Flutamide is soluble in ethanol (50 mg/ml) with heat as needed, yielding a clear to hazy, yellow to yellow-green solution. It is also soluble in acetone, ethyl acetate, methanol, chloroform, and ether.
The molecular weight of flutamide is 276.21, and this value can be used to calculate the amount needed to make a 1nM solution in the desired volume:
https://www.sigmaaldrich.com/support/calculators-and-apps/mass-molarity-calculator
Using freshly made solutions whenever possible is always recommended, but storing them at -20°C in non-diluted aliquots should maintain their stability for at least one month.