The invention provides a drug delivery system for a combination of therapeutic agents. The system includes a water soluble biodegradable ABA-type triblock copolymer that possesses thermosensitive gelation properties. The system can form a stable thermogel that includes a combination of therapeutic agents including, for example, rapamycin, paclitaxel, and 17-AAG. After administration to a subject, the drugs are released at a controlled rate from the thermogel, which biodegrades into non-toxic components. The polymer system can also function to increase the solubility and stability of drugs in the composition.