JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Pharmacy II PHYSICAL PHARMACEUTICS-II Previous Question Paper (November/December-2020 - R17)

Chemical degradation of pharmaceutical products is influenced by various physical and chemical factors that affect the stability of the active pharmaceutical ingredient (API) and excipients. These factors must be controlled to ensure product shelf-life and efficacy.

Physical Factors

1. Temperature: According to the Arrhenius equation, the rate of chemical degradation increases significantly with temperature. Higher temperatures provide the activation energy required for degradation reactions such as hydrolysis or oxidation.
2. Light (Photolysis): Exposure to ultraviolet or visible light can trigger photochemical degradation. Molecules with chromophores absorb light energy, leading to free radical formation or molecular cleavage.
3. Moisture/Humidity: Water acts as a reactant in hydrolysis and as a medium for other degradation pathways. High humidity can also affect the physical state of solid dosage forms, promoting chemical instability.
4. Physical State: Drugs in solution are generally more susceptible to degradation than those in solid dosage forms due to increased molecular mobility and collision frequency.

Chemical Factors

1. pH: The pH of the medium is critical for drugs prone to acid-base catalysis. Most drugs exhibit a specific pH range of maximum stability; deviation from this range often leads to rapid hydrolysis or oxidation.
2. Oxidation: Many drugs are susceptible to atmospheric oxygen. This process is often catalyzed by trace metal ions (e.g., Fe, Cu), light, or heat, leading to the formation of peroxides or free radicals.
3. Solvent Effects: The dielectric constant and polarity of the solvent influence the reaction rate. Solvents can stabilize or destabilize transition states, thereby affecting the rate of degradation.
4. Ionic Strength: Changes in ionic strength can alter the activity coefficients of reacting species, potentially accelerating or retarding degradation reactions depending on the charge of the reactants.
5. Excipient Interactions: Incompatibility between the API and excipients (e.g., binders, lubricants, or antioxidants) can lead to chemical degradation. For example, the presence of trace impurities in excipients can catalyze drug decomposition.

Hydrolysis and oxidation are the primary pathways for chemical degradation of pharmaceutical substances, leading to loss of potency and potential toxicity.

1. Hydrolysis

Hydrolysis involves the cleavage of chemical bonds by reaction with water. It is the most common degradation pathway for drugs containing functional groups such as esters, amides, lactams, and imides.

Reaction: For an ester, the reaction is: R-COOR' + H₂O → R-COOH + R'-OH. For example, the hydrolysis of aspirin (acetylsalicylic acid) produces salicylic acid and acetic acid.

Effect: It results in the loss of therapeutic activity, change in physical appearance (e.g., precipitation), and potential formation of toxic degradation products.

Prevention: Strategies include:

• Formulating in non-aqueous vehicles (e.g., oils, propylene glycol).
• Adjusting pH to the range of maximum stability.
• Reducing moisture content by using desiccants or moisture-proof packaging (e.g., blister packs).
• Solid-state formulation (e.g., tablets instead of solutions).

2. Oxidation

Oxidation involves the loss of electrons or hydrogen, or the addition of oxygen. It often proceeds via a free-radical chain mechanism initiated by light, heat, or metal ions.

Reaction: The process typically occurs in three stages: initiation, propagation, and termination. For example, the oxidation of ascorbic acid (Vitamin C) involves the loss of two hydrogen atoms to form dehydroascorbic acid.

Effect: Oxidation leads to discoloration (yellowing or browning), development of rancidity in fats/oils, loss of pharmacological activity, and potential formation of harmful free radicals.

Prevention: Strategies include:

• Addition of antioxidants (e.g., BHA, BHT, ascorbic acid, or sodium metabisulfite) to act as free-radical scavengers.
• Displacement of atmospheric oxygen with inert gases like nitrogen or carbon dioxide during manufacturing and packaging.
• Storage in light-resistant (amber) containers to prevent photo-oxidation.
• Chelation of trace metal ions (which catalyze oxidation) using agents like EDTA.
• Storage at lower temperatures to slow down the reaction rate.