Small-molecule APIs: Characteristics
Why tiny impurities matter more than purity in medicines
What this tab covers The physical and chemical properties that set its grade, price and uses, with the commercial grades and the standards that define them.An API is sold against a pharmacopoeia monograph and a regulatory dossier, not just a chemical specification. Assay, impurity profile, polymorph and particle size decide whether a formulator can use it without re-filing. Changing supplier requires validation and often a regulatory variation, which makes supply sticky.
Properties that matter commercially
| Property | Value / specification | Why it matters | Source |
|---|---|---|---|
| Assay and impurities | typically 98.0-102.0% assay; individual unknown impurities at or below 0.10% (ICH Q3A thresholds) | drives safety and regulatory acceptance | ICH Q3A |
| Nitrosamine and genotoxic impurities | nanogram-level limits per day of exposure | NDMA findings since 2018 forced route changes | FDA and EMA nitrosamine guidance |
| Polymorph and particle size | specified per product | affects dissolution and tableting | qualitative |
Commercial grades
- USP, Ph. Eur., IP and JP grades
- DC (directly compressible) grades for some high-volume APIs
- Sterile API for injectables
- Micronised grades for inhalation