(USP 44/BP 2022)

  1.  Drug Substance General Information (ICH 3.2.S.1)

1.1. Nomenclature (ICH 3.2.S.1.1)

International non-proprietary name: Azathioprine (Brand Name: Azasan, CellCept, Simulect, Imuran, Rapamune)

Compendial name:    Azathioprine

Chemical name: 6-(1-methyl-4-nitro-1H-imidazol-5-yl) thio] 1H-purine-6-[(1-methyl-4-nitro-5-imidazolyl-5yl) mercaptopurine

Arasto’s code: AZA

CAS Registry Number: [446-86-6]

Empirical formula: C9H7N7O2S

Molecular Weight: 277.26

  1. Drug Substance General Information (ICH 3.2.S.1)

1.3. General Properties (ICH 3.2.S.1.3)

Azathioprine is a light yellow to light greenish-yellow crystalline compound.  It is orally administered for the treatment of renal homotransplantation and rheumatoid arthritis.  Azathioprine is insoluble in water or organic solvents (except hot DMSO).  Azathioprine pKa has been reported as 8.07 at 25 oC (Am. J. Ana. Chem20111, 14-24).    The addition of one molar equivalent of alkali affords Azathioprine salt which is water soluble. The salt of Azathioprine is sufficiently soluble to make a 10 mg/ml water solution which is stable for 24 hours at 15° to 25°C. Azathioprine is stable in solution at neutral or acid pH but hydrolysis to mercaptopurine occurs in excess sodium hydroxide (0.1N), especially on warming. Conversion to mercaptopurine also occurs in the presence of sulfhydryl compounds such as cysteine, glutathione, and hydrogen sulfide. LD50has been reported to be 535 mg/kg Oral in rat (oral) /AZ/ Azathioprine.html.

The determination of purity and assay of APIs require comparison of the product with their respective Reference Standards (RS) and Related Compounds (RC or known impurities).  Accordingly, ICH regulations on the purity and assay of reference standard and related compounds are clearly defined and must be followed by drug substance and drug product manufacturers.

According to ICH Q7, 11.1 there are 3 types of standards.  This is summarized in the following chart and discussed in detail below.

The impurities provided in the following table represent Secondary Reference Standards (SRS) that are prepared in-house by synthesis or by isolation.  Each SRS has undergone extensive characterization (IR, UV, 1HNMR, 13CNMR. Mass Spec) and determination of its purity and assay (HPLC).  For specification of the SRS of those products that have a monograph, the SRS is compared with a pharmacopoeia Primary Reference Standard (UV, HPLC retention time).  For specification of those products that do not have a monograph (known as House Primary Standard), we compare their UV ε or ג/max, IR major absorptions, 1HNMR d(ppm), 13CNMR d(ppm) or HPLC retention time with values reported in the chemical literature for these compounds.

  1. Primary and Secondary Reference Standard (ICH 3.2.S.5)

 5.1. Active Pharmaceutical Ingredient

Primary Reference Standard for Azathioprine is available from United States Pharmacopoeia. We will use a Secondary Reference Standard (previously referred to as Working Standard) for direct control of all batches of azathioprine.

As per ICH (Q7, 11.1) and ICH (Q6, 2.11, 3.2, 3.3) the Secondary Reference Standards, which include the API and its Related Compounds, must be examined for their proof of structure (characterization), assay and purity and specification (identification by comparison). Furthermore, ICH Guideline on the Preparation of Common Technical Document (Q4M) requires that the data obtained from characterization, assay and purity and specification must be included in section 3.2.S.3.2 for Related Compounds (already discussed in that section) and section 3.2.S.5 of the DMF for the API. To this end, the Secondary Reference Standards of the API azathioprine has undergone extensive characterization (UV, IR, 1 H NMR, 13C NMR, and Mass Spec) to assure its structure, assay and purity (HPLC and/or titration) and specification (comparison of its HPLC retention time and UV ג/max with USP Primary Reference Standard.

The Secondary Reference Standard for azathioprine was produced from a released batch of azathioprine by subjecting it to an additional crystallization from the final solvent system used in the production of the API to avoid the possibility of other polymorph formation.


Product: AzathioprineCAS No.: 446-86-6Spec. No.: APC-QC-SPEC-371-00
Issue Date: Apr, 2023Valid up to: Apr, 2024Reference: BP2022


AppearancePale-yellow powder.
SolubilityPractically insoluble in water and in ethanol (96%). It is soluble in dilute solutions of alkali hydroxides and sparingly soluble in dilute mineral acids.
IdentificationInfrared absorption spectrophotometry
Loss on dryingMax. 1.0%
Sulphated AshMax. 0.1%
Residual SolventsMethanol: Max. 3000 ppm (Class II)
Related substances (HPLC)Impurity A: Max. 0.15%

Impurity B: Max. 0.15%

Unspecified impurities: Max. 0.10%

Total impurities: Max. 0.5%

Assay98.5% to 101.0% (dried substance)
Prepared by: M. Shahbazi, B.Sc.Chem.Checked by: A. Forghani, B.Sc.Chem.
Approved by: F. Javadizadeh, M.Sc.Chem.
Storage: Protected from light.