Drug Excretion, Clearance and Plasma Half-Life for RRB, KGMU, KSSSCI, MPESB Pharmacist Exams / GPAT / DPEE / Drugs Inspector Exams / D.Pharm, B.Pharm, and Pharm.D.
o Drug excretion is the irreversible removal of drugs and/or their metabolites from the body by all possible routes of excretion.
o The excretion of drugs via kidneys is known as renal excretion. Excretion via all other possible routes is known as non-renal excretion of drugs.
o Depending upon the drugs pKa, changes in urine pH will change the ratio of ionised to unionised drug, affecting the rate of drug excretion. For example alkalinization of urine by the administration of Sodium bicarbonate will increase the excretion of salicylates (weak acids).
o The cycle in which the drug is absorbed, secreted in bile and reabsorbed is known as enterohepatic cycling (Fig.).
o Clearance is the hypothetical volume of body fluids containing drug from which the drug is removed or cleared completely in a specific period of time. Thus clearance is a measurement of drug elimination from the body.
o Elimination of drug from the body involves two processes, biotransformation or drug metabolism and drug excretion.
o Clearance is the second primary pharmacokinetic parameter. It is represented as Cl. Its unit is ml/min or liters/hour. It is constant for any given plasma drug concentration.
o Secondary pharmacokinetic parameters are derived from primary pharmacokinetic parameters. These are half-life (t½), bioavailability, AUC, Cmax, etc.
o Elimination half-life of drug i.e. time required to decrease plasma drug concentration by half, can be obtained as, t1/2 = 0.693/ K Where, K is Elimination rate constant and t1/2 is Elimination half-life. Unit of Elimination half-life is hours or minutes.
o Half-life is a hybrid/secondary pharmacokinetic parameter and is determined by the apparent volume of distribution (V) and clearance (Cl).
o t1/2 (min) = (0.693 X V (mL/kg)) / Cl (mL/min/kg)
o Half-life is directly proportional to V and inversely proportional to Cl.
MCQs
1. Total body clearance is
(a) the drug elimination rate divided by the plasma drug concentration
(b) the drug elimination rate divided by the Vd
(c) the amount of drug in body divided by the plasma drug concentration
(d) None of the above
2. Renal excretion of drug depend on
(a) urine flow
(b) pH of urine
(c) Physicochemical properties of drug
(d) All of above
3. Elimination is expressed as follows:
(a) Rate of renal tubular reabsorption
(b) Clearance speed of some volume of blood from substance
(c) Time required to decrease the amount of drug in plasma by one-half
(d) Clearance of an organism from a xenobiotic
4. Below mentioned all factors increases plasma elimination half life except
(a) Higher protein binding
(b) Higher tissue binding
(c) Higher apparent Vd
(d) Higher renal/ hepatic clearance
5. The biological half-life of drug:
(a) is a constant physical property of the drug
(b) is a constant chemical property of the drug
(c) may be increased in patients with impaired renal failure
(d) may be decreased in patients by giving the drug by rapid I.V. injection
6. The half life of a drug eliminated by first order elimination kinetics will be longer in individuals who have an:
(a) increased volume of distribution or increased clearance
(b) increased volume of distribution or decreased clearance
(c) decreased volume of distribution or increased clearance
(d) decreased volume of distribution or decreased clearance
7. Half life (t½) is the time required to:
(a) change the amount of a drug in plasma by half during elimination
(b) metabolize a half of an introduced drug into the active metabolite
(c) absorb a half of an introduced drug
(d) bind a half of an introduced drug to plasma proteins
8. Elimination rate constant (Kelim) is defined by the following parameter:
(a) rate of absorption
(b) maximal concentration of a substance in plasma
(c) highest single dose
(d) half life (t½)
9. Systemic clearance (Cl) is related with:
(a) only the concentration of substances in plasma
(b) only the elimination rate constant
(c) volume of distribution, half life and elimination rate constant
(d) bioavailability and half life
10. Biological Half life equation for first order process is
(a) t1/2 = a/2K
(b) t½ = 0.693/K
(c) t½ = 1/aK
(d) t½ = 3/2 K
Check your answers below:
Recommended book for the preparation of GPAT and Drugs Inspector Exams, "ESSENTIAL PHARMACY REVIEW FOR DRUGS INSPECTOR EXAMS", NIRALI PRAKASHAN, PUNE
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Recommended book for the preparation of RRB Pharmacist Exam, "PHARMACIST RECRUITMENT EXAM", NIRALI PRAKASHAN, PUNE
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Check your Answers:
1. (a)
2. (d)
3. (d)
4. (d)
5. (c)
6. (b)
7. (a)
8. (d)
9. (c)
10. (b)
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