Impurities in Pharmaceutical Analysis: Types, Sources, USP & ICH Guidelines, and Advanced Profiling Explained

What is Impurity?

Impurities defined as a foreign particle that affects the purity of a substance.

Impurities are chemical substances inside a confined amount of liquid, gas, or solid, which differ from the chemical composition of the material or compound.

Impurities in pharmaceuticals are the unwanted chemicals that remain with the active pharmaceutical ingredients (APIs), or develop during formulation.

The presence of these unwanted chemicals even in small amounts may influence the efficacy and safety of the pharmaceutical products. Impurity can:

• Reduce drug effectiveness.
• Cause unwanted side effects.
• Shorten the product’s shelf life.

Example: If aspirin tablets are stored in a humid environment, they may decompose to form salicylic acid and acetic acid, both of which are impurities.

Introduction

Pharmaceutical analysis ensures that medicines are safe, effective, and of high quality. Since patients directly consume medicines, purity is crucial. Trace impurities, if not controlled, may lead to harmful effects.

Impurities in Pharmaceutical Analysis

Regulatory agencies such as:

• United States Pharmacopeia (USP)
• European Pharmacopoeia (EP)
• International Council for Harmonisation (ICH)

set strict guidelines on allowable impurity levels. Impurity profiling is now an integral part of drug discovery, manufacturing, and quality assurance.

Example: During drug approval, regulatory authorities require a complete impurity profile before granting permission to market the product.

Types of Impurities

Impurities are classified differently based on common practice, USP guidelines, and ICH guidelines.

1. Common Names of Impurities

• Organic Impurities: Related to the drug substance, such as starting materials, intermediates, or degradation products.
  Example: Paracetamol may degrade to 4-aminophenol, a harmful organic impurity.
• Inorganic Impurities: Residues from catalysts, metals, or reagents.
  Example: Traces of palladium may remain in a drug synthesized using palladium catalysts.
• Residual Solvents: Solvents used in synthesis or purification.
  Example: Methanol or acetone may remain after crystallization.
• Degradation Products: Formed during storage.
  Example: Vitamin C can oxidize into dehydroascorbic acid.
• Contaminants: External substances like dust or microbes.
  Example: Microbial contamination in eye drops can cause infections.

2. According to the United States Pharmacopeia (USP)

• Specified Impurities: Known impurities that must be controlled.
  Example: In ibuprofen, specific related compounds are tested.
• Unspecified Impurities: Not individually listed but limited by a general threshold (usually 0.1%).
• Total Impurities: Combined sum of all impurities.

Residual Solvents in USP:

• Class 1: To be avoided (e.g., benzene).
• Class 2: Limited use (e.g., methanol).
• Class 3: Less toxic (e.g., ethanol).

3. According to ICH Guidelines

ICH classifies impurities as:

1. Organic Impurities
2. Inorganic Impurities
3. Residual Solvents

The International Council for Harmonisation (ICH) provides very clear rules about impurities, especially in its Q3A, Q3B, and Q3C guidelines. These guidelines are used worldwide to make sure that drugs are safe, effective, and consistent in quality.

ICH classifies impurities into three main categories:

• Organic Impurities: These are chemical impurities that are related to the drug substance. They may include starting materials, intermediates, by-products, degradation products, or even reagents that remain after the manufacturing process.
  Example: In the synthesis of paracetamol, 4-aminophenol may remain as a residual impurity. Over time, paracetamol may also degrade to form the same compound, making it both a process- and degradation-related impurity.
• Inorganic Impurities: These come from the manufacturing process but are not organic in nature. They may include residual metals (from catalysts), inorganic salts, reagents, ligands, or filter aids.
  Example: Traces of platinum or palladium from catalysts used in chemical reactions may remain in the final drug product. Another example is excess chloride or sulfate ions from reagents used during synthesis.
• Residual Solvents: These are solvents used during the synthesis or purification of the drug that may remain in the final product. Solvents are necessary in drug manufacturing, but if not removed completely, they can be harmful. ICH Q3C categorizes solvents into three classes based on their toxicity:
  • Class 1 (to be avoided): Solvents like benzene and carbon tetrachloride, which are carcinogenic or highly toxic.
  • Class 2 (to be limited): Solvents like methanol, chloroform, or toluene, which can cause reversible or irreversible toxicity and must be kept within strict limits.
  • Class 3 (low toxic potential): Solvents like ethanol or acetone, which are less toxic but still need monitoring.

