I am using culture media to dissolve my drug as the drug is not stable in DMSO, so I am keeping fresh drug solution for when I want to carry any experiment (cytotoxicity, cell cycle analysis, apoptosis assay, etc.). Can I prepare stock solution of my drug and store it in 4c or -20c, like what I do when I am using DMSO? Is this drug solution contaminated or unstable to be stored for long period?