Temperature excursion (revision 31)

A temperature excursion is an episode in which a product is held outside the storage conditions stated on its label. The term is used in pharmaceutical distribution and manufacturing for a deviation whose consequences must be assessed before the product is released for further use, and the assessment is a documented judgement about degradation rather than a rule about temperature.^[1]

An excursion is defined relative to a label, not to an absolute temperature. Because compendial definitions of controlled cold and controlled room temperature build permitted excursion bands into the definitions themselves, a departure from the nominal range is not automatically an excursion in the regulatory sense; a product labelled for storage at 2–8 °C that reaches 15 °C for an hour may remain within its labelled conditions, whereas the same profile applied to a product labelled for frozen storage would not.^[2]

Assessment proceeds from three quantities: how far outside the range the product went, for how long, and how much such exposure it has already accumulated. Duration and magnitude are combined through the mean kinetic temperature, a degradation-weighted average that gives disproportionate weight to warm intervals and can exceed the permitted range even when the ordinary time-weighted mean does not. Cumulative accounting matters because a product that has spent three separate periods outside its range has consumed three portions of whatever stability margin the manufacturer's programme established.^[3][1]

The evidence that makes a disposition possible is the product's own stability data, generated under the conditions specified in ICH Q1A(R2) and supplemented where necessary by deliberate stress studies. A product with no such programme — which includes essentially all peptides distributed for research use — cannot have an excursion assessed in this sense, because there is nothing against which to compare the exposure.^[4]

Distribution and compendial guidance treat an excursion as a deviation requiring investigation. The European good distribution practice guidelines require that deviations be recorded and their effect on quality evaluated, and that product not be released for further distribution until that evaluation is complete. WHO model guidance takes the same approach and adds the concept of a stability budget: the total permitted exposure outside labelled conditions, allocated across the product's life, against which each event is charged.^[5][6]

Three framings of the same event coexist and are worth distinguishing.

The compliance framing

Did the product leave its labelled conditions? This is answered by comparing a record against a label, and is a question about documents.

The kinetic framing

How much additional degradation is expected? This is answered by applying a rate model to the exposure, and is a question about chemistry.

The analytical framing

Can any change be measured? This is answered by testing, and is a question about method capability.

The three can disagree, and the disagreement is informative rather than paradoxical. A brief excursion may be a compliance failure while producing degradation far below the detection limit of any available assay; conversely a profile within the permitted band may still cause measurable change in a product whose stability is marginal. Assessment practice therefore uses all three: compliance to determine whether an investigation is required, kinetics to predict, and testing to confirm.^[1][7]

An excursion is also not a single class of event. Warming above the labelled range and cooling below it have different mechanisms and are not interchangeable in an assessment, because freezing damage is not governed by the Arrhenius relationship on which the warming arithmetic depends. Guidance for products labelled 2–8 °C accordingly treats a freezing event as a separate category, often with a stricter disposition, and freeze indicators exist precisely because the mean kinetic temperature calculation cannot detect the event.^[8][9]

Modern labels for injectable incretin therapies do not state a single storage condition; they state a cold-storage condition for the unopened product and a permitted period at higher temperature, either before first use or after it. These allowances are excursion tolerances that have been established by stability testing and written into the label, which converts what would otherwise be a deviation into ordinary use.

Allowances are as stated in the respective prescribing information and differ between jurisdictions; the figures above follow United States labelling and are cited to the labels rather than to secondary summaries.^{[10][11][12][13][14]}

Three features of this table matter for interpretation. The allowances are single, non-renewable periods, not a budget that resets: a pen that has spent 21 days at room temperature has consumed the allowance whether or not it is subsequently refrigerated. They differ substantially between products with similar chemistry, which reflects differences in formulation, container and the stability data generated rather than differences in the peptide. And they are stated as maxima at a ceiling temperature, so a period at 35 °C is outside the allowance entirely rather than being a shorter permitted period.^[4]

The corresponding position for lyophilised research peptides is that no such allowance exists in a documented form. Supplier storage statements are recommendations, and the periods sometimes quoted for ambient tolerance are not supported by an identifiable stability study. The physical argument for the robustness of a dry cake is set out at Lyophilisation and is a reason to expect tolerance, not a substitute for having measured it.^[15]

Where an excursion consists of warming, its cumulative effect is summarised by the mean kinetic temperature. For intervals of duration t_i at absolute temperatures T_i:

T_MKT = (ΔH/R) ÷ [ −ln( Σ t_i e^{−ΔH/(R T_i)} ÷ Σ t_i ) ]

with ΔH/R taken as 10,000 K by convention.^[3][1]

A realistic worked case demonstrates the property that makes the statistic useful. A consignment labelled for storage at 2–8 °C is held for 30 days, of which 96 hours are spent at 22 °C — a courier depot over a long weekend, repeated — and the remaining 624 hours at 5 °C.

The time-weighted arithmetic mean is

(96 × 22 + 624 × 5) ÷ 720 = (2112 + 3120) ÷ 720 = 7.27 °C

which lies inside the labelled range. The mean kinetic temperature, however, weights by degradation. With T = 295.15 K and 278.15 K:

e^{−10000/295.15} = e^−33.8811 and e^{−10000/278.15} = e^−35.9518

The colder term is smaller by a factor of e^−2.0707 = 0.1261. The weighted sum is therefore

e^−33.8811 × (96 + 624 × 0.1261) ÷ 720 = e^−33.8811 × 174.7 ÷ 720 = e^−33.8811 × 0.2426

The divergence is a direct consequence of convexity. Degradation rate is a convex function of temperature, so the average of the rates at two temperatures exceeds the rate at their average temperature. Mean kinetic temperature is defined as the temperature whose rate equals the average rate, and therefore always lies at or above the time-weighted arithmetic mean, with equality only for a constant profile.

The gap widens with the spread of the profile. For the 30-day case above, a spread of 17 °C produced a gap of 2.9 °C. For the extreme cycle worked at Cold chain, a spread of 28 °C produced a gap of 8.1 °C. A record with a narrow spread can be summarised by its arithmetic mean with little error; a record with a wide spread cannot, and it is precisely the wide-spread records that arise from excursions.^[3]

The rate constant for a chemical degradation pathway is conventionally described by the Arrhenius equation, in which rate rises exponentially with the reciprocal of absolute temperature. With the compendial activation energy, the multiplier for a 10 K rise near 25 °C is

exp[10000 × (1/298.15 − 1/308.15)] = exp(1.0884) = 2.97

• Cold chain
• Peptide stability table
• Beyond-use date
• Deamidation
• Methionine oxidation