These classifications help companies focus on which impurities are most important and how much risk they present. Regulatory authorities also define three threshold levels:

• Reporting Threshold: Above this level, the impurity must be reported.
• Identification Threshold: Above this level, the impurity must be chemically identified.
• Qualification Threshold: Above this level, the impurity must be studied for its safety profile.

Threshold levels depend on the maximum daily dose (MDD) of the drug.

Example: If a drug has an MDD of less than 2 g/day, any impurity greater than 0.1% must be identified and evaluated.

Source of Impurities

Impurities can enter a drug during various stages:

1. Raw Materials: Poorly purified raw materials may bring impurities.
   Example: Impure starting materials in penicillin fermentation can introduce by-products.
2. Manufacturing Process: By-products or intermediates.
   Example: In sulfonamide synthesis, incomplete reactions leave unreacted amines.
3. Catalysts and Metals: Traces of metals may remain.
   Example: Cisplatin, a cancer drug, may contain platinum residues.
4. Environmental Factors: Storage conditions can affect purity.
   Example: Exposure of tetracycline to light produces toxic degradation products.
5. Packaging and Storage: Packaging materials may leach chemicals.
   Example: Plasticizers from PVC packaging may contaminate the drug.
6. Cross-Contamination: Manufacturing multiple products in the same plant may cause mixing.
   Example: Powder from one tablet batch may contaminate another.
7. Human Handling: Poor practices may introduce contaminants.
   Example: Improper glove use can cause microbial contamination.

Case Study: In 2018, several batches of valsartan (used for hypertension) were recalled because they contained NDMA, a probable carcinogen. This impurity was formed during a change in the manufacturing process.

Impurity Profiling

Impurity profiling involves identifying, characterizing, and quantifying impurities.

Steps in Impurity Profiling

1. Detection – Finding impurities using techniques.
   Example: HPLC can detect trace impurities in aspirin.
2. Identification – Determining chemical structure.
   Example: LC-MS may identify an unknown impurity in antibiotics.
3. Quantification – Measuring impurity levels.
   Example: GC can measure ethanol levels in cough syrups.
4. Toxicological Evaluation – Checking safety.
   Example: NDMA in valsartan was evaluated as unsafe even in trace amounts.
5. Regulatory Submission – Documentation for approval.

Techniques Used

• HPLC – For separating and quantifying impurities.
• GC – For volatile impurities.
• MS – For identifying unknown compounds.
• NMR – For structural analysis.
• FTIR – For functional group identification.
• Hyphenated Methods (LC-MS, GC-MS, LC-NMR) – For advanced impurity profiling.

Example: In paracetamol analysis, HPLC coupled with MS identifies 4-aminophenol impurity.

Case Studies

To understand the impact of impurities, several real-world case studies highlight their significance:

1. Heparin Contamination (2008)

• In 2008, certain heparin batches were contaminated with oversulfated chondroitin sulfate (OSCS).
• This impurity was not part of the intended product and was toxic.
• The contamination led to severe allergic-type reactions and even patient deaths.
• This incident highlighted the importance of screening for unexpected impurities in biologically sourced drugs.

2. Nitrosamine Impurities in Sartans (2018)

• In 2018, common antihypertensive drugs such as valsartan, losartan, and irbesartan were recalled.
• They were found to contain NDMA (N-Nitrosodimethylamine) and NDEA (N-Nitrosodiethylamine).
• Both are probable human carcinogens.
• The impurities were traced back to changes in the manufacturing process.
• This case emphasized the role of process monitoring and regulatory vigilance.

