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| Losses mount with cycling. Even compounds stored for prolonged periods at room temperature last longer than compounds that undergo repeated freeze–thaw cycles. (Adapted from Kozikowski, B. A.; et al. The effect of freeze/thaw cycles on the stability of compounds in DMSO. Presented at the Society for Biomolecular Screening conference, Baltimore, Sept 2001.) |
In the repeated therapeutic testing of potential drugs from a combinatorial library, the compounds are often stored in a frozen solution of dimethyl sulfoxide (DMSO) to maintain their shelf lives. Unfortunately, frozen samples cannot be tested, but must be thawed and then refrozen for storage and future testing. This constant back and forth between the freezer and benchtop takes its toll on the samples. But the impact of repeated freeze–thaw cycles has rarely been tested directly.
Barbara Kozikowski and colleagues at Procter & Gamble Pharmaceuticals (Mason, OH) decided to measure the effect of temperature cycling on samples from the company’s own chemical repository. They presented their findings at the recent Society for Biomolecular Screening conference in Baltimore. The group stored microtiter plates containing DMSO solutions of 320 library compounds (20 mM in 100% DMSO) at 4 °C under argon in pressure canisters to simulate a low-humidity environment. The plates were then subjected to 25 freeze–thaw cycles and exposed at room temperature to mimic normal sampling conditions. Control plates were stored at room temperature or 4 °C under argon and were tested at the study’s mid- and end points. Aliquots of the thawed solutions were then tested every fifth cycle for sample integrity using high-pressure liquid chromatography/evaporative light-scattering detection/atmospheric pressure chemical ionization/mass spectrometry.
Regardless of how the samples were maintained, there was clear compound deterioration; but the samples that underwent freeze–thaw cycling suffered the most, showing a drop of more than 10% within 10 cycles. Samples stored continuously at 4 °C suffered the least. On the basis of this analysis, the researchers have decided that future samples will undergo a maximum of 15 freeze–thaw cycles before being discarded, and they are now trying to determine whether there is a relationship between freeze–thaw stability and chemical structure.
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