For instances in which a reference-standard material is not available from a commercial source, the material must be synthesized.
For APIs, the material may start out as a lot of drug substance with sufficient purity to be designated as the reference-standard
material, or it may require further purification. Known impurities or degradants will require custom synthesis. Their purity
requirements, hoewver, are generally not as stringent. Although there is no set guideline to characterize a reference-standard
material, Figure 1 depicts a decision-tree approach involving broad range analytical techniques.
Figure 1: Decision-tree for reference-standard qualification. MS is mass spectroscopy; NMR is nuclear magnetic resonance;
UV is ultra-violet; FTIR is Fourier Transform Infrared Spectroscopy; HPLC is high-performance liquid chromatography; KF is
Karl Fischer; GC is gas chromatography; and LC is liquid chromatography. (FIGURE 1 IS COURTESY OF THE AUTHOR.)
The analytical procedures shown in Figure 1 are dependent on the evaluation of the development process. Minimal required tests
for initial characterization are typically performed using the following tests:
- Organic impurity—HPLC with UV detection
- Metals impurity—ICP with MS detection or ICP with optical-emission spectroscopy detection
- Noncombustible impurities—residue on ignition
- Residual solvents—GC with flame ionization detection
- Water content—Karl Fischer titration
- Structural confirmation: hydrogen and carbon–13 NMR, LC–MS, or FTIR
- Empirical confirmation—C, H, N analysis
- Appearance confirmation—visual inspection.
Other tests may include chiral evaluation (HPLC with UV detection), melting point, differential scan calorimetry, and polymorph
evaluation by X-ray powder diffraction. Different types of reference-standard materials and the qualification tests recommended
are presented in Table II.
Table II: Types of reference-standard material compared with recommended test.
Initial qualification and requalification.
Initial characterization of the reference standard should include a full suite of analytical tests. Requalification at subsequent
points may include a reduced suite of analysis, depending on initial results. It is recommended that a three-tiered approach
be adopted to avoid interruption in stability or clinical programs, as outlined below.
- Tier 1: The reference-standard material qualification program should be started at least one month before the stability or
clinical program begins to allow for requalification and assignment of a new expiration date. This timeframe will also help
to avoid delays in testing for subsequent programs due to an expired reference standard.
- Tier 2: At least two lots of reference-standard material should be placed in the qualification program three months apart.
If the primary lot degrades, the secondary lot may be used during the interim period while a new, third batch is manufactured,
characterized, and qualified.
- Tier 3: At least two storage conditions should be chosen: the intended storage condition and an alternative storage condition
as a contingency. For example, if the intended storage condition is 2–8 °C then the reference standard should also be stored
at –20 °C as a contingency. This may allow for an extended life of the reference standard if it is proven to be unstable for
a long period of time at its anticipated storage condition. In addition, useful stability information may be ascertained if
the contingency conditions samples are tested as well as the intended storage condition.