3. Paracetamol Degradation

• Paracetamol (acetaminophen) can degrade in humid conditions, producing 4-aminophenol.
• This impurity is toxic to the liver and can cause hepatic injury.
• Proper packaging and humidity control are essential to prevent this.
• Example: In tropical countries, paracetamol tablets stored without moisture-resistant packaging degrade faster.

4. Tetracycline Degradation

• This case illustrates why stability studies and proper storage conditions are critical.
• Tetracyclines are antibiotics prone to degradation on exposure to light and heat.
• They can form epianhydrotetracycline, a nephrotoxic compound.
• Consumption of degraded tetracycline has been associated with Fanconi-like syndrome, a kidney disorder.

Importance of Impurity Analysis

• Patient Safety: Ensures medicines do not cause harm.
• Regulatory Compliance: Meets USP and ICH standards.
• Drug Stability: Identifies degradation pathways.
• Process Improvement: Optimizes manufacturing.
• Global Acceptance: Facilitates international drug approval.

Example: The recall of contaminated valsartan emphasized the need for strict impurity monitoring.

Conclusion

Impurities in pharmaceuticals cannot be completely avoided, but they must be strictly controlled. Guidelines from USP and ICH ensure global safety standards. By applying advanced analytical techniques and impurity profiling, pharmaceutical companies ensure that drugs remain pure, stable, and safe.

Examples from history, such as the valsartan recalls, heparin contamination, and paracetamol degradation, highlight why impurity monitoring is one of the most important aspects of modern drug development.

Frequently Asked Questions (FAQ)

Q1. Why are impurities important in pharmaceutical analysis?
Impurities are important because they can impact drug safety, reduce therapeutic effectiveness, and shorten shelf life. Monitoring ensures medicines remain safe and effective.

Q2. What are the main types of pharmaceutical impurities?
The main types include organic impurities, inorganic impurities, and residual solvents, as classified by ICH guidelines.

Q3. How are impurities detected in drugs?
Impurities are detected using advanced analytical techniques such as HPLC, GC, LC-MS, GC-MS, and NMR spectroscopy.

Q4. What are residual solvents and why are they controlled?
Residual solvents are organic volatile chemicals used during manufacturing. Some are toxic (e.g., benzene), so their presence must be controlled within safe limits.

Q5. Can impurities form after manufacturing?
Yes. Impurities may form during storage due to exposure to heat, light, oxygen, or humidity. For example, paracetamol forms 4-aminophenol under humid conditions.

Q6. What was the issue with nitrosamine impurities in valsartan?
Valsartan and other sartans were recalled in 2018 due to NDMA and NDEA impurities, which are probable human carcinogens formed during changes in the manufacturing process.

Q7. How does impurity profiling help?
Impurity profiling helps in identifying, quantifying, and qualifying impurities, ensuring drugs are safe and comply with regulatory guidelines.

Q8. What role do packaging materials play in impurity formation?
Poor-quality packaging can leach chemicals (e.g., plasticizers) into the drug, introducing impurities. Hence, high-quality, compatible packaging is essential.

Q9. What are some real-world examples of harmful impurities?
Examples include OSCS in heparin (2008), nitrosamines in sartans (2018), 4-aminophenol in paracetamol, and epianhydrotetracycline in tetracyclines.

Q10. How do regulatory bodies control impurities?
Regulatory bodies like ICH, USP, and EMA provide strict guidelines, set impurity limits, and require extensive impurity profiling in pharmaceutical quality control.

References

1. ICH Harmonised Guideline – Impurities in New Drug Substances (Q3A(R2)) and Impurities in New Drug Products (Q3B(R2)).
2. ICH Harmonised Guideline – Impurities: Guideline for Residual Solvents (Q3C(R6)).
3. United States Pharmacopeia (USP) General Notices and Requirements.
4. European Medicines Agency (EMA) – Guidelines on the Limits of Genotoxic Impurities.
5. WHO Guidelines for Stability Testing of Pharmaceutical Products.
6. Relevant case studies from FDA safety communications (2008 Heparin contamination, 2018 Nitrosamine impurities).