Star Structure of Antibody-Targeted HPMA Copolymer-Bound Doxorubicin:  A Novel Type of Polymeric Conjugate for Targeted Drug Delivery with Potent Antitumor Effect

The aim of this study was to compare the properties and antitumor potential of a novel type of antibody-targeted N-(2-hydroxypropyl)methacrylamide (HPMA) copolymer-bound doxorubicin conjugates with star structure with those of previously described classic antibody-targeted or lectin-targeted HPMA copolymer-bound doxorubicin conjugates. Classic antibody-targeted conjugates were prepared by aminolytic reaction of the multivalent HPMA copolymer containing side-chains ending in 4-nitrophenyl ester (ONp) reactive groups with primary NH(2) groups of the antibodies. The star structure of antibody-targeted conjugates was prepared using semitelechelic HPMA copolymer chains containing only one reactive N-hydroxysuccinimide group at the end of the backbone chain. In both types of conjugates, B1 monoclonal antibody (mAb) was used as a targeting moiety. B1 mAb recognizes the idiotype of surface IgM on BCL1 cells. The star structure of the targeted conjugate had a narrower molecular mass distribution than the classic structure. The peak in the star structure was around 300-350 kDa, while the classic structure conjugate had a peak around 1300 kDa. Doxorubicin was bound to the HPMA copolymer via Gly-Phe(D,L)-Leu-Gly spacer to ensure the controlled intracellular delivery. The release of doxorubicin from polymer conjugates incubated in the presence of cathepsin B was almost twice faster from the star structure of targeted conjugate than from the classic one. The star structure of the targeted conjugate showed a lower binding activity to BCL1 cells in vitro, but the cytostatic activity measured by (3)Hthymidine incorporation was three times higher than that seen with the classic conjugate. Cytostatic activity of nontargeted and anti-Thy 1.2 mAb (irrelevant mAb) modified HPMA copolymer-bound doxorubicin was more than hundred times lower as compared to the star structure of B1 mAb targeted conjugate. In vivo, both types of conjugates targeted with B1 mAb bound to BCL1 cells in the spleen with approximately the same intensity. The classic structure of the targeted conjugate bound to BCL1 cells in the blood with a slightly higher intensity than the star structure. Both types of targeted conjugates had a much stronger antitumor effect than nontargeted HPMA copolymer-bound doxorubicin and free doxorubicin. The star structure of targeted conjugate had a remarkably higher antitumor effect than the classic structure: a single intravenous dose of 100 microg of doxorubicin given on day 11 completely cured five out of nine experimental animals whereas the classic structure of targeted conjugate given in the same schedule only prolonged the survival of experimental mice to 138% of control mice. These results show that the star structure of antibody-targeted HPMA copolymer-bound doxorubicin is a suitable conjugate for targeted drug delivery with better characterization, higher cytostatic activity in vitro, and stronger antitumor potential in vivo than classic conjugates.