The kinetic principle that connects a defined stability limit with performance under long-term storage conditions
Shelf life is reached when a drug substance or drug product no longer meets an applicable stability specification based on a critical quality attribute (CQA). CQAs that limit shelf life include growth of an individual degradant or total degradation products, potency loss, dissolution change, and change in activity.
Accelerated stability studies use elevated temperature and, when relevant, relative humidity to produce measurable change sooner than using real-time measurements. Accelerated changes in CQAs must be related back to the intended storage conditions. Isoconversion helps provide that connection.
Isoconversion in Pharmaceutical Stability
Isoconversion is the time required for a defined CQA to reach a specified level of change, the specification limit. If an impurity limit is 0.50%, for example, the isoconversion time at each study condition is the time at which the impurity reaches 0.50%.
Limitations of Initial Degradation Rates
Conventional kinetic treatments assume changes in CQAs behave linearly with time. Pharmaceutical systems frequently depart from this behavior. A primary degradant may undergo secondary degradation, causing its accumulation rate to slow. A secondary degradant may appear only after a precursor accumulates, producing a lag followed by growth. Depletion of an antioxidant or another inhibitor can accelerate degradation later in the study. Autocatalysis, diffusion control, and heterogeneous solid-state environments can also produce curvature. In one common scenario, drug molecules within a crystal lattice, at a particle surface, in an amorphous region, or at an API-excipient interface all react at different rates. The observed degradation represents the combined behavior of molecules in several microenvironments rather than one simple kinetic process.
Isoconversion and Arrhenius Shelf-Life Modeling
At each accelerated temperature (and humidity) condition, data are used to estimate the time at which the CQA reaches its specification limit. Because the extent of change is held constant, the analysis compares equivalent kinetic points. The reciprocal of the isoconversion time functions as a rate associated with that defined endpoint. The temperature dependence of those endpoint rates can then be modeled using an Arrhenius relationship. For solid products affected by moisture, a humidity term can be incorporated. This treatment reduces dependence on knowing the complete shape of the degradation curve.
Estimating Isoconversion
Sample exposure times to accelerated conditions should ideally bracket the failure point at each condition. Results far beyond the limit may reflect a different portion of a nonlinear curve and contribute little to estimating the crossing time. Results far below the limit require long extrapolation and lead to larger errors in estimating the isoconversion times.
Analytical variability propagates into uncertainty in the estimated isoconversion time, especially when the data provide only a shallow slope or a long projection to the limit.
Scientific review remains necessary when a point appears anomalous, degradation greatly exceeds the specification, an accelerated condition causes a physical change, or the predicted crossing lies far beyond the measured period.
Isoconversion Analysis in ASAPprime®
ASAPprime® is FreeThink Technologies’ software for designing and analyzing accelerated predictive stability studies based on the Accelerated Stability Assessment Program. For a selected stability-limiting attribute, the software estimates the isoconversion time at each temperature and humidity condition, models the condition dependence, and estimates shelf life and its associated statistical uncertainty under the proposed storage condition.
Applications in Pharmaceutical Development
An isoconversion model can show how quickly a CQA approaches its specification under defined storage conditions. That information can support:
- Selection among candidate formulations
- Assessment of temperature and humidity sensitivity
- Packaging determination
- Storage and excursion impact
- Clinical-supply planning
- Stability sections of regulatory submissions
Regulatory acceptance of ASAPprime® for setting shelf life of drug substances and drug products depends on the development stage, product, jurisdiction, study quality, and scientific
justification presented.
FreeThink combines accelerated study design, analytical testing, data analysis, and scientific interpretation through its ASAPprime® stability software and predictive stability services. Learn more about isoconversion analysis and shelf-life modeling with ASAPprime®.
Primary Scientific References
- Waterman, Kenneth C., J. T. Swanson, and B. L. Lippold. 2014. “A Scientific and Statistical Analysis of Accelerated Aging for Pharmaceuticals. Part 1: Accuracy of Fitting Methods.”
Journal of Pharmaceutical Sciences 103 (10): 3000–3006. https://doi.org/10.1002/jps.24075 - Waterman, Kenneth C., and Roger C. Adami. 2005. “Accelerated Aging: Prediction of Chemical Stability of Pharmaceuticals.”
International Journal of Pharmaceutics 293 (1–2): 101–125. https://doi.org/10.1016/j.ijpharm.2005.01.013. - Waterman, Kenneth C., Anthony J. Carella, Michael J. Gumkowski, Patrick Lukulay, Bruce C. MacDonald, Michael C. Roy, and Sheri L. Shamblin. 2007. “Improved Protocol and Data Analysis for Accelerated Shelf-Life Estimation of Solid Dosage Forms.”
Pharmaceutical Research 24 (4): 780–790. https://doi.org/10.1007/s11095-006-9201-4