It is difficult to help without the information of the drug. Since you will do the vivo experiment, water solution will be safe. If the molecular weight is 300 g/mol, then its concentration will be 18 mg/mL, which is not bad for your purpose. Since most of drugs are amine, then you can convert it from the free base into the salt form by adjusting pH 4.0 using HCl. If it is in salt form already, then its solubility in water can be up to 18 mg/mL. Since I don't know your drug, this is all I can have. Hopefully, it is helpful for you.
It can be well dissolved in water, at least in acidic water solution. However, from cayman, they don't recommend to prepare its aqueous solution >1 day (see attachment). You can dissolve it in Ethanol, but it only goes up to 30 mM.