Classification of Impurities

Impurities can be classified into the following categories:

  1. Organic Impurities (Process And Drug-Related)
  2. Inorganic Impurities
  3. Residual Solvents
  1. Organic Impurities :

Organic impurities can arise during the manufacturing process and/or storage of the new drug substance.

They can be identified or unidentified, volatile or non-volatile, and include:  

  • Starting materials
  • By-products
  • Intermediates
  •  Degradation products
  •  Reagents, ligands and catalysts

2. Inorganic Impurities:

  • Inorganic impurities can result from the manufacturing process.
  • They are normally known and identified and include:
    • Reagents, ligands and catalysts
      • Heavy metals or other residual metals
      • Inorganic salts
      • Other materials (e.g., filter aids, charcoal)

3. Residual Solvents:

  • Residual are inorganic or organic liquids used as vehicles for the preparation of solutions or suspensions in the synthesis of a new drug substance.

3.1 Classification of Residual Solvents by Risk Assessment

a. Class 1 Solvents : Solvents to be avoided

  • Solvents in Class 1 should not be employed in the manufacture of drug substances, excipients, and drug products because of their unacceptable toxicity or their deleterious environmental effect.
  • However, if their use is unavoidable in order to produce a drug product with a significant therapeutic advance, then their levels should be restricted as shown in Table.

-Known human carcinogens

-Strongly suspected human carcinogens,

-Environmental hazards.

b. Class 2 solvents: Solvents To Be Limited

  • Solvents in Table should be limited in pharmaceutical products because of their inherent toxicity.
  • PDE*s are given to the nearest 0.1 mg/day, and concentrations are given to the nearest 10 ppm.

-Non-genotoxic animal carcinogens

-Possible causative agents of other irreversible toxicity such as neurotoxicity or teratogenicity.

-Solvents suspected of other significant but reversible toxicities.

c. Class 3 Solvents: Solvents With Low Toxic Potential

  • Solvents in Class 3 may be regarded as less toxic and of lower risk to human health.
  • Class 3 includes no solvent known as a human health hazard at levels normally accepted in pharmaceuticals.

-Solvents with low toxic potential to man; no health-based exposure limit is needed.

-Class 3 solvents have PDEs of 50 mg or more per day

3.2 Solvents for which No Adequate Toxicological Data was Found

* PDE: Permitted Daily Exposure is the maximum acceptable intake per day of residual solvent in pharmaceutical products.

Reference : ICH Q3 

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