While product viability over time is an important consideration, calorimetry has not been a popular tool for its study in the past. Now, however, a new technique has been developed to determine stability of low molecular weight materials in both aqueous and non-aqueous solvents. Its primary application areas include drug formulation and stability.
Estimates of product stability are often made using accelerated thermal decomposition studies at high temperatures followed by extrapolation of the data to the temperature of interest.
When the temperature range under investigation is much higher than the temperature of interest, the extrapolation of results is less certain, since the mechanism of decomposition can be different at different temperatures.
Stability studies
Calorimetry has not been very popular for stability studies since limitations in sensitivity prevented meaningful measurements at low temperatures (025oC) where decomposition rates are very low.
In this temperature range heats of decomposition are so small as to be virtually undetectable by most calorimeters.
However, US company MicroCal has overcome this limitation with the development of the new, ultra-sensitive VP-DSC (differential scanning calorimeter). As is shown below, this can be used to provide stability information at lower temperatures previously unattainable by differential scanning calorimetry.
Its primary application areas are drug formulation and stability, small molecule characterisation, nucleic acid analogues and pesticide development.
Methods
DSC scans were performed using a 9.5mg/ml aqueous solution of the anti-bacterial drug Cefalozin (from Sigma Biochemicals), without further purification. The sample was scanned using the VP-DSC from approximately 20oC to 130oC at a heating rate of 3.8oC per hour. After rapid cooling, the sample was scanned a second time.
A water-water baseline was determined at the same heating rate over a 25-hour period.
For isothermal studies, another solution of Cefalozin at the same concentration was observed for 16.3 hours at 60oC using the isothermal mode of the VP-DSC. Results were compared against a water-water baseline using the same experimental conditions.
Decomposition
In the first scan, the heat of decomposition is readily apparent at temperatures above 35oC, leading to a large exotherm with a minima at 92oC.
Above 120oC another exothermic process is apparent. Below 92oC the shape of the repeat scan is almost identical to the baseline. Above this temperature the second exothermic process is again shown.
The excess heat associated with the primary mechanism of decomposition at lower temperatures is obtained by the subtraction of the second scan from the first.
Since more than one decomposition process occurs at high temperature, the decision was taken to work with the low temperature data (060oC), uncomplicated by multiple decomposition processes.
For example, isothermal scans at 60oC including a water-water control scan which establishes the zero heat baseline.
At this temperature there are two separate decomposition processes occurring in the solution of Cefalozin.
During the first five hours of the isothermal study, a minor impurity appears to decompose faster than Cefalozin itself. After this period, Cefalozin decomposition shows a near-linear decrease in the rate of decomposition with time, which is the expected behaviour for a first-order decay.
If one compares the total integrated area between the control line and the Cefalozin line with that fraction of area in the early region, the early phase if found to represent only 2.5 per cent of the total heat for the process.
Hence it is justified to remove it from the calculations by using an integration based on an extrapolation from data obtained after the early phase.
Enthalpy change
Over a period of 16.3 hours, the total heat obtained from this integration is -55 mcal for the decomposition process.
Assuming the process is first order, the total heat for decomposition of the entire sample (1.0 x 10-5 moles) is -170 µcal which gives an estimated enthalpy change for the process (eH) of -17.0kcal/mole.
To calculate the rates of decomposition (moles/minute) for the decomposition process at all temperatures, the excess heat capacity (eCp) data (mcal/minute) is divided by eH for the process (mcal/mole) at each temperature. The rate constant (k) can then be calculated from a knowledge of the eCp and eH at a given temperature, along with the molar amount of unreacted material at that temperature.
If the natural logarithm of the rate constant is plotted as a function of the inverse temperature, as shown in Fig. 1, a linear correlation is achieved, whose slope is equal to the activation energy (Ea) of the first-order decomposition process.
In the case of Cefalozin this is equal to 21.1kcal/mole. If this relationship is extrapolated to 20oC, the rate of decomposition at this temperature is found to be 7.9 x 10-6 min-1, consistent with a half-life of 61 days.
Summary
In many areas, product viability over time is an important consideration. This article demonstrates the use of calorimetry to determine the stability of an aqueous sample of Cefalozin.
This approach is widely applicable to low molecular weight materials in both aqueous and non-aqueous solvents.
It is important to note that thermal measurements of this type describe thermal stability which may or may not be related to the presence of contaminants in, or potency of, a compound.
This article is based on a MicroCal LLC Application Note and is copyright of MicroCal Incorporated. MicroCal's web address is www.microcalorimetry.com
