacokinetics Questions 2) Answer



After 2 hrs = 120 mins, drug has reduce to half from
160 mg to 80mg,
After 4 hrs, the drug will reduce from 80 to 40mg.
Finally after 6 hrs, the drug will reduce from 40 to
20 mg.
Pharmacokinetics Questions
3. An IV infusion of a drug is started at 400mg/hour. If the
clearance = 50L / hour. What is the anticipated plasma level
at steady state?
 2 mg/L
 4 mg/L
 8 mg/L
 16 mg/L
 32 mg/L
Pharmacokinetics Questions 3) Answer



Infusion rate = CL X Css
Css = Infusion rate / CL
Css = 400 mg/hour / 50L/hr = 8 mg/L
Pharmacokinetics Questions
4. Piroxicam with a half life of 40 hr being dosed once daily
(every 24 hrs), steady state will be reached shortly following
which dose (not half life)?
 1st dose
 2nd dose
 5th dose
 8th dose
 20th dose
Pharmacokinetics Questions 4) Answer




Half life = 40hrs
Dose admin rate = 24 hrly
Drug reaches steady state in 5 half lives = 5x 40 =
200 hrs.
Therefore, 200 hrs are reached after = 200 / 24 = ~
8th dose
Pharmacokinetics Questions
5. At 12 hr after the IV administration of the bolus dose, the
plasma level of the drug is 3 mg/L.
If the Vd= 10L and the elimination half life= 6 hrs.
What is the dose administered?
 120mg
 180mg
 240mg
 480mg
 600mg
Pharmacokinetics Questions 5) Answer







Half life = 6 hrs
Vd = 10L
Loading dose = Vd x Plasma conc at zero hour –
immediately after IV
Plasma conc after 12 hrs = 3 mg/L, half life = 6hrs ;
Plasma conc at 6 hrs = 6 mg/L
Plasma conc at zero hour = 12 mg/L
Loading dose = 10L x 12mg/L = 120mg.
Pharmacokinetics Questions
6. Mr. John is admitted to the general hospital with pneumonia due to gram
negative bacteria. The antibiotic Tobramycin was ordered. The CL and Vd
of Tobramycin in Mr John are 80mL/min and 40L respectively.
i).What is the maintenance dose that must be administered I.V at every 6 hrs
to eventually obtain average steady state of 4mg/L?





100 mg
115 mg
175 mg
200 mg
250 mg
Pharmacokinetics Questions
6 ii).If you wish to give Mr. John a loading dose to
achieve the therapeutic plasma concentration of
6mg/L, How much should be given?





100 mg
115 mg
175 mg
200 mg
240 mg
PK Questions
6 i). Steady state required of 4mg/L
 Vd
= 40L
 Cl
= 80ml/ min
 CL = Rate of elimination

---------------------------
Plasma concentration
PK Questions 6 i) Answer







At steady state,
Rate of elimination = Rate of infusion
Rate of infusion = CL x CSS
Rate of infusion = 80ml/min x 4 mg/L
= 0.080 L/min X 4mg/L
= 0.32 mg/min
= 0.32 X 60 mg/hr
= 19.2mg/hr
PK Questions 6 i) Answer


Rate of infusion = 19.2 mg /hr
Maintenance dose every 6 hr
= 19.2 x 6 = 115.2 gm
PK Questions 6 ii) Answer
6 ii). Loading dose = Vd x Plasma concentration at
zero time
 Vd = 40L
 Therapeutic concentration required = 6mg/L
 Loading dose = 40 L x 6mg/L
 Loading dose = 240 mg
PK Questions
7. Despite your careful adherence to basic pharmacokinetic principles,
your patient on digoxin therapy has developed digoxin toxicity. The
plasma digoxin level is 4ng/L. Renal function is normal and the
plasma half life is 1.6 days. How long should you withhold digoxin to
reach a safe therapeutic plasma conc of 1ng/L?
 1.6 days
 3.2 days
 2.4 days
 4.8 days
 9.6 days
PK Question 7) Answer

Since the blood level drops by 50% during each half
life (1.6 days) the plasma level will be 2 ng/L after 1.6
days and 1 ng/L after 3.2 days.
PK Questions
8. A 47-yr-old patient with myocardial infarction had severe cardiac arrhythmia.
You have decided to give lidocaine to correct arrhythmia. A continuous
infusion of lidocaine 1.92 mg/min is started at 8.A.M. The PK parameters of
lidocaine are
Vd = 77L, CL = 640 ml/min, half life = 1.8 hours.
The expected steady state plasma concentration is approximately
 40 mg/L
 3.0 mg/L
 7.2 mg/L
 8.46 mg/L
 10.20 mg/L
PK Question 8) Answer








