Drug Stability & Degradation

Drug stability refers to a pharmaceutical product's ability to maintain its intended chemical, physical, microbiological, toxicological, and therapeutic properties within specified limits throughout the period of storage and use. Understanding degradation pathways enables pharmacists to advise on storage, counsel patients about expired medicines, and recognise products that may have lost efficacy or become toxic.

Major Chemical Degradation Pathways

1. Hydrolysis

Hydrolysis is the most common chemical degradation mechanism for pharmaceutical compounds. It involves the cleavage of a chemical bond by water. Susceptible functional groups include esters, amides, lactones, lactams, and carbonates.

Ester hydrolysis is acid- and base-catalysed. The rate depends on pH, temperature, and the steric and electronic properties of the ester. Classic examples:

• *Aspirin (acetylsalicylic acid)* hydrolyses to salicylic acid and acetic acid — the vinegar odour in an old aspirin bottle signals significant degradation.
• *Procaine* hydrolyses to PABA and diethylaminoethanol.
• *Atropine* hydrolyses to tropic acid and tropanol.

Amide hydrolysis is generally slower than ester hydrolysis due to the partial double-bond character of the C–N bond. Examples: chloramphenicol, penicillins (beta-lactam ring hydrolysis).

Beta-lactam antibiotics are particularly susceptible to ring-opening hydrolysis. Penicillin G is inactivated rapidly at pH < 2 (gastric acid) and pH > 8. This dictates the need for buffered oral formulations and explains why amoxicillin tablets have a relatively short reconstituted suspension shelf-life (typically 7 days refrigerated).

Mitigation of hydrolysis: Use of anhydrous dosage forms (dry powder inhalers, lyophilised injectables), pH optimisation in solution formulations, use of co-solvents to reduce water activity, moisture-barrier packaging.

2. Oxidation

Oxidative degradation is the second most common degradation pathway. It involves loss of electrons from the drug molecule, typically mediated by reactive oxygen species (ROS: superoxide, hydroxyl radical, singlet oxygen) or direct reaction with molecular oxygen.

Auto-oxidation proceeds via a free radical chain mechanism:

• Initiation: RH → R• + H• (catalysed by trace metal ions, UV light)
• Propagation: R• + O₂ → ROO•; ROO• + RH → ROOH + R•
• Termination: R• + R• → R–R (non-radical product)

Drugs susceptible to oxidation include:

• Phenols: Morphine, catecholamines (adrenaline/epinephrine, dopamine) → quinone formation (darkening of solution, loss of potency)
• Thiols and thioethers: Captopril → disulfide dimer; omeprazole → sulfoxide and sulfone metabolites (also in vivo)
• Polyunsaturated fatty acids in lipid emulsions: Rancidification
• Vitamins: Ascorbic acid (Vit C) → dehydroascorbic acid; Vit A, D, E, K oxidation

Antioxidants as excipients: Sodium metabisulfite, BHA (butylhydroxyanisole), BHT, ascorbic acid, EDTA (chelates pro-oxidant metal ions), nitrogen overlay in vials.

3. Photodegradation

Photodegradation occurs when drug molecules absorb UV or visible light energy, causing excited-state reactions including bond cleavage, isomerisation, and oxidation.

Examples of photosensitive drugs:

• *Nifedipine*: Rapid degradation to nitroso derivative under UV light; amber or opaque packaging essential; IV bags must be covered with light-protective wrap.
• *Sodium nitroprusside*: Degrades to toxic cyanide; IV infusions must be wrapped in foil.
• *Furosemide (frusemide)*: Photodegrades to saluamine (pharmacologically inactive).
• *Riboflavin (Vitamin B2)*: Converts to lumiflavin and lumichrome.
• *Methotrexate*: Photo-decomposition products retain some cytotoxic activity.
• *Ciprofloxacin*: Degrades under UV irradiation.

Packaging solutions: Amber glass or plastic (filters UV < 470 nm); opaque white HDPE bottles; foil-overwrap for IV bags; light-protective IV giving sets.

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Arrhenius Equation and Shelf-Life Prediction

The Arrhenius equation describes the temperature-dependence of chemical reaction rates, including drug degradation:

k = A · e^(−Ea/RT)

Where:

• k = rate constant
• A = pre-exponential (frequency) factor
• Ea = activation energy (J/mol)
• R = universal gas constant (8.314 J/mol/K)
• T = temperature in Kelvin

Accelerated Stability Testing

The Q10 rule states that for every 10°C rise in temperature, the reaction rate approximately doubles (Q10 ≈ 2 for many pharmaceutical degradation reactions). This allows accelerated stability studies at elevated temperature (40°C/75% RH) to predict shelf life at room temperature (25°C) or refrigeration (5°C).

ICH Stability Guidelines (Q1A-Q1F)

The International Council for Harmonisation (ICH) Stability Guidelines specify conditions for formal stability testing:

• Long-term: 25°C/60% RH (12-month minimum for room temperature products)
• Intermediate: 30°C/65% RH
• Accelerated: 40°C/75% RH (6 months)
• Refrigerated products: 5°C/ambient RH long-term; 25°C accelerated

Shelf-Life Calculation

The expiry date on a pharmaceutical product represents the time point at which the product, stored under labelled conditions, retains at least 90% of its labelled potency and complies with all specification criteria. A first-order degradation model is most commonly applied:

C(t) = C₀ · e^(−kt)

Shelf-life (t90) where 10% degradation is acceptable:

t90 = ln(0.9) / (−k) ≈ 0.105 / k

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Expiry Dating and Its Interpretation

Expiry Date vs Use-By Date

In NZ, the Medicines Regulations 1984 and Medsafe guidelines require that solid oral dosage forms display an expiry date (month/year). The product should not be dispensed on or after the expiry date. For multi-dose containers, an in-use expiry (e.g., "use within 28 days of opening") supplements the manufacturer expiry date.

Pharmacist Responsibility

Pharmacists must not dispense products that have expired. Stock rotation (FIFO — first in, first out) and regular expiry date checks (monthly in most pharmacy SOPs) are mandatory. Expired products must be segregated and returned to the waste contractor (typically via the Take-Back programme in NZ).

Reconstituted Products

Many oral antibiotic suspensions and injectables require reconstitution. Once reconstituted, the hydrolysis rate increases dramatically due to water availability. For example:

• Amoxicillin/clavulanate suspension: 7 days in refrigerator
• Azithromycin suspension: 10 days at room temperature
• Reconstituted IV vancomycin: 14 days refrigerated

The pharmacist must label reconstituted products with the in-use expiry date at the time of dispensing.

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Storage Requirements

Cold Chain

The cold chain maintains vaccine and biological efficacy from manufacture to patient. In NZ, the National Immunisation Programme specifies 2–8°C storage for most vaccines. Cold chain breaches (excursions above 8°C or below 0°C) require reporting to the vaccine coordinator and Immunisation Advisory Centre (IMAC) for a case-by-case assessment of vaccine viability.

NZ Medicines Cold Chain Requirements include:

• Insulin: 2–8°C (unopened); once opened 15–25°C for up to 28–30 days (product-specific)
• Ergometrine: 2–8°C, protect from light
• Oxytocin: 2–8°C
• All vaccines: 2–8°C, no freezing (unless specified, e.g., varicella, yellow fever, some flu vaccines)

Light-Sensitive Medicines

Storage in original packaging is essential. Dispensary amber vials are the standard for light-sensitive oral liquids and tablets where the original packaging is impractical. Patients should be counselled to keep medicines in the original packaging, in a cool, dark place — not in the bathroom cabinet (temperature and humidity fluctuations) or on a sunlit windowsill.