Key points
- Identify the mechanism first - enzymatic, oxidative, light, pH or adsorption - then choose the stabiliser.
- Stabilise at the point of collection: what happens in the first minutes at the clinic often decides the result.
- Every stabilisation step must be part of validation: whole-blood, bench-top, freeze-thaw, long-term and processed-sample stability under the real conditions.
Why stability fails
A stability failure rarely announces itself. QCs prepared in the laboratory may be stable while study samples are not, because the problem happens before the samples reach the lab - in whole blood waiting to be centrifuged, at room temperature in a busy clinic, or in a freezer that is opened ten times a day. The first step is always to find out what breaks the molecule.
Enzymatic degradation
Esters, amides and some prodrugs are hydrolysed by plasma and red-cell esterases, often within minutes.
- Esterase inhibitors added to the collection tube: sodium fluoride (often as NaF/potassium oxalate tubes), dichlorvos, PMSF, eserine (physostigmine) or bis(4-nitrophenyl) phosphate (BNPP) - chosen and tested for the specific compound.
- Temperature: collect into pre-chilled tubes, keep on wet ice, centrifuge in a refrigerated centrifuge and freeze plasma quickly.
- Acidification of plasma can stop enzymatic activity for some compounds - check that it does not cause chemical degradation instead.
Oxidation and light
- Antioxidants for catechols, thiols and other easily oxidised groups: ascorbic acid, sodium metabisulfite, BHT or EDTA (to bind catalytic metal ions).
- Thiols may need derivatisation at collection to prevent disulfide formation.
- Light-sensitive compounds: amber tubes, yellow light in the laboratory, foil-wrapped racks - and a light-protected autosampler.
pH-dependent conversion
Some molecules exist in two forms that interconvert depending on pH - the classic example is the lactone and hydroxy-acid forms of statins. If both are measured, the pH of plasma and of every solution in the method must be controlled (often buffering to a mildly acidic pH and keeping samples cold) so that the ratio does not change between collection and analysis.
Back-conversion of metabolites
Acyl glucuronides, N-oxides and some conjugates can convert back to the parent drug during storage, extraction or in the ion source. The result is a falsely high parent concentration - and a classic cause of failed incurred sample reanalysis.
- Stabilise acyl glucuronides by lowering plasma pH and keeping samples cold.
- Separate the metabolite chromatographically from the parent, so in-source conversion cannot add to the parent peak.
- Test it: spike the metabolite (if available) into blank matrix and measure how much parent appears.
Adsorption losses
Lipophilic and some basic compounds stick to plastic and glass, especially in low-protein matrices such as urine and CSF. Losses look like instability but are not. Add a surfactant or protein (for example Tween 20 or BSA) to the matrix, use low-binding tubes and keep transfer steps to a minimum.
Proving it works
Stability experiments in ICH M10 use low and high QCs (at least three replicates) under conditions that cover the real handling of study samples; the mean accuracy should be within ±15% of nominal. Typical experiments:
| Experiment | Covers |
|---|---|
| Whole-blood stability | Time and temperature between collection and centrifugation |
| Bench-top (short-term) | Thawed samples during sample preparation |
| Freeze-thaw | As many cycles as study samples may undergo; frozen for at least 12 hours between cycles |
| Long-term | Storage temperature(s) and duration covering the study |
| Processed sample / autosampler | Extracts waiting for injection, including re-injection |
| Stock and working solutions | Storage of the reference solutions |
If a stabiliser is used, validate with stabilised matrix and make sure the clinic instructions match exactly what was validated.
Educational summary for chromatographic (LC-MS/MS) bioanalysis. Always apply the current ICH M10 text, the expectations of the regulator concerned and your laboratory's SOPs. Ligand-binding assays have wider limits in ICH M10.
ARISENTO Insights is published by Autofai Technologies Private Limited, the makers of ARISENTO bioanalytical software.