PREPARATION AND FUNCTIONAL-EVALUATION OF NEW DOXORUBICIN IMMUNOCONJUGATES CONTAINING AN ACID-SENSITIVE LINKER ON SMALL-CELL LUNG-CANCER CELLS

The anthracycline doxorubicin (DOX) is one of the most effective drugs for the treatment of small-cell lung cancer (SCLC), but its clinical application is limited by unspecific side-effects like cardiotoxicity. In the present study doxorubicin was conjugated to the monoclonal ant ibodies (mAb) SEN7, MOC31, and SWA11 via a novel acid-sensitive hydraz one linker. These mAb recognize SCLC-associated antigens of cluster 1 (NCAM), cluster 2 (EGP-2/GA733-2), and cluster 4 (CD24) respectively. To assess their potential therapeutic use against SCLC, the antigen-bi nding activities, the rates of internalization and the cytotoxic effec ts of the immunoconjugates were examined on tumour cell lines. The pre paration procedure preserved the antigen-binding activities of the mAb and yielded immunoconjugates with average drug:mAb ratios of 7:1. The hydrazone linker was found to be stable at neutral pH but to release doxorubicin under acidic conditions. In contrast to SEN7-DOX, MOC31-DO X and SWA11-DOX were rapidly internalized into SCLC target cells upon binding to their specific cell-surface antigens. Accordingly, both imm unoconjugates proved to be highly cytotoxic agents, inhibiting thymidi ne incorporation by 50% at concentrations between 0.5 mu M and 1 mu M and were 100-fold more selective than free doxorubicin. The results su ggest that binding to selective cell-surface antigens, rapid internali zation and efficient release of doxorubicin from the mAb by acid hydro lysis are required for the selective and potent function of the immuno conjugates. In particular, the use of MOC31-DOX for targeted cytotoxic therapy might be promising because of the limited cross-reactivity of the mAb with normal human tissues and its recently demonstrated tumou r localization potential in SCLC patients.