JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Pharmacy II Year PHARMACOLOGY-I Previous Question Paper (August/September - 2022 - R17)

Pharmacokinetics is the study of the time course of drug absorption, distribution, metabolism, and excretion (ADME). It describes what the body does to the drug, focusing on the relationship between drug concentration in the body and time.

The order of reactions describes the kinetics of drug elimination and metabolism, determining how the rate of change in drug concentration depends on the concentration itself.

1. Zero-Order Kinetics (Linear Elimination): The rate of elimination is constant and independent of the drug concentration in the plasma. This occurs when elimination mechanisms (enzymes or carriers) are saturated. The rate equation is:

Where C is concentration, t is time, and k₀ is the zero-order rate constant. A plot of concentration versus time is a straight line. Examples include high doses of ethanol, phenytoin, and salicylates.

2. First-Order Kinetics (Non-linear Elimination): The rate of elimination is directly proportional to the drug concentration. Most drugs follow this pattern at therapeutic doses. The rate equation is:

Where k is the first-order elimination rate constant. A plot of the logarithm of concentration versus time is a straight line. The drug has a constant half-life, meaning a fixed percentage of the drug is eliminated per unit of time.

3. Mixed-Order Kinetics (Michaelis-Menten Kinetics): Many drugs exhibit mixed-order kinetics, where the process follows first-order kinetics at low concentrations and shifts toward zero-order kinetics as the concentration increases and saturates the elimination pathway. The rate is described by:

Where Vmax is the maximum rate of elimination and Km is the Michaelis constant (the concentration at which the rate is half of Vmax).