Improving IC 50 Results with Acoustic Droplet Ejection

IC 50 analyses are typically sample, time, and labor intensive. They commonly require multiple dilution steps and consume significant amounts of sample compound. Aqueous intermediate dilutions of concentrated stock solutions can lead to rapid sample precipitation and the generation of false negatives (Spicer, T.; Fitzgerald, Y.; Burford, N.; Matson, S.; Chatterjee, M.; Gilchrist, M.; Myslik, J.; O'Connell, J. Pharmacological evaluation of different compound dilution and transfer paradigms on an enzyme assay in low volume 384-well format, Poster presented at Drug Discovery Technology, August 2005, Boston, MA). Hydrophobic compounds may stick to pipette tips or intermediate dilution vessels, reducing the concentration of the analyte in the dilution and also increasing the possibility of cross contamination. The requirement for multiple serial dilutions in common IC 50 analyses causes significant accumulated error. Concentrations of dimethyl sulfoxide (DMSO), the typical solvent used to solubilize compound libraries, as low as 1% in the final assay solution can significantly affect the results of the experiment. Finally, the cost of pipette tips and intermediate dilution vessels, and the frequency of the DMSO washes of tips grows significantly as the number of compounds being analyzed is increased. A system incorporating acoustic droplet ejection of compounds improves IC 50 results by reducing the amount of sample used in the analysis to nanoliters, eliminating intermediate aqueous dilutions and accumulated pipetting error, lowering DMSO concentrations in the final assay to below 1%, and reducing costs of consumables (plastics, solvents, and their disposal). (JALA 2006;11:240–6)