Abstract: The present invention provides long-acting sustained-release formulations of drugs such as proteins or peptides, which are injectable, completely biodegradable and safe, as well as ensuring efficient encapsulation of the drugs such as proteins or peptides without inhibiting their biological activity. It is possible to achieve injectable sustained-release formulations that ensure efficient encapsulation and long-term sustained release of drugs such as proteins or peptides while retaining their biological activity, when a solution containing a drug and a polysaccharide derivative such as hyaluronic acid having a crosslinkable functional group(s) or a salt thereof is dehydrated in microparticulate form starting from a dilute state, where crosslinking proceeds slowly, to reach a concentration which facilitates crosslinking, to thereby cause crosslinking reaction during concentration, so that the drug is encapsulated into the crosslinked polysaccharide to give drug-carrying microparticles.

Abstract: The present invention aims to provide completely biodegradable and biocompatible sustained-release carriers for proteins or peptides, which allow encapsulation of the proteins or peptides at high encapsulation rates without inhibiting their biological activity. The present invention provides a method for preparing a sustained-release carrier, wherein in a solution in the presence of a protein or a peptide, a hyaluronic acid derivative having an unsaturated bond(s) or a mercapto group(s) introduced into hyaluronic acid is chemically crosslinked with a mercapto group-containing compound or an unsaturated bond-containing compound, respectively, to give a hydrogel. The method of the present invention enables efficient encapsulation of proteins or peptides while retaining their biological activity.