0.1% DMSO in water? 48.2 mg of pure "drug"?

If so -

You have 48.2(mg)/332(g/mol) = 0.1452 mmol of material. (n=m(g)/MW(g/mol)

Final volume 0.1452(mmol)/20(mM)= 7.259 mL (v=n/c)

o.1% DMSO in RPMI and not in water, it is extract and not the pure drug.

Thank you...

Without knowing how much if that 48.2 mg is your drug there is no way of accurately preparing a stock solution. You need to setermine the %by mass of drug in your extract.