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

Reaction kinetics describes the rate at which chemical reactions proceed and the factors affecting these rates. In pharmaceutical systems, the order of reaction is determined by how the concentration of reactants influences the rate of degradation.

1. Zero-order kinetics: The rate of reaction is independent of the concentration of the reactant. The rate equation is dC/dt = -k₀. A plot of concentration versus time yields a straight line with a slope of -k₀. Example: Degradation of drugs in suspensions where the concentration in solution remains constant as the drug dissolves from the solid phase.
2. First-order kinetics: The rate of reaction is directly proportional to the concentration of the reactant. The rate equation is dC/dt = -kC, which integrates to log C = log C₀ - (kt/2.303). A plot of log concentration versus time is linear. Most drug degradation processes follow first-order kinetics.
3. Second-order kinetics: The rate of reaction is proportional to the product of the concentrations of two reactants or the square of the concentration of one reactant. The rate equation is dC/dt = -kC².
4. Pseudo-first-order kinetics: This occurs when a second-order reaction is made to behave like a first-order reaction because one reactant is present in such large excess that its concentration remains effectively constant throughout the reaction.

Drug stability is influenced by various environmental and chemical factors that can accelerate the degradation of active pharmaceutical ingredients. Key factors include:

• Temperature: According to the Arrhenius equation, the rate of chemical degradation increases significantly with temperature. Higher temperatures provide more kinetic energy, increasing the frequency of molecular collisions.
• pH: Many drugs are susceptible to acid-base catalysis. The rate of hydrolysis or oxidation is often highly dependent on the pH of the solution, with specific pH values often representing the point of maximum stability.
• Light (Photolysis): Exposure to ultraviolet or visible light can trigger photochemical reactions, leading to the breakdown of light-sensitive drugs.
• Oxygen (Oxidation): Atmospheric oxygen can cause oxidative degradation, especially in drugs containing phenolic groups, unsaturated bonds, or aldehydes. This is often catalyzed by metal ions.
• Moisture (Hydrolysis): Water is a common cause of degradation for esters, amides, and lactams. Moisture in solid dosage forms or the solvent in liquid formulations can facilitate hydrolytic cleavage.
• Microbial Contamination: Microorganisms can metabolize drug components or excipients, producing enzymes or acidic/alkaline byproducts that destabilize the formulation.
• Packaging Materials: Interactions between the drug and the container (e.g., leaching of plasticizers or adsorption of the drug onto the container walls) can affect stability.