Storage of pharmaceutical discovery compounds dissolved in dimethylsulfoxide (DMSO) is commonplace
within industry. Often, the DMSO stock solution is added to an aqueous system (e.g. in bioassay or kinetic solubility
testing)- since most test compounds are hydrophobic, precipitation could occur. Little is known about the factors affecting
this precipitation process at the low (µM) concentrations used in screening analyses. Here, a poorly water soluble test
compound (tolnaftate) was used to compare manual and automated pipetting, and explore the effect of mixing variables
on precipitation. The amount of drug present in the supernatant after precipitation and centrifugation of the samples was
quantified. An unusual result was obtained in three different laboratories: results of experiments performed initially were
statistically significantly higher than those performed after a few days in the same lab. No significant differences were
found between automated and manual pipetting, including in variability. Vortex mixing was found to give significantly
lower supernatant amounts compared to milder mixing types. The mixing employed affects the particle growth of the
precipitate. These findings are of relevance to discovery stage bioassay and kinetic solubility analyses.
Keywords: Drug precipitation, drug screening, growth, mixing, NTA, particle.
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