Contents
1. The short answer
Classic Arrhenius/MKT, ASAP and AccelStab are often discussed as rivals. They are not — they answer different questions. Three of them predict a development shelf life from short, stressed studies; the operational job of an excursion review is different: "given this product's already-characterised kinetics and this actual logger trace, is this batch releasable, and with what confidence?" That is a probabilistic Arrhenius question, not a predictive-modelling one.
2. The shared foundation
All three inherit the Arrhenius temperature dependence of the degradation rate constant:
A = pre-exponential factor · Eₐ = activation energy · R = 8.314 J/(mol·K) · T = kelvin
What each method adds on top of this base — a humidity term, a non-linear degradation form, an uncertainty model — is exactly what distinguishes them.
3. The three approaches
Classic Arrhenius & MKT
The baseline: rate depends on temperature via Arrhenius; MKT collapses a varying profile into one equivalent temperature. Simple, well-understood, sufficient when temperature is the dominant stress. Its blind spots are humidity and non-linear degradation.
ASAP — humidity-corrected Arrhenius
The Accelerated Stability Assessment Program adds a relative-humidity term to the Arrhenius rate, so it models moisture-driven degradation. Its strength is fast, predictive shelf-life estimates from short high-stress studies — a development tool for products where humidity matters.
AccelStab — Šesták–Berggren kinetics
An open-source R package fitting the Šesták–Berggren kinetic model, which describes non-linear degradation curves (autocatalytic, sigmoidal) that a straight Arrhenius line misses. Also a predictive/development tool, stronger where the degradation shape is complex.
4. Side by side
| Aspect | Classic Arrhenius/MKT | ASAP | AccelStab |
|---|---|---|---|
| Models humidity | No | Yes | Not primarily |
| Degradation shape | Linear (log) | Linear (log) | Non-linear |
| Primary job | Baseline / MKT | Predict shelf life | Predict shelf life |
| Best when | Temperature dominates | Humidity-sensitive | Complex kinetics |
| Uncertainty stated | Optional | Yes (predictive) | Yes (fit CI) |
5. Which one, when
For development shelf-life prediction from short stressed studies: ASAP if humidity drives the failure, AccelStab if the degradation curve is non-linear, classic Arrhenius if temperature dominates and simplicity is fine.
For an operational release decision after an excursion, none of the predictive tools is the right frame. The product's kinetics are already characterised; the question is how much of the stability budget this specific event consumed, with a defensible confidence interval. That is what Synlogica Terminus Quality (Terminus M4) computes — a probabilistic Arrhenius model over the actual logger trace with Monte-Carlo uncertainty propagation, packaged as a sealed decision. The full four-way comparison, with equations and assumptions, is in the stability models compared white paper.
6. FAQ
Which stability model should I use for excursion review?
For an operational release decision after an excursion, a probabilistic Arrhenius model over the actual profile (with stated uncertainty) fits best — it answers "is this batch releasable and with what confidence". ASAP and AccelStab are development tools that predict shelf life from short stressed studies, a different question.
What is the difference between Arrhenius, ASAP and AccelStab?
All three build on Arrhenius temperature dependence. Classic Arrhenius/MKT is the baseline; ASAP adds a humidity term; AccelStab fits non-linear (Šesták–Berggren) curves. They differ in what they model on top of the shared kinetics and whether the job is prediction or release.
Is ASAP better than classic Arrhenius?
Not better, different. ASAP is stronger when humidity drives degradation and you want a fast predictive shelf life from a short study. Classic Arrhenius/MKT is simpler and sufficient when temperature dominates. Pick by the failure mode and the question, not by which is newest.
7. References
- Waterman K.C. et al. — The Application of ASAP to QbD for Drug Product Stability. AAPS PharmSciTech.
- Humidity-corrected Arrhenius equation — Int. J. Pharmaceutics (2016).
- AccelStab — Accelerated Stability Kinetic Modelling (CRAN / github.com/AccelStab/AccelStab).
- ICH Q1A(R2); ICH Q1E — Stability Testing and Evaluation.