Infusion rate = 1.92 mg/min
Vd = 77L
CL = 640ml/min
Half life = 1.8 hrs
CL = Elimination rate
------------------Plasma conc
At steady state, Infusion rate = Elimination rate
PK Question 8) Answer

Infusion rate = CL x CSS

Css = Infusion rate
-----------------CL
PK Question 8) Answer






Css =
1.92 mg/min
-----------------640 ml/min
Plasma concentration at steady state
= 1.92 mg
-------------640 ml
PK Question 8) Answer


Plasma conc = 1.92 mg
--------------------0.64 L
Css = 3mg/L
PK Questions
9. A 56-yr-old patient of CAD requires an infusion of procainamide. Its
half life is 2hrs. The infusion is begun at 9.00 a.m. At 1.00 p.m the
same day, a blood sample is taken; the drug concentration is found
to be 3 mg/L. What is the probable steady state concentration?
 3 mg/L
 4 mg/L
 6 mg/L
 10 mg/L
 15 mg/L
PK Q 9) Answer

A patient requires an infusion of procainamide. Its half life is 2hrs.

The infusion is begun at 9.00 a.m and at 1.00 p.m the same day, a
blood sample is taken; the drug concentration is found to be 3 mg/L.

The plasma concentration after 4 hrs (i.e., after 2 half lives) is
3 mg/L.
The drug reaches 75% of the steady concentration after 2 half lives.
The steady state concentration is 100 X 3 mg/ L / 75 = 4 mg /L


PK Qs
10. A young man is brought to the emergency room in a deep coma. His friends state
that he self administered a large dose of morphine 6 hrs earlier. The immediate
blood analysis in the hospital shows a morphine plasma level of 0.25mg/L.
Assuming that the PK of morphine in this patient are Vd 200L and half life is 3 hrs,
how much morphine did the patient inject 6h earlier?
 A.25 mg
 B. 50 mg.
 C. 100 mg.
 D. 200 mg
 E. Incomplete data
PK Question 10) Answer







Large dose of morphine injected 6 hrs before.
The immediate blood analysis shows a morphine plasma
level of 0.25 mg/L ( i.e after 6 hrs.)
Kinetics of morphine, Vd = 200L and half life = 3 hrs.
Plasma concentration after 3 hrs could be = 0.50 mg/ L
Plasma concentration at zero hour or immediately after
injection of morphine = 1.00mg/L.
Loading dose = Vd X plasma conc at zero hour
Loading dose = 1.00 mg/L X 200 L = 200 mg
PK Q
11. The following are pharmacokinetic data for Drug X: Clearance,
20 L/hr in a 70 kg adult; Effective plasma concentration, 5 mg/L;
Oral availability 50% or 0.5. Calculate the oral maintenance dosing
rate for Drug X in a 70 kg person.
 A. 10 ug/hr
 B. 200 mg/hr
 C. 1 mg/hr
 D. 4 mg/hr
 E. 50 mg/hr
PK Q 11) Answer

Clearance = 20 L/hr

Effective plasma concentration, 5 mg/L
Oral availability 50% or 0.5
Rate of infusion (intravenous) = CL x Css
Maintenance dose (oral) = CL X Css / F
Maintenance dose = 20L/hr X 5 mg/L /0.5
Maintenance dose = 100 mg / 0.5 hr
Maintenance dose = 200 mg/hr






CL =
Elimination rate
-----------------------------Plasma concentration
Practice questions
Q12. A pharmacologist is determining the PK parameters of
a novel antibiotic in order to determine the proper dosage.
The drug is a weak acid with a pKa of 5.0. Assuming a
pH of 8.0 in the urine, approximately what percent of the
drug will be in a form that can be rapidly excreted?
 0.1
 1
 90
 99
 99.9
PHARMACOKINETICS
Practice questions




10
Drug is a weakly acid
pKa of acidic drug is 5.0
pH of the surroundings 8.0 in the urine
pH-pKa = 3
{ ionized}
pH-pKa
= 10 3 ---------------------- for an Acidic drug
{ non-ionized }
1000
{ ionized }
---------- = ------------------ for an Acidic drug
1
{ non-ionized }
1000 is ionized and 1 is non-ionized; in % 99.9% is ionized and water soluble
and excreted in urine.