pH-Sensitive dextran hydrogels were prepared by activation of dextran (T-70) with 4-nitrophenyl chloroformate, followed by conjugation of the activated dextran with 4-aminobutyric acid and cross-linking with 1,10-diaminodecane. The cross-linking efficiencies determined by mechanical measurements were in the range of 52-63%. Incorporation of carboxylpropyl groups in dextran hydrogels led to a higher equilibrium and faster swelling under high pH conditions. The swelling reversibility of hydrogels was also observed after repeated changes in buffers between pH 2.0 and 7.4. The slow rates of swelling and deswelling in response to changes in pH were attributed to the hydrophilic nature of dextran and formation of hydrogen bonds between the hydroxyl groups of dextran with water molecules. The pronounced effect of carboxylic acid content on degradation of hydrogels was observed after 4 h of incubation with dextranase and the influence significantly decreased after exposure to the enzyme for 8 h. The mechanism of bulk degradation of hydrogels under high swelling extent was substantiated using Coomassie blue protein assay. The release rate of bovine serum albumin from hydrogels was primarily determined by the swelling extent. The release rate was further enhanced by addition of dextranase in buffer solutions.
Three kinds of polymeric adriamycin (ADR) conjugates of dextran were synthesized, namely a dextran-Gly-Leu-Gly-ADR (DGLGA) conjugate with a lysosomally degradable tripeptide spacer group, a dextran-Gly-Leu-Gly-ADR-galactosamine (DGLGA-Ga) conjugate with a targeting moiety of galactosamine on DGLGA, and a dextran-C6H10-ADR (DC6A) conjugate with a hexamethylen spacer group. The content of the ADR moiety in the polymeric-drug conjugate was about 3 mol%. Enzyme hydrolysis of DGLGA and DC6A was carried out by incubation with papain. The total amount of ADR released after 48 h was 43 mol% for DGLGA and less than 1 mol% for DC6A. In an in vitro cytotoxicity experiment, the DGLGA-Ga conjugate has higher cytotoxic efficacy than the other conjugates for incubation with Hep-3B cells and consequently, the capability of targeting hepatoma cells of the galactosamine residue was determined. In contrast, for the incubation with SiHa cells of these conjugates, there was no significant cytotoxicity effect. The in vivo cytotoxic efficacy of each conjugate (20 mg ADR equiv./kg) against CT-26 mice colon cells implanted subcutaneously in Balb-C mice was studied. The DGLGA conjugate generated the best therapeutic effect with the presence of long-term survival (LTS) at day 50 (2/6).
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