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 .