Synthesis of poly(vinyl alcohol)-doxorubicin conjugates containing cis-aconityl acid-cleavable bond and its isomer dependent doxorubicin release.

Aconityl-doxorubicin (ADOX) was synthesized by the modified method of Shen and Ryser. Two isomers of cis-ADOX (cis-configuration) and trans-ADOX (trans-configuration) were generated in the reaction of DOX and cis-aconitic anhydride. These products were separated completely by using HPLC and analyzed by TOF-MS spectroscopy and (1)H- and (13)C-NMR experiments. The yields of cis-ADOX and trans-ADOX were 36.3 and 44.8%, respectively. The free gamma-carboxylic group of ADOX molecule was coupled to poly(vinyl alcohol) (PVA) via ethylenediamine spacer, resulting the macromolecular conjugates of PVA-cis-ADOX and PVA-trans-ADOX, respectively. The DOX content of the conjugates estimated by the hydrolysis method detected the aglycone of DOX which can be estimated as the PVA-bound DOX selectively was 4.4 w/w% which was similar to 4.6 w/w% by the ordinary UV method. Both PVA-cis-ADOX and PVA-trans-ADOX were very stable at neutral pH, but the release of DOX was increased markedly under acidic conditions. Half-life of the release of DOX from PVA-cis-ADOX at pH 5.0 was 3 h which was 4.7-fold shorter than that from PVA-trans-ADOX (14 h). The cytotoxicities of PVA-cis-ADOX and PVA-trans-ADOX were evaluated by using J774.1 cells employing a [(3)H]uridine incorporation assay as a measure of RNA synthesis. A significant difference in antitumor activity between PVA-cis-ADOX and PVA-trans-ADOX was observed where the former was much active than the later. It was suggested that the conjugate enters the cells and reaches the lysosomal/endosomal compartment, and that the aconityl spacer releases DOX from the conjugate in the acidic compartment of lysosomes/endosomes due to the participation of a free carboxylic group.