Provided are compositions comprising a hydrogel formed from a cyclodextrin-modified branched polyethyleneimine and an adamantane-modified, eight-arm polyethyelene glycol. The hydrogel may contain an active agent, and can provide sustained release the active agent. Beneficial delivery characteristics may result from electrostatic or lipophilic interactions between the active agent and the hydrogel. Delivery of valproic acid from the present compositions can provide treatment following surgical resection of glioblastoma brain tumors.
