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 antibodies (mAb) SEN7, MOC31, and SWA11 via a novel
acid-sensitive
hydrazone 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-binding activities, the rates of internalization and the cytotoxic effects of the
immunoconjugates were examined on tumour cell lines. The preparation 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-DOX and SWA11-DOX were rapidly internalized into SCLC target cells upon binding to their specific
cell-surface antigens. Accordingly, both
immunoconjugates proved to be highly
cytotoxic agents, inhibiting
thymidine incorporation by 50% at concentrations between 0.5 microM and 1 microM and were 100-fold more selective than free
doxorubicin. The results suggest that binding to selective
cell-surface antigens, rapid internalization and efficient release of
doxorubicin from the mAb by
acid hydrolysis are required for the selective and potent function of the
immunoconjugates. 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 tumour localization potential in SCLC patients.