Eradication of CD19+ Leukemia by Targeted Calicheamicin θ

Children with relapsed and refractory acute lymphoblastic leukemia (ALL) still face a critical prognosis. We tested the hypothesis that targeted calichearnicin theta (theta) using an anti-CD19-immunoconjugate may provide ail effective treatment strategy for CD19(+) ALL. Calicheamicin theta is a rationally designed prodrug Of the natural enediyene calichearnicin gamma, obtained by total synthesis. It offers the advantage of increased in vivo stability and 1000-fold hi-her antitumor potency over calicheamicin gamma. First, we demonstrate efficacy of calicheamicin theta against primary pre-B leukemic cells and multidrug-resistant leukemia cell lines (IC50 = 10(-9) to 10(-12) M), Second, conjugation of calicheamicin theta to an internalizing murine anti-CD19 monoclonal anti body was demonstrated to affect neither calichearnicin theta mediated cytotoxicity nor binding of the antibody to the target molecule. Third, anti-CD19-calicheamicin theta immunoconjugate revealed a maximum tolerated dose of 10 mu g/kg and CD19-specific and long-lasting eradication of established leukemia was demonstrated in a xenograft model. Finally, we show that the antileukemic effect of anti-CD19-calicheamicin theta is mediated by induction of apoptosis proceeding through the caspase-mediated mitochondrial pathway, On the basis of these results, we conclude that anti-CD19-calicheamicin theta immunoconjugates may offer a novel and effective approach for the treatment of relapsed CD19(+)ALL.