A sustained-release preparation is provided which comprises a drug and a bioabsorbable polymer hydrogel, wherein a concentration gradient of the drug is formed in the hydrogel. Also disclosed is a method of sustained release of a drug in vivo using the sustained-release preparation of the invention. The directionality of the drug release may be controlled by employing the sustained-release preparation of the invention. The sustained-release preparation of the invention is particularly useful as an anti-cancer agent.
