this according to the application, if you use the extract by injection for animal, you most redissolve by normal solution ph 7.2, primarily, wight the powder, then dissolve to prepare a stock solution, then you can preparation other concentration.

This is for cancer cell and neurotoxicity for leech. So i can use distilled water to make stock solution?

are you use microtiter plate, MTT assay, if you that, you can dissolve by ddH2O, but if the extract cant dissolve easily, you can use DMSO with final concentration 1-2% due to it toxicity on cell line,

commonly, we used DMSO instead of ddH2O in all bioactivity assay of extract on cell line.

thanks