Your calculation is correct. The solubility data might be wrong, or was determined using another salt form of the compound. Try to scale it up to 10 mL, sonicate, or dissolve it a a higher concentration in organic solvent and than add water to the desired concentration. Make sure that the organic content in the final solution is no more than 10%.

Thanks Andrei Blasko , but this drug is poorly soluble in organic solvent and totally soluble in water.

i think i will reduce the initial stock to 50mM in this way i will dissolve the drug in 388.4ul which is enough to be soluble, also this stock i will prepare from it my working stock.

I think this is is the best thing.

Drug solubility sometimes depends on pH. try to change pH it may affect solubility. you can use Chemicalize site ( chemicalize .com) to know what happens to drug solubility with different